The Formation of Tribofilms of MoS2 Nanotubes on Steel and DLC-Coated Surfaces

Solid-lubricant nanoparticles as additives in oil provide good tribological properties based on the physical lubrication mechanisms in the contact. For this reason, they are strong candidates for use in the lubrication of diamond-like carbon (DLC) coatings, which only poorly interact with the traditional, chemically based additives. In this study, we focused on how a tribofilm formed from MoS2 nanotubes is related to the tribological properties of these nanotubes, and then, we analysed such a tribofilm on steel and DLC-coated surfaces using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy. We demonstrated that when using oil containing MoS2 nanoparticles, the formation of a tribofilm is a key factor in decreasing the friction for the steel and DLC-coated contacts. The major difference between the steel and the DLC contacts is the extent to which the MoS2-based tribofilm covers the surface, which is 20 % in the case of the DLC/DLC contacts, but almost 40 % in the case of the steel/steel contacts. Moreover, the MoS2-based tribofilm was found to be more oxidized on the DLC surface than on the steel surface. Nevertheless, we found that the chemical and functional properties of the MoS2-based tribofilm are very similar, or even the same, for both the steel and DLC-coated surfaces. No direct evidence of any chemical reactions between the MoS2 and the steel or DLC coating was observed.