Two different lubricants containing zinc dialkyldithiophosphate (ZDDP) additive were tested in a rolling–sliding contact test rig (micropitting rig) at different relative humidities. The effect of relative humidity on the bulk properties (e.g., viscosity, water concentration, water saturation level) of the lubricants and their tribological performance (e.g., friction, wear, micropitting level) as well as the related tribochemistry was extensively explored. Relative humidity had a limited effect on the viscosity of the tested lubricants. However, the friction and micropitting level decreased, while the wear increased at higher relative humidity. This increased wear was attributed to a thinner tribofilm and shorter chain length of the polyphosphates derived from the ZDDP additive. Hydrolysis of the ZDDP additive occurred, and the polar water molecules limited the access of the ZDDP additive to the substrate. The different polarities of the two base oils (Ester, polyalphaolefin) also led to different tribological and tribochemical performance.
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