Additive effects on the tribological performance of WC/a-C:H and TiC/a-C:H coatings in boundary lubrication

Although diamondlike carbon coatings like WC/a-C:H have a proven history of tribological performance in many mechanical applications, studies have reported that these coatings can sometimes experience high wear rates when operating in lubricants with large concentrations of phosphorous- and sulfur-based additives. In this study, WC/a-C:H and TiC/a-C:H coatings have been tribologically tested against AISI 52100 steel and Si3N4 counter faces under boundary lubrication in ISO 320 mineral oil, mineral oil with 1% aryl zinc dithiophosphate and 1% molybdenum dithiocarbamate, and a fully formulated wind turbine gear box oil. Results revealed that the wear coefficients of TiC/a-C:H were much smaller than those of WC/a-C:H in testing performed in lubricants with additives. A thionization wear mechanism was proposed to explain these results. •Wear more severe in WC/a-C:H than TiC/a-C:H from S-containing additives.•Tribochemical wear mode of metal DLCs is consistent with a thionization mechanism.•Composition of counterface influences amount of tribochemical wear of coatings.