WC/C:H films synthesized by an hybrid reactive magnetron sputtering/Plasma Enhanced Chemical Vapor Deposition process: An alternative to Cr (VI) based hard chromium plating

WC/C:H thin films have been synthesized by a hybrid Plasma Enhanced Chemical Vapor Deposition/Physical Vapor Deposition method consisting in the reactive sputtering of a tungsten target in Ar-C₂H₂ atmospheres. The influence of the gas mixture on the physico-chemical properties of WC/C:H layers including their chemical composition, morphology, tribological and mechanical features has been studied. X-ray photoelectron spectroscopy measurements reveal an increase in the carbon content and a higher aliphatic bonds density in detriment to the carbide ones as a function of the acetylene flow. These results, in combination with Raman spectroscopy, X-ray diffraction, transmission electron microscopy and atomic force microscopy data, allow to depict the material as crystallized W_xC_{1 −y} nano-spheres (corresponding to aggregate of smaller nanograins of W₂C and WC_1−x) diluted in a hydrogenated carbon matrix; the ratio between the hard crystallized WC phase and the softer hydrogenated carbon matrix being reduced when increasing the acetylene flow. The film nanohardness and tribological properties (i.e. friction coefficient and wear rate) are directly connected to the film chemical composition and structure. High nanohardness (~ 20 GPa), friction coefficient of μ = 0.5 and high wear rate (5.4 · 10⁻¹³ m²/N) are obtained for the lowest carbon content of 37 at.%. When increasing the carbon content in the film (77 at.%), the hardness strongly decreases to ~ 10 GPa and a lower friction coefficient (μ = 0.2) and wear rate (4.7 · 10⁻¹⁵ m²/N) are obtained. These properties are similar and even better than the ones of Cr(VI) based electroplating coatings, demonstrating the attractiveness of our WC/C:H films as a potential alternative to these coatings. - Highlights: • WC/C:H thin films are synthesized by an hybrid process. • WC/C:H thin film structure depends on argon/acetylene ratio. • Mechanical and tribological properties can be tailored. • Mechanical and tribological properties better than hard chromium are obtained.