Analytic results and experiments in ultrahigh vacuum indicate that the static friction between two clean crystalline surfaces should almost always vanish, yet macroscopic objects always exhibit static friction. A simple and general explanation for the prevalence of static friction is proposed. “Third bodies,” such as small hydrocarbon molecules, adsorb on any surface exposed to air and can arrange to lock two contacting surfaces together. The resulting static friction is consistent with experimental behavior, including Amontons' laws.

A microscopic theory for the ubiquitous phenomenon of static friction is presented. Interactions between two surfaces are modeled by an energy penalty that increases exponentially with the degree of surface overlap. The resulting static friction is proportional to load, in accordance with Amontons's laws. However, the friction coefficient between bare surfaces vanishes as the area of individual contacts grows, except in the rare case of commensurate surfaces. An area independent friction coefficient is obtained for any surface geometry when an adsorbed layer of mobile atoms is introduced between the surfaces. The predictions from our simple analytic model are confirmed by detailed molecular dynamics simulations.

Wear limits the life-span of many mechanical devices with moving parts. To reduce wear, lubricants are frequently enriched with additives, such as zinc phosphates, that form protective films on rubbing surfaces. Using first-principles molecular dynamics simulations of films derived from commercial additives, we unraveled the molecular origin of how antiwear films can form, function, and dissipate energy. These effects originate from pressure-induced changes in the coordination number of atoms acting as cross-linking agents to form chemically connected networks. The proposed mechanism explains a diverse body of experiments and promises to prove useful in the rational design of antiwear additives that operate on a wider range of surface materials, with reduced environmental side effects.

This paper is concerned with the principles of Green’s function-based molecular dynamics (GFMD) simulations of semi-infinite elastic solids and their application to various contact mechanical problems. A methodology to compute the (renormalized) elastic interactions of surface atoms is presented first. It is based on the fluctuation-dissipation theorem, with the help of which thermal fluctuations of atomic displacements can be related to the elastic Green’s functions and thus to the effective coupling between surface atoms. We suggest a sparse representation of these renormalized spring constants and present numerical results for some simple two- and three-dimensional lattices. The renormalized elastic interactions can be obtained for relatively small systems and then be extrapolated to large systems. They incorporate the full elastic response of semi-infinite solids in a way that only surface atoms have to be considered in molecular dynamics simulations. The usefulness of GFMD is demonstrated by applying it to various idealized contact models, such as nonadhesive Hertzian contacts as well as nonadhesive contacts between flat, semi-infinite elastic solids and a self-affine, rigid substrate. In all cases, a zero probability density $P\left(p\right)$ for infinitesimally small contact pressures $p$ is found, as predicted theoretically. If the self-affine, nonadhesive surfaces are under such high loads that the contact is complete, the pressure histogram can be represented by a Gaussian also in accordance with theoretical predictions. However, if the topography of the substrate resembles that of industrial steel surfaces and the loads are moderate, $P\left(p\right)$ decays exponentially for medium and large $p$ in contradiction to theoretical predictions for randomly rough surfaces.