Tribological behavior and lubrication mechanisms of a-C:H films in vacuum from −200 to 25 °C

Lubrication failure of moving parts at extremely cryogenic temperatures poses a major challenge for advancements in space exploration, superconductivity, and other technologies. This study systematically investigates the tribological behavior of hydrogenated amorphous carbon (a-C:H) films in vacuum from −200 to 25 °C. Notably, as the temperature decreases, the friction coefficient and the wear life of the a-C:H films exhibit an abnormal increase. At −200 °C, the wear life exhibits a remarkable enhancement of at least two orders of magnitude. Introducing in situ mass spectrometry and cryogenic micro/nano indentation, the dynamic monitoring of interface damage, hydrogen passivation, and hardness evolution was conducted during the friction process. The work indicated that cryogenic temperatures significantly reduce the damage of a-C:H films, leading to changes in the synergistic lubrication involving hydrogen passivation, graphitization, and transfer films, resulting in high friction and low wear. This is fundamentally attributed to cryogenic temperatures altering the interfacial activity, which is the key factor in activating the synergistic lubrication of the above mechanisms. Crucially, with a suitable interfacial activity at −75 °C, a-C:H films can achieve an ultralow friction coefficient of ~0.015 and a wear rate of ~10^-8 mm³/(N·m). This work provides critical insights and establishes a foundation for deploying a-C:H films for cryogenic applications.