Friction dependence on processing priority for graphitization/passivation coupled amorphous carbon films

Abstract

In this study, amorphous carbon (a-C) films were modified using different process sequences-H passivation followed by graphitization (a-C@H@G2000K) and graphitization followed by H passivation (a-C@G2000K@H). The friction dependence on the surface H content and the processing priority was comparatively investigated at the atomic scale, with a focus on the coupling mechanism for achieving low friction. The results indicated that the friction properties closely depended on the H content of the contacted a-C surfaces. An appropriate H content significantly improved the friction property through the coupling effect of the lubrication between surface graphitized structures and the repulsion between H atoms, resulting in a rapid decrease in the friction coefficient; however, the graphitization mechanism remained dominant. Excessive H reduced the repulsion between the contacted graphitized structures and hindered the sliding of these structures (shear susceptible), resulting in a slow increase in the friction coefficient. Most importantly, compared with the a-C@H@G2000K systems, the a-C@G2000K@H system exhibited higher effectiveness in reducing the friction coefficient, achieving a lower friction coefficient under the same surface H content; this was attributed not only to the small surface roughness and the low fraction of unsaturated bonds but also to the well stress distribution of the surface H atoms.