Deposition and characterisation of carbon-containing tungsten coatings prepared by reactive magnetron sputtering

Carbon-containing tungsten coatings were deposited by magnetron sputtering onto AISI 316 stainless steel substrates in Ar/C₂H₂ gas mixtures. Optical emission spectroscopy was used to control the _C2H₂ gas flow. The goal of this investigation was to study the influence of the carbon content in W_1-xC_x coatings (with x being in the range of 0.05 to 0.19) on the microstructure, hardness, Young's modulus, adhesion and corrosion behaviour. The carbon concentration in the coatings was evaluated by glow discharge optical emission spectroscopy (GDOES) and wavelength-dispersive X-ray analysis (WDX) and was found to be linearly dependent on the relative optical intensity of the tungsten 400.8 nm line, used as a control signal for the reactive gas concentration at the vapour source. X-ray diffraction (XRD) was used for phase- and peak shape parameter analysis, whilst the coating morphology was evaluated by scanning electron microscopy. Knoop microhardness (H_K), Young's modulus by ultrasonic surface acoustic waves (USAW) and scratch adhesion tests were performed to evaluate the mechanical properties of the coatings. The corrosion resistance was examined by potentiodynamic electrochemical testing. It was found that all coatings consist mainly of the b.c.c. αW phase. With increasing carbon content from W_0.95C_0.05 to W_0.81C_0.19 an expansion of the αW lattice occurs progressively, associated with an increase in the X-ray peak full width at half maximum (FWHM) and a decrease in intensity. The films density, hardness and corrosion values increased with increasing carbon content up to 15 at.% C, where the hardness reached maximum values of 4000 H_K0.025. The best adhesion results were observed for W_1-xC_x coatings with x ≤S 0.08.