LRCS-UPJV

after 1802 days I can say… Bada Bing, Bada bang, Bada boom! Table.1

List of Figures

List of Tables

Introduction

This text presents the comprehensive result of my Ph.D. research. This project deals with the various factors that can influence the tribocorrosion process and how to reduce the surface damage produced by the combined effect of mechanical wear and corrosion. A wide variety of sectors from industry, transportation, construction, and even health, can be affected by tribocorrosion. The produced damage diminishes the lifetime of mechanical systems which can lead to major security problems and economical losses. For the purpose of this work materials used for the construction of food processing machines were studied. Moreover, a protective thin film was proposed and its tribocorrosion process was studied.

The experimental work was performed at the "Laboratoire de Physique de la Matière Condensée" (LPMC) from the "Université de Picardie Jules Verne" (UPJV) and the "Instituto de Investigaciones en Materiales" (IIM) at the "Universidad Nacional Autónoma de México" (UNAM).

The manuscript is divided in an introduction and six chapters.

Chapter 1 presents the state of the art in tribocorrosion along with the description of different experimental methods applied in this research.

Chapter 2 presents the apparatus utilized for the tribocorrosion study and the thin film obtention, along with a detailed description of the changes and adaptations carried out. Such adaptations were needed for specific tasks in this project.

A comprehensive study of the substrates is presented in Chapter 3. The mechanical and corrosive behavior of both AISI D2 steel and AISI 431 steel are presented. The combined effect of corrosion and mechanical wear was explored in order to establish the performance of each steel under the desired conditions. The performed tests were carried out in view of the tribocorrosion characterization.

The thin films synthesis by rf magnetron sputtering and the characterization of the obtained thin films are presented in chapter 4. The mechanical, corrosion and tribocorrosion behavior of the thin films are reported in chapter 5.

Finally, chapter 6 presents the general conclusions and perspectives of this work.

Food Industry and construction materials

During the last decades, the food industry has changed and grown along with the consumer markets, this evolution has produced development in the wide range of steps that comprise the process from preparation to packing and shipping food [START_REF] Heldman | Food Science[END_REF][START_REF] Shan | Canned Citrus Processing Techniques, Equipment, and Food Safety[END_REF][START_REF] Moore | Automation in the Food Industry[END_REF][START_REF] Moerman | Materials of Construction for Food Processing Equipment and Services : Requirements , Strengths and Weaknesses[END_REF].

In general, the materials that are used for the manufacture of the machines employed in this process are not controlled by the standards and guidelines created by the worldwide legislative and standardization bodies. Nevertheless, such materials are regularly selected to prevent the contamination of food by microorganisms, dirt, and chemical substances. However, the materials must be durable and mechanically stable, easy to machine, compatible with other materials used for the construction, chemically resistant and temperature resistant (hot and/or cold) [START_REF] Moerman | Materials of Construction for Food Processing Equipment and Services : Requirements , Strengths and Weaknesses[END_REF].

Due to this large number of requirements, to select the best materials of construction the manufacturers must know about the physical, chemical and thermal behavior, and at the same time, they must respond to laws, regulations, standards, and applicable guidelines. In the European Union, the legislation is given by the European Commission Regulation (EC), the general regulation is the EC No.

1935/2004 for materials intended to be in contact with food, but there are specific regulations for specific types of materials[Ekaterina [START_REF] Karamfilova | Food Contact Materials -Regulation (EC) 1935/2004[END_REF]. The materials used for food processing equipment can be classified as metals and alloys, plastics, elastomers, composites, films, and others such as glass, ceramics, wood, etc. [START_REF] Aguilera | Food Materials Science. Food Materials Science: Principles and Practice[END_REF][START_REF] Moerman | Materials of Construction for Food Processing Equipment and Services : Requirements , Strengths and Weaknesses[END_REF]. In the present work, the interest is focused on metals and thin films.

The current European regulation, the "Resolution CM/Res(2013) on metals and alloys used in food contact materials", recommends the use of aluminum, chromium, copper, gold, iron, manganese, magnesium, molybdenum, nickel, platinum, silicon, silver, titanium, zinc, cobalt, vanadium and carbon. The most common metallic alloys used in the food industry, are cast iron, steel (mild, galvanized, and stainless), nickel, copper (brass and bronze), and titanium alloys.

Since these materials can be exposed to extreme conditions where they are supposed to perform well for an extended lifetime, some improvements, such as surface thin films, have been developed. The desired properties of a thin film depend on the specific application, but in a broad sense of this work, thin films are intended to protect the equipment from an aggressive environment, reduce friction between two surfaces in contact and improve the chemical, and mechanical properties of the coated surface.

According to the European legislation, the general principles of regulation which need to be respected are: 'thin films, if used, shall be free from surface delamination, pitting, flaking, spalling, blistering and distortion when exposed to the conditions encountered in the environment of intended use, and to cleaning, bactericidal treatment or sterilization' [START_REF] Moerman | Materials of Construction for Food Processing Equipment and Services : Requirements , Strengths and Weaknesses[END_REF].

Among the different options, ceramic coatings are resistant to temperature, abrasion, high-temperature corrosion, and erosion-corrosion resistant. To use them as construction materials for food machinery they must be [START_REF] Moerman | Materials of Construction for Food Processing Equipment and Services : Requirements , Strengths and Weaknesses[END_REF]]:

• Inert.

• Nonporous and non-absorbent.

• Smooth, unbroken surfaces, entirely free of crazing, i.e., small hairline cracks • Resistant to scratching, scoring, and distortion when exposed to the conditions encountered in the environment of intended use

• Nontoxic, free of leachable lead, cadmium, and other potentially toxic heavy metals.

Canned food systems

Canning was invented in France in 1810 as a way to preserve food at room temperature for the military, originally the containers were corked glass, handmade canisters [START_REF] Yam | Encyclopedia of Packaging Technology[END_REF]]. This invention has evolved to become a common way to preserve food within a multi-billionaire industry. For example, since the 1980s, more than 45 million tons of canned food have been produced, of which more than half are tinned fruits and vegetables [START_REF] Shan | Canned Citrus Processing Techniques, Equipment, and Food Safety[END_REF]]. Only in the U.S. canned food represented a business of 15.8 billion dollars in 2016 [Grand View Research 2018].

During the canning process, different machines and equipment are used. The common sequence of the full canning process is, first, to prepare the product to be canned, then to fill and seal the containers (cans) and finally to sterilize them, regularly by pasteurization.

During the hermetically sealing step, there are two key points to consider. One is that a proper vacuum must be achieved so that the container is sealed while the food is near or at boiling point. The most common method of producing an internal vacuum is by displacing the air in the can with steam [START_REF] Yam | Encyclopedia of Packaging Technology[END_REF]]. The other key point is to guarantee the correct closure of the can since this will assure a long shelf life and prevent microbiological spoilage. This step is made using a seaming machine [START_REF] Featherstone | Canning Operations[END_REF][START_REF] Shan | Canned Citrus Processing Techniques, Equipment, and Food Safety[END_REF]] composed of pressure heads (to prevent cans from shifting while they are being sealed), trays (to transport the cans), chucks, and rolls (round-shaped pieces to fold and seam the cans) [START_REF] Yam | Encyclopedia of Packaging Technology[END_REF]; [START_REF] Shan | Canned Citrus Processing Techniques, Equipment, and Food Safety[END_REF].

Figure 1 shows an illustration of the seaming system based on chucks and rolls for closing cans. In it, one can see how the chuck will keep the can lid in position while the chuck presses the lid to the can body. To close the can there are two possibilities,

(1) the can does not move while the roller pushes and turns, or (2) the can turns while the roller just applies pressure. The turning speed is regularly measured in terms of cans per minute, and this can go from 25 up to 275 in a range of can diameters from 200 mm to 603 mm.

Figure 1 Scheme of a can seaming system [START_REF] Yam | Encyclopedia of Packaging Technology[END_REF]].

Construction materials for seaming rolls

The rollers are normally manufactured of hard and wear resistance materials steel like AISI D2 steel since they shouldn't be in contact with lubricant oil, metal debris, sewage, and other pollutants [START_REF] Yam | Encyclopedia of Packaging Technology[END_REF]]. To improve the performance and make the parts last longer, it is possible to find rolls with metal nitrides or carbides films, which due to the metallic character of their chemical bonds, are well known for good electrical and thermal conductivities, metallic lustre and mechanical hardness, wear resistance and very low chemical reactivity.

Commercially it is possible to find AISI D2 steel rollers uncoated and coated with titanium nitride and titanium carbide thin films. Both have been extensively studied,

and it has been shown that they exhibit good corrosion resistance [START_REF] Chen | On the Corrosion Behavior of TiN-Coated AISI D2 Steel[END_REF][START_REF] Fenker | Corrosion Behaviour of Decorative and Wear Resistant Coatings on Steel Deposited by Reactive Magnetron Sputtering -Tests and Improvements[END_REF][START_REF] Kral | Critical Review on the Elastic Properties of Transition Metal Carbides, Nitrides and Carbonitrides[END_REF][START_REF] Igual Muoz | Tribocorrosion Mechanisms in Sliding Contacts[END_REF], as well as an excellent wear resistance under a variety of conditions [START_REF] Henry | Wear Mechanisms of Titanium and Aluminium Nitride Coatings: A Microtribological Approach[END_REF][START_REF] Samano | Mechanical Properties Optimization of Tungsten Nitride Thin Films Grown by Reactive Sputtering and Laser Ablation[END_REF][START_REF] Ching | Effects of Surface Coating on Reducing Friction and Wear of Orthopaedic Implants[END_REF]. A summary of the hardness and common thickness values of coatings on rollers is presented in Table 1. The transition metal nitrides commonly have crystalline structures where the atoms of nitrogen occupy interstitial sites in the metal lattice. This kind of arrangement and type of bonding can confer these compounds with excellent chemical, physical and mechanical properties such as resistance to deformation, abrasive wear and corrosion along with high melting points, metallic, lustre, and even superconductivity.

Due to their properties, these nitrides can be used for different applications as structural, electronic, magnetic, optical, and catalytic materials. Elements that belong to the same group and present similar behavior and properties, such as molybdenum and tungsten and the nitrides of these elements can present similar performance and properties as chromium nitride.

Often, to obtain wear-resistance nitride thin films, methods such as Chemical

Vapor Deposition (CVD) and Physical Vapor Deposition (PVD) are employed. In general, the mechanical and electrochemical properties of different nitride thin films have been reported, but there are almost no publications about their tribocorrosion properties.

Tungsten nitride as a protective film

As we mentioned before, among the nitride compounds tungsten nitride presents a range of useful properties, such as excellent hardness, chemical stability and thermal stability [START_REF] Gonohe | Tungsten Nitride Deposition by Thermal Chemical Vapor Deposition as Barrier Metal for Cu Interconnection[END_REF][START_REF] Samano | Mechanical Properties Optimization of Tungsten Nitride Thin Films Grown by Reactive Sputtering and Laser Ablation[END_REF][START_REF] Addonizio | Influence of Process Parameters on Properties of Reactively Sputtered Tungsten Nitride Thin Films[END_REF]Polcar, Parreira, and Cavaleiro 2007b;[START_REF] Mischler | Triboelectrochemical Techniques and Interpretation Methods in Tribocorrosion: A Comparative Evaluation[END_REF][START_REF] Nakajima | Reaction of Metal, Carbide, and Nitride of Tungsten with Hydrogen Peroxide Characterized by 183 W Nuclear Magnetic Resonance and Raman Spectroscopy[END_REF][START_REF] Wen | Growth, Stress and Hardness of Reactively Sputtered Tungsten Nitride Thin Films[END_REF]]. Because of that, different efforts to produce this nitride as a thin film have been carried out.

Some of the reported methods are reactive magnetron sputtering [START_REF] Hones | Structural and Mechanical Properties of Chromium Nitride, Molybdenum Nitride, and Tungsten Nitride Thin Films[END_REF][START_REF] Hones | Mechanical Properties of Hard Chromium Tungsten Nitride Coatings[END_REF][START_REF] Addonizio | Influence of Process Parameters on Properties of Reactively Sputtered Tungsten Nitride Thin Films[END_REF][START_REF] Prakash | Structural, Corrosion and Mechanical Properties of Sputtered Deposited Chromium Tungsten Nitride (Cr1-XWxN) Nanocomposite Thin Films[END_REF][START_REF] Baker | Reactive Sputter Deposition of Tungsten Nitride Thin Films[END_REF]Polcar, Parreira, and Cavaleiro 2007b;[START_REF] Mischler | Triboelectrochemical Techniques and Interpretation Methods in Tribocorrosion: A Comparative Evaluation[END_REF]Shen and Mai 2000;[START_REF] Wen | Growth, Stress and Hardness of Reactively Sputtered Tungsten Nitride Thin Films[END_REF], pulsed laser deposition [START_REF] Soto | Tungsten Nitride Films Grown via Pulsed Laser Deposition Studied in Situ by Electron Spectroscopies[END_REF]], atomic layer deposition, and chemical vapor deposition [START_REF] Gonohe | Tungsten Nitride Deposition by Thermal Chemical Vapor Deposition as Barrier Metal for Cu Interconnection[END_REF]].

Due to its properties, one can consider that tungsten nitride might be a suitable protective thin film to be used on construction materials for the food industry. The present study aims for an industrial application, and one of the most suitable synthesis methods is reactive sputtering, as sputtering is already industrially used to produce thin films at industrial level. Some of the advantages of this method are that precursor materials are often not hazardous [Beitz and Kuttner 1994;[START_REF] Addonizio | Influence of Process Parameters on Properties of Reactively Sputtered Tungsten Nitride Thin Films[END_REF], highquality films can be applied to large areas, and the film surface can have very few defects [START_REF] Fenker | Corrosion Behaviour of Decorative and Wear Resistant Coatings on Steel Deposited by Reactive Magnetron Sputtering -Tests and Improvements[END_REF].

With reactive sputtering, it is possible to obtain tungsten nitride in a range of various compositions and structures depending on the experimental settings. The stoichiometric phase W2N has been reported as the most stable phase, which is a crystalline face centered cubic structure as shown in Figure 2. Moreover, to obtain such structure, by reactive sputtering, it is recommended to use around 12% of N2 in the mixture of gases during the deposition process [START_REF] Addonizio | Influence of Process Parameters on Properties of Reactively Sputtered Tungsten Nitride Thin Films[END_REF][START_REF] Baker | Reactive Sputter Deposition of Tungsten Nitride Thin Films[END_REF].

As for the known properties of this material, hardness and Young's modulus can have values from 29 to 39 GPa and 300 to 390 GPa, respectively [Polcar, Parreira, and Cavaleiro 2007b]. Such values depend on the composition and structure of the tungsten nitride thin film. Moreover, large nitrogen contents are associated with the higher hardness while the largest Young's modulus is related to the smallest nitrogen concentration.

In general, the studies of tungsten nitride have been focused on its mechanical properties [Polcar, Parreira, and Cavaleiro 2007a;[START_REF] Hones | Mechanical Properties of Hard Chromium Tungsten Nitride Coatings[END_REF][START_REF] Wen | Growth, Stress and Hardness of Reactively Sputtered Tungsten Nitride Thin Films[END_REF]Polcar, Parreira, and Cavaleiro 2007b;[START_REF] Mischler | Triboelectrochemical Techniques and Interpretation Methods in Tribocorrosion: A Comparative Evaluation[END_REF][START_REF] Samano | Mechanical Properties Optimization of Tungsten Nitride Thin Films Grown by Reactive Sputtering and Laser Ablation[END_REF]], but some people had explored other applications such as electrodes [START_REF] Villaseca | Synthesis and Characterization of Tungsten Nitride (W2N) from WO3 and H2WO4 to Be Used in the Electrode of Electrochemical Devices[END_REF], and optical devices [START_REF] Addonizio | Influence of Process Parameters on Properties of Reactively Sputtered Tungsten Nitride Thin Films[END_REF]]. To the best of our knowledge very little literature exists about its electrochemical behavior and corrosion resistance. There are only reports about metallic tungsten [START_REF] Stansbury | Effect of Electrode Potential on the Tribocorrosion of Tungsten[END_REF]] and nitrides of other metals combined with tungsten [START_REF] Prakash | Structural, Corrosion and Mechanical Properties of Sputtered Deposited Chromium Tungsten Nitride (Cr1-XWxN) Nanocomposite Thin Films[END_REF][START_REF] Fenker | Corrosion Behaviour of Decorative and Wear Resistant Coatings on Steel Deposited by Reactive Magnetron Sputtering -Tests and Improvements[END_REF][START_REF] Fathollahzade | Electrochemical Evaluation of Corrosion and Tribocorrosion Behaviour of Amorphous and Nanocrystalline Cobalt-Tungsten Electrodeposited Coatings[END_REF]. As for the combined effect of tribocorrosion of tungsten nitride, no information has been published at the moment of drafting this thesis.

Because of its known properties, the use of tungsten nitride as a thin film to protect against tribocorrosion was explored. Such thin films were synthesized by rf magnetron sputtering under different conditions. To probe the tribocorrosion resistance a special cell was designed and constructed.

Figure 2 Representation of W2N fcc crystalline structure [START_REF] Baker | Reactive Sputter Deposition of Tungsten Nitride Thin Films[END_REF] Chapter 1

State of the art and experimental techniques

1.1 Surface wastage, a tribocorrosion approach.

A physical surface is part of a solid that presents certain electronic, mechanical, and chemical properties that can influence the behavior of the bulk material. It can be broadly defined as a geometrical boundary between the solid and the contact with other solids or the environment [START_REF] Menezes | Tribology for Scientists and Engineers From Basics to Advanced Concepts[END_REF]]. The study of solid surfaces, although complex, can lead to an understanding of superficial phenomena and partial or total control of the surface properties. The preparation of a material surface is fundamental for the industry when the interaction between surfaces and the environment predefines functions related to aspects such as friction, resistance to mechanical stress, wear, and corrosion resistance. The structure of a surface can be described by its roughness, waviness, lay, and flaws all of which determine its texture.

When a solid is in contact with another solid surface, several changes can happen for example, elastic and plastic deformations, chemical reactions, and material loss. In Essence, tribology has been developed to study the mechanical interactions between two surfaces in static contact or relative motion [START_REF] Heldman | Food Science[END_REF][START_REF] Street | Guidance on Food Additives[END_REF]Imeta Srl 2018].

Surface tribology and wastage

Tribology covers a broad spectrum of arrangements from micro and nanomechanical systems up to the field of traditional macroscopic tribology and to the contacts between tectonic plates [START_REF] Menezes | Tribology for Scientists and Engineers From Basics to Advanced Concepts[END_REF]].

To understand the relevance of tribology, one only need to think about any phenomena whenever two solids touch each other. Here the forces of action and reaction are brought into play and surface interaction dominates. Due to this interaction, surface structure changes can occur, and matter may be transferred between surfaces, or the surfaces can break.

Wear generalities

Around the year 1500 Leonardo da Vinci got interested in the study of mechanics that had been abandoned after by the ancient Greeks. His research about movement gave him ideas of concepts such as inertia, momentum, efficiency, friction, and center of gravity. The fundamental laws of Leonardo da Vinci, which were re-discovered two centuries later by Amontons:

• "The friction force is proportional to the weight of the body that moves over a horizontal plane and to the normal load applied to a surface."

• "Friction is independent of the apparent contact area."

• "Friction depends on the materials and the state of their surfaces."

These laws were completed by Coulomb who described the difference between static and dynamic friction forces, by saying: "The friction force is practically independent of the sliding speed of surfaces."

These laws are now the base of tribology which comprises the study of friction, wear, and lubrication. In tribology, we study the phenomena that concern the set of contact factors that play a role in friction, ergo, mechanical stress, temperature, third body behavior, etc. These factors are the mechanical, physical, and chemical properties of the materials.

Friction is a tribology phenomenon observed in a great variety of sliding and rolling situations, and it can only be defined in a system of at least two bodies in contact. Furthermore, it can be defined as the force that acts against the relative motion between two bodies in contact. Friction can be dry or fluid; dry friction, also known as Coulomb friction, occurs under dry conditions while fluid friction occurs in lubricated conditions [START_REF] Menezes | Tribology for Scientists and Engineers From Basics to Advanced Concepts[END_REF]. Friction is often described by a dimensionless scalar value called the coefficient of friction (µ) which is defined as the ratio of the friction force F to the normal load FN.

µ = F 𝐹 𝑁 𝑒𝑞. (1.1)
Where the normal force is defined as the net force compressing two parallel surfaces together and the friction force is the resisting force which acts in an the opposite direction to the direction of motion [START_REF] Menezes | Tribology for Scientists and Engineers From Basics to Advanced Concepts[END_REF]Beitz and Kuttner 1994].

When there is relative motion between two surfaces, the progressive loss of material from any of these caused by mechanical influences is called wear. The material loss is regularly exhibited by the appearance of small detached particles and changes in the surfaces exposed to a normal force [Beitz and Kuttner 1994;[START_REF] Scharf | Solid Lubricants: A Review[END_REF][START_REF] Godet | Hard Coatings for Tribological Applications: A Pluridisciplinary Approach[END_REF][START_REF] Menezes | Tribology for Scientists and Engineers From Basics to Advanced Concepts[END_REF]]. This effect is normally undesired therefore, many systems have been studied to understand how wear occurs and how to find ways to diminish the material loss [START_REF] Jiang | Modelling Sliding Wear: From Dry to Wet Environments[END_REF][START_REF] Bailey | Effect of Roughness and Sliding Friction on Contact Stresses[END_REF][START_REF] Roberts | The Effect of Substrate Surface Roughness on the Friction and Wear of Sputtered MoS2 Films[END_REF][START_REF] Castillejo | Wear and Corrosion Resistance of Niobium-Chromium Carbide Coatings on AISI D2 Produced through TRD[END_REF][START_REF] Henry | Wear Mechanisms of Titanium and Aluminium Nitride Coatings: A Microtribological Approach[END_REF][START_REF] Cho | Wear Behavior of AISI D2 Steel by Enhanced Ion Nitriding with Atomic Attrition[END_REF] Wear is produced by the effect of physical and chemical processes which occur during the relative motion of two bodies in contact. 'Depending on the physical and mechanical processes occurring in the body surfaces it is possible to recognize different types of wear such as:

• adhesion, which is produced by the formation and breaking of atomic links between the surfaces in contact.

• abrasion, that refers to the scratching and micromachining of surfaces and by an increase in the roughness.

• surface disruption, this can happen when at least one of the surfaces cracks or propagates a crack in the opposing surface, with separation of particles due to alternating loads in the system.

• tribo-chemical reactions, this is present when there are chemical reactions of the bodies in contact, or the ambient, and the tribological reaction can then be enhanced due to the appearance of friction.'

• lubrication refers to the presence of a lubricant.

Lubricants are used to lower the friction between two sliding bodies and therefore reduce wear. Additionally, they can help to distribute heat, remove contaminants, and improve the efficiency of mechanical systems. In general, lubricants are classified as liquid, solid, or gaseous [START_REF] Menezes | Tribology for Scientists and Engineers From Basics to Advanced Concepts[END_REF]. Solid lubricants are commonly used when lubrication under extreme conditions is required, such as very high or low temperatures, aggressive environment, or vacuum [Beitz and Kuttner 1994].

Contact mechanics

In general, contact can be described by a set of external efforts, forces, and momentum applied over two surfaces. The contact of two ideal surfaces of two different bodies under the action of a normal force (FN) will create strain and deformations over both bodies. Here the contact pressure (P) will appear over the contact area (A), in between the two surfaces. Such pressure is defined by:

𝑃 = 𝐹 𝑁 𝐴 𝑒𝑞. (1.2)
In the region where the contact is made, there will be a certain deformation. From a mechanics point of view, deformations have two regimes, each with different properties. The nature of the contact will depend on these regimes, and therefore on its mechanical properties.

One of these regimes is the Hertzian contact, in which there are only plastic deformations, commonly found in a sphere/sphere, sphere-plane and parallel cylinders contact types. It is possible to consider a Hertzian contact if:

• The material is homogeneous and isotropic.

• The deformation is considered small.

• There is no friction between the surfaces (absence of tangential forces).

• The contact area ratio is much smaller than the curvature ratio.

In the case of contact between a sphere and a plane (Figure 1.1), it is possible to calculate the deformation and stress by applying the theory of Hertz. According to this theory, if we have a contact with a FN different from zero, both bodies in contact will be deformed in a circular region of radius (a) given by the following expression:

𝑎 = ( 4 3 𝑅 𝐸 ′ 𝐹 𝑁 ) 1 3 ⁄ 𝑒𝑞. (1.3)
Where R is the radius of the sphere and E' is the combined Young's modulus given by the Young's modulus of each material (E1, E2) and their Poisson ratio (𝜈1, 𝜈2) in the following expression:

1 𝐸 ′ = 1 -𝜈 1 2 𝐸 1 + 1 -𝜈 2 2 𝐸 2 𝑒𝑞. (1.4)
Now it is possible to define the contact area (A) as:

𝐴 = 𝜋𝑎 2 𝑒𝑞. (1.5)
With an average pressure or Hertz pressure (Pm) given by:

𝑃 𝑚 = 𝐹 𝑁 𝜋𝑎 2 𝑒𝑞. (1.6)
Taking into consideration all the aspects affecting wear, it is not possible to reduce the concept to a simple material loss. One must keep in mind that this definition hides numerous different phenomena, which produce a set of geometric and physicochemical changes to the superficial layers exposed to friction.

Main wear mechanisms in sliding contacts

To determinate the wear mechanisms in a given sliding contact it is important to consider a set of phenomena not always easy to interpret. These phenomena are the emission of debris, mass loss, the shape of the wear track, and the physical and chemical changes in the surface in contact. For sliding contacts, four main types of wear mechanisms are recognized [START_REF] Ionescu | Erisation Des m ´ Ecanismes d ' Usure Par Tribocorrosion d ' Alliages Mod ` To Cite This Version : Caractérisation Des Mécanismes d ' Usure Par Tribocorrosion d ' Alliages Modèles Ni -Cr[END_REF]Cao and Mischler 2018b] and described next. between a sphere and a plane.

Adhesive wear

When in a sliding contact a surface loses material, the material is transferred and attached to the surface of another body so junctions of asperities might be formed.

Frequently, the material transfer will occur from a soft to a hard material surface, so subsequent sliding will produce sheared and loose wear debris. This kind of wear will occur when the two surfaces experience short-range Van der Waals forces, which come into action at about 1 nm to form strong adhesive junctions. For this type of mechanism, the amount of wear depends on the location of the junctions sheared. A schematic representation of this wear mechanism is shown in Figure 1.2 c.

Abrasive wear

Abrasive wear comes from material displacement produced by hard particles or hard protuberances. When a hard material slides along a soft surface with protuberances, due to the shear strength there will be two-body abrasive wear (Figure 1.2 a), but if wear debris gets caught between two surfaces and at least one of the surfaces is abraded, then it is called third-body abrasive wear (Figure 1.2 b).

The model to describe abrasive wear assumes that the asperities will act as a sharp tool that will displace a volume of material. The surfaces damaged by abrasion present wear marks of variable depth parallel to the movement. Wear is constant in function of time while the debris volume grows linearly with the applied load and the distance.

The material lost depends on the wear and antagonist materials, the wear marks give further information about the origin of the asperities [START_REF] Ionescu | Erisation Des m ´ Ecanismes d ' Usure Par Tribocorrosion d ' Alliages Mod ` To Cite This Version : Caractérisation Des Mécanismes d ' Usure Par Tribocorrosion d ' Alliages Modèles Ni -Cr[END_REF]. When the wear marks of little depth and shine, the asperities of the antagonist piece correspond to oxide layers.

If the wear marks are isolated, and severely interrupted, then hard particles have been introduced in between the surfaces, hence such hard particles are found to be incrusted. Whereas, when they are interrupted and have scratched the hard surface, then the abrasive particles are embedded in the soft surface. Finally, if they are interrupted and present over the soft material, it means that the antagonist piece is very rough.

Fatigue wear

Surface fatigue can be observed at microscopic levels when asperities are found between sliding surfaces. The repeated loading and unloading cycles (stress cycling) may induce the formation of cracks so that a critical number of cycles will result in the breakup of the surface, leaving large pits. Unlike adhesion and abrasion, the asperities in fatigue wear make contact without adhering or abrading, but instead deforming plastically the surface and subsurface until cracks nucleate [Cao and Mischler 2018b;[START_REF] Menezes | Tribology for Scientists and Engineers From Basics to Advanced Concepts[END_REF]. Once the cracks are formed, further loading will make cracks extend and propagate, a scheme of this type of wear is shown in Figures 1.2d and 1.2e.

Corrosive wear

When sliding takes place in a corrosive environment, the wastage of the surface can be called corrosive wear. In such a process, a chemical or electrochemical reaction contributes to material removal. The reaction layers in the contact lead to the formation of wear debris consisting mainly of reaction products. Currently, this type of wear is studied by tribocorrosion methods [Cao and Mischler 2018b;[START_REF] Landolt | Electrochemical Methods in Tribocorrosion: A Critical Appraisal D[END_REF].

Corrosion of materials

Corrosion occurs when a material reacts by exchanging electrons (oxidationreduction) with the surrounding environment. When the material and the environment (electrolyte) are electrical conductors, this phenomenon occurs as electrochemical corrosion. For example, a metal immersed in a solution. When corrosion occurs, there are modifications in the composition of the material surface, generally as a functional degradation of the material and a modification of the environment. Redox reactions can be described in a simplified way, written as follows:

𝑂𝑥𝑖𝑑𝑎𝑡𝑖𝑜𝑛 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛: 𝑀 → 𝑀 As wear, corrosion resistance is not an intrinsic property of the material. It depends completely on the physicochemical conditions and the environment.

Interface solid/liquid

When a metal is in contact with an aqueous solution, an interface is formed. This interface (commonly named electrical double layer) is constructed of different layers, as displayed in Figure 1.3. The nearest to the metal surface (inner Helmholtz layer) is where electric charges of the adsorbed ions are located. This is followed by a second layer called Helmholtz outer layer, which contains solvated ions. Finally, a "diffuse layer" is made of ions distributed in a three-dimensional region between the Helmholtz outer layer and the bulk [START_REF] Fernandez-Solis | Fundamentals of Electrochemistry[END_REF].

When this interface is formed an equilibrium is reached. This equilibrium has an associated potential, also known as the open circuit potential (EOC/OCP), which represents an electrochemical cell when no current is flowing [START_REF] Amemiya | Corresponding Authors[END_REF]].

The electrochemical potential (𝜇 𝑖 ̅ ), represents the mechanical work required to change 1 mol of ions with valency z and charge ze from a standard state to a specific Where µi represents the chemical potential of the species i, F is the Faraday constant and Φ the local electrostatic potential.

1.1.6 Corrosion rate determinations.

In general, the corrosion rate is defined by the corrosion mechanisms. These mechanisms are established by the atomic, molecular, or ionic transport processes that take place at the interface. The processes can occur over several steps, where the slowest of them commonly controls the rate of the overall reaction. Since it is not possible to directly observe on an atomic scale these steps, it is necessary to infer probable mechanisms from indirect measurements and observations. For example, measurements of electrical potential and current.

The corrosion rate of a material depends on both the material properties and the environment. For the study of a material, it is important to consider composition, crystallinity, grain size, and surface roughness. In the case of the environment: conductivity, pH, and corrosion products.

Corrosion rate determinations are frequently done using a cell in a three electrodes arrangement. Those electrodes are a Working Electrode (WE) that will be the analyzed sample, a Counter Electrode (CE) which will be an electrode of a material immune to the conductive environment or electrolyte and a Reference Electrode (RE). An image of this arrangement is shown in Figure 1.4.

Figure 1.4 representation of a three electrodes cell [START_REF] Amemiya | Corresponding Authors[END_REF] 1.1.6.1 Potentiodynamic curves and Tafel extrapolation method

One option to estimate the corrosion rate is to take current measurements while the working electrode is polarized from the EOC. The measured data is expressed as an E (log i) curve as shown in Figure 1.5. To apply this method, the resulting polarization curve must present a linear or "Tafel region" in at least one of the branches (cathodic or anodic) [START_REF] Fernandez-Solis | Fundamentals of Electrochemistry[END_REF]. A potential scan of around ± 100 mV about Ecorr is required to determine whether a linear section of at least one decade of current is present so that a reasonably accurate extrapolation can be made. The linear section must be present in at least one of the redox branches. Tafel regions can be observed at some tens millivolts from the corrosion potential (Ecorr).

Corrosion currents can be converted into corrosion rates because the oxidation at a specific current corresponds to a particular loss of material. To transform the corrosion current to corrosion rate (CR) eq.1.11 can be applied. Where: EW =Equivalent weight of the material, K= 3.27 x 10 -3 mm g/µA cm year, ρ= Material density, and A = sample area 1.1.6.2 Electrochemical Impedance Spectroscopy (EIS).

EIS tests consist of the application of small sinusoidal perturbations of a potential

(5-10 mV), typically in a range of frequencies between 10 -4 and 10 6 Hz [START_REF] Stansbury | Effect of Electrode Potential on the Tribocorrosion of Tungsten[END_REF][START_REF] Fernandez-Solis | Fundamentals of Electrochemistry[END_REF]]. Therefore, it is considered a non-destructive method for the evaluation of a wide range of materials. The response to these perturbations is the electrical current, and it can be analyzed as a sum of sinusoidal functions (Fourier series). This analysis can provide information about parameters such as corrosion rate, electrochemical reaction mechanisms, and kinetics.

The excitation signal, expressed as a time function, can be written as:

𝐸 𝑡 = 𝐸 0 sin(𝜔𝑡) 𝑒𝑞. (1.12)
Where Et is the potential at the time t, E0 is the signal amplitude and, ω is the radial frequency defined as:

𝜔 = 2𝜋𝑓 𝑒𝑞. (1.13)
In a linear system, the response as electrical current (It) is shifted in phase (φ) with a different amplitude (I0):

𝐼 𝑡 = 𝐼 𝑜 cos(𝜔𝑡 -𝜑) 𝑒𝑞. (1.14)
The analysis of the electric current measurements allows to determine the complex impedance:

𝑍 (𝜔) = |𝛥𝐸| |𝛥𝐼| 𝑒𝑞. (1.15)
Z represents the ratio between the sinusoidal electric potential and the electric current. Often a complex plane representation of the elements, that describes the impedance, is used. Here the potential can be written as:

𝐸 𝑡 = 𝐸 0 𝑒 -𝑖𝜔𝑡 𝑒𝑞. (1.16)
With 𝑖 = √-1. We have,

𝐼 (𝑡) = 𝐼 0 𝑒 𝑖(𝜔𝑡-𝜑) 𝑒𝑞. (1.17)
Which allows simplifying the radio in eq. 1.15 as:

|𝑍|𝑒 𝑖𝜑 = |𝑍|[cos(𝜑) + 𝑖 sin(𝜑)] 𝑒𝑞. (1.18)
To analyze EIS results the equivalent circuits method is commonly used. Regularly the Randles circuit, which describes the response as a combination of capacitors and resistors, is the starting point [START_REF] Fernandez-Solis | Fundamentals of Electrochemistry[END_REF][START_REF] Orazem | Electrochemical Impedance Spectroscopy[END_REF].

To have an idea of the behavior, the results are frequently displayed as Nyquist and Bode plots (Figure1.6).

Tribocorrosion

Tribocorrosion is the research area that combines the domains of tribology and corrosion, which seeks to understand the surface degradation mechanisms when mechanical wear and electrochemical processes interact with each other [START_REF] Wood | Tribo-Corrosion of Coatings: A Review[END_REF][START_REF] Fathollahzade | Electrochemical Evaluation of Corrosion and Tribocorrosion Behaviour of Amorphous and Nanocrystalline Cobalt-Tungsten Electrodeposited Coatings[END_REF][START_REF] Gracia-Escosa | Tribocorrosion Behavior of TiBxCy/a-C Nanocomposite Coating in Strong Oxidant Disinfectant Solutions[END_REF][START_REF] Obadele | Tribocorrosion Behaviours of AISI 310 and AISI 316 Austenitic Stainless Steels in 3.5% NaCl Solution[END_REF][START_REF] Nix | Mechanical Properties of Thin Films[END_REF][START_REF] Arunkumar | A Review of Indentation Theory[END_REF]]. This research area began in the late 1980s and has merged in recent years driven by the increasing demand for biomedical implants, power generation, marine, and offshore industries [START_REF] Garcia | Food Contact Materials[END_REF][START_REF] Obadele | Tribocorrosion Behaviours of AISI 310 and AISI 316 Austenitic Stainless Steels in 3.5% NaCl Solution[END_REF][START_REF] Nix | Mechanical Properties of Thin Films[END_REF][START_REF] Kempf | Nanohardness Measurements for Studying Local Mechanical Properties of Metals[END_REF]]. This research area aims to understand and control tribocorrosion processes to improve the useful lifetime of man-made objects and machines, and to allow safe operation of machines, and devices. [START_REF] Fathollahzade | Electrochemical Evaluation of Corrosion and Tribocorrosion Behaviour of Amorphous and Nanocrystalline Cobalt-Tungsten Electrodeposited Coatings[END_REF][START_REF] Gracia-Escosa | Tribocorrosion Behavior of TiBxCy/a-C Nanocomposite Coating in Strong Oxidant Disinfectant Solutions[END_REF][START_REF] Obadele | Tribocorrosion Behaviours of AISI 310 and AISI 316 Austenitic Stainless Steels in 3.5% NaCl Solution[END_REF][START_REF] Nix | Mechanical Properties of Thin Films[END_REF] The degradation of a surface by tribocorrosion depends on a wide range of parameters related to the corrosive environment and the materials in contact with it.

Figure1.7 shows a scheme of such parameters. The details of the contact play a critical role in determining the effect of corrosion on the wastage rate, to understand the mixed contribution first we need to understand how each contribution works separately. Total wear is caused by friction, changes of the electrochemical state, and kinetics of the reactions that are expressed as the destruction of surface layers. The destruction of the surface layers will result in a heterogeneous surface which can provoke galvanic coupling between the slid track and the passive film.

Influence of sliding over corrosion resistance and influence of corrosion over sliding.

Some materials create a passive film over the surface (passivating materials). This passive film can be damaged or partially removed in a sliding system. When this happens in a corrosive environment [Richard and Geringer 2019], a redox reaction occurs. Such a reaction occurs to rebuild the passive film. In certain systems, this will also imply a parallel dissolution of material into the environment. In general, corrosion produces modifications in the surface composition, and so in the mechanical properties. Similarly, the formation of oxides or corrosion products on the surface can originate or modify a third body, and therefore the friction and the wear mechanism.

In an aggressive environment, modifications in composition and surface properties can produce selective dissolution, which can be accelerated by the galvanic coupling effect [Richard and Geringer 2019].

Nevertheless, tribocorrosion can also be controlled by mechanical phenomena, where the corrosion of the surface indirectly participates in the production of abrasive particles.

Synergism between corrosion and wear

Simultaneous mechanical and chemical interaction can occur in a great variety of conditions. Two or three bodies in contact and relative motion put in an aggressive environment, often behave as tribocorrosion systems. The relative movement of the contact surfaces can be unidirectional or alternating.

The consequences of friction and corrosion coupling are complex to study and master. To deduce the tribocorrosion behavior of a system it is not enough to know the tribological system in absence of a corrosive environment and the performance of an electrochemical system not exposed to friction. Friction affects the corrosion behavior as much as corrosion modifies the overall response of the tribological system. So, it is possible to say that there is a synergy between corrosion and friction, which will affect the wear mechanisms and the total material loss.

Since tribocorrosion is an independent phenomenon, to try to estimate the details.

Various groups have proposed to measure the synergism between tribology and corrosion, so the total wear (Wt) will be given by the sum of a mechanical contribution (Wm), an electrochemical one (Wc) and, a synergism term (WS). The last term represents the amount of material loss due to the combined effect of corrosion and wear. The simplest way to express this relation is given by eq. 1.19.

𝑊 𝑡 = 𝑊 𝑚 + 𝑊 𝑐 + 𝑊 𝑠 𝑒𝑞. (1.19)
The synergism term is defined here as the sum of the change of wear due to corrosion (Wcm) and the modification in corrosion due to wear (Wmc).

𝑊 𝑠 = 𝑊 𝑐𝑚 + 𝑊 𝑚𝑐 𝑒𝑞. (1.20)
If we substitute 1.20 in 1.19, we get:

𝑊 𝑡 = 𝑊 𝑚 + 𝑊 𝑐 + 𝑊 𝑐𝑚 + 𝑊 𝑚𝑐 𝑒𝑞. (1.21)
The effect of wear on corrosion is referred to as the 'additive effect'. The 'synergistic effect' is defined as the enhancement of wear due to corrosion and the 'negative synergism' or 'antagonistic effect' is the protection of the surface against further wear due to the electrochemical formation of a protective layer [START_REF] Levy | DEPOSITION OF TUNGSTEN NITRIDE[END_REF][START_REF] Igual Muoz | Tribocorrosion Mechanisms in Sliding Contacts[END_REF][START_REF] Landolt | Tribocorrosion of Passive Metals and Coatings[END_REF][START_REF] Levy | ASTM G119-09: Guide for Determining Synergism Between Wear and Corrosion[END_REF]].

Tribocorrosion evaluation by the American Society for Testing and Materials, Standard guide for determining synergism between wear and corrosion (ASTM G119-09)

To determinate the synergism effect in a tribocorrosion system, there are two frequently used ways. The first one is proposed by the ASTM in the standard guide G119-09[ASTM G119-93 1998]. This standard proposes a series of steps and tests to estimate the synergism on tribocorrosion of metals. The steps and description of the measurements are summarized below. With these results, it is possible to see if the "synergism effect" enhances the wear due to corrosion (positive synergism) or protect the surface (negative synergism). The main limitation of this method resides in the fact that under the experimental set up it is not possible to truly separate the different contributions in the total wear loss, for example in the cathodic polarization tests described in III it is possible to assume that material loss is only due to wear, even though the absence of corrosion products can affect the response of the sample tested. So, this will not correctly represent the reallife wear behavior of a surface [START_REF] Celis | Testing Tribocorrosion of Passivating Materials Supporting Research and Industrial Innovation : Handbook[END_REF]. Moreover, applying eq. 1.28 the ratio between corrosion and mechanical material losses (Kc) can be calculated.

Electrochemical Impedance Spectroscopy as a tool for tribocorrosion

𝐾 𝑐 = 𝑊 𝑎𝑐𝑡 𝑐 + 𝑊 𝑟𝑒𝑝𝑎𝑠𝑠 𝑐 𝑊 𝑎𝑐𝑡 𝑚 + 𝑊 𝑟𝑒𝑝𝑎𝑠𝑠 𝑚 𝑒𝑞. (1.28)
In general, the Kc value can describe three behaviors; i. Kc > 1, the material loss is predominantly due to corrosion. The total wear is mainly controlled by the reactivity of the substrate with the environment. ii. Kc<1, the material loss is predominantly caused by mechanical wear and iii. Kc << 1, the contribution due to the acceleration of corrosion induced by the destruction of the passive film is negligible compared to the total wear.

Additionally, the wear experienced by the combined effect can be evaluated to determine the electrochemical and mechanical components of the total wear by applying the following equations. 𝑊 𝑎𝑐𝑡 𝑙 𝑚 = Material lost due to mechanical wear of the active material in the wear track during the continuous sliding test.

Furthermore, the information acquired during this test allowed evaluating of the effect of the formation of the passive film on the mechanical wear by the ratio between the specific mechanical wear on the active and passivated parts of the wear track (Km), defined by eq. 1.31. Two cases can be distinguished; "i. Km >1, the passive material protects the material against mechanical removal. ii. Km < 1, the formation of the passive film accelerates the mechanical removal of the material.

𝐾 𝑚 = 𝑊 𝑎𝑐𝑡 𝑙𝑎𝑡 𝑚 𝑊 𝑟𝑒𝑝𝑎𝑠𝑠 𝑙𝑎𝑡 𝑚 𝐴 𝑟𝑒𝑝𝑎𝑠𝑠 𝑙𝑎𝑡 𝐴 𝑎𝑐𝑡 𝑙𝑎𝑡 𝑒𝑞. (1.31)
1.3 Characterization techniques

Scanning electron microscopy (SEM)

Scanning electron microscopy is widely used to examine surface topography and morphology by producing high resolution images. SEM works at high electron energies (typically, 5 -100 keV) and it is this highly focused beam that scans the surface of a sample. Figure 1.8 presents a schematic image of the core components of a scanning electron microscope.

The electron beam arrives at the surface with an energy E0 which generates elastic and inelastic interactions. The elastic interactions are produced by collisions "incident electron-nucleus", while the inelastic are mainly due to collisions "incident electronmaterial electron".

The expected interactions are divided in several groups (Figure 1.9) [START_REF] Ruste | Microscopie Électronique à Balayage. Principe et Équipement[END_REF]]:

• Primary electrons from the beam can exit the sample after the elastic and inelastic collision. This phenomenon is called backscatter electron emission.

• Secondary electron emission occurs due to an inelastic collision between an incident electron and the electron cloud of the atoms in the sample. The collision leads to the ejections of an electron from the cloud, this is known as ionization.

• Material ionization can produce a deexcitation of the atom with the emission of Auger electrons or photons (fluorescence). This is a function of the atomic composition of the sample and the geometry of the features under observation.

• For semiconductors and insulating materials, a photomagnetic emission is possible. This cathodoluminescence, most of the time, is due to the displacement of an electron from the valence band to the conduction band.

From all these interactions, an image of the surface can be spatially reconstructed on a phosphor screen (or CCD detector) from the intensity of the secondary electron emission at each point scanned by the incident beam. Because of the shallow excitation depth of low-energy electrons produced by the primary electron beam, only the secondary electrons and the backscattering electrons can escape and be detected.

The backscattered electrons come from deeper regions of the sample, while the secondary electrons originate from surface regions. Therefore, backscattered electrons and the secondary electrons carry different types of information. Images obtained from the backscattered electrons show high sensitivity to differences in atomic number; the higher the atomic number, the brighter the material appears in the image, while secondary electrons imaging can provide more detailed surface information, useful for the inspection of the topography of the sample surface.

If the SEM is equipped with an X-ray detector, it is possible to perform energy-dispersive X-ray spectroscopy (EDX/EDS).

Energy-dispersive X-ray spectroscopy (EDX)

EDX can provide the elemental composition of solids. The high-energy primary electron beam penetrates deeply into a specimen (~1-5 μm). So, the X-rays produced from the interaction of these electrons with atoms deep in the bulk of the specimen can pass through the material and be detected. Therefore, EDX cannot be considered a surface analysis method. Nevertheless, in a few seconds, a qualitative survey of the elements present in almost any sample can be made, and in only a few minutes, enough data can be collected for quantification. The most frequent application is on electric conductor materials, although nonconducting samples can be studied.

However, only elements with atomic number greater than four can be detected in optimal conditions.

X-ray diffraction

The X-ray diffraction is used to identify the crystalline phases present in a material. This nondestructive technique relies on the fact that X-rays are a form light, with wavelengths on the order of nanometers. A sample is radiated with an X-ray beam and then the intensities and scattering angles of the X-ray that leave the material are measured.

In materials with a crystalline structure, the scattered X-ray undergoes constructive or destructive interference. In other words, if the diffracted beam is in phase with the incident beam, it can result in harmonic values of the wavelength frequency (Figure 1.10). This constructive interaction is described by Bragg's law:

𝑛𝜆 = 2𝑑 ℎ𝑘𝑙 sin 𝜃 𝑒𝑞. (1.32)
Where: n is an integer, λ the wavelength of the x-ray, dhkl is the interplanar distance and θ the incidence angle of the x-ray beam This law indicates that a given family of planes produces constructive interference only at a certain incident angle and the interferences are detected in a specific direction.

Taking advantage of this law, nowadays the X-ray diffraction as a method to analyze the structure of materials. This equipment consists of an x-ray beam source (a metallic cathode or tube of Cu or Co) and an x-ray detector. In a XRD machine, the sample, the X-ray source and the ray detector are attached to motors, which allows control of the incident and the detection angles.

There are two Bragg-Brentano methods currently used. The first one is a θ-θ mode where the x-ray source and the detector moves simultaneously leaving the sample motionless. The second one is a θ-2θ mode, the source does not move, but the sample and the detector move at an angle θ and 2θ, respectively. Figure 1.11 shows a diagram of these two schemes.

To characterize thin films, there is a special configuration, called razing beam. Here the x-ray beam is kept steady at a very small angle θi while the detector sweeps in θs.

This configuration helps to increase the intensity of the detected signal since the volume of the layer that contributes to the diffraction is larger and constant. In the case of films, this configuration also helps to decrease the contribution of the substrate.

X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy is a technique that can provide a comprehensive qualitative and quantitative overview of a surface. This method is based on the photoelectric effect. The bombardment of X-ray induces the ionization of the irradiated material. This ionization is accompanied by photoelectron emissions. These emitted electrons are core level (inner shell) electrons. The energy of these electrons is measured and their values provide information about the nature and environment of the atoms [START_REF] Ratner | Properties of Materials[END_REF]]. This process is described by the eq. 1.34:

𝐾𝐸 = ℎ𝜈 -𝐵𝐸 -𝜙 𝑠 𝑒𝑞. (1.33)
Where:

BE: energy binding the electron to an atom KE: kinetic energy of the emitted electron h𝜈: energy of the photon

s: spectrometer work function

The resultant spectrum is a plot of the number of detected electrons per energy interval as a function of its kinetic energy. This characterization technique can identify the elements in the first 10 nm of the surface, on the assumption that the element of interest exists at >0,05atomic %. A schematic representation of an XPS device is shown in Figure 1.12.

The binding energy depend on the chemical environment of the atom, the first observation of this dependance was made by [START_REF] Sokolowski | Chemical Shift Effect in Inner Electronic Levels of Cu Due to Oxidation[END_REF] where a shift of several eV was found between metallic copper and copper oxide.

Using the relationship between the binding energy and the chemical environment of the atom, XPS can be used to evaluate the bonds that an atom takes part of. The threshold energy is determined by the energy difference between the core level (initial state) and the lowest energy level in the unoccupied band (the final state). The energies of the initial and final states change with the chemical environment.

Contact profilometry

Profilometry is used to measure the surface topography of a sample which can then be used to determine factors such as surface roughness, coating thickness, changes in volume, among others. These measurements can be made by contact and Figure 1.12 XPS device schema [START_REF] Ratner | Properties of Materials[END_REF] non-contact profilometry. in the current work, contact profilometry was used to make all the measurements.

Contact profilometry is performed with a diamond stylus point. The stylus takes measurements by electromechanically moving a diamond-tipped stylus over the sample surface while applying a small load. The load is big enough to assure contact with the surface but small enough to avoid damaging the surface. The stylus is linked to a Linear Variable Differential Transformer (LDVT), which produces and processes electrical signals that correspond to surface variations of the sample [START_REF] Bruker | DektakXT User Manual[END_REF]].

After being converted to digital format, these surface variations are stored for display as a two-or three-dimensions graph as shown in Figure1.13.

A contact profilometer can measure small surface variations in vertical stylus displacement as a function of position. A typical profilometer can measure small vertical features ranging in height from 10 nm to 1 mm. The radius the stylus ranges from 20 nm to 50 μm, and the horizontal resolution is controlled by the scan speed and data signal sampling rate. The stylus tracking force can range from less than 1 to 50 milligrams. In the present work a DektakXT stylus profilometer with up to a 4Å repeatability

Hardness measurement

Hardness implies the resistance to plastic deformation as an indicator of a material's ability against to resist deformation. To measure hardness, two main methods exist, quasi-static (indentation) and rebound or dynamic. The most widely used is the static hardness test, which can be described as a penetration process, in which the penetration resistance of defined bodies (spheres, pyramids, cones) is determined at a material surface. Depending on the test method, penetration resistance is either determined as the ratio of test force to the surface area of the impression (Brinell hardness, Vickers hardness) or as the remaining depth of penetration of the penetrating body (Rockwell hardness).

Generally speaking, indentation methods consist of a point that applies a load over (some newtons) to the material, leaving a mark on the surface. Such indentation mark is measured (depth or diameter), the size value is then used to calculate the hardness of the material.

When one works with bulk materials many of the indentation methods perform well, but when working with thin films it is better to use micro or nanoindentation methods so that the influence of the substrate can be neglected. In general, it is considered that HC is given by a simple law of mixtures (eq. 1.34) which involves the substrate hardness (HS) and the film hardness (HF).

𝐻 𝐶 = 𝐻 𝑠 + 𝛼(𝐻 𝐹 -𝐻 𝑠 ) 𝑒𝑞. 1.34
Where the coefficient α is associated to the contribution of the film. Such coefficient varies from author to author but Lesage et al defined it as a function of the film thickness (t)and the indentation diagonal (d). Rewriting eq. 1.34:

𝐻 𝐶 = 𝐻 𝑠 + 𝑓 ( 𝑡 𝑑 ) (𝐻 𝐹 -𝐻 𝑠 ) 𝑒𝑞. 1.35
Since the hardness varies with ISE, hence with the hardness. The variation of d as a function of the applied load (P), when expressed as natural algorithms of the values, presents a linear tendency. Hence, f can be defined as: Where m corresponds to the slope of the linear regression for the logarithmic graph of the indentation diagonal as a function of the applied load, ln d (ln P). Such linear regression will be expressed as:

𝑓 = ( 𝑡 𝑑 ) 𝑚 𝑒𝑞. 1.36
ln 𝑑 = 𝑚 ln 𝑃 + 𝑏 𝑒𝑞. 1.37

Under these conditions HC can be expressed by the general relation:

𝐻 𝐶 = (1 -𝑓) ( 1 𝐻 𝑠 + 𝑓 ( 1 𝐻 𝐹 - 1 𝐻 𝑆 ))
+ 𝑓 (𝐻 𝑠 + 𝑓(𝐻 𝑓 -𝐻 𝑠 )) 𝑒𝑞. 1.38

Such expression can be rewritten as a polynomial relation in terms of HF that allows the calculation of the film hardness which corresponds to the positive root of the equation:

𝐴 𝑝 𝐻 𝐹 2 + 𝐵 𝑃 𝐻 𝐹 + 𝐶 𝑃 = 0, with { 𝐴 𝑃 = 𝑓 2 (𝑓 -1) 𝐵 𝑃 = (-2𝑓 3 + 2𝑓 2 -1)𝐻 𝑠 + (1 -𝑓)𝐻 𝐶 𝐶 𝑃 = 𝑓𝐻 𝐶 𝐻 𝑆 + 𝑓 2 (𝑓 -1)𝐻 𝑆 2 𝑒𝑞. 1.39

Scratch test

The scratch test consists of a point contact with the sample where an increasing load is applied during the movement of the point over the surface of a coating (Figure1.15). This test generates increasing stress to the surface, this will show specific regions of damage (at critic loads) that can be seen with an optic microscope [START_REF] Berthout | Tests de Rayure[END_REF]. Among the visible damage, it is possible to recognize if the film is detached from the surface or if it breaks within the film.

The specific type of damage depends on the mechanical properties of the film and the substrate. When an increasing load is applied to a film there will be elastic and plastic deformations will occur, then the substrate will also be deformed and finally at higher loads fracture of the coating and detachment can occur. Commonly a first critical load charge (Lc1) is when the first cracks are observed, a second critical load (Lc2) is when cracks are continuously observed and the third critical load, (Lc3) is when spalling or detachment of the coating is first seen. These critical loads depend on intrinsic and extrinsic parameters of the film-substrate system and the counter body used. As follow:

• Intrinsic parameters of the film o Substrate properties: hardness, elastic modulus, thermic expansion coefficient, and roughness are also linked to residual stress over the film.

o Film properties: hardness, elastic modulus, residual stress, roughness, and thickness have a direct influence over the critic load.

o The friction coefficient and friction force between the point and the film.

• Thin films can develop large intrinsic stress during their growth process. The magnitude of such stress can exceed the tensile strength of the bulk material. A way to estimate such intrinsic stress is by the wafer curvature method. This nondestructive method is widely used to estimate the coating stress. The principle behind the method it to measure the curvature of the substrate before and after deposition of the thin film [Abadias et al. 2018a].

To determinate the average stress in the film (𝜎 ̅) the Stoney equation (eq.1.40) is applied. This equation relates the average stress in the film to the change in the

Adaptation of the sputtering machine

Most of the thin films, obtained during the present work, were deposited by rf sputtering magnetron using a MP300S by Plassys, acquired by the LPMC in 1997. Such a system has a cylindrical vacuum chamber with a capacity of three different 3 inches circular targets. Such targets can only be used one at a time.

For the present work, the equipment was powered by an eni power system OEM-6AM-1 power supply. A diagram of the exterior of the vacuum chamber is presented in Figure 2.1. Such a vacuum chamber is connected to a pre-chamber intended to charge and discharge samples into the main chamber. The samples are suspended in a sample holder attached to a rotatory feedthrough in the lid. This feedthrough is intended to move the position of the sample holder over the desired target position. Originally the Plasyss MP300S was equipped with a sample heater and system to measure the substrate temperature. However, these parts were broken and had not been repaired at the beginning of the present work. Additionally, the manufacturer (Plasyss) informed us that they no longer produced the necessary parts for the system.

Hence an alternative sample heater and thermocouple were installed. The choice made for the heating system was a MeiVac Inc. 2" resistive substrate heather. The heater was powered by a CPX200D supply from Aim-TTi. Finally, to measure the temperature of the sample a type k thermocouple was installed. This thermocouple was connected to an external multimeter to give the temperature value. The installation of the thermocouple was not completely satisfactory since the sample holder was attached to the lid of the chamber, and hence, the contact between the sample and the thermocouple point was difficult. Nevertheless, after several attempts, a successful installation of the thermocouple was achieved.

Tribocorrosion test apparatus

To perform a tribocorrosion test, the test equipment must allow control and monitoring of the mechanical and electrochemical parameters [START_REF] Mathew | Construction of a Tribocorrosion Test Apparatus for the Hip Joint: Validation, Test Methodology and Analysis[END_REF][START_REF] López-Ortega | Tribocorrosion of Passive Materials: A Review on Test Procedures and Standards[END_REF]Mathew et al. 2009a]. An alternative to acquiring new equipment and a specific module for tribocorrosion measurements is the construction of a tailored cell. When building such a cell one needs to consider several technical aspects such as (i) geometry and construction of the corrosion cell (appropriate for installation in the existing tribometer), (ii) proper/consistent locations of electrode (iii) possible leakage of the solution/electrolyte, (iv) collection and synchronization of the data from the tribometer and potentiostat, and (v) proper electrical insulation [Mathew et al. 2009a;[START_REF] López-Ortega | Tribocorrosion of Passive Materials: A Review on Test Procedures and Standards[END_REF] For the present work, an in house designed and made testing cell, to be used in a UTM Tribolab by Bruker, was develop for discs of 2.5 or 3.0 cm diameter. Different configurations and ways to perform the tribocorrosion tests were explored. Each cell was designed and draw using Solid works. Such cell was print using a Raise 3D printer, and a PLA filament.

Three different models were conceived and tested. The first model shown in Figure 2.4 consisted of a cell with a downer cap to keep the sample in place, and two lateral supports one for the counter electrode and the other for the reference electrode. Such design allowed to be in a fix position for the electrodes and at the same time, the sample was properly isolated. Nevertheless, the type tribological test was limited to reciprocating movement and the screw base resulted in electrolyte leaks. Furthermore, different printing qualities were tested with similar results. Consequently, the cell did not close correctly, and the electrolyte leakage prevailed.

Another approach was to use as a base the UMT-Tribolab module (Figure2.5). Such a module was designed to be used in lubricated pin on disc tests. Such a module was intended for discs of 3 inches in diameter and the main container was metallic. Hence to be able to use samples of a smaller size and avoid the effect of the container, a cover for the sample support was designed. Figure 2.6 shows such a piece, which was attached to the sample container using the screws used to attach the container to the base of the module. To assure that the sample was well fixed PTFE tape was added around the sample. This approach allowed the system to be used to perform pin on disc tests and the leaking problems were diminished.

With such a piece, tribocorrosion tests were attempted with the experimental setup shown in Figure 2.7. Nevertheless, it was not possible to have direct electric contact with the sample, and the electric current circulated through the entire machine. Since the electric contact was not isolated it was not possible to carried out the electrochemical measurements.

To ensuring that the electric contact with the sample was isolated from the rest of the system and such that the electrical contact could be sustained as the sample rotated a sliding contact system was developed. To achieve these sliding contact a highly conductive electrolyte was used as a bridge between the potentiostate and the sample to be tested sample. Figure 2.8 shows the diagram of the proposed cell, which worked as a cover as the previously presented one with two levels. The bottom level was built to contain the 'bridge electrolyte" and the top level for the samples and the electrolyte for the electrochemical test. The used 'bridge electrolyte' used for the sliding contact was a 1.0 M aqueous solution of potassium chloride. The sample was attached to a silver wire, and the ends on the wire were submerged in the bridge electrolyte. To make the electrical contact between the sliding contact and the potentiostat a second silver wire was submerged in the potassium chloride and directly connected to the potentiostat. Chapter 3

Evaluation of the substrates; a tribocorrosion approach 3

This section presents the wear behavior of AISI D2 and AISI 341 steel at room temperature and relative humidity, the type of wear and the wear rate were assessed with the support of techniques such as contact profilometry, scanning electronic microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). Moreover, the evaluation of the corrosion was explored using electrochemical impedance spectroscopy (EIS). Finally, the tribocorrosion behavior of AISI 341steel was studied by the combination of a pin on disc tests and EIS. We chose the protocol proposed by [START_REF] Diomidis | A Methodology for the Assessment of the Tribocorrosion of Passivating Metallic Materials[END_REF] because it has been used to characterize different stainless steels uncoated and coated with protective films.

Steel generalities.

Steel is a widely used alloy and is mainly based on iron and carbon, but it can also contain manganese, sulfur, phosphorus, chromium, among other elements to adjust its chemical, mechanical and physical properties such as corrosion resistance, strength, and ductility[J. D. Verhoeven 2007]. Regularly we speak of steel when the amount of carbon in the alloy is not larger than 2%.

Along with the different compositions of the alloy, it is possible to find it with different phases and crystalline structures. The most common phases of steel are body-centered cubic (bcc) ferrite and face-centered cubic (fcc) austenite, the same as those for pure iron but it is also possible to find cementite and perlite, Figure 3 to their composition they can be considered as stainless or semi-stainless, nevertheless, their corrosion resistance is very limited since most of the chromium content tends to precipitate.

Among the 'tool steels', one can find the AISI D2 steel or 1.2379, according to the EN 10027-2. This steel is commonly used in industrial applications for the fabrication of pieces such as punches, piercing and blanking dies, spinning tools, shear blades, slitting cutters, as well as a variety of woodworking tools. [START_REF] Tang | Tribology International Dry Sliding Friction and Wear Behaviour of Hardened AISI D2 Tool Steel with Different Hardness Levels[END_REF]. Its reported composition is 1.5% carbon, up to 12.0% of chromium, 0.3% silicon, 0.8% molybdenum, 0.9% vanadium with the rest being iron. Due to its composition, such steel is found regularly as cementite in a tetragonal body-centered crystalline structure (Figure 3.2a). In general, it is heat-treated with two tempered processes to increase its hardness, this heat treatment will change the crystalline structure to a face-centered cubic structure (austenitic phase, Figure 3.2b). Moreover, depending on the heat treatment conditions metallic carbides of different sizes can be formed. In particular for the AISI D2 steel it is possible that Cr, Mo, and V form metallic carbides. The modification of the crystalline structure of the steel and the size of the carbides in the steel can highly impact the overall hardness of the material [START_REF] Rahim | A Comprehensive Review on Cold Work of AISI D2 Tool Steel[END_REF].

Regardless that this alloy has good mechanical properties, the lifetime of pieces and accessories fabricated with AISI D2 steel are affected by the evolution and type of work conditions in industrial processes. For example, pieces manufactured from AISI D2 steel are used as chuck and rolls in the process of canned food production, where liquid and vapor from the products being canned can come in contact with the pieces [START_REF] Yam | Encyclopedia of Packaging Technology[END_REF][START_REF] Shan | Canned Citrus Processing Techniques, Equipment, and Food Safety[END_REF]]. This environment can negatively impact the performance of the pieces and reduce the lifetime. A reduction of the useful lifetime of the pieces impacts the energy and the consumptions of spare parts consumptions.

Therefore, one is looking for new technologies that can enhance the lifetime [START_REF] Ramírez | Influence of Substrate Microstructure on the Contact Fatigue Strength of Coated Cold-Work Tool Steels[END_REF][START_REF] Díaz-Guillén | Electrochemical Corrosion Performance of AISI D2 Tool Steel Surface Hardened by Pulsed Plasma Nitriding[END_REF]].

To extend the useful life of pieces such as rolls, some films (TiN, TiC, Cr Nevertheless, studies of the corrosion have not been extensively reported [START_REF] Findik | Latest Progress on Tribological Properties of Industrial Materials[END_REF][START_REF] Gauvent | Corrosion of Materials Used as Cutting Tools of Wood[END_REF][START_REF] Sørensen | Anticorrosive Coatings: A Review[END_REF][START_REF] Fenker | Corrosion Behaviour of Decorative and Wear Resistant Coatings on Steel Deposited by Reactive Magnetron Sputtering -Tests and Improvements[END_REF].

Moreover, the combined effect of mechanical wear and corrosion only has been explored for the coated material [START_REF] Castillejo | Wear and Corrosion Resistance of Niobium-Chromium Carbide Coatings on AISI D2 Produced through TRD[END_REF][START_REF] Karakaş | Tribocorrosion Behavior of Surface-Modified AISI D2 Steel[END_REF]].

AISI 431 / 1.4057-stainless steel

Stainless steel was discovered by accident in 1912 in Germany and England. Such type of steel must contain at least 11 % of chromium and have a low carbon concentration, between 0.03% and 1.00%. Stainless steels are well known for their corrosion resistance, such resistance occurs when a thin film (~2 nm) of chromium oxide grows over the alloy surface, such a layer is dense and almost inert, so it is an efficient passivation layer. The AISI 431 steel (EN 1.4057) reported composition of 15-17% Cr, 1.5-3% Ni, 0-1% Si, 0-1% Mn, 0.1-0.2% C, 0.04 % P and 0.03% S. Commonly this type of steel has a martensitic phase. Such phases correspond to a body-centered tetragonal crystalline structure (bct)[J. [START_REF] Verhoeven | Steel Metallurgy for the Non-Metallurgist[END_REF].

Some of the most common properties for this type of steel are high hardness, good wear resistance, fair corrosion resistance, and good performance at high temperatures.

Due to this, AISI 431 steel is frequently used for gas turbines, airplane engine parts, laboratory equipment, and aeronautic parts [START_REF] Paxton | Kirk-Othmer Encyclopedia of Chemical Technology[END_REF]J. D. Verhoeven 2007].

Similar to other types of steel, the final hardness of the material depends on the heat treatment received. Such heat treatment can modify the crystalline structure and the carbides presents on the alloy.

Materials -substrate's characterization

The present work involves studies of the tribological and electrochemical

properties via a tribocorrosion study of these alloys. This chapter presents the results of the characterization of the AISI D2 tool steel and AISI 431 stainless steel used as substrates in this study. To verify the tribological, corrosion, and tribocorrosion behavior of the samples a series of tests were performed. Such tests were pin on disc, electrochemical impedance spectroscopy, and coupled pin on disc and EIS tests for the corrosion resistant material.

Sample preparation

Steel discs of 3.0 cm in diameter and 3 mm in thickness were used as substrates in the present work. To assure a similar initial surface for each test, the steel samples were polished up to mirror finishing using either a Buehler-MetaServ 250 at 'Institut After the polishing process, each sample was cleaned consecutively with acetone (Fisher chemicals, general purpose grade) and isopropanol (Sigma-Aldrich ≥99,7%)

for 10 min using an Elma ultrasonic bath. To finish up each sample was dried out using filtered compressed air.

Roughness

Roughness refers to the variations in the height of the surface relative to a reference plane. The surface roughness of the samples was measured along a standard linear distance using a Dektak XT contact profilometer from Bruker, with a diamond tip of 2.5 µm radial curvature at 3 mg of force. The measurement length was 3 mm crossing the center of the sample. Typical profiles for each of the studied steels are shown in

Scanning electronic microscope and Energy Dispersive X-ray Spectroscopy

Once the samples were polished, scanning electronic microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDX) of the substrates were taken. For both steel samples, the images were obtained under high vacuum at 20 KeV. The images and EDX analysis were obtained at the 'Plateforme de Microscopie Électronique' at the UPJV with a Quanta 200FEG microscope with a microanalyzer X INCA Oxford type SDD 80 mm 2 .

Hardness

Hardness is known as the measure of the resistance offered by a material against penetration. One way to measure it is by micro indentation as explained in Chapter1.

Micro

Experimental conditions

This section gives the working conditions for the different tests performed for the tribological and electrochemical characterization of the substrates.

Pin on disc tests

The measurements were performed in a UTM-Tribolab by Bruker in a pin on disc machine with a load capacity of 0.5 N to 20.0 N. The counter body for the tests were 12 mm of diameter grade 20 aluminum oxide balls from Redhill Materials. Normal loads of 0.5 N and 1.5 N were applied. The rotation speed was 60 rpm, which is equivalent to 0.02 m/s and for a total length of 72 m.

The UMT-Tribolab takes measurements with the upper sample (pin) fixed to a strain-gauge force sensor which processes mechanical and electrical signals that correspond to the sample's contact behavior. The system processes the signals by converting the mechanical behavior to a digital format.

The contact area and the average pressure in the contact region are given by Hertz's Law, described in chapter one. A summary of the obtained values applying eq. 1.5 and eq.1.6 are shown in Table 3-1. The values indicate that an increment of 1N corresponds to an increment of 25% in the average pressure at the contact point.

Electrochemical tests

As a part of the characterization of the substrate, electrochemical measurements such as open circuit potential (OCP), and electrochemical impedance spectroscopy (EIS) were performed with a PalmSens 4 potentiostat. All measurements were made using a three-electrodes arrangement with a silver/silver chloride (Ag/AgCl) electrode as the reference, a platinum grid as the counter electrode, and the analyzed sample as the working electrode. The measurements were performed on the cell conceived for the tribocorrosion measurements, hence the contact between the working electrode and the potentiate was made through an aqueous solution of potassium chloride. A diagram of such a cell is shown in Figure 3.10.

Electrolyte

Citric acid (2-hydroxypropane-1,2,3-tricarboxylic acid) is a weak acid naturally found on several foods. Thanks to its flavor and pH it is frequently used as a protective additive for beverages, jams, jellies, canned food, and even meat [START_REF] Taylor | Canned Food A Review on Acidifying Treatments[END_REF] substance at a pH of around 4 since a large number of bacteria do not survive at this pH value [START_REF] Taylor | Canned Food A Review on Acidifying Treatments[END_REF][START_REF] Shan | Canned Citrus Processing Techniques, Equipment, and Food Safety[END_REF].

Given that citric acid is extensively used in the food industry, an aqueous solution of citric acid at pH 4 was used as the electrolyte for all the electrochemical measurements. The citric acid solution was made using citric acid (NortemBio/ food grade). The acid was weighed using an Ohaus Scout Pro 600 balance, then it was transferred to a 200 ml Hirschmann EM volumetric flask. The desired volume was completed with distilled water. To confirm the pH of the solution, pH measurements were performed with an ezodo mp-103 pH meter.

Open circuit potential and reaction time

Open circuit potential measurements were carried out for up to five hours, to verify the system's steady-state and determine the characteristic reaction time (treact) [START_REF] Celis | Testing Tribocorrosion of Passivating Materials Supporting Research and Industrial Innovation : Handbook[END_REF]. After the OCP measurements an EIS test was performed.

Electrochemical impedance spectroscopy.

EIS measurements were carried out using the same arrangement as for the OCP measurements in a frequency interval from 10 5 Hz to 10 -2 Hz with at least three points per decade and amplitude of ± 10 mV vs OCP. The results were modeled using the equivalent circuit approach to determine the corrosion resistance of the system which multiplied by the exposed area of the sample gave the specific corrosion resistance.

Tribocorrosion evaluation

For the passive alloy, the tribocorrosion behavior was evaluated. Given that tribocorrosion is a mixed phenomenon, such evaluation was carried out combining the pin on disc tests and the electrochemical measurements. For such measurements, a three-electrode cell and an aluminum oxide ball were used. Figure 3.11 shows a diagram of the employed cell.

Continuous test

After a steady OCP value was achieved, EIS and pin on disc tests were performed simultaneously. Continuous sliding pin on disc tests at 1.5 N normal load and 0.02 m/s of angular speed were performed, for a total length of 72 m. While the pin on disc test was running, EIS measurements were carried out in a frequency interval from 10 5

Hz to 10 -2 Hz with at least three points per decade. From such tests, the volume loss and the corrosion current density of the active material were determined. Contact profilometry and SEM measurements were used to assess the final state of the wear surface and the material loss volume. Furthermore, the EIS results were modeled using the equivalent circuit approach.

Latency tests

For the latency tests, successive identical cycles of active pin on disc and latency were performed until a total length of 72 m was reached. The first part of the cycle corresponds to the active mechanical wear of the surface while the second (latency) allows the wear surface to passivate. The applied normal load was 1.5 N. Figure 3.12 shows a schematic top view of a disc at the different stages of a tribocorrosion latency test, where different areas can be identified during the test. A0 represents the total exposed area (45 mm 2 ), Aact (4.2 mm 2 ) the area that corrodes actively and Arepass (4.7x10 - 2 mm 2 ) the area that re-passivates during the latency time [START_REF] Diomidis | A Methodology for the Assessment of the Tribocorrosion of Passivating Metallic Materials[END_REF]].

During such tests, the corrosion potential fluctuates hence the corrosion current density cannot be obtained from EIS measurements. Nevertheless, the total volume loss can be used to calculate the ratio which indicates the effect of the material passivation in the mechanical material removal (Km) and the contribution due to the re-passivation as described on the state of the art.

Results

The following section presents the tribological and electrochemical characterization of the substrates. The performed test followed the tribocorrosion evaluation protocol described in section 1.3.4.

Pin on disc tests

During the pin on disc test the normal force and the friction force were sensed and then transformed into the coefficient of friction. After the pin on disc test, a circular wear track was obtained on the steel surface, moreover, some changes over the surface of the counter body (aluminum oxide ball) were also examined. The resulting wear track and the contact point in the aluminum oxide ball were analyzed.

Coefficient of friction

Examples of the temporal evolution of the coefficient of friction for AISI D2 steel at 0.5 N and 1.5 N are shown in Figure 3.13, the yellow line represents the smoothed data using an adjacent average filter for every 500 points. Each of the presented curves shows sharp variations during the first few minutes, such drastic change is common in most of the tribological tests, and it is known as 'break in' or 'running in' period.

This period is described as a series of different stages during which small adjustments in the geometry of interacting parts as well as changes in surface roughness occurs [START_REF] Heilmann | Running-in Processes Affecting Friction and Wear. THE RUNNING-IN PROCESS IN TRIBOLOGY[END_REF]. Once the break-in has passed it is possible to consider that the COF of the system is being measured, hence the average coefficient of friction was calculated in the steady state range. The estimated COFs were 0.67±0.04 and 0.53 ± 0.10 at 0.5 and 1.5 N respectively. Moreover, in the steady state range the curves presented a large dispersion, such 'noisy behavior' has been reported for AISI D2 steel and it can be attributed to stick-slip motion during the pin on disc test [START_REF] Tang | Tribology International Dry Sliding Friction and Wear Behaviour of Hardened AISI D2 Tool Steel with Different Hardness Levels[END_REF]]. During the stick phase, the friction force builds up until reaching a critical value.

Once the critical force overcomes the static friction, slip occurs at the interface, and Figure 3.13 Graph of coefficient of friction as function of time for the system alumina (pin) AISI D2 (disc)at 0,02m/s of speed and Left-hand @ 0.5N, right-hand @ 1.5N of load energy is released so that the frictional force decreases[X. [START_REF] Han | An Investigation into the Tribological Property of Coatings on Micro-and Nanoscale[END_REF][START_REF] Menezes | Tribology for Scientists and Engineers From Basics to Advanced Concepts[END_REF]].

Furthermore, for AISI 431 steel the pin on disc tests were performed at 1.5 N.

Figure 3.14 presents the temporal evolution of the COF, the COF presents a curve that shows drastic changes during the first minutes of the tests, breaking in of the test.

After the break in period a steady state was achieved. During the steady state a large dispersion was also observed. Such a form of dispersion can be attributed to external vibrations and the type of wear mechanism. The average coefficient of friction was 0.48 ± 0.21.

Wear track analysis by contact profilometry

In 1953 John Archard proposed eq. 3.1 at present known as Archard's equation. To estimate the wear volume, mapping profilometries were carried out with a Dektak XT contact profilometer. The total area analyzed was a square of 7 mm of length with 15 µm of separation between each single profilometry step. The obtained surface 3D map was analyzed with the software Visio 64. A topographical image of a sample after the pin of disc test at 0.5 N of load is shown in Figure 3.15 along with an example of a single profile at 0.5N, the topographical data was used to estimate the material loss by applying the volume analysis in the region of the wear track. The estimated material accumulated at 0.5N was 2.4x10 -5 ±1.8x10 -6 mm 3 giving a wear coefficient of 6.7x10 -7 mm 3 /Nm. Similar measurements were performed over the samples subjected to 1.5 N, an example of a single profile (Figure 3.16) showed that there was accumulated material in the wear track and a smooth surface in the surrounding area. The obtained accumulated material volume was 2.1x10 -4 ±4.2x10 -5 mm 3 giving a wear coefficient of 1.9x10 -6 mm 3 /Nm. Furthermore, for AISI 431 steel the same type of profilometry was performed.

Figure 3.17 shows a topographical image of the obtained wear track alongside a single profile. The single profile better illustrates the roughness change over the wear track.

The estimated accumulated material was 1.6x10 -4 ±1.0x10 -4 mm 3 giving a wear coefficient, k, of 1.4 X10 -6 mm 3 /Nm. Additionally, map profilometries were carried out over the surface of the counter bodies. An image of an Al2O3 ball is shown on Figure 3.18, such a topographical image revealed the presence of accumulated material in the surrounding area of the contact point.

Wear track analysis by scanning electronic microscope and Energy dispersive X-ray spectroscopy.

SEM images of the steel surface, before and after the pin on disc tests, were taken.

the left-handed image on Figure 3.19 shows the surface of an AISI D2 steel disc before the test, while the right-handed image shows the wear track after the pin on disc test at 0.5 N. In each image the regions of accumulated material can be identified along with wear lines in the direction of rotation. The width of the wear track was about 50 μm, a comparable size to the one calculated by the Hertz eq. ( 1.3).

Furthermore, SEM images of the wear track using 1.5 N are displayed in Figure 3.20. The image shows a wear track width of around 90 μm, with many lines in the direction of the rotation. Additionally, there was evidence of accumulation over most of the surface. Similar results were found for AISI 431 steel, a comparison of the surface before and after the pin on disc test is shown in Figure 3.21. Such comparison makes evident the damage produced by the pin on disc test, the region of the wear track presents a different topology to the rest of the surface with a track width of about 100 µm. Nevertheless, Cr is at the limit between corrosion and passivation, and consideration of the percentage of chromium suggests that the AISI 431 steel might also passivate.

Electrochemical tests

Open circuit potential and reaction time

Electrochemical impedance spectroscopy

An example of a typical Nyquist diagram and its simulation is presented in Figure 3.27. The diagrams show two semicircles, hence two-time constants, that have been shifted from the origin. Such behavior is commonly found during corrosion processes and can be described by the equivalent circuit shown in Figure 3.28. The proposed equivalent circuit consists of a resistance R1 and two RQ parallel circuits. The first RC parallel circuit along with R1 represents the ohmic resistance of the solution between the working and the reference electrodes. Such behavior is commonly attributed to the geometry of the electrochemical cell and the reference electrode geometry [START_REF] Edwards | Experimental Limitations in Impedance Spectroscopy: Part V. Apparatus Contributions and Corrections[END_REF][START_REF] Hsieh | Experimental Limitations in Impedance Spectroscopy: Part III. Effect of Reference Electrode Geometry/Position[END_REF]]. Finally, in the second RC component, R3 corresponds to the polarization resistance of the material (Rp); the Q element is a constant phase element (CPE) which describes a non-ideal capacitor when the capacitor phase angle is different from -90° [START_REF] Walter | A Review of Impedance Plot Methods Used for Corrosion Performance Analysis of Painted Metals[END_REF]]. CPEs are generally used to describe surface reactivity, surface heterogeneity, and surface roughness as well as the distribution of the current and potential caused by the electrode geometry and porosity [START_REF] Montemor | Electrochemical Study of Modified Bis-[Triethoxysilylpropyl] Tetrasulfide Silane Films Applied on the AZ31 Mg Alloy[END_REF]. The Rp value was multiplied by the exposed area to obtain the specific corrosion current (rp). The estimated value of rp was 3.3 ±0.1 kΩcm 2 , with a chi-square of 0.0052 suggesting a well-adjusted circuit. For a specific corrosion resistance lower than 100 kΩcm 2 the material is considered to be under corrosion [START_REF] Ningshen | Hydrogen Effects on the Passive Film Formation and Pitting Susceptibility of Nitrogen Containing Type 316L Stainless Steels[END_REF][START_REF] Diomidis | A Methodology for the Assessment of the Tribocorrosion of Passivating Metallic Materials[END_REF]], hence AISI D2 steel corrodes in contact with citric acid 10 -4 M.

The corrosion resistance was used to calculate the corrosion current icorr applying the Stern-Geary equation (eq.3.2) where B is a term that varies between 13 and 35 mV depending on the nature of the material and the environment. For metallic materials, in acidic environments, B can be considered as 24 mV as suggested by some authors [Mathew et al. 2009b;[START_REF] Diomidis | Tribocorrosion of Stainless Steel in Sulfuric Acid: Identification of Corrosion-Wear Components and Effect of Contact Area[END_REF][START_REF] Celis | Testing Tribocorrosion of Passivating Materials Supporting Research and Industrial Innovation : Handbook[END_REF]. The estimated icorr was 9.3x10 -6 ±2x10 -8 A/cm 2 , a comparable value to those obtained by other authors using different electrolytes [START_REF] Chen | On the Corrosion Behavior of TiN-Coated AISI D2 Steel[END_REF][START_REF] Sista | Electrochemical Boriding and Characterization of AISI D2 Tool Steel[END_REF][START_REF] Díaz-Guillén | Electrochemical Corrosion Performance of AISI D2 Tool Steel Surface Hardened by Pulsed Plasma Nitriding[END_REF]]. This type of behavior has been reported for other pH values, due to precipitation of the alloying elements [START_REF] Chen | On the Corrosion Behavior of TiN-Coated AISI D2 Steel[END_REF][START_REF] Castillejo | Wear and Corrosion Resistance of Niobium-Chromium Carbide Coatings on AISI D2 Produced through TRD[END_REF][START_REF] Díaz-Guillén | Electrochemical Corrosion Performance of AISI D2 Tool Steel Surface Hardened by Pulsed Plasma Nitriding[END_REF]. Furthermore, precipitation of the alloying elements is commonly found in iron-alloys with the cementite phase [Beitz and Kuttner 1994].

In addition, a Nyquist diagram for AISI 431 steel and its simulation are shown in

Tribocorrosion evaluation

Continuous sliding

For AISI D2 steel the influence of the electrolyte on the material wear was studied, nevertheless, the tribocorrosion synergy could not be evaluated since it is not a passive material. To evaluate the influence of the electrolyte in the wear of AISI D2, steel pin on disc tests of the steel in contact with the electrolyte were performed. Figure 3.31 and Figure 3.32 show profiles of the wear track region. For the sample tested at 0.5 N, the wear track was difficult to identify since the surrounding area showed considerable damage due to corrosion. However, for the 1.5 N test, the wear track was more evident with a maximum depth of about 800 nm. As a result of the pin on disc tests the surface roughness changed, and the average surface roughness of the areas close to the wear track were, respectively, for the 0.5 and 1.5 N loads, 720 ± 60 nm and 773 ± 90 μm. Moreover, the material volume loss was 2.3x10 -3 ±4.9x10 -4 mm 3 at 1.5 N.

In contrast, after the dry pin on disc test a smear accumulation of material was found, therefore it is evident that the damage over the steel surface is modified by presence of a corrosive environment than under normal room temperature and humidity conditions. As mentioned above the surrounding area of the wear track presented significant damage. Such damage made it difficult to correctly identify the wear track and increased the associated error.

In order to evaluate the tribocorrosion behavior of AISI 431 steel, coupled measurements of pin on disc and EIS were performed. As a first approach, the EIS tests were performed under continuous sliding. Figure 3.33 presents the temporal For AISI 431 steel the rpass corresponded to the rp value calculated in section 3.4.2.2, 160.6 ±5.0 kΩcm 2 . Knowing Rps and rpass we could calculate ract applying eq. 3.6. such resistance value represents the corrosion resistance value of the surface being exposed to mechanical wear.

𝑟 𝑎𝑐𝑡 = 𝐴 𝑡𝑟 𝑅 𝑝𝑠 𝑟 𝑝𝑎𝑠𝑠 𝑟 𝑝𝑎𝑠𝑠 -𝑅 𝑝𝑠 (𝐴 0 -𝐴 𝑡𝑟 ) 𝑒𝑞. (3.6)
The estimated ract value was 3.5 kΩcm 2 , applying eq.3.2 , the corrosion current was found to be 7.6 x10 -6 A/cm 2 . Such corrosion resistance and corrosion current values represent a system under corrosion. Hence, the surface of the material, being exposed to mechanical wear corrodes.

Additionally, Figure 3.35 shows the mapping profilometry of the wear track and a single profile. The wear loss was 4.1x10 -3 ±4.0x10 -7 mm 3 with a K value of 3.7 x10 -5 mm 3 /Nm. In comparison with the dry test, instead of material accumulation over the wear track we found material loss. Such difference made evident that the presence of the electrolyte modified the tribological response.

Latency tests

During the tribocorrosion latency tests, the surface material undergoes successive events of de-passivation (the passive layer is removed) and re-passivation (re-growth of the passive layer). The latency time was selected so that the re-growth of the passivation film, was not negligible [START_REF] Celis | Testing Tribocorrosion of Passivating Materials Supporting Research and Industrial Innovation : Handbook[END_REF].

The resulting wear track was measured by contact profilometry as for the previous experiments. Figure 3.36 shows the resulting mapping and a single profile. Such results were used to calculate the total volume loss over the wear track. The estimated loss volume was 2.8x10 -3 ±4.0x10 -7 mm 3 with a K of 2.6 x10 -5 mm 3 /Nm.

The volume loss value measured after the latency tests along with the previous corrosion evaluation results for AISI 431 steel were used to calculate Kc and Km as described in 1.2.2. The estimated value of Kc was 3.6 x10 -1 , and such a value describes a system where the contribution, to tribocorrosion, due to the acceleration of corrosion induced by the destruction of the passive film is low compared to the total wear.

Furthermore, Km was estimated to be 3.0x10 -3 , and a Km < 1 is typical for a system where the formation of the passive film accelerates the mechanical removal of the material [START_REF] Celis | Testing Tribocorrosion of Passivating Materials Supporting Research and Industrial Innovation : Handbook[END_REF]. This means that when the passive film has time to regrow the mechanical wear removes a larger amount of material than when the passive film was not present.

Chapter's conclusions

In this chapter, the tribological and electrochemical behavior was studied of the substrate material was studies. For the tribological part, pin on disc tests were performed. The obtained experimental results confirmed the good wear resistance of both AISI D2 steel and AISI 431 steel. For AISI D2 steel, the analysis of the obtained wear tracks showed accumulated material over the wear track area. Moreover, the presence of material adhered was responsible for the drastic change on the coefficient of friction and its presence was confirmed by the EDS analysis, where important amounts of iron over the counter body surface were found.

The electrochemical study was performed using an aqueous solution of citric acid Finally, the tribocorrosion evaluation showed that the combined damage over the surface modified the overall response of the system. Moreover, the determination of the ratios Kc and Km showed that the acceleration of the corrosion induced by the destruction of the passive film was negligible compared to the total wear and that the formation of the passive film accelerated the mechanical material removal.

Chapter 4

Synthesis and characterization of the tungsten nitride thin film 4

This section presents the theoretical and experimental parts concerning the synthesis of the tungsten nitride thin films. The deposition of tungsten nitride and the role of the deposition parameters on the properties of the thin film are described.

Physical vapor deposition

Physical vapor deposition (PVD) is a well-known technology, widely used for the deposition of thin films. Such films are generally applied to increase tools lifetime, decrease friction, and to improve thermal properties [START_REF] Kelly | Magnetron Sputtering: A Review of Recent Developments and Applications[END_REF][START_REF] Baptista | Sputtering Physical Vapour Deposition (PVD) Coatings: A Critical Review on Process Improvement and Market Trend Demands[END_REF]. PVD is a film deposition process in which a film grows on the substrate atom by atom, hence, it requires the atomization or vaporization of a material from a solid source (target). The atomic deposition can be carried out in vacuum, gas, plasma or electrolytic environment. When performed in vacuum environment, the reduction in residual gas reduces the contamination during the deposition process. Moreover, the PVD process can be plasma assisted; such plasma assisted PVD is divided into different power sources technologies. The different power sources used in PVD are direct current, radio frequency, pulsed plasma, ion beam, etc.

The most common surface coating methods in a gaseous state regarding the PVD process are thermal evaporation, arc evaporation and sputtering [START_REF] Depla | Sputter Deposition Processes[END_REF][START_REF] Baptista | Sputtering Physical Vapour Deposition (PVD) Coatings: A Critical Review on Process Improvement and Market Trend Demands[END_REF]]. These techniques allow for particles to be extracted from the target at very low pressure and be transported and deposited on to the substrate [START_REF] Mubarak | Review of Physical Vapour Deposition (PVD) Techniques for Hard Coating[END_REF].

Magnetron Sputtering

Coating technologies are classified as physical or chemical deposition methods.

The deposition methods can be divided into vacuum evaporation, glow-discharge processes, gas-phase chemical processes, and liquid phase chemical techniques [START_REF] Aliofkhazraei | Nanocoatings[END_REF][START_REF] Seshan | Handbook of Thin-Film Deposition Processes and Techniques[END_REF]. Amongst the glow discharge methods, different types of sputtering can be found. The most common approach to growing films by sputtering is the use of a magnetron source. A magnetic field is applied to a sputtering target so that it traps primary and secondary electrons in a localized region close to the cathode into the "racetrack" [START_REF] Swann | Magnetron Sputtering[END_REF][START_REF] Alfonso | Thin Film Growth Through Sputtering Technique and Its Applications[END_REF][START_REF] Depla | Sputter Deposition Processes[END_REF]. The electron trajectory is elongated so that the probability of ionizing a gas atom increases.

The sputtering technique was developed at the end of the 19th century and has been adapted to large scale production in the last decades [START_REF] Wasa | Preface[END_REF]]. The main advantages of this technique are: (1) deposition of virtually any material, (2) high deposition rates,(3) high purity films, (4) good film adhesion, (5) uniform films over large areas, (6) and relatively low cost [START_REF] Depla | Sputter Deposition Processes[END_REF][START_REF] Swann | Magnetron Sputtering[END_REF]J. G. Han, n.d.]. To obtain thin films by magnetron sputtering first, high-energy ions strike a solid surface (target) and through the momentum interchange cause the emission of atoms from the surface. Second, the ejected atoms are transported towards the object to be coated [START_REF] Swann | Magnetron Sputtering[END_REF]]. To allow the atoms to move freely low gas pressures are required.

To power a magnetron it is possible to choose from radio frequency (rf), direct current (dc), pulsed dc and high-power impulse magnetron sputtering (HIPIMS) [START_REF] Depla | Sputter Deposition Processes[END_REF]. Among them, the most extensively used is dc magnetron sputtering. In dc sputtering, the target works as the cathode electrode, and the substrate is placed on the anode, which is electrically floating or biased to some negative potential. During the dc discharge, a plasma is generated near the cathode (-ve) surface, typically the gas pressure must be > 0.1 Pa. The magnetron traps the electrons close to the target surface creating a high density plasma from which ions (+ve) can be extracted to sputter the target material[Edgar [START_REF] Alfonso | Thin Film Growth Through Sputtering Technique and Its Applications[END_REF].

This type of sputtering requires that the target be a conductive material. A diagram of a dc magnetron sputtering is presented in Figure 4.1.

Reactive magnetron sputtering

The use of electrical conductors as targets limits the choice of materials that can be deposited. To mitigate this problem, a reactive gas can be added to the discharge. The reactive gas, e.g. nitrogen, oxygen, hydrogen sulfide, etc. can react with the sputtered material to form a compound on the substrate [START_REF] Depla | Sputter Deposition Processes[END_REF][START_REF] Swann | Magnetron Sputtering[END_REF]]. Reactive sputtering is commonly used for the deposition of oxide and nitride thin films using metallic substrates because of the high deposition rate, controlled deposition, and the possibility to use either dc or rf power [START_REF] Seshan | Handbook of Thin-Film Deposition Processes and Techniques[END_REF] A possible drawback of reactive sputtering is that the reactive gas can react with the target surface. This is known as target "poisoning", which results in a significantly reduced sputter yield and, thereby, reduced deposition rate. A further consequence of poisoning is a hysteresis of the reactive gas partial pressure at increasing/decreasing reactive gas flow, and, associated with an unstable operation regime between high and low target uptake of the reactive gas at a constant partial flow of that reactive gas, which often requires additional means of stabilization in practical applications [START_REF] Safi | Recent Aspects Concerning DC Reactive Magnetron Sputtering of Thin Films: A Review[END_REF][START_REF] Güttler | Mechanisms of Target Poisoning during Magnetron Sputtering as Investigated by Real-Time in Situ Analysis and Collisional Computer Simulation[END_REF]]. The significance of this phenomenon is that quite extensive investigation is required to establish the optimum experimental conditions to deposit a given compound in a particular deposition system.

Radiofrequency magnetron sputtering

Radiofrequency sputtering was developed to enable the sputtering of dielectric materials, but it can also be used to sputter metals as well. The rf sputtering operates analogously to dc sputtering only that for a small part of the rf cycle, the cathode and anode are electrically reversed in each electrical cycle. This eliminates charge buildup on an insulating surface by providing an equal number of ions, then electrons [START_REF] Seshan | Handbook of Thin-Film Deposition Processes and Techniques[END_REF]]. This allows insulators or metals to be sputtered in reactive environments. The use of an insulating target together with the difference in mobility of the electrons and ions (because of the difference in mass), and the existence of a capacitor in the rf circuit, means that a large negative space charge accumulates in the surface of the target. It is this negative charge that attracts the plasma ions and produces the sputtering.

Nevertheless, rf sputtering has a notable lower deposition rate, this is because the intensity of the negative space oscillates and the magnetron trap alternately opens and closes, allowing the electrons to escape when the trap is open and forcing electrons to cross the magnetic field line. This leads to a drop in the discharge power and consequently to a decrease of the available power at the target [START_REF] Depla | Sputter Deposition Processes[END_REF].

Experimental part

In this section, all the materials, methods, and experimental conditions for the growth and evaluation of the thin films are described. After the polishing process, each sample was cleaned consecutively with acetone (Fisher chemicals, general purpose grade) and isopropanol (Sigma-Aldrich ≥99,7%)

Substrate's preparation

for 10 min using an Elma ultrasonic bath. To finish each sample was dried using filtered compressed air.

Coating conditions by RF magnetron sputtering

To select the deposition conditions several publications were consulted and used as a guide. Such references reported good mechanical and tribological properties but none of them reported the corrosion behavior or the tribocorrosion behavior of the resulting thin films [P. [START_REF] Hones | Mechanical Properties of Hard Chromium Tungsten Nitride Coatings[END_REF]Polcar, Parreira, and Cavaleiro 2007a;[START_REF] Mischler | Triboelectrochemical Techniques and Interpretation Methods in Tribocorrosion: A Comparative Evaluation[END_REF][START_REF] Brett | Corrosion of Sputtered W-Ni-N Hard Coatings in Chloride Media[END_REF][START_REF] Baker | Reactive Sputter Deposition of Tungsten Nitride Thin Films[END_REF][START_REF] Samano | Mechanical Properties Optimization of Tungsten Nitride Thin Films Grown by Reactive Sputtering and Laser Ablation[END_REF]Shen and Mai 2000;[START_REF] Addonizio | Influence of Process Parameters on Properties of Reactively Sputtered Tungsten Nitride Thin Films[END_REF]Polcar, Parreira, and Cavaleiro 2007b;[START_REF] Parreira | Synthesis, Structural and Mechanical Characterization of Sputtered Tungsten Oxide Coatings[END_REF][START_REF] Louro | Hardness versus Structure in W-Si-N Sputtered Coatings[END_REF]. A summary of the deposition conditions in such publications is presented in Table 4.1, most of the references used dc reactive sputtering in a wide range of deposition powers and pressures. Moreover, most of them have shown that at a 12 % of nitrogen in the gas mixture crystalline W2N could be obtained. In the present work, tungsten nitride thin films were obtained by magnetron sputtering using a MP300S Plasyss system. The vacuum chamber was evacuated at a pressure of 1.25x10 -4 Pa (1x10 -6 Torr) before deposition. The target was a metallic tungsten disc (99.99%) provided by Plasmaterials, Inc. The target-sample distance was fixed at 4.5 cm. The target was cleaned by starting the plasma with argon, at least 5 minutes before each deposition. During this time, the substrate was covered with a shield. Figure 4.2 shows a diagram of the used system.

For each experiment a 300nm metallic tungsten buffer layer was first deposited.

For the tungsten nitride deposits, a mixture of argon (88 %) and nitrogen (12 %) was used. The pressure was fixed at 1.0, 0.7 or 0.5 Pa (7.5, 5.2 or 3.2 mTorr). The deposition power was 35 W, 150 W or 200 W. The substrates were heated at 220°C, before starting the deposition and the heating system was kept on during the deposition process.

Each substrate was coated with at least 1µm of thickness. Following to sputtering, the samples were left to cold down inside the vacuum chamber overnight. Subsequently, the samples were transferred to a tubular furnace provided by 'Services for research'.

Once in the furnace, the samples were heat-treated for 90 min at 500 °C under a nitrogen atmosphere. The nitrogen flow was started at least five minutes before the furnace was turned on. The heating rate was fixed at 10°C/min for all the samples. The samples were left to cold down in the furnace to a temperature lower than 80 °C before the nitrogen flow were stopped. After the heat treatment the samples were stored in a closed acrylic box in presence of a desiccant.

Results

This section presents the experimental details of the coating process, the thin film properties, and the properties optimization in terms of tribocorrosion protection.

4.

3.1 Thin films by dc vs rf sputtering magnetron.

As a film, tungsten nitride is an electric conductor; hence it can be obtained by accumulations. Additionally, Figure 4.3b and 4.3d present the EDS spectrum for each of the samples where the same elements were detected.

For a film to protect against corrosion, they must be the smoothest possible to avoid problems of diffusion of the electrolyte in between defects or pores in the thin films. Considering the observed surfaces, the thin film deposited by dc magnetron sputtering might present corrosion due to the observed texture. The electrolyte could travel in between the granules and hence directly attack the substrate and/or trigger the detachment of the thin film. Preliminary corrosion tests showed similar corrosion resistance for the coatings deposited by dc and rf sputtering. Since no significant difference between the coatings deposited by rf or dc sputtering was found and most of the experimental work was carried out in France, it was decided to continue the study with thin films deposited by rf magnetron sputtering (the deposition system available in France).

Growth rate and thickness

To estimate the deposition time, thickness measurements by profilometry were performed using different coated glass samples. Samples at each gas pressure and power were obtained. at 0.5 Pa, 11 nm/min at 0.7 Pa and 16 nm/min at 1 Pa. On the other hand, high powers and low pressures have been linked to increments on the thin film density [START_REF] Sun | Effects of Film Density on Electrochromic Tungsten Oxide Thin Films Deposited by Reactive Dc-Pulsed Magnetron Sputtering[END_REF][START_REF] Long | Structural and Mechanical Properties of Amorphous Si-C-Based Thin Films Deposited by Pulsed Magnetron Sputtering under Different Sputtering Powers[END_REF][START_REF] Musil | Mechanical and Tribological Properties of Sputtered Mo-O-N Coatings[END_REF]].

The mean free path defined by eq 4.1 represents the average distance traveled by the sputtered particle of diameter, δm. Such distance is enhanced at low pressure (P)

and high temperature (T). Such enhancement has been linked to larger relative densities on the resulting thin film.

𝜆 = 2.3 𝑥 10 20 𝑇 𝑃 𝛿 𝑚 2 𝑒𝑞. 4.1
For applications such as corrosion protection, a dense film is recommended to avoid the migration of the electrolyte up to the substrate. Hence for the present work, a dense thin film is looked for.

Surface roughness

After deposition, contact profilometry was performed to measure the surface roughness of the thin films. The average roughness (Ra) and the maximum peak to valley (Rz) were evaluated for the different deposition conditions. Table 4.2 presents a summary of the results. They show that an increment in the deposition power led to an increase in the surface roughness. However, no significant change was found when the pressure was changed.

Thin film stress

A common characteristic we can find in films obtained by sputtering is residual stress [Thornton and Hoffman 1989a;Shen et al. 2000;[START_REF] Clyne | Residual Stresses in Coated and Layered Systems[END_REF]]. The stress can come from intrinsic, thermal, and mechanical origins. The presence of stress can impact the performance reliability and durability of material components and devices.

Nevertheless, stress can have a positive effect on conductivity, dielectric permittivity, and charge carrier mobility. On the other hand, stress can lead to low adherence, fracture, or blistering of the thin films. In the case of films intended to enhance mechanical properties, it is important to determine the stress and reduce it to avoid problems such as peeling off and cracking.

Intrinsic stress is present as a result of the combined chemical and microstructural defects incorporated during the condensation process [Abadias et al. 2018b]. Thermal stress is caused by the difference in thermal expansion coefficients of the film and the substrate since the use of an insulating target together with the difference in mobility of the electrons and ions (because of the difference in mass), and the existence of a capacitor in the rf circuit, means that a large negative space charge accumulates in the surface of the target. it is this negative charge that attracts the plasma ions and produces the sputtering. [Thornton and Hoffman 1989b] To decrease intrinsic stress and hence the blistering and fracture risk, the thin films obtained in the present work were heat treated at different temperatures and in order to estimate the stress, the wafer curvature technique was utilized.

This technique is one of the most common ways to determine stress. The principle behind it is to measure the curvature of the substrate before and after deposition of the film.

To determine the stress, of 2 cm x 1 cm x 100 µm thick pieces of silicon wafers [100] were coated. Standard profilometries measurements were performed over the center line of the pieces of silicon before and after the coating process. The Stoney equation was applied to calculate the thin films stress at the different deposition conditions.

In a first approach, to estimate the heat treatment temperature, thin films obtained at 35 W and 1 Pa of pressure were synthetized. The heat treatment of independent samples was performed under continuous nitrogen flow at either 300 °C, 400 °C or 500 °C, the heat rate was 10 °C per minute. The sample was kept at the aimed temperature for 90 min. Table 4.3 presents the results of the stress found for the different heat treatment temperatures. It can be observed that there was a decrease in the stress with the increasing temperature of the heat treatment. For the tested thin films, it was impossible to heat treat the sample over 500 °C because at larger temperatures the thin film tended to delaminate probably due to thermally induced stress or oxidation [Polcar, Parreira, and Cavaleiro 2008]. Stress can also be caused by chemical and structural defects present in a thin film depending on the deposition conditions. Hence, the stress was determined for the different deposition conditions (power and pressure) and heat treated at 500°C. Table 4.3 presents a summary of the found values. Such values showed that the coating stress was not significantly different for the different deposition conditions.

X ray diffraction

The measurements were made either at the UNAM with a x ray diffractometer Rigaku Ultima IV (1,5406Å) or at LPMC using x ray diffractometer D8 by Bruker

(1,5406Å) in either θ-2θ or grazing incidence arrangement with a rotatory detector from 10° to 100°. WO3 thin films [START_REF] Baker | Reactive Sputter Deposition of Tungsten Nitride Thin Films[END_REF]Shen and Mai 2000;[START_REF] Wen | Growth, Stress and Hardness of Reactively Sputtered Tungsten Nitride Thin Films[END_REF][START_REF] Tesfamichael | Thin Film Deposition and Characterization of Pure and Iron-Doped Electron-Beam Evaporated Tungsten Oxide for Gas Sensors[END_REF][START_REF] Addonizio | Influence of Process Parameters on Properties of Reactively Sputtered Tungsten Nitride Thin Films[END_REF]].

Figure 4.13a shows the high-resolution data for tungsten (W 4f), the spectrum shows the typical doublet for tungsten along with a small signal commonly assigned to WO3. The core level spectrum for nitrogen (N 1s) and oxygen (O 1s) are displayed in Figure 4.13b and 4.13c, respectively. Similar have been reported for tungsten nitride thin films by other authors [START_REF] Wen | Growth, Stress and Hardness of Reactively Sputtered Tungsten Nitride Thin Films[END_REF][START_REF] Addonizio | Influence of Process Parameters on Properties of Reactively Sputtered Tungsten Nitride Thin Films[END_REF]Shen and Mai 2000].

Since the Ar/N2 ratio was kept constant, no large changes in the final composition of the thin films were expected. This was confirmed from the XPS analysis where the average atomic composition was found to be 51.1 ± 3.5 % of W, 35.4 ± 3.4% of O and 13.3 ± 2.1% of N. Such composition obtained from the sample surface after erosion.

Moreover, composition analysis measurements were performed at different depths. Moreover, analysis of the chemical shift showed that the tungsten detected was bonded to oxygen and nitrogen, and both nitrogen and oxygen are only bonded to tungsten. Hence the obtained coatings are tungsten oxynitride.

Hardness measurement by micro indentation.

Micro indentations measurements were made to the samples using a Nanovea PB1000 Hardness Tester with a Vickers diamond point in a load range between 0.1 to 2 N. To estimate the thin film hardness, the mixed hardness for the substrate-thin film was considered. Different loads were applied the model proposed by Lesage et al [START_REF] Lesage | A Model to Determine the Surface Hardness of Thin Films from Standard Micro-Indentation Tests[END_REF]] was used to estimate hardness of the thin films.

During the indentation process delamination frequently occurred, hence only estimated hardness values could be obtained to give a general tendence of the hardness values as a function of the preparation conditions and not the actual hardness values.

For thin films deposited at 35 W and 1.0 Pa the estimated value by the Lesage model was 8.9 ± 0.2 GPa, whereas for a thin film made at 200 W and 1.0 Pa the estimated value was 9.1 ± 1.7 GPa. Clearly these values are not significantly different.

For the samples obtained at other deposition conditions the obtained results could not be adjusted due to the delamination problems.

Conclusions

The XRD measurements of the obtained thin films showed the existence of a combination of two crystalline structures, a fcc structure of tungsten nitride and a monoclinic phase of tungsten oxide (WO3). The crystallinity was somehow promoted by using higher deposition powers and lower gas pressures.

The heat treatment of the coated samples caused a decrement in the stress and improved crystallinity of the films. Such diminution in the stress might be expected to improve the tribological response. Nevertheless, the heat treatment boosted the formation of the oxide, and the oxidation of the thin film has been linked to delamination problems.

The scratch tests showed that an increase in the deposition power, slightly diminished the resistance towards fracture of the thin films giving a LC2 of 10.9 N for the sample sputtered at 200 W and 1.0 Pa.

The XPS results showed that elements in the thin film not affected by the deposition conditions. As the XPS depth profile showed, the detected elements and their atomic percentages remain constant, and the thin films are tungsten oxynitride.

The same elements were detected by EDS. Hence, the variations in the response of the thin film are not related to the composition but to other factors such as the crystalline structure, roughness, thin film's density, etc.

For this work a dense thin film with good adherence should present the best performance as a protective thin film to tribocorrosion. Therefore, the best deposition

b a

conditions for tribocorrosion protection of the proposed substrates are probably low deposition pressures and medium to high powers.

Electrochemical impedance spectroscopy

EIS measurements were carried out using the same arrangement as for the OCP studies in a frequency interval from 10 5 Hz to 10 -2 Hz with at least 3 points per decade and an amplitude of ± 10 mV vs OCP. The results were modeled using the equivalent circuit approach to determinate the corrosion resistance of the system which when multiplied by the exposed area of the sample gave the specific corrosion resistance.

Tribocorrosion evaluation

The tribocorrosion behavior of the passivated coatings was evaluated. Given that tribocorrosion is a mixed phenomenon, such evaluation was carried out coupling the pin on disc tests and the electrochemical measurements. For such studies a threeelectrode cell and an aluminum oxide ball were used.

Continuous sliding test

After a steady OCP value was achieved, EIS and pin on disc tests were performed simultaneously. A continuous sliding pin on disc tests at 1.5 N of normal load and 0.02 m/s of angular speed were performed, for a total length of 72 m. While the pin on disc test was running, EIS measurements were carried out in a frequency interval from 10 5 Hz to 10 -2 Hz with at least 3 points per decade. From such tests the volume loss and the corrosion current density of the active material were determined. Contact profilometries and SEM measurements were used to assess the final state of the wear surface. Furthermore, the EIS results were fit using the equivalent circuit approach.

Latency tests

For the latency tests, successive identical cycles of active pin on disc and latency were performed until a total wear test length of 72m was reached. Each cycle comprised a full turn (1 s) and a time off (toff)of 20 s. The applied normal load was 1.5 N.

By Combining the EIS results, the continuous sliding volume loss and the total volume loss during on the latency tests, the ratios Kc and Km were calculated. As described in Chapter 1, the Kc value can be used to define if the corrosion process of the mechanical wear dominates over the tribocorrosion process. Furthermore, Km indicates the effect of the material passivation in the mechanical material removal, and the contribution due to the re-passivation as described in the first chapter.

Results

Pin on disc

During the pin on disc test the normal force and the friction force were sensed, and then used to calculate the coefficient of friction. The pin on disc test produced a circular wear track on the thin film.

Coefficient of friction

The temporal evolution of the COF from the pin on disc tests of the thin films sputtered at 1.0 Pa and different powers are shown in Figure 5.1. The red line is the smoothed data using the adjacent average filter of 500 points. In each case the curves showed the typical sharp variation at the beginning of the test. Corresponding to the running in period, once the steady state regime was achieved the COF was estimated, the results for the plasma powers of 35, 150 and 200 W were 0.37 ± 0.02, 0.27 ± 0.05 and 0.32 ± 0.03 respectively. Such results present no clear tendency with respect to the deposition power and are almost within the experimental uncertainty of the measurement. Considering the obtained XRD presented in 4.3.5 one can infer that the crystallinity of the thin film and the COF do not depend on the plasma power used for the deposition process.

Additionally, thin films sputtered at 200 W but different gas pressures were tested, and Figure 5.2 presents the comparison of the temporal evolution of samples sputterer at 200 W and 1.0, 0.7 or 0.5 Pa. The curves have a similar behavior than the one found for the thin films made using different deposition powers. The average COF was estimated in the steady state range of the test giving the following values: 0.32 ± 0.03, 0.33 ± 0.03 and 0.42 ±0.05 for 1.0, 0.7 and 0.5 Pa respectively. Such values of COF are slightly smaller than the ones found for AISI D2 steel (0.53 ± 0.10) and AISI 431 steel (0.48 ± 0.21). Such smaller COF is linked to a smaller friction force, so probably the thin film acts as a lubricant and smaller damage over the wear track was expected.

Wear track analysis

After the pin on disc test, mapping profilometries of the wear track were acquired. Such profilometries were carried out using a Dektak XT contact profilometer by Bruker, with a tip of 2.5 µm at 3 mg of normal load. The obtained topographic images were used to estimate the surface changes on the wear track region, an example of the topographical image acquired is presented in Figure 5.3. In the image it is possible to distinguish the circular wear track; such type of image was used to estimate the material loss volume reported in Table 5.1.

Table 5.1 Summary of profilometry results for the tungsten nitride thin films after the pin on disc test at 1.5 N of normal load and 0.02m/s of speed. A summary of the wear track roughness and the material loss is presented in Table 5.1. The material loss values were consistently smaller than the ones found for the substrates, 2.1 x10 -4 mm 3 for AISI D2 steel and 1.6 x10 -4 mm 3 for AISI 431 steel. Such values show that the thin film is more resistant to mechanical wear, than any of the tested steels.

Wear track analysis by scanning electronic microscope and Energy

dispersive X-ray spectroscopy.

As for the steel substrate samples, images were acquired by scanning electronic microscopy, Using a Quanta 200FEG microscope with a microanalyzer X INCA Oxford type SDD 80 mm 2 . For the thin films, the images were obtained under high vacuum at 20 KeV while for the counter bodies the images were obtained at low vacuum and energies of about 10 KeV. In general, the wear track presented lines in the direction of the rotation and evidence of material accumulation. EDS tests revealed a difference of detected elements before and after the pin on disc test. On average, for the thin film before a pin on disc test 87 % of W, 3 % of N, 6% of O and 4% C were detected. Such percentages changed after the pin on disc test. Finally, SEM images and an EDS measurement of the counter body were taken.

Figure 5.12 presents a SEM image along with the EDS mapping of a counter body after a pin on disc test of AISI D2 coated with tungsten nitride. The SEM image shows the contact point with some accumulated material. This region (red square) was analyzed by EDS, most of the detected elements corresponded to the counter body elements Aluminum (35 %) and Oxygen (55%) with approximately 10% of W from the thin film.

Since the thin film material adhered to the counter body and the type of marks found over the wear tracks, one can infer that the thin film wear occurred by third body abrasion.

Electrochemical tests

This presents the electrochemical evaluation of AISI D2 and AISI 431 steel coated with tungsten nitride. The tests were performed in a three electrode arrangement as described in section 3.2.2.

Open circuit potential and reaction time

The temporal evolution of the open circuit potential of the samples with tungsten nitride coatings prepared at different power and pressure values was recorded. Figure 5.13 displays a comparison the evolution for the uncoated and coated AISI D2 at 1.0 Pa and different powers. Each of the curves show a sharp decrement of the potential, until a steady value was achieved. For steel, the steady value was reached after only 23 minutes, whereas, for the coated steel this took various hours. The OCP steady state value of the thin film s sputtered at 35 W was -0.4 V vs Ag/AgCl or -0.23 V vs NHE after a reaction time of 3 h 40 min. The thin film at 150W reached the steady state at-0.35 V vs Ag/AgCl ( -0.14 V vs NHE), with a reaction time about 2h 30 min. Finally, for the sample sputtered at 200 W a steady state was reached at -0.29 V vs Ag/ AgCl (-0.09 V vs NHE) with an estimated reaction time of 3h 40 min. In general, when comparing two electrochemical systems, the system with the higher OCP value is more likely to be corrosion resistant. Therefore, the increment on the deposition power presented a beneficial effect in the OCP.

Moreover, the reaction time is significantly larger than the one found for AISI D2. This larger reaction time indicates that the system equilibrium took longer to be reached. This can be due to changes in the thin film surface, which alters the measured interface. hence the passivation process took longer. The measured OCP for the coated sample was -0.04V vs Ag/AgCl (0.15 V vs ENH) a similar value than that found for the uncoated sample. Furthermore, the uncoated silicon sample achieved a steady state at about 0.06 V vs Ag/AgCl (0.26V vs NHE), and for the coated sample had an OCP value was 0.03V vs Ag/AgCl (0.23V vs NHE) at a treact of 30 min. Such OCP represents a system which tends to passivation, indicating that the coatings deposited on conductive silicon and AISI 431 could passivate.

Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) measurements were carried out with a 10.0 mV perturbation in a frequency range from 10 5 Hz to 10 -2 Hz. For each decade of frequency, at least three measurements were made. The obtained response was fitted using the equivalent circuit method. The Z circuit fitting tool in the PS Trace 5.8 software was used applying the Levernberg-Marquarat algorithm. An area of 0.45cm 2 was exposed during the experiments. Once the system achieved steady state, the electrochemical impedance spectroscopy test was performed. Figure 5.17 The obtained resistance values were used to calculate the specific corrosion resistance (rcorr) and the corrosion current (icorr), and Table 5.2 presents a summary of the values. The rcorr was slightly larger for the coated samples, moreover, the rcorr value increased with the deposition pressure used. Nevertheless, these values correspond to a system under corrosion, hence, the tungsten nitride partially protects the substrate. proposed in Figure 5.18. The Rp of the thin film deposited on the AISI 431 was 410 ± 50 kΩ, which corresponds to a rcorr of 207 ±22 kΩ cm 2 , such specific polarization resistance corresponded to a passive system with an icorr of 5.8x10 -8 A/cm 2 . Such values are of a better passivated surface than for the uncoated steel. The rcorr estimated value represents the passivation resistance value of the system (rpass). As for the silicon samples, the estimated Rp value was 188MΩ ± 5MΩ (84 MΩ cm 2 ) for the uncoated sample and for the 617 ± 20 kΩ for the coated sample. The Rp value for the coated sample corresponds to a rcorr of 277 kΩ cm 2 and again this specific polarization resistance corresponds to a passive system with an icorr of 8.6x10 -8 A/cm 2 .

The specific corrosion resistance found for the thin film in both AISI 431 steel and Si were very similar, 207 and 277 kΩ cm 2 . Hence, one can infer that such a value truly corresponds to the corrosion resistance value of the tungsten nitride thin film.

SEM and EDS

To determine if surface changes occurred due to the electrochemical tests, SEM images and EDS measurements on the thin films before and after the electrochemical tests were. (deposited at 150W and 200W respectively). This indicates that an increment on the plasma power helps to diminish the corrosion regions helping to control the corrosion damage. Furthermore, EDS measurements were performed and for the thin film as deposited the average composition was 87 % of W, 3 % of N, 6% of O and 4% C. As for the samples after the electrochemical tests, EDS measurements were performed on the regions of the texture (red squares), such regions had a different composition than that of the as deposited samples. The analyzed regions in b, c and d had similar compositions. The average of such compositions was: Fe 75%, Cr 9%, O 8%, V 3%, C 3%, and W 4%. The most abundant elements correspond to the substrate, hence in such regions the thin films had been almost completely removed. On the area where no texture changes were detected the composition of the surface corresponded to that of the sample before the electrochemical test. Similarly, to the thin films obtained on steel, SEM images and EDS measurements were performed on the thin films deposited on the conductive silicon. the electrochemical test. One can consider that the amounts of the elements in the thin films deposited on AISI 431 steel and silicon were virtually unaltered by the electrochemical test. Nevertheless, the thin films deposited over AISI D2 presented localized corroded regions. The total area of the corroded region was found to decrease for the films deposited at the higher plasma powers.

Thin film adherence and buffer layers.

Due to the prevailing adherence problems, explained in chapter 4, and the recurrent corrosion points of the thin films deposited on AISI D2 steel, four different buffer layers were explored. Each type of buffer layer was deposited on the AISI D2 substrates prior to the deposition of the tungsten nitride coatings. The buffer layers thickness was about 10 nm, the elected materials were tungsten, platinum, silicon, and platinum. The tungsten was used as buffer layer since it is the base metal for the nitride, the use of the base metal of hard coatings deposited by sputtering has been reported to help enhance the adherence. Silicon, platinum and carbon are materials known to have good corrosion resistance; hence a priori they could help to protect the substrate surface from potential corrosion.

Tungsten, silicon, and carbon layers were deposited by rf sputtering magnetron. As for platinum, a DC sputtering metallizer was used. After the buffer layer was deposited the tungsten nitride thin film was deposited by rf magnetron sputtering at 150W and 0.5 Pa, furthermore the thin films where heat treated at 500°C for 90 min under nitrogen atmosphere. Finally, a region of the sample was put in contact with a 10 -4 M aqueous citric acid solution. The changes over the surface were investigated by SEM and EDS using as a guide some notable mark in the sample surface. As a reference SEM and EDS measurements were performed before the deposition for each sample.

Tungsten buffer

The tungsten buffer layer was sputter deposited at 150 W and 0.5 Pa using pure Ar for 2 min, followed by the deposition of the tungsten nitride thin film by simple replacing the Ar with a mixture of N2 12% in Ar. analyzed region. The elements and their percentages were Fe 76.5 %, Cr 15.5%, C 4.5%, V 1.4%, Si 1.5%, and Mo 0.7%. The distribution showed iron deficiency (less intensity color) regions; moreover, such regions were rich in vanadium and carbon indicating the probable presence of carbides in such regions.

After the deposition of the tungsten nitride coatings, SEM images of the sample were acquired. Figure 5.28 shows a smooth and uniform surface of the samples. Moreover, EDS tests were performed to identify the main elements, with their percentages found to be for W 88%, O 7.5%, for N 4.5%.

Finally, the sample was put in contact with the aqueous solution of citric acid for five hours. Subsequently, SEM images and EDS tests were performed. Figure 5.29 shows the images and mapping EDS of the sample. In the left-handed image it is possible to distinguish a small, damaged region, a close up to this region is given in the right-handed image. The EDS maps show the distribution of each detected element. The elements that correspond to the thin film are virtually absent from the region where a change in the texture is notable. The zones where the thin film was removed correspond to the areas of carbon.

As a way to comparison, similar experiments were performed using a substrate of AISI 431 steel., Figure 5.30 shows the SEM images of the uncoated sample, coated and after being in contact with the citric acid solution. In the first image one can see a reasonably uniform surface on the nearby of the mark, some small dark points (valleys on the surface) can be identified as a part of the surface texture. EDS measurements of the uncoated substrate detected the following elements Fe 78%, 17% Cr, 3% Ni and 2% C. For the AISI 431 sample coated with the tungsten buffer layer and the tungsten nitride thin film, the EDS test gave a composition of W 89%, O 7%, and N 4%. The same elements at similar percentages were found for the sample after being exposed Figure 5.29 SEM images and EDS mapping results of AISI D2 coated with a tungsten buffer layer followed by tungsten nitride after being exposed to a 10 -4 citric acid aqueous solution. to citric acid. Hence, for this sample the use of the buffer layer of tungsten and the tungsten nitride film effectively protected the substrate's surface from corrosion, moreover, one can confirm that the presence of the buffer layer does not produce the delamination of the tungsten nitride layer seen in the AISI D2 substrate.

Silicon buffer over AISI D2

Since the thin films deposited over silicon had good electrochemical characteristics the use of a silicon layer as buffer between the substrate and the thin film was explored. The silicon layer was deposited by rf magnetron at 1Pa and 200W using pure Ar in the Plasyss system, immediately followed by the sputtering of the tungsten nitride thin film. Figure 5.31 contains the SEM images of AISI D2 sample coated with the silicon buffer the tungsten nitride thin film before and after being in contact with the citric acid solution. The left-handed side image is of the thin film as deposited, the sample has a and smooth surface with no large visible defects. However, for the sample after contact with the acid, one can see a change on the surface texture in some regions. EDS measurements of these parts (Figure 5.32) showed the presence of iron from the substrate indicating that the thin film had been partially. The average weight percentage of each element on the corroded sample were 76% W, 12% O, 2% N, 4% C and 6% Fe. The EDS maps show again corrosion regions, in such corrosion regions the elements of the buffer layer and tungsten were not detected, once again the presence of carbon an iron from the substrate was detected. tungsten nitride film. The EDS analysis gave an average composition of W 84%, O 12%, N 3%, and Ar 1%.

After exposition to the citric acid solution the surface was considerable damaged as shown in the right-handed image of Figure 5.31. The Figure 5.34 shows the EDS maps of the damaged region, as before each image is the distribution of each detected element. The detected elements and their average percentages were W 45%, Fe 24%, 11 O%, C 10%, Cr 6%, N 3%, and Pt 2%.

The EDS maps for W, Fe, Pt, C and Cr demonstrate that the acid treatment removed the tungsten nitride film and the buffer layer.

Carbon buffer layer

Finally, AISI D2 substrates were coated with a buffer carbon layer by rf magnetron sputtering using pure argon at 0.5 Pa and 250W using the Plasyss system, immediately followed by the sputtering of the tungsten nitride film at the same condition of the previous samples. In between deposits (buffer and tungsten nitride) the coatings were keep inside the same vacuum chamber. Once both deposits were finished the samples were taken off the vacuum chamber and the coatings delaminates instantly in contact with air. Figure 5.35 shows an example of a coated samples just after the deposition process had finished. As one can see the sample suffered generalized delamination of the tungsten nitride coatings due to the presence of the carbon coating. This helped to corroborate that when the tungsten nitride film is in contact with high carbon concentrations the thin film delaminates.

Tribocorrosion evaluation

Continuous sliding

The measurements were carried out on samples coated with tungsten nitride deposited by rf magnetron sputtering at 0.5 Pa and 150 W were under continuous sliding test conditions at 1.5N of load and 0.02 m/s, in contact with 10 -4 M citric acid aqueous solution.

For the thin films sputtered over AISI D2 the influence of the electrolyte pin on disc test was evaluated, however, the tribocorrosion synergy could not be evaluated because of the dominant localized corrosion regions. Hence, to evaluate the influence of the electrolyte in the wear of the AISI D2 steel coated with tungsten nitride, pin on disc tests were performed. Figure 5.36 shows an example of a wear track profile. Here is not evident to identify the wear track region due to severe damage over the samples. The estimated material loss was 2.2x10 -3 mm 3 , a value two magnitude's order larger than for the dry test (1.910 -5 mm 3 ) and one magnitude's order larger than for the uncoated steel (4.91x10 -4 mm 3 ). Such large increment is probably due a galvanic effect between the thin film and the steel [START_REF] Diomidis | A Methodology for the Assessment of the Tribocorrosion of Passivating Metallic Materials[END_REF][START_REF] López-Ortega | Tribocorrosion of Passive Materials: A Review on Test Procedures and Standards[END_REF]. The galvanic effect occurs when two conductive materials (in general metals) are in contact with an electrolyte and one of the materials corrodes preferentially [START_REF] Bagotsky | Fundamentals of Electrochemestry[END_REF]]. In this case, when the thin film presents delamination and fractures due the mechanical wear and corrosion points, caused by the incompatibility of the thin film with the rich in carbon regions the AISI D2 steel. Then the areas where the coating has failed will be exposed to accelerated corrosion.

As for the AISI 431 coated samples, the combined electrochemical measurements and pin on disc tests were performed. The OCP was measured before and during the pin on disc test. Figure 5.37 shows the temporal evolution of the COF. Such a curve presents significant changes during the first minutes of the test which corresponded to the breaking in of the passivation layer before a steady state was established. The average COF during the steady state was 0.04 ±0.01, a significantly smaller value than that found for the pin on disc test without electrolyte (0.28 ± 0.07). This reduction in the COF can be attributed to the aqueous solution acting as a lubricant and the formation of a good passive layer. Furthermore, the resulting Nyquist diagram for the test under continuous sliding is presented in Figure 5.38, such a diagram has two semi circles as for the previously analyzed systems. The obtained results were modeled using the equivalent circuit shown in Figure 3.28 composing of two parallel RC circuits connected in series. Here the second resistance (R2) corresponds to the polarization resistance Rps of the system, which is considered to be the combination of two polarization resistances, Ract associated with the area that corrodes and Rpass the area that passivates. The estimated of Rps value was 154.1 ±2.2 kΩ. Such value, along with the specific passivation resistance (rpass) obtained from the static EIS tests, was employed to estimate the specific corrosion resistance of the active region or wear track (ract). The ract was calculated as described in Chapter 3. The estimated ract value was 9.3 kΩcm 2 which corresponds to a corrosion current of about 2.6x10 -6 A/cm 2 . Furthermore, profilometry mapping was performed over the sample surface after the coupled test. Figure 5.39 presents an example of a wear track profile; the wear track region is highlighted with the red rectangles to assist on the wear tack detection. The coatings proved to diminish the mechanical wear for both substrates with wear rates typically an order of magnitude smaller than that obtained for the uncoated steel samples (AISI D2 2.1x10 -4 mm 3 , AISI 431 1.6x10 -4 mm 3 . Nevertheless, the coatings deposited on AISI D2 presented adhesion problems and such problems resulted in localized corrosion. To overcome this problem the use of different buffer layers was explored. Unfortunately, the proposed buffer layers did not reduce the localized corrosion. In particular, the adherence problems were found to be associated to high carbon concentrations in the substrate in some localized regions of the substrate

The EIS tests revealed an increment in the corrosion resistance for the thin films deposited over AISI D2 steel. Nevertheless, the increment was not enough to achieve passivity.

The thin films deposited over AISI 431 steel increased the corrosion resistance of the sample surface.

Comparing the specific corrosion resistance from the thin films obtained on conductive silicon and AISI 431 steel, it was possible to estimate that the specific resistance corrosion value for the tungsten nitride thin film was about 242 ± 35 kΩcm 2 .

Hence the tungsten nitride thin film was corrosion resistant on this substrate.

The tribocorrosion of the thin film deposited on AISI 431steel was studied. After the pin on disc test under room temperature and relative humidity, the thin film presented a total material accumulation of 7.1x10 -6 ±1.8x10 -7 mm 3 a significantly lower value than the one obtained for the uncoated substrate, 1.6x10 -4 ±1.0x10 -4 mm 3 . Hence the thin film protected the surface against mechanical wear. Moreover, the specific corrosion of the coated sample (207 ±25 kΩcm 2 ) was found to be larger than the one for the uncoated AISI 431 steel (160±5 kΩcm 2 ). Additionally, when exposed to tribocorrosion the volume loss over the wear track was smaller for the coated sample (5.1x10 -4 for continuous sliding and. 7.0 x10 -4 mm 3 for latency), than for the uncoated sample with (1.8x10 -3 mm 3 for continuous sliding and.8x10 -3 mm 3 for latency). Finally, the estimated Kc and Km values were 3.6x10 -1 , 3.0x10 -3 for AISI 431 steel and 5.1x10 -1 , 6.1x10 -3 for the tungsten nitride thin film. Hence the tribocorrosion mechanism was for the uncoated and coated AISI 431 steel samples. This indicated that the acceleration of the corrosion due to the destruction of the passive film was low and that the formation of the passive film accelerates the mechanical removal of the material.

6

Chapter 6

General conclusion and perspectives

This thesis presents a tribo-electrochemical study (tribocorrosion) of thin tungsten nitride films aimed for protective thin films against tribocorrosion concerning tools used in the food industry. Tribocorrosion is a complex wear phenomenon that occurs when a mechanical action takes place on a metal surface in a corrosive environment. It depends on both; the used tribological system, in terms of the mechanical properties and geometry of the bodies constituting the contact, and on the chemical and electrochemical properties of the medium in which the wear process takes place.

Therefore, when studying tribocorrosion, it is crucial to highlight the mechanisms at work and to identify the relevant parameters to better understand tribocorrosion.

With this in mind, a methodology has been put in place to determine the various contributions to the total wear by isolating mechanical wear, corrosion, and synergy between both, the substrates, and the tungsten nitride film.

In order to satisfactorily perform the tribo-electrochemical measurements, a custom-made cell was created. This cell had an appropriate geometry for the tribometer used, allowed the proper arrangement of electrodes for electrochemical tests, avoided electrolyte leaks and allowed the adequate isolation of the electrical contact of the sample.

For the substrates it was found that the AISI D2 steel resists the mechanical wear but corrodes when in contact with citric acid. Since it spontaneously corrodes if exposed to simultaneous mechanical wear and corrosion the material loss will be dominated by the material corrosion. As for AISI 431 steel it was found that it exhibits good wear and corrosion resistances when in contact with citric acid. Both steels wear by third body abrasion with aluminum oxide as counter body.

Moreover, when exposed to tribocorrosion it was found that the general damage was significantly larger than when exposed to wear or corrosion independently.

Furthermore, the destruction of the passive film does not affect in a significative way the corrosion speed. Nevertheless, the formation of the passive film accelerates the mechanical material removal. Hence, if relative movement between the steel surface and the counter body is slow enough to allow the regrowth of the passive film, the amount of damage due to the tribocorrosion process is larger.

The AISI D2 steel and the AISI 431 were coated with tungsten nitride towards the improvement of the tribological, electrochemical and tribo-electrochemical properties of the material surface. The obtained thin films presented a combination of two crystalline structures, a fcc as the structure found for tungsten nitride and a monoclinic as the one found on tungsten oxide (WO3).

The tungsten nitride thin films proved to diminish the mechanical wear for both substrates with wear rates one magnitude order smaller than the one obtained for the uncoated steel samples. Nevertheless, the thin films obtained over AISI D2 steel presented localized corrosion. Different deposition conditions were attempted to improve the corrosion resistance. It turned out that denser thin films deposited at low pressures and high powers presented the best corrosion resistance. However, even for the optimized films the surface corrosion was not completely eliminated due to persistent delamination problems in carbon rich regions on AISI D2 steel. Hence, tungsten nitride will protect from corrosion only in certain systems with less carbon content.

When exposed to tribocorrosion, the AISI 431 steel coated with tungsten nitride lose less material than the uncoated substrate, indicating that the thin film protects against tribocorrosion. Moreover, it was found that the corrosion speed is not affected by the mechanical removal of the passive film, nevertheless, the formation of such passive film accelerates the mechanical removal of material.

The results found in this thesis provides a new approach on the use of metallic nitrides as tribocorrosion resistant materials, leaving the door open to study the influence of the pH in the corrosion and tribocorrosion process towards the use of the thin film in different applications. As for a tungsten nitride as a protective film in construction material for the food industry, the present work gives a first approach highlighting the resistance against tribocorrosion at room temperature, nevertheless, additional studies at higher temperatures (up to 60°C) should be carried out along with toxicity panels of the resulting products of the tribocorrosion.

Furthermore, the influence of the composition of the tungsten nitride thin films and their corrosion and tribocorrosion resistance should be studied to cross with the extensive literature on their tribological properties. Such type of studies can help to better understand the factors that affects the tribocorrosion of tungsten nitride and its limitations.

Figure 1

 1 Figure 1 Scheme of a can seaming system [Yam 2009]. .....................................................

Figure 2

 2 Figure 2 Representation of W2N fcc crystalline structure [Baker and Shah 2002] .........

Figure 1 . 1

 11 Figure 1.1 Representation of a Hertz contact between a sphere and a plane. .............. between a sphere and a plane. ............................................................................................

Figure 1 . 2

 12 Figure 1.2 Illustrations of different wear types. (a) third body abrasive wear; (b) three bodies abrasive wear; (c) adhesive wear, (d) and (e) fatigue wear. ...............................

Figure 1 . 3

 13 Figure 1.3 Representation of the layers at a solid/liquid interface. 1 Inner Helmholtz layer, 2 Outer Helmholtz layer, 3 Diffuse layer, 4 Solvated ions (cation), 5 Electrolyte solvent, 6 Specifically adsorbed ions. Image adapted from [Fernandez-Solis et al. 2016] ........................................................................................................................................

Figure 1 .

 1 Figure 1.4 representation of a three electrodes cell [Amemiya et al. 2007] ..................

Figure 1 . 5

 15 Figure 1.5 Polarization curve with Tafelian behavior where the corrosion current icorr is found at the Ecorr [Amemiya et al. 2007] .........................................................................

Figure 1 . 6

 16 Figure 1.6 Randles circuit and it equivalent as Nyquist and Bode plots, modified from reference [Fernandez-Solis et al. 2016] .....................................................................

Figure 1 . 7

 17 Figure 1.7 Representation of the elements parameters and system characteristics which can influence the response of tribocorrosion processes. Translated from [Richard and Geringer 2019]. ..............................................................................................

Figure 1 . 8

 18 Figure 1.8 Schematic of the core components of a scanning electron microscope. Image taken from reference [Inkson 2016] ........................................................................

Figure 1 . 9

 19 Figure 1.9 Representation of the interaction electron matter in a scanning electron microscope. Where Ej is the ionization energy, Zd is the full diffusion depth, Zu useful penetration depth and Zm the total penetration depth. Adapted from reference[Ruste 2013] ........................................................................................................................................

Figure 1 .

 1 Figure 1.10 Representation of Bragg's law for constructive interferences. ...................

Figure 1 .

 1 Figure 1.11 Representation of the measurement geometry for the measurements by X-ray diffraction. The image on the left represents the mode θ-θ, the image on the right the mode θ-2θ. .............................................................................................................

Figure 1 .

 1 Figure 1.13 To the left a graph of a sample profile, to the right a 3D profile. ..............

Figure 1 .

 1 Figure 1.14 Schematic comparison of Knoop and Vickers micro-indentations [Broitman 2017] .....................................................................................................................

Figure 1 .

 1 Figure 1.15 Schematic illustration of a scratch test[Berthout, Randall, and Randall 2017]. .......................................................................................................................................

Figure 2 . 2

 22 Figure 2.2 Top view of the vacuum chamber without lid ...............................................

Figure 2 . 1

 21 Figure 2.1 Diagram of the vacuum chamber of the Plasyss MP 300S ...........................

Figure 2 . 3

 23 Figure 2.3 Transversal cut diagram of the vacuum chamber. ........................................

Figure 2 . 4

 24 Figure 2.4 Diagram of the tribocorrosion cell for reciprocating test (left) and a picture of such a cell (right)...............................................................................................................

Figure 2 . 5

 25 Figure 2.5 Picture of the lubricated pin on disc tests module. .......................................

Figure 2 . 6

 26 Figure 2.6 Diagram of the cover for the pin on disc UTM tribolab base (left) and a picture of the final cover (right). .........................................................................................

Figure 2 . 7

 27 Figure 2.7 Picture of the experimental set up for the module cover. ............................

Figure 2 . 8

 28 Figure 2.8 Diagram of the tailored tribocorrosion test. ...................................................

Figure 2 . 9

 29 Figure 2.9 Picture of the tribocorrosion cell and the experimental set up. ...................

Figure 3 . 1

 31 Figure 3.1 Phase diagram iron-carbon for the steel and cast iron regions[Uhlenhaut et al. 2006] ...............................................................................................................................

Figure 3

 3 Figure 3.2 (a) Example of a body centered tetragonal crystalline structure, structure of the cementite. (b) example or a face centered cubic crystalline structure, crystallune estructur which coresponds to austenite. .....................................................

Figure 3 . 3

 33 Figure 3.3 Example of a standard profile for mirror polished samples of AISI D2 (right) and AISI 431 (left). ....................................................................................................

Figure 3 .

 3 Figure 3.5 Top SEM of the section analyzed by EDX. Bottom counts per second of each element along the analyzed line for AISI D2 steel. .................................................

Figure 3

 3 Figure 3.4 SEM micrography and mapping EDX of an AISI D2 steel sample, each image represents the dispersion of each detected element. ............................................

Figure 3

 3 Figure 3.6 SEM micrography and mapping EDX of an AISI 431 sample, each image represents the dispersion of each detected element. .......................................................

Figure 3 . 7

 37 Figure 3.7 Top SEM of the section analyzed by EDX. Bottom counts per second of each element along the analyzed line for AISI 431. .........................................................

Figure 3 . 8

 38 Figure 3.8 Diffractogram of a substrate of AISI D2 ..........................................................

Figure 3 . 9

 39 Figure 3.9 Load as a function of depth for a micro-indentation measure of AISI D2 steel at 5N ...............................................................................................................................

Figure 3 .

 3 Figure 3.11 Diagram of the cell for tribocorrosion measurements. ...............................

Figure 3 .

 3 Figure 3.12 Schematic top view of a disc shaped test sample: (a) without sliding, (b) during sliding tests, and (c) during sliding tests at long latency times resulting in a partially active sliding track [Celis and Ponthiaux 2017]. ...............................................

Figure 3 .

 3 Figure 3.13 Graph of coefficient of friction as function of time for the system alumina (pin) AISI D2 (disc)at 0,02m/s of speed and Left-hand @ 0.5N, right-hand @ 1.5N of load ..........................................................................................................................................

Figure 3 .

 3 Figure 3.14 Graph of coefficient of friction as function of time for the system alumina (pin)-AISI 431 (disc)at 0,02m/s of speed at 1.5 N of normal load..................................

Figure 3 .

 3 Figure 3.15 Right 3D mapping profilometry for AISI D2 after pin on disc test at 0.5N and room temperature and relative humidity. Left profile of the wear track region. room temperature and relative humidity..........................................................................

Figure 3 .

 3 Figure 3.16 Profile of the wear track region for AISI D2 after a pin on disc test at 1.5 N, and room temperature and relative humidity. ...........................................................

Figure 3 .

 3 Figure 3.17 Right 3D mapping profilometry of an AISI 431 steel disc after pin on disc test at 1.5N and room temperature and relative humidity. Left profile of the wear track region. ...........................................................................................................................

Figure 3 .

 3 Figure 3.18 Al2O3 ball 3D image of profilometry after 60 min of pin on disc test at 0.5 N; in contact with AISI D2 steel. .........................................................................................

Figure 3 .

 3 Figure 3.19 -Left-SEM micrography of the AISI D2 steel surface non exposed to wear at 1000 x. -Right-SEM micrography at 1000 x of steel after a pin on disc test at 0.5 N ..................................................................................................................................................

Figure 3 .

 3 Figure 3.20 SEM micrography at 1000 x of AISI D2 steel after a pin on disc test at 1.5 N ..............................................................................................................................................

Figure 3 .

 3 Figure 3.21 -Left-SEM micrography of the AISI 431 steel surface non exposed to wear. -Right-SEM micrography of AISI 431 steel after a pin on disc test at 1.5 N .....

Figure 3 .

 3 Figure 3.23 Mapping EDS images for a selected region in the counted body after a dry pin on disc test at 0.5 N .................................................................................................

Figure 3 .

 3 Figure 3.22 SEM image of the contact point of an Al2O3 ball after a pin on disc test at 0.5N of normal load in contact with a disc of AISI D2 steel. A corresponds to the surface of the ball not exposed to wear and B corresponds to the region in contact with steel during the pin on disc test. ................................................................................

Figure 3 .

 3 Figure 3.24 Temporal evolution of the OCP for AISI D2 steel in contact with 10 -4 M aqueous solution of citric acid .............................................................................................

Figure 3 .

 3 Figure 3.25 E-pH diagrams for the elements on AISI D2 steel, the dashed lines a and b indicates the stability range of water and the intersection of the red lines the current working conditions [Pourbaix 1974]. ...................................................................

Figure 3 .

 3 Figure 3.26 E-pH diagrams for the elements on AISI 431 steel, the dashed lines a and b indicates the stability range of water and the intersection of the red lines the current working conditions [Pourbaix 1974]. ...................................................................

Figure 3 .

 3 Figure 3.27 Nyquist diagram for the EIS of AISI D2 steel in contact with the citric acid 10-4 M solution. .............................................................................................................

Figure 3 .

 3 Figure 3.28 Equivalent circuit for the EIS response of AISI D2 steel in contact with the citric acid 10 -4 M solution. ..............................................................................................

Figure 3 .

 3 Figure 3.29 Nyquist diagram for AISI 431 steel in contact with the citric acid 10 -4 M solution. ..................................................................................................................................

Figure 3 .

 3 Figure 3.30 Equivalent circuit for the EIS response of AISI 431 steel in the citric acid 10 -4 M solution. ......................................................................................................................

Figure 3 .

 3 Figure 3.31 Profile of the wear track region after AISI D2 steel after a pin on disc at 0.5 N test in contact with citric acid 10 -4 M. .......................................................................

Figure 3 .

 3 Figure 3.32 Profile of the wear track region after AISI D2 steel after a pin on disc at 1.5 N test in contact with citric acid 10 -4 M. .......................................................................

Figure 3 .

 3 Figure 3.33 Temporal evolution of the COF for the system AISI 431 steel -Al2O3 at 1.5N when in contact with the citric acid 10 -4 M solution................................................

Figure 3 .

 3 Figure 3.34 Nyquist diagram for of AISI 431 steel in contact with the citric acid 10 -4 M solution while in continuous sliding at 1.5 N ...................................................................

Figure 3 .

 3 Figure 3.35 Right 3D mapping profilometry of an AISI 431 steel disc after continuous pin on disc test at 1.5N and in contact with the electrolyte. Left profile of the wear track region. ...........................................................................................................................

Figure 3 .

 3 Figure 3.36 Right 3D mapping profilometry of an AISI 431 steel disc after latency pin on disc test at 1.5N and in contact with the electrolyte. Left profile of the wear track region. .....................................................................................................................................

Figure 4 . 1

 41 Figure 4.1 Schematic diagram of a dc magnetron sputtering system. Image adapted from reference [Edgar Alfonso and Cubillos 2012] ..........................................................

Figure 4 . 2

 42 Figure 4.2 Diagram of the experimental sputtering configuration. ...............................

Figure 4

 4 Figure 4.3 SEM image and EDS spectrum for the thin film sputtered by dc magnetron sputtering (a,b) and rf magnetron sputtering (c,d) at 5000 x. ....................

Figure 4 . 4

 44 Figure 4.4 Tungsten nitride growth rate at 1 Pa of pressure and different deposition powers ....................................................................................................................................

Figure 4 . 5

 45 Figure 4.5 Tungsten nitride growth rate at 150 W and different pressures. ................

Figure 4

 4 Figure 4.6 XRD patterns for thin films obtained at 1 Pa of pressure and different powers. ...................................................................................................................................

Figure 4

 4 Figure 4.7 XRD patterns for thin films obtained at different pressures and 200 W of power. .....................................................................................................................................

Figure 4

 4 Figure 4.8 XRD patterns for thin films at 150W and 0.5 Pa before and after heat treatment at 500°C. θ-2θ configuration .............................................................................

Figure 4 . 9

 49 Figure 4.9 Resulting scratch track by increasing load scratch tests for tungsten nitride thin films deposited at different powers and 1 Pa of gas pressure. ..............................

Figure 4 .

 4 Figure 4.10 Resulting scratch track by increasing load scratch tests for tungsten nitride thin films deposited at different gas pressure and 200W. ..................................

Figure 4 .

 4 Figure 4.11 XPS survey spectrum for thin films deposited at 1 Pa of gas pressure and different plasma powers. .....................................................................................................

Figure 4 .

 4 Figure 4.12 XPS survey spectrum XPS survey spectrum for thin films deposited at 150 W of plasma power and different gas pressures. ......................................................

Figure 4 .

 4 Figure 4.13 High resolution spectrum for a) W 4f, b) N1s, and c) O1s. d) Atomic percentage detected at different depths. ...........................................................................

Figure 4 .

 4 Figure 4.14 (a) Indentation trace for a 0.5 N applied load over AISI D2 coated with tungsten nitride at 200 W and 1.0 Pa. (b) Indentation trace for a 0.5 N applied load over AISI D2 coated with tungsten nitride at 200 W and 0.7 Pa. ...................................

Figure 5 . 1

 51 Figure 5.1 Temporal evolution of the coefficient of friction for thin films sputtered at 1.0 Pa and different powers. ................................................................................................

Figure 5 . 2

 52 Figure 5.2 Temporal evolution of the coefficient of friction for thin films sputtered at 200 W and different pressure. .............................................................................................

Figure 5 . 3

 53 Figure 5.3 Resulting mapping profilometry image of the tungsten nitride thin fillm (150W and 0.5 Pa) after a pin on disc test under room temperature and humidity. .. Table 5.1 Summary of profilometry results for the tungsten nitride thin films after the pin on disc test at 1.5 N of normal load and 0.02m/s of speed. ...............................

Figure 5 .

 5 Figure 5.4 Left-Example of average profiles after a pin on disc test at 1.5 N of normal load for tungsten nitride thin films sputtered at 1.0 Pa and different powers Right-Example of average profiles after a pin on disc test at 1.5 N of normal load for tungsten nitride thin films sputtered at 200 W and different pressures. ......................

Figure 5 . 5

 55 Figure 5.5 Al2O3 ball 3D image of profilometry after 60 min of pin on disc test at 1.5 N; in contact with a tungsten nitride thin film. ................................................................

Figure 5 . 7

 57 Figure 5.7 Profile of the wear track region for AISI 431 coated with tungsten nitride after a pion on disc test at 1.5 N, and room temperature and relative humidity. .......

Figure 5

 5 Figure 5.8 SEM images of the tungsten nitride thin films a) before the pin on disc test, and after the pin on disc b) 35 W and 1.0 Pa, c) 150 W and 1.0 Pa, d) 200 W and 1.0 Pa. ....................................................................................................................................

Figure 5 . 9

 59 Figure 5.9 Mapping EDS of a tungsten nitride thin film (150 W and 1.0 Pa) after a pin on disc test at room temperature and relative humidity. .............................................

Figure 5 .

 5 Figure 5.10 SEM image of AISI 431 coated with tungsten nitride at 150 W and 1.0 Pa, a) before and b) after a pin on disc test. ...........................................................................

Figure 5 .

 5 Figure 5.11 EDS mapping images for the elements detected in an AISI 431 steel sample coated with tungsten nitride after a dry pin on disc test.................................

Figure 5 .

 5 Figure 5.12 SEM image and EDS mapping of a counter body after a pin on disc test in contact with AISI D2 steel coated with tungsten nitride. .........................................

Figure 5 .

 5 Figure 5.13 OCP curves for AISI D2 steel, and thin films obtained at different deposition powers. ..............................................................................................................

Figure 5 .

 5 Figure 5.15 OCP curves for AISI D2 steel, and thin films obtained at different deposition pressure.............................................................................................................

Figure 5 .

 5 Figure 5.14 E-pH diagram for tungsten [Pourbaix 1974]. .............................................

Figure 5 .

 5 Figure 5.16 Temporal evolution of the OCP for -left-AISI 431 steel uncoated and coated -right-un coated and coated conductive silicon in contact with 10 -4 M citric acid. .......................................................................................................................................

Figure 5 .

 5 Figure 5.17 Nyquist diagram for the EIS of AISI D2 steel uncoated and coated (1.0 Pa and different powers) in contact with the citric acid 10 -4 M solution. .........................

Figure 5 .

 5 Figure 5.18 Equivalent circuit for the EIS response of coated AISI D2 steel in contact with 10 -4 M citric acid solution. .........................................................................................

Figure 5 .

 5 Figure 5.19 Nyquist diagram for the EIS of AISI D2 steel coated at 200 W and different pressures in contact with the citric acid 10-4 M solution. ..............................

Figure 5 .

 5 Figure 5.20 Nyquist diagram for the EIS of -right-AISI 431steel uncoated and coated and -left-silicon uncoated and coated in contact with the citric acid 10 -4 M solution. ................................................................................................................................................

Figure 5 .

 5 Figure 5.21 SEM images of AISI D coated with tungsten nitride at (a) 150 W and 1.0 Pa before electrochemical test (b) 35 W and 1.0 Pa after electrochemical tests (c) 150 W and 1.0 Pa after electrochemical tests (d) 200 W and 1.0 Pa after electrochemical tests. .......................................................................................................................................

Figure 5 .

 5 Figure 5.22 EDS spectrum for AISI D2 coated with tungsten nitride -left-as deposited and -right-after electrochemical test. ............................................................

Figure 5 .

 5 Figure 5.23 SEM image of AISI 431 coated with tungsten nitride, left hand, as sputtered and right hand after the electrochemical tests. .............................................

Figure 5 .

 5 Figure 5.24 EDS mapping of AISI 431 coated with tungsten nitride as deposited....

Figure 5 .

 5 Figure 5.26 EDS mapping of conductive silicon coated with tungsten nitride after the electrochemical test. ............................................................................................................

Figure 5 .

 5 Figure 5.27 Top SEM images of uncoated AISI D2, bottom EDS mapping of the sample. ..................................................................................................................................

Figure 5 .

 5 Figure 5.28 SEM images of AISI D2 coated with tungsten buffer layer + tungsten nitride. ...................................................................................................................................

Figure 5 .

 5 Figure 5.29 SEM images and EDS mapping results of AISI D2 coated with a tungsten buffer layer followed by tungsten nitride after being exposed to a 10 -4 citric acid aqueous solution. ................................................................................................................

Figure 5 .

 5 Figure 5.30 SEM images for AISI 431, left to right, uncoated, as coated and coated after being exposed to citric acid solution. ......................................................................

Figure 5 .

 5 Figure5.31 SEM image for AISI D2 coated with a silicon buffer layer and tungsten nitride (left)as coated an (right) after exposed to 10 -4 citric acid aqueous solution. ..

Figure 5 .

 5 Figure 5.32 Mapping EDS for AISI D2 coated with a silicon buffer layer and tungsten nitride after contact with 10 -4 citric acid aqueous solution. ..........................................

Figure 5 .

 5 Figure5.33 SEM image for AISI D2 coated with a platinum buffer layer and tungsten nitride (left)as coated an (right) after exposed to 10 -4 citric acid aqueous solution. ..

Figure 5 .

 5 Figure 5.34 Mapping EDS for AISI D2 steel coated with a platinum buffer layer and tungsten nitride at 150 W and 0.5 Pa after contact with 10 -4 citric acid aqueous solution. ................................................................................................................................

Figure 5 .

 5 Figure 5.35 AISI D2 steel coated with a buffer layer of carbon and tungsten nitride at 150W and 0.5 Pa. .................................................................................................................

Figure 5 .

 5 Figure 5.36 Profile of the wear track region after AISI D2 steel coated with tungsten nitride after a pin on disc at 1.5 N test in contact with citric acid 10 -4 M. ...................

Figure 5 .

 5 Figure5.37 Temporal evolution of the COF for the system AISI 431 steel coated with tungsten nitride-Al2O3 at 1.5N when in contact with the citric acid 10 -4 M solution.

Figure 5 .

 5 Figure 5.38 Nyquist diagram for of AISI431 steel coated with tungsten nitride in contact with the citric acid 10 -4 M solution while in continuous sliding at 1.5 N. .....

Figure 5 .

 5 Figure 5.39 Profile of the wear track region for AISI 431 steel coated with tungsten nitride in contact with the citric acid 10 -4 M solution while in continuous sliding at 1.5 N. .....................................................................................................................................

Figure 5 .

 5 Figure 5.40 Profile of the wear track region for AISI 431 coated with tungsten nitride in contact with the citric acid 10 -4 M solution while in latency sliding at 1.5 N. .......

  Hardness values and coatings thickness of common materials for the manufacture of rolls for can seaming systems....................................................................Table 3-1 Summary of the Hertzian contact area and average pressure of the working conditions. .............................................................................................................................. Table 4.1 Summary of the deposition conditions for tungsten nitride films ................ Table 4.2 Summary of the thin film's surface roughness at different depositionconditions. .............................................................................................................................. Table 4.3 Summary of surface stress results of thin films at different heat treatment temperatures. ......................................................................................................................... Table 4.4 Surface stress values for thin films sputtered at different conditions. ......... Table 5.1 Summary of profilometry results for the tungsten nitride thin films after the pin on disc test at 1.5 N of normal load and 0.02m/s of speed. ............................... Table 5.2 Summary of corrosion resistance and specific corrosion resistance for AISI D2 coated with tungsten nitride. ......................................................................................

Figure 1 . 1

 11 Figure 1.1 Representation of a Hertz contact between a sphere and a plane.between a sphere and a plane.

Figure 1 . 2

 12 Figure 1.2 Illustrations of different wear types. (a) third body abrasive wear; (b) three bodies abrasive wear; (c) adhesive wear, (d) and (e) fatigue wear.

Figure 1 . 3

 13 Figure 1.3 Representation of the layers at a solid/liquid interface. 1 Inner Helmholtz layer, 2 Outer Helmholtz layer, 3 Diffuse layer, 4 Solvated ions (cation), 5 Electrolyte solvent, 6 Specifically adsorbed ions. Image adapted from [Fernandez-Solis et al. 2016]

Figure 1 . 5

 15 Figure1.5 Polarization curve with Tafelian behavior where the corrosion current icorr is found at the Ecorr[START_REF] Amemiya | Corresponding Authors[END_REF]

Figure 1 . 6

 16 Figure 1.6 Randles circuit and it equivalent as Nyquist and Bode plots, modified from reference [Fernandez-Solis et al. 2016]

Figure 1 . 7

 17 Figure 1.7 Representation of the elements parameters and system characteristics which can influence the response of tribocorrosion processes. Translated from [Richard and Geringer 2019].

  An alternative methodology, based on the time cyclic evolution of the wear track, has been proposed and tried by different authors[START_REF] Mischler | Triboelectrochemical Techniques and Interpretation Methods in Tribocorrosion: A Comparative Evaluation[END_REF][START_REF] Igual Muoz | Tribocorrosion Mechanisms in Sliding Contacts[END_REF] Cao and Mischler 2018a; 2018a;[START_REF] Landolt | Electrochemical Methods in Tribocorrosion: A Critical Appraisal D[END_REF][START_REF] Celis | Testing Tribocorrosion of Passivating Materials Supporting Research and Industrial Innovation : Handbook[END_REF]. It entails electrochemical impedance spectroscopy (EIS) measurements under continuous and intermittent sliding. The combination of these experiments allows gathering information about the evolution of the surface with time as well as the identification of the mechanisms of material loss and surface degradation. This protocol is designed as a series of successive steps that provide important information about the tribocorrosion of passivating materials. Such steps are listed as follows: I. Electrochemical tests on a passive material without sliding. These tests should be carried out in a three electrodes arrangement. a. Estimation of the reaction time characteristic of the system by the measurement of the open circuit potential (EOC) until a steady value (±1mV/min). b. Polarization resistance (Rp) of the passivated material is measured by EIS and the specific polarization resistance of passive material (rpass) is calculated for a sample of area A0 applying eq. 1.25. 𝑟 𝑝𝑎𝑠𝑠 = 𝑅 𝑝 𝐴 0 𝑒𝑞. (1.25) c. Calculation of the corrosion current density of the material covered by a passive film ipass can be calculated with eq. 1.26. 𝑖 𝑝𝑎𝑠𝑠 = 𝐵 𝑟 𝑝𝑎𝑠𝑠 (1.26) II. Electrochemical tests on active sliding track during sliding tests. In a threeelectrode configuration, sliding tests at the desired conditions are performed while EIS measurements are made. In the case of rotation (ball or pin on disc), it is recommended to take as rotation period (trot) treact/10000. This allows to estimate the corrosion current density of an active material. III. Electrochemical tests on partially active sliding track during sliding tests. In a configuration as in II, EIS measurements are carried out on intermittent sliding. The time of sliding pause or latency time (tlat) is recommended to be treact/100. Such a series of steps give unique information about the surface at different stages of the tribocorrosion process. This protocol defines the material loss at the wear track is defined by eq. 1.27 𝑊 𝑡𝑟 = 𝑊 𝑎𝑐𝑡 𝑐 + 𝑊 𝑎𝑐𝑡 𝑚 + 𝑙𝑜𝑠𝑠 𝑑𝑢𝑒 𝑡𝑜 𝑐𝑜𝑟𝑟𝑜𝑠𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑎𝑐𝑡𝑖𝑣𝑒 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝑖𝑛 𝑡ℎ𝑒 𝑤𝑒𝑎𝑟 𝑡𝑟𝑎𝑐𝑘 𝑊 𝑎𝑐𝑡 𝑚 = 𝑀𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝑙𝑜𝑠𝑠 𝑑𝑢𝑒 𝑡𝑜 𝑚𝑒𝑐ℎ𝑎𝑛𝑖𝑐𝑎𝑙 𝑤𝑒𝑎𝑟 𝑜𝑓 𝑎𝑐𝑡𝑖𝑣𝑒 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝑖𝑛 𝑡ℎ𝑒 𝑤𝑒𝑎𝑟 𝑡𝑟𝑎𝑐𝑘 𝑊 𝑟𝑒𝑝𝑎𝑠𝑠 𝑐 = 𝑀𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝑙𝑜𝑠𝑠 𝑏𝑦 𝑐𝑜𝑟𝑟𝑜𝑠𝑖𝑜𝑛 𝑜𝑓 𝑟𝑒𝑝𝑎𝑠𝑠𝑖𝑣𝑎𝑡𝑒𝑑 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝑖𝑛 𝑡ℎ𝑒 𝑤𝑒𝑎𝑟 𝑡𝑟𝑎𝑐𝑘 𝑊 𝑟𝑒𝑝𝑎𝑠𝑠 𝑚 = Material loss by mechanical wear of the repassivated material in the wear track

=

  Material lost due to corrosion of the active material in the wear track during the continuous sliding test. CF = Sum of the ratios of the atomic weight and number of electrons involved in the reaction for each metal. N= Number of cycles during the pin of disc test iact l= corrosion current of the active region during the latency tests. Aact1= Active area during the continuous pin on disc test 𝑊 𝑎𝑐𝑡1 𝑚 = Material lost due to mechanical wear of the active material in the wear track during the continuous sliding test.

Figure 1 . 8

 18 Figure 1.8 Schematic of the core components of a scanning electron microscope. Image taken from reference [Inkson 2016] Figure 1.9 Representation of the interaction electron matter in a scanning electron microscope. Where Ej is the ionization energy, Zd is the full diffusion depth, Zu useful penetration depth and Zm the total penetration depth. Adapted from reference[Ruste 2013]

Figure 1 .

 1 Figure 1.10 Representation of Bragg's law for constructive interferences.

Figure 1 .

 1 Figure 1.11 Representation of the measurement geometry for the measurements by X-ray diffraction. The image on the left represents the mode θ-θ, the image on the right the mode θ-2θ.

Figure 1 .

 1 Figure 1.13 To the left a graph of a sample profile, to the right a 3D profile.

  To neglect the influence of the substrate the indentation depth should not exceed 10% of the thin film thickness. To estimate the hardness values of films by micro indentation several methods exist, such methods consider different properties of the material involved and the geometry of the indentation mark. Nowadays, nanoindentation has become a standard technique for the determination of the hardness and elastic properties of films. Nevertheless, controversy has arisen when such methods are used on coatings to measure the tribological properties since the low indentation depths only represent the hardness of the first few nanometers of the films. Such material is commonly removed at the beginning of the tribological test during the running in period. Hence, for the present work micro-indentation was the elected indentation method. 1.3.6.1 Micro-indentation test and composite hardness The estimation of hardness by micro-indentation is characterized by applied loads (L) in the range of L< 2 N and penetrations depths (h) < 0.2 μm, depending on the hardness of the sample [Broitman 2017]. The two most employed tests in microindentation are Vickers and Knoop. Such indentation tests use a diamond indenter with a pyramidal shape. Figure 1.14 shows a diagram of indentation marks for Vickers and Knoop micro-indentation test. In general, the diagonal (d) corresponds to 30.5 times h for Knoop and ~7 times h for Vickers. When measuring the hardness of films by micro indentation it has been observed that the final estimated value will depend on the obtained indentation size. The larger the indentation size is, the closest it is to the substrate value. Such a phenomenon is known as the indentation size effect (ISE)[Puchi-Cabrera, Staia, and Iost 2015; Lesage et al. 2006]. Hence the observed hardness value corresponds to a composite hardness (HC).

Figure 1 .

 1 Figure 1.14 Schematic comparison of Knoop and Vickers micro-indentations [Broitman 2017]

  Extrinsic parameters o Contact point area o Velocity of the point speed o Ambient temperature and relative humidity o Velocity of application of the load o Composition and form of the counter body o Wear of the spherical point 1.3.8 Coating stress by wafer curvature method.

Figure 1 .

 1 Figure 1.15 Schematic illustration of a scratch test[Berthout, Randall, and Randall 2017].

Figure 2 .

 2 Figure 2.2 shows a top view of the vacuum chamber without the lid; the drawing shows the distribution of targets inside the chamber and the shield attached to the rotatory feedthrough. The tungsten target, the one mainly used target in the present work, was mounted at the back of the chamber in the right-handed support. To ensure a good mechanical, electrical and thermal contact between the target and the target support.

Figure 2 . 1

 21 Figure 2.1 Diagram of the vacuum chamber of the Plasyss MP 300S

Figure 2 . 3

 23 Figure 2.3 Transversal cut diagram of the vacuum chamber.

Figure 2 . 4

 24 Figure 2.4 Diagram of the tribocorrosion cell for reciprocating test (left) and a picture of such a cell (right).

Figure 2 . 5

 25 Figure 2.5 Picture of the lubricated pin on disc tests module.

Figure 2 . 6

 26 Figure 2.6 Diagram of the cover for the pin on disc UTM tribolab base (left) and a picture of the final cover (right).

Figure 2 . 7

 27 Figure 2.7 Picture of the experimental set up for the module cover.

Figure 2 .

 2 Figure 2.9 shows a picture of the final cell, the counter electrode and the reference electrode were added using external supports to keep the same distance between electrodes. This cell was used during the tribocorrosion test in the experimental part of the present work.

Figure 2 . 8

 28 Figure 2.8 Diagram of the tailored tribocorrosion test.

Figure 2 . 9

 29 Figure 2.9 Picture of the tribocorrosion cell and the experimental set up.

  .1 shows a phase diagram for the system iron-carbon, such diagram shows the possible phases of iron and metallic carbides that can be present in a steel at different carbon concentrations and temperatures. Besides iron and carbon, a wide variety of elements can be found in steel. Such elements are added to modulate its properties. Due to the complexity and variety of compositions several efforts to classify steel have been made. One of the most popular, in USA, is the one proposed by the American Iron & Steel Institute (AISI) which classifies steel in big groups of carbon steel, alloy steel, tool steel, and stainless steel.Within these groups, there are subdivisions based on specific chemical composition, physical or mechanical properties and their use[START_REF] Paxton | Kirk-Othmer Encyclopedia of Chemical Technology[END_REF] Roussak, n.d.]. The European system is a classification of materials with a sequence of numbers. This classification is given by the norm EN 10027-2. The classification is composed of five digits (X.XXXX) where the first digit indicates the group of material (1 for the group of steels), the following digit indicates if it is or not an alloy or a special type of alloy.For steel, the third digit represents the type of steel, quality, composition, or resistance, while the last two digits correspond to its place on the list. In both classifications, it is possible to find tool steels and stainless steels. Both types of steel are extensively used in a great variety of machining and manufacture operations[START_REF] Murry | Désignation Normalisée Des Aciers Désignation Normalisée Des Aciers[END_REF]], i.e. steel is the most used material in processing material and household utensils in the food industry[START_REF] Garcia | Food Contact Materials[END_REF].

Figure 3 . 1

 31 Figure 3.1 Phase diagram iron-carbon for the steel and cast iron regions[Uhlenhaut et al. 2006]

  plating) are currently used to enhance the mechanical performance (reduction of the wear rate) with up to 31 GPa of hardness[Imeta Srl 2018]. Most of the work on the improvement of AISI D2 steel has been focused on its mechanical properties[Ramírez et al. 2012; Díaz-Guillén et al. 2013; Novak and Polcar 2014; Cho, Lee, and Lee 2015; Hamidzadeh, Meratian, and Mohammadi Zahrani 2012; Lesyk et al. 2017; Tang et al. 2013; Toboła et al. 2017; Bourithis, Papadimitriou, and Sideris 2006; Yasavol and Ramalho 2015].

Figure 3

 3 Figure 3.2 (a) Example of a body centered tetragonal crystalline structure, structure of the cementite. (b) example or a face centered cubic crystalline structure, crystallune estructur which coresponds to austenite.

  Universitaire de Technologie d'Amiens' (IUT Amiens) or a Strueurs Tegra Pol-21 polishing machine at the 'Laboratoire de Réactivité et Chimie des solides' (LRCS-UPJV). The polishing condition were 15 N of load and 300 rpm with different abrasive papers of SiC (from 600 up to 1200) to finish them with a cloth and a solution of Al2O3 of 0.1 μm of size. The abrasive papers, cloth and Al2O3 solution were provided by PRESI in France.

Figure 3 . 3 .

 33 Figure 3.3. Each sample was measured five times. Each measurement was separated by an angle of 60°.

Figure 3 .

 3 Figure 3.4 shows an example of a typical SEM image along with examples of EDX mapping for AISI D2 steel. The SEM image shows a relatively smooth surface. The EDX analysis confirmed the AISI D2 steel composition, identifying the following weight percentages: 79.5 % Fe, 13.4 % Cr, 1.25 % Si 1.1 % V, 0.8 % Mo and 4.0% C. The mapping shows the dispersion of each element in the analyzed surface, the colored region represents the measured part where the element was found while the black parts represent an absence of such element. To confirm such uneven dispersion an EDX linear measure was performed. The result of such measurement is shown in

Figure 3 . 5 .

 35 Figure 3.5. The bottom figure shows a graph of the 'counts per second (cps) for eachelement. This analysis helped to identify the distribution of each element along the measured line, where it was possible to see that there were areas of accumulation of carbon combined with an iron deficiency. Such carbon accumulations, common in high carbon steels help to increase the steel hardness.

Figure 3 .

 3 Figure 3.5 Top SEM of the section analyzed by EDX. Bottom counts per second of each element along the analyzed line for AISI D2 steel.

Figure 3

 3 Figure 3.4 SEM micrography and mapping EDX of an AISI D2 steel sample, each image represents the dispersion of each detected element.

Figure 3

 3 Figure 3.6 SEM micrography and mapping EDX of an AISI 431 sample, each image represents the dispersion of each detected element.

Figure 3 . 8

 38 Figure 3.8 Diffractogram of a substrate of AISI D2

  -indentation tests were performed over mirror polished discs of AISI D2 and AISI 431 steel using a Nanovea PB1000 Hardness Tester at 5.0N of applied load with a Vickers indenter. An example of a load-indentation depth curve is shown in Figure3.9. The estimated hardness for each of substrates were 351 (3.4 GPa) for AISI D2 steel and 348 (3.3 GPa) for AISI 431 steel , such hardness values corresponds to those reported for each material[START_REF] Tang | Tribology International Dry Sliding Friction and Wear Behaviour of Hardened AISI D2 Tool Steel with Different Hardness Levels[END_REF][START_REF] Díaz-Guillén | Electrochemical Corrosion Performance of AISI D2 Tool Steel Surface Hardened by Pulsed Plasma Nitriding[END_REF]].

Figure 3 . 9

 39 Figure 3.9 Load as a function of depth for a micro-indentation measure of AISI D2 steel at 5N

Figure 3 .

 3 Figure 3.10 Diagram of the cell for electrochemical measurements

Figure 3 .

 3 Figure 3.11 Diagram of the cell for tribocorrosion measurements.

Figure 3 .

 3 Figure 3.12 Schematic top view of a disc shaped test sample: (a) without sliding, (b) during sliding tests, and (c) during sliding tests at long latency times resulting in a partially active sliding track [Celis and Ponthiaux 2017].

  This equation establishes the proportional relation of the wear volume, V, with the normal load, FN, and the total sliding distance, L, on a tribological test. The proportionality constant represents the wear coefficient, K, which allows predicting the useful life of a tribological system, as long as the system does not change the wear type[START_REF] Jiménez | 2 -Friction and Wear[END_REF][START_REF] Kenneth | Guide to Friction , Wear , and Erosion Testing[END_REF]].

Figure 3 .

 3 Figure 3.14 Graph of coefficient of friction as function of time for the system alumina (pin)-AISI 431 (disc)at 0,02m/s of speed at 1.5 N of normal load

Figure 3 .

 3 Figure 3.15 Right 3D mapping profilometry for AISI D2 after pin on disc test at 0.5N and room temperature and relative humidity. Left profile of the wear track region. room temperature and relative humidity.

Figure 3 .

 3 Figure 3.16 Profile of the wear track region for AISI D2 after a pin on disc test at 1.5 N, and room temperature and relative humidity.

Figure 3 .

 3 Figure 3.18 Al2O3 ball 3D image of profilometry after 60 min of pin on disc test at 0.5 N; in contact with AISI D2 steel.

Figure 3 .

 3 Figure 3.17 Right 3D mapping profilometry of an AISI 431 steel disc after pin on disc test at 1.5N and room temperature and relative humidity. Left profile of the wear track region.

Figure 3 .

 3 Figure 3.19 -Left-SEM micrography of the AISI D2 steel surface non exposed to wear at 1000 x. -Right-SEM micrography at 1000 x of steel after a pin on disc test at 0.5 N at 0.5 N

Figure 3 .

 3 Figure 3.22 shows the SEM image, such image presents the contact point (B) with residues of material. The presence of material adhered to the Al2O3 surface affects the COF, and it is related to the breaking process. As seen in Figure 3.13, the coefficient of friction changes drastically after approximately 15 minutes. This indicates a change on the contact. The image of the analyzed region and the map of the identified elements are shown in Figure 3.23. Among the elements found in abundance over the surface, there are oxygen, aluminum, and iron. The oxygen and aluminum signals correspond to the ball while the iron corresponds to the adhered material. The amount of iron found in the counter body is relatively small and most of the accumulated material in the surrounding area of the contact point is aluminum and oxygen,

Figure 3 .

 3 Figure 3.20 SEM micrography at 1000 x of AISI D2 steel after a pin on disc test at 1.5 N

Figure 3 .

 3 Figure 3.24 shows the curve of the temporal evolution of the OCP. The curve presents a sharp decrement at the beginning of the measurement until a steady state was achieved around -0.5 V vs Ag/AgCl. The estimated treact was 28 min.

Figure 3 .

 3 Figure 3.22 SEM image of the contact point of an Al2O3 ball after a pin on disc test at 0.5N of normal load in contact with a disc of AISI D2 steel. A corresponds to the surface of the ball not exposed to wear and B corresponds to the region in contact with steel during the pin on disc test.

Figure 3 .

 3 Figure 3.23 Mapping EDS images for a selected region in the counted body after a dry pin on disc test at 0.5 N

Figure 3 .

 3 Figure3.25 E-pH diagrams for the elements on AISI D2 steel, the dashed lines a and b indicates the stability range of water and the intersection of the red lines the current working conditions[START_REF] Pourbaix | Atlas of Electrochemical Equilibria in Aqueous Solutions[END_REF]].

Figure 3 .

 3 Figure 3.24 Temporal evolution of the OCP for AISI D2 steel in contact with 10 -4 M aqueous solution of citric acid

Figure 3 .

 3 Figure 3.26 E-pH diagrams for the elements on AISI 431 steel, the dashed lines a and b indicates the stability range of water and the intersection of the red lines the current working conditions [Pourbaix 1974].

Figure 3 .

 3 Figure 3.27 Nyquist diagram for the EIS of AISI D2 steel in contact with the citric acid 10-4 M solution.

Figure 3 .

 3 Figure 3.28 Equivalent circuit for the EIS response of AISI D2 steel in contact with the citric acid 10 -4 M solution.

Figure 3 .

 3 Figure 3.29. The diagram is similar to the results for AISI D2 steel; the diagram presented two time constants, hence, the proposed equivalent circuit consists of two RQ parallel circuits (Figure 3.30), such circuit presents a slight difference from the one proposed in Figure 3.28, due to the electrode arrangement when the tests were performed. For the circuit in Figure 3.30 the first RC parallel circuit represents the ohmic resistance of the solution between the working and the reference electrodes, while in the second RC component, R2 corresponds to the polarization resistance of the material (Rp); the Q element is a constant phase element (CPE). The estimated value of rp was 160.6 ±5.0 kΩcm 2 , with a chi-square of 0.002 suggesting a well modelled circuit. The rp value was used to calculate the icorr applying eq. 3.2. for AISI 431 the estimated value was 1.5x10 -7 A/cm 2 . The corrosion current and corrosion potential correspond to those of a passive material, hence AISI 431 steel passivates under citric acid pH 4.

Figure 3 .

 3 Figure 3.30 Equivalent circuit for the EIS response of AISI 431 steel in the citric acid 10 -4 M solution.

Figure 3 .

 3 Figure 3.29 Nyquist diagram for AISI 431 steel in contact with the citric acid 10 -4 M solution.

Figure 3 .

 3 Figure 3.31 Profile of the wear track region after AISI D2 steel after a pin on disc at 0.5 N test in contact with citric acid 10 -4 M.

Figure 3 .±6Figure 3 .

 33 Figure 3.32 Profile of the wear track region after AISI D2 steel after a pin on disc at 1.5 N test in contact with citric acid 10 -4 M.

Figure 3 .

 3 Figure 3.34 Nyquist diagram for of AISI 431 steel in contact with the citric acid 10 -4 M solution while in continuous sliding at 1.5 N

Figure 3 .

 3 Figure 3.35 Right 3D mapping profilometry of an AISI 431 steel disc after continuous pin on disc test at 1.5N and in contact with the electrolyte. Left profile of the wear track region.

Figure 3 .

 3 Figure 3.36 Right 3D mapping profilometry of an AISI 431 steel disc after latency pin on disc test at 1.5N and in contact with the electrolyte. Left profile of the wear track region.

pH 4 .

 4 Under such pH conditions, the open circuit potential had a steady state at about -0.5 V vs Ag/AgCl. At this pH-E condition, most of the elements in the alloy could be expected to corrode. Moreover, the linear polarization and EIS experiments revealed that the AISI D2 steel corroded with an estimated value of specific corrosion resistance of 3.3 ±0.01 kΩcm 2 . Hence to prevent the corrosion of the alloy without affecting its wear resistance the use of a protective thin film was proposed.Similar to the AISI D2 steel, the AISI 431 steel presented accumulated material over the wear track area. The electrochemical tests showed that AISI 431 steel passivated with a specific corrosion resistance of 160.6 ±5.0 kΩcm 2 .

Figure 4 . 1

 41 Figure 4.1 Schematic diagram of a dc magnetron sputtering system. Image adapted from reference [Edgar Alfonso and Cubillos 2012]

  Before coating the substrate, each substrate was polished to mirror finishing utilizing either a Buehler-MetaServ 250 at IUT Amiens or a Strueurs Tegra Pol-21 polishing machine at the 'Laboratoire de Reactivite et Chimie des solides'. The polishing condition were 15 N of load and 300 rpm with different abrasive papers of SiC (from 600 up to 1200) to finish them with a cloth and a solution of Al2O3 of 0,1 μm of size. The abrasive papers, cloth and Al2O3 solution were provided by PRESI in France.

Figure 4 . 2

 42 Figure 4.2 Diagram of the experimental sputtering configuration.

  Figure 4.3 SEM image and EDS spectrum for the thin film sputtered by dc magnetron sputtering (a,b) and rf magnetron sputtering (c,d) at 5000 x.

  Figure 4.4 shows the temporal evolution of the tungsten nitride thickness for 1 Pa at different deposition power. Such a figure illustrates how at larger power the growth rate increases due to an increment on the ion bombardment. The estimated growth rates were 8 nm/min at 35 W, 15 nm/min at 150 W and 18nm/min.

Figure 4 .

 4 Figure 4.5 presents the growth rate of the film at 150 W and different deposition pressures; the curves show that the deposition rate is approximately linearly proportional to the deposition power. The estimated deposition rates were 8 nm/min

Figure 4 . 5

 45 Figure 4.5 Tungsten nitride growth rate at 150 W and different pressures.

Figure 4 .

 4 Figure 4.6 shows the diffraction pattern for thin films deposited on silicon wafers at the same deposition pressure and temperature, and different powers. Such patterns were obtained using the grazing incidence configuration. The obtained signals corresponded mainly to two different types of crystal WO3 and W2N. Moreover, it is evident that at larger power deposition the crystallinity of the tungsten nitride phase increased.

Figure 4 .

 4 Figure 4.7 presents the diffraction patterns for thin films synthesized at the same deposition power but at different pressures. The spectra show that an increment in the deposition pressure probably reduces the crystallinity of the thin film.

Figure 4 .

 4 Figure 4.8 shows the θ-2θ diffraction patterns for the thin film obtained at 150 W and 0.5 Pa, the red line is the pattern for the thin films as deposited. While the black line presents the pattern of the thin film after the heat treatment process. Such comparison shows that the heat treatment promotes the crystallinity of the thin film. The estimated crystalline size was of about 25 nm for the coatings deposited at 1 Pa and different powers, 162 nm and 198 nm for 200 W and 0.5 and 0.7 Pa respectively.

Figure 4

 4 Figure 4.6 XRD patterns for thin films obtained at 1 Pa of pressure and different powers.

Figure 4

 4 Figure 4.7 XRD patterns for thin films obtained at different pressures and 200 W of power.

Figure 4

 4 Figure 4.8 XRD patterns for thin films at 150W and 0.5 Pa before and after heat treatment at 500°C. θ-2θ configuration

Figure 4 .

 4 Figure 4.10 displays images of the obtained scratch tracks for samples at different pressure depositions. The thin film deposited at 0.7 Pa a LC1 was 8.3 ±0.4 N and LC2 is present at 10.5 ±0.4N. Finally, for the sample coated at 0.5 Pa LC1 was 7.9 ±0.3N and LC2 10.8 ± 0.5 N. No significant difference was found in the film adhesion of coatings deposited at different gas pressures.

Figure 4 . 9

 49 Figure 4.9 Resulting scratch track by increasing load scratch tests for tungsten nitride thin films deposited at different powers and 1 Pa of gas pressure.

Figure 4 .

 4 Figure 4.11 presents the XPS survey spectrums for thin films made at 1.0 Pa of gas pressure and various plasma powers. The survey spectrum shows the spectral signatures of tungsten, nitrogen, and oxygen. Similar results were found for thin films deposited at 150 W power and different gas pressures; the survey spectra are shown in Figure 4.12. Such type of survey spectra has been previously reported for WNx and

Figure 4 .

 4 Figure 4.11 XPS survey spectrum for thin films deposited at 1 Pa of gas pressure and different plasma powers.

Figure 4 .

 4 Figure 4.12 XPS survey spectrum XPS survey spectrum for thin films deposited at 150 W of plasma power and different gas pressures.

Figure 4 .

 4 Figure 4.10 Resulting scratch track by increasing load scratch tests for tungsten nitride thin films deposited at different gas pressure and 200W.

Figure 4 .

 4 Figure 4.13d shows the atomic percentage profile of each element from the surface down to 160 nm within the thin film, the lines represent the average value of each element and the points the value measured at the given depth. The percentage of the

Figure 4 .

 4 Figure 4.13 High resolution spectrum for a) W 4f, b) N1s, and c) O1s. d) Atomic percentage detected at different depths.

  Figure 4.14 shows examples of the indentation marks.

  Figure 4.14 (a) shows an indentation mark for a measurement with no delamination, hence the hardness value obtained could be used in the thin film's hardness estimation.

  Figure 4.14 (b) shows an example of an indentation mark obtained at the same indentation conditions as for (a), nevertheless, this thin film presented evident delamination in the indentation region. When this type of behavior was encountered attempts to estimate the thin film hardness with the different existing models were unsuccessful. The delamination problems will be treated in more detail in the following chapter.

Figure 4 .

 4 Figure 4.14 (a) Indentation trace for a 0.5 N applied load over AISI D2 coated with tungsten nitride at 200 W and 1.0 Pa. (b) Indentation trace for a 0.5 N applied load over AISI D2 coated with tungsten nitride at 200 W and 0.7 Pa.

Figure 5 . 1

 51 Figure 5.1 Temporal evolution of the coefficient of friction for thin films sputtered at 1.0 Pa and different powers.

Figure 5 . 2

 52 Figure 5.2 Temporal evolution of the coefficient of friction for thin films sputtered at 200 W and different pressure.

Figure 5 . 3

 53 Figure 5.3 Resulting mapping profilometry image of the tungsten nitride thin film (150W and 0.5 Pa) after a pin on disc test under room temperature and humidity.

  of single profiles for thin films obtained at different deposition conditions are presented in Figure 5.4. Each profile shows a significant change in the surface roughness on the wear track region (highlighted by the dotted red rectangles).

Figure 5 . 4

 54 Figure 5.4 Left-Example of average profiles after a pin on disc test at 1.5 N of normal load for tungsten nitride thin films sputtered at 1.0 Pa and different powers Right-Example of average profiles after a pin on disc test at 1.5 N of normal load for tungsten nitride thin films sputtered at 200 W and different pressures.

  Figure 5.7 shows an example of a profile of the AISI 431 coated with tungsten nitride after the pin on disc test at 1.5 N, and room temperature and relative humidity. The region of the wear track is highlighted by red rectangles. The estimated volume of accumulated material was 7.1x10 -6 ±1.8x10 -7 mm 3 .

Figure 5 . 5

 55 Figure 5.5 Al2O3 ball 3D image of profilometry after 60 min of pin on disc test at 1.5 N; in contact with a tungsten nitride thin film.

Figure 5 . 7

 57 Figure 5.7 Profile of the wear track region for AISI 431 coated with tungsten nitride after a pion on disc test at 1.5 N, and room temperature and relative humidity.

Figure 5 . 6

 56 Figure 5.6 Temporal evolution of the coefficient of friction for the system alumina (pin)-tungsten nitride (coated disc) at 0,02m/s of speed at 1.5 N of normal load and, room temperature and relative humidity

Figure 5 .

 5 Figure 5.9 presents an EDS mapping for the sample presented in Figure 5.6 c after the pin on disc test. The detected elements and their percentage were: W 76%, N 3 %, O 10%, C 5%, and Fe 6%. The mapping images present the distribution of the elements

Figure 5 .

 5 Figure 5.11 shows the element maps of the sample after the pin on disc test. The colored parts represent the points where the element was detected, there was a decrease in the oxygen concentration in the wear track, probably due to the removal of an external oxide film. Moreover, no iron was detected hence, the thin film in the analyzed region was not removed.

Figure 5 .

 5 Figure 5.11 EDS mapping images for the elements detected in an AISI 431 steel sample coated with tungsten nitride after a dry pin on disc test.

Figure 5 .

 5 Figure 5.12 SEM image and EDS mapping of a counter body after a pin on disc test in contact with AISI D2 steel coated with tungsten nitride.

Figure 5 .

 5 Figure 5.14 shows the E-pH or 'Pourbaix diagram' for tungsten, such a diagramrepresents the chemical species which can be found in aqueous solution for a given element at a specific E-pH values. The red dots represent the OCP for the different thin film s at pH 4. At these points, tungsten is in the passive region, and this means that the metal will react to form a passive film. Hence, it was expected that the thin film passivated. Nevertheless, the shape of the curve (decreasing values) is commonly associated with systems under corrosion[START_REF] Celis | Testing Tribocorrosion of Passivating Materials Supporting Research and Industrial Innovation : Handbook[END_REF].

Figure 5 .

 5 Figure 5.13 OCP curves for AISI D2 steel, and thin films obtained at different deposition powers.

Figure 5 .

 5 Figure5.14 E-pH diagram for tungsten[START_REF] Pourbaix | Atlas of Electrochemical Equilibria in Aqueous Solutions[END_REF]].

Figure 5 .

 5 Figure 5.16 Temporal evolution of the OCP for -left-AISI 431 steel uncoated and coated -right-un coated and coated conductive silicon in contact with 10 -4 M citric acid.

  shows in the left-handed graph the Nyquist diagrams for the uncoated and coated AISI D2 steel. The coatings were made at different plasma powers and 1.0 Pa of gas pressure, and the right-handed figure presents the modelled Nyquist diagram for the samples just described. Each diagram is composed by two semicircles. The first semicircle represents the electrolyte resistance whereas the second can be used to evaluate the sample corrosion resistance. To evaluate the material resistance values the data were fitted using the equivalent circuit method. The proposed circuit is presented in Figure 5.18. The circuit contains constituted by the same elements as for uncoated AISI D2 steel and has two RC components. The first RC parallel circuit represents the ohmic resistance of the solution between the working and the reference electrodes, modified by the geometry and the distance between the electrodes. The second RC component, R2 corresponds to the polarization resistance of the material (Rp); the Q element is a constant phase element (CPE).

Figure 5 .

 5 Figure 5.18 Equivalent circuit for the EIS response of coated AISI D2 steel in contact with 10 -4 M citric acid solution.

Figure 5 .

 5 Figure 5.17 Nyquist diagram for the EIS of AISI D2 steel uncoated and coated (1.0 Pa and different powers) in contact with the citric acid 10 -4 M solution.

  Figure 5.19 Nyquist diagram for the EIS of AISI D2 steel coated at 200 W and different pressures in contact with the citric acid 10-4 M solution.

  Figure 5.21 shows representative SEM images of the AISI D2 coated samples as deposited and after the electrochemical tests. Figure 5.21a is an example of a coating as deposited, such image shows a smooth and uniform surface. Figure 5.21 b, c and d show the changes of texture surface of the samples. For the coating deposited at 35 W and 1 Pa (b) a large incidence of corrosion areas was observed. The total corroded area diminished when the power increased, as observed on c and d

Figure 5 .

 5 Figure 5.20 Nyquist diagram for the EIS of -right-AISI 431steel uncoated and coated and -left-silicon uncoated and coated in contact with the citric acid 10 -4 M solution.

Figure 5 .

 5 Figure 5.21 SEM images of AISI D coated with tungsten nitride at (a) 150 W and 1.0 Pa before electrochemical test (b) 35 W and 1.0 Pa after electrochemical tests (c) 150 W and 1.0 Pa after electrochemical tests (d) 200 W and 1.0 Pa after electrochemical tests.

Figure 5 .

 5 Figure5.22 presents a comparison of the EDS spectrum of the AISI D2 steel as coated and after the electrochemical test, for the coating as deposited only tungsten, oxygen and nitrogen from the coating were found but for the coating after the electrochemical test elements from the substrate, Fe, Cr and V were detected, confirming that part of the coatings has been removed.

Figure 5 .

 5 Figure 5.23 SEM image of AISI 431 coated with tungsten nitride, left hand, as sputtered and right hand after the electrochemical tests.

Figure 5 .

 5 Figure 5.22 EDS spectrum for AISI D2 coated with tungsten nitride -left-as deposited and -right-after electrochemical test.

  Figure 5.25 shows the SEM images for the coated silicon sample before and after the electrochemical test. Both images show a uniform surface without significant changes in the texture. Figure 2.26 is of the EDS mapping of the sample after the electrochemical tests, the images show an even distribution of the element in the thin film The weight percentages of each element were 84% W, 10% O, 3% N, 2% C, and 1% Ar, for the thin film as deposited and 82% W, 12% O, 3% N, 2% C, and 1% Ar after

Figure 5 .

 5 Figure 5.24 EDS mapping of AISI 431 coated with tungsten nitride as deposited.

Figure 5 .

 5 Figure 5.25 SEM images of conductive silicon coated with tungsten nitride, left hand, as sputtered and right hand after the electrochemical tests.

Figure 5 .

 5 Figure 5.26 EDS mapping of conductive silicon coated with tungsten nitride after the electrochemical test.

  Figure 5.27 Top SEM images of uncoated AISI D2, bottom EDS mapping of the sample.

Figure 5 .

 5 Figure 5.28 SEM images of AISI D2 coated with tungsten buffer layer + tungsten nitride.

Figure 5 .

 5 Figure 5.30 SEM images for AISI 431, left to right, uncoated, as coated and coated after being exposed to citric acid solution.

Figure 5 .

 5 Figure 5.31 SEM image for AISI D2 coated with a silicon buffer layer and tungsten nitride (left)as coated an (right) after exposed to 10 -4 citric acid aqueous solution.

  Figure 5.33 SEM image for AISI D2 coated with a platinum buffer layer and tungsten nitride (left)as coated an (right) after exposed to 10 -4 citric acid aqueous solution.

Figure 5 .

 5 Figure 5.32 Mapping EDS for AISI D2 coated with a silicon buffer layer and tungsten nitride after contact with 10 -4 citric acid aqueous solution.

Figure 5 .

 5 Figure 5.34 Mapping EDS for AISI D2 steel coated with a platinum buffer layer and tungsten nitride at 150 W and 0.5 Pa after contact with 10 -4 citric acid aqueous solution.

Figure 5 .

 5 Figure 5.35 AISI D2 steel coated with a buffer layer of carbon and tungsten nitride at 150W and 0.5 Pa.

Figure 5 .

 5 Figure 5.36 Profile of the wear track region after AISI D2 steel coated with tungsten nitride after a pin on disc at 1.5 N test in contact with citric acid 10 -4 M.

Figure 5 .

 5 Figure 5.37 Temporal evolution of the COF for the system AISI 431 steel coated with tungsten nitride-Al2O3 at 1.5N when in contact with the citric acid 10 -4 M solution.

Figure 5 .

 5 Figure 5.38 Nyquist diagram for of AISI431 steel coated with tungsten nitride in contact with the citric acid 10 -4 M solution while in continuous sliding at 1.5 N.

Figure 5 .

 5 Figure 5.39 Profile of the wear track region for AISI 431 steel coated with tungsten nitride in contact with the citric acid 10 -4 M solution while in continuous sliding at 1.5 N.

  Afin d'effectuer des mesures de tribocorrosion, une cellule faite sur mesure a été conçue. Ladite cellule et le montage expérimental sont présentés sur la Figure i. La cellule a fonctionné comme une couverture pour un support existant auparavant. Le niveau inférieur a été conçu pour contenir un contact électrique glissant composé par un « électrolyte de pont » et un fil conducteur tandis que le niveau supérieur a été utilisé pour l'échantillon et l'électrolyte pour le test électrochimique (solution aqueuse 10 -4 M d'acide citrique). L'électrolyte de pont utilisé pour le contact électrique coulissant était une solution aqueuse 1,0 M de chlorure de potassium. L'échantillon a été attaché à un fil d'argent, les extrémités du fil ont été immergées dans l'électrolyte. Pour établir le contact électrique entre le contact électrique glissant et le potentiostat, un deuxième fil d'argent a été immergé dans le chlorure de potassium puis connecté au potentiostat. Etude des substrats. L'usure mécanique, la corrosion et le processus de tribocorrosion des aciers utilisés comme substrats ont été analysés. Pour les tests tribologiques, un contre-corps en oxyde d'aluminium d'un diamètre de 12 mm a été utilisé. Lesdits tests ont été effectués à 1,5 N de charge normale, 0,02 m/s de vitesse (équivalent à 60rmp) et une distance parcourue de 72 m. D'autre part, les tests électrochimiques ont été réalisés à l'aide d'un potentiostat

Figure i

 i Figure i Schéma de la cellule de tribocorrosion et du montage expérimental

Figure

  Figure ii Image de la trace d'usure sur acier AISI D2 et acier AISI 431 après le test 'pin on disc' à 1.5N de charge normale et à température ambiante et humidité ambiante.

Figure

  Figure ii Image de la trace d'usure sur acier AISI D2 et acier AISI 431 après le test 'pin on disc' à 1.5N de charge normale et à température ambiante et humidité ambiante.

Finalement, afin

  Figure iv Image de la trace d'usure sur nitrure de tungstène déposée a 150W and 0.5 Pa sur acier AISI D2 (à gauche) et acier AISI 431 (à droit) après le test 'pin on disc' à 1.5N de charge normale et à température ambiante et humidité ambiante.

Figure

  Figure iv Image de la trace d'usure sur nitrure de tungstène déposée a 150W and 0.5 Pa sur acier AISI D2 (à gauche) et acier AISI 431 (à droit) après le test 'pin on disc' à 1.5N de charge normale et à température ambiante et humidité ambiante.

Table . 1

 . Hardness values and coatings thickness of common materials for the manufacture of rolls for can seaming systems.

	Material	Hardness (GPa)	Thickness (µm)
		[Puchi-Cabrera 2002; Chen et al. 1999]	[Imeta Srl 2018]
	Titanium Nitride (TiN)	23	6-8
	Titanium Carbide (TiC)	31	6-8
	AISI D2	7	---
	Transition metals nitrides as thin films for construction materials in the food
	industry

Table 3 -

 3 1 Summary of the Hertzian contact area and average pressure of the working conditions.

Table 4 .

 4 1 Summary of the deposition conditions for tungsten nitride films

	Author	Sputtering	Power density (W/cm 2 )	Temperature	Pressu re (Pa )	Pressure (mTorr)	N2 %	Distance substrate-target (cm)
	Hones, P. et al	rf	5.05	500 ± 20 K	0.66	5	0.1-0.3	10
	Samano, E.C. et al	dc and laser ablation	5.09	400 °C	0.53-2.13	4 -17	0 -50	3
	Polcar, T: et al	dc	6 mA cm -2	<350 °C	0.3	2	0-75	6.5
	Polcar, T: et al	dc	10 mA cm -2	<350 °C	0.3	2	0-75	6.5
	Louro, Cavaleiro	dc	10	450 °C	0.3	2	0.1 -0.6	--
	Addonizio , et al	Pulsed dc	0.65-1.97	'Room temperature'	0.73	5	7.7-20	8
	Shen, Mai	d c	4.28	200 °C	0.8	6		7

Table 4

 4

	.2 Summary of the thin film's surface roughness at different deposition
	conditions.
	Sputtering conditions
			Ra (nm)	Rz (nm)
	Power (W)	Pressure (Pa)
	35		120 ± 20	850 ± 180
	150	1	270 ± 50	700 ±150
			280 ± 20	800 ± 200
	200	0.7	260 ± 10	1000± 300
		0.5	250 ± 20	800 ± 150

Table 4 .

 4 3 Summary of surface stress results of thin films at different heat treatment temperatures.

	Heat treatment temperature (°C)	Stress (MPa)
	Without heat treatment	11.3 ±9.9
	300	5.4 ±3.4
	400	2.8 ±1.2
	500	2.5 ±1.2

Table 4 .

 4 4 Surface stress values for thin films sputtered at different conditions.

	Power (W)	Pressure (Pa)	Stress (MPa)
	35	1	2.5 ± 1.2
	150	1	2.6 ± 0.6
		1	2.6 ± 0.3
	200	0.7	3.4 ± 0.5
		0.5	3.8 ± 0.3

Table 5 .

 5 2 Summary of corrosion resistance and specific corrosion resistance for AISI D2 coated with tungsten nitride.

	Power (W)	Pressure (Pa)	Rp(kΩ)	rcorr (kΩcm 2 )
		AISI D2	7.3	3.3 ± 1.0
	35	1.0	8.6	3.9 ± 0.2
	150		17.1	7.6 ±0.6
	200		26.4	12.1 ±0.3
		0.7	22.7	10.1 ±0.3
		0.5	17.7	7.2 ± 0.5
	Moreover, EIS measurements were performed for AISI 431 steel and conductive
	silicon coated with tungsten nitride sputtered at 150 W and 0.5 Pa.

thank my supervisors, Andreas Zeinert and Stephen Muhl, for their valuable guidance. Your insightful feedback pushed me to sharpen my thinking and brought my work to a successful end.

I acknowledge the generous financial support from the 'Consejo Nacional de Ciencia y Tecnología' through their international funding system (270930/439637) and PAPIIT through the project IG 101220.

Professors Maxence Bigerelle and Iván Campos for accepting to judge the present work and the invaluable feedback provided. Also, thanks to my jury members for the support, thank you for everything.

My gratitude and acknowledgement to everyone from IIM-UNAM, LPMC -UPJV,

Chapter 5

Tribological and tribocorrosion properties of the thin film 5 This chapter presents the wear behavior at room temperature and relative humidity of the1µm of thick tungsten nitride thin films. The type of wear and the wear rate were determined. Additionally, the corrosion resistance was evaluated by electrochemical impedance. The influence of the sputtering parameters and the type of substrate were appraised. Finally, the tribocorrosion behavior of the passive thin films was determined.

Experimental conditions

Pin on disc tests

The tribology experiments were performed using a UTM-Tribolab by Bruker in a pin on disc configuration at the conditions described in 3.3.1

Electrochemical tests

Electrochemical measurements such as open circuit potential, and electrochemical impedance spectroscopy were performed with a PalmSens 4 potentiostat at the conditions described in 3.3.2.

Open circuit potential

Open circuit potential (OCP) measurements were carried out for up five hours to verify the steady-state and determine the characteristic reaction time (treact). After the OCP measurements either a linear polarization or an EIS test was performed.

The estimated wear loss was 5.1x10 -4 ±4.0x10 -7 mm 3 one order of magnitude larger than the one found for substrate without coating (1.8x10 -3 ±4.0x10 -7 mm 3 ). Hence, the thin film increased the tribocorrosion resistance.

Latency tests

During the latency tests, pin on disc test were performed with the sample in contact with the electrolyte. For such tests, successive identical cycles of rotation(1s)-latency (20s) were repeated until a total sliding distance of 72 m was reached.

The resulting wear track was measured by contact profilometry as for the previous experiments. Figure 5.40 shows a wear track profile. Such results were used to calculate the total volume loss in the wear track giving a value of 7 x10 -4 ±4.0x10 -7 mm 3 .

The volume loss value measured after the latency tests along with the EIS the coated sample were used to calculate Kc and Km as described in 1.2.2. The estimated Kc ratio was 5.1x10 -1 , and as mentioned earlier when Kc <<1 one can consider that corrosion is virtually not affected by the presence of the tribological test. Hence the system was dominated by the mechanical wear of the thin film. Furthermore, the estimated Km value was 6.1x10 -3 , and such a value indicates that the mechanical removal was accelerated by the formation of the passive layer of the thin film.

Chapter's conclusions

In this chapter the results of tribological and electrochemical studies of the tungsten nitride coated steel substrates has been reported. Additionally, an estimation of the tribocorrosion behavior of the coating deposited over AISI 431 was performed.

Résumé étendu

Le processus de tribocorrosion se manifeste lorsque la dégradation d'une surface se produit en raison de l'interaction de l'usure mécanique et de la corrosion, c'est-à-dire lorsque deux matériaux solides en contact et en mouvement relatif sont exposés à un environnement corrosif. Le présent travail explore l'utilisation d'une couche mince de nitrite de tungstène comme protection contre le processus de tribocorrosion des aciers utilisés pour des outils de production de l'industrie alimentaire. Le présent travail a abouti à l'adaptation du UTM-tribolab pour l'évaluation de la tribocorrosion par 'pin on disc'. De plus, des couches minces de nitrite de tungstène ont été déposées avec succès par pulvérisation cathodique rf. Cette couche mince s'est avérée être une option viable pour protéger des matériaux tels que l'acier AISI 431 contre la tribocorrosion. En revanche son utilisation comme film protecteur sur des matériaux à haute teneur en carbone tels que l'AISI D2 ne peut pas être validée, en raison de délaminations récurrentes qui exposent la surface du substrat.

Les résultats obtenus présentent une première approche à l'exploration de l'utilisation du nitrite de tungstène comme film résistant à la tribocorrosion. Il reste à explorer l'utilisation d'additifs dans le film pour améliorer l'usure survenue lors du processus de tribocorrosion ainsi qu'à étudier le comportement sur d'autres types de substrats ; sans oublier la recherche d'autres alternatives de films tampons pour éviter le délaminage lorsque le film est déposé sur des substrats riches en carbone.

Résumé

Le travail présenté ici a pour objet la synthèse de couches minces de nitrure de tungstène ainsi que l'étude de leurs propriétés par rapport à la tribocorrosion. La méthodologie utilisée comprend : (1) l'adaptation et l'optimisation du système de synthèse de couche minces (pulvérisation cathodique RF) et du système de mesure de la tribocorrosion, (2) la caractérisation électrochimique et tribologique des substrats (acier AISI 431 et acier AISI D2), (3) et la réalisation, l'optimisation et la caractérisation de couches minces de nitrure de tungstène et de leurs propriétés tribo-électrochimiques (tribocorrosion).

Le revêtement obtenu a amélioré la résistance à l'usure et à la corrosion des deux substrats. Néanmoins, seul pour le cas de l'AISI 431, la vitesse de corrosion s'est révélée suffisamment faible, le système est donc considéré comme passif et relativement plus résistant à la tribocorrosion. La meilleure réponse a été trouvée pour les revêtements obtenus à une puissance de dépôt élevée et à de faibles pressions de dépôt, suggérant qu'une densité plus élevée de la couche mince améliore ses réponses tribologiques et électrochimiques. Enfin la synergie entre l'usure mécanique et la corrosion de la couche mince de nitrure de tungstène a été évaluée. Avec un rapport Kc d'environ 5,1x10 -1 , l'accélération de la corrosion induite par l'usure mécanique est négligeable, et le rapport Km de 6,14x10 -3 suggère que l'usure mécanique est accélérée par la formation du film passif.

Les résultats obtenus présentent une première approche à l'exploration de l'utilisation du nitrite de tungstène comme couche mince résistant à la tribocorrosion. Ceci ouvre la porte à une étude plus approfondie et plus complète de ce type de couches minces comme couches protectrices contre la tribocorrosion en contact avec différents électrolytes.

Mots clés : tribocorrosion, pulvérisation cathodique, couches minces

Abstract

The present work is focused on the synthesis and tribocorrosion study of tungsten nitride as a tribocorrosion resistant thin film. The methodology used comprised: (1) the adaptation and optimization of the synthesis system (radio frequency magnetron sputtering) and the tribocorrosion evaluation system, (2) the electrochemical and tribological evaluation of the substrates (AISI 431 steel and AISI D2 steel) and (3) the synthesis, optimization and triboelectrochemical (tribocorrosion) evaluation of the tungsten nitride thin film.

The obtained thin film improves the wear and corrosion resistance of both, AISI 431 steel and AISI D2 steel. Nevertheless, only for AISI 431 the corrosion speed is low enough to be considered as a passive system and a suitable tribocorrosion resistant system. The best response was found for the thin films obtained at high deposition power and low deposition pressures, suggesting that a higher film density enhance the tribological and electrochemical responses. Moreover, the tribocorrosion synergism between wear and corrosion of the tungsten nitride thin film was evaluated finding a Kc ratio of about 5.1x10 -1 , indicating that the corrosion acceleration induced by the mechanical wear is negligible, and a Km ratio of 6.14x10 -3 , revealing that the mechanical material removal is accelerated by the formation of the passive film.

The obtained results represent an initial approach to the exploration of the tungsten nitrite thin film as a protective film against tribocorrosion. Such results open the path towards more extensive and comprehensive studies of such type of thin films as protection against tribocorrosion when in contact with different electrolytes.

Key words: tribocorrosion, sputtering, thin films