Friction Control in Thin-Film Lubrication

A novel method is proposed for controlling and reducing friction in thin-film boundary lubricated junctions, through coupling of small amplitude (of the order of 1 Å) directional mechanical oscillations of the confining boundaries to the molecular degrees of freedom of the sheared interfacial lubricating fluid. Extensive grandcanonical molecular dynamics simulations revealed the nature of dynamical states of confined sheared molecular films, their structural characteristics, and the molecular scale mechanisms underlying transitions between them. Control of friction in the lubricated junction is demonstrated, with a transition from a high-friction stick-slip shear dynamics of the lubricant to an ultralow kinetic friction state (termed as a superkinetic friction regime), occurring for Deborah number values De ) τf/τosc > 1, where τosc is the time constant of the boundary mechanical oscillations normal to the shear plane and τf is the characteristic relaxation time for molecular flow and ordering processes in the confined region. A rate and state model generalized to include the effects of such oscillations is introduced, yielding results in close correspondence with the predictions of the atomistic simulations.