Tribological properties of hydrogenated amorphous carbon films in different atmospheres

•Tribological properties of a-C:H films in vacuum, air, dry nitrogen, argon and oxygen atmospheres were investigated.•Superlubricity was achieved for self-mated a-C:H film counterparts in dry nitrogen atmosphere.•Synergistic effect of electrostatic repulsion and tribofilm of a-C:H film friction was discussed to disclose the superlubricity mechanism.

In this study, hydrogenated amorphous carbon (a-C:H) films prepared by plasma enhanced chemical vapor deposition had distinguishing tribology behaviors in dry nitrogen, oxygen, argon, humid air and vacuum. Both the friction and wear rates of a-C:H films sliding against Al2O3 in nitrogen and argon were lower than that in oxygen and humid air atmospheres. And superlubricity (0.009) was achieved in dry nitrogen atmosphere for self-mated a-C:H counterpart. From the viewpoint of various gas molecules' adsorption, the a-C:H films' friction behavior was investigated. Micro-Raman and scanning electron microscope (SEM) were employed to study the interfacial structural evolution and tribofilm distribution in different atmospheres. X-ray photoelectron spectroscopy (XPS) was used to reveal the relation between wear rate and tribo-oxidation reaction in various atmospheres. We confirmed that nitrogen molecule provided repulsion force at sliding interface and reduced friction of a-C:H film. Meantime, graphitic tribofilm also played a significant role. It was stated that electronic repulsion force aroused by adsorption was enough for ultra-low friction, while the presence of graphitic tribofilm had synergetic effect with it in achieving superlubricity. This study provided considerations and discussions about interfacial electronic character's effect on friction behaviors of carbon based film.

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