From Continuous to Molecular Scale in Modelling Elastohydrodynamic Lubrication: Nanoscale Surface Slip Effects on Film Thickness and Friction

Abstract

The aim of this article is to propose an original approach in modelling elastohydrodynamic lubrication (EHL), combining a classical model derived from continuum mechanics and a nanoscale investigation carried out by Molecular Dynamics simulations. In particular, nanoscale slip is numerically quantified and a semi-analytical model for surface slip variation with pressure, film thickness and sliding velocity is presented. A composite model involving both continuum mechanics and nanoscale effects allows for a better understanding of dimple formation and offers a new basis towards a physically based friction prediction. This tentative represents a new direction towards a more realistic modelling of lubricated contacts with ultra thin film for the present industrial needs.

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…These simulations have shown that the slip length generally increases with increasing sliding velocity and pressure [168][169][170][171]. At very high sliding velocity, the slip length asymptotes towards a constant value [57].…”

“…This can lead to large reductions in friction compared to the Couette case [146]. Slip has been commonly observed in NEMD simulations of thin fluid films confined between atomically-smooth surfaces [57,[168][169][170][171]. Many of these studies have quantified the slip length, which is defined as an extrapolated distance into the wall where the tangential velocity component vanishes (see Fig.…”

“…Inflexible molecules, which can conform to the surface only with a greater expense of conformational energy generally exhibit more slip [177]. Conversely, the slip length decreases with increasing surface-fluid attraction strength and film thickness [169][170][171], and is virtually eliminated in the presence of atomic-scale surface roughness, which frustrates molecular layering [153,154]. This final observation is critical since most experimental surfaces will contain fractal roughness, even down to the atomic scale [178], which is likely to discourage boundary slip on conventional surfaces.…”

Elastohydrodynamic Lubrication

“…As the EHL films in contacts lubricated by very low viscosity oils can be ultra thin, i.e., in the range of few nanometres, the use of a continuum model of lubricant flow becomes questionable [78]. In these ultra-thin films molecular characteristics control the onset of slip inside the EHL contact.…”

“…The regulating factor is that the amount of slip is inversely proportional to the film thickness, i.e., thicker film results in smaller slip. In such cases the molecular dynamic simulations provide better understanding of the phenomena occurring than the continuum models [78].…”