Tribological properties of a tetrahedral amorphous carbon (ta-C) film under boundary lubrication in the presence of organic friction modifiers and zinc dialkyldithiophosphate (ZDDP)

Diamond-like carbon (DLC) films provide low friction and high wear resistance under lubricated conditions. In particular, tetrahedral amorphous carbon (ta-C) film has shown ultralow friction behavior under boundary lubrication with an organic friction modifier (FM), such as glycerol mono-oleate (GMO). On the other hand, there are many studies on the tribological properties of DLC films that are lubricated with oils with added zinc dialkyldithiophosphate (ZDDP). ZDDP, however, shows quite different tribological performance, which depends on the type of DLC film. The lubrication mechanism of ZDDP against DLC films has not yet been fully clarified. This research studied the effects of ZDDP additives on the tribological properties of a ta-C film under boundary lubrication with oils that contained organic friction modifiers. The tribological properties were evaluated using the reciprocating type cylinder-on-disk tribo-tester. ZDDP and three types of organic additives (GMO, glycerol mono-oleoyl ether (GME), and tallow diethanol amine (TDEA)) were added to poly-alpha olefin (PAO). Friction tests were carried out for the DLC/DLC tribo-pair under lubrication with PAO and PAO-based oils containing GMO, GME, TDEA, ZDDP, GMO+ZDDP, GME+ZDDP, and TDEA+ZDDP additives. Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and atomic force microscope (AFM) were used for analyses of the ta-C surface after the friction tests. The ta-C film lubricated with PAO+FMs reached an ultralow friction coefficient of 0.02-0.03. On the other hand, the films with PAO+GMO+ZDDP and PAO+GME+ZDDP had increased friction coefficients and wear volumes compared to the PAO+FMs oils. The ta-C film lubricated with PAO+TDEA+ZDDP reached an ultralow friction coefficient of 0.028. The surface analyses demonstrated that the formation rate and chemical composition of the ZDDP-derived tribofilm depended on the combination of ZDDP and each FM. It was concluded that the formation rate and chemical composition of the ZDDP-derived tribofilm played an important role in the low friction behavior of the ta-C films that were lubricated with any of the oils with added ZDDP.