XPS analysis on the influence of water on the evolution of zinc dialkyldithiophosphate–derived reaction layer in lubricated rolling contacts

The presence of water in lubricating oil affects the performance of lubricated rolling contacts. This work particularly focuses on the role of water in altering the evolution of zinc dialkyldithiophosphate (ZDDP)–derived reaction layers. A series of tribological tests were carried out using a rolling four-ball machine. Different initial water amounts were added to 2 wt% ZDDP in poly-alpha-olefin (PAO). The reaction layer evolution and its chemical properties were studied using XPS and SEM. SEM analyses show that water seems to inhibit the growth of the ZDDP-derived reaction layer. XPS analyses reveal that the reaction layer undergoes a depolymerisation of the polyphosphate chain when water is present in lubricated contacts. Surface distress was observed in 2 wt% ZDDP in poly-alpha-olefin, whereas when water was mixed with pure base oil, the surface distress increased. Different mechanisms of the influence of water on the ZDDP-derived reaction layer evolution are discussed.