Linda Aissani
email: lindaaissani2004@yahoo.fr

Mamoun Fellah

Lamia Radjehi

Corinne Nouveau

Alex Montagne

Akram Alhussein
email: akram.alhussein@utt.fr

Effect of

Keywords:

Introduction

Chromium thin films with exclusive super saturationed metalloid elements such as carbon or nitrogen have received wide technological interest due to their remarkable performance, such as high hardness, high elasticity modulus, good wear resistance and chemical stability under extreme conditions [START_REF] Shah | Influence of sputtering parameters and nitrogen on the microstructure of chromium nitride thin films deposited on steel substrate by direct-current reactive magnetron sputtering[END_REF][START_REF] Lakra | Synthesis and characterization of Cr-ZrO 2 , composite coating formed by DC and pulse electrodeposition[END_REF][START_REF] Akkera | Thickness dependent structural and electrical properties of magnetron sputtered nanostructured CrN thin films[END_REF][START_REF] Khalil Ibrahim | Annealing effects on microstructural, optical, and mechanical properties of sputtered CrN thin film coatings: experimental studies and finite element modeling[END_REF]. Nowadays, the most commonly used PVD coatings are chromium nitrides [START_REF] Fuentes | Recent advances in the chromium nitride PVD process for forming and machining surface protection[END_REF] and carbides [START_REF] Fellah | Thermal treatment effect on structural and mechanical properties of Cr-C coatings[END_REF]. These hard coatings (H = 15-40 GPa, E = 200-500 GPa) have a high wear resistance which is many times better than steel substrates. Chromium carbides and nitrides, each having several interesting properties, become very attractive candidates for protective coatings in industrial metal cutting tools. Concerning the chromium carbide coating, the presence of carbon in the chromium-based metal coating increases hardness and wear resistance by a lubricating effect [START_REF] Pengfei | Study on tribological property of CrCN coating based on magnetron sputtering plating technique[END_REF]. The Cr 7 C 3 is one of the Cr carbides having excellent corrosion and wear resistance and is used largely in severe environmental conditions [START_REF] Fellah | Thermal treatment effect on structural and mechanical properties of Cr-C coatings[END_REF][START_REF] Pengfei | Study on tribological property of CrCN coating based on magnetron sputtering plating technique[END_REF][START_REF] Xiao | Mechanical properties and chemical bonding characteristics of Cr 7 C 3 type multicomponent carbides[END_REF]. Among the chromium nitrides, the CrN and Cr 2 N are widely used for industrial applications because of their good hardness, high chemical stability and toughness [START_REF] Lin | High rate deposition of thick CrN and Cr 2 N coatings using modulated pulse power (MPP) magnetron sputtering[END_REF][START_REF] Basu | Structure and properties of Cr x N coating[END_REF][START_REF] Zou | The effect of composition on the structure, mechanical properties, and thermal stability of sputter coated ternary chromium-molybdenum-nitride coatings[END_REF].

Furthermore, improving mechanical properties and increasing the cutting speed limit of a machining tool are necessary for its durable performance. This requires new hard based chromium coatings with high thermal stability and good wear resistance under more severe workload conditions. Generally, the enhanced tribological and mechanical properties of sputtered binary chromium coatings can be achieved by varying the microstructure and chemical composition. This is typically controlled by the deposition parameters and the applied treatment [START_REF] Aissani | Influence of vanadium on structure, mechanical and tribological properties of CrN coatings[END_REF]. Recently, researchers have developed coatings based on forming hard chromium phases embedded in the nitrogen in parallel with the carbon (CreC and CreN) [START_REF] Lin | High rate deposition of thick CrN and Cr 2 N coatings using modulated pulse power (MPP) magnetron sputtering[END_REF][START_REF] Basu | Structure and properties of Cr x N coating[END_REF][START_REF] Zou | The effect of composition on the structure, mechanical properties, and thermal stability of sputter coated ternary chromium-molybdenum-nitride coatings[END_REF]. Many studies have been conducted to develop the mechanical properties of chromium-based coatings, especially hardness and wear resistance [START_REF] Aissani | Influence of vanadium on structure, mechanical and tribological properties of CrN coatings[END_REF][START_REF] Kim | Syntheses and mechanical properties of Cr-Mo-N coatings by a hybrid coating system[END_REF]. The presence of carbon will confine the grain growth of the chromium nitride phase, which inhibits the mobility of plastic deformation during solid-solid interactions [START_REF] Gilewicz | Wear and corrosion behavior of CrCN/CrN coatings deposited by cathodic arc evaporation on nitrided 42CrMo4 steel substrates[END_REF][START_REF] Almer | Microstructure and thermal stability of arc evaporated Cr-C-N coatings[END_REF]. The resulting chromium carbonitride phases combine the properties of chromium carbides (hardness and ductility) and chromium nitrides (hardness, wear and corrosion resistance). Tests of hardness, tribology behaviour, corrosion and thermal resistance showed that chromium carbonitride coatings have great potential for tool protection [START_REF] Lin | High rate deposition of thick CrN and Cr 2 N coatings using modulated pulse power (MPP) magnetron sputtering[END_REF]. However, only limited reports on sputtered CreCeN coatings are available; these either discuss the microstructure and mechanical properties with different carbon content [START_REF] Basu | Structure and properties of Cr x N coating[END_REF], or the influence of (C + N)/Cr ratio on the mechanical and tribological behaviour without comprehensive information on the chemical bonding structure [START_REF] Gilewicz | Wear and corrosion behavior of CrCN/CrN coatings deposited by cathodic arc evaporation on nitrided 42CrMo4 steel substrates[END_REF].

In earlier experiments (to be published elsewhere) we found that our CreN [START_REF] Aissani | Influence of vanadium on structure, mechanical and tribological properties of CrN coatings[END_REF] and CreC [START_REF] Fellah | Thermal treatment effect on structural and mechanical properties of Cr-C coatings[END_REF] films show rather high friction coefficients between 0.53 and 0.62 and correspondingly high wear rates. Therefore, in this paper, our objective is to improve the coating's mechanical and tribological behaviour by increasing the carbon content. We investigated the influence of the coating composition on the mechanical and tribological properties of CreCeN coatings, especially with regards to the diffusion of carbon from the substrate during annealing temperature and comparing the properties of CreN and CreC films. The present study is aimed at preparing, characterizing and testing CreCeN coatings with high hardness and high wear resistance. Therefore, the chemical composition and structure as well as mechanical and tribological properties of the annealed CreCeN coatings deposited by reactive magnetron sputtering were thoroughly presented.

Experimental details

Film deposition

High carbon steel samples (XC100 in the French standard AFNOR-10027, containing 1.17 wt% C) were used to deposit chromium nitrides. The small disc substrates (15 × 3 mm 2 ) were initially polished to obtain an average roughness R a of about 30 nm. Before deposition, all substrates were cleaned ultrasonically in acetone for 5 min, and were then dried in hot air.

The RF reactive magnetron sputtering (NORDIKO type 3500, 13.56 MHz) was used to deposit CrN coatings. A chromium target (99.98% purity, Ø 101.6 × 3 mm 2 ) was used and its distance from the substrate holder was kept constant at 80 mm. The deposition chamber was pumped down to a low pressure (< 10 -5 Pa). Prior to deposition, the substrates were sputter-cleaned for 5 min by argon plasma at 1 Pa and -700 V in order to remove impurities. The deposition procedure was carried out, at 150 °C, for 90 min, under a reduced pressure of 0.4 Pa with an N 2 (purity 99.999%)/Ar (purity 99.999%) ratio of 0.25 [START_REF] Aissani | Influence of vanadium on structure, mechanical and tribological properties of CrN coatings[END_REF]. The electrical parameters applied to the Cr target and substrates were kept constant at 650 W and -900 V, respectively. The Cr-N/ XC100 samples were used for the evaluation of film structure, morphology, thickness, hardness, adhesion and friction coefficient.

Heat treatment

The Cr-N/XC100 samples were heat-treated under controlled vacuum at temperatures ranging from 700 to 1000 °C, in a tubular furnace. In order to study the effect of annealing temperature on the properties of the coatings, the thermal cycle started with an increase of 10 °C/min, followed by maintaining at the desired temperature for 1 h at various fixed temperatures. Finally, cooling under controlled vacuum (about -10 °C/min) was done. The vacuum pressure of 10 -5 Pa, assured by a diffusion pump, was kept constant.

Film morphology, chemical composition and microstructure analysis

Film thicknesses and morphologies were observed by means of a scanning electron microscope (Jeol JSM-6400F, 10 kV), equipped with Energy dispersive spectroscopy (EDS) microanalysis to determine the chemical composition of the coating. The nature of the chemical bonds was investigated by X-ray photoelectron spectroscopy (Riber SIA 100 XPS). CasaXPS software tool was used for this analysis. The K αAl radiation (hν = 1468.68 eV, 300 W) was applied as the excitation source. The vacuum in the spectrometer was 4 × 10 -8 Pa. The spectrometer calibration was performed using binding energy level of C1s at 284.8 eV, which was absorbed on the surface of the samples. Then, the corresponding Cr2p, N1s and C1s spectra were fitted under a multipeak fitting method, and the ratio of Lorentzian to Gaussian was 20% [START_REF] Aissani | Influence of vanadium on structure, mechanical and tribological properties of CrN coatings[END_REF].

The structure and phase composition of the coatings were assessed by X-ray diffraction (XRD) patterns using a Siemens D5000 X-ray θ-2θ apparatus applying Co Kα radiation (λ = 1.781 Å, 45 kV and 40 mA). The average crystalline size of the coatings was determined by Scherrer's method [START_REF] Aissani | Influence of vanadium on structure, mechanical and tribological properties of CrN coatings[END_REF]:

= D 0.9 λ β•cosθ (1)
where 0.9 is the shape factor, λ represents the X-ray wavelength used for the measurement (λ Co = 0.178 nm), β is the line width (full width half maximum, FWHM) in radians and θ is the Bragg's angle. The Cr 2 N (200) and Cr 7 C 3 (410) diffraction peak positions were used to estimate crystalline sizes.

Mechanical and tribological characterizations

The hardness and Young's modulus of CreN films were determined by means of nanoindentation tests using a Nano Indenter XP from MTS, working with the Continuous Stiffness Measurement mode. By using a three-sided pyramidal diamond tip (Berkovich indenter), a maximum depth of 200 nm was considered, leading to a maximum load of approximately 10 mN. The loading and unloading of the indentations were carried out under loading control with a nominal rate of 0.05 mN/ s. Indentation curves were described by means of the general methodology for the films around 2 μm thick [START_REF] Staia | Assessment of the mechanical and tribological performance of a ZrN PVD coating[END_REF].

To investigate the mechanical properties of the film/substrate interface zone, hardness and Young's modulus were determined by load-displacement data obtained by using a model MTS System equipped with Berkovich tip, performing 20 indentations per sample. The hardness measurements as a function of the film depth were performed on specimen's cross-sections going from the film surface zone to the steel substrate where indentation spacing was 0.2 μm.

The Young's modulus of the coating E is calculated with the following equation:

= - + - E ν E ν E 1 (1 ) ( 1
) i i m m 2 2 (2)
where E m and ν m are Young's modulus and Poisson's ratio for the specimen, E i (1140 GPa) and ν i (0.07) are parameters proper to the Berkovich indenter. The hardness and Young's modulus values for each depth position were obtained by averaging 3 indentations.

The roughness of film surface was measured by an atomic force microscope (AFM 100, APE research), operated in contact mode with a scan range of 3 × 3 μm 2 .

The tribological characterization was carried out by sliding wear tests using a tribometer (TRIBOtester) with a ball-on-disc configuration. Tests were carried out at ambient temperature with the following conditions: a 100Cr6 ball (Ø6 mm) was used as a counterpart, with an applied normal load of 2 N (> 1 GPa), a sliding distance of 200 m and a constant speed of 5 cm•s -1 . The morphology and composition of damaged samples were evaluated with an SEM equipped with an EDS. The wear volume and the surface roughness were determined with an optical profilometer (VEECO, Wyko-NT 1100) [START_REF] Staia | Assessment of the mechanical and tribological performance of a ZrN PVD coating[END_REF].

Results and discussion

Structural characterization

Chemical composition

In order to compare the properties of CreN, CreC and CreNeC phases, it was necessary to prepare samples with the same conditions (e.g. deposition time). We investigated the influence of the annealing temperature on the variation of chemical composition of these samples. Fig. 1 shows the Cr, Fe, N, C and O contents of Cr-N/XC100 coatings at different annealing temperatures measured by EDS. It can be seen that oxygen contamination is < 5 at.%. The main sources of oxygen impurity are probably the post-deposition contaminations and the annealing treatment carried out under mediocre vacuum. The oxygen concentration was reduced gradually as a function of increasing annealing temperature which leads to the decomposition of chromium oxides from 800 °C. The Cr and N concentrations changed slightly without C presence at 800 °C. This is attributed to the stability of chromium nitrides. At T ≥ 900 °C, we can see a marked decrease in the Cr and N concentrations, which is more significant at 1000 °C. In contrast, we noticed a progressive increase in the Fe and C concentrations. This can be explained by the diffusion of Cr and N elements and their reactions with the C of the XC100 substrate at 1000 °C to form the chromium carbide and carbonitride phases [START_REF] Almer | Microstructure and thermal stability of arc evaporated Cr-C-N coatings[END_REF]. Fig. 2 presents the evolution of N/Cr and C/Cr ratios as a function of the annealing temperatures. We can see that the C/Cr ratio increased with an increasing annealing temperature. We obtained a C/Cr ratio of 0.45 and 0.60 at 900 and 1000 °C, respectively. However, the N/Cr ratio decreased between 900 and 1000 °C from 0.6 to 0.38. This is mainly due to the disappearance of nitrogen in the Cr-N system. Similar behaviour was observed on annealed Zr/XC100 samples [START_REF] Benouareth | Study of the interaction between a zirconium thin film and a C100 steel substrate: temperature effect[END_REF].

Depth profiles obtained by XPS analysis at 900 °C showed that the composition of the coatings was heterogeneous throughout the thickness. The profiles present short-range discrepancy around a constant mean value for all the elements in the coatings (Fig. 3). In the middle of the interface zone, the profiles indicated that the carbon signal gained in intensity and reached a maximum value of about 20 at.%. The carbon content decreased gradually between the middle and superficial regions confirming that its diffusion from the substrate occurred at this annealing temperature (900 °C). In the meantime, the intensity of chromium and nitrogen signals decreased in the interface zone. A slight degree of oxygen contamination was evident which probably originated from some impurity. This oxygen contamination was confirmed previously by the X-ray microanalysis results (Fig. 1).

Crystalline structure

Fig. 4 presents the XRD patterns of coatings before and after heat treatment. Due to the small thickness of the coating deposited on the substrate, the Fe α intensity of the diffraction peaks is at a maximum. XRD analysis showed that the as-deposited coatings have a very shortrange order, where the small (111) and ( 200) diffraction lines at 44.32°a nd 53.81°correspond to CrN (cubic structure, JCPDS 00-011-0065). Also, ( 200) and (111) Cr 2 N diffraction peaks can be observed at 47.99°a nd 50.29°(hexagonal structure, JCPDS 35 0803), which are similar to the observations in earlier studies [START_REF] Aissani | Influence of vanadium on structure, mechanical and tribological properties of CrN coatings[END_REF][START_REF] Nouveau | The influence of deposition parameters on the wear resistance of Cr x N y magnetron sputtering coatings in routing of oriented strand board[END_REF]. In the early stages of the annealing treatment, all coatings were fully crystallized, as confirmed by the number of sharp peaks shown on Fig. 4. Between 700 and 800 °C, there was a significant growth of CrN (111) and (200) signals, proving the stability of this nitride from 750 °C [START_REF] Knotek | Applications to cuttings tools: CVD-coated cutting tools by R.F[END_REF]. In addition, a new small (410) peak of Cr 7 C 3 appears at 46.19°(orthorhombic structure, JCPDS 00-006-0683), which confirms the beginning of the carbon diffusion from the substrate to the film.

As the annealing temperature increases to 900 °C, (111) CrN preferential orientation becomes weaker and broader. However (410), (421) and (611) Cr 7 C 3 , (511), ( 112) and (200) Cr 2 N preferential orientations become stronger with minor (420) Cr 23 C 6 peaks (cubic structure, JCPDS 035 0694). Every time the annealing temperature rises, we stimulate the migration of a greater amount of carbon atoms to the film, which leads to the observed increase of the (410) Cr 7 C 3 peak intensity. This carbide is the most stable and the hardest among all the other carbide phases, especially when it is used as a coating for high carbon steel [START_REF] Fellah | Thermal treatment effect on structural and mechanical properties of Cr-C coatings[END_REF][START_REF] Benouareth | Study of the interaction between a zirconium thin film and a C100 steel substrate: temperature effect[END_REF][START_REF] Neves | In situ structural evolution of arc-deposited Cr-based coatings[END_REF]. From Fig. 4, we can also see a gradual disappearance of Cr 2 N peaks by the reaction of chromium coating with XC100 substrate. This is due to the difference in crystal structures of carbon, chromium carbide and chromium nitride. The carbon atoms can easily replace the nitrogen atoms in the chromium nitride crystal structure to form chromium carbide [START_REF] Lupicka | The adhesion of CrN thin films deposited on modified 42CrMo4 steel[END_REF][START_REF] Gheriani | Effect of substrate surface ion bombardment etching on reaction between chromium thin films and steel substrates[END_REF][START_REF] Warcholinski | Arc-evaporated CrN, CrN and CrCN coatings[END_REF][START_REF] Vyas | Nano-structured CrN/CNx multilayer films deposited by magnetron sputtering[END_REF]. Thus, the carbon element perturbs the normal crystal arrangement of chromium nitrides. In addition, they transform into chromium carbonitride by the appearance of (111) and (131) Cr 3 N 0.6 C 0.4 at 41.02°and 51.20°, respectively (cubic structure, JCPDS 01 0892540) [START_REF] Schuster | Influence of organochromium precursor chemistry on the microstructure of MOCVD chromium carbide coatings[END_REF][START_REF] Lin | Structure and properties of selected (Cr-Al-N, TiC-C, Cr-B-N) nanostructured tribological coatings[END_REF][START_REF] Aissani | Structural and mechanical properties of Cr-Zr-N coatings with different Zr content[END_REF][START_REF] Ye | Doping carbon to improve the tribological performance of CrN coatings in seawater[END_REF]. This agrees well with earlier works, where carbides and carbonitrides have been successfully formed from transition-metal-nitride-based thin films deposited on high carbon steel [START_REF] Benouareth | Study of the interaction between a zirconium thin film and a C100 steel substrate: temperature effect[END_REF][START_REF] Lupicka | The adhesion of CrN thin films deposited on modified 42CrMo4 steel[END_REF]. XRD analysis reveals that the Cr 2 N phase is thermally stable at 1000 °C which can be attributed to the presence of sufficient chromium and nitrogen in the deposited coating. This allowed the creation of more stable positions and to consequently facilitate the stability of nitrogen in chromium which was justified with a high N/Cr ratio (0.38) at 1000 °C (Fig. 2). If we consider the formation energy of these compounds, the Cr 2 N can be formed in the interval of 7-10 at.% N 2 at 650 °C and requires formation energy of about 397.4 ± 0.4 eV [START_REF] Aissani | Influence of vanadium on structure, mechanical and tribological properties of CrN coatings[END_REF][START_REF] Staia | Assessment of the mechanical and tribological performance of a ZrN PVD coating[END_REF][START_REF] Nouveau | The influence of deposition parameters on the wear resistance of Cr x N y magnetron sputtering coatings in routing of oriented strand board[END_REF]. This value is higher than that of CrN phase which is formed at higher nitrogen concentration (> 15% N 2 in the gas mixture) at 750 °C and needs formation energy of about 396.8 ± 0.4 eV [START_REF] Staia | Assessment of the mechanical and tribological performance of a ZrN PVD coating[END_REF]. Thus, on one hand, the formation of different phases of chromium nitrides can be justified and attributed to the nitrogen percentage. On the other hand, the transformation of chromium nitrides to chromium carbonitrides in the coatings from 900 °C consolidates the properties of carbides and nitrides, which can be explained by the smallest carbon atomic radius among the other elements (Cr, Fe, and N). Moreover, the activation energy for carbon diffusion in polycrystalline chromium is 1.14 eV and that of carbon into iron is about 1.4 eV [START_REF] Fellah | Thermal treatment effect on structural and mechanical properties of Cr-C coatings[END_REF][START_REF] Gheriani | Effect of substrate surface ion bombardment etching on reaction between chromium thin films and steel substrates[END_REF]. These values are less than the activation energy of nitrogen into chromium. So, the carbon can be diffused and stabilized in chromium more easily than nitrogen. Consequently, annealing treatment facilitates the rearrangement of chromium, carbon and nitrogen to give different compounds. Nouveau et al. [START_REF] Nouveau | The influence of deposition parameters on the wear resistance of Cr x N y magnetron sputtering coatings in routing of oriented strand board[END_REF] and Warcholinski et al. [START_REF] Warcholinski | Arc-evaporated CrN, CrN and CrCN coatings[END_REF] have confirmed the formation of chromium nitrides and carbonitrides. At 900 °C, the existence of oxygen shows the presence of (104) Cr 2 O 3 oxide at 34.2°( hexagonal structure, JCPDS 85-0869), with a minor intensity diffraction peak. Many studies confirm that the presence of chromium oxide is still observed in based-chromium films deposited by PVD processes at high vacuum [START_REF] Aissani | Influence of vanadium on structure, mechanical and tribological properties of CrN coatings[END_REF] and even after their annealing under vacuum [START_REF] Fellah | Thermal treatment effect on structural and mechanical properties of Cr-C coatings[END_REF].

The annealing treatment favourises the diffusion of carbon in the CrN coatings, which confirms the formation of binary hard chromium carbide. However, the chemical and structural characterization doesn't exhibit the incorporation of the Fe atoms in the CrN lattice. Indeed, the diffusion activation energy of Fe into polycrystalline Cr (3.45 eV) is higher than the diffusion energy of C into polycrystalline Cr (1.15 eV). This justifies the fact that the diffusion of C has preceded the diffusion of Fe from the substrate towards the CrN coatings [START_REF] Fellah | Thermal treatment effect on structural and mechanical properties of Cr-C coatings[END_REF]. XRD analysis reveals that the shift in the peak positions towards lower 2θ angles as annealing temperature increases is proportional to an increase in the lattice parameter a. The Cr 7 C 3 (410) reflection is broadened and becomes increasingly asymmetric while the Cr 2 N (200) slightly decreases with increasing both annealing temperature and C content as shown in Figs. 4 and5: (200) Cr 2 N located at 50.29°(at 800 °C), 50.40°(at 900 °C) and 50.52°(at 1000 °C), and (410) Cr 7 C 3 located at 46.01°(at 800 °C); 46.64°(at 900 °C) et 46.547°(at 1000 °C), respectively. Further heating of the sample provides additional energy helping the carbon atoms to displace which leads to a reduction in the peak width as the coating composition becomes more homogeneous. The presence of further narrow peaks suggests the main effect of heat treatment and the significant variation in the chemical composition of coatings. This result is strongly supported and confirmed with the composition measurements described in Section 3.1.1 (Fig. 1).

The inter-reticular distance d (200) of Cr 2 N decreases progressively in contrary to the increase of d (410) of Cr 7 C 3 during the annealing treatment with the replacement of nitrogen by smaller carbon atoms within the mixed chromium carbonitride solid solution and the formation of chromium carbides (Fig. 5). This tendency is consistent with the lattice parameter (a) of both binary nitride (Cr 2 N: a = 4.78 Å) and carbide (Cr 7 C 3 : a = 7.014 Å) assuming stoichiometric compositions. As each binary compound (Cr-N, Cr-C) is reported as fully mutually soluble, it should be possible to interpolate between the lattice parameters of the chromium nitride and carbide and derive the lattice parameter of the chromium carbonitride Cr 3 C 0.4 N 0.6 (a = 4.81 Å). Recent studies of carbonitride Me(C, N) phases produced by high-energy ball milling are in line with these observations [START_REF] Vyas | Nano-structured CrN/CNx multilayer films deposited by magnetron sputtering[END_REF]. This is explained by the increase of microstructural strain or crystallite size effects in the film [START_REF] Benouareth | Study of the interaction between a zirconium thin film and a C100 steel substrate: temperature effect[END_REF].

The changes in the bonding structure of Cr-N coatings were analysed. The C1s, Cr2p (1/2, 3/2) and N1s spectra in the XPS analysis of Cr-N coatings were detected (Fig. 6). At 900 °C, the Cr2p XPS spectra (Fig. 6.a) can be decomposed with different binding energies of 574.23 eV (Cr-C), 575.46 eV (Cr-N), 576.50 eV (Cr-N), 583.63 eV (Cr-C) and 585.34 eV (Cr-O), which correspond to Cr 7 C 3 , CrN, Cr 2 N, Cr 7 C 3 and Cr 2 O 3 , respectively [START_REF] Fellah | Thermal treatment effect on structural and mechanical properties of Cr-C coatings[END_REF][START_REF] Aissani | Influence of vanadium on structure, mechanical and tribological properties of CrN coatings[END_REF]. Fig. 6.b illustrates the N1s spectra fitting curves of Cr-N coating, where the N1s peak is decomposed into two peaks with binding energies of 396.84 eV and 397.89 eV, which correspond to Cr-N [START_REF] Aissani | Influence of vanadium on structure, mechanical and tribological properties of CrN coatings[END_REF]. This gives additional evidence to support the bonding of N to Cr after annealing treatment at 900 °C and it is maybe due to the higher electronegativity of nitrogen [START_REF] Schuster | Influence of organochromium precursor chemistry on the microstructure of MOCVD chromium carbide coatings[END_REF]. These results mean that CrN and Cr 2 N coexist in the sample as shown from the XRD analysis (Fig. 4) [START_REF] Lin | Structure and properties of selected (Cr-Al-N, TiC-C, Cr-B-N) nanostructured tribological coatings[END_REF][START_REF] Aissani | Structural and mechanical properties of Cr-Zr-N coatings with different Zr content[END_REF]. As shown in Fig. 6.c, the C1s peak, can be fitted into two peaks, which centre on 282.98 eV and 284.34 eV corresponding to C-Cr and sp 2 C-C, respectively presenting the Cr 7 C 3 and free carbon [START_REF] Schuster | Influence of organochromium precursor chemistry on the microstructure of MOCVD chromium carbide coatings[END_REF][START_REF] Ye | Doping carbon to improve the tribological performance of CrN coatings in seawater[END_REF]. At 1000 °C, we notice a significant decrease in the Cr2p spectra intensity and changes in the N1s and C1s spectra (Fig. 6.d). The For the N1s (Fig. 6.e), the main binding energy at 397.03 eV was decomposed into two overlapping peaks at 396.98 and 397.12 eV, corresponding to NeCr bond due to CrN and Cr 2 N, respectively [START_REF] Schuster | Influence of organochromium precursor chemistry on the microstructure of MOCVD chromium carbide coatings[END_REF]. However, the minor peak located at 398.62 eV corresponds to NeC bond [START_REF] Tillmann | Development and tribological investigation of the coating system chromium carbonitride (CrCN) to different surface designs[END_REF]. The main C1s fine spectra (at 1000 °C, Fig. 6.f) is located at a binding energy of 283.12 eV with a shoulder at a lower binding energy positioned at 283.34 eV. Through Gaussian fitting analysis, the peaks at 282.97 eV, 284.50 eV and 285.51 eV correspond to CeCr (1st peak) and CeN bonds. According to Ye et al. [START_REF] Ye | Influences of bias voltage on the microstructures and tribological performances of Cr-C-N coatings in seawater[END_REF], the CeN bond is composed of sp 2 CeN and sp 3 CeN. This indicates that the CeN binding is strongly formed in the CreN coatings after incorporation of carbon atoms and confirms the coexistence of chromium nitride, carbide and carbonitride phases in the CreN system at high annealing temperature as shown with XRD analysis. It is noteworthy that the N1s peak has a higher intensity than that of the C1s peak. This difference probably means that nitrogen is rather less linked to chromium during the transformation of chromium nitrides to chromium carbonitrides by the diffusion of carbon into the CreN film during annealing. This can be explained by the lattice size effect (r C < r N ), which facilitates the preferential diffusion of C in Cr as compared to N. Also, the standard enthalpies of Cr 7 C 3 and CrN formation are respectively 3.47 and 1.14 eV [START_REF] Gheriani | Effect of substrate surface ion bombardment etching on reaction between chromium thin films and steel substrates[END_REF], indicating that the CreC bond is stronger than the CreN one.

Films' morphology

The surface of the CreN films shows a reflective metallic grey colour with the microstructure characterized by fine crystallite in dome form (D = 13 nm). The surface morphology shows the atom agglomeration during the growth of films (Fig. 7). The cross section SEM images, presented in Fig. 8, show the existence of a columnar structure with good adherence in the interfacial Cr-N/XC100 zone due to the low deposition temperature (Fig. 7.a). This structure is typical to the transition zone (T) according to Thornton's model [START_REF] Thornton | High rate thick film growth[END_REF]. In our previous work [START_REF] Aissani | Influence of vanadium on structure, mechanical and tribological properties of CrN coatings[END_REF], this structure was observed for the CreN films deposited by the same process. Between 800 and 900 °C (Fig. 7.c, d), the film surface becomes porous with nodular grains (D = 18 nm). After annealing at 1000 °C, the SEM images present large and rounded grains (D = 34 nm) with the appearance of black holes within these grains (Fig. 7.e). The presence of these holes can be explained by the diffusion of Cr and N elements towards the substrate. This structure is characteristic of the transition zone (T) according to Thornton's model [START_REF] Thornton | High rate thick film growth[END_REF].

In order to demonstrate the influence of temperature on the film thickness and consequently confirm the diffusion mechanism, different measurements of thickness were performed by using a Scanning Electron Microscope (SEM). It was observed that the film thickness and the surface roughness increased during the annealing treatment: e = 2.60 μm, Ra = 11 nm at 800 °C (Fig. 8.c) and e = 2.92 μm, Ra = 57 nm at 1000 °C (Fig. 8.e). Moreover, we did not observe any cracks through the film thickness after annealing treatment at different temperatures suggesting that a high concentration of grain boundaries contributes to the relaxation of stresses in films and prevents the propagation of cracks [START_REF] Benouareth | Study of the interaction between a zirconium thin film and a C100 steel substrate: temperature effect[END_REF].

The coating thickness increased significantly and the interface zone enlarged with the increase in annealing temperature, confirmed by the diffusion of carbon atoms from the substrate to the CrN film which is thermally activated. At the same time, chromium and nitrogen signal intensity decreased while annealing temperature increased, which suggests a reaction with the carbon to form carbide and carbonitride. Another reason promoting the increase of film thickness is the film microstructure composed of fine columnar grains elongated perpendicularly to the film surface. This corresponds with other work, where film thicknesses of zirconium carbide coatings have been greatly increased by raising annealing temperatures [START_REF] Benouareth | Study of the interaction between a zirconium thin film and a C100 steel substrate: temperature effect[END_REF].

During annealing treatment, the structure of steel substrates was changed from ferrite to lamellar austenite (phase transformation in XC100 steel). The annealing of the hyper-eutectoid XC100 steel (containing 1.17 wt% of C) up to 800 °C transforms the eutectoid, the ferrite and the cementite to austenite, which would lead to the dissolution of more carbon and promote the formation of chromium carbides and carbonitrides [START_REF] Benouareth | Study of the interaction between a zirconium thin film and a C100 steel substrate: temperature effect[END_REF].

Mechanical properties of films: hardness (H) and Young's modulus (E)

Surface properties

Fig. 9 presents the hardness (H) and Young's modulus (E) of CreN films annealed at different temperatures. The curves of both H and E show the same tendency. For untreated CreN film, the hardness and Young's modulus are 18.2 and 198 GPa, respectively. These values are close to that obtained by Ibrahim et al. [START_REF] Khalil Ibrahim | Annealing effects on microstructural, optical, and mechanical properties of sputtered CrN thin film coatings: experimental studies and finite element modeling[END_REF] for CrN coatings deposited by magnetron sputtering. However, the film hardness is a little bit lower than that reported in the literature (19-24 GPa) [START_REF] Aissani | Influence of vanadium on structure, mechanical and tribological properties of CrN coatings[END_REF][START_REF] Lee | Nano mechanical properties evaluation of R. F. magnetron sputtered chromium nitride thin film[END_REF][START_REF] Ehiasarian | Comparison of microstructure and mechanical properties of chromium nitride-based coatings deposited by high power impulse magnetron sputtering and by the combined steered cathodic arc/unbalanced magnetron technique[END_REF]. This may be attributed to the heat produced by high energetic ion bombardment [START_REF] Ehiasarian | Comparison of microstructure and mechanical properties of chromium nitride-based coatings deposited by high power impulse magnetron sputtering and by the combined steered cathodic arc/unbalanced magnetron technique[END_REF]. A gradual increase in the coating hardness and Young's modulus with an increase in the annealing temperature from 800 to 900 °C was observed (Fig. 9). The CreN coatings hardness increased from 18.3 to 22.4 GPa after annealing temperatures of 800 and 900 °C, respectively. This enhancement can be attributed to several factors: the grain size refinement according to Hall-Petch [START_REF] Hall | The deformation and ageing of mild steel: III discussion of results[END_REF], the grouping of defects at the film-substrate interface and the formation of the hardest chromium carbide Cr 7 C 3 compound in the film with the chromium nitrides. All these parameters will probably contribute to the film hardness increase [START_REF] Gheriani | Effect of substrate surface ion bombardment etching on reaction between chromium thin films and steel substrates[END_REF]. However, we observed a decrease in the hardness value from 22.4 GPa at 900 °C to 14.9 GPa at 1000 °C. This drop can be explained by the interaction between carbon, nitrogen and chromium atoms forming the chromium carbonitrides. In addition, the increase in grain size and the appearance of black holes, which decrease coating density and release the compressive stress in the coatings, can reduce coating resistance to the indenter penetration and decrease hardness. This result was also similarly confirmed by the formation of vanadium carbonitrides films deposited on steel tools by thermal diffusion processes [START_REF] West | Facile synthesis of bimetallic carbonitrides, V1-xTix(C,N), by microwave carbothermal reduction-ammonolysis/ carburisation (MW-CRAC) methods[END_REF]. Furthermore, the maximum measured hardness (22.4 GPa) is low compared to the values reported in the literature for CreCeN films deposited by different PVD techniques, which range between 25 and 32 GPa [START_REF] Ye | Influences of bias voltage on the microstructures and tribological performances of Cr-C-N coatings in seawater[END_REF][START_REF] Wu | Microstructure, mechanical and tribological properties of Cr-C-N coatings deposited by pulsed closed field unbalanced magnetron sputtering[END_REF]. Firstly, this can be explained by the under-stoichiometry of Cr 7 C 3 phase (C/Cr = 0.4, at 900 °C) [START_REF] Tong | Effects of carbon content on the microstructure and mechanical property of cathodic arc evaporation deposited CrCN thin films[END_REF]. Secondly, this low value can be related to the oxygen contamination of film. The hardness of chromium oxide is lower than the carbide or nitride and the oxygen incorporation in the CrN crystal lattice would weaken the covalent CeC and CeN bonding. The favourable bindings of ionic CreO lead to a decrease in the mechanical behaviour law [START_REF] Benouareth | Study of the interaction between a zirconium thin film and a C100 steel substrate: temperature effect[END_REF].

Considering the Young's modulus (Fig. 9) we can clearly see the variation of E as a function of annealing temperatures. The Young's modulus decreased slightly from 198 GPa before annealing to 192 GPa at 700 °C with more oxygen contamination (12 at.%). Schalk et al. [START_REF] Schalk | Microstructure, mechanical and optical properties of TiAlON coatings sputter-deposited with varying oxygen partial pressures[END_REF] also reported a decrease of Young's modulus with an increasing oxygen concentration for sputter-deposited TiAlON coatings. The further increase of carbon content in film, while increasing the annealing temperature from 800 to 900 °C, led to a significant increase in the Young's modulus and to achieve its maximum value of 264 GPa. This is probably due to the amorphous carbon phase coming from the formation of CeC bonds [START_REF] Schalk | Microstructure, mechanical and optical properties of TiAlON coatings sputter-deposited with varying oxygen partial pressures[END_REF], which was confirmed by the XPS analysis.

It has been revealed that the aptitude of CrN films to resist mechanical failure at different annealing temperatures is improved by a high H/E ratio [START_REF] Lin | High rate deposition of thick CrN and Cr 2 N coatings using modulated pulse power (MPP) magnetron sputtering[END_REF]. It implies that a longer elastic strain to failure for the CreCeN thin films allows the redistribution of the applied load over a large area as a function of annealing temperature (Fig. 9) [START_REF] Pengfei | Study on tribological property of CrCN coating based on magnetron sputtering plating technique[END_REF]. Before annealing treatment, the CrN coatings exhibited a higher H/E ratio (0.082) as compared to those of the CreN coatings after annealing (0.065-0.077). Therefore, this result indicates that the untreated CreN coatings have a high resistance against abrasive wear, low Young's modulus and higher toughness [START_REF] Lin | High rate deposition of thick CrN and Cr 2 N coatings using modulated pulse power (MPP) magnetron sputtering[END_REF][START_REF] Shan | Effect of N 2 flow rate on microstructure and mechanical properties of PVD CrN x coatings for tribological application in seawater[END_REF].

Evolution of H and E through the film's thickness

Fig. 10 shows the hardness and Young's modulus of the CreN coatings, annealed at 900 °C, as a function of the penetration depth. The CreN coatings exhibited lower hardness and Young's modulus values in the middle zone than those at the surface or the interface zones (Fig. 10). The CreN coatings displayed high mechanical properties (H = 22.32 GPa, E = 264 GPa) and (H = 21.50 GPa, E = 253 GPa) at the film interface and surface, respectively. According to the XPS analysis (Fig. 3), the replacement of CreN bonds by harder CreC and CreNeC bonds led to enhance the mechanical properties of the film. Therefore, the formation of chromium carbide adjacent to the chromium nitride hinders dislocation movement and leads to an increase in hardness [START_REF] Benouareth | Study of the interaction between a zirconium thin film and a C100 steel substrate: temperature effect[END_REF]. The gradual decrease in the coating hardness and Young's modulus in the middle zone can be attributed to the migration of nitrogen to the interface between film and substrate. This migration results in less interstitial elements and leads to the absorption or destruction of defects. Consequently the coating hardness decreases.

Tribological behaviour of deposited films

Friction coefficient

After annealing treatment of Cr-N/XC100 at different temperatures, the tribological properties of coatings were measured. Fig. 11 and Table 1 show friction coefficient, critical loads and wear volume values of untreated and annealed coatings. The tendency of friction coefficient values is opposite to those of hardness and the Young's modulus. Before annealing, the friction coefficient of the CreN film was 0.53 [START_REF] Aissani | Influence of vanadium on structure, mechanical and tribological properties of CrN coatings[END_REF] and initially increased to 0.62 under heat treatment at 700 °C. This can be due to removal of the superficial oxidative layer formed during the film deposition. The carbon diffusion into the CreN coating can form a self- lubricating layer, which can significantly reduce the friction coefficient of CreCeN coatings [START_REF] Shan | Effect of N 2 flow rate on microstructure and mechanical properties of PVD CrN x coatings for tribological application in seawater[END_REF]. Increasing the C content in the CreN film treated between 800 and 900 °C led to a decrease in the friction coefficient. Its lowest value of 0.42 was obtained at 900 °C because of the formation of CreC bond in the Cr 7 C 3 compound [START_REF] Pengfei | Study on tribological property of CrCN coating based on magnetron sputtering plating technique[END_REF]. However, at 1000 °C, the friction coefficient of the CrN coating increased to its highest value of 0.86 which can be attributed to the grain coalescence of the CreN coating favouring the initiation of defects through the film and accelerating the propagation of cracks during sliding [START_REF] Pengfei | Study on tribological property of CrCN coating based on magnetron sputtering plating technique[END_REF][START_REF] Shan | Effect of N 2 flow rate on microstructure and mechanical properties of PVD CrN x coatings for tribological application in seawater[END_REF]. In addition, the high surface roughness (Ra = 57 nm at 1000 °C) reduces the real contact area between the ball and CreN coating [START_REF] Pengfei | Study on tribological property of CrCN coating based on magnetron sputtering plating technique[END_REF]. Although the values of friction coefficient are not very low, this may limit the use of CreCeN films in lubricious applications. This result corresponds with previous studies performed on CreCeN and TieCeN coatings against an alumina ball [START_REF] Tillmann | Development and tribological investigation of the coating system chromium carbonitride (CrCN) to different surface designs[END_REF]. The friction coefficient in the tests with a CreCeN coating with 2.60 μm (at 900 °C) was consistently higher than for the thicker coating. It may be attributed to the small grain size, dense structure and defect of the coatings, which is consistent with the result of SEM and roughness. H. Pengfei et al. [START_REF] Pengfei | Study on tribological property of CrCN coating based on magnetron sputtering plating technique[END_REF] pointed out that the friction coefficient significantly improved as the grain size decreased.

As shown in Fig. 11, the wear volume has a similar trend to that of the friction coefficient. By increasing the annealing temperature, the wear volume increased from 3 × 10 5 μm 3 (before annealing) to 4.05 × 10 5 μm 3 (at 700 °C) and then decreased to a minimum value of 1.25 × 10 5 μm 3 at 900 °C with a further diffusion of carbon (10 at.%), corresponding to the stability of the hard chromium carbide phase. It is noteworthy that the wear volume of the very hard CreCeN coatings annealed at 900 °C is even lower than that of the relatively soft CrN nitride coating. In general, a lower friction and wear volume with increasing carbon content can be observed, which can be related to the increase of carbon bond fraction which is in agreement with literature [START_REF] Riyadh | The influence of roughness on the wear and friction coefficient under dry and lubricated sliding[END_REF].

Adhesion performance

An oscillating tribometer was used to investigate the adhesion between the CreN coatings and the XC100 steel substrates. Two critical loads were defined: the cohesive failure critical load (Lc 1 ) and the adhesion failure critical load (Lc 2 ) (Fig. 11). It is obvious that the critical loads are inversely proportional to that of the friction coefficient.

Good adhesion between coatings and substrate is a main goal for good tribological behaviour [START_REF] Lin | High rate deposition of thick CrN and Cr 2 N coatings using modulated pulse power (MPP) magnetron sputtering[END_REF]. For the CreN coating (Fig. 12.a), before annealing treatment, the wear track was wide with a critical load of Lc 2 = 24 N (Fig. 11), which makes the coating poorly worn with the appearance of Cr (30 at.%) and N (25 at.%) in the wear track due to fatigue fracture. This indicates that the friction coefficient between the coating and 100Cr6 ball is high (μ = 0.53) and suggests that the wear mechanism is generally adhesive together with slight abrasive wear.

At 700 °C (Fig. 12.b), the coating adhesion was significantly weaker than that of untreated CreN coating, which was indicated by the severe cracking and large delamination that occurred at a lower load (5 and 7 N). Taking into account the existence of chromium nitrides at 700 °C, this may be due to high oxidation of the films (18 at.% of O). Also, the difference between both the Young's modulus of coating and substrate leads to a higher concentration of shear stresses in the interfacial zone (coating-substrate), which contributes to the increase in wear volume [START_REF] Lin | High rate deposition of thick CrN and Cr 2 N coatings using modulated pulse power (MPP) magnetron sputtering[END_REF].

At 800 °C, the wear rate was slightly higher than that at 900 °C with a critical load of L c1 = 14 N (Figs. 11 and 12.c). This could be due to a more pronounced oxidation rate (O = 13 at.%) as shown by XRD analysis (Fig. 4).

At 900 °C (Fig. 12.d), the wear track became narrower and the debris on the wear track was in small amounts. The CreCeN coating presented a medium adhesive (Lc 1 = 14 N) and cohesive (Lc 2 = 28 N) critical loads. The EDS analysis of the wear track region showed that the chemical composition of the wear debris was 38 at.% Cr, 38 at.% N, 13 at.% O, 6 at.% Fe and 5 at.% C. This indicates that the adhesion decreased gradually with an increasing annealing temperature and abrasive wear. Fig. 11 shows the decrease in the wear volume with an increase in annealing temperature achieving its lowest value of 1.25 × 10 5 μm 3 at 900 °C then increased to 3.05 × 10 5 μm 3 at 1000 °C. These may be attributed to higher hardness values, grain size refinement and the formation of stronger CeC and CeN bonds. Also, good adhesion and a significant reduction of the friction coefficient depicted strong interfacial bonding between the film and the substrate, as observed during scratch testing of our coatings [START_REF] Gilewicz | Tribological properties of CrCN/CrN multilayer coatings[END_REF].

After annealing treatment at 1000 °C (Fig. 12.e), superficial chipping was observed at the edge of the scratch track which appears to be in the initial stage for the adhesive failure of chromium nitride film at Lc 2 = 12 N. This may be due to the low concentrations of Cr (14 at.%) and N (12 at.%) compared to those of Fe (44 at.%) and C (12 at.%) in the analysed scratch track. This can also be correlated with the poor adhesion and deformation of the films as observed during the scratch test. It can be concluded that the deposited coatings exhibited better mechanical and tribological properties at 900 °C. Furthermore, the presence of a high quantity of oxygen (15-20 at.%) in the wear track compared to those in the pure CrN coatings contributes to high wear rate at room temperature in ambient air.

Conclusion

The effects of annealing on CreN magnetron sputtered XC100 samples were investigated on the structure, morphology, mechanical and tribological properties of the coatings.

The stability of chromium nitrides deposited on XC100 samples was achieved between 700 and 800 °C. The formation of Cr 7 C 3 carbide was observed which is due to the diffusion of carbon atoms from steel substrate towards the deposited film from 800 °C. However, the Cr 23 C 6 carbide only appeared at T ≥ 900 °C.

The diffusion of Cr and N led to partial transformation of the chromium nitrides to chromium carbonitrides Cr 3 N 0.6 C 0.4 at 1000 °C.

The best mechanical properties of the CreN films were obtained at 900 °C with the formation of a chromium carbide and nitride mixture. Hardness and Young's modulus of films gradually decreased with the transformation of chromium nitrides to carbonitrides.

The best wear resistance and coefficient of friction in CreCeN coatings were observed at 900 °C and were 1.25 × 10 -5 μm 3 and 0.42, respectively. However, it is noteworthy that annealing at 700 and 1000 °C led to the formation of pores and oxides and, as a consequence, to the damage and total delamination of all the CreN films.

Fig. 1 .

 1 Fig. 1. Variation of Cr, Fe, C, N and O composition as a function of annealing temperatures for Cr-N/XC100 samples.

Fig. 2 .

 2 Fig.2. Variation of stoichiometric ratios as a function of annealing temperatures for Cr-N/XC100 samples.

Fig. 3 .

 3 Fig. 3. XPS profiles of Cr-N/XC100 sample annealed at 900 °C.

Fig. 4 .

 4 Fig. 4. XRD patterns of CrN/XC100 samples for different annealing temperatures.

Fig. 5 .Fig. 6 .

 56 Fig. 5. Inter-reticular distance of Cr 2 N (200) and Cr 7 C 3 (410) as a function of annealing temperature.

  Cr2p spectra can be fitted into four components, located at about 574.21 eV, 575.7 eV, 576.31 eV, 583.60 eV and 586.41 eV, corresponding to Cr 7 C 3 and Cr 2 N [25-28].

Fig. 7 .

 7 Fig. 7. SEM surface morphology of Cr-N/XC100 samples: a) before and after annealing at: b) 700 °C, c) 800 °C, d) 900 °C and e) 1000 °C.

Fig. 8 .

 8 Fig. 8. Cross section SEM morphology of Cr-N/XC100 samples: a) before and after annealing at: b) 700 °C, c) 800 °C, d) 900 °C and e) 1000 °C.

Fig. 9 .

 9 Fig. 9. Hardness, Young's modulus and H/E ratio of CreN coatings for different annealing temperatures.

Fig. 10 .

 10 Fig. 10. Hardness and Young's modulus of the CrN coatings, annealed at 900 °C, as a function of the penetration depth.

Fig. 11 .

 11 Fig. 11. Friction coefficient, critical loads (Lc 1 , Lc 2 ) and wear volume of CreN coatings for different annealing temperatures.

Fig. 12 .

 12 Fig. 12. SEM images and EDS analysis of wear tracks of Cr-N/XC100: a) before and after annealing at: b) 700 °C, c) 800 °C, d) 900 °C and e) 1000 °C.

Table 1

 1 Mechanical and tribological properties of CreCeN coatings.

	Temperature, °C Mechanical properties	Tribological properties
		H, GPa	E, GPa	H/E	CoF W v (×10 5 ),	Lc 1 Lc 2
					μm 3
	Before annealing	18.22	198	0.092 0.53	3.00	11 24
	700	16.00	190	0.084 0.62	4.05	5	7
	800	18.26	200	0.091 0.55	1.50	10 24
	900	22.39	256	0.087 0.42	1.25	14 28
	1000	14.86	200	0.074 0.83	3.05	7