Simulation of thermal stability and friction: a lubricant confined between monolayers of wear inhibitors on iron oxide

To understand antiwear phenomena in motor engines at the atomic level and provide evidence in selecting future ashless wear inhibitors, we studied the thermal stability of the self-assembled monolayer (SAM) model for dithiophosphate (DTP) and dithiocarbamate (DTC) molecules on the iron oxide surface using molecular dynamics. The interactions for DTP, DTC and Fe₂O₃ are evaluated based on a force field derived from fitting to ab initio quantum chemical calculations of dimethyl DTP (and DTC) and Fe(OH)₂(H₂O)₂-DTP (DTC) clusters. MD simulations at constant-NPT are conducted to asses relative thermal stabilities of the DTP and DTC with different pendant groups (n-propyl, i-propyl, n-pentyl, and i-pentyl). To investigate frictional process, we employ a steady state MD method, in which one of the Fe₂O₃ slabs maintained at a constant linear velocity. We obtain the time averaged normal and frictional forces from the interatomic forces. Then, we calculated the friction coefficient at the interface between SAMs of DTP and the confined lubricant, hexadecane, to assess the shear stability of DTPs with different pendant groups.