A Multiscale Study on the Wall Slip Effect in a Ceramic–Steel Contact With Nanometer Thick Lubricant Film by a Nano to Elastohydrodynamic Lubrication Approach

Abstract

A novel nanotoelastohydrodynamic lubrication (EHL) multiscale approach, developed to integrate molecularscale phenomena into macroscopic lubrication models based on the continuum hypothesis, is applied to a lubricated contact problem with a ceramic–steel interface and a nanometric film thickness. Molecular dynamics (MD) simulations are used to quantify wall slip occurring under severe confinement. Its dependence on the sliding velocity, film thickness, pressure, and different wall materials is described through representative analytical laws. These are then coupled to a modified Reynolds equation, where a noslip condition applies to the ceramic surface and slip occurring on the steel wall is described through a Naviertype boundary condition. The results of this nanotoEHL approach can contradict the wellestablished lubrication theory for thin films. In fact, slip can occur over the whole contact length, leading to a significant modification of the lubricant flow and consequently of the film thickness. If both walls move at the same velocity, the flow is reduced at the contact inlet and the film thickness decreases. If the nonslipping wall entrains the fluid, this one is accelerated resulting in a larger mass flow; nevertheless, the surface separation is reduced as the lubricant flows even faster in the contact center. The opposite effect occurs if the slipping surface entrains the fluid, causing a lower mass flow but higher film thickness. Finally, friction is generally smaller compared to the classical noslip case and becomes independent of the sliding velocity as total slip is approached.