Synthesis and tribological properties of WSe_x films prepared by magnetron sputtering

WSe_x films with variable Se/W ratio were deposited by non-reactive r.f. magnetron sputtering from WSe₂ target changing the applied d.c. pulsed bias conditions and substrate temperature. The structural and chemical properties were measured by cross-sectional scanning electron microscopy (X-SEM), energy dispersive analysis (EDX), X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS). The tribological properties were measured in ambient air (RH = 30-40%) and dry nitrogen by means of a reciprocating ball-on-disk tribometer. A clear correlation was found between the Se/W ratio and the measured friction coefficient displaying values below 0.1 (in ambient air) and 0.03 (in dry N₂) for ratios Se/W ≥ 0.6 as determined by electron probe microanalysis (EPMA). The results demonstrated that notable tribological results could be obtained even in ambient air (friction ≤ 0.07 and wear rate ≈10^-7 mm³ Nm^-1) by controlling the film microstructure and chemical composition. By incorporating carbon, wear and chemical resistance can be gained by formation of non-stoichiometric carbides and/or alloying into the defective WSe_x hexagonal structure. The existence of a WSe₂ rich interfacial layer (either on the ball scar or embedded in the film track) was evidenced by Raman in low friction conditions. The improvement in tribological performance is therefore obtained by means of layered WSe_x, the formation of gradient composition from metallic W (hard) to WSe₂ (lubricant) and carbon incorporation.