When wall slip wins over shear flow: A temperature-dependent Eyring slip law and a thermal multiscale model for diamond-like carbon lubricated by a polyalphaolefin oil

Abstract

The quantitative description of lubricant flow in nanoscale channels is complicated by various finite-size effects that are not taken into account in conventional thermo-elasto-hydrodynamic lubrication (TEHL) models. One of these effects is wall slip, a phenomenon that has been extensively studied both theoretically and experimentally. The relationship between wall slip and thermal effects is intricate, and some authors debate whether the friction reduction observed in their experiments in the TEHL regime can be explained by either slip or viscosity reduction in heated lubricants. To disentangle these mechanisms, a comprehensive molecular dynamics study of the relationship between temperature and slip in the shear flow of a 4 cSt polyalphaolefin (PAO4) base oil in a nanoscale diamond-like carbon (DLC) channel is performed here. An Eyring law describes the relationship between slip velocity and shear stress at the solid-liquid interface for a given temperature and pressure. The same simulation campaign provides a pressure-dependent law for the interface thermal resistance (ITR) between DLC and PAO4. These constitutive laws are employed in a continuum model for lubricated parallel channels. By taking heat conduction into account and combining the slip and ITR laws with a temperature- and pressure-dependent Eyring viscosity law, questions about the competition of slip and thermal thinning of the lubricant can be answered. For DLC film thicknesses compatible with tribological experiments and applications, this model shows that slip is only relevant for very thin lubricant films that are typical of boundary lubrication, suggesting the dominance of thermal thinning in the TEHL regime.