Impact of the Surface and Microstructure on the Lubricative Properties of MoS₂ Aging under Different Environments

Molybdenum disulfide (MoS₂) with a hydrophobic property and layered structure possesses an excellent lubricative property and has been widely used as a lubricant in various areas, including satellites, aircraft, and new energy vehicles. Aging is a ubiquitous phenomenon in MoS₂ and plays a key role in its tribological application for shortening its service life. The effect of the surface and microstructure on the lubricative properties of MoS₂ aging under different environments, including deionized water (DI water), ultraviolet/ozone (UV/Ozone), and high-temperature, was investigated. First, the lubrication of MoS₂ transiently degrades because of physical adsorption and recovers after mechanical removal. The lubrication of MoS₂ also degrades slightly when its surface becomes hydrophilic, thereby enhancing the adhesion energy due to atomic oxygen interaction under UV/Ozone exposure. Second, the lubrication of MoS₂ degrades irreversibly because of the formation of stripes with the destroyed structures under accelerated aging. The lubrication of MoS₂ further degrades with the formation of small triangular pits under high-temperature annealing. Finally, the lubrication of MoS₂ deteriorates due to the destroyed structure and complete oxidation. The severe aging of MoS₂ is accompanied with large triangular pits due to anisotropic oxidation etching of MoS₂. The lubrication failure of MoS₂ was determined on the basis of structural defect formation and surface property degradation induced by the extent of oxygen diffusion. The enhanced out-of-plane deformation due to the reduced out-of-plane stiffness and the increased energy barriers of defects are fundamentally responsible for the lubrication degradation of MoS₂ at the atomic scale. These findings can provide new insights into the atomic-scale mechanism underlying the lubrication failure of MoS₂ and pave the way for the realization of MoS₂-based lubrication application under various environments.