The distance upon contact: Determination from roughness profile

TL;DR

This paper investigates how the initial contact distance between rough surfaces depends on surface roughness and contact area, providing a scale-dependent model crucial for understanding dispersive forces.

Contribution

It introduces a simple expression for the contact distance as a function of roughness and contact area, highlighting scale dependence in contact mechanics.

Findings

Contact distance depends on surface roughness and contact area.

Uncertainty in separation decreases with increasing contact scale.

Model applies to gold films and similar rough surfaces.

Abstract

The point at which two random rough surfaces make contact takes place at the contact of the highest asperities. The distance upon contact d_0 in the limit of zero load has crucial importance for determination of dispersive forces. Using gold films as an example we demonstrate that for two parallel plates d_0 is a function of the nominal size of the contact area L and give a simple expression for d_0(L) via the surface roughness characteristics. In the case of a sphere of fixed radius R and a plate the scale dependence manifests itself as an additional uncertainty \delta d(L) in the separation, where the scale L is related with the separation d via the effective area of interaction L^2\sim\pi Rd. This uncertainty depends on the roughness of interacting bodies and disappears in the limit L\to \infty.