Slip of Hexadecane on Organic Friction Modifier Monolayers

Organic friction modifiers (OFMs) are amphiphilic surfactants added to lubricants in order to reduce friction and wear. They achieve this by preventing the contact of surface asperities and perhaps also by promoting boundary slip. The structure, flow, and friction behaviour of nanometer-thick OFM films are challenging to probe directly through experiment. In this study, large-scale nonequilibrium molecular dynamics (NEMD) simulations have been used to provide unique insights into the physical origins of friction reduction by OFMs under a wide range of conditions. The OFM films are shown to be resilient to high pressures and sliding velocities, as well as nanoscale surface roughness. Both slip and friction are shown to be sensitive to the amount of interdigitation between OFM and hexadecane layers. The formation of films with a high surface coverage is shown to be critical to form smooth sliding interfaces, promote slip and ultimately reduce friction. Slip and friction trends from the NEMD simulations are consistent with experimental results.