LAUSR

The moving components of combustion engines are operated under harsh conditions of high pressures and temperatures. Extreme‐pressure anti‐wear additives, such as tricresyl phosphate (TCP), are mixed with base oil to prevent wear through the formation of a lubricant film on the substrate. We studied the effect of liquid pressure on the decomposition pathway of TCP in base oil molecules (2,5‐dimethylhexane) using hybrid quantum‐classical simulations with density functional theory for electrons. At a temperature of 300 K, we found that: (i) bond‐breaking barrier energies of both the OC and PO bonds of TCP decrease monotonically as the liquid pressure increases; (ii) the bond‐breaking barrier energy of PO is lower than that of OC at pressures of 0 and 2.0 GPa, but is higher at a pressure of 5.0 GPa; and (iii) the applied pressure significantly lowers the bond‐breaking barrier energies of both OC and PO when the PO bond of TCP is directed upward from the substrate. These findings are explained by the inhomogeneous distribution of base oil molecules around TCP and the steric repulsion of the PO bond of TCP. These results indicate that the internal structures of the lubricant films are pressure‐dependent.