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Article The Structure, Morphology, and Mechanical Properties of Ta-Hf-C Coatings Deposited by Pulsed Direct Current Reactive Magnetron Sputtering

Alexis de Monteynard 1,2 , Huan Luo 3, Mohamed Chehimi 4, Jaafar Ghanbaja 5, Sofiane Achache 1,2, Manuel François 1 , Alain Billard 3 and Frédéric Sanchette 1,2,*

1 ICD LASMIS: Université de Technologie de Troyes, Antenne de Nogent, Pôle Technologique de Haute-Champagne, 52800 Nogent, France; [email protected] (A.M.); sofi[email protected] (S.A.); [email protected] (M.F.) 2 Nogent International Center for CVD Innovation, LRC CEA-ICD LASMIS, UT.T, Antenne de Nogent-52, Pôle Technologique de Haute-Champagne, 52800 Nogent, France 3 Institut FEMTO-ST: CNRS, UTBM, Univ. Bourgogne Franche-Comté, Site de Montbéliard, F-90010 Belfort Cedex, France; [email protected] (H.L.); [email protected] (A.B.) 4 Université Paris Est: ICMPE (UMR 7182), CNRS, UPEC, 2-8 rue Henri Dunant, 94320 Thiais, France; [email protected] 5 Institut Jean Lamour (UMR CNRS 7198), Université de Lorraine, 54000 Nancy, France; [email protected] * Correspondence: [email protected]



Received: 31 January 2020; Accepted: 27 February 2020; Published: 28 February 2020


Abstract: Ta, Hf, TaCx, HfCx, and TaxHf1-xCy coatings were deposited by reactive pulsed Direct Current (DC) magnetron sputtering of Ta or Hf pure metallic targets in Ar plus CH4 gas mixtures. The properties have been investigated as a function of the carbon content, which is tuned via the CH4 flow rate. The discharge was characterized by means of Optical Emission Spectroscopy and, in our conditions, both Ta-C and Hf-C systems seem to be weakly reactive. The structure of the as-deposited pure tantalum film is metastable tetragonal β-Ta. The fcc-MeCx carbide phases (Me = Ta or Hf) are {111} textured at low carbon concentrations and then lose their preferred orientation for higher carbon concentrations. Transmission Electron Microscopy (TEM) analysis has highlighted the presence of an amorphous phase at higher carbon concentrations. When the carbon content increases, the coating’s morphology is first compact-columnar and becomes glassy because of the nano-sized grains and then returns to an open columnar morphology for the higher carbon concentrations. The hardness and Young’s modulus of TaCx coatings reach 36 and 405 GPa, respectively. For HfCx coatings, these values are 29 and 318 GPa. The MeCx coating residual stresses increase with the addition of carbon (from one-hundredth of 1 MPa to 1.5 GPa approximately). Nevertheless, the columnar morphology at a high carbon content allows the residual stresses to decrease. Concerning TaxHf1-xCy coatings, the structure and the microstructure analyses have revealed the creation of a nanostructured coating, with the formation of an fcc superlattice. The hardness is relatively constant independently of the chemical composition (22 GPa). The residual stress was strongly reduced compared to that of binary carbides coatings, due to the rotation of substrates.

Keywords: UHTCs: TEM microscopy; Ta-Hf-C coatings; reactive magnetron sputtering

1. Introduction Nowadays, surface treatments are widely used at industrial scale to improve the lifetime of many components or tools in many applications fields [1]. Surface functionalities can be addressed by a wide

Coatings 2020, 10, 212; doi:10.3390/coatings10030212 www.mdpi.com/journal/coatings Coatings 2020, 10, 212 2 of 17 variety of methods. Mechanical treatment, such as SMAT (Surface Mechanical Attrition Treatment), can significantly enhance mechanical properties by severe plastic deformation for producing superficial ultrafine-grained alloys and metals [2]. Deposition technologies at atmospheric pressure (sol-gel, spraying based technologies, etc.) can provide the surface protection of biomedical materials to improve corrosion resistance and enhance biocompatibility [3]. Plasma spray techniques are also used for coatings in extreme environments such as thermal barrier coatings (TBCs), which are advanced ceramic coatings that are applied to metallic surfaces such as gas turbine blades and aerospace engines [4,5]. Since the 1980s, low-pressure physical or chemical vapor deposition techniques are also widely used for thin film deposition in many application fields, including antibacteria [6], anticorrosion [7,8], and hard coatings [9,10]. Ultra High Temperature Ceramics (UHTCs) offer an exceptional combination of properties as high mechanical and thermal resistances (a melting point over 3000 K) [11]. These materials can be used in various applications working in extreme conditions, such as hypersonic vehicles or solar cells [12–15]. TaC and HfC are known as the two binary carbides with the highest melting temperatures (Tm)[16]. This has recently been confirmed by Cedillos-Barraza et al., who measured melting temperatures with a laser system on bulk materials sintered by Spark Plasma Sintering (SPS); Tm(TaC) = 4041 K and Tm(HfC) = 4232 K [17,18]. Some authors have studied the synthesis of TaCx and HfCx coatings by reactive or non-reactive magnetron sputtering. However, within the Ta-Hf-C system, some ternary alloys can offer melting points beyond the HfC and TaC binary carbides. Lasfargues et al. have deposited HfC and TaC coatings from compound targets [19]. They obtained chemical composition ranging from HfC0.66 to HfC0.76 and from TaC0.61 to TaC0.81 by tuning the substrate’s bias voltage and temperature. The authors attribute the composition variation to the enhancement of the adatom’s mobility by increasing those two parameters. All the HfC coatings exhibit a faced-centered cubic (fcc) structure with a {111} texture (which becomes random when the bias substrate increases), whereas TaC coatings (also fcc) show a small fraction of Ta2C (hcp-structured) [19]. They obtained a compact, smooth, and V-shaped columnar growth morphology for all coatings. Evans et al. have synthesized TaC/a-C:H coatings by the reactive magnetron sputtering of a pure metallic target in Ar-C2H2 gas mixtures [20–22]. They show that a coating’s hardness drops from 18 to 12 GPa when the fraction of the amorphous phase (a-C:H) increases [20]. A bias voltage (between –50 and –150 V) applied during the deposition favors TaC crystallization, which allows the hardness to increase from 12 to 20 GPa [21]. Vargas et al. have used CH4 as a reactive gas to synthesize TaC thin films [23]. A nanocrystallized TaC film was deposited, and the composition range was between TaC0.29 and TaC1.17. HfC coatings can be synthesized in a reactive mode with C2H2 or CH4 [24,25]. The same effect was observed in TaC coatings; that is, the hardness decreases due to the appearance of an amorphous phase. The aforementioned references, concerning the synthesis of TaCx and HfCx coatings by magnetron sputtering, are all that are found in the literature. Thus, there is a lack of data on the influence of both deposition parameters and carbon content on the structure, morphology, and mechanical properties of TaCx and HfCx coatings synthesized by reactive magnetron sputtering. Indeed, Jansson et al. have published an extensive review about the sputter deposition of transition metal carbide, and they strongly insist on the role of carbon content in tuning certain coating properties (such as the structure or the microstrucuture) [26]. Moreover, the correlation between the characterization of plasma species from the Ta-Ar-CH4 and Hf-Ar-CH4 discharges and the resulting coatings properties is lacking in the literature. The aim of this work is to synthesize TaCx and HfCx thin films by pulsed DC reactive magnetron sputtering, with CH4 as the reactive gas. The use of CH4 helps to provide better control of both the coating microstructure and the composition compared to C2H2, due to the lower amount of carbon brought. Indeed, if those coatings are used in applications implying a high temperature, the amorphous a-C:H phase easily formed by C2H2 limits the thermal stability. Furthermore, the relationship between the discharge characterization, the deposition parameters, and the coating’s properties was established. Only one reference was found in the literature on TaxHf1-xCy ternary alloy Coatings 2020, 10, 212 3 of 17

coatings obtained by magnetron sputtering [27]. Due to the lack of literature on TaxHf1-xCy coatings, it is necessary to control the process with binary carbides, in order to determine the experimental conditions necessary to synthesize ternary alloys. In this work, we, for the first time, synthesized TaxHf1-xCy coatings by reactive magnetron sputtering with a characterization of the structure, the microstructure, the morphology, and the mechanical properties (hardness, Young’s modulus, and residual stress).

2. Experimental Details

2.1. Deposition

Ta, TaCx, Hf, HfCx, and TaxHf1-xCy thin films were deposited by pulsed DC reactive magnetron sputtering of disc targets (200 mm in diameter) in a 100 L Alcatel vacuum chamber in pure Ar or in 4 Ar-CH4 reactive discharges. The deposition chamber was pumped down to 10− Pa before each run. Substrates (Si wafers, glass slides, and M2 steel) were degreased in acetone by an ultrasonic bath and then successively rinsed by ethanol and dried with warm air. The target-to-substrate distance was 13 cm. A substrate etching was performed in pure argon before each deposition stage with biasing at 600 V for 30 min. Ta (99.9% purity) and Hf (99.9% purity) targets were also etched in pure argon at − 2 A. Solvix (50 kHz) and Advanced Energy MDX pulsed DC (50 kHz) power supplies powered the substrate holder and targets, respectively. The argon flow rate (DAr) was kept constant at 30 sccm for all experiments, and the CH4 flow rate (DCH4) increased from 1 to 6 sccm for the coating’s synthesis in reactive conditions. The working pressure was 1 Pa, and the substrate holder was kept at floating potential for all experiments. Optical Emission Spectroscopy (OES) measurements were carried out with a triple optical fiber placed at mid-distance between the target and the substrate. The Ar0, Ta0, Hf0, and H wavelengths Ta0 H f 0 Hα used to calculate the I , I , and I ratios ((I) being the peak intensity) were 750.4, 696.9, 706.4, and IAr0 IAr0 IAr0 656.3 nm respectively. Experimental conditions of the Ta, TaCx, Hf, and HfCx coatings synthesized by pulsed DC reactive and non-reactive magnetron sputtering are summarized in Table1.

Table 1. Deposition conditions of Ta, TaCx, Hf, and HfCx.

Deposited DCH4 Discharge Total Pressure DCH4 (sccm) DAr (sccm) (%) Material DCH4+DAr Current (A) (Pa) Ta 0 30 0 2 1

TaCx 1–6 30 3.2–16.7 2 1 Hf 0 30 0 2 1

HfCx 1–6 30 3.2–16.7 2 1

Concerning TaxHf1-xCy coatings, the discharge current on each (Ta and Hf) target was tuned between 1 and 3 A by maintaining the total current dissipated on both targets at 4 A. The CH4 flow rate was 8 sccm (indeed, the pure TaC and HfC phases were obtained with 4 sccm of CH4, as shown in Section 3.2). The substrate holder rotation was set to 10 rpm. Table2 summarizes the experimental conditions of the ternary alloy synthesis. Coatings 2020, 10, 212 4 of 17

Table 2. Deposition conditions of TaxHf1-xCy coatings.

Ta Target Hf Target Discharge current (A) 1–3 1–3 Frequency (kHz) 50 50 Target–substrate distance (cm) 13 13 Pressure (Pa) 1 1

DCH4 (sccm) 8 8 DAr (sccm) 30 30

2.2. Characterizations The coatings’ thickness was measured by an Altisurf 500 profilometer, and it remained between 2 and 3 µm. The coatings’ cross-sectional morphology was observed with an FEG (Field Emission Gun), Hitachi SU 8030 SEM. X-Ray Diffraction (XRD) was performed with a Brüker D8 Advance diffractometer using Cu K radiation (= 1.54060 Å; accelerating voltage = 40 kV; current = 40 mA) in θ–2θ mode from 20◦ to 120◦ with a 0.02◦ step. Young’s modulus and hardness were obtained by means of a Nano Indenter XP, MTS Systems Corporation, with a Continuous Stiffness Measurement (CSM) option. The indenter is a three-side pyramidal diamond tip (Berkovitch). The values of hardness and Young’s modulus were measured from an average of 10 indents. The depth of penetration was 200 nm. Ta4 f The coating’s composition was estimated via XPS measurement by calculating (Ta4 f +C1s) and H f 4 f + (H f 4 f +C1s) area peak ratios. Ar etching was performed to eliminate the surface’s contamination layer (60 s for TaCx coatings and 2 min for HfCx coatings). A resolution of 0.1 eV was used with K (Thermo) fitted with a monochromatic Al Kα X-ray source (spot size: 400 µm) and a flood gun for static charge compensation. The transmission electron microscope (TEM) was a JEM - ARM 200F Cold FEG TEM/STEM operating at 200 kV and equipped with a spherical aberration (Cs) probe and image correctors (a point resolution of 0.12 nm in TEM mode and 0.078 nm in STEM mode). The residual stresses are measured from an iron-substrate curvature before and after deposition, using an Altisurf profilometer. The stress was then calculated using Stoney’s equation [28], as in [29].

3. Results and Discussion

3.1. Reactive Discharge Characterization

The Ta-Ar and Hf-Ar discharges seem to have a similar behavior when the CH4 flow rate increases. 0 ITa0 IH f A continuous decrease of IAr0 (Figure1a) and IAr0 (Figure1c) ratios was observed with an increasing C2H2 flow rate. These evolutions are due to the decrease in the metallic species density, which can first be explained by the target’s poisoning associated with the compound’s sputtering yield, which is lower than that of the metal. The decrease in the argon partial pressure when CH4 is introduced at a constant total pressure also leads to a decrease in the sputtering rate. The electronic energy was also + shared between the Ar excitation/ionization and the CH4 dissociation, leading to an Ar rarefaction. IHα The IAr0 (Figure1b,d corresponding to Ta-Ar-CH 4 and Hf-Ar-CH4 discharges) ratio increases when CH4 is introduced into the chamber. This behavior is due to the increase of the H radicals concentration following the different dissociation reactions of CH4 molecules. CoatingsCoatings 20202020,, 1010,, x 212 FOR PEER REVIEW 55 of of 17 17 Coatings 2020, 10, x FOR PEER REVIEW 5 of 17

0 α ITa 0 IH 훼 Figure 1. Evolution of (a) the IAr퐼푇푎0 ratio and (b) the IAr0퐼퐻ratio according to the CH4 flow rate (from 1 to 6 Figure 1. Evolution of (a) the 0 ratio and (b) the 0 ratio0 according to the CH4 flow rate (from 1 Figure 1. Evolution of (a) the 퐼퐴푟 ratio and (b) the 퐼퐴푟 ratioIH f according to the CHIH4α flow rate (from 1 sccm) within Ta-Ar discharges and0 the evolution of (c)𝛼𝛼 the ratio퐼퐻 and푓0 (d) the ratio according퐼퐻훼 to to 6 sccm) within Ta-Ar discharges𝐼𝐼𝐼𝐼𝑎𝑎 and the evolution𝐼𝐼𝐼𝐼 ofIAr (c0) the ratio andIAr 0 (d) the ratio 0 0 퐼퐴푟0 퐼퐴푟0 tothe 6 CHsccm)4 flow within rate Ta (from-Ar 1discharges to 6𝐼𝐼𝐼𝐼 sccm)𝑟𝑟 and within the Hf-Ar evolution discharges.𝐼𝐼𝐼𝐼𝑟𝑟 of (c) the For both0 ratio Ta-Ar and and (d Hf-Ar) the discharges,𝛼𝛼 ratio according to the CH4 flow rate (from 1 to 6 sccm) within Hf-Ar discharges.𝐼𝐼𝐼𝐼𝑓𝑓 For both Ta-Ar and𝐼𝐼𝐼𝐼 Hf-Ar accordingthe total pressureto the CH and4 flow discharge rate (from current 1 to are6 sccm) kept within constant Hf at-Ar 1 Padischarges. and 20 A, For respectively both Ta-Ar and0 Hf-Ar discharges, the total pressure and discharge current are kept constant𝐼𝐼𝐼𝐼𝑟𝑟 at 1 Pa and 2 A, respectively𝐼𝐼𝐼𝐼𝑟𝑟 discharges, the total pressure and discharge current are kept constant at 1 Pa and 2 A, respectively Figure2 shows the evolution of the TaC x (Figure2a) and HfC x (Figure2b) coatings’ growth Figure 2 shows the evolution of the TaCx (Figure 2a) and HfCx (Figure 2b) coatings’ growth rate rateFigure when the2 shows inlet CHthe 4evolutionflow rate of increases. the TaCxThe (Figure growth 2a) and rate HfC dropsx (Figure as DCH 2b)4 increases coatings’ for growth both TaCratex when the inlet CH4 flow rate increases.µ The growth rate drops as µDCH4 increases for both TaCx and whenand HfC the xinletcoatings CH4 flow (from rate 4 to increases. 1.5 m/h The for TaCgrowthx and rate from drops 5 to as 2 DCHm/h4 forincreases HfCx). for This both is aTaC commonx and HfCx coatings (from 4 to 1.5 µm/h for TaCx and from 5 to 2 µm/h for HfCx). This is a common HfCphenomenonx coatings (from due to 4 the to sputtering1.5 µm/h for yield TaC ofx theand compound from 5 to lower2 µm/h than for thatHfC ofx). theThis metal. is a common This also phenomenon due to the sputtering yield of the compound lower than that of the metal. This also phenomenonconfirms the target’sdue to the poisoning sputtering as seen yield in of Figure the compound1. Moreover, lower the Arthan partial that pressureof the metal. declines This when also confirms the target’s poisoning as seen in Figure 1. Moreover, the Ar partial pressure declines when confirmsCH4 is introduced the target’s becausepoisoning the as total seen pressure in Figure is 1. kept Moreover, constant the at Ar 1 partial Pa. Hence, pressure the sputteringdeclines when rate CH4 is introduced because the total pressure is kept constant at 1 Pa. Hence, the sputtering rate CHdecreases,4 is introduced which is because consistent the with total the pressure previous is observationskept constant (Figure at 1 Pa.1). Hence, the sputtering rate decreases, which is consistent with the previous observations (Figure 1). decreases, which is consistent with the previous observations (Figure 1).

FigureFigure 2 2.. DepositionDeposition rate as aa functionfunction ofof CH CH44flow flow rate rate at at constant constant current current and and pressure pressure (2 A(2 andA and 1 Pa) 1 Figure 2. Deposition rate as a function of CH4 flow rate at constant current and pressure (2 A and 1 Pa)for (fora) TaC(a) TaCx coatingsx coatings and and (b) ( HfCb) HfCx coatings.x coatings. Pa) for (a) TaCx coatings and (b) HfCx coatings. 3.2. Composition, Structure, and Microstructure 3.2. Composition, Structure, and Microstructure 3.2. Composition, Structure, and Microstructure The measured atomic composition of TaCx and HfCx coatings according to the CH4 flow rates are The measured atomic composition of TaCx and HfCx coatings according to the CH4 flow rates summarizedThe measured in Table atomic3. The composition carbon content of TaC increasesx and asHfC thex coatings CH 4 flow according rate increases to the for CH both4 flow TaC ratesx and are summarized in Table 3. The carbon content increases as the CH4 flow rate increases for both TaCx areHfC summarizedx coatings. Nevertheless,in Table 3. The the carbon composition content increases of TaCx coatings as the CH seems4 flow to rate remain increases constant for fromboth TaC 4 tox 6 and HfCx coatings. Nevertheless, the composition of TaCx coatings seems to remain constant from 4 andsccm HfC of CHx coatings.4 while keepingNevertheless, a stoichiometric the composition compound of TaC withoutx coatings forming seems an to over-stoichiometric remain constant from coating. 4 to 6 sccm of CH4 while keeping a stoichiometric compound without forming an over-stoichiometric to 6 sccm of CH4 while keeping a stoichiometric compound without forming an over-stoichiometric coating. coating.

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Table 3. Chemical composition of TaCx and HfCx coatings measured by XPS.

CH Flow Rate TaC Coatings HfC Coatings 4 Ta4f (atom%) x Hf4f (atom%) x (sccm) (Ta4f+C1s) Formula (Hf4f+C1s) Formula

1 17.6 TaC0.35 10.1 HfC0.20

2 32.9 TaC0.66 16.8 HfC0.34

3 37.9 TaC0.76 30.2 HfC0.60

4 51.7 TaC1.03 40.5 HfC0.81

5 51.8 TaC1.04 45.1 HfC0.90

6 50.0 TaC1.00 47.4 HfC0.95

The evolution of TaCx and HfCx coatings structure as a function of the CH4 flow rate is shown in Figures3 and4, respectively. When pure Ta is sputtered (Figure3a), a β-Ta tetragonal structure textured {002}, which is not referenced in the pressure-temperature diagram of Ta [30,31], is obtained. The stable phase α-Ta has a body-centered cubic (bcc) structure. The β-Ta metastable structure is commonly obtained in sputtering processes [32–36]. Collin et al. have proposed a theory explaining the precipitation of this phase [36] by thermodynamic calculations. TEM imagery has revealed the presence of an amorphous tantalum phase (a-TaSi) at the substrate/β-Ta interface. Thermodynamic calculations have shown that growth on the amorphous a-TaSi phase of the β-Ta phase is more favorable than that of the α-Ta phase. A growth with planes (002) parallel to the substrate’s surface, characterized by a strong {002} texture in the XRD pattern (Figure3a), is an expected result because the film grows according to the planes with the lowest surface energy. However, Bernoulli et al. have proposed another hypothesis based on the observed results in the literature [37]. They assume that the chemical composition of the substrate’s surface (the presence of a native oxide, for example) has a strong influence on the Ta germination and growth. When 1 sccm of CH4 is introduced (Figure3b), the α-Ta bcc phase with a {110} texture is stabilized. This could be due to the insertion of carbon in the lattice, which creates a local compressive stress, allowing a tetragonal to bcc transition via lattice contraction. Furthermore, the shortest Ta-Ta distance in the β-Ta tetragonal cell is smaller than that of the α-Ta bcc cell (i.e. 0.265 nm against 0.286 nm, respectively) [38]. It should be easier for carbon to be inserted in the bcc structure, and this could be a reason why tetragonal β Ta is transformed into bcc α-Ta. A fcc nanocristalined TaC structure with a {111} texture is obtained for 2 sccm of CH4. The texture is lost when DCH4 increases until 6 sccm. The loss of preferential orientation can be explained by a reduced adatom mobility associated with alower bombardment of the growing film. The evolution of TaC0.76 microstructure across the film by TEM is shown in Figure5. The Selected Area Electron Diffraction (SAED) patterns confirm the presence of a TaC fcc crystalline phase. However, both the microstructure and crystalline orientation evolve following the selected zone within the film. Near the substrate/film’s interface (Figure5a,b), a polycrystalline film growth with equiaxed grain oriented following the <111> direction is observed. When the coating becomes thicker (Figure5c–f), a columnar microstructure appears. The <111> preferential orientation is also reinforced. This is a common phenomenon of thin film growth [39], and this is consistent with the XRD pattern in Figure3d. Twin grains can also be observed (Figure5c). CoatingsCoatings 20202020, 10, 10, 212, x FOR PEER REVIEW 7 of 17 7 of 17

Figure 3. XRD patterns for different flow rates. (a) Pure Ta, (b) 1 sccm of CH4, (c) 2 sccm of CH4, (d) 3 Figure 3. XRD patterns for different flow rates. (a) Pure Ta, (b) 1 sccm of CH4, (c) 2 sccm of CH4, (d) 3 sccm of CH4, (e) 4 sccm of CH4, (f) 5 sccm of CH4, and (g) 6 sccm of CH4. sccm of CH4, (e) 4 sccm of CH4, (f) 5 sccm of CH4, and (g) 6 sccm of CH4.

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Figure 4. XRD patterns for different flow rates. (a) Pure Hf, (b) 1 sccm of CH4, (c) 2 sccm of CH4, (d) 3

sccm of CH4, (e) 4 sccm of CH4, (f) 5 sccm of CH4, and (g) 6 sccm of CH4.

Figure 4. XRD patterns for different flow rates. (a) Pure Hf, (b) 1 sccm of CH4, (c) 2 sccm of CH4, (d) 3 sccm of CH4, (e) 4 sccm of CH4, (f) 5 sccm of CH4, and (g) 6 sccm of CH4.

The evolution of TaC0.76 microstructure across the film by TEM is shown in Figure 5. The Selected Area Electron Diffraction (SAED) patterns confirm the presence of a TaC fcc crystalline phase. However, both the microstructure and crystalline orientation evolve following the selected zone within the film. Near the substrate/film’s interface (Figure 5a,b), a polycrystalline film growth with equiaxed grain oriented following the <111> direction is observed. When the coating becomes thicker (Figure 5c–f), a columnar microstructure appears. The <111> preferential orientation is also reinforced. This is a common phenomenon of thin film growth [39], and this is consistent with the XRD pattern in Figure 3d. Twin grains can also be observed (Figure 5c).

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Figure 5. TEM cross section images of TaC0.76 coating. (a) The substrate-film interface, (b) the corresponding SAED, (c) at the middle of the film, (d) the corresponding SAED, (e) at the top of the film, andFigure (f) the 5. correspondingTEM cross section SAED. images of TaC0.76 coating. (a) The substrate-film interface, (b) the Figure 5. TEMcorresponding cross section SAED, (c )images at the middle of ofTaC the0.76 film, coating. (d) the corresponding (a) The substrate SAED, (e) at-film the top interface, of the (b) the film, and (f) the corresponding SAED. correspondingThe evolution SAED, of the (c )HfC at thex coatings middle structureof the film, is (similard) the corresponding to that of the TaC SAED,x coatings. (e) at the Nevertheless, top of the film, and (f) the corresponding SAED. the stable hcpThe α evolution-Hf metallic of the HfCstructurex coatings with structure a {002} is similar texture to that is offirst the TaCobtainx coatings.ed when Nevertheless, the Hf target is sputteredthe in stable pure hcpargonα-Hf (Figure metallic 4a). structure When with 1 sccm a {002} of texture CH4 is firstintroduced obtained ( whenFigure the 4b), Hf targetthe α is-Hf phase remainsThe evolution hcpsputtered, but of inthe purethe insertion HfC argonx (Figurecoa of tingscarbon4a). Whenstructure leads 1 sccm to theis of similar CHpeak4 is shift introducedto that toward of (Figure the smaller TaC4b),x the anglescoatings.α-Hf because phase Nevertheless, of the the lattice’sstable hcpdistortionremains α-Hf hcp, metallicassociated but the insertion structure with of the carbon carbonwith leads a atom’s{002} to the peak textureinsertion. shift towardis Figurefirst smaller obtained 6a anglesconfirms becausewhen the ofthecoating’s the Hf target hcp is lattice’s distortion associated with the carbon atom’s insertion. Figure6a confirms the coating’s hcp sputteredstructure. instructure. pureIndeed argon, Indeed, the FFT (Figure the carried FFT carried4a). out When out with with an1 an sccm HRTEM HRTEM of image,CH image,4 is coupled introducedcoupled to the to theoretical the (Figure theoretical pattern 4b), withthpatterne α-Hf with phase remainsthe [100] hcp thezone, but [100] axisthe zone ,insertion calculated axis, calculated of using carbon using jems jems leads software software to the (Figure(Figure peak6b), shift6b) helps, helps toward to highlight to highlight smaller this structure. anglesthis structure. because of the lattice’s distortion associated with the carbon atom’s insertion. Figure 6a confirms the coating’s hcp structure. Indeed, the FFT carried out with an HRTEM image, coupled to the theoretical pattern with the [100] zone axis, calculated using jems software (Figure 6b), helps to highlight this structure.

(a) (b)

Figure 6. (a) HRTEM image of an HfC0.20 coating. (b) Simulated indexed electron diffraction pattern with a [100] zone axis.

Figure 6. (a) HRTEM image of an HfC0.20 coating. (b) Simulated indexed electron diffraction pattern Figurewith 6. a ([100]a) HRTEM zone axis. image of an HfC0.20 coating. (b) Simulated indexed electron diffraction pattern with a [100] zone axis. From 2 to 6 sccm of CH4, a NaCl fcc structure of the carbide phase (HfC) is obtained where the

{111}From texture 2 to 6 issccm also of progressively CH4, a NaCl lost. fcc structureTEM microscopy of the carbide (Figure phase 7) also (HfC) shows is obtainedan fcc structure. where the {111}Nevertheless, texture is alsoa halo progressively on the FFT, Figurelost. TEM 7c, reveals microscopy the presence (Figure of 7)an also amorphous shows anphase. fcc Thisstructure. is Neverthelessprobably an, a a -haloC:H phaseon the commonly FFT, Figure encountered 7c, reveals in transition the presence metal ofcarbide an amorphous coatings [26] phase.. This is probably an a-C:H phase commonly encountered in transition metal carbide coatings [26].

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From 2 to 6 sccm of CH4, a NaCl fcc structure of the carbide phase (HfC) is obtained where the {111} texture is also progressively lost. TEM microscopy (Figure7) also shows an fcc structure. Nevertheless, a halo on the FFT, Figure7c, reveals the presence of an amorphous phase. This is probably an a-C:H phaseCoatings commonly 2020, 10, x FOR encountered PEER REVIEW in transition metal carbide coatings [26]. 10 of 17 Coatings 2020, 10, x FOR PEER REVIEW 10 of 17

FigureFigure 7.7.( a(a)) TEM TEM image image of of HfC0.95 HfC0.95 coating. ( (bb)) The corresponding SAED. ((c)) TheThe HRTEMHRTEM imageimage ofof Figure 7. (a) TEM image of HfC0.95 coating. (b) The corresponding SAED. (c) The HRTEM image of thethe HfC0.95 HfC0.95 coating. coating. the HfC0.95 coating.

ConcerningConcerning TaTaxxHfHf1-x1-xCy coatings,coatings, Figure Figure 88 showsshows thethe microstructuremicrostructure ofof aa coating coating where where 3A 3A and and Concerning TaxHf1-xCy coatings, Figure 8 shows the microstructure of a coating where 3A and 1A1A werewere appliedapplied toto thethe TaTa and and Hf Hf targets, targets, respectively. respectively. It Itreveals reveals a ananostructured nanostructured coatingcoating wherewhere thethe 1A were applied to the Ta and Hf targets, respectively. It reveals a nanostructured coating where the periodperiod isis 2.22.2 nm.nm. IfIf thethe periodperiod isis predictedpredicted withwith EquationEquation (1),(1), thetheexperimental experimental periodperiod isis consistentconsistent period is 2.2 nm. If the period is predicted with Equation (1), the experimental period is consistent withwith thethe calculated calculated period: period: with the calculated period: e  = n . =  = 𝑒𝑒𝑛𝑛 n.t where. (nm) is the period, en (nm) is the coating’s thickness, n (rpm) is the rotation speed, and t (min) 𝑛𝑛𝑒𝑒𝑛𝑛𝑡𝑡 wherewhere (nm) (nm) is is the the period, period, e nen(nm) (nm) is is the the coating’s coating’s thickness, thickness, n n (rpm) (rpm) is is the the rotation rotation speed, speed and, and t (min) t (min) is is the𝑛𝑛 deposition𝑡𝑡 time. The calculated period is about 2 nm in our conditions. theis the deposition deposition time. time. The The calculated calculated period period is aboutis about 2 nm 2 nm in ourin our conditions. conditions.

Figure 8. (a) STEM image of a TaxHf1-xCy coating with 3A and 1A applied on Ta and Hf targets; (b) Figure 8. (a) STEM image of a TaxHf1-xCy coating with 3A and 1A applied on Ta and Hf targets; (b) Figurethe measured 8. (a) STEM period. image of a TaxHf1-xCy coating with 3A and 1A applied on Ta and Hf targets; (b) the measuredthe measured period. period. The XRD patterns (Figure 9) show a crystalline monophasic fcc material, which is a result coming TheThe XRDXRD patternspatterns (Figure(Figure9 )9) show show a a crystalline crystalline monophasic monophasic fcc fcc material, material which, which is is a a result result coming coming from the superlattice TaC/HfC coating. The chemical composition does not influence the coatings’ fromfrom thethe superlatticesuperlattice TaCTaC/HfC/HfC coating.coating. TheThe chemicalchemical compositioncomposition does not influenceinfluence the coatings’coatings’ crystallographic structure. crystallographiccrystallographic structure.structure.

CoatingsCoatings 20202020, 10, 10, x, FOR 212 PEER REVIEW 1111 of of1717 Coatings 2020, 10, x FOR PEER REVIEW 11 of 17

Figure 9. XRD patterns of Ta Hf C coatings. (a) 1A on Ta and 3A on Hf, (b) 2A on Ta and 2A on Hf, Figure 9. XRD patterns of TaxHfx 1-xC1-xy coatings.y (a) 1A on Ta and 3A on Hf, (b) 2A on Ta and 2A on Hf, Figure 9. XRD patterns of TaxHf1-xCy coatings. (a) 1A on Ta and 3A on Hf, (b) 2A on Ta and 2A on Hf, andand (c) ( c3A) 3A on on Ta Ta and and 1A 1A on on Hf. Hf. and (c) 3A on Ta and 1A on Hf. 3.3. Morphology 3.3. Morphology3.3. Morphology The evolution of TaCx and HfCx coatings’ morphology is shown in Figures 10 and 11, respectively. The evolution of TaCx and HfCx coatings’ morphology is shown in Figures 10 and 11, They exhibitThe ae veryvolution similar of evolutionTaCx and when HfC DCHx coatings’4 increases. morphology Pure metals is (Tashown and Hf)in Figures have a columnar 10 and 11, respectively. They exhibit a very similar evolution when DCH4 increases. Pure metals (Ta and Hf) morphology,respectively. probably They dueexhibit to the a very high similar working evolution pressure when (1 Pa), DCH which4 increases. limits the Pure adatom’s metals mobility (Ta and to Hf) have a columnar morphology, probably due to the high working pressure (1 Pa), which limits the createhave a compact a columnar morphology. morphology, When probably CH4 is introduced, due to the thehigh morphology working pressure loses its (1 columnar Pa), which character limits the adatom’s mobility to create a compact morphology. When CH4 is introduced, the morphology loses to becomeadatom’s glassy. mobility This to e ffcreateect can a compact be observed morphology. in Figures When 10c and CH 114 isc whenintroduced, 2 sccm the of morphology CH4 are used. loses its columnar character to become glassy. This effect can be observed in Figure 10c and Figure 11c Thisits eff columnarect must becharacter related to thebecome size ofglassy. the grain, This whicheffect can might be beobse veryrved fine. in Figure This is consistent10c and Figure with 11c when 2 sccm of CH4 are used. This effect must be related to the size of the grain, which might be very XRDwhen patterns 2 sccm (Figures of CH3c4 are and used.4c) where This theeffect peaks must are be wide, related traducing to the size a nanocrystalline of the grain, which structure. might The be very fine. This is consistent with XRD patterns (Figure 3c and Figure 4c) where the peaks are wide, columnarfine. This growth is consistent is observed with when XRD DCH patterns4 continues (Figure to increase 3c and due Figure to the 4c) carbide where phase the crystallization peaks are wide, traducing a nanocrystalline structure. The columnar growth is observed when DCH4 continues to andtraducing the growth a ofnanocrystall grains. ine structure. The columnar growth is observed when DCH4 continues to increaseincrease due to due the to carbide the carbide phase phase crystallization crystallization and the and growth the growth of grains. of grains.

Figure 10. Evolution of TaCx coatings morphology. (a) Pure Ta, (b) TaC0.35 with 1 sccm of CH4, Figure 10. Evolution of TaCx coatings morphology. (a) Pure Ta, (b) TaC0.35 with 1 sccm of CH4, (c) (c) TaC0.66Figure with10. Evolution 2 sccm of of CH4, TaC (dx )coatings TaC0.76 morphology. with 3 sccm of(a CH4,) Pure and Ta, ( e(b) TaC1.00) TaC0.35 withwith 61 sccmsccm of of CH4. CH4, (c) TaC0.66 with 2 sccm of CH4, (d) TaC0.76 with 3 sccm of CH4, and (e) TaC1.00 with 6 sccm of CH4. TaC0.66 with 2 sccm of CH4, (d) TaC0.76 with 3 sccm of CH4, and (e) TaC1.00 with 6 sccm of CH4.

Coatings 2020, 10, 212 12 of 17 Coatings 2020, 10, x FOR PEER REVIEW 12 of 17 Coatings 2020, 10, x FOR PEER REVIEW 12 of 17

FigureFigure 11. 11. EvolutionEvolution of of HfC HfCxx coatings coatings morphology. morphology. (a) (Purea) Pure Hf, Hf, (b) ( bHfC) HfC0.200.20 with with 1 sccm 1 sccm of CH of4, CH4, (c) Figure 11. Evolution of HfCx coatings morphology. (a) Pure Hf, (b) HfC0.20 with 1 sccm of CH4, (c) HfC(c) HfC0.340.34 with 2 with sccm 2 sccmof CH of4, ( CH4,d) HfC (d0.60) HfC0.60 with 3 sccm with 3of sccm CH4, of and CH4, (e) andHfC (0.95e) HfC0.95with 6 sccm with of 6 CH sccm4. of CH4. HfC0.34 with 2 sccm of CH4, (d) HfC0.60 with 3 sccm of CH4, and (e) HfC0.95 with 6 sccm of CH4.

TheThe e evolutionvolution of of the the morphology morphology of of Ta TaxxHfHf1-1-xxCCy coatingsy coatings as as a function a function of of the the applied applied current current target target The evolution of the morphology of TaxHf1-xCy coatings as a function of the applied current target isis shown shown in in Figure Figure 12. 12 .The The obtained obtained morphologies morphologies are are relatively relatively compact; compact; nevertheless, nevertheless, when when 2A 2A is is is shown in Figure 12. The obtained morphologies are relatively compact; nevertheless, when 2A is appliedapplied on on both both targets targets,, the the appearance appearance of of columns columns can can be be highlighted. highlighted. applied on both targets, the appearance of columns can be highlighted.

Figure 12. Evolution of TaxHf1-xCy coatings morphology. (a) 3A on Ta and 1A on Hf, (b) 2A on Ta and Figure 1 12.2. EvolutionEvolution of Ta xHfHf11-x-xCCy ycoatingscoatings morphology. morphology. (a (a) )3A 3A on on Ta Ta and and 1A 1A on on Hf, Hf, ( (bb)) 2A 2A on on Ta Ta and and 2A on Hf, and (c) 1A on Ta and 3A on Hf. 2A on Hf, and ( c) 1A on Ta andand 3A3A onon Hf.Hf. 3.4.3.4. Mechanical Mechanical P Propertiesroperties 3.4. Mechanical Properties 3.4.1.3.4.1. Nanoindenta Nanoindentationtion 3.4.1. Nanoindentation TheThe evolution evolution of of the the coating’s coating’s hardness hardness and and Young’s Young’s modulus modulus is is shown shown in in Figure Figure 13.13. The evolution of the coating’s hardness and Young’s modulus is shown in Figure 13. For both TaCx and HfCx coatings, mechanical properties are enhanced when DCH4 increases from 1 to 4 sccm. This should be due to carbide precipitation (TaC and HfC fcc-structured) with a higher covalent bond degree (between Ta or Hf and C) and to the film’s morphology (corresponding to the observed morphologies in Section 3.4). Nevertheless, when DCH4 = 5 and 6 sccm, both the coating’s hardness and Young’s modulus drop. This is a well known effect due to the presence of a carbon-based amorphous phase (a-C:H) (as can be seen in Figure7), which is harmful to the hardness and Young’s modulus, as shown in [20]. Moreover, the morphology is open-columnar for the higher CH4 flow rates. Pure Ta and Hf reach a hardness and Young’s modulus of about 17 and 213 GPa for Ta, respectively, and 8 and 156 GPa for Hf, respectively. The hardness and Young’s modulus of TaCx are between 18 and 36 GPa and between 236 and 405 GPa, respectively. The hardness and Young’s modulus of HfCx are between 11 and 29 GPa and between 175 and 318 GPa, respectively. This is consistent with values found in the literature and reported in Table4.

Figure 13. Evolution of (a) hardness and (b) Young’s modulus of TaCx (full lines) and HfCx coatings Figure 13. Evolution of (a) hardness and (b) Young’s modulus of TaCx (full lines) and HfCx coatings (dotted lines) according to the CH4 flow rate. The corresponding coating’s structures are reported on (dotted lines) according to the CH4 flow rate. The corresponding coating’s structures are reported on the figure. Beyond the horizontal line, the coating’s structure remains fcc. the figure. Beyond the horizontal line, the coating’s structure remains fcc.

Coatings 2020, 10, x FOR PEER REVIEW 12 of 17

Figure 11. Evolution of HfCx coatings morphology. (a) Pure Hf, (b) HfC0.20 with 1 sccm of CH4, (c) HfC0.34 with 2 sccm of CH4, (d) HfC0.60 with 3 sccm of CH4, and (e) HfC0.95 with 6 sccm of CH4.

The evolution of the morphology of TaxHf1-xCy coatings as a function of the applied current target is shown in Figure 12. The obtained morphologies are relatively compact; nevertheless, when 2A is applied on both targets, the appearance of columns can be highlighted.

Figure 12. Evolution of TaxHf1-xCy coatings morphology. (a) 3A on Ta and 1A on Hf, (b) 2A on Ta and 2A on Hf, and (c) 1A on Ta and 3A on Hf.

3.4. Mechanical Properties

3.4.1. Nanoindentation Coatings 2020, 10, 212 13 of 17 The evolution of the coating’s hardness and Young’s modulus is shown in Figure 13.

(a)

(b)

Figure 13. Evolution of (a) hardness and (b) Young’s modulus of TaCx (full lines) and HfCx coatings (dotted lines) according to the CH4 flow rate. The corresponding coating’s structures are reported on the figure. Beyond the horizontal line, the coating’s structure remains fcc.

Table 4. Reported mechanical properties of TaCx and HfCx coatings included in this work.

Deposited Elaboration Hardness Young’s Modulus Authors Ref. Material Method (GPa) (GPa) D. Bernoulli et al. [38] Ta DC magnetron sputtering 14 not measured TaC Unbalanced magnetron 40 400–500 Lasfargues et al. [19] x HfCx sputtering system 34–36 400–500 Reactive unbalanced magnetron Evans et al. [20] TaC 9–20 109–220 x sputtering DC reactive magnetron Shuo et al. [24] HfC 10–34 not measured x sputtering Ta 17 213 This work TaCx Reactive pulsed DC magnetron 18–36 236–405 Hfsputtering 8 156 HfCx 11–29 175–318

The measured hardness and Young’s modulus of TaxHf1-xCy coatings are reported in Table5. The measured values are consistent with other values found in the literature on bulk materials [40–43]—i.e., 23 GPa for hardness and 240 GPa for Young’s modulus. Nevertheless, the composition does not influence strongly either the hardness or Young’s modulus.

Table 5. Hardness and Young’s modulus of TaxHf1-xCy coatings.

3 A on Ta 2 A on Ta 1 A on Ta Coatings 1 A on Hf 2 A on Hf 3 A on Hf

DCH4 (sccm) 8 8 8 H (GPa) 22.6 23.8 22.8 E (GPa) 235 249 238 Coatings 2020, 10, 212 14 of 17

3.4.2. Residual Stresses

Coatings 2020, 10Figure, x FOR 14 PEER shows REVIEW the evolution of the residual stresses of TaCx (Figure 14a) and HfCx (Figure 14b) 14 of 17 coatings according to the CH4 flow rate. The compressive stresses in both coatings rise significantly lead to awith compressive the increase macro in carbon-state content; stress i.e.,in the it increases film [45] from. At –350a high MPa carbon to –1750 content, MPa for the TaC compressivex and stresses becomefrom –200 weaker. MPa to –1600This MPais consistent for HfCx. with This isthe linked coatings to the’ latticecolumnar deformation morphology due to the(Figure carbon 10e and incorporation. This is consistent with Windishmann’s model, based on volumetric distortions caused Figure 11e). According to Hoffman's model, the interaction of the columns between them generates by the atom’s displacement [44]. Moreover, the coating compaction due to the insertion of carbon Coatings 2020, 10, x FOR PEER REVIEW 14 of 17 tensile stresses(Figures [46,10b47] and. Hence,11b) participates the diminution in this phenomenon. of residual stresses Indeed, thewithin interatomic TaCx and repulsive HfCx forces coatings at high carbonlead tocontent a compressive results macro-statemacro from-state a competition stress in thethe filmfilm between [[45]45].. At thea high tensile carbon and content, compressive the compressive stresses. A similar effectstresses is becomeshown weaker. in Figure ThisThis 15 isis consistent consistentfor TaxHf withwith1-xC thethey coatings coatings’coatings,’ columnarwherecolumnar the morphologymorphology residual (Figuresstresses(Figure 10e10 aree and reduced when 2A11Figur ise). applied Accordinge 11e). According on toboth Hoffman’s targets,to Hoffman's model, where model, the a interaction co thelumnar interaction of morphology the columnsof the columns between is found. between them The generates them residual generates tensile stresses of stresses [46,47]. Hence, the diminution of residual stresses within TaC and HfC coatings at high ternary tensilecoatings stresses are [46,strongly47]. Hence, reduced the diminution compared of residual to the stresses binary within coatings,x TaCx and duex HfC tox coatingsthe rotation at of carbon content results from a competition between the tensile and compressive stresses. A similar substrates,high which carbon limits content their results bombardment. from a competition between the tensile and compressive stresses. A similareffect is effect shown is inshown Figure in 15 Figure for Ta x15Hf for1-x CTay xcoatings,Hf1-xCy coatings where the, where residual the stressesresidual arestresses reduced are whenreduced 2A whenis applied 2A is on applied both targets, on both where targets, a columnarwhere a co morphologylumnar morphology is found. is The found. residual The residual stresses stresses of ternary of ternarycoatings coatings are strongly are reducedstrongly compared reduced tocompared the binary to coatings, the binary due tocoatings, the rotation due ofto substrates, the rotation which of substrates,limits their which bombardment. limits their bombardment.

Figure 14.Figure Residual 14. Residual stress stressevolution evolution of ( ofa)( aTaC) TaCxx and and ( (bb)) HfCxHfCx coatingscoatings as as a function a function of CH4 of flowCH4 rate.flow rate. The correspondingTheFigure corresponding 14. Residual coating’s coating’s stress structure evolution structure is of shown is (a shown) TaC inx inand the the ( bfigure. figure.) HfCx Beyond cBeyondoatings the as the horizontala function horizontal line,of CH theline,4 flow coating’s the rate. coating’s structureThestructure remains corresponding remains fcc. fcc. coating’s structure is shown in the figure. Beyond the horizontal line, the coating’s structure remains fcc.

Figure 15. Residual stresses of TaxHf1-xCy coatings as a function of Hf target’s discharge current.

Figure 15. Residual stresses of TaxHf1-xCy coatings as a function of Hf target’s discharge current. Figure4. Conclusion 15. Residual stresses of TaxHf1-xCy coatings as a function of Hf target’s discharge current.

The properties (structure, morphology, and mechanical properties) of TaxHf1-xCy thin films 4. Conclusionsynthesized by reactive magnetron sputtering (with CH4 as the reactive gas) have been evaluated. Concerning binary coatings, the deposition rate progressively decreases as the CH4 flow rate The properties (structure, morphology, and mechanical properties) of TaxHf1-xCy thin films increases due to the target’s poisoning. The HfCx or TaCx coating structure can be controlled by synthesized by reactive magnetron sputtering (with CH4 as the reactive gas) have been evaluated. tuning the CH4 flow rate. A β-Ta (with a tetragonal structure {002} textured) phase is firstly obtained, 4 Concerningand the α- Tabinary phase coatings(bcc structure), the isdeposition stabilized when rate DCH progressively4 = 1 sccm. The decreases TaC phase as crystalizes the CH from flow rate increases2 sccmdue ofto CHthe4 withtarget’s a {111} poisoning. texture, which The is HfClost whenx or TaCDCHx4 coatingincreases untilstructure 6 sccm. can Concerning be controlled the by tuning theHf -CHC system,4 flow therate. α- HfA β(the-Ta hcp (with structure a tetragonal with a {002} structure texture) {002} is primarily textured) sputtered phase and is firstlykeeps this obtained , structure when 1 sccm of CH4 is used. The HfC phase fcc {111} textured is synthesized at 2 sccm of and the α-Ta phase (bcc structure) is stabilized when DCH4 = 1 sccm. The TaC phase crystalizes from CH4. The texture is also progressively lost as DCH4 increases. Both Ta and Hf coatings exhibit a 2 sccm of CH4 with a {111} texture, which is lost when DCH4 increases until 6 sccm. Concerning the compact-columnar morphology. When CH4 is introduced in the gas mixture, the coating’s Hf-C system, the α-Hf (the hcp structure with a {002} texture) is primarily sputtered and keeps this structure when 1 sccm of CH4 is used. The HfC phase fcc {111} textured is synthesized at 2 sccm of CH4. The texture is also progressively lost as DCH4 increases. Both Ta and Hf coatings exhibit a compact-columnar morphology. When CH4 is introduced in the gas mixture, the coating’s

Coatings 2020, 10, 212 15 of 17

4. Conclusions

The properties (structure, morphology, and mechanical properties) of TaxHf1-xCy thin films synthesized by reactive magnetron sputtering (with CH4 as the reactive gas) have been evaluated. Concerning binary coatings, the deposition rate progressively decreases as the CH4 flow rate increases due to the target’s poisoning. The HfCx or TaCx coating structure can be controlled by tuning the CH4 flow rate. A β-Ta (with a tetragonal structure {002} textured) phase is firstly obtained, and the α-Ta phase (bcc structure) is stabilized when DCH4 = 1 sccm. The TaC phase crystalizes from 2 sccm of CH4 with a {111} texture, which is lost when DCH4 increases until 6 sccm. Concerning the Hf-C system, the α-Hf (the hcp structure with a {002} texture) is primarily sputtered and keeps this structure when 1 sccm of CH4 is used. The HfC phase fcc {111} textured is synthesized at 2 sccm of CH4. The texture is also progressively lost as DCH4 increases. Both Ta and Hf coatings exhibit a compact-columnar morphology. When CH4 is introduced in the gas mixture, the coating’s morphology becomes glassy due to the nano-grain sizes. However, when more reactive gas is used, an open-columnar growth is observed. TEM microscopy has allowed one to highlight the amorphous a-C:H phase when 6 sccm of CH4 is added to the Ar-Hf discharge. The hardness and Young’s modulus of TaCx coatings reach 36 and 405 GPa, respectively. Concerning HfCx coatings, the hardness and Young’s modulus reach 29 and 318 GPa. Nevertheless, the mechanical properties, with a high carbon content, drop. This is assumed to be related to the presence of an amorphous a-C:H phase. Concerning ternary coatings, nanostructured TaxHf1-xCy coatings were synthesized by co-sputtering in reactive conditions. The period between TaC/HfC nanolayers was measured at 2 nm, while forming an fcc superlattice. The hardness and Young’s modulus are not influenced by the coating’s composition and were found to be 23 and 240 GPa, respectively. The compressive stresses around 10 MPa are relatively weak, due to the rotation of substrates.

Author Contributions: Conceptualization, M.F., A.B. and F.S.; Methodology, M.C., J.G., S.A. and F.S.; Validation, A.B. and F.S.; Formal analysis, A.d.M., H.L., M.C., A.B. and F.S.; Investigation, A.d.M., H.L. and J.G.; Resources, F.S.; Data curation, A.M., H.L. and J.G.; Writing—original draft preparation, A.d.M.; Writing—review and editing, A.B. and F.S.; Supervision, F.S.; Project administration, A.d.M., and F.S.; Funding acquisition, F.S. All authors have read and agreed to the published version of the manuscript. Acknowledgments: The authors would like to thank GIP 52 for their financial support. Conflicts of Interest: The authors declare no conflict of interest.

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