Time-of-Flight Secondary Ion Mass Spectroscopy investigation of the chemical rearrangement undergone by MoS2 under tribological conditions

MoS2 is a well-known lubricant for vacuum application. However, the role of contamination in achieving low friction and long wear life in industrial applications remains unclear as the literature presents some contradictions. A former study performed in ultrahigh vacuum showed that in a sliding macro contact, the origin of low friction was primarily due to the formation of a 3rd body layer, its trapping and its ability to flow plastically inside the contact. The study showed a homogenisation of the chemical elemental composition of the material and the internal coating contaminants throughout the contact and the 3rd body. To study that homogenisation and to verify if a chemical rearrangement occurred at a molecular level under friction, Time-of-Flight Secondary Ion Mass Spectroscopy analyses are performed. Firstly, they show that the MoS2 coating made by Physical Vapour Deposition is not a “MoS2” but is a complex MoxSyOz structure. Secondly, they show that a chemical rearrangement of the material with the internal contaminants effectively occurs in ultrahigh vacuum and leads to the formation of a unique MoSxOy phase. Such a phase shows the beneficial role of reasonable contamination on the tribological behaviour.