Mechanisms of cation-induced superlubricity transition of poly(vinylphosphonic acid) coatings

Abstract

Water-based superlubricity systems have unique advantages, such as low friction and environmental friendliness. While control of water-based superlubricity is highly attractive, it is also challenging. The superlubricity transition behavior of poly(vinylphosphonic acid) (PVPA) lubricated using a NaCl solution when mixed with multiple lubricants was investigated in this study. The mechanisms of superlubricity transition were investigated by exploring the internal structural changes of a PVPA coating and the molecular behaviors at the PVPA interface affected by cations through X-ray photoelectron spectroscopy (XPS) depth profiling, quartz crystal microbalance with dissipation (QCM-D), and molecular dynamics simulation. The type and magnitude of the superlubricity transition caused by cations with different valence states are clearly different. The introduced monovalent cation that can stably adsorb on the PVPA surface, exhibits a strong attraction toward the surrounding water molecules, instantaneously strengthening the lubrication at the PVPA interface. These changes at the interface are reversible and would cause a sudden decrease in the coefficient of friction (COF). Adsorbing an introduced divalent cation on the PVPA molecular chain leads to the transformation of the PVPA coating from a stable network structure to an unstable bridging structure. The sudden increase in the COF, caused by the altered internal structure of the coating, is irreversible. This study provides theoretical support for the adaptive control of water-based superlubricity and intelligent lubrication.