Effect of Water Content on the Tribological Performance of DLC Films under Oil Lubrication

Diamond-like carbon (DLC) films have been extensively applied in tribological protection of core components in engines.In order to investigate the effect of water content on the tribological properties of DLC films under engine operating conditions，a water-PAO6 emulsion was used as the lubricant to examine the friction and wear behavior of two types of DLC films commonly applied in engines (a-C and a-C:H films) at different temperatures.Results showed that under the lubrication conditions of the water-PAO6 emulsion，the friction coefficient was slightly influenced by the water content，showing a minor overall increase.Partial failure occurred in the a-C films，whereas no substrate exposure was observed in the a-C:H films.The wear rates of both films decreased with the increase in water content.Surface analysis revealed that the increase in water content inhibited the degradation of molybdenum dithiocarbamate (MoDTC)，thereby reducing the formation of the lubricant additive MoS2.Moreover，the rise in water content promoted the transfer of carbon to the counter-ball surface.The combined effects of the carbon transfer film，MoS2 and iron oxides influenced the tribological behavior of the DLC films.