Effect of hydrogen content in a-C:H coatings on their tribological behaviour at room temperature up to 150 °C

Hydrogenated amorphous carbon (a-C:H) coatings deposited onto steel substrates by plasma assisted CVD, using different precursor gases (1 < H/C ratio < 4) were tested for their tribological behaviour. The H content in these coatings ranged from 25 to 29 at.%. Fretting mode I tests were performed on different couples consisting of coated and/or uncoated first bodies. Some tests were performed after a heat treatment of the coatings. As-deposited a-C:H/corundum couples tested at 23 °C and 50% RH showed lowering of the coefficient of friction at increasing normal load. Graphitisation is taking place in sliding contacts at high normal loads. For a-C:H/corundum couples a clear minimum in the coefficient of friction was noticed at 100 °C for coatings containing 27 at.% H. The coefficient of friction recorded on such couples is high compared to the one recorded on as-deposited a-C:H/a-C:H couples. However for the a-C:H/a-C:H couple, a lowering of the coefficient of friction with increasing fretting test temperature was noticed. The decreasing coefficient of friction was accompanied by an increasing wear. Graphitisation is causing severe degradation of a-C:H coatings at high test temperatures. An energetic analysis of the wear is finally reported. It appeared that the wear volume recorded at RT on as-deposited a-C:H coatings varies linearly with the cumulative dissipated energy. The wear rate coefficient decreases with increasing H-content. A stabilization of the sp³ bonds with increasing H-content might explain this behaviour. Confirmation was found by performing high temperature fretting tests. Interesting is the finding that fretting tests at RT performed after a thermal treatment of a-C:H coatings at either 100 or 150 °C, show a friction and wear behaviour identical to the ones recorded on as-deposited coatings tested at RT.     

Structure and properties of selected (Cr–Al–N,TiC–C,Cr–B–N) nanostructured tribological coatings

The paper will present the state-of-art in the process, structure and properties of nanostructured multifunctional tribological coatings used in different industrial applications that require high hardness, toughness, wear resistance and thermal stability. The optimization of these coating systems by means of tailoring the structure (graded, superlattice and nanocomposite systems), composition optimization, and energetic ion bombardment from substrate bias voltage control to provide improved mechanical and tribological properties will be assessed for a range of coating systems, including nanocrystalline graded Cr_1−xAl_xN coatings, superlattice CrN/AlN coatings and nanocomposite Cr–B–N and TiC/a-C coatings. The results showed that the superlattice CrN/AlN coating exhibited a super hardness of 45 GPa when the bilayer period Λ was about 3.0 nm. Improved toughness and wear resistance have been achieved in the CrN/AlN multilayer and graded CrAlN coatings as compared to the homogeneous CrAlN coating. For the TiC/a-C coatings, increasing the substrate bias increased the hardness of TiC/a-C coatings up to 34 GPa (at −150 V) but also led to a decrease in the coating toughness and wear resistance. The TiC/a-C coating deposited at a −50 V bias voltage exhibited an optimized high hardness of 28 GPa, a low coefficient of friction of 0.19 and a wear rate of 2.37 × 10⁻⁷ mm³ N⁻¹ m⁻¹. The Cr–B–N coating system consists of nanocrystalline CrB₂ embedded in an amorphous BN phase when the N content is low. With an increase in the N content, a decrease in the CrB₂ phase and an increase in the amorphous BN phase were identified. The resulting structure changes led to both decreases in the hardness and wear resistance of Cr–B–N coatings.     

Preparation of a-C:H/a-C:H:Si:O and a-C:H/a-C:H:Si multilayer coatings by PACVD

Carbon based multilayer coatings were prepared by plasma assisted chemical vapor deposition (PACVD) using methane (CH₄) and hexamethyldisiloxane (HMDSO) or methane and tetramethylsilane (TMS) as precursors. These coatings were deposited in a modified plasma nitriding plant operated at relatively high working pressures of 20 Pa. The multilayer design consisted of a-C:H and a-C:H:Si:O or a-C:H and a-C:H:Si single layers, respectively. The number of single layers and the material of the top layer were varied at constant total coating thicknesses. These multilayer coatings were investigated with regard to their morphology and composition as well as indentation hardness, abrasive wear, lubricant free friction and wetting behavior via contact angle measurements. The multilayer coatings exhibited lower wear rates and higher hardness values than a-C:H:Si:O or a-C:H:Si single layers and lower friction coefficients than pure a-C:H coatings under unlubricated test condition. Utilizing duplex processes, combining plasma nitriding pre-treatment and a following coating deposition, the adhesion of the multilayer coatings on high speed and cold working steel substrates could be considerably improved.     

Preparation and characterisation of a-C and a-C:H coatings deposited by pulsed magnetron sputtering

The amorphous carbon coatings of a-C and a-C:H type were deposited by pulsed magnetron sputtering in argon and argon/acetylene atmosphere, respectively. The deposition rate, chemical composition, structure and mechanical properties of these coatings were studied as a function of acetylene flow rate. The adding of acetylene to working atmosphere caused increase of deposition rate and hydrogen content in coatings, and at the same time decrease in their hardness. The friction and wear behaviour of a-C and a-C:H coatings in ambient air are highly dependent on kind of counterparts material. The “true” friction coefficients of a-C and a-C:H coatings sliding against a-C and a-C:H coatings, respectively, are similar in values (0.06-0.08) and wear rates are similar too. Significantly higher friction coefficients (0.2-0.3) and wear rates were observed for both a-C and a-C:H coatings sliding against 100Cr6 steel. The lowest friction coefficients (0.02-0.04) and wear rates were obtained for a-C and a-C:H coatings sliding against Alumina counterpart.     

Friction and wear behavior of plasma assisted chemical vapor deposited nanocomposites made of metal nanoparticles embedded in a hydrogenated amorphous carbon matrix

Nanocomposite coatings consisting of preformed silver or chromium nanoparticles embedded into a hydrogenated amorphous carbon matrix (a-C:H) were synthesized by Electron Cyclotron Resonance plasma assisted Chemical Vapor Deposition (ECR-CVD). In a first step, the nanoparticles were distributed on silicon substrates by dipping in an ethanol suspension. In a second step, the ECR-CVD deposition of the a-C:H layer was done. The effect of the incorporation and the concentration on the friction and wear behavior was derived from unlubricated reciprocating sliding tests performed in ambient air. A decrease in the coefficient of friction, more intense with Cr incorporation, is induced by the preferential metal interaction with environment. In addition, for both metals, the coefficient of friction becomes lower as the metal concentration increases. A gradual increase in the coefficient of friction is detected for increasing the number of sliding cycles, which is attributed to the combined effect of surface smoothing and oxidation in the sliding contact. In conclusion, the valuable protective properties of the fullerene-like a-C:H coatings are enhanced by metal addition. As a consequence, a considerable reduction of the surface roughness and the volume loss in the wear tracks is especially noticeable for 10,000 cycles tests.     

Optimizing the tribological properties and high-speed drilling performance of a-C:H coatings via nitrogen addition

a-C:H:N_x% coatings with various levels of nitrogen addition ranging from 0 to 29 at.% are deposited on high-speed steel substrates and micro-drills utilizing a Closed Field Unbalanced Magnetron (CFUBM) sputtering technique. The tribological properties of the various coatings are evaluated by performing reciprocating sliding wear tests against an AISI 1045 steel cylinder under an applied load of 100 N. Additionally, the machining performance of the coated micro-drills is investigated by conducting high-speed through-hole drilling tests utilizing Printed Circuit Board (PCB) specimens. The experimental results reveal that the a-C:H:N_8% coating possesses the best tribological properties, namely the lowest wear depth, the lowest friction coefficient and the longest lifetime. In addition, it is shown that the a-C:H:N_8% coating increases the lifetime of the micro-drill by a factor of three compared to that of an uncoated micro-drill.     

TiBCN:CNx multilayer coatings deposited by pulsed closed field unbalanced magnetron sputtering

In this study, a combination of nanocomposite and multilayer coating design was investigated in an effort to reduce the coefficient of friction (COF) while maintaining good mechanical properties of the TiBCN coatings. The TiBCN:CN_x coatings consist of TiBCN and CN_x nanolayers which were deposited alternately by reactive sputtering a TiBC composite target (80 mol% TiB₂ + 20 mol% TiC) and a graphite target in an Ar:N₂ mixture using a pulsed closed field unbalanced magnetron sputtering system. Low angle X-ray diffraction and transmission electron microscopy characterizations confirmed that the coatings consist of different bilayer periods in a range of 3.5 to 7.0 nm. The TiBCN layers exhibited a nanocomposite structure, whereas the CN_x layers were in an amorphous state. The mechanical properties and wear resistance of the TiBCN:CN_x multilayer coatings were investigated using nanoindentation and ball-on-disk wear test. The TiBCN:CN_x coatings exhibited high hardness in a range of 20-30 GPa. The highest hardness of 30 GPa was achieved in the coating with a bilayer period of 4.5 nm. A low COF of 0.17 sliding against a WC-Co ball was obtained at a bilayer period of 4.5 nm, which is much lower than those of the single layer TiBCN and TiBC nanocomposite coatings (0.55-0.7).     

Controlling friction and wear of nc-WC/a-C(Al) nanocomposite coating by lubricant/additive synergies

Owing to increasing demands for reductions in emissions and improvements in fuel economy in the automotive industry, there is an urgent need to improve tribological performances of components. In the current paper, an nc-WC/a-C(Al) carbon-based nanocomposite coating was fabricated successfully via the magnetron sputtering process. The microstructure and mechanical properties of the as-fabricated nanocomposite coating were investigated. In particular, its friction and wear behaviors under poly-alpha-olefin oil lubricant added with anti-wear (AW), extreme-pressure (EP), or molybdenum dialkyldithiocarbamate (MoDTC) additive were systemically evaluated. Results show that the nc-WC/a-C(Al) nanocomposite coating has a typical nanocrystallite/amorphous microstructure and good mechanical properties. The significant improvement in the tribological performance of the boundary-lubricated nc-WC/a-C(Al) coating is mainly attributed to the WS₂ or MoS₂ + WS₂-containing tribofilm when S-based EP or MoDTC additive was used. Superior tribological performance of nc-WC/a-C(Al) nanocomposite coating was achieved by lubricant/additive synergies, indicating its potential application as a protective coating for automotive tribo-components.     

Mechanical, tribological and erosion behaviour of super-elastic hard Ti-Si-C coatings prepared by PECVD

Titaniun carbide (TiC) based coatings prepared by low temperature Plasma Enhanced Chemical Vapor Deposition (PECVD) are investigated as attractive candidates for wear resistance, and particularly for protection against solid particle erosion. In the present work, we incorporated silicon (Si) as an alloying element to TiC, to obtain ternary nanostructured Ti-Si-C films. The incorporation of Si in TiC resulted in significant microstructural, mechanical and tribological modifications. By controlling the Si content in the films, we observed a transition between films consisting of fine nano-sized TiC crystallites (nc-TiC) embedded in an amorphous C:H matrix (a-C:H) to a microstructure formed by nc-TiC encapsulated in a-SiC/a-C:H matrix. This allowed one to selectively control the main mechanical characteristics, namely the hardness (H), the Young's modulus (E), and the friction coefficient (μ), in the range of 14-32 GPa, 140-240 GPa, and 0.16-0.6, respectively. For films prepared under optimized conditions, high elastic strain to failure and high resistance to plastic deformation of the Ti-Si-C films, expressed by H/E and H³/E² ratios, resulted in an 8 fold increase of the erosion resistance at an impact angle of 90° compared to a bare steel substrate. Erosion resistance at 30° increased by a factor of 22 compared to bare substrate due to a simultaneous combination of high H and low μ. Taking into consideration the severe erosion test conditions and the Ti-Si-C film thickness of less than 5 μm in this work, further improvement is expected for thicker films.     

Microstructure and electrical properties of Ti-Si-C-Ag nanocomposite thin films

Ti-Si-C-Ag nanocomposite coatings consisting of nanocrystalline TiC in an amorphous Si matrix with segregated Ag were deposited by dual magnetron sputtering from Ti₃SiC₂ and Ag targets. As evidenced by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, for Ag contents below 10 at.%, the Ag forms ∼ 10 nm large crystallites that are homogeneously distributed in the films. For higher Ag contents, coalescence during growth results in the formation of > ∼ 100 nm Ag islands on the film surface. The electrical resistivity of the coatings was measured in a four-point-probe setup, and ranged from 340 μΩcm (for Ti-Si-C coatings without Ag) to 40 μΩcm (for high Ag content).     

Comparison of tribological properties of industrial low friction coatings

MX2(M= Mo, W; X=S, Se) and DLC (a-C: H and WC/C) are the two kinds of typical low friction coatings widely used in industry. The friction and wear properties of these two kinds of coatings marked as MOVIC,MOST, MoSez/Ni, WSez, a-C: H and WC/C coatings were determined by fretting tests in ambient air of different humidity. The results show that the coefficient of friction of MXz coatings increases when the relative humidity of air increases whereas the coefficient of friction DLC coatings decreases with the increasing of relative humidity. MOVIC and WSe2 coatings have a poor friction and wear resistance because of non-basal planes (100) and ( 101 ) parallel to the surface in the MOVIC coating, or the rough and porous surface of WSe2 coatings. Among these six coatings, MoSe2/Ni and WC/C eoatinas have the highest wear resistance which seems to be unaffected by the relative humidity.     

Microstructure and mechanical properties of W-C:H coatings deposited by pulsed reactive magnetron sputtering

Reported are results of microstructure, mechanical and tribological properties studies for thin, amorphous hydrogenated carbon based coatings with tungsten content from 4.7 at.% up to 10.3 at.%. Studied coatings have been deposited by pulsed, reactive magnetron sputtering on substrates under planetary rotation. Resulting coatings, characterized by transmission electron microscopy (TEM) also at high resolution (HREM), show multilayer structure consisting of sub-layers of W-C:H type, with alternately high and low tungsten concentration. Thickness and number of sub-layers depend on rotation speed of planetary substrate holder. An average tungsten concentration decreases with increasing partial pressure of reactive gas (C₂H₂) during deposition. More insight into the microstructure of coatings provided HREM analysis showing crystalline precipitations of about 1-2 nm in size as well as tungsten-rich and tungsten-poor W-C:H sub-layers. Raman spectra confirm presence of amorphous, hydrogenated carbon (a-C:H) phase in the coatings. Microhardness of studied coatings depends on tungsten content and increases from 10.7 GPa to 13.7 GPa, for 5.1 at.% and 10.3 at.% of tungsten content, respectively. The highest cracking resistance and best adhesion (L_c2 = 78 N and HF1) has been achieved for coatings containing 4.9 at.% of tungsten and a sub-layer thickness of 5 nm. Tribological processes occurring in the coating-coating contact zone are dominated by graphitization and oxidation of W-C:H coating. Very low friction coefficient (0.04) and low wear rate seems to be an effect gaseous micro-bearing by tribo-generated carbon oxides and methane as well as hydrogen released from the coating. In the W-C:H-steel contact zone a tribo-layer composed of iron and tungsten oxides mixed with graphite-like products is growing at the surface of steel counterpart. This tribo-layer becomes a barrier restricting direct contact of steel with the coating and thus preventing it from further intense wear.     

Electrodeposition and tribological characterisation of nickel nanocomposite coatings reinforced with nanotubular titanates

Metal nanocomposite nickel coatings reinforced with regularly shaped nanotubular titanates (multi-layered wall structure with ca. 5 nm internal diameter and 30 to 500 nm tube length) were electrodeposited from a modified Watts nickel electrolyte. Tribological properties of the coatings are characterised via measurements of the coating hardness, surface friction, wear rate and elastic modulus. Surface microstructures of the coatings were imaged (SEM and TEM analyses). The nanotubular titanates were shown to be embedded within the bulk of the coating and some particles protruded from the top surface. The nanotubular titanates in the nickel coating acted akin to a cross-linked and mesh-like matrix to enhance the dispersion-strengthening mechanism against external load. Nanocomposite nickel coatings reinforced with nanotubular titanates have shown (a) ~ 22% reduction in surface friction against a spherical diamond tip, (b) ~ 29% enhancement in wear resistance in a 3-body slurry abrasive wear test (steel counter body and 5 μm SiC particles), (c) ~ 102% improvement in coating hardness and (d) ~ 26% improvement in elastic modulus when compared with a nickel coating containing irregularly shaped nanosized titanium dioxide particles.     

Comparative investigation of Al- and Cr-doped TiSiCN coatings

The aim of this work was a comparative investigation of the structure and properties of Al- and Cr-doped TiSiCN coatings deposited by magnetron sputtering of composite TiAlSiCN and TiCrSiCN targets produced by self-propagating high-temperature synthesis method. Based on X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy data, the Al- and Cr-doped TiSiCN coatings possessed nanocomposite structures (Ti,Al)(C,N)/a-(Si,C) and (Ti,Cr)(C,N)/a-SiC_xN_y/a-C with cubic crystallites embedded in an amorphous matrix. To evaluate the thermal stability and oxidation resistance, the coatings were annealed either in vacuum at 1000, 1100, 1200, and 1300 °C or in air at 1000 °C for 1 h. The results obtained show that the hardness of the Al-doped TiSiCN coatings increased from 41 to 46 GPa, reaching maximum at 1000 °C, and then slightly decreased to 38 GPa at 1300 °C. The Cr-doped TiSiCN coatings demonstrated high thermal stability up to 1100 °C with hardness above 34 GPa. Although both Al- and Cr-doped TiSiCN coatings possessed improved oxidation resistance up to 1000 °C, the TiAlSiCN coatings were more oxidation resistant than their TiCrSiCN counterparts. The TiCrSiCN coatings showed better tribological characteristics both at 25 and 700 °C and superior cutting performance compared with the TiAlSiCN coatings.     

Improvement in load support capability of a-C(Al)-based nanocomposite coatings by multilayer architecture

Due to severe operating conditions and long lifetime requirements for mechanical components, the great challenge is to develop coatings with anti-wear and high load support capability. The designs for nanocomposite protective coatings are very promising and provide an attractive alternative to take into account the multilayer architecture. In this work, a-C(Al)-based nc-TiC/a-C(Al), nc-CrC/a-C(Al) and nc-WC/a-C(Al) nanocomposites were constructed by Cr/CrN/CrCN multilayer. The microstructure, mechanical properties, friction and wear behaviors for these multilayer coatings were systemically investigated. Results showed that the top-layered nc-TiC/a-C(Al), nc-CrC/a-C(Al) and nc-WC/a-C(Al) nanocomposites were dominated by typically nanocrystallite/amorphous microstructure, and these nanocomposites constructed by multilayer approach presented superior mechanical properties which possessed relatively high hardness, low internal stress as well as high adhesion strength. Particularly, the as-fabricated nc-TiC/a-C(Al), nc-CrC/a-C(Al) and nc-WC/a-C(Al) multilayer coatings exhibited superior anti-wear capability under relatively high applied Hertzian contact pressure compared to corresponding monolayer coatings. The improvement in friction and wear performances of as-fabricated multilayer coatings was mainly attributed to superior mechanical properties and formation of graphitized tribofilm as well as high load support capability by multilayer architecture, indicating that these coatings might be good candidates as solid lubrication materials in engineering applications.     

Tribological behaviour of titanium carbide/amorphous carbon nanocomposite coatings: From macro to the micro-scale

The tribological behaviour of nanocomposite coatings made of nanocrystalline metal carbides and amorphous carbon (a-C) prepared by PVD/CVD techniques is found to be very dependant on the film deposition technique, synthesis conditions and testing parameters. Focusing in the TiC/amorphous carbon-based nanostructured system, this paper is devoted to an assessment of the factors governing the tribological performance of this family of nanocomposites using a series of TiC/a-C films prepared by magnetron sputtering technique varying the power applied to each target (titanium or graphite) as model system to establish correlations between film microstructure and chemical compositions and tribological properties measured by a pin-on-disk tribometer. The film microstructure goes from a quasi-polycrystalline TiC to a nanocomposite formed by nanocrystals of TiC embedded in an amorphous carbon matrix as observed by transmission electron microscopy (TEM). The nanocrystalline/amorphous ratio appears to be the key-parameter to control the tribological properties and its quantification has been done by electron energy-loss spectroscopy (EELS). A significant change in the tribological performance is observed for nanocomposites with amorphous carbon phase contents above 60–65%. The friction coefficient decreases from 0.3 to 0.1 and the film wear rates by a factor of 10. Examination of the wear scars on ball and film surfaces by laser micro-Raman spectroscopy has allowed to determine the presence of metallic oxides and carbonaceous compounds responsible of the observed friction behaviour. The revision of the literature results in view of the conclusions obtained enabled to explain their apparent dispersion in the tribological performance.     

Comparison testing of diamond-like a-C:H coatings prepared in plasma cathode-based gas discharge and ta-C coatings deposited by vacuum arc

The method of amorphous carbon coating deposition based on decomposition of acetylene in a non-self-sustained hollow cathode pulsed-DC discharge is investigated. The discharge is maintained by the electron emission of a grid-stabilized plasma cathode based on a DC glow discharge. The method allows the gas pressure in the discharge gap and the non-self-sustained discharge parameters to be varied in a wide range. It makes it possible to optimize the properties of the deposited coating and to perform in situ the preliminary ion cleaning of sample surface and the plasma immersion ion implantation to form an interface and to improve the coating's adhesion. The 0.1-10-μm-thick a-C:H films were deposited on tungsten carbide and stainless steel substrates at a deposition rate of 0.5-8 μm/h. The coatings were investigated using the methods of atomic-force microscopy (AFM), scanning electron microscopy (SEM) and Raman spectroscopy. The arithmetic average surface roughness (9-34 nm), the friction coefficients (0.01-0.3), the density (2.2-2.4 g/cm³), the microhardness (16-75 GPa) and the internal stresses in the films (3-7 GPa) were measured. Comparison was made between the properties of the resulted a-C:H coating with the properties of the ta-C coating obtained by cathodic vacuum arc deposition.     

Deposition of alpha-WC/a-C nanocomposite thin films by hot-filament CVD

Nanostructured tungsten carbide coatings containing amorphous carbon (a-C) phases are interesting composite materials. The incorporation of the a-C phase simultaneously improves the thermal stabilization of the carbide phase and the coatings' friction coefficient. Such nanocomposite coatings are also electrically conducting with resistivity values comparable to the transition metals, which make them useful for a number of electrochemical and electronic applications. Many deposition techniques have been used for the synthesis of these coatings. However, most of them lead to the formation of complex crystalline structures consisting of more than one carbide phase and varying amorphous contents.The novelty of this work is the formation of WC coatings with controllable film thickness and a-C content, almost fully composed by α-WC phase. Tungsten carbide coatings were deposited on silicon substrates using a hot filament chemical vapor deposition (HFCVD) equipment with hydrogen and methane as the deposition gasses. Two types of nanocomposite coatings were obtained with significant variations of the carbide phase and a-C contents.The results point to the W vaporization time as the main parameter influencing the film thickness. We also conclude that the formation of α-WC phase is the combining result of W filaments vaporization in vacuum and the carbon incorporation at low substrate temperature. The small crystallite size of the carbide grains (5-6 nm) could also explain the rapid diffusion of C through the tungsten-containing layer. Preliminary results show that the amount of a-C incorporated in the film is not only dependent on the CH₄/H₂ ratio but also on the substrate temperature.     

Structure and mechanical properties of nickel/carbon nanocomposite films formed by microwave plasma-assisted deposition technique from argon-acetylene gas mixture

The structure and mechanical properties of nickel/hydrogenated amorphous carbon (Ni/a-C:H) films formed by microwave plasma-assisted deposition technique were investigated as a function of the carbon content using various methods: Rutherford backscattering spectroscopy (RBS), Raman spectroscopy and tribometry. The size of carbon clusters determined by Raman spectroscopy in Ni/a-C:H films deposited in gas mixtures containing 40 and 60% of C₂H₂, and in nickel free a-C:H films was 1 and 4 nm, respectively. However, the amorphous Ni/a-C:H films deposited from a gas mixture containing 60% of C₂H₂ exhibited the lowest friction coefficient (∼ 0.04), at the same time the nanohardness of these films was ∼ 7 GPa.     

Annealing and oxidation study of Ta-Ru hard coatings

Refractory metal alloy coatings have been widely used as protective coatings on glass molding dies. The formation of intermetallic compounds in the coatings inhibits grain growth at high-temperature environment in the mass production of optical components. The current work presents Ta-Ru coatings with a Cr interlayer on cemented carbide substrates and silicon wafers deposited by direct current magnetron co-sputtering at 400 °C. The as-deposited Ta-Ru coatings possessed a hardness of 13-14 GPa and a surface roughness of 1.3-4.0 nm. The annealing treatments were carried out at 600 °C under two vacuum levels of 3 × 10^− 3 and 3 Pa, respectively. After annealing in vacuum at 3 × 10^− 3 Pa, the Ta-Ru coatings showed grain size, hardness, surface roughness and phase stability comparable to those of the as-deposited coatings. While annealing in vacuum at 3 Pa, preferential oxidation of Ta in the Ta-Ru coatings was verified by X-ray photoelectron spectroscopy, a variation of the chemical composition in depth was analyzed by Auger electron spectroscopy and the internal oxidation zone consisting of a laminated structure was observed by transmission electron microscopy.