Origin of higher graphitization under higher humidity on the frictional surface of self-mated hydrogenated carbon films

Abstract The ‘friction-induced graphitization’ mechanism for hydrogenated amorphous carbon (a-C:H) films has been postulated, but it requires concrete analyses at specific conditions. Here, we have discussed the reason for higher sp2 bond content on the interface of self-mated fullerene-like a-C:H films when the relative humidity (RH) increases from dry N2 to 70%. The reason is the failure of the role of FL structures and the change of interface structure evolution. Before RH 50%, FL structures work properly, and the films remain low graphitization to form nanoparticle-like debris having onion-like structures. After RH 50%, the failure of the role of FL structures leads to the abruptly increasing wear, and the formation of larger flake-like debris. Higher sp2 content, is not only caused by formation of longer and order graphene sheets than the original surfaces, but also linked with the increase of C O bonds. This is unlike the general friction-induced graphitization that higher sp2 content indicates higher graphitization. The result reminds us to further understand friction-induced graphitization mechanism at special condition.