Modeling Bearing and Shear Forces in Molecularly Thin Lubricants

Abstract

Under the effects of high shear rate and confinement between solid surfaces, the behavior of a thin lubricant film deviates from that of the bulk, resulting in significant increases of lubricant viscosity and interfacial slip. A semi-empirical model accounting for the breakdown of continuum theory at the nanoscale is proposed-based on film morphology and chemistry from available experimental and molecular dynamics simulation data-to describe lubricant behavior under shear. Viscosity stiffening and interfacial slip models are introduced into the formulations of the normal (bearing) and shear forces acting on a sphere that moves within a thin lubricant film parallel to a rigid plane. The experimentally measured 'apparent' viscosity confounding the effects of both stiffening and slip is used to predict the hydrodynamic forces acting on a fully or partially submerged sphere for the purposes of describing lubricant contact in magnetic storage. The proposed sphere-on-flat model forms the basis of a future, dynamic contact with friction model that will account for lubricant contact in the context of molecularly thin lubricated rough surface contact.

Modeling Sliding Contact of Rough Surfaces with Molecularly Thin Lubricants

“…A rough surface model of molecularly thin lubricant (MTL) contact is presented in this study, which builds on a single asperity model [1] that accounts for dynamic shearing experiments with polymeric thin lubricants [2]. This model is coupled with an existing rough surface dynamic contact model with friction [3,4] that has been extended to include variable lubricant surface energy according to recent experimental measurements [5,6].…”

“…However, experimental evidence shows that the rheology of lubricants confined in nanoscale gaps is markedly different from that of the bulk resulting in solid-like behavior [7,8], while viscous forces that are primarily dependent on the sliding velocity dominate the interface prior to solid contact [7]. Furthermore, results from the single asperity model suggest that lubricant contact forces are not negligible [1]. This study aims to calculate the contribution of a lubricant monolayer to the contact and friction forces of a rough sliding interface, especially under high shear rates, such as those encountered in the head-disk interface of magnetic storage harddisk drives.…”

“…The single asperity model [1] investigated the normal and shear forces under the effects of increasing viscosity with decreasing separation and increasing shear rate based on experimental data by Fukuzawa et al [2]. Lubricant thicknesses used in the dynamic shearing experiments were of the order of a few hundred microns.…”

“…Molecular dynamics (MD) simulations have shown that wall interaction and molecular end-group functionality affect the behavior of the lubricant layer, while lubricant confinement (separation between the solid surfaces) and shear rate were found to play a critical role in determining the lubricant's liquid-or solid-like response [7,8]. It was proposed [1] that the forces in MTL layers could be adequately extrapolated up to the expulsion of mobile lubricant molecules from the interface because the main effects of confinement and wall interactions/end-group functionality (slip behavior) are captured in the dynamic shearing experiments [2,9,10].…”

“…The solid surface root-mean-square (RMS) roughness was sufficiently small in the experiments [2]-of the order of the average combined thickness of the bonded molecule layers-to be engrossed in the transition region between steady lubricant and steady solid contact; this allowed for the effective decoupling of hydrodynamic contact from the transition region where lubricant film rupture and roughness effects (i.e., asperity contact) come into play [1]. However, while displacement-control results in clearly distinguishable regimes of steady lubricant, transitional, and solid contact in the experimental data, the statistical nature of contact would dominate the interface otherwise such that the response would transition directly from lubricant to solid contact in non-displacement-control settings.…”