Tribochemical reaction of Si-DLC coating in water studied by stable isotopic tracer

Tribochemical reaction of Si-DLC coating in water was investigated by stable isotopic tracer. Heavy water (deuterium oxide, D2O) was used to carry out the friction test of the coating against a 440C ball. The worn surfaces were analyzed by ToF-SIMS. The results showed that tribochemical reaction of Si-DLC occurred with D2O, and CD, OD groups were formed on the contact surfaces. The formation of SiOD and CO2D groups was also confirmed on the worn ball surface. The tribochemical products containing hydrophilic hydroxyl and carboxyl groups are considered to be responsible for low friction and wear of Si-DLC coating and the counter part in a water environment. On the other hand, the Si amount of the coating decreased due to the rubbing. The decrease in the Si amount was not only caused by the removal of the surface layer in which Si concentrated, but also by the tribochemical reaction. The presence of Si may accelerate the tribochemical reaction resulting in the lower friction and the wear of the counter ball, although the wear of the coating increases due to chemical wear, compared with Si-free DLC coating.