Decrypting third body flows to solve dry lubrication issue – MoS2 case study under ultrahigh vacuum

Feedback from former applications and parametric studies often give recommendations on how to use solid lubricants but, based on those elements, it is difficult to predict how they will behave and for how long lubrication can be asserted. Mastering friction and wear requires understanding the contact life. Decryption of the contact life needs considering dynamic flows of 3rd body inside the contact. Those flows depend on a mechano-physico-chemical coupling effect occurring at and between all scales (nano, micro, and macro). In this study, an experimental procedure to decrypt those flows is proposed. This procedure couples both real time measurements (Friction forces, Mass spectrometry, Optical Visualization) and post-mortem analysis (Scanning Electron Microscope, Energy Dispersive X-ray Spectroscopy). As mechanical and physico-chemical measurements are performed simultaneously, the relationship between them both can be studied. The method is applied to MoS2 PVD coatings. The decryption of the 3rd body flows and the identification of velocities accommodation sites and modes allowed to identify columnar structure as the main reason for low friction and long life. It also raised the question of a beneficial role of coating original contamination on the tribological behavior of the studied MoS2 coating.