Effects of concentration of zinc dialkyldithiophosphate on the tribological properties of tetrahedral amorphous carbon films in presence of organic friction modifiers

The effects of the concentration of zinc dialkyldithiophosphate (ZDDP) on the tribological properties of a tetrahedral amorphous carbon (ta-C) film in the presence of organic friction modifiers (FMs) were investigated. Friction tests were conducted under lubrication with poly-alpha-olefin (PAO) and PAO containing glycerol-mono-oleate (GMO), tallow-diethanol-amine (TDEA), ZDDP, GMO+ZDDP, and TDEA+ZDDP. For PAO+GMO+ZDDP, the friction coefficients increased as the concentration of ZDDP increased. For PAO+TDEA+ZDDP, the ultralow friction was observed at the ta-C/ta-C contact regardless of the concentration of ZDDP. Surface analyses demonstrated the combination of TDEA and ZDDP promoting the formation of sulfur-rich tribofilms, which play important roles in the low friction of the ta-C film. •The frictional properties of the ta-C film depended on the combination of additives.•The frictional properties of the ta-C film depended on the concentration of ZDDP.•The ta-C film lubricated with PAO+TDEA+ZDDP showed an ultralow friction.•The chemical composition of tribofilms depended on the combinations of additives.•The high-sulfur rate tribofilm dominated an ultralow friction of the ta-C film.