摘要： The purpose of the present work is to probe the friction mechanism of hydrogenated diamond-like carbon (H-DLC) ?lm in air by varying sliding velocity (25–1000 mm/s). Friction tests of Al2O3 ball against H-DLC ?lm were conducted with a rotational ball-on-disk tribometer. As the sliding velocity increases, both the friction coef?cient and the surface wear of H-DLC ?lm decrease, reach the minimum values, and then increase in the high sliding velocity region. Based on the observed results, three main friction mechanisms of H-DLC ?lm—namely graphitization mechanism, transfer layer mechanism, and passivation mechanism—are discussed. Raman analysis indicates that the graphitization of worn surface on the H-DLC ?lm has a negligible contribution to the variation of the friction coef?cient and the surface wear. The origin of the sliding velocity dependence is due to the synergistic interaction between the graphitized transfer layer formation and the surface passivation. The present study will not only enrich the understanding of friction mechanism of H-DLC ?lms in air, but will also help to promote their practical engineering applications.