Nanocrystalline WC and WC/a-C composite coatings produced from intersected plasma fluxes at low deposition temperatures

Low temperature vacuum deposition of tungsten carbide coatings, W1-yCy with compositions that varied from y=0 to 0.9, was investigated. Special attention was given to the production of nanocrystalline carbides with coatings of y>0.5. Previous attempts at producing WC with excess carbon at near room temperatures resulted in the formation of amorphous phases. In this study, crystalline WC was produced at 45 and 300 degrees C by the intersection of plasma fluxes from magnetron sputtering of tungsten and laser ablation of graphite. At both temperatures, formation of WC chemical bonding and nanocrystalline cubic beta-WC1-x was observed using x-ray photoelectron spectroscopy and grazing angle x-ray diffraction when the carbon content was increased more than 30%. Increasing the substrate temperature to 300 degrees C did not affect the percentage of WC bonding, but it did promote considerable crystallization of cubic WC. As the carbon content was increased to more than 50% a second phase consisting of amorphous carbon (a-C) was observed together with amorphitization of beta-WC1-x. The a-C phase was identified as amorphous diamond-like carbon (DLC) by Raman spectroscopy. At 60-80 at. % C, a two phase structure was produced, which was composed of nanocrystalline beta-WC1-x with 5-10 nm grains and amorphous DLC. The hardness of the WC/DLC composites was about 26 GPa based on nanoindentation tests. Correlation of the chemistry, microstructure, and mechanical properties of WC and WC/a-C coatings is discussed. (C) 1999 American Vacuum Society. [S0734-2101(99)00203-1].