Progresses on cryo-tribology: lubrication mechanisms, detection methods and applications

Tribology at cryogenic temperatures has attracted much attention since the 1950s with the acceleration of its applications in high-tech equipment such as cryogenic wind tunnels, liquid fuel rockets, space infrared telescopes, superconducting devices, and planetary exploration, which require solid lubrication for moving parts at low temperatures down to 4 K in cryogenic liquid, gaseous, or vacuum environments. Herein, the research progress regarding cryo-tribology is reviewed. The tribological properties and mechanisms of solid lubricants listed as carbon materials, molybdenum disulfide, polymers, and polymer-based composites with decreasing temperature are summarized. The friction coefficient increases with decreasing temperature induced by thermally activated processes. The mechanism of transfer film formation should be considered as a significant way to enhance the tribological properties of solid lubricants. In addition, applications of solid lubrication on moving parts under cryogenic conditions, such as spherical plain bearings and roller bearings, are introduced. The technology for tribological testing of materials and bearings at cryogenic temperatures is summarized, where the environmental control, motion and loading realization, as well as friction and wear measurement together in a low-temperature environment, result in the difficulties and challenges of the low-temperature tribotester. In particular, novel technologies and tribotesters have been developed for tribotests and tribological studies of solid lubricants, spherical plain bearings, and roller bearings, overcoming limitations regarding cooling in vacuum and resolution of friction measurement, among others, and concentrating on in-situ observation of friction interface. These not only promote a deep understanding of friction and wear mechanism at low temperatures, but also provide insights into the performance of moving parts or components in cryogenic applications.