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.

This study demonstrates that magnetron-sputtered NbSe₂ film can be used as a lubricant for space current-carrying sliding contact, which accommodates both metal-like conductivity and MoS₂-like lubricity. Deposition at low pressure and low energy is performed to avoid the generation of the interference phase of NbSe₃. The composition, microstructure, and properties of the NbSe₂ films are further tailored by controlling the sputtering current. At an appropriate current, the film changed from amorphous to crystalline, maintained a dense structure, and exhibited excellent comprehensive properties. Compared to the currently available electrical contact lubricating materials, the NbSe₂ film exhibits a significant advantage under the combined vacuum and current-carrying conditions. The friction coefficient decreases from 0.25 to 0.02, the wear life increases more than seven times, and the electric noise reduces approximately 50%.

Recent studies have reported that adding nanoparticles to graphene enables macroscale superlubricity to be achieved. This study focuses on the role of nanoparticles in achieving superlubricity. First, because graphene nanoscrolls can be formed with nanoparticles as seeds under shear force, the applied load (or shear force) is adjusted to manipulate the formation of graphene nanoscrolls and to reveal the relationship between graphene-nanoscroll formation and superlubricating performance. Second, the load-carrying role of spherical nano-SiO₂ particles during the friction process is verified by comparison with an elaborately designed fullerene that possesses a hollow-structured graphene nanoscroll. Results indicate that the incorporated nano-SiO₂ particles have two roles in promoting the formation of graphene nanoscrolls and exhibiting load-carrying capacity to support macroscale forces for achieving macroscale superlubricity. Finally, macroscale superlubricity (friction coefficient: 0.006–0.008) can be achieved under a properly tuned applied load (2.0 N) using a simple material system in which a graphene/nano-SiO₂ particle composite coating slides against a steel counterpart ball without a decorated diamond-like carbon film. The approach described in this study could be of significance in engineering.