Tribological performance and tribochemistry of nanocrystalline WC/amorphous diamond-like carbon composites

The tribological performance and tribochemistry of vacuum deposited nanocry stalline WC/amorphous diamond-like carbon (DLC) composite coatings were inv estigated. Coating chemistry was varied by regulation of carbon content bet ween 30 and 90 at.%. Deposition temperature was used to adjust the degree o f beta-WC crystallinity from near amorphous with 1-3 nm sized grains to nan ocrystalline with 5-10 nm sized grains, which were embedded in unhydrogenat ed DLC matrix. The microstructure critically influenced both the hardness a nd tribological properties of WC/DLC composites. Coatings with poorly cryst allized WC were softer (15 Cpa) and exhibited lower friction and higher wea r rates in comparison to harder (26 Cpa) composites with larger WC nanocrys tals. The hardness of WC/DLC composites was at least twice as high as the h ardness of metal doped DLC coatings reported in the literature. Micro Raman and X-ray photoelectron analysis was performed on transfer films formed on the surface of steel balls in sliding against WC/DLC composites in humid a ir and dry nitrogen, which found both oxides and graphite carbon, providing self-lubrication. The high hardness and self-lubricating properties of WC composites resulted in low wear rates and friction coefficients of about 0. 2. Relationships among coating chemistry, structure, hardness, friction, we ar rates, as well as, tribofilm composition and structure are discussed. (C ) 1999 Elsevier Science S.A. All rights reserved.