Evolution of tribo-induced interfacial nanostructures governing superlubricity in a-C:H and a-C:H:Si films.

Hydrogenated amorphous carbon (a-C:H) is capable of providing a near-frictionless lubrication state when rubbed in dry sliding contacts. Nevertheless, the mechanisms governing superlubricity in a-C:H are still not well comprehended, mainly due to the lack of spatially resolved structural information of the buried contact surface. Here, we present structural analysis of the carbonaceous sliding interfaces at the atomic scale in two superlubricious solid lubricants, a-C:H and Si-doped a-C:H (a-C:H:Si), by probing the contact area using state-of-the-art scanning electron transmission microscopy and electron energy-loss spectroscopy. The results emphasize the diversity of superlubricity mechanisms in a-C:Hs. They suggest that the occurrence of a superlubricious state is generally dependent on the formation of interfacial nanostructures, mainly a tribolayer, by different carbon rehybridization pathways. The evolution of such anti-friction nanostructures highly depends on the contact mechanics and the counterpart material. These findings enable a more effective manipulation of superlubricity and developments of new carbon lubricants with robust lubrication properties.