Pressure-assisted decomposition of tricresyl phosphate on amorphous FeO using hybrid quantum-classical simulations

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.