Tribological Properties of Tricresyl Phosphate Blended in Base Oils between Iron-Based Surfaces

Studying the lubrication of tricresyl phosphate (TCP) blends with base oils provides a better understanding of the reaction mechanism of TCP. However, given that some studies point out the weak role of base oils, this aspect is frequently overlooked in research. To explore the distinctions between pure TCP and TCP-base oil blended systems as well as to elucidate the underlying mechanism of the tribological properties, we employed reactive force field molecular dynamics (ReaxFF-MD) to investigate the tribochemical reactions and tribological properties of different ratios of TCP and octene molecules confined between iron and iron oxide substrates at different loads. Results indicate that the addition of octene molecules to TCP reduces friction and wear of the system, while the ameliorating effect of octene is influenced by surface materials and loads. On the Fe surface, octene inhibits the dissociation of C_TCP-O_TCP bonds, the formation of P-O_TCP bonds, and the polymerization of TCP, resulting in a slowdown in interfacial shear and a reduction in the formation of bridge bonds. Consequently, the tribological properties are improved, particularly under high loads. In contrast, this enhancement is not pronounced on the Fe₂O₃ surface, where TCP primarily undergoes dissociation of P-O_TCP bonds, with octene only affecting the polymerization of TCP. The steady-state friction coefficient of the unfailed models on the Fe surface is lower than that on the Fe₂O₃ surface due to the nucleophilic substitution with TCP, generating remarkable interfacial shear from polyphosphate clusters, whereas TCP forms stabilized adsorbed carbon films by carbon rings on the Fe surface. Additionally, the Fe substrate readily forms iron-carbon bonds, producing numerous bridge bonds that cause severe wear. On the contrary, the Fe₂O₃ substrate constructs fewer bridge bonds on account of unstable adsorption of carbon rings, presenting superior wear resistance.