Durable Near-Zero Wear Behavior Achieved by Polymer-Based Protic Ionic Liquids on Engineering Steel Surfaces

Near-zero wear on engineering steel surfaces is a promising solution to extend the service life of mechanical equipment. However, most existing strategies offer only limited low wear under particular conditions and friction pairs. To address this, we design a polymer-based proton ionic liquid (PPILs) lubricant, leveraging the proton exchange between polyethylenimine, which is rich in active nitrogen groups, and bis(2-ethylhexyl) phosphate. The results indicate that under high Hertzian contact pressure (2.15 GPa), the friction coefficient of PPILs is ∼0.08, and the wear rate of the steel disk is as low as 1.46 × 10^-10 mm³·N^1-·m^-1. Even in multiple environments (long-term friction or high frequency), PPILs continue to exhibit durable near-zero wear behavior. It is related to the combined effects of adsorption films, tribochemical films, and hydrodynamic effects. Moreover, the PPILs were combined with polyol aqueous solutions to create a superlubrication system (μ = 0.007) with an ultrashort running-in period (<3 s) under Si₃N₄/glass friction pairs, thereby enriching the research on superlubrication materials. This study addresses the existing limitations of ultralow wear on engineering steel surfaces and introduces a new category of polymer-based protic ionic liquid lubricants, significantly expanding the application of ultralow wear materials.