Regulation Mechanism of Trivalent Cations on Friction Coefficient of a Poly(Vinylphosphonic Acid) (PVPA) Superlubricity System

Abstract

The application range of superlubricity systems can be extended effectively by realizing an adjustable friction coefficient. In this study, a stable poly(vinylphosphonic acid) (PVPA) superlubricity system was developed using sodium chloride (NaCl) solution as the lubricant. A sudden increase in the friction coefficient occurred when a trivalent salt solution was introduced to the base lubricant during the friction process. The changes in surface microstructure and interfacial molecular behavior induced by trivalent cations were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and molecular dynamics simulation. The regulation mechanism of trivalent cations with respect to the friction coefficient of the PVPA superlubricity system was explored. Although La3+ and Fe3+ in solutions exist in two forms (La3+ and Fe(OH)3), both can destroy the stable coating structure through a bridging effect, resulting in a sudden increase in the friction coefficient. The ability of various cations to attract the surrounding water molecules is an important reason for the varying degree of change of the friction coefficient. In addition, the degree of sudden increase in the friction coefficient is dependent on the concentration of trivalent cations. There is an extreme concentration at which the maximum sudden increase degree in friction coefficient can be obtained. This study provides insights into the realization of oil-based superlubricity through interface regulation.