Lubrication degradation mechanism of Mo-S-Ti composite films irradiated by heavy ions

Previous works show that the ion irradiation damage leads to the MoS₂ films lose short-range ordering and the reduction of lubrication properties. However, its lubrication degradation mechanism remained unclear. In this work, mechanical tests revealed that with the increase of ion fluence, irradiation damaged Mo-S-Ti film exhibits an increasing tendency in hardness as well as the improving fracture toughness. Nevertheless, tribotests found that Mo-S-Ti films show the higher wear rates as well as the reduced friction coefficient with the increase of ion fluence. Raman spectra found that peak intensity collected from wear tracks increased with the increase of ion fluence, while, the intensity of transferred layer from counterparts decreased. Based on this nearly inverse tendency of Raman peaks from wear tracks and counterparts, a lubrication degradation mechanism was reasonably proposed. Although a well-crystallized MoS₂ layers forming on the wear tracks may play a key role in reducing the friction coefficient during friction, but after irradiation the lack of transferred layer formed on the counterface leads to the higher wear rate and short wear duration.