Cu doping and friction-induced self-migration in a-C:H films to improve anti-wear life in vacuum

Hydrogenated amorphous carbon (a-C:H) films can achieve superlubricity, but the problem of rapid failure in vacuum remains a challenge for their applications. In this study, a novel solution route is demonstrated by friction-induced Cu self-migration to improve the anti-wear life of Cu/a-C:H films. The results show that Cu doping promotes the increase of sp(2)C, reduces the internal stress and improves the adhesion. Additionally, the doped Cu atoms mainly exist in a metallic state, which is benefit for the diffusion of Cu atoms in the matrix under friction effect. Further results imply that the anti-wear life of the Cu/a-C:H films with different Cu content are improved, especially, the most significant increase in anti-wear life occurring at a Cu content of 1.15 at%, which is related to the Cu-rich transfer films that form a Cu/C friction interface. Furthermore, the distinct Cu migration channels consisting of crystal clusters aggregated by the Cu atoms dispersed in the matrix are observed in the cross-section HRTEM and EDS characterizations of the wear track, which verifies the self-migration behavior of Cu atoms. The formed Cu/C friction interface eliminates the strong covalent interactions and maintains a stable lubricity state, thus prolonging the anti-wear life.