Journal Article

Friction laws at the nanoscale

Turner, Kevin T., Mo, Yifei, Szlufarska, Izabela

2009

Overview

Friction at the nanoscale For large objects sliding over one another, the friction force is proportional to the true contact area between the two bodies — which is smaller than the apparent contact area because the surfaces are rough, consisting of a large number of smaller features (asperities) that actually make the contact. The situation for nanomaterials, however, has been unclear, since the continuum contact theory that can account for macroscale effects has been predicted to break down at the nanoscale. Using large-scale molecular dynamics simulations of scanning force microscopy experiments, Yifei Mo et al . show that, despite this, simple friction laws do apply at the nanoscale: the friction force depends linearly on the number of atoms, rather than the number of asperities, that are chemically interacting across the sliding interfaces. For large objects sliding over one another, the friction force is proportional to the true contact area between the two bodies — this is smaller than the apparent contact area as the surfaces are rough, consisting of a large number of smaller features (asperities) that actually make contact. Here a related idea holds for contacts at the nanoscale: the friction force depends linearly on the number of atoms (rather than asperities) chemically interacting across the sliding interfaces. Macroscopic laws of friction do not generally apply to nanoscale contacts. Although continuum mechanics models have been predicted to break down at the nanoscale 1 , they continue to be applied for lack of a better theory. An understanding of how friction force depends on applied load and contact area at these scales is essential for the design of miniaturized devices with optimal mechanical performance 2 , 3 . Here we use large-scale molecular dynamics simulations with realistic force fields to establish friction laws in dry nanoscale contacts. We show that friction force depends linearly on the number of atoms that chemically interact across the contact. By defining the contact area as being proportional to this number of interacting atoms, we show that the macroscopically observed linear relationship between friction force and contact area can be extended to the nanoscale. Our model predicts that as the adhesion between the contacting surfaces is reduced, a transition takes place from nonlinear to linear dependence of friction force on load. This transition is consistent with the results of several nanoscale friction experiments 4 , 5 , 6 , 7 . We demonstrate that the breakdown of continuum mechanics can be understood as a result of the rough (multi-asperity) nature of the contact, and show that roughness theories 8 , 9 , 10 of friction can be applied at the nanoscale.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: Adhesion / Chemicals / Condensed matter: structure, mechanical and thermal properties / Continuum mechanics / Exact sciences and technology / Friction / Friction force microscopes / Humanities and Social Sciences / Instruments, apparatus, components and techniques common to several branches of physics and astronomy / letter / Mechanical and acoustical properties; adhesion / multidisciplinary / Physics / Scanning probe microscopes, components and techniques / Science / Science (multidisciplinary) / Shear strength / Solid surfaces and solid-solid interfaces / Surface chemistry / Surface roughness / Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties)