Abdelkrim Kehal

Nadia Saoula
email: nsaoula@cdta.dz

Seddik-El-Hak Abaidia

Corinne Nouveau

Effect of Ar/N 2 flow ratio on the microstructure and mechanical properties of Ti-Cr-N coatings deposited by DC magnetron sputtering on AISI D2 tool steels

Keywords:

Growing stage

A B S T R A C T Ti-Cr-N coatings were deposited on Si (100) and AISI D2 tool steel substrates by reactive DC magnetron cosputtering technique from titanium and chromium target in mixed Ar/N 2 atmosphere. The Ar/N 2 ratio effects on the chemical composition, structure, morphology, intrinsic stress and mechanical properties of the Ti-Cr-N coatings were investigated. The growing process of Ti-Cr-N coatings can be divided into three stages: Stage I, in poisoning mode (low flow ratio 1 < Ar/N 2 ≤ 1.4), Stage II, in transition mode (intermediate flow ratio 1.4 ≤ Ar/N 2 ≤ 3) and Stage III in metallic mode (Ar/N 2 > 3). For all samples, XRD analysis shown the formation of mixed nitrides phases. In stage I, Ti 2 N, TiN 0.3 , and hexagonal-Cr 2 N phases were observed. In Stage II, TiN 0.3 , Cr 2 N, and cubic-TiN phases were formed, while only TiN and Cr 2 N are observed in stage III. The coatings deposited with Ar/N₂ ratio of 3 shows the largest hardness of 24 GPa which is attribute to the dense structure and smoother surface morphology. The properties of the films are discussed in terms of evolution growth stages resulting by the variation of Ar/N 2 flow ratios.

Introduction

Binary hard coatings of metal nitrides like TiN and CrN are commonly used as protective coatings in industry because of their high hardness, low friction coefficient and their ability to extend tool life during abrasive and chemical aggressive conditions [START_REF] Zhang | Fatigue and mechanical behavior of Ti-6Al-4V alloy with CrN and TiN coating deposited by magnetic filtered cathodic vacuum arc process[END_REF][START_REF] Paksunchai | Effect of sputtering current on structure and morphology of (Ti 1-xCrx)N thin films deposited by reactive unbalanced magnetron co-sputtering[END_REF]. However, the performance and reliability of these nitrides do not meet the requirements for specific applications such as in high speed machining, dry cutting, drilling and jet engine turbine blades [START_REF] Paksunchai | Effect of sputtering current on structure and morphology of (Ti 1-x Cr x) N thin films deposited by reactive unbalanced magnetron co-sputtering[END_REF]. As a result, ternary nitride coatings such as Ti-Cr-N have attracted great attention due to the combined properties of TiN and CrN. Several research groups [START_REF] Mendoza-Leal | Tribological performance of ternary TiMN films (M=AL, B, and Cr) deposited by cathodic arc on M2 steel[END_REF][START_REF] Taweesup | Corrosion behavior of Ti-Cr-N coatings on tool steel substrates prepared using DC magnetron sputtering at low growth temperatures[END_REF] have shown that the Ti-Cr-N system has good mechanical properties and good chemical stability. Indeed, it has been reported that CrN has a higher corrosion resistance than TiN [START_REF] Suo | Corrosion behaviour of TiN and CrN coatings produced by magnetron sputtering process on aluminium alloy[END_REF]; Ti-Cr-N is more resistant to corrosion in artificial seawater compared to CrN [START_REF] Kong | Influence of titanium or aluminum doping on the electrochemical properties of CrN coatings in artificial seawater[END_REF]. Mendoza-Leal et al. [START_REF] Mendoza-Leal | Tribological performance of ternary TiMN films (M=AL, B, and Cr) deposited by cathodic arc on M2 steel[END_REF] found that the coefficient of friction Ti-Cr-N decreases significantly with Cr content. However, with the addition of Cr into TiN to form Ti 0.5 Cr 0.5 N coating, the coefficient of friction decreases significantly of 50% than that of TiN [START_REF] Mendoza-Leal | Tribological performance of ternary TiMN films (M=AL, B, and Cr) deposited by cathodic arc on M2 steel[END_REF]. Nainaparampil et al. [START_REF] Nainaparampil | Formation and characterization of multiphase film properties of (Ti-Cr)N formed by cathodic arc deposition[END_REF] attributed this low friction coefficient to the plastic deformation of the wear debris when the TiN and CrN were combined into a single film of Ti-Cr-N. Similar results have been reported in the literature where it has been shown that the ternary nitride Ti-Cr-N coating provides the best wear properties [START_REF] Quazi | Scratch adhesion and wear failure characteristics of PVD multilayer CrTi/CrTiN thin film ceramic coating deposited on AA7075-T6 aerospace alloy[END_REF]. In terms of mechanical properties, it has been observed that the Ti-Cr-N compound coating has higher hardness than that of TiN (21 GPa) and CrN (19 GPa) coatings [START_REF] Taweesup | Corrosion behavior of Ti-Cr-N coatings on tool steel substrates prepared using DC magnetron sputtering at low growth temperatures[END_REF]. The transition from binary CrN to ternary Ti 0.2 Cr 0.4 N 0.4 leads to an increase in hardness from 19 to 24 GPa. With further increase in Ti content, the hardness of ternary Ti 0.4 Cr 0.2 N 0.4 reaches a maximum of 26 GPa [START_REF] Taweesup | Corrosion behavior of Ti-Cr-N coatings on tool steel substrates prepared using DC magnetron sputtering at low growth temperatures[END_REF] In addition to the chemical composition effect, the mechanical properties of ternary nitride-coated steel are influenced by the structure of the coating. For example, it has been shown that the synthesis of ternary coatings generates multiphase structures often resulting in fine-grained and distorted structures [START_REF] Hones | Sputter deposited chromium nitride based ternary compounds for hard coatings[END_REF] and provides ternary nitrides with higher mechanical properties than binary nitrides [START_REF] Huang | Microstructure and properties of TiCrN coatings by arc ion plating[END_REF].

According to Musil et al. [START_REF] Musil | Hard and superhard Zr-Ni-N nanocomposite films[END_REF] and Vishnyakov et al. [START_REF] Vishnyakov | Ion assisted deposition of titanium chromium nitride[END_REF], the microstructure and hardness of the ternary hard coatings are sensitively affected by the energy and density of the ion flux of the plasma. The ion flux consists mainly of Ar + and/or N 2+ ions, whereas, in the case of the DC sputtering, the ionization rate of the sputtered species is typically less than 1% [START_REF] Sarakinos | High power pulsed magnetron sputtering: a review on scientific and engineering state of the art[END_REF]. This indicates that Ar/N 2 flow ratio is expected to be a key factor, which affects not only the film's stoichiometry but also its structure and mechanical properties of the films. Few research groups have reported the effect of the main plasma parameters, such as the effect on the mechanical properties of the Ti-Cr-N films of the variation in discharge power [START_REF] Vishnyakov | Ion assisted deposition of titanium chromium nitride[END_REF][START_REF] Chen | Investigation of the properties of TixCr1-xN coatings prepared by cathodic arc deposition[END_REF][START_REF] Feng | Effect of Ti content on structure and mechanical properties of Cr-Ti-N films[END_REF] or the partial pressure of the N 2 flow [START_REF] Huang | Microstructure and properties of TiCrN coatings by arc ion plating[END_REF][START_REF] Kosari Mehr | Nanoindentation and nanoscratch studies of submicron nanostructured Ti/TiCrN bilayer films deposited by RF-DC co-sputtering method[END_REF][START_REF] Abbas Kosari Mehr | Raman and ultraviolet-visible spectroscopy of titanium chromium nitride thin films[END_REF]. However, to our knowledge, there is no study on the evolution of the properties of Ti-Cr-N thin film related to the Ar/N 2 flow ratio. The effect of this parameter on the properties of the ternary nitride Ti-Cr-N film may be studied. For this purpose, Ti-Cr-N coatings were deposited on Si and AISI D2 substrates using DC magnetron sputtering.

During these experiments, we kept all the process parameter constant while varying Ar/N 2 flow ratio. The effect of the Ar/N 2 ratio on the evolution of the deposition rate, structure, chemical composition, and mechanical properties of sputtered Ti-Cr-N coatings were investigated systematically.

Experimental

Samples preparation

In this work, silicon wafers (100) (1 × 1 cm 2 , 450 μm) and AISI D2 steel (15 × 15 × 5 mm 3 ) were used as substrates. The chemical composition of AISI D2 steel is 1.6C, 0.35 Mn, 12 Cr, 0.25 Si, 0.5 Ni, 0.9 Mo, 0.5 V and Fe balance, in weight percent (wt%). The steel samples were subjected to heat treatment in order to increase their hardness. A Rockwell C hardness of 55 HRC was obtained after the heat treatment, and the working faces of the substrates were polished with silicon carbide papers ranging from (180 to 800 grit). The average surface roughness of the polished substrates was Ra = 90 nm. Before the deposition process, the substrates were cleaned and rinsing in an ultrasonic bath of acetone and then dried. Then, the substrates were loaded into the rotating substrate holder inside the deposition chamber.

Deposition of coatings

The Ti-Cr-N coatings were deposited by DC reactive magnetron sputtering (KENOSISTEC -model KS40V -113K12). Ti-Cr-N films were obtained by co-sputtering of titanium and chromium targets (with a purity of 99.9% and a size of 408 × 127.5 × 7.5 mm 3) . High purity Ar (99.999%) and N 2 (99.999%) gas were introduced during the deposition process. The distance between the targets and the vertical rotating substrate-holder is 105 mm. The substrate-holder rotates in an oscillatory motion between the target of Ti and Cr at a speed of 3 rpm. The number of oscillations for each deposition was fixed to 151 cycles, resulting in uniform coating thickness. The chamber was evacuated to a base pressure of ~1.5 × 10 -6 mbar. In order to remove the surface oxide or any contamination, the targets and the substrates were cleaned by ion etching in Ar plasma discharge.

The sputtering conditions were set at two different applied powers: 1500 W and 3000 W for the chromium and titanium targets, respectively. The deposition time was 22 min. The temperature of the substrate during the deposition was about 100-140 • C. Different Ar/N 2 gas mixtures were used and the total working pressure was fixed to 3 × 10 -3 mbar.

Coatings characterization

The coatings were analysed to investigate their structural, morphological and mechanical properties. X-ray diffraction (XRD) and Raman spectroscopy were used to identify the structure of the coatings. The Xray diffraction was carried out using a Philips System X'Pert diffractometer with CuKα radiation (λ = 1.541 Å) in Bragg-Brentano configuration (θ-2θ). The diffractometer acquiring range is between 15 • and 90 • , using a scanning speed of 3.55 • /min and a precision of 0.03 • . The Raman spectra of the coatings were recorded on a Horiba LabRAM HR spectrometer using the 633 nm line of Ar + Laser as an excitation source. The spectra were recorded over the range of 50 to 1300 cm -1 .

The surface morphology of the coatings was observed with a JEOL field emission scanning electron microscope FE-SEM (JEOL JSM 7610F). The composition of the coatings was determined by dispersive energy Xray spectroscopy (EDS operated at 5 kV). The coating's thicknesses were measured from cross sectional scanning electron microscopy (SEM) micrographs. The surface roughness of the films was determined by the MFP-3D from Asylum Research atomic force microscopy (AFM).

To determine the residual stress, thin rectangular sheets of silicon Si (100) (25 × 10 × 0.38 mm 3 ) were used as substrates. The curvature radius was measured by the optical profilometer (VEECO, Wyko NT-1100) before and after the Ti-Cr-N deposition. The values and the directions of the curvature radii were obtained by Gwydion software, which determines the curvature of a surface with two orthogonal directions. The residual stress was then calculated using the modified Stoney formula [START_REF] Aissani | Influence of film thickness and Ar-N 2 plasma gas on the structure and performance of sputtered vanadium nitride coatings[END_REF].

The hardness of the nitride coatings was measured by a nanoindentation tester CSM module with Berkovich diamond indenter tip under a fixed load of 2 mN. In the nano-indentation test, the maximum penetration depth of the indentation should not exceed 10% of the thickness of the film to avoid the influence of the substrate. Multiple indentations were made at different locations on the film's surface. The hardness and Young's modulus were then determined by the Oliver and Pharr method [START_REF] Oliver | An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments[END_REF].

Results and discussion

Chemical composition and deposition rate

The chemical compositions of the Ti-Cr-N coatings determined by EDS analysis are listed in Table 1. The results confirm the presence of nitrogen, chromium and titanium in the coatings. It should be also noted that in all the coatings, the presence of oxygen (up to 11.5 at.%), which is possibly due to the adsorbed residual oxygen during deposition [START_REF] Jeng | Effects of substrate bias and nitrogen flow ratio on the resistivity, composition, crystal structure, and reflectance of reactively sputtered hafnium-nitride film[END_REF]. Also, the results indicates that increase in Ar/N 2 ratio results in decrease of the content of N atoms in the coatings from about 36 to 13.5 at.%. The Cr/Ti atomic ratio in the coatings stays nearly constant in the range of 0.87-1. [START_REF] Feng | Effect of Ti content on structure and mechanical properties of Cr-Ti-N films[END_REF].

Fig. 1 shows that the nitrogen ratio to metallic species (N/Ti + Cr), decreases with the increase of the Ar/N 2 ratio. Fig. 2 shows the correlation between the deposition rate and the concentration of nitrogen and oxygen as a function of the Ar/N 2 ratio. The amount of oxygen observed in all samples of the coatings is relatively constant, except for the coatings deposited in the atmosphere of Ar/N 2 = 2 where the samples contain a higher amount of oxygen. The cross-section analysis (Table 1) clearly show that the thickness increased with the Ar/N 2 ratio, from 750 nm to 1310 nm. This ratio has a significant effect on the deposition rate. This effect is a well-known for the reactive sputtering process. Indeed, at the lower Ar/N 2 , the process deposition is in poisoning mode. With the increase of Ar/N 2 flow ratio, the overall sputtering yield of the target increases. Under these conditions, the process is in metallic mode [START_REF] Alaksanasuwan | Preparation and characterization of nanostructured TiCrN thin films deposited from Ti-Cr mosaic target by reactive DC magnetron sputtering[END_REF] and the deposition rate of metallic-mode is nearly 1.76 times that of poisoned-mode. There is also an intermediate Ar/N 2 regime (from 1.4 to 3) that indicates a simple transition between the metallic and poisoned modes. From these observations, the composition of the film is the result of the competition between two processes: (i) the quantity of metallic species (Ti + Cr) arriving from the target related to its sputtering yield, and (ii) the flow rates of reactive gases Ar/N 2 . To illustrate this dependence, the growing process of Ti-Cr-N coatings can be divided into three stages (Fig. 2). Stage I corresponds to a high nitrogen concentration with a low deposition rate and a low oxygen concentration. Stage II corresponds to an intermediate nitrogen concentration and deposition rate with a high oxygen concentration; and Stage III corresponds to a low nitrogen concentration, a high deposition rate, and a low oxygen concentration. This behaviour can be defined in more details as follows:

Stage I corresponds to the lower gas ratio (from Ar/N 2 = 1 to 1.4) . Two phenomena predominate. Firstly, the high collision frequency of particles species in the plasma caused by the high molecular diameter of N 2 (Ф = 3.14 Å) or N 2+ ions in gaseous phase compared to the Ar atoms (Ф = 1.76 Å) or Ar + ions (Ф = 1.54 Å) [START_REF] Dubey | Effect of nitrogen flow rate on structural and mechanical properties of Zirconium Tungsten Nitride (Zr-W-N) coatings deposited by magnetron sputtering[END_REF]. Secondly, the formation of a ceramic compound on the target surface (poisoning mode deposition) [START_REF] Safi | Recent aspects concerning DC reactive magnetron sputtering of thin films: a review[END_REF]. Under these conditions, the high amount of N 2 molecules induces more collisions, which leads to increase the molecules dissociation and the formation of N + ions. These finally will be implanted into the film [START_REF] Castillo | The role of the nitrogen flow rate on the transport properties of CrN thin films produced by DC magnetron sputtering[END_REF]. Consequently, the deposition rate is lower in this first stage, because of the poisoning of the target. For the presence of oxygen, this element is at its lower level. Thus, the efficiency of removal of the surface oxygen by the nitrogen ions in this regime is almost identical to the ions flux dominated by the argon ions gas as mentioned by Vishnyakov et al. [START_REF] Vishnyakov | Ion assisted deposition of titanium chromium nitride[END_REF].

Stage II (from Ar/N 2 = 1.4 to 3) corresponds to the transition mode (intermediate in the values of the deposition rate and nitrogen concentration). The mean free path of the sputtering argon gas increases as the nitrogen flow rate decreases [START_REF] Castillo | The role of the nitrogen flow rate on the transport properties of CrN thin films produced by DC magnetron sputtering[END_REF]. Under these conditions, the collision frequency between sputtered particles and sputtering gas molecules is lower. The deposition rate was found almost constant. According to Patsalas et al. [START_REF] Patsalas | Surface kinetics and subplantation phenomena affecting the texture, morphology, stress, and growth evolution of titanium nitride films[END_REF] increasing ion flux (below the subplantation threshold) can in certain conditions decrease the deposition rate and provide enough time for the migration of adatoms and so increases their surface diffusion length without increasing the thickness. The content in nitrogen decreases from 35 to 22.5% while the oxygen content increases from 5 to 11.5%. These results are consistent with the fact that in the pressure range studied in this stage, the sputtering yield increases with the pressure, but the pressure is not high enough to promote a denser film with vertical growing and less oxygen contamination. However, the sputtering pressure is well known to be efficient in inhibiting the adsorption of oxygen if it is used at an appropriate level of flow rate.

In the stage III, the flow and energy of ions flux of sputter-ejected species increase further as the Ar/N2 ratio (>3) increases. As a result of the substantial increase in deposition rate, the adsorbed oxygen impurities can be easily re-sputtered by the incident ions particles, especially Ar + [21] however, the concentration of oxygen decreased to 6.2 at.% for Ar/N 2 = 4. Finally, since the chemical composition and velocity of the deposited species play a significant role in the film properties, these stages are important for the following discussion.

Structure analyses

The X-ray diffraction patterns of Ti-Cr-N films deposited using various Ar/N 2 ratios are shown in Fig. 3. All patterns display a broad peak at a diffraction angle of about 22 • , suggesting an amorphous-like phase. The XRD reflections indicate a presence of binaries nitrides of (Ti 2 N, TiN 0.3 and Cr 2 N) and their reflections are overlapped to a single large peak. This suggests a good solubility of these binaries nitrides as the Ar/N 2 ratio was increased [START_REF] Chernogor | The impact of Ni and Mo on growth-morphology and mechanical properties of arc evaporated Ti-Cr-N hard coatings[END_REF]. Furthermore, no peaks referring to the oxide phase were observed by XRD, implying that oxygen could be present in the grain boundaries and/or forming an amorphous oxide phase [START_REF] Vaz | Property change in ZrN x O y thin films: effect of the oxygen fraction and bias voltage[END_REF].

In stage I (from Ar/N 2 = 1 to 1.4), the Ti-Cr-N films contain predominantly a mixture of two rich metallic nitrides Ti 2 N (112) and Cr 2 N (110) orientations, with a small volume fraction of another mixture of TiN 0.3 (110) and Cr 2 N (211) crystalline orientations. When the Ar/N 2 ratio increases to 1.4, the [TiN 0.3 + Cr 2 N] diffraction peak increase and the peak of [Ti 2 N + Cr 2 N] tends to disappear. Moreover, the width of the diffraction peak increases and its intensity decreases suggesting increased stresses in the coating and a decrease in the grains size [START_REF] Uglov | Composition and phase stability upon annealing of gradient nitride coatings[END_REF].

For the film deposited at the transition stage (Ar/N 2 = 2), the dominant texture is composed of cubic TiN (200) and hexagonal Cr 2 N (111) phases. The distortion mechanism, caused by the transition phase of the Ti-N in a mixture with Cr 2 N, led to the interstitial insertion of the nitrogen atoms into the hexagonal structure of TiN 0.3 (a = 2.9737, b = 2.9737, c = 4.7917 Å) resulting in a distorted fcc-TiN, i.e. with a large lattice parameter (a = b = c = 4.2417 Å) [START_REF] Lin | Effect of negative substrate bias voltage on the structure and properties of CrN films deposited by modulated pulsed power (MPP) magnetron sputtering[END_REF]. However, with dense grain boundaries this can indicate that compressive stress occurred within the coating at Ar/N 2 = 2.

At higher energies of incident ions of the third stage, (from Ar/N 2 = 3 to 4), the grains grow with the [TiN + Cr 2 N] predominated orientation. A clear increase in the width of the peak can be observed (Fig. 3), indicating a decrease in the grain size and an increase in defect density due to enhanced collisions of incident particles with the coating surface, which is likely to lead to the generation of high intrinsic stresses during the growing film [START_REF] Klima | Evolution of structure and residual stress of a fcc/hex-AlCrN multi-layered system upon thermal loading revealed by cross-sectional X-ray nano-diffraction[END_REF].

In all the films, it could be observed the chromium nitride Cr 2 N in the solid solution (Ti,Cr)N, whereas, the titanium nitride which coexists with Cr 2 N phase, evolves toward three phases: Ti 2 N, TiN 0.3 and TiN. This could be explained by the enthalpy formation of the nitride of Cr and Ti (ΔH TiN = -337.65 kJ/mol and ΔH CrN = -117.15 kJ/mol), which shows that the Ti-N is easier to be formed than Cr-N [START_REF] Paksunchai | Growth and Characterization of Nanostructured TiCrN Films Prepared by DC Magnetron Cosputtering[END_REF].

Raman spectroscopy

In order to confirm and obtain additional information about the different observed phases in XRD patterns, the structure of Ti-Cr-N coatings was analysed by Raman spectroscopy in the range between 50 and 1200 cm -1 . The Raman spectra of the obtained coatings are presented in Fig. 4.

The Ti-Cr-N Raman spectrum is similar to those collected for Ti-N [START_REF] Kainz | Microstructure and mechanical properties of CVD TiN/TiBN multilayer coatings[END_REF] and Cr-N [START_REF] Qi | Effect of Sputtering Power on the Chemical Composition, Microstructure and Mechanical Properties of CrNx Hard Coatings Deposited by Reactive Magnetron Sputtering[END_REF] from 50 to 720 cm -1 . This indicates that the substitutional solid solution of Cr in TiN does not affect the Raman spectra significantly. A similar observation was reported for the Raman spectra of Ti-Cr-B-N [START_REF] Kiryukhantsev-Korneev | Thermal stability and oxidation resistance of Ti -B -N, Ti -Cr -B -N, Ti -Si -B -N and Ti -Al -Si -B -N films[END_REF] and Al-doped TiN films [START_REF] Barshilia | Raman spectroscopy studies on the thermal stability of TiN, CrN, TiAlN coatings and nanolayered TiN/CrN, TiAlN/CrN multilayer coatings[END_REF]. In addition, this confirms the results of the XRD patterns where the (Ti-N) system seems to be more sensitive to the variation of the Ar/N 2 ratio than the stable (Cr 2 N) phase.

However, the Raman spectrum of Ti-Cr-N film can be deconvoluted into six Gaussian peaks. Table 2 gives the derived wavenumbers for the TA, LA, TO or LO modes of Ti-Cr-N coatings.

The low-frequency scattering caused by acoustical phonons (TA and LA modes) is related to vibrations of the heavier metal atoms, while optical range scattering (TO and LO) refers to vibrations of the lighter non-metal ions [START_REF] De Freitas | Influence of the plasma nitriding conditions on the chemical and morphological characteristics of TiN coatings deposited on silicon[END_REF]. Furthermore, an increase in the intensity of the optical phonon band suggests the formation of metallic vacancies, while an increase in the intensity of the acoustical band indicates a nitrogen vacancy [START_REF] Ponon | Effect of deposition conditions and post deposition anneal on reactively sputtered titanium nitride thin films[END_REF]. In comparison to the binary TiN and CrN, the Raman spectra of the Ti-Cr-N coating show the existence of a broad peak with low intensity corresponding to the second-order transitions (A + O) at the range of 740 to 795 cm -1 , which is in good agreement with those reported by Mehr et al. [START_REF] Abbas Kosari Mehr | Raman and ultraviolet-visible spectroscopy of titanium chromium nitride thin films[END_REF]. At 52 cm -1 , all spectra present a very weak line that corresponds to the laser plasma line. This line is used to calibrate the frequency of the Raman shift [START_REF] György | Structural, morphological and local electric properties of TiO2 thin films grown by pulsed laser deposition[END_REF].

The study of the intensity ratio of the nitrogen peak to metallic peak I = (TO + LO) / (TA + LA) is a semi-quantitative method to analyse the variations in vacancy concentrations [START_REF] Ponon | Effect of deposition conditions and post deposition anneal on reactively sputtered titanium nitride thin films[END_REF]. It is obvious from Fig. 5 that the Raman spectra of the different films are affected by the flux of ions and the ion energy caused by the variation of the Ar/N 2 ratio. In addition, Fig. 5 show that the global tendency of the scattering intensity decreases as a function of the increase of the Ar/N 2 ratio. This indicates that the crystalline phase is decreasing [START_REF] Kiryukhantsev-Korneev | Thermal stability and oxidation resistance of Ti -B -N, Ti -Cr -B -N, Ti -Si -B -N and Ti -Al -Si -B -N films[END_REF][START_REF] Kaiser | Raman scattering, superconductivity, and phonon density of states of stoichiometric and nonstoichiometric TiN[END_REF], while the metallic phase is increasing continuously with the Ar/N 2 ratio. This is in perfect consistency with chemical results (Table 1). Due to the high reflectivity of the metallic surfaces, the incident light penetration depth limits the Raman scattering in the case of the metallic materials; which consequently reduces the intensity in the Raman spectra as the Ar/N 2 flux increases [START_REF] Cheng | Raman spectroscopy and x-ray diffraction studies of (Ti,Al)N films deposited by filtered cathodic vacuum arc at room temperature[END_REF]. The variation of the intensity ratio I = (TO + LO) / (TA + LA) can be explained as follows: in stage I, the intensity ratio I = (TO + LO) / (TA + LA) changes slightly from 0.8415 (at Ar/N 2 = 1) to 0.8293 (at Ar/N 2 = 2) which corresponds to a slight decrease in nitrogen content from 36 to 35 at.% (Table 1). In the second stage (Fig. 5), the intensity ratio I = (TO + LO) / (TA + LA) reached the minimum value of 0.7074 at Ar/N 2 = 2, which corresponds to a higher reduction in nitrogen content (from 35 to 22 at.%) and the (Ti + Cr) increased continuously with the Ar/N 2 ratio (Table 1). However, it should be noted that the intensity ratio of I = (TO + LO) / (TA+LA) = 0.7074 at the Ar/N 2 = 2, should normally be at intermediate values between 0.8293 and 0.7734 in order to express the continuous decrease of the nitrogen content analysed by EDS (Table 1). The intensity ratio I = (TO + LO) / (TA + LA) of 0.7074 represents the transition zone between the poisoning mode and the metallic mode as a function of the Ar/N 2 ratio. For the third stage (Fig. 5), the intensity ratio I = (TO + LO) / (TA + LA) increases further more at Ar/N 2 = 4 while we observe an opposite tendency with the nitrogen content. An increase in the nitrogen peak Raman intensity will imply the formation of metallic vacancies, as described above. This can be explained by the increase of the concentration of titanium vacancy in the films because of the preferential resputtering phenomenon of Ti observed at the higher ions flux (gas ratio = 4) (Table 1). Under these conditions, the Ti vacancies with N atoms as their first nearest neighbours, contribute to the rise in the optical scattering [START_REF] Kaiser | Raman scattering, superconductivity, and phonon density of states of stoichiometric and nonstoichiometric TiN[END_REF], which is observed in Fig. 4e.

Microstructure and morphology

The microstructure and surface morphology of the Ti-Cr-N coatings on (Si) substrates are shown in Fig. 6. The SEM micrographs show that the coatings have the typical columnar structure that is expected for PVD coatings. The cross-section SEM images of the Ti-Cr-N coating show a clear evolution of the thickness with the Ar/N 2 ratio. The surface morphology evolves into two main forms: from 1 to 1.4 of Ar/N 2 , the surface morphology is pyramidal (Fig. 6f andg) and presents the greatest surface roughness RMS = 4.4 nm at Ar/N 2 = 1.4. When the gas flow ratio increases from 2 to 4, the coatings exhibit a cauliflower-like structure (Fig. 6h, i andj) and smoother surface with a low surface roughness of RMS = 1.9 nm obtained at Ar/N 2 = 3.

Based on the three stages of transition mentioned above, the evolution of the morphology and microstructure of the Ti-Cr-N coatings can be interpreted as follows:

Stage I (Ar/N 2 = 1 to 1.4): the evolution of the growing structure is governed by the coarsening of the columns structure, resulting in deep cups between columns and open column boundaries in combination with the atomic shadowing effect [START_REF] Lin | Effect of asynchronous pulsing parameters on the structure and properties of CrAlN films deposited by pulsed closed field unbalanced magnetron sputtering (P-CFUBMS)[END_REF]. For this stage, a slight increase in roughness from 3.8 nm to 4.4 nm was noticed (Fig. 7). The width of the columnar grains increases with a faceted surface. At this early stage, we can see that the thickness increases by more than 30% from 750 to 1088 nm (Fig. 6a andb). However, this behaviour can be related to the limited penetration of particles at the top of the layer [START_REF] Patsalas | Surface kinetics and subplantation phenomena affecting the texture, morphology, stress, and growth evolution of titanium nitride films[END_REF], which could be due to the insufficient ion energy to densify the surface growing film.

At the second stage, from Ar/N 2 = 1.4 to 3; the roughness decreases when the flow ratio increases. This indicates the beginning of the densification and smoothening of the films. The transition from roughening mechanism, due to the limited mobility of surface defects (Stage I) [START_REF] De Freitas | Influence of the plasma nitriding conditions on the chemical and morphological characteristics of TiN coatings deposited on silicon[END_REF] to the smoothing mechanism controlled by the surface diffusion effect (Stage II) [START_REF] Wang | Bias effect on microstructure and mechanical properties of magnetron sputtered nanocrystalline titanium carbide thin films[END_REF] is clearly demonstrated by the disappearance of the faceted surface (Fig. 6h). Both of these opposite effects of rougheningsmoothing are associated with surface energy, as the thin film growth the system tends to form the most energetically favourable configuration [START_REF] Liu | Atomic force microscopy study of surface roughening of sputter-deposited TiN thin films[END_REF]. However, the increase in the Ar/N 2 ratio induces a higher surface diffusion that allows the adatoms to avoid the direct sticking on the top of the columns and diffuse superficially and efficiently, filling in more the voids between them, resulting in the disappearance of the faceted surface [START_REF] Vaz | Property change in ZrN x O y thin films: effect of the oxygen fraction and bias voltage[END_REF]. Compared to the first stage, where the deposition rate lags the process of the surface diffusion [START_REF] Liu | Stress and structural studies of ZnO thin films on polymer substrate under different RF powered conditions[END_REF], the slight decrease in thickness (deposition rate) at the second stage (Ar/N 2 = 2) (Fig. 6h) indicates the beginning of the densification of the film under the increase in ion bombardment [START_REF] Zou | Characterization and properties of CrN films deposited by ion-source-enhanced middle frequency magnetron sputtering[END_REF]. The smoother surface obtained at Ar/N 2 = 3, indicates that the optimum or/and the critical energy delivered to the growing film per deposited particle was reached [START_REF] Griepentrog | Properties of TiN hard coatings prepared by unbalanced magnetron sputtering and cathodic arc deposition using a uni-and bipolar pulsed bias voltage[END_REF]. According to Dubey et al. [START_REF] Dubey | Effect of bias induced microstructure on the mechanical properties of nanocrystalline zirconium tungsten nitride coatings[END_REF] the bombardment of the growing film surface with a certain critical threshold of kinetic energy during the deposition process of a sputter atom results in the non-penetration of ions beyond the first atomic layer. Then their energy is essentially transferred to the adatoms, thereby increasing the surface mobility, the surface diffusion reaching its maximum and decreasing the surface roughness to its lower RMS = 1.9 nm.

At the third stage, Ar/N 2 > 3, the sputtered atoms induced by the increase of the gas ratio to 4 arrive at the surface of the growing film with enhanced energy, corresponding to a change from smoothing to a roughening mechanism. The surface roughness RMS increases from 1.9 to 2.7 nm, which could be attributed to the bulk displacements of adatoms and the re-sputtering of the near-surface atoms [START_REF] Dubey | Effect of bias induced microstructure on the mechanical properties of nanocrystalline zirconium tungsten nitride coatings[END_REF]. The result of chemical analysis (Table 1) confirms that the process of growing film under this stage was enlarged by preferential re-sputtering of the lighter

Table 2

Summary of assignments for the acoustic and optic phonon modes (cm -1 ) and relative intensity ratio of I (TO+LO) / I (TA+LA) for the Ti-Cr-N films. titanium atom [START_REF] Wolfe | Nanolayer (Ti,Cr)N coatings for hard particle erosion resistance[END_REF], which decreases the content of titanium from 39 to 37 at.% at Ar/N 2 = 4, resulting in surface defects and roughening of the film surface.

Residual stress

The Stoney formula was used to calculate the residual stress σ in the coatings:

σ = 1 6 ( 1 Ra - 1 Rb ) Et 2 s (1 -ν)t f (1)
where E, is the Young's modulus (181 GPa) of the substrate, ν the Poisson's ratio (0.28) of the substrate, t s the thickness of the substrate, t f the thickness of the film, R b and R a the curvature radii of the substrate before and after deposition, respectively [START_REF] Aissani | Influence of film thickness and Ar-N 2 plasma gas on the structure and performance of sputtered vanadium nitride coatings[END_REF].

As shown in Fig. 8, the stress in all films is compressive, and tends to be proportional to the deposition rate, except at Ar/N 2 = 3, then it increases for a flow ratio = 4.

According to the results obtained by Stoney formula, the increase in the residual compressive stress is more affected by the deformed shape related to the curvature than the conthing's thickness. These two dimensional factors (thickness and the radius of curvature) are inversely proportional in our study, i.e. that the induced compressive surface stress pulls the surface outwards, making the curvature of film/substrate system more convex; the radii of the convex shape of substrate tends to decrease, which leads to increase further more the residual compressive stress with the higher atomic mobility induced by the increase in Ar/N 2 ratio.

However, the total residual stress can be considered as a result of dynamic competition between the tensile and compressive stress developed during growth of the film [START_REF] Jena | Effect of O2/Ar gas flow ratio on the optical properties and mechanical stress of sputtered HfO2 thin films[END_REF]. To understand the stress data, the morphology structure-stress relationship is studied fellow as function of the stage mentioned above:

Stage I (from Ar/N 2 = 1 to 1.4); the grain size and thickness of the films increase and the stresses are compressive. The increase of the column size caused by the coarsening grains boundary mechanism (Fig. 6a andb), usually induces tensile stress [START_REF] Shamsutdinov | The effect of Cu interlayers on grain size and stress in sputtered Fe-Cu multilayered thin films[END_REF], while the total intrinsic stress calculated by the Stoney formula is compressive (increases from -136 ± 0.1 to -162 ± 10 MPa at Ar/N 2 = 1.4). This proves that the tensile stress induced by the reduction of the grain boundaries was competed by another mechanism that generates a significant compressive stress state. According to the models proposed by Sheldon et al. [START_REF] Sheldon | Competition between tensile and compressive stress mechanisms during Volmer-Weber growth of aluminum nitride film[END_REF], the tensile and compressive mechanisms can be both operating during the film growth. Indeed, the tensile stress occurring at the early stage of the film coalescence and post-coalescence, can be compensated by the compressive stress induced by the insertion of extra atoms to the grain boundary leading to a compressive stress state. This can explain, that the final residual stress state of the 1088 nm thick film, is the result of the dominant contribution of the inserted extra atoms against the coarsening grain boundary mechanism during the film growth.

Stage II (from Ar/N 2 = 1.4 to 3); we observe an increase in compressive stress, from -162 ± 10 to -615 ± 8 MPa at Ar/N 2 = 2. This could be explained by the competition between the tensile stress induced by the increase in surface mobility of the adatoms [START_REF] Floro | The dynamic competition between stress generation and relaxation mechanisms during coalescence of Volmer-Weber thin films[END_REF] and the stress induced by the insertion of oxygen atoms into the grains boundary [START_REF] Zhao | The growth behavior and stress evolution of sputtering-deposited LaNiO3 thin films[END_REF]. However, the effect of the segregation mechanism of oxygen atoms on the grain boundary is dominant and tends to be contrasted with the tensile stress state by restricting the length of the surface diffusion at the very early stage of the film growth. The result is a compressive macrostress state in the film at Ar/N 2 = 2. Then, the increase in the ions flux of the sputtered atoms/ions at Ar/N 2 = 3 leads to reach the threshold energy of deposition. The surface diffusion becomes dominant which allows the films layers to grow in quasi-two-dimensional growth mode. Consequently, the adsorption of oxygen and the point defects were reduced. Indeed the annihilation of Frankel pairs or the annihilation of interstitials were found to be relaxed on the surface growing film [START_REF] Wen | Growth, stress and hardness of reactively sputtered tungsten nitride thin films[END_REF]. During this process, the distance between the columns leads to the development of attractive interatomic forces through the interstices, i.e. tensile stress in the film. Below the critical inter-column distance, the repulsive forces generated between the cylindrical structures will play a predominant role and the resulting residual stresses will remain in the compressive state [START_REF] Tranchant | Relation between residual stresses and microstructure in Mo(Cr) thin films elaborated by ionized magnetron sputtering[END_REF]. In other studies, this phenomenon of evolution in stress is called "grain boundary relaxation" or "grain boundary shrinkage" [START_REF] Tranchant | Relation between residual stresses and microstructure in Mo(Cr) thin films elaborated by ionized magnetron sputtering[END_REF].

Finally at the third stage (Ar/N 2 > 3), the oxygen impurities were reduced at a relatively constant level. The compressive stress at Ar/N 2 = 4 increase from -499 ± 10.6 to -798 ± 7 MPa, this can be related to knock-on (or atomic peening) processes [START_REF] Davis | A simple model for the formation of compressive stress in thin films by ion bombardment[END_REF]. According to the model of Davis et al. [START_REF] Davis | A simple model for the formation of compressive stress in thin films by ion bombardment[END_REF] and Windischmann et al. [START_REF] Windischmann | An intrinsic stress scaling law for polycrystalline thin films prepared by ion beam sputtering[END_REF], if the energy of the deposited particles species exceeds the threshold of the critical energy, the ions or atoms deposited at the surface will penetrate into the structure and result in the increased defects. In this case, the compressive stresses can be caused by the direct effect of the subplantation ion of bombarding species in the films and the displacement of atoms from a lattice of the film to interstitial positions [START_REF] Zhang | Effects of substrate bias voltage on the microstructure, mechanical properties and tribological behavior of reactive sputtered niobium carbide films[END_REF].

Mechanical properties

Hardness is defined as the resistance of the material to plastic deformation and is mainly sensitive to compressive stress, grain size, preferred orientation and defect density [START_REF] Huang | Effect of film thickness and Ti interlayer on the structure and properties of nanocrystalline TiN thin films on AISI D2 steel[END_REF]. The Young's modulus is considered as a measurement of the stiffness of the material and it's variation is consistent with the hardness of the coating [START_REF] Shi | Structural and tribological properties of CrTiAlN coatings on Mg alloy by closed-field unbalanced magnetron sputtering ion plating[END_REF]. The hardness (H), Young's modulus (E) and H 3 /E 2 ratio of the Ti-Cr-N coatings are shown in Fig. 9. All of the coatings are much harder than the uncoated AISI D2 substrate (6.4 GPa). The hardness of the coatings ranges from 10 to 24 GPa. The maximum hardness was achieved for the coating deposited at Ar/N 2 = 3. The same trend was observed for Young's modulus ranging from 190 to 255 GPa with a maximum observed for Ar/N2 = 3. Further increase in ions flux to Ar/N2 = 4, results in a decrease in hardness and Young's modulus to 15 GPa and 245 GPa, respectively. These values of hardness and Young's modulus are in agreement with the results obtained by other studies (Table 4) [START_REF] Feng | Effect of Ti content on structure and mechanical properties of Cr-Ti-N films[END_REF][START_REF] Hones | Sputter deposited chromium nitride based ternary compounds for hard coatings[END_REF][START_REF] Hsieh | Characterization of (Tix Cr0.6-x) N0.4 coatings and their tribological behaviors against an epoxy molding compound[END_REF][START_REF] Akbarzadeh | Characterization of TiN, CrN and (Ti, Cr) N coatings deposited by cathodic ARC evaporation[END_REF][START_REF] Wang | Evaluating mechanical properties and crack resistance of CrN, CrTiN, CrAlN and CrTiAlN coatings by nanoindentation and scratch tests[END_REF].

The evolution in hardness and Young's modulus of Ti-Cr-N films can be related to solid solution hardening, lattice distortion, densification/ strengthening mechanism of the grain boundaries and the nature of chemical bonding strengthening. The effect of these factors on the mechanical properties can be explained as follows:

At the first stage, from Ar/N 2 = 1 to 1.4, the minimum hardness was observed for the films with a similar amount of chromium and titanium (Table 1) obtained at Ar/N 2 = 1. Similar results were reported by Hones et al. [START_REF] Hones | Sputter deposited chromium nitride based ternary compounds for hard coatings[END_REF]. In terms of structure, this corresponds to the preferred orientation related to the coexistence of two metallic rich nitrides (Ti 2 N + Cr 2 N) phases. This (Ti 2 N + Cr 2 N) peak tends to disappear at a gas ratio of 1.4 when the peak intensity of the solid solution (TiN 0.3 + Cr 2 N) further increases. As a consequence, the hardness increases from 10 to 13 GPa at Ar/N 2 = 1.4 indicating the beginning of the hardening by the mixed multi-phase structure [START_REF] Hsieh | Characterization of (Tix Cr0.6-x) N0.4 coatings and their tribological behaviors against an epoxy molding compound[END_REF]. However, the ceramics in mixed phases are well known to have a higher hardness than each phase on its own [START_REF] Holmberg | Tribology of thin coatings[END_REF].

In the second stage, when the Ar/N 2 ratio is increased from 1.4 to 2 a higher strength of the material was observed. This may be due to the combined effect of two parameters. The first is a greater increase in compressive stress induced by impurity segregation of oxygen into the grain boundary, and the second is a distortion mechanism caused by the transition phase of the (Ti-N) in a mixture of Cr 2 N. However, the interstitial insertion of the nitrogen atoms into the hexagonal structure of TiN 0.3 results in a distorted fcc-TiN [START_REF] Lin | Effect of negative substrate bias voltage on the structure and properties of CrN films deposited by modulated pulsed power (MPP) magnetron sputtering[END_REF]. This mechanism accompanied by the expansion in lattice parameter contributes to the enhancement of hardness by a dislocation blocking effect [START_REF] Wang | Fabrication of CrAlN nanocomposite fi lms with high hardness and excellent anti-wear performance for gear application[END_REF]. As mentioned above, for the Ar/N 2 ratio of 3, the surface diffusion is at its maximum level. In this state, two processes can be produced and contribute to reach the maximum in mechanical properties of the film: (i) according to Carter et al. [START_REF] Carter | Surface roughening during ion-assisted film deposition[END_REF] the surface diffusional relaxation process leads to maximise neighbour bonding between atoms, whereas the decrease in the interatomic distance was related to the covalent band gap in an inversely proportional way. The higher energy of the covalent bonding led to stronger interatomic forces and contributes to the improvement in film hardness [START_REF] Cavaleiro | The Influence of the Addition of a Third Element on the Structure and Mechanical Properties of Transition-Metal-Based Nanostructured Hard Films: Part I-Nitrides[END_REF]; (ii) the second process is the reduction of the fraction of grain boundary voids, thereby strengthening the grain boundaries mechanism [START_REF] Zou | Characterization and properties of CrN films deposited by ion-source-enhanced middle frequency magnetron sputtering[END_REF]. Finally, the result is the improvement of the hardness of (TiN + Cr 2 N) structure by the densification of the film and strengthening mechanism of the grain boundaries.

In the third stage (Ar/N 2 > 3), the hardness dropped to 15 GPa. This can be explained by two factors: first, an increase in the metallic character of the bonding due to a decrease in nitrogen content and thus a decrease in its covalency [START_REF] Yu | Phase stability, chemical bonding and mechanical properties of titanium nitrides: a first-principles study[END_REF], and second, excessive densification caused by higher ion bombardment. According to Su et al. [START_REF] Su | Effects of bias voltage and annealing on the structure and mechanical properties of WC0.75N0.25 thin films[END_REF], the increase of the hardness in thin films is affected by high compressive stress. Therefore, proper stress may be required to obtain a higher hardness. Otherwise, the material may undergo a plastic flow, and a reduction in hardness will occur if the energy of incident ions exceeds the critical level of energy. Besides hardness and elastic modulus, the ratio H 3 /E 2 is also important to evaluate the resistance of the coating to plastic deformation in loaded contact [START_REF] Zegtouf | Influence of substrate bias voltage on structure, mechanical and corrosion properties of ZrO2 thin films deposited by reactive magnetron sputter deposition[END_REF]. From the nanoindentation data, the plastic deformation resistance (H 3 /E 2 ) of Ti-Cr-N films was calculated and the values are presented in Table 3. The (H 3 /E 2 ) is related to the surface plastic deformation response to sliding contact load, which was associated with the hardness [START_REF] Ou | Mechanical and tribological properties of CrN/TiN multilayer coatings deposited by pulsed dc magnetron sputtering[END_REF]. According to the nanoindentation data (Table 3), a higher hardness corresponds to a higher H 3 /E 2 ratio, which is an indication of higher plastic deformation resistance for the film obtained at Ar/N 2 = 3. As a result, high hardness and low elastic modulus are desirable, as it allows the load to be distributed over a wider area and therefore a good improvement wear resistance [START_REF] Macías | Effect of nitrogen flow ratio on microstructure, mechanical and tribological properties of TiWSiNx thin film deposited by magnetron co-sputtering[END_REF].

Conclusion

Titanium chromium nitride films were deposited on Si (100) and AISI D2 substrates using DC magnetron sputtering in a gas mixture of N 2 and Ar. The effects of the variation of the Ar/N 2 ratio on the chemical composition, structure, surface morphology, residual stress and mechanical properties of the Ti-Cr-N films were investigated.

Based on this study, the growing process of Ti-Cr-N coatings is divided into three distinct stages depending on the Ar/N 2 ratio. The hardness and Young's modulus were related to the structure evolution and compressive residual stress for the three stages of the film growth.

For the films prepared at low values of Ar/N 2 ratio (Stage I, Ar/N 2 = 1 to 1.4), the composition and structure reveal rich metallic nitrides films with a slight increase in residual stress, hardness and Young's modulus. The films corresponding to the intermediate gas ratio (Stage II, Ar/N 2 = 1.4 to 3) presented an increase of their hardness and Young's modulus with the Ar/N 2 ratio. Maximum hardness and Young's modulus of 24 GPa and 255 GPa were respectively reached at Ar/N 2 = 3 while the residual stress was relaxed and a smoother surface was observed. This optimum in properties is attributed to the combined effect of film densification and the strengthening mechanism of the grain boundaries. Finally, the films prepared with the highest gas ratio (Stage III, Ar/N 2 > 3), lead to the re-sputtering of the surface, an increase of the surface roughness and compressive residual stress. The dominant metallic character of the bonding due to the decrease in nitrogen content and hence, decreases its covalency, combined with the excessive densification induced by the higher ion bombardment, leads to a drastic decrease in hardness and Young's modulus of the film obtained for Ar/N 2 = 4. It has also been shown that the hardening caused by the densification of the structure with an optimum level of compressive residual stress contributed to a maximum of resistance to plastic deformation (H 3 /E 2 ).

Finally, this study demonstrated that the Ti-Cr-N coatings are very sensitive to the variation of the gas ratio Ar/N 2 and showed a wide range of mechanical properties.

Fig. 1 .

 1 Fig. 1. The N/(Ti + Cr) ratio of the Ti-Cr-N films deposited with different Ar/ N 2 flow ratios.

Fig. 2 .Fig. 3 .

 23 Fig. 2. Deposition rate, nitrogen and oxygen content deposited with different Ar/N 2 flow ratios.

Fig. 4 .

 4 Fig. 4. Raman spectra of Ti-Cr-N films deposited at the Ar/N 2 flow ratio a) 1, b) 1.4, c) 2, d) 3 and e) 4.

Fig. 5 .

 5 Fig. 5. Dependence of the intensities I (TA+LA) , I (TO+LO) and relative intensities I (TO+LO) / I (TA+LA) of Ti-Cr-N films on Ar/N 2 flow ratios.

Fig. 6 .

 6 Fig. 6. FE-SEM cross-section and surface morphology of the Ti-Cr-N coatings deposited with different Ar/N 2 flow ratios.

Fig. 7 .

 7 Fig. 7. Surface roughness RMS of the Ti-Cr-N films deposited with different Ar/ N 2 flow ratios.

Fig. 8 .

 8 Fig. 8. Residual stress of Ti-Cr-N films deposited with different Ar/N 2 flow ratios .

Fig. 9 .

 9 Fig. 9. Hardness, Young's modulus and plastic deformation resistance (H 3 /E 2 ) ratio of the Ti-Cr-N films deposited with different Ar/N 2 flow ratios.

Table 3

 3 Surface roughness and mechanical proprieties of Ti-Cr-N films. Ar/N 2 ratio Roughness (RMS) (nm) Hardness (H) (GPa) Young's modulus (E) (GPa) Plastic deformation resistance (H 3 /E 2 )

Table 1

 1 Film thickness, composition, metal concentration ratio, atomic ratio, and film stoichiometry as a function of Ar/N 2 ratio.

	Ar/N 2 ratio		Thickness (nm)	Elemental composition (at.%)		Atomic ratio		Film stoichiometry (Ti x Cr 1-x ) N y
				Cr	Ti	O	N	Cr + Ti	Cr/Ti	X = Ti/(Ti + Cr)	Y=N/(Ti + Cr)
	40/40	1.0	750	28.6	28.8	6.6	36.0	57.4	0.99	0.50	0.62	(Ti 0.50 Cr 0.50 ) N 0.62
	47/33	1.4	1088	28.0	32.0	5.0	35.0	60.0	0.87	0.53	0.58	(Ti 0.53 Cr 0.47 ) N 0.58
	54/26	2.0	1070	34.0	32.0	11.5	22.5	66.0	1.06	0.48	0.34	(Ti 0.48 Cr 0.52 ) N 0.34
	60/20	3.0	1200	35.4	39.0	6.8	18.8	74.4	0.90	0.52	0.25	(Ti 0.52 Cr 0.48 ) N 0.25
	64/16	4.0	1310	43.0	37.0	6.5	13.5	80.0	1.16	0.46	0.17	(Ti 0.46 Cr 0.54 ) N 0.17

Acknowledgements

The authors are grateful to the MESRS (Ministère de l'Enseignement Supérieur et de la Recherche Scientifique), of Algeria, for financial support. They also want to thank Mr. Khelladi, Mohamed Redha from LCIMN (Laboratoire de Chimie, Ingénierie Moléculaire et Nanostructures) of University Ferhat-Abbas, Sétif in Algeria, for his technical expertise.

CRediT authorship contribution statement

Abdelkrim Kehal: Investigation, Writingoriginal draft. Nadia Saoula: Supervision, Writingreview & editing. Seddik-El-Hak Abaidia: Supervision, Writingreview & editing. Corinne Nouveau: Supervision, Writingreview & editing.

Declaration of competing interest

All authors have no conflict of interest to declare.

Table 4

Hardness and Young modulus reported in different works.

Hardness (GPa)

Young module (GPa)

Present study 10-24 190-255 Feng et al. [START_REF] Feng | Effect of Ti content on structure and mechanical properties of Cr-Ti-N films[END_REF] 12.6-25.8 195.6-362.7 Hones et al. [START_REF] Hones | Sputter deposited chromium nitride based ternary compounds for hard coatings[END_REF] 14-18 215-250 Hsieh et al. [START_REF] Hsieh | Characterization of (Tix Cr0.6-x) N0.4 coatings and their tribological behaviors against an epoxy molding compound[END_REF] 23-26 254-293 Akbarzadeh et al. [START_REF] Akbarzadeh | Characterization of TiN, CrN and (Ti, Cr) N coatings deposited by cathodic ARC evaporation[END_REF] 22.5-26 373-415 Wang et al. [START_REF] Wang | Evaluating mechanical properties and crack resistance of CrN, CrTiN, CrAlN and CrTiAlN coatings by nanoindentation and scratch tests[END_REF] 13.9


Fatin Shamimi

Mohd Zuki

Frédéric Merienne
email: frederic.merienne@ensam.eu

Suziah Sulaiman
email: suziah@utp.edu.my

Mohamad Naufal

Mohamad Saad

Dayang Rohaya
email: dayangrohaya.ar@utp.edu.my

Awang Rambli

Sensory Feedback and Interactivity: Enhancing Motivation and Engagement for VR Stroke Rehabilitation

Keywords: Virtual reality, sensory feedback, stroke, rehabilitation

Stroke is a condition that happens when the brain is cut off from blood supply. Patients are at risk of disabilities. To help patient recover, gain mobility, and independence, rehabilitation starts as soon as possible. Unfortunately for some patients, they are forced to undergo long-term rehabilitation period. Since the activities during therapy are repetitive, many patient losses their motivation to continue therapy thus unable to recover. Virtual reality has been found to increase motivation for stroke rehabilitation. Its key elements, especially sensory feedback and interactivity have been found to increase motivation and engagement especially when applied to gamification. However, practitioners in related field have raised concern on the relevancy of some VR elements used for rehabilitation. Inappropriate elements may not be effective for the program; thus, requires further investigation. This paper explores various elements that could encourage motivation, bring better engagement, and enhance task performance of stroke patients via VR-based rehabilitation. A literature survey was conducted. The findings signal for the importance of sensory feedback and interactivity when used in VR environment for stroke patients in their rehabilitation programme.

I. INTRODUCTION

A cut off of blood supply to the brain due to blockage by a clot or ruptures of the arteries leads to a stroke. Brain damage caused by stroke usually affects one side of the brain, either on the right or left hemisphere. The side of the damaged brain will cause disability on the opposite side of the body.

Often, disabilities caused by stroke are long-term disabilities. When this happens patients lose their independence and would require a long-term care. Rehabilitation is important for the recovery of stroke patients due to loss of mobility and other functions. It helps patients relearn skills lost and most importantly to improve function so patients can regain their independence. The process of rehabilitation is to treat most of function lost, improving the functional and intellectual capacities from neuroplasticity, and re-educating motor cognitive functions [START_REF] Sherman | Understanding Virtual Reality-Interface[END_REF], [START_REF] Szmekova | The efficiency of cognitive therapy using virtual reality on upper limb mobility in stroke patients[END_REF].

There are various approaches of therapies in stroke rehabilitation including physical activities, and cognitive and emotional activities. Physical activities involve motor-skill exercises, mobility training, and constraint-induced therapy. Cognitive and emotional activities are therapy for cognitive disorders, communication disorders, psychological evaluation and treatment, and medication. The speed of recovery differs by the severity and other complications patient may face. This affects the duration of stroke rehabilitation.

Prolonged rehabilitation can cause motivational issues in patients. As time goes by, patients are inclined to get tired of the therapy exercises and would consider the training program as monotonous [START_REF] Navarro | Adaptive, Multisensorial, Physiological and Social: The Next Generation of Telerehabilitation Systems[END_REF]. Frequent training with high intensity of repetitions turn rehabilitation into a mundane and depressing task for the patients. As the patients get tired and bored of the training process, they will find themselves losing motivation for rehabilitation.

Other corresponding factors to the problem of rehabilitation is patients' engagement in the therapy sessions. Engagement has been defined by [START_REF] Varni | Multimodal (multisensory) integration, in technology[END_REF] in their study as patients' intentional effort to work toward recovery by participating fully in rehabilitation therapies. The study revealed that the most frequently rated encountered barriers impacting patient engagement are fear of pain, depressed mood, and cognitive issues. These issues damper the motivation of patients to start or continue with rehabilitation. In the end, patients will be noncompliant with rehabilitation program. Motivation and engagement of patients in receiving rehabilitation is also interlinked with task performance of the patients. It has been concluded by [START_REF] Teo | Corrigendum: Does a Combination of Virtual Reality, Neuromodulation and Neuroimaging Provide a Comprehensive Platform for Neurorehabilitation? -A Narrative Review of the Literature[END_REF] that highly repetitive training can result in improved recovery. Repetition combined with active engagement by the patients undergoing rehabilitation encourages reorganization.

To keep patients undergoing rehabilitation interested and motivated, interactive and appealing set of training procedures should be introduced to patients. The trend has shown that interactive gaming being put forward for rehabilitation with the purpose of making training fun and contextual [START_REF] In | Effects of visual feedback with a mirror on balance ability in patients with stroke[END_REF]. While interactive gaming for rehabilitation can be achieved through pc-based games and mobile applications, previous studies by [START_REF] Thikey | Augmented visual feedback of movement performance to enhance walking recovery after stroke: Study protocol for a pilot randomised controlled trial[END_REF], [START_REF] Thikey | A virtual avatar to facilitate gait rehabilitation post-stroke[END_REF] showed that robot training using virtual reality (VR) has been revealed to enhance stroke rehabilitation. Preliminary studies by [START_REF] Moussard | Music as a Mnemonic to Learn Gesture Sequences in Normal Aging and Alzheimer's Disease[END_REF] also indicated the potential of VR as a tool for physical rehabilitation. It is then further proved that VR provides a unique medium that achieves the requirements for effective rehabilitation intervention [START_REF] Pantev | Plasticity of the human auditory cortex related to musical training[END_REF]. VR application can provide therapy within a functional, purposeful, and motivating context. Bohil et al. [START_REF] Wan | Music making as a tool for promoting brain plasticity across the life span[END_REF] add that VR simulations can be highly engaging, providing crucial motivation for rehabilitative applications requiring consistent, repetitive practice. Furthermore, improvement of brain plasticity and regenerative processes is possible through real and ecological demands of the real world offered by VR [START_REF] Rosati | On the Role of Auditory Feedback in Robot-Assisted Movement Training after Stroke: Review of the Literature[END_REF].

Despite the potential benefits VR could bring to stroke rehabilitation, practitioners in related field have raised concern on the relevancy of some VR elements used for rehabilitation. Inappropriate elements may not be effective for the program [START_REF] Sihvonen | Music-based interventions in neurological rehabilitation[END_REF]. It has been observed that less is being discussed on the relationship of VR key elements and theories of gamification involving incentives and rewards in supporting the rehabilitation. Without this understanding, the effectiveness of such a VR system may be difficult to evaluate. Questions that include (i) what constitutes in the VR elements that make stroke patients perform during rehabilitation, and (ii) how do the elements relate to patients' motivation and engagement are two fundamental issues that need investigation. This paper aims to explore various elements that could encourage motivation, bring better engagement, and enhance task performance of stroke patients via VR-based rehabilitation. It presents a literature review on VR multisensory feedback, and engagement retention in relation to the feedback. The findings will be used for discussion purposes at the end of this paper.

II. LITERATURE REVIEW

A. Key elements of virtual reality experience

The popularity of VR could also be credited to how relatable and real a simulated situation or environment is when VR is used. Information now is no longer static and non-dimensional, which also means VR adds value to user experience. According to Sherman and Craig [START_REF] Tong | Music-supported therapy (MST) in improving post-stroke patients' upper-limb motor function: a randomised controlled pilot study[END_REF], there are four key elements of virtual reality experiencea virtual world, immersion, sensory feedback, and interactivity.

How does VR allow engagement and motivation for rehabilitation for the patients? The four key elements provide a suitable environment exclusively for the rehabilitation that is immersive physically and mentally through the augmentation of the senses. The use of sensory feedback through various modalities such as the visual, auditory, and tactile feedback as well as interactive design of the rehabilitation system could add fun into the therapy session. Therefore, patients will be more motivated to go through therapy and will be more engaged in the session.

B. Multi-sensory feedback in virtual reality

Not only sensory feedback is an essential element in virtual reality experience, it also provides VR users with significant benefit. Sensory feedback stimulates brain centers due to CNS plasticity [START_REF] Särkämö | Music and speech listening enhance the recovery of early sensory processing after stroke[END_REF]. Bohil et al. remark that VR engages the sensorimotor system much more by providing realistic stimulation to multiple sensory channels simultaneously. One of the interesting issues in stroke rehabilitation using VR is the use of multi-sensory feedbacks.

Multi-sensory is the integration of different senses (sight, touch, hearing, smell, and taste), however, Navarro et al. [START_REF] Forsblom | The Effect of Music and Audiobook Listening on People Recovering From Stroke: The Patient's Point of View[END_REF] stated that vestibular or balance sense as part of hearing and proprioception is also a part of multi-sensory component. Feedback sent through multi-sensory channels in VR means that user can receive information not only visually but through other senses as well. The term multi-sensory and multimodality can also be used interchangeably as defined by Varni and Castagné [START_REF] Hoffman | Physically touching and tasting virtual objects enhances the realism of virtual experiences[END_REF]: a modality is identified as a perceptual modality while multimodality is identified as multi-sensory. Teo et al. [START_REF] Weber | Visual, Vibrotactile, and Force Feedback of Collisions in Virtual Environments: Effects on Performance, Mental Workload and Spatial Orientation[END_REF] claim that VR environment produces multi-sensory feedback which in turn contributes toward better memory consolidation and retention. VR allows a sense of overall well-being due to the simulation of multiple channels by use of auditory and visual feedback which stimulate patient's awareness of his performance.

C. Visual feedback

In et al. [START_REF] Ramírez-Fernández | Haptic Feedback in Motor Hand Virtual Therapy Increases Precision and Generates Less Mental Workload[END_REF] examined the effects of visual feedback with a mirror on stroke patients' balance ability. The outcome demonstrates benefits of visual feedback from a mirror for improving balance ability during quiet standing on an unstable surface in patients with stroke. Although the experiment was conducted outside of the VR domain, such result is a benchmark proving that visual feedback does improve task performance for stroke rehabilitation. However, [START_REF] Sveistrup | Motor rehabilitation using virtual reality[END_REF] have another opinion on mirrors or videorecordings visual feedback. Although these methods provide patients with visual feedback of the movement performance and allow them to be aware and correct their movements, they are not optimal after a stroke due to cognitive and selfimage issues. The authors noted that when using these methods, viewpoints are often limited and aspect of movement sequences that would benefit from observation in different planes are failed to be captured. From the patient point of view, they can be distracted by the clinical background or even distressed by their appearance post stroke. Additionally, difficulties such as views being obscured by clothing and problems with differentiating left from right among stroke patients due to lowered cognitive function when looking at mirror reflection are some of the weaknesses of mirror therapy.

The use dynamic visualizations of the patient's movements have been suggested as able to facilitate gait reeducation [START_REF] Sveistrup | Motor rehabilitation using virtual reality[END_REF]. A stick figure representing the patient was shown on a monitor and mimics the patient's movement in either real time or after the completion of an action depending on the tasks and the patient's capability. A follow up of the study [START_REF] Shing | The study of auditory and haptic signals in a virtual reality-based hand rehabilitation system[END_REF] revealed improvement in walking velocity, step length, spatial symmetry, and temporal symmetry. The study found that the participants were able to engage with the avatar and expressed that they found it helpful to be able to see how they moved. With VR, environments and scenarios can be created according to the ideal situation with lesser distraction.

D. Auditory feedback

Audio could also act as an effective medium in a VR stroke rehabilitation program as there are strong correlation between music and motor functions; music aids motor learning [START_REF] Barrett | The use and effect of video game design theory in the creation of gamebased systems for upper limb stroke rehabilitation[END_REF]. Researches [START_REF] Widmer | Does motivation matter in upper-limb rehabilitation after stroke? ArmeoSenso-Reward: Study protocol for a randomized controlled trial[END_REF], [START_REF] Sugawara | Social Rewards Enhance Offline Improvements in Motor Skill[END_REF] found strong plasticity effects on motor brain areas from long-and short-term musical practice. Rosati et al. [START_REF] Richter | Studying Gamification: The Effect of Rewards and Incentives on Motivation[END_REF] supports the findings through reviewing studies in neurosciences. Many of the studies suggest that auditory stimulation can improve brain plasticity by affecting specific mechanisms that contribute significantly to recovery from neurological damage.

According to Sihvonen et al. [START_REF] Thon | Cognition and motor skill learning[END_REF], the scope of formal music-based intervention can be active intervention or receptive intervention. Active intervention includes activities such as creating music, playing an instrument, singing, or musical improvisation. On the other hand, receptive intervention involves listening to music that is administered by a credentialed music therapist. Sihvonen et al. reviewed sixteen randomized controlled trials that used music as a component of therapy for stroke-related neurological and neuropsychiatric disturbances. From the review, eight studies revealed rehabilitation with a musicbased intervention enhanced motor recovery in stroke patients. In one of the studies [START_REF] Bai | Development of a Novel Home Based Multi-Scene Upper Limb Rehabilitation Training and Evaluation System for Post-Stroke Patients[END_REF], the improvement in motor skills was found to be specifically caused by music rather than by motor training alone as patients practicing with mute instruments showed less improvement. The study by Cha et al. shows improvement in balance, gait velocity, cadence, stride length, and double support period on the affected side on the group that underwent movement therapy with rhythmic music accompaniment. These findings indicate that auditory feedback does add value on task performance of the patients.

Besides improving task performance, Särkämö et al. [START_REF] Särkämö | Music and speech listening enhance the recovery of early sensory processing after stroke[END_REF] recommend listening to music to enhance cognitive recovery and mood in post-stroke patients. Other than that, auditory feedback such as music might bring better engagement towards the rehabilitation process of stroke patients. As found by Forsblom et al. [START_REF] Forsblom | The Effect of Music and Audiobook Listening on People Recovering From Stroke: The Patient's Point of View[END_REF], music listening provides better relaxation, increased motor activity, and improved mood. Not only does auditory feedback improve mood hence elevating the motivation of patients to undergo rehabilitation, Rosati et al. [START_REF] Rosati | On the Role of Auditory Feedback in Robot-Assisted Movement Training after Stroke: Review of the Literature[END_REF] point out that informative feedback on errors in movement can sustain motivation during learning. Aspects of persuasion can be applied as auditory feedback to elevate motivation during therapy. Application of human-computer dialogue support such as praises, suggestions, and walkthrough may brighten patient's mood and keep the engagement level high.

E. Haptic feedback

Haptic feedback is classified into two classes: kinesthetic feedback and tactile feedback. Simply put, kinesthetic feedback can be felt from sensors of the muscles, joints, and tendons. This kind of feedback gives the brain ability to approximate size, weight, and position of and object relative to the body. Meanwhile, tactile feedback can be felt in fingers or on the surface. The sensors for sensing tactile feedback are embedded in the skin and right beneath it. Such feedback allows brain to process feels such as vibration, pressure, touch, and texture. Kinesthetic feedback will be beneficial for gross motor therapy while tactile feedback will benefit fine motor therapy in stroke rehabilitation using VR. In VR, haptic feedback is usually presented through wearables such as haptic gloves or full body suits.

As claimed by Hoffman et al. [START_REF] Hoffman | Physically touching and tasting virtual objects enhances the realism of virtual experiences[END_REF], tactile augmentation is an effective technique for adding texture and force feedback to virtual objects. With haptic feedback in VR, the participants are more engaged in the virtual world. They had more fun, found the event more realistic, and showed slight inclination to feel more present in the virtual world. Besides, Weber et al. [START_REF] Weber | Visual, Vibrotactile, and Force Feedback of Collisions in Virtual Environments: Effects on Performance, Mental Workload and Spatial Orientation[END_REF] recommend active force feedback as it significantly enrich interaction in VR, resulting in higher sensation of presence and or immersion.

Other than engagement and motivation, haptic feedback is found to improve task performance of patients undergoing motor therapy using VR [START_REF] Ramírez-Fernández | Haptic Feedback in Motor Hand Virtual Therapy Increases Precision and Generates Less Mental Workload[END_REF]. This complements the statement by [START_REF] Sveistrup | Motor rehabilitation using virtual reality[END_REF] whom identified haptic as substantial for improving a patient's performance in more difficult tasks. A report by Shing et al. [START_REF] Shing | The study of auditory and haptic signals in a virtual reality-based hand rehabilitation system[END_REF] demonstrated that haptic feedback is particularly beneficial when added to an upper extremity movement when the difficulty of the task was high. Findings by Weber et al. show that participant had better performance in term of precision, mental workload, and spatial orientation when assisted with force feedback as compared to vibrotactile and visual feedback.

F. Engagement retention through multi-sensory feedback reward system

Barett et al. [START_REF] Barrett | The use and effect of video game design theory in the creation of gamebased systems for upper limb stroke rehabilitation[END_REF], reviewed various game design principles and the theoretical grounding of its presence. These principles could be utilized in the creation of stroke rehabilitation systems. The study shows that games can be a powerful therapeutic tool when intelligently integrated into rehabilitation to combat concerns such as low motivation and engagement. Core factors in game design such as meaningful play, feedback, goals, rewards, challenge, difficulty, failure and flow could be integral to the creation of custom rehabilitative system or games as it is to commercial games.

Widmer et al. [START_REF] Widmer | Does motivation matter in upper-limb rehabilitation after stroke? ArmeoSenso-Reward: Study protocol for a randomized controlled trial[END_REF] suggested the use of monetary reward as a motivation in rehabilitation after stroke. The result of the trial done by Widmer et al. demonstrated positive outcome on the motivation of patients in rehabilitation. However, the brain process compliment is equivalent of a social reward as being rewarded with money [START_REF] Sugawara | Social Rewards Enhance Offline Improvements in Motor Skill[END_REF]. This finding was made through investigating the effect of praise on motor skill consolidation; the study found that participants that receive praises for their performance indicated improvement in their motor skills memory. Even though Widmer et al. used money as reward to promote motivation, the authors suggest that other forms of reward such as social rewards (smiles, praises), food rewards (sweets, dietary allowance), or token programs may be applied in rehabilitation. Reward can be presented to patients in the form of multisensory feedbacks in VR based on the core factors in game design as previously mentioned.

Ricther et al. [START_REF] Richter | Studying Gamification: The Effect of Rewards and Incentives on Motivation[END_REF] summarized the theoretical base of incentives and rewards in gamification as shown in Table 1. Personal investment theory is represented by PIT in the table.

G. Existing Literature on VR-based Stroke Therapy

A systematic search was conducted on IEEE Xplore, Elsevier (Science Direct), and Scopus journal databases in January to September 2019. The year of publications is set for 5 years from 2015 to 2019 due to the evolutionary nature of technology especially virtual reality. This search is to explore various virtual reality-based rehabilitation design which increase patient's engagement level in rehabilitation for stroke patient.

The search yielded 30 articles, of which 19 are potentially relevant papers to be retrieved in full text for further evaluation. Then, the 19 articles are screened by contents which excludes criteria such as: article is general literature on technology-based stroke rehabilitation, article is a clinical practice or a guideline, research does not include

III. DISCUSSION

From the literature, it is found that the itself supports a better engagement and higher motivation through its multisensory feedbacks and interactivity. Studies also found that sensory feedback stimulates brain centers due to CNS plasticity. The processing of sensory feedback and knowledge of results allows cognitive acquisition of and memorization of movements. Thon [START_REF] Thon | Cognition and motor skill learning[END_REF] stated that generally, the association of cognitive process effects motor learning positively. Thus, the inclusion of sensory feedback to stroke rehabilitation might increase task performance of stroke patients undergoing therapies.

There are many ways to design the interactivity of VR, but the most popular technique is through gamification. Coincidentally, the core factors in game design have been found to support rehabilitation. Gamification factor such as rewards and feedbacks are included in the theoretical base of incentives and rewards. In this theory, each motivation goal has its own design and role. Hence, it is possible to target the type of motivation that is needed to be boosted for rehabilitation purposes during design and development of system. Interactivity could also integrate multi-sensory feedback as the input and output to increase patients' engagement. Besides, the incentives and rewards could be presented in these modalities, for example, points and praises through audio and visual cues.

Based on this theory, multi-sensory feedback in VRbased stroke rehabilitation system encourage motivation, bring better engagement, and enhance task performance of stroke patients.

Beside theoretical studies, there are also existing studies with prototypes developed for VR-based stroke rehabilitation as shown in table 2. The studies are able to prove that VR, gamification, and some sensory modalities indeed enhanced motivation and engagement. Unfortunately, there are lack of emphasize on the type of modalities used in the experiments.

This research topic lacks studies on how much of the relative weight of each sensory feedback (visual, auditory, or haptic) adds value to the whole experience during therapy. Although all modalities positively impact stroke rehabilitation, there could be modalities that are less effective than the others in aiding recovery. This proposition is in parallel with that highlighted by rehabilitation practitioners reported in [START_REF] Sihvonen | Music-based interventions in neurological rehabilitation[END_REF]. The focus should no longer be on the usefulness of VR technology anymore but more specific on to the VR elements i.e. the relevancy and appropriateness of such elements for implementation. This shift of focus could be useful to address the lack of guidelines in the application of multi-sensory modalities in VR-based stroke rehabilitation system.

As shown in table 2, various studies have explored and proved the effectiveness of interactivity and multi-sensory feedback in rehabilitation via VR. However, most of studies, as pointed out earlier, lack details on why and how specific elements of sensory feedbacks are used. Most studies are limited in their findings of cognition, engagement, and motivation factor of their rehabilitation system. Furthermore, it was not explicitly shown that the selections of gameplay or pattern as well as the sensory feedbacks are user-centered. Inability to consider how users experience and use a system for rehabilitation might affect the effectiveness of the therapy by demotivating and disengaging users. This will override the effectiveness of sensory feedback and interactivity offered by VR, causing higher rate of noncompliance. • Games can be a powerful therapeutic tool when intelligently integrated into rehabilitation to combat concerns such as low motivation and low engagement.

• Core factors in game design such as meaningful play, feedback, goals, rewards, challenge, difficulty, failure and flow could be integral to the creation of custom rehabilitative games as it is to commercial games. Particularly, if these core factors which are found to add to motivation, engagement and commercial success are combined with technologically advanced control systems that compensate for motor impairment.

• Limited research elucidating these game design concepts and advocating their utilisation in stroke rehabilitation.

Massetti et al., (2018)

[27]

• Evidences from the review show the potentials of VR to have beneficial effects on rehabilitation of patients with neurological disorders.

• VR is reportedly to have psychological and cognitive benefits when adapted to the patient.

• Elderly patients using VR as rehabilitative device was found to have improvement in attention or memory stimulation.

• VR is likely to be an increasingly important option to complement traditional therapies.

• There are needs for innovative, functional, and interactive VR devices that are available and accessible for individuals with such disorders.

• More studies are required to prove the effectiveness of existing VR technology.

Bai & Song (2019)

[28]

• Hospital clinical trial results show that the system may increase patients' motivation in training.

• Varieties of rehabilitation training virtual scenes were added to execute rehabilitation training for patients and effectively improve their enthusiasm.

•

The study does not mention any auditory aspect of the games.

•

It is not mentioned whether these games are able to deeply engage subjects with its visual display or possibly with any auditory support.

•

No cognitive benefits of the games were discussed.

Ghassemi, Ochoa, Yuan, Tsoupikova, & Kamper (2018)

[29]

• Some serious game based systems may undoubtedly help with user engagement however they are limited in the range of therapy they can provide.

• Cyclical stretching of the hand muscles through imposed joint rotation can reduce muscle hyper-excitability in stroke survivors.

• It also improves task performance with the impaired hand.

•

Haptic feedback from the glove can aid sensorimotor learning.

•

The study does not mention any auditory aspect of the games.

Pham, Phan, Nguyen, & Nguyen (2016) [START_REF] Pham | Game-based virtual rehabilitation system for upper extremity using low-cost camera[END_REF] • Rehabilitative games should motivate patients to exercise, should be repeated many times.

• Each session should be short and the different level of difficulty should be present

•

Hand motions are able to be detected during therapy with game although low cost camera is used.

•

No result of study with actual stroke patient using the game system.

• Lack of evidence showing the game pattern and the designed game helps in stroke rehabilitation.

From the literature, we have learnt that while sensory feedbacks are found to be effective and are widely studied, not much research have been focusing on which specific elements of the sensory feedbacks enhance motivation and engagement factors during rehabilitation. This is followed by sparse information and design model or framework on developing a VR-based rehabilitation system with multisensory feedback. On top of that, most studies are not design with a user-centered approach, thus not enough evidence showing that the users can still be compliant to use the systems in an extended duration.

IV. CONCLUSION AND FUTURE WORK

This paper presented the literature of key elements of VR, multi-sensory feedbacks, and theories of gamification incentives and rewards in supporting the rehabilitation of stroke patients. The outcome of this study shows that these features have potential in assisting recovery. More importantly, the features are said to be able to encourage motivation, bring better engagement, and enhance task performance during rehabilitation.

Studies on the design and framework of rehabilitative games would be conducted in the future. Then the effectiveness and efficiency of each modality would be measured in order to obtain the hierarchy of the modalities for developmental guidelines.

Table 1 . Theoretical base of incentives and rewards in gamification

 1

	Motivation Theory	Incentives/ Rewards	Role
	Self-efficacy Self-efficacy, goal-setting, PIT, expectancy value, need achievement Self-efficacy, goal-setting, PIT, expectancy value, social comparison	Audio/verbal/ visual/music/ sound effect Progress bar Points/bonus/ dividend Mini games/ challenges/quests Badges	Feedback Feedback, achievements Feedback, reward, status, achievements, competition, progression, ownership Reward, status, competition, achievements Status and reputation, achievements and past accomplishments, collection, competition, ownership
		Virtual goods	Reward, social, status, achievements, ownership, self-expression
		Leaderboard	Status and reputation, achievements, competition
	Self-efficacy, goal-setting, PIT, expectancy value, need achievement, social comparison	Rewards-choosing colours, power Achievements Levels	Achievements Collection, status, competition, discovering, progression Feedback, status and reputation, achievements, competition, moderate challenge
	Social comparison, personal investment theory, expectancy value	Avatar	Social, self-expression, ownership
	any visual, haptic, or auditory modalities in rehabilitation programme, no theories for design of VR-based stroke rehabilitation, engagement level is not increased. The findings are tabulated in table 2.

Table 2 . Existing studies on VR-based stroke rehabilitation

 2

	Reference	Main outcomes	Gap
	Barrett, Swain, Gatzidis, & Mecheraoui (2016)
	[22]