Research Progress of Cryogenic Tribology

Recently, with the development of deep space exploration, superconductivity, quantum computing, and other technologies, there is more and more equipment that needs to be in service under extremely low temperature conditions, and the lubrication problems caused by cryogenic temperature are increasingly apparent. The research of cryogenic tribology mainly relies on the development of space technology in America, the Soviet Union, Europe and other countries in the last century, in the actual research, people are seemly to evaluate its cryogenic tribological performance from the material point of view. We review the existing material systems ranging from polymer composites, metal materials, bonded solid coatings, and vacuum sputtering lubricating films, and find there are relatively few studies on cryogenic tribology, mainly focusing on the study of changes in the tribological properties of materials, which results in a lack of in-depth research into the mechanisms of scientific action. The wear resistance of most solid lubricant materials becomes poor at cryogenic temperature, making it difficult to meet the demand for long service. The tribological behavior of materials is affected not only by the temperature but also by the type and state of the cryogenic medium, thus showing different trends. From a material point of view, differences in the type of material and the methods it is prepared also lead to different variations in tribological properties at cryogenic temperatures, thus having various effects on the tribological performance. In general, this influence process is very complex, resulting in a lack of in-depth and integrated understanding of the tribological mechanism of cryogenic tribology. The motion of atoms and electrons is inhibited, which leads to changes in the

tribological properties of materials at cryogenic temperature. The slowing down of atomic thermal motion leads to changes in the interatomic distance of the material, which affects the mechanical properties of the material. The changes in mechanical properties affect the deformation and contact area of the friction interface, which shows different tribological properties. Meanwhile, it also leads to changes in material structure, causing the change of phase structure and friction interface structure, which affects the generation of transfer film and the strength and mode of interfacial interaction, leading to the change of tribological behavior. In terms of electrons, the suppression of electron motion and the reduced probability of collisions between electrons lead to weaker interactions between the friction interfaces at cryogenic temperature, changing the tribochemical activity and the state of the transfer film between the friction substrates. When the temperature is lower than the superconducting transition temperature, the contribution of electrons to the energy dissipation in the friction process is reduced, leading to a lower friction coefficient. Based on the essence of the inhibition effect of cryogenic temperature on the motion of atoms and electrons, we integrate and generalize the mechanism of the influence of cryogenic on the tribological properties of solid lubricating materials in this review. The influence mechanism of cryogenic temperature induces mechanical properties changes, tribochemical reactivity changes between frictional interfaces, phase and interface structure changes, electron-phonon coupling, and other micro energy dissipation forms are introduced. Last but not least, a conceptual framework summarizing the information discussed is presented, which is expected to help pave the way for future studies on cryogenic tribology to provide ideas for researchers to carry out follow-up research work.