Effect of Chain Length on the Tribological Behavior of Polymer-Based Protic Ionic Liquids as Green High-Performance Lubricants

Despite the remarkable success in synthesizing various functional ionic liquids (ILs) by taking advantage of the structural designability, developing unique ILs as green lubricants to further mitigate friction and wear remains a compelling challenge. In this study, a series of polymer-based protic ionic liquids (PPILs) were designed as green lubricants using polyethylenimine as cations and fatty acids with different chain lengths as anions. These PPILs exhibit excellent wettability on a steel substrate, appropriate pH values, high viscosity, commendable rheological behavior, and anticorrosion performance, thereby mitigating safety concerns and stability issues under severe conditions. Notably, PPILs based on nonanoic acid (PEIC9), with medium chain length, show a low friction coefficient (0.059) and wear volume (8.55 × 10³ μm³) under high load conditions (2.15 GPa), which are mainly related to the formation of a densely ordered and thicker protective layer on the contact surface. Furthermore, the synergy of adsorption films, tribochemical films, and hydrodynamic lubrication mechanism offers durable and efficient protection for steel substrate surfaces. This study will serve as an effective addition to a green liquid lubricating material.