Effects of Surface Chemistry on the Mechanochemical Decomposition of Lubricant Additives

The growth of protective tribofilms from lubricant antiwear additives on rubbing surfaces is initiated by mechanochemical decomposition reactions. These processes are not well understood at the molecular scale for many important additives, such as tricresyl phosphate (TCP). One aspect that needs further clarification is the extent to which the surface properties affect the mechanochemical decomposition rate. In this study, we use nonequilibrium molecular dynamics (NEMD) simulations with a reactive force field (ReaxFF) to study the decomposition of TCP molecules confined and pressurised between sliding ferrous surfaces at a range of temperatures. We compare the decomposition of TCP on native iron, iron carbide, and iron oxide surfaces. We show that the decomposition rate of TCP molecules increases exponentially with temperature and shear stress, implying that this is a stress-augmented thermally activated process. The rates and products of decomposition depend on the properties of the confining surfaces. The activation energy, activation volume, and pre-exponential factor are similar for TCP decomposition between iron and iron carbide surfaces, but significantly different for iron oxide surfaces. These findings provide new insights into the mechanochemical decomposition of TCP and have important implications for the design of novel lubricant additives for use in high-temperature and high-pressure environments'