The effect of surface roughness on the adhesion of solid surfaces for systems with and without liquid lubricant

Abstract

We present molecular dynamics results for the interaction between two solid elastic walls during pull-off for systems with and without octane (C8H18) lubricant. We used two types of substrate—flat and corrugated—and varied the lubricant coverage from ∼1/8 to ∼4 ML (monolayers) of octane. For the flat substrate without lubricant the maximum adhesion was found to be approximately three times larger than for the system with the corrugated substrate. As a function of the octane coverage (for the corrugated substrate) the pull-off force first increases as the coverage increases from 0 to ∼1 ML, and then decreases as the coverage is increased beyond monolayer coverage. It is shown that at low octane coverage, the octane molecules located in the substrate corrugation wells during squeezing are pulled out of the wells during pull-off, forming a network of nanocapillary bridges around the substrate nanoasperities, thus increasing the adhesion between two surfaces. For greater lubricant coverages a single capillary bridge is formed. The adhesion force saturates for lubricant coverages greater than 3 ML. For the flat substrate, during pull-off we observe discontinuous, thermally activated changes in the number n of lubricant layers (n−1→n layering transitions), whereas for the corrugated substrate these transitions are “averaged” by the substrate surface roughness.

A molecular dynamics simulation of boundary lubrication: The effect of n-alkanes chain length and normal load

“…It has been found that a layered structure of thin film [1] and stick-slip motion [2] existed in thin film lubrication. There have also been wide discussions about the effects of the chain length of lubricant molecule [3,4], chain branch [5][6][7][8] and roughness of wall surfaces [9][10][11][12] on viscosity, distributions of lubricant density and velocity, but these works ignored asperity contact and deformation of the wall.…”

Molecular dynamics characterization of the contact between clean metallic surfaces with nanoscale asperities

“…Under normal operating conditions noble metal surfaces exhibit a layer of adsorbed molecules that play a large role in contact mechanics. These surface films provide damping for the impact during closing and also affect adhesion by influencing capillary forces and limiting direct metal-metal interactions 11 . Despite these pervasive molecular films, in most cases we expect some direct metalmetal bridging due to the large local stresses, plastic deformation within the asperities, as well as to the large local temperatures caused by Joule heating when electrical currents run through the contacts.…”

Capillary adhesion between elastic solids with randomly rough surfaces

“…The limiting case of molecular thin fluid films must be treated using atomistic methods, e.g., Molecular Dynamics (see, e.g., Ref. [20,21]). …”