Michael Ougier

Alexandre Michau
email: alexandre.michau@cea.fr

Frederic Schuster

Hicham Maskrot

Michel L Schlegel

Effects of HiPIMS discharges and annealing on Cr-Al-C thin films

Keywords: HiPIMS, Substrate bias potential, MAX phase, Cr2AlC coating, Annealing, Microstructure

Cr2AlC coatings were synthesized by high power impulse magnetron sputtering (HiPIMS) from Cr2AlC compound target and subsequent thermal annealing in Ar atmosphere. The effect of HiPIMS duty cycle and substrate bias potential (UB) on the thin film composition were investigated. All initial Cr-Al-C coatings exhibit similar compositions close to the target stoichiometry, and dense and amorphous structure independently of the duty cycle.

Meanwhile, Al deficiencies up to 15 % are observed with UB increasing to -200 V. Based on the measured fraction of ionized metal species flux at the substrate, highly energetic bombardment of the coating with ionized inert gas and metal plasma species causes preferential resputtering of Al. Partially crystallized Cr2AlC thin films were obtained by annealing as-deposited Cr-Al-C coatings at 550°C for 4 h. The annealed coating is made of an amorphous inner layer and a crystalline Cr2AlC outer layer. A higher annealing temperature of 650°C led to complete transformation from amorphous phase to crystallized Cr2AlC, and to micro-cracking. These results indicate that the synthesis temperature of MAX phase could be reduced, and the annealing time increased, to obtain protective coatings of Cr2AlC on heatsensitive components without alteration of the substrate metallurgical properties.

Introduction

Cr2AlC belongs to the Mn+1AXn phases, where 'M' stands for an early transition metal element, 'A' is an element of the A group, mostly from the IIIA and IVA columns of the Mendeleev periodic table, and 'X' is C or N. MAX phases are hexagonal ternary carbides or nitrides with a nanolayered structure, resulting in a unique combination of both metal and ceramics properties [START_REF] Barsoum | The MAX phases A new class of solids -Thermodynamically stable nanolaminates[END_REF][START_REF] Barsoum | Elastic and Mechanical Properties of the MAX Phases[END_REF][START_REF] Sun | Progress in research and development on MAX phases: a family of layered ternary compounds[END_REF]. MAX phases exhibit for instance high stiffness [START_REF] Schneider | Elastic properties of Cr2AlC thin films probed by nanoindentation and ab initio molecular dynamics[END_REF], easy machinability [START_REF] Barsoum | Elastic and Mechanical Properties of the MAX Phases[END_REF], ductile-to-brittle transition temperatures from 800 to 1000°C [START_REF] Sun | Synthesis and Characterization of a Metallic Ceramic Material-Ti3SiC2[END_REF][START_REF] Tian | Mechanical properties of pulse discharge sintered Cr2AlC at 25-1000[END_REF][START_REF] Naik Parrikar | The Effect of Grain Size on Deformation and Failure of Ti2AlC MAX Phase under Thermo-Mechanical Loading[END_REF], good resistance to aggressive media and high thermal and electrical conductivity [START_REF] Hettinger | Electrical transport, thermal transport, and elastic properties of M2AlC (M=Ti, Cr, Nb, and V)[END_REF][START_REF] Ying | Effect of Cr7C3 on the mechanical, thermal, and electrical properties of Cr2AlC[END_REF]. Furthermore, the MAX phase Cr2AlC has an excellent resistance to high-temperature oxidation in air due to the formation of a continuous, dense and adherent alumina scale [START_REF] Tallman | A Critical Review of the Oxidation of Ti2AlC, Ti3AlC2 and Cr2AlC in Air[END_REF][START_REF] Ougier | Hightemperature oxidation behavior of HiPIMS as-deposited Cr-Al-C and annealed Cr2AlC coatings on Zr-based alloy[END_REF]. Aluminum diffusion and oxidation in cracks also result in self-healing properties and thermal-shock resilience [START_REF] Ougier | Hightemperature oxidation behavior of HiPIMS as-deposited Cr-Al-C and annealed Cr2AlC coatings on Zr-based alloy[END_REF][START_REF] Pei | Crack healing behaviour of Cr2AlC MAX phase studied by X-ray tomography[END_REF][START_REF] Berger | Crack healing in Y-doped Cr2AlC-MAX phase coatings[END_REF]. These attractive properties make Cr2AlC a suitable candidate for harsh environments, especially as protective coatings for accident-tolerant fuel claddings (ATF) [START_REF] Ougier | Hightemperature oxidation behavior of HiPIMS as-deposited Cr-Al-C and annealed Cr2AlC coatings on Zr-based alloy[END_REF][START_REF] Tang | Improvement of the High-Temperature Oxidation Resistance of Zr Alloy Cladding by Surface Modification with Aluminum-Containing Ternary Carbide Coatings[END_REF][START_REF] Imtyazuddin | Radiation resistance and mechanical properties of magnetron-sputtered Cr2AlC thin films[END_REF][START_REF] Zhang | On the chemical compatibility between Zr-4 substrate and well-bonded Cr2AlC coating[END_REF]. Following the Fukushima Dai'ichi accident in 2011, international efforts focused on the development of ATF nuclear claddings for light water reactors. Several solutions are being investigated worldwide, including surface treatments on existing Zr-based alloys or replacement of these alloys [START_REF] Duan | Current status of materials development of nuclear fuel cladding tubes for light water reactors[END_REF][START_REF] Tang | Protective coatings on zirconiumbased alloys as accident-tolerant fuel (ATF) claddings[END_REF]. Protective coatings on Zrbased claddings appear to be the most promising short-term solution. To this end, various coating materials are investigated, such as Cr [START_REF] Bischoff | AREVA NP's enhanced accidenttolerant fuel developments: Focus on Cr-coated M5 cladding[END_REF][START_REF] Brachet | Early studies on Cr-Coated Zircaloy-4 as enhanced accident tolerant nuclear fuel claddings for light water reactors[END_REF], metallic alloys [START_REF] Zhong | Performance of iron-chromium-aluminum alloy surface coatings on Zircaloy 2 under high-temperature steam and normal BWR operating conditions[END_REF], ceramics including nitrides [START_REF] Meng | Study of the oxidation behavior of CrN coating on Zr alloy in air[END_REF][START_REF] Alat | Ceramic coating for corrosion (c3) resistance of nuclear fuel cladding[END_REF][START_REF] Alat | Multilayer (TiN, TiAlN) ceramic coatings for nuclear fuel cladding[END_REF] and nanolaminated ceramics such as MAX phases [START_REF] Yeom | Laser surface annealing and characterization of Ti2AlC plasma vapor deposition coating on zirconium-alloy substrate[END_REF][START_REF] Maier | Cold spray deposition of Ti2AlC coatings for improved nuclear fuel cladding[END_REF][START_REF] Tang | Evaluation of magnetron sputtered protective Zr-C-Al coatings for accident tolerant Zircaloy claddings[END_REF][START_REF] Wang | Microstructural evolution of epitaxial Ti3AlC2 film on sapphire under ion irradiation and nanoindentation-induced deformation[END_REF].

Additionally, MAX phase coatings are also under development for other applications, e.g. to improve the corrosion resistance of steel bipolar plates in fuel cells [START_REF] Lu | Synthesis and characterization of conductive ceramic MAX-phase coatings for metal bipolar plates in simulated PEMFC environments[END_REF][START_REF] Lu | Characterization of Ti3SiC2-coating on stainless steel bipolar plates in simulated proton exchange membrane fuel cell environments[END_REF]. Coatings of MAX phases (for example Cr2AlC and Ti2AlC) are also investigated for aircraft gas turbines, to increase the lifetime and operating temperature of thermal barrier coatings [START_REF] Smialek | Oxidative durability of TBCs on Ti2AlC MAX phase substrates[END_REF][START_REF] Smialek | Oxidation of Al2O3 Scale-Forming MAX Phases in Turbine Environments[END_REF][START_REF] Gonzalez-Julian | Cr2AlC MAX phase as bond coat for thermal barrier coatings: Processing, testing under thermal gradient loading, and future challenges[END_REF] or to protect turbine blades from erosion [START_REF] Naveed | Study of high velocity solid particle erosion behaviour of Ti2AlC MAX phase coatings[END_REF][START_REF] Eichner | Solid particle erosion behavior of nanolaminated Cr2AlC films[END_REF].

In the past few years, thick coatings of MAX phases have been elaborated by various methods, mainly physical vapor deposition (PVD) at moderate temperatures compared to those required for the fabrication of bulk materials (around 1100°C) [START_REF] Eklund | The Mn+1AXn phases: Materials science and thin-film processing[END_REF]. Other techniques, such as cold spray, have been successfully applied for the deposition of relatively thick, oxidation-resistant coatings of Cr2AlC [START_REF] Go | Cold spray deposition of Cr2AlC MAX phase for coatings and bond-coat layers[END_REF]. However, the microstructure of such films is relatively loose and highly porous compared to other deposition techniques. The coatings also contain a significant amount of secondary phases (CrxCy and/or Cr-Al intermetallics), resulting in premature failure. In contrast, PVD and specifically magnetron sputtering are able to synthesize dense and crystalline films with minor or no secondary phases. The coating is obtained either from the sputtering of elemental [START_REF] Schneider | Ab initio calculations and experimental determination of the structure of Cr2AlC[END_REF][START_REF] Field | A combinatorial comparison of DC and high power impulse magnetron sputtered Cr2AlC[END_REF] and compound targets [START_REF] Eichner | Solid particle erosion behavior of nanolaminated Cr2AlC films[END_REF][START_REF] Walter | Towards large area deposition of Cr2AlC on steel[END_REF][START_REF] Berger | Characterization of Cr-Al-C and Cr-Al-C-Y films synthesized by High Power Impulse Magnetron Sputtering at a low deposition temperature[END_REF][START_REF] Obrosov | Chemical and Morphological Characterization of Magnetron Sputtered at Different Bias Voltages Cr-Al-C Coatings[END_REF] or by reactive sputtering [START_REF] Su | Synthesis of Cr2AlC thin films by reactive magnetron sputtering[END_REF]. Crystallization of the MAX phase can be favored in situ by heating the substrate during sputtering, or ex situ by annealing a stack of nanolayers of amorphous of partially crystallized films [START_REF] Abdulkadhim | Crystallization kinetics of amorphous Cr2AlC thin films[END_REF][START_REF] Grieseler | Nanostructured plasma etched, magnetron sputtered nanolaminar Cr2AlC MAX phase thin films[END_REF][START_REF] Tang | Textured growth of polycrystalline MAX phase carbide coatings via thermal annealing of M/C/Al multilayers[END_REF][START_REF] Crisan | Incipient low-temperature formation of MAX phase in Cr-Al-C films[END_REF]. The formation of Cr2AlC MAX phase from amorphous Cr-Al-C thin films occurs in the temperature range from 580°C to 600°C [START_REF] Walter | Towards large area deposition of Cr2AlC on steel[END_REF][START_REF] Abdulkadhim | Crystallization kinetics of amorphous Cr2AlC thin films[END_REF][START_REF] Stelzer | Remote Tracking of Phase Changes in Cr2AlC Thin Films by In-situ Resistivity Measurements[END_REF]. Obviously, the phase transition is significantly dependent on the deposition temperature, heating rate, annealing temperature and time. Therefore, it is crucial to clarify the correlation between the annealing parameters and the coating microstructure as it may significantly affect its performance, e.g. as an ATF coating [START_REF] Tang | Improvement of the High-Temperature Oxidation Resistance of Zr Alloy Cladding by Surface Modification with Aluminum-Containing Ternary Carbide Coatings[END_REF][START_REF] Yeom | Laser surface annealing and characterization of Ti2AlC plasma vapor deposition coating on zirconium-alloy substrate[END_REF].

Recently, dense and crystalline Cr2AlC coatings have been synthesized on Zr-based alloys [START_REF] Tang | Improvement of the High-Temperature Oxidation Resistance of Zr Alloy Cladding by Surface Modification with Aluminum-Containing Ternary Carbide Coatings[END_REF][START_REF] Roberts | Magnetron Sputtering and Corrosion of Ti-Al-C and Cr-Al-C coating for Zr alloy Nuclear Fuel Cladding[END_REF] by magnetron sputtering below the recommended upper limit of 580°C required to avoid metallurgical degradation of these substrates [START_REF] Gilbon | Irradiation creep and growth behavior, and microstructural evolution of advanced Zr-base alloys[END_REF]. The coated samples displayed a better corrosion and oxidation resistance than the uncoated Zr-based alloys. However, the coating performances were hampered by their limited thickness and poor adherence. These drawbacks led to an accelerated degradation of the coating both in nominal regime and under conditions of loss-of-coolant accident (LOCA). Based on the above findings, it can be concluded that several challenges have to be overcome for the development of Cr2AlC MAX phase coatings on Zr-based substrates. They include the elimination of secondary phases, the decrease of deposition or annealing temperatures down to acceptable values for Zr-alloys metallurgy, and the improvement of the coating density and adhesion.

High power impulse magnetron sputtering (HiPIMS) is a modern PVD technique, which consists in powering the target with short current pulses (<200 μs) at frequencies approximatively between 100 and 500 Hz. In this way, high ionization of the sputtered species can be obtained without significant target heating. The high ion-to-neutral ratio and intense ionic bombardment then allows for an improved control of the coating composition, microstructure, and mechanical properties [START_REF] Sarakinos | High power pulsed magnetron sputtering: A review on scientific and engineering state of the art[END_REF][START_REF] Elmkhah | Microstructural and electrochemical comparison between TiN coatings deposited through HIPIMS and DCMS techniques[END_REF][START_REF] Hovsepian | Performance of HIPIMS deposited CrN/NbN nanostructured coatings exposed to 650 °C in pure steam environment[END_REF][START_REF] Hovsepian | Novel HIPIMS deposited nanostructured CrN/NbN coatings for environmental protection of steam turbine components[END_REF]. For example, exceptional protection of nuclear fuel cladding coated with chromium has been demonstrated, both in nominal and LOCA conditions [START_REF] Bischoff | AREVA NP's enhanced accidenttolerant fuel developments: Focus on Cr-coated M5 cladding[END_REF]. Furthermore, the higher energy of incident species enhances the surface mobility of deposited atoms, favoring the crystallization of e.g., Cr2AlC and V2AlC at lower temperatures compared to conventional magnetron sputtering [START_REF] Berger | Characterization of Cr-Al-C and Cr-Al-C-Y films synthesized by High Power Impulse Magnetron Sputtering at a low deposition temperature[END_REF][START_REF] Jiang | Growth of V-Al-C thin films by direct current and high power impulse magnetron sputtering from a powder metallurgical composite target[END_REF]. However, the correlations between the properties of Cr-Al-C coating and process parameters, especially HiPIMS pulse parameters and substrate bias, have been scarcely investigated [START_REF] Rueß | HPPMS deposition from composite targets: Effect of two orders of magnitude target power density changes on the composition of sputtered Cr-Al-C thin films[END_REF]. For industrial coating processes, the substrate bias is a major parameter as it controls the coating microstructure [START_REF] Obrosov | Chemical and Morphological Characterization of Magnetron Sputtered at Different Bias Voltages Cr-Al-C Coatings[END_REF], has a direct effect on the film growth rate [START_REF] Bobzin | Investigations on the substrate bias influence on reactive HPPMS plasmas[END_REF], and can increase adhesion between the substrate and coating [START_REF] Broitman | Industrial-scale deposition of highly adherent CNx films on steel substrates[END_REF][START_REF] Santiago | Adhesion enhancement of DLC hard coatings by HiPIMS metal ion etching pretreatment[END_REF]. Hence, HiPIMS is a processing route that opens many possibilities to produce optimized Cr2AlC coatings.

Another important aspect is the use of compound targets, which is generally preferred in industrial applications for its simplicity and repeatability. Yet, it has been recognized that the elemental composition of MAX phase films obtained by magnetron sputtering can deviate from the target composition depending on the process parameters [START_REF] Jiang | Growth of V-Al-C thin films by direct current and high power impulse magnetron sputtering from a powder metallurgical composite target[END_REF][START_REF] Rueß | HPPMS deposition from composite targets: Effect of two orders of magnitude target power density changes on the composition of sputtered Cr-Al-C thin films[END_REF][START_REF] Alami | High-power impulse magnetron sputtering of Ti-Si-C thin films from a Ti3SiC2 compound target[END_REF][START_REF] Magnuson | Compositional dependence of epitaxial Tin+1SiCn MAX-phase thin films grown from a Ti3SiC2 compound target[END_REF]. This deviation is frequently observed for compound targets containing elements with large differences in atomic masses [START_REF] Jo | Single elementary targetsputtered Cu2ZnSnSe4 thin film solar cells[END_REF][START_REF] Achenbach | Correlative Experimental and Theoretical Investigation of the Angle-Resolved Composition Evolution of Thin Films Sputtered from a Compound Mo2BC Target[END_REF] and, especially in HiPIMS, is exacerbated by differences in ionization energies. As a consequence, the film structure and composition can be significantly influenced by process parameters such as temperature [START_REF] Frodelius | Sputter deposition from a Ti2AlC target: Process characterization and conditions for growth of Ti2AlC[END_REF], pressure [START_REF] Eklund | Magnetron sputtering of Ti3SiC2 thin films from a compound target[END_REF], substrate inclination angle [START_REF] Mráz | Angle-resolved evolution of the composition of Cr-Al-C thin films deposited by sputtering of a compound target[END_REF] and substrate bias [START_REF] Jiang | Growth of V-Al-C thin films by direct current and high power impulse magnetron sputtering from a powder metallurgical composite target[END_REF][START_REF] Rueß | HPPMS deposition from composite targets: Effect of two orders of magnitude target power density changes on the composition of sputtered Cr-Al-C thin films[END_REF][START_REF] Alami | High-power impulse magnetron sputtering of Ti-Si-C thin films from a Ti3SiC2 compound target[END_REF].

The present work investigates the synthesis of Cr2AlC coatings on Zr-based substrates. Focus is placed on the influence of the deposition and annealing parameters on the film properties.

Cr-Al-C films were first deposited on two different substrates, stainless steel and Zr-based alloy, by HiPIMS from a composite target. The crystallization and microstructure evolution of the as-deposited Cr-Al-C coating upon annealing in argon atmosphere from 500 to 650°C was then assessed. The results provide an insight into the relationship between the HiPIMS deposition parameters and the annealing treatment on the film properties, and establish a strategy to fabricate Cr2AlC MAX phase coating on Zr-based alloys.

Materials and methods

Synthesis

Stainless steel (SS304L, 15 × 15 × 3 mm) and Zr-based alloy (Zr702, 30 × 30 × 2 mm) coupons were polished using SiC paper up to 2000 grit. Prior to deposition, the samples were sonicated in water, acetone and ethanol. Deposition of Cr-Al-C films on SS304L was performed in an in-house HiPIMS PVD equipment using a Cr2AlC circular target (6 inches diameter) with a Cr:Al:C stoichiometry of 2:1:1 (99.9%; Neyco Vacuum & Materials). A SPIK3000A pulsing unit (MELEC) fed by a direct current (DC) generator (ADL GmbH) powered the cathode. During the deposition process, the cathode was operated in powercontrolled mode with an average power of 250 W (≈ 1.37 W.cm -2 ) with a target-substrate distance of 8 cm. The argon working pressure was set to 0.55 Pa. The frequency and pulse duration of the HIPIMS cathodes were varied between 300 to 1100 Hz and 20 to 200 μs, corresponding to a duty cycle from 2.4 to 11.1 %. Furthermore, the effect of a DC substrate bias (UB) was investigated in the range of floating potential to -200 V. The deposition time was adjusted to grow an approximate 2 µm-thick coating. The substrates were not intentionally heated and were coated only on one side since they were laying on a flat substrate-holder, facing the target.

The deposition of Cr-Al-C films on Zr702 was performed in an industrial chamber developed by DEPHIS (Etupes, France) with a base pressure of 4 × 10 -5 mbar. The setup is equipped with one Cr2AlC and one Cr rectangular target (99.8% purity; Nano&Micro PVD) with areas of 840 cm² (560 × 150 mm) each, and a sample holder with a triple rotation axis that allows for the coating growth on all faces of coupons. Unipolar pulse was applied to the targets using a GX100/1000 DC power supply (ADL GmbH) combined with a SPIK2000USB Dual HiPIMS pulse controller (MELEC) operated in a power-controlled mode. In order to remove contaminants and improve adhesion of coatings, substrates were pre-etched with Cr + produced in a HIPIMS plasma discharge of the Cr target (target voltage 950 V, pulse frequency 1923 Hz and pulse width 20 μs for an average power of 3000 W) in a 0.5 Pa Ar atmosphere for 10 min. A negative UB value of -900 V was applied to the sample holder.

After etching, Cr-Al-C coatings were deposited in 0.35 Pa Ar and substrates were left at a floating potential. The average power, pulse width and frequency were 2500 W (≈ 2.98 W.cm -2 ), 70 μs, and 310 Hz, respectively. These parameters resulted in a peak power density of 280 W.cm -2 and a duty cycle of 2.2%. The vacuum chamber was heated up to 400°C during film growth by two radiant heaters which were not available in the first in-house equipment.

Annealing of the coatings deposited on Zr702 was performed in a flowing argon atmosphere (99.99 %). The samples were loaded in a horizontal alumina tube furnace (Carbolite; Gero 301 PID controller) at room temperature and then heated to the desired annealing temperature at 4 °C.min -1 . Samples were subsequently cooled inside the furnace down to room temperature.

A compact retarding field energy analyzer (RFEA) with an embedded quartz crystal microbalance (QCM) (Quantum system, Impedans) was used to measure the fraction of metal ions in the ionized flux arriving at the substrate surface [START_REF] Sharma | Measurement of deposition rate and ion energy distribution in a pulsed dc magnetron sputtering system using a retarding field analyzer with embedded quartz crystal microbalance[END_REF]. The sensor consists of a QCM mounted behind a RFEA grid stack. The RFEA is composed of five stainless steel grids and can be configured to selectively repulse the ion flux from the quartz crystal. Typically, a positive potential is applied to block metal ions, and a negative potential is applied to permit the fluxes of both metal ions and the neutral species to reach the QCM. Thus, direct measurement of the total and neutral-related deposition rates at the substrate location can be determined when both neutrals and ions are allowed, or when only neutrals are allowed, respectively. From these two measurements, the contribution of ionized species to the deposition rate (or ion-related deposition rate) can be obtained. The ionized fraction of the flux can be calculated as the ratio of the ion-related to total deposition rates. For the measurements, the sensor was placed at the substrate level, i.e. at 8 cm from the target surface. For each experiment, 20 individual measurements (ions + neutrals and only ions) were carried out. The acquisition time of one measurement was set to 20 s.

Coating structural characterization

X-ray diffraction (XRD) was performed with a D8 Discover (Bruker) diffractometer in grazing incidence configuration using Cu Kα radiation (40 kV and 40 mA). In order to obtain lattice parameters of annealed Cr-Al-C coatings, all XRD patterns were fitted using MAUD [START_REF] Lutterotti | Texture, residual stress and structural analysis of thin films using a combined X-ray analysis[END_REF] software to do Rietveld refinement. Raman spectra were collected using a LabRam HR spectrometer (Horiba Jobin Yvon) coupled with an Olympus BX41 microscope. A frequencydoubled Nd-YAG laser (wavelength of 532 nm) was used as source, and the signal was collected using a 100× objective. Integration times were adjusted to optimize the signal-tonoise ratio.

The surface and cross-section of the as-deposited and annealed coatings were imaged using a scanning electron microscope (JEOL JSM 7000F) equipped with a field emission gun (15 kV, 2 nA). For nanoanalysis, a thin foil was prepared using the focused ion beam (FIB) lift-out technique in a NanoLab HELIOS 600i (FEI; UMS 3623 Raimond Castaing, Toulouse, France) microscope. Transmission electron microscopy (TEM) was performed using a JEM 2100F (JEOL; UMS 3623 Raimond Castaing, Toulouse, France) operated at 200 kV and equipped with an EDS analyser (Bruker SDD Xflash 5030). The selective area electron diffraction (SAED) patterns were analyzed using CaRIne software.

Compositional analysis of the coatings was carried out by glow-discharge optical emission spectroscopy (GDOES; Horiba GD-Profiler 2™). X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermofisher Escalab 250 XI spectrometer equipped with a monochromatic X-ray Al-Kα source (hυ = 1486.6 eV). A constant pass energy of 20 eV was used for high-resolution spectra. All samples were characterized in ultra-high vacuum after etching by Ar + ion at 500 eV for 40 min to remove the topmost contaminated layers.

Results and discussion

Effect of deposition parameters on Cr-Al-C films

The chemical compositions of the Cr-Al-C films deposited on SS304L as a function of duty cycle and UB were assessed by GDOES. Clearly, the chemical composition is not significantly affected by the duty cycle in the investigated range (Fig. 1a) and is close to the Cr2AlC stoichiometry. The contents of Al lies within the range of 24 to 26 at.% while the Cr/C ratio varies between 2.0 and 2.2. In addition, small traces of O can be found (≈ 0.8 at.%), likely due to residual water vapor within the PVD chamber during the growth. These slight compositional deviations should not affect the crystallization of Cr2AlC MAX phase upon annealing, as this phase is known to form even at compositions deviating significantly from the stoichiometric ratio [START_REF] Eichner | Solid particle erosion behavior of nanolaminated Cr2AlC films[END_REF][START_REF] Baben | Oxygen incorporation in M2AlC (M=Ti, V, Cr)[END_REF].

In contrast, when UB is increased from the floating potential to -200 V at a fixed duty cycle, the Al content is strongly modified (Fig. 1b). At UB = -200 V, an Al depletion up to 10 % with respect to Cr2AlC stoichiometry is observed. As the Cr/C ratio is approximately constant, the Al loss is compensated by an increase in both Cr and C contents. To identify the origin of this deviation, the ionized flux fraction was investigated using the RFEA-QCM. The deposition rate, the fraction of ionized metal species and the ratio of ionized over neutral species were measured as a function of UB (Fig. 2). Experimental results indicate the presence of metallic ions during HiPIMS discharge for all conditions. For example, for UB = -50 V, the ionized fraction of the metal flux is about 0.18, meaning that metallic ions make 18 % of the metal vapor assuming that the sticking coefficient of ions and neutrals on the quartz are identical. This high fraction is in agreement with the known properties of plasma obtained by HiPIMS [START_REF] Bohlmark | The ion energy distributions and ion flux composition from a high power impulse magnetron sputtering discharge[END_REF][START_REF] Ferrec | Mass spectrometry analyzes to highlight differences between short and long HiPIMS discharges[END_REF]. However, the ionized metal flux fraction appears to be low, which may be attributed to the sputtered ions returning back to the target [START_REF] Anders | Discharge physics of high power impulse magnetron sputtering[END_REF][START_REF] Anders | Deposition rates of high power impulse magnetron sputtering: Physics and economics[END_REF] and to only marginal ionization of C in Ar discharge [START_REF] Rueß | HPPMS deposition from composite targets: Effect of two orders of magnitude target power density changes on the composition of sputtered Cr-Al-C thin films[END_REF]. The small ionization of C is due to its higher ionization energy (11.3 eV) compared to Cr (6.77 eV) and Al (5.99 eV). As UB increases from -50 V to -200 V, the ionized metal flux fraction increases from 18 to 85 % while the deposition rate decreases. The ratio of ionized species over neutral ones increases up to 600% approximately.

These results can be explained with the influence of the polarization UB of the substrate, which primarily affects the plasma properties at the substrate surface. On the one hand, highly negative UB promotes the transport of ionized species to the substrate surface, which results in a plasma extension towards the substrate, and an increasing proportion of ionized species [START_REF] Bobzin | Investigations on the substrate bias influence on reactive HPPMS plasmas[END_REF]. On the other hand, the resputtering of species already adsorbed on the substrate or the coating can be caused by the highly energetic incident ions [START_REF] Sarakinos | High power pulsed magnetron sputtering: A review on scientific and engineering state of the art[END_REF]. Thus, Al depletion can be attributed to preferential resputtering of Al triggered by the very energetic bombardment by both metal ions and Ar ions (not measured). Our results corroborate those obtained previously for Cr-Al-C thin films deposited by HiPIMS at high |UB| [START_REF] Rueß | HPPMS deposition from composite targets: Effect of two orders of magnitude target power density changes on the composition of sputtered Cr-Al-C thin films[END_REF]. Possible explanations for this Al preferential resputtering are i) the weaker Cr-Al bond energy compared to Cr-C bond; ii) the higher efficiency of energy transfer in an elastic collision between argon ions and Al; and iii) the smaller mass of Al compared to Cr, which amplifies resputtering [START_REF] Rueß | HPPMS deposition from composite targets: Effect of two orders of magnitude target power density changes on the composition of sputtered Cr-Al-C thin films[END_REF].

Effect of annealing temperature on properties of Cr-Al-C films

Annealing was performed on Cr-Al-C thin films deposited on Zr702 substrates at a floating potential and a duty cycle of 2.2 %. The XRD patterns of as-deposited CrAlC films exhibit a broad diffraction hump at about 42° indicating that the films are X-ray amorphous (Fig. 3). This result is consistent with previous studies in which the substrate temperatures are too low to achieve crystallization of a highly ordered MAX phase structure [START_REF] Abdulkadhim | Crystallization kinetics of amorphous Cr2AlC thin films[END_REF][START_REF] Liu | Fabrication and mechanical properties of high purity of Cr2AlC coatings by adjustable Al contents[END_REF].

In order to promote crystallization of the as-deposited Cr-Al-C films, an annealing posttreatment was performed at 500-650°C for 4-50 h in Ar (Fig. 3). The comparison of XRD patterns of thin films as-deposited or annealed at 500°C for 4 h (not shown) does not reveal noticeable structural changes. However, unambiguous diffraction peaks are observed after 50 h of annealing at 500°C, and could be assigned to the (100)/(101), (103)/(006), (106)/(200), ( 110), ( 109) and ( 116)/(203) bands of Cr2AlC MAX phase (JCPDS card 00-058-0267). Annealing at 550°C for 4 h also results in the formation of pure Cr2AlC phase, in agreement with formation of MAX phase by a diffusion process that depends on the annealing temperature and time, as reported elsewhere [START_REF] Wang | Diffusion-controlled intercalation approach to synthesize the Ti2AlC MAX phase coatings at low temperature of 550 °C[END_REF]. No noticeable diffraction signal of Cr-Al intermetallic compounds or Cr-C carbides is detected. As the annealing temperature increases to 650°C, the intensity of Cr2AlC diffraction peaks increase, which implies an improved crystallinity, and shift in position, suggesting a change in lattice parameters. For a temperature increase from 550°C to 650°C, the a lattice parameter derived from the Rietveld refinement slightly increases from 2.84 to 2.85 Å, and the c lattice parameter decreases from 13.04 to 12.96 Å. This evolution is consistent with previous studies on Cr2AlC [START_REF] Abdulkadhim | Crystallization kinetics of amorphous Cr2AlC thin films[END_REF][START_REF] Stelzer | Remote Tracking of Phase Changes in Cr2AlC Thin Films by In-situ Resistivity Measurements[END_REF]. It is worth to mention that all annealed Cr2AlC films exhibit similar preferred crystallographic orientation of (103), which is the frequently observed preferred orientation in MAX phase film under conditions of non-basal plane growth [START_REF] Walter | Towards large area deposition of Cr2AlC on steel[END_REF][START_REF] Feng | Preparation of Ti2AlC MAX Phase Coating by DC Magnetron Sputtering Deposition and Vacuum Heat Treatment[END_REF][START_REF] Naveed | Sputtering Power Effects on Growth and Mechanical Properties of Cr2AlC MAX Phase Coatings[END_REF][START_REF] Li | Deposition of Phase-pure Cr2AlC Coating by DC Magnetron Sputtering and Post Annealing Using Cr-Al-C Targets with Controlled Elemental Composition but Different Phase Compositions[END_REF].

However, it should be pointed out that the annealed films may also partially contain transition products such as the disordered (Cr,Al)2Cx solid solution, also known as γ-Cr2AlC. This phase can form at annealing temperatures below 585°C and has a hexagonal structure [START_REF] Berger | Characterization of Cr-Al-C and Cr-Al-C-Y films synthesized by High Power Impulse Magnetron Sputtering at a low deposition temperature[END_REF][START_REF] Abdulkadhim | Crystallization kinetics of amorphous Cr2AlC thin films[END_REF][START_REF] Bugnet | Chemically sensitive amorphization process in the nanolaminated Cr2AC (A=Al or Ge) system from TEM in situ irradiation[END_REF].

For the ordered MAX phase, the lattice parameter c is exactly three times longer than for γ-Cr2AlC [START_REF] Berger | Characterization of Cr-Al-C and Cr-Al-C-Y films synthesized by High Power Impulse Magnetron Sputtering at a low deposition temperature[END_REF]. Generally, the (002) peak at 2θ around 13° as well as the (101) peak at 36.90° attest to the formation of MAX phase. However, in our case and as it was previously reported for these coatings in [START_REF] Ougier | Hightemperature oxidation behavior of HiPIMS as-deposited Cr-Al-C and annealed Cr2AlC coatings on Zr-based alloy[END_REF], the (002) contribution is absent and the (101) one is certainly overlapped by the (100) peak. The (002) diffraction peak may be hidden in the baseline, because our diffraction geometry leads to reflection shift and broadening at low diffraction angles. Moreover, grazing incidence XRD was completed with two-theta scans but unfortunately the contribution of the Zr-based substrate prevents any exploitation regarding the coating due to its too small thickness. Hence phase identification based on diffraction data alone is not possible in our case. A parasite contribution was identified in the coating annealed at 600°C and was attributed to a carbon-based contamination originating from a pollution of the oven used for the thermal treatment.

Raman spectroscopic analyses were performed to confirm the formation of the Cr2AlC MAX phase during annealing. Generally, four Raman bands (1a, 1b, 1c and 1d) can be observed in the 150 to 400 cm -1 wavenumber range [START_REF] Barsoum | Elastic Properties, Raman and Infrared Spectroscopy[END_REF]. However, the second and third contributions (1b and 1c) usually overlap in experimental data. As shown in Fig. 4, only a very broad hump around 220 cm -1 is observed for as-deposited Cr-Al-C, confirming that this film is amorphous. After annealing at 500 and 550°C, two bands appear around 245 and 330 cm -1 , the expected positions of the 1b (mixed with 1c) and 1d contributions. This suggests atomic rearrangement and supports a partial crystallization of Cr2AlC MAX phase. As the annealing temperature increases to 600°C, a peak at 165 cm -1 characteristic of the 1a Raman contribution appears, implying a significant increase in the crystallinity of MAX phase. The Raman signals attributed to the Cr2AlC phase become narrower and stronger with an increasing annealing temperature to 650°C. The evolution of the Raman spectra correlates the phase change from amorphous to Cr2AlC MAX phase during annealing, as observed in XRD. However, it is still uncertain if the coating contains only Cr2AlC or a mixture of Cr2AlC and a less ordered phase.

SEM images reveal the surface morphologies of the annealed coatings (Fig. 5). Micro-cracks appear on the coatings annealed at 600°C and above. This is likely a consequence of the large mismatch in thermal expansion coefficient between the coating and the substrate (around 11-13 × 10 -6 K -1 for Cr2AlC and 5 × 10 -6 K -1 for Zr702). A suitable interlayer (bond coat) may mitigate this mismatch and prevent coating damage for annealing temperatures above 550°C.

Internal stresses are also expected to contribute because of the ionic bombardment seen during the deposition and mostly during pre-etching with Cr ions. However, their precise evaluation could not be performed.

To further check the elemental diffusion during annealing, GDOES profiles of as-deposited

Cr-Al-C coatings on Zr702 and annealed coatings at different temperatures were measured (Fig. 6). The interface between the coating and the substrate is not obvious on the GDOES profiles. It was arbitrary set to the intersection point of Cr and Zr elemental profiles (see Fig. 6). No obvious difference appears between the as-deposited coating and the sample annealed at 550°C, with relatively sharp changes in concentrations at the interface between the coating and the substrate (Fig. 6a-b). The transitions in profiles are broadened at ≥ 600°C, suggesting interdiffusion of Cr, Al, C, and Zr (Fig. 6c-d), which can lead to the improvement of coating adhesion to the substrate [START_REF] Schroeter | Fabrication and characterisation of PVD coatings on the base of Cr2AlC max phase[END_REF][START_REF] Tang | Deposition, characterization and high-temperature steam oxidation behavior of single-phase Ti2AlC-coated Zircaloy-4[END_REF]. It is also well known that Al-containing MAX phases possess high chemical reactivity with Zr-based alloys, which promotes thermal interdiffusion [START_REF] Tang | Improvement of the High-Temperature Oxidation Resistance of Zr Alloy Cladding by Surface Modification with Aluminum-Containing Ternary Carbide Coatings[END_REF][START_REF] Tang | Synthesis and characterization of Ti2AlC coatings by magnetron sputtering from three elemental targets and ex-situ annealing[END_REF][START_REF] Liang | Synthesis of Zr2Al3C4 coatings on zirconium-alloy substrates with AlC/Si interlayers as diffusion barriers[END_REF][START_REF] Mráz | Thermal stability enhancement of Cr2AlC coatings on Zr by utilizing a double layer diffusion barrier[END_REF]. This is a consequence of the significant solubility limit of Al in Zr (1.2 at.%), which is 17 times greater than that of Zr in Al (0.07 at.%) at temperatures of 565-625°C [START_REF] Laik | Intermetallics in the Zr-Al diffusion zone[END_REF].

To gain insight into the coating microstructural evolution during annealing, the coatings annealed at 550°C and 650°C for 4 h were characterized by TEM. The bright field TEM image of a FIB lamella for the Cr-Al-C film annealing at 550°C (Fig. 7a) reveals a doublelayered structure. The outer layer (noted 1 in Fig. 7a) exhibits a columnar structure with elongated grains, the classical structure for thin films of MAX phase deposited by PVD using compound targets [START_REF] Berger | Characterization of Cr-Al-C and Cr-Al-C-Y films synthesized by High Power Impulse Magnetron Sputtering at a low deposition temperature[END_REF][START_REF] Obrosov | Chemical and Morphological Characterization of Magnetron Sputtered at Different Bias Voltages Cr-Al-C Coatings[END_REF]. The typical diffraction spot from the (002) basal plane of the MAX phase is detected on SAED patterns (Fig. 7c). Several additional diffraction bands were observed and match within 10% with theoretical interplanar distances calculated for Cr2AlC using parameters from [START_REF] Schuster | The ternary systems: CrAlC, VAlC, and TiAlC and the behavior of H-phases (M2AlC)[END_REF]. No diffraction rings characteristic of amorphous phases were observed. Compared to the associated XRD pattern, the (002) contribution confirms the preferential formation of the crystallized MAX phase over the γ-Cr2AlC phase.

In contrast, the inner layer (noted 2 in Fig. 7a) does not exhibit any microstructural features of crystallization, and only two diffuse rings are present on the SAED pattern (Fig. 7c). The rings are centered on interplanar distances of 2.15 and 1.25 Å, which correspond to theoretical XRD diffraction angles of 42.0° and 76.4°, respectively. The 2.15 Å value of the inner-ring is consistent with the hump of the XRD pattern (around 42°) of the as-deposited coating that evolves into (103)/(006) reflections upon annealing at ≥ 500°C. The outer-ring is in agreement with the (109) reflection theoretically obtained for Cr2AlC at 77.1° that shifts to 76.4° and 76.8° at 500 and 650°C, respectively. A sub-layer (500 nm) within the inner layer is also visible close to the substrate interface in Fig. 7a, and will be discussed later. Hence, the 4 hours annealing period obviously did not result in complete crystallization from amorphous Cr-Al-C to Cr2AlC, in agreement with the Raman results at 550°C.

It is interesting to note that crystallization starts from the outer surface of the film, possibly due to the presence of defects on the surface of the amorphous layer which may act as nucleation and growth sites of MAX phases [START_REF] Eklund | The Mn+1AXn phases: Materials science and thin-film processing[END_REF]. For example, the oxygen impurity on the film surface lowers the activation barrier for nucleation and thus reduces the transition temperature [START_REF] Eklund | The Mn+1AXn phases: Materials science and thin-film processing[END_REF]. A local thermal emission due to phase transition from amorphous to crystalline Cr2AlC phase may then occur [START_REF] Abdulkadhim | Crystallization kinetics of amorphous Cr2AlC thin films[END_REF][START_REF] Stelzer | Remote Tracking of Phase Changes in Cr2AlC Thin Films by In-situ Resistivity Measurements[END_REF]. This heat would diffuse into neighboring areas, inducing further crystallization, and results in the formation of a crystallization front moving towards the substrate [START_REF] Ohdaira | Variation of crystallization mechanisms in flash-lamp-irradiated amorphous silicon films[END_REF].

Distinct from the double layered structure observed at 550°C, the coating annealed at 650°C reveals a simple single-layered polycrystalline structure (Fig. 8a,b). It can be seen from TEM images that the film is dense with no noticeable macro-size defects. Cr2AlC is clearly identified by SAED patterns with experimental interplanar distances deviating by less than 10% from expected ones (Fig. 8c). There is no significant differences between the coating annealed at 650°C and the outer crystalline layer from the coating annealed at 550°C. Grains are still elongated with a columnar structure parallel to the growth direction. Fig. 8d shows the SAED pattern of the circled area, with a [033 1] zone axis. The measured interplanar distances and angles are quite consistent (<10 %) with the theoretical values of the Cr2AlC MAX phase. These results are reminiscent of single layers observed in studies, in which nanolaminates were growing perpendicular to the substrate surface and parallel to the (001) plane [START_REF] Berger | Characterization of Cr-Al-C and Cr-Al-C-Y films synthesized by High Power Impulse Magnetron Sputtering at a low deposition temperature[END_REF][START_REF] Obrosov | Chemical and Morphological Characterization of Magnetron Sputtered at Different Bias Voltages Cr-Al-C Coatings[END_REF].

As shown in Fig. 7a and Fig. 8a, a sub-layer (around 500 nm) is located at the coating/substrate interface in the Cr-Al-C coating annealed at 650°C. It should be pointed out that this sublayer is also visible in the SEM observations of the as-deposited coating, which are not presented here. This area in Fig. 9a shows repeating broader bright and narrower dark segments, indicating a slight difference in the chemical composition of its constitutive layers.

EDX measurements revealed moderate fluctuations of the Cr and Al compositions (Fig. 9b,c). This layered structure was attributed to periodic switch-on of the substrate-holder bias, as confirmed by the recording of parameters applied during deposition. Such phenomenon has been reported for cathodic arc deposition of MAX phase coatings from a compound target with a rotating substrate [START_REF] Eriksson | Layer formation by resputtering in Ti-Si-C hard coatings during large scale cathodic arc deposition[END_REF][START_REF] Li | Phase formation and microstructure evolution of arc ion deposited Cr2AlC coating after heat treatment[END_REF]. The layering can be attributed to resputtering of lighter elements (Al in our case) by the energetic ion flux in high plasma incidence angle segments of the rotation at a substrate bias less than -30 V [START_REF] Eriksson | Layer formation by resputtering in Ti-Si-C hard coatings during large scale cathodic arc deposition[END_REF], as well as in absence of a bias [START_REF] Li | Phase formation and microstructure evolution of arc ion deposited Cr2AlC coating after heat treatment[END_REF]. The resulting fluctuations of Al and Cr contents in this sublayer further hindered local crystallization into the MAX phase during annealing, as shown in the SAED pattern inset. Furthermore, although Cr/C ratio is slightly greater than 2 [START_REF] Mertens | Effect of the Composition on the Structure of Cr-Al-C Investigated by Combinatorial Thin Film Synthesis and ab Initio Calculations[END_REF], no obvious impurity phases are detected within the coating by XRD or TEM. A possible explanation is that deviations from stoichiometry are accommodated by the formation of stacking and substitution (Cr by Al) defects as well as interstitial defects. It can also be mentioned that the coating/substrate interface is slightly enriched in Fe originating from Zr702, as measured by EDX analysis (data not shown). The dissolution of Laves Zr(Fe,Cr)2 precipitates initially present within the α-Zr matrix, and the subsequent diffusion of Fe to Cr/Zr interface during annealing can explain this observation [START_REF] Brachet | Early studies on Cr-Coated Zircaloy-4 as enhanced accident tolerant nuclear fuel claddings for light water reactors[END_REF][START_REF] Ribis | Atomic-scale interface structure of a Cr-coated Zircaloy-4 material[END_REF].

XPS analyses were performed to study the chemical environment of Al, Cr and C atoms in the as-deposited film and the one annealed at 550°C (Fig. 10). As the Al 2p and Cr 3s bands strongly overlap around 74.4 eV, only the Al 2s bond was analyzed for Al. A single peak is detected for Cr 2p3/2, Cr 2p1/2, Al 2s and C 1s contributions, indicating single chemical binding states. These peaks are found at 574.1 eV, 583.4 eV, 117.5 eV and 282.7 eV respectively, in accordance with the characteristic bonding in Cr2AlC [START_REF] Obrosov | Chemical and Morphological Characterization of Magnetron Sputtered at Different Bias Voltages Cr-Al-C Coatings[END_REF][START_REF] Zamulaeva | Conditions for formation of MAX phase Cr2AlC in electrospark coatings deposited onto titanium alloy[END_REF][START_REF] Abdelkader | Molten salts electrochemical synthesis of Cr2AlC[END_REF]. No traces of Al4C3 (282.4 eV) [START_REF] Coulombe | Cryolitic vein XPS imaging from an industrial grade carbon[END_REF] or elementary carbon (C-C bonds) are observed (see Fig. 10b andc), meaning that the C atoms do not form an amorphous polymorph. These results confirm that Cr2AlC crystallized at 550°C with no secondary phases, in good agreement with the XRD, Raman and TEM analyses. Furthermore, the full width at half maximum (FWHM) of all peaks remains roughly unchanged after annealing, suggesting no formation of additional phases.

Based on the microstructural investigations, it is evident that amorphous Cr-Al-C is gradually transformed to polycrystalline Cr2AlC MAX phase. As mentioned above, the solid phase transformation of amorphous to polycrystalline Cr2AlC MAX phase generally consists in two steps. First, solid-state diffusion promotes the formation of a disordered (Cr,Al)2CX solid solution. Second, local reorganization drives the conversion to ordered Cr2AlC MAX phase.

An important thermal activation is indeed required to promote element diffusion over important distances inducing the growth of highly ordered MAX phases [START_REF] Eklund | The Mn+1AXn phases: Materials science and thin-film processing[END_REF]. However, no direct evidence of the disordered solid solution was found here. In addition, the threshold temperature triggering the formation of the MAX phase by annealing an amorphous film is slightly lower compared to previously reported temperatures of around 600°C [START_REF] Abdulkadhim | Crystallization kinetics of amorphous Cr2AlC thin films[END_REF][START_REF] Stelzer | Remote Tracking of Phase Changes in Cr2AlC Thin Films by In-situ Resistivity Measurements[END_REF][START_REF] Li | Phase formation and microstructure evolution of arc ion deposited Cr2AlC coating after heat treatment[END_REF]. On the one hand, this difference may be caused by the extended annealing time as previously reported for a Ti2AlC [START_REF] Wang | Diffusion-controlled intercalation approach to synthesize the Ti2AlC MAX phase coatings at low temperature of 550 °C[END_REF]. On the other hand, it can be expected that the highly-energy conditions of HIPIMS process should also favor the growth of MAX phase from a disordered, metastable solid solution. This is because the major energy flux towards the growing thin film in HiPIMS is provided by Al + and Cr + ions [START_REF] Rueß | HPPMS deposition from composite targets: Effect of two orders of magnitude target power density changes on the composition of sputtered Cr-Al-C thin films[END_REF]. Thus, the increased ionization and ion energies of metal species in the HIPIMS plasma increase the reactivity of depositing species. This probably resulted in the formation of a homogeneous atomic mixture of Cr, Al and C, locally storing chemical energy. Similar to the reactive multilayer thin films [START_REF] Grieseler | Nanostructured plasma etched, magnetron sputtered nanolaminar Cr2AlC MAX phase thin films[END_REF][START_REF] Tang | Textured growth of polycrystalline MAX phase carbide coatings via thermal annealing of M/C/Al multilayers[END_REF], the stored chemical energy can be released to bolster the growth of the MAX phase during annealing. Hence, the formation of crystalline Cr2AlC without intermediate phases is believed to result from the initial atomic arrangement of the amorphous structure.

As mentioned above, Cr2AlC based-coatings are promising candidates as protective metallurgical coatings [START_REF] Ougier | Hightemperature oxidation behavior of HiPIMS as-deposited Cr-Al-C and annealed Cr2AlC coatings on Zr-based alloy[END_REF][START_REF] Imtyazuddin | Radiation resistance and mechanical properties of magnetron-sputtered Cr2AlC thin films[END_REF][START_REF] Mráz | Thermal stability enhancement of Cr2AlC coatings on Zr by utilizing a double layer diffusion barrier[END_REF]. So far, these coatings can be obtained only after hightemperature deposition or annealing, with the constraint that temperatures should not exceed metallurgical thresholds. This is especially critical in the case of Zr alloys used for cladding of nuclear fuel. In our case, an annealing treatment at 550°C is a good compromise, as it allows for the formation of a partially crystalline layer without inducing micro-cracking, maximizing protection against O diffusion [START_REF] Ougier | Hightemperature oxidation behavior of HiPIMS as-deposited Cr-Al-C and annealed Cr2AlC coatings on Zr-based alloy[END_REF]. Nevertheless, residual stresses are still expected to be present within the coatings, which could facilitate cracks formation and their propagation for higher temperatures like in accident regime, but also at lower temperatures. Hence, stresses induced by the deposition and by the thermal annealing should be quantified. An adjusted deposition process coupled with an architectured coating might emerge from these future results.

Conclusions

In this study, HiPIMS and thermal annealing were used to synthesize coatings of Cr2AlC MAX phase on Zr-based substrate. Dense and amorphous Cr-Al-C films were deposited at room temperature. An amorphous structure and film compositions close to the target composition are achieved independently of the duty cycle. However, an increase in the substrate bias UB up to -200 V leads to Al deficiencies up to 15 %, due to the preferential resputtering of Al triggered by the high energetic bombardment of the surface with ionized inert gas and metal plasma species.

The formation of Cr2AlC MAX phases was confirmed after annealing at 550°C for 4 h. The annealed coating has a double-layered structure, with an amorphous inner layer and a crystalline Cr2AlC outer layer. Annealing at 650°C resulted in a simple single-layered crystalline Cr2AlC coating but led to micro-cracking. Hence, with respect to protective applications, a lower annealing temperature should be preferred. These results suggest that the synthesis temperature of MAX phase could be reduced while the annealing time increased and support a valid alternative strategy to fabricate MAX phase coatings. This two-step process has been implemented for investigations of protective coatings against high temperature oxidation of Zr-based substrates [START_REF] Ougier | Hightemperature oxidation behavior of HiPIMS as-deposited Cr-Al-C and annealed Cr2AlC coatings on Zr-based alloy[END_REF].

List of figure captions

Fig. 1 :

 1 Fig. 1: Al concentration and Cr/C ratio of Cr-Al-C films deposited by HiPIMS on SS304L at (a) various duty cycles for negatively biased substrates (-50 V) and (b) at different substrate bias potentials with a fixed duty cycle (4.35 %). The dashed line indicates the Cr/C ratio of stoichiometric Cr2AlC.

Fig. 2 :

 2 Fig. 2: Cr-Al-C deposition rate, ionized metal flux fraction (IMFF) and ratio of ionized over neutral species (Ri/Rn) in HiPIMS with varying bias voltage UB at a fixed duty cycle (4.35 %).

Fig. 3 :

 3 Fig.3: X-ray diffraction patterns of the as-deposited Cr-Al-C coatings on Zr702 and annealed at 500°C, 550°C, 600°C and 650°C along with indexed peaks of Cr2AlC MAX phase (JCPDS card 00-058-0267). C stands for a carbon-based contamination and S for the substrate.

Fig. 4 :

 4 Fig.4: Raman spectra of the as-deposited Cr-Al-C coatings on Zr720 and annealed at 500°C, 550°C, 600°C and 650°C.

Fig. 5 :

 5 Fig. 5: Surface morphologies of the Cr-Al-C coatings deposited on Zr702 and annealed at (a) 500°C, (b) 550°C, (c) 600°C and (d) 650°C.

Fig. 6 :

 6 Fig. 6: Elemental GDOES profiles of the Cr-Al-C coatings grown on Zr702: (a) as-deposited, (b) annealed at 550°C, (c) 600°C and (d) 650°C.

Fig. 7 :

 7 Fig. 7: (a) Bright-field overview TEM image of the partially crystallized Cr2AlC coating on the Zr702 substrate annealed at 550°C for 4 h. Enlarged view and corresponding SAED pattern of the (b, c) outer and (d, e) inner layers.

Fig. 8 :

 8 Fig. 8: TEM observation of the crystallized Cr2AlC coating on the Zr702 substrate annealed at 650°C for 4 h. (a, b) Bright-field images of the coating at various magnifications, (c) representative SAED pattern of the coating and (d) of the circled area.

Fig. 9 :

 9 Fig. 9: (a) Bright-field TEM image and SAED pattern of the multi-layer structure in the Cr-Al-C coating annealed at 650°C and (b, c) the corresponding EDX mapping of Cr and Al.

Fig. 10 :

 10 Fig. 10: XPS spectra of (a) Cr 2p, (b) Al 2s, and (c) C 1 s for as-deposited and annealed Cr-Al-C thin films at 550°C.

  Figure 3

Figure 10

 10 Figure 10

Acknowledgements

This work was partially funded by the CEA Cross-Cutting Program on Materials and Processes Skills, France. J. Varlet and C. Maurin are thanked for their assistance in sample preparation. The authors warmly acknowledge C. Josse and A. Pugliara (UMS 3623 Raimond Castaing, Toulouse, France) for the preparation of TEM samples by FIB and for TEM analyses, respectively. The authors want to acknowledge M. Tabarant and F. Miserque for their contribution in GDOES/XPS analysis and constructive discussions.

List of figures