Molecular Insights into the Transition from Elastohydrodynamic to Boundary Lubrication: Unveiling Mechanistic Changes

The continued miniaturization of mechanical systems under increasing loads confines lubricants to only a few molecular layers, driving a transition from elastohydrodynamic lubrication (EHL) to boundary lubrication (BL) governed by molecular-scale transport mechanisms. How momentum and energy transport evolve across this transition remains unclear. This article establishes a unified molecular picture linking elastohydrodynamic lubrication and boundary lubrication for confined polyalphaolefin (PAO) films, achieved by continuously reducing their thickness from the continuum regime to a monolayer. It is shown that the lubrication transition is governed by a fundamental shift in the dominant mechanisms of momentum and heat transport. In the EHL regime, wall slip arises from a mismatch between molecular momentum relaxation and the imposed shear time scale. Under ultrahigh pressure, this mismatch is amplified, strongly enhancing slip, suppressing bulk viscous dissipation, and producing a pressure-induced temperature plateau within the lubricant. With increasing confinement, cross-wall molecular adsorption dominates interfacial momentum transfer, leading to anomalously large slip even at low pressure and culminating in near-complete slip in the monolayer limit. Upon entering the BL regime, heat generation becomes interface-dominated, breaking down the classical parabolic temperature profile and yielding an almost uniform film temperature. By explicitly accounting for velocity slip and temperature jump, a universal expression for the average lubricant temperature that rationalizes the confinement-induced transition in heat-generation mechanisms is set up. Moreover, a qualitative analysis is performed on the film-thickness dependence of density, viscosity, and the coefficient of friction, followed by fitting a viscosity-thickness relationship. Finally, we reveal that ultrahigh pressure drives a glass-like amorphization of confined lubricants, strongly suppressing thermal conductivity through enhanced phonon scattering, while shear-induced molecular ordering prior to compression mitigates this collapse. These results bridge EHL and BL at the molecular level and provide a physically grounded framework for extending continuum lubrication models into the nanometer regime.