Steric Hindrance Effects on the Formation Mechanism of Tribofilms from Phosphorus-Based Extreme Pressure Additives

Traditional trial-and-error approaches to studying the lubrication mechanisms of extreme-pressure (EP) antiwear additives are costly and time-consuming. ReaxFF molecular dynamics simulations combined with macroscopic tribological experiments were employed to elucidate the regulatory effect of substituent steric hindrance on phosphate ester tribological performance and unravel the atomic-scale tribofilm formation process on iron oxide surfaces. Polyphosphates were identified as the core component governing EP performance: reduced steric hindrance enhanced phosphate ester adsorption on the metal oxide surface, evidenced by increased Fe–H, Fe–C and C–O interfacial bonds that formed a denser initial adsorption layer. This enhanced adsorption promoted active site-surface contact, driving molecular multistage decomposition and carbon skeleton reorganization; generated phosphorus-containing and p-methylphenoxyl radicals synergistically facilitated efficient polyphosphate formation. The simulated atomistic tribofilm formation mechanism was experimentally validated by QCM-D, SEM-EDS and XPS analyses of interfacial adsorption and tribofilm composition. This work clarifies the mechanistic link between molecular steric hindrance, interfacial tribochemical reactions and EP performance, proposes a rational molecular design strategy for high-performance phosphate-based EP additives, and advances fundamental understanding of organic–inorganic interfacial tribochemistry.