Influence of W content on microstructural, mechanical and tribological properties of sulfurized W-doped diamond-like carbon coatings

A series of sulfurized W doped diamond-like carbon (W-DLC) coatings were prepared using a two-step method as depositing W-DLC coating firstly, and then treated by low temperature ion sulfuration. A designing thought of a composite DLC with sulfide coating with optimized mechanical and tribological properties was realized. The structural analyses were performed on the scanning electron microscope (SEM), X-ray photoelectron spectroscope (XPS), scanning Auger microprobe (SAM), X-ray diffraction (XRD) and Raman spectroscope. The mechanical and tribological properties were evaluated by the nanoindenter and ball-on-disk friction and wear tester. The results show that the microstructures, mechanical and tribological properties of sulfurized W-DLC coatings exhibited a significant dependence on the W concentration. A high fraction of W in W-DLC coating promoted the formation of WC which could prevent the S diffusion and the extent of graphitization for sulfurized W-DLC coatings. The hardness of sulfurized W-DLC coating gradually increased with higher W concentration, and it was lower than that of the untreated W-DLC coating. The coefficient of friction and wear rate of sulfurized W-DLC coating were reduced with the increase of W content. The best tribological properties with the lowest friction coefficient and wear rate were achieved on sulfurized 27.7% W-DLC coating. The tribological mechanism was attributed to the graphite-rich and sulfide composite top layer with lower shear strength and combining a support of the sublayer W-DLC coating with better mechanical property.