On the Derjaguin Offset in Boundary-Lubricated Nanotribological Systems

Abstract

We performed molecular dynamics simulations of boundary-lubricated sliding, varying the boundary lubricant type, its molecular surface coverage, the substrate roughness, and the load. The resulting load versus friction behavior was then analyzed to study how changes in lubricant type, coverage, and roughness affect the extrapolated friction force at zero load, the so-called Derjaguin offset. A smooth-particle-based evaluation method by the authors, applied here for the first time to visualize the sliding interface between the two layers of boundary lubricant, allowed the definition and calculation of a dimensionless normalized sliding resistance area, which was then related to the Derjaguin offset. This relationship excellently reflects the molecular surface coverage, which determines the physical condition of the lubricant, and can differentiate between some lubricant-specific frictional properties.

A Comparison of Classical Force-Fields for Molecular Dynamics Simulations of Lubricants

“…The friction coefficients presented are the average of those calculated for the top and bottom slabs. The validity of the high load approximation has been confirmed from previous NEMD simulations of stearic acid layers with a thin, separating layer of n -hexadecane [8,9,10]. …”

“…The pressure ( P z = 0.5 GPa) was controlled by applying a constant normal force to the outermost layer of atoms in the upper slab, keeping the z -coordinates of the outermost layer of atoms in the lower slab fixed, as is common in confined NEMD simulations [6,7,8,9,10] (Figure 1b).…”

“…Despite these challenges, nonequilibrium molecular dynamics (NEMD) simulations using classical force-fields have shed light on the nanoscale behavior of many experimentally-important lubricant systems. These have included simulations investigating the rheological and tribological behavior of several base oils [1,2,3,4,5] as well as many important additives [6,7,8,9,10]. …”

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…Eder et al [239] performed NEMD simulations of several OFMs on iron surfaces, varying the OFM type (stearic acid, oleic acid, methyl stearate), OFM surface coverage (2.54−4.57 nm −2 ), the pressure (0.07−1.60 GPa), and the surface roughness. The resulting load versus friction behaviour was then analysed to study these parameters affected the friction coefficient and the extrapolated friction force at zero load (Derjaguin offset).…”

Confined fluid density of a pentaerythritol tetraheptanoate lubricant investigated using molecular dynamics simulation

“…Most of the published studies on thin film properties focused on boundary slip, viscosity and friction [5][6][7][8][9]. Studies relating to compressibility are limited.…”