Theory of the noncontact friction forces between sliding nanoasperity and a surface

A theory is firstly developed of the noncontact friction forces between the sliding nanoasperity and substrate. The linear force-velocity dependence is substantiated. We differ two principally distinct types of the friction forces: the static force, independent of the tip velocity V, being of adhesive nature, and the dynamical one, proportional to V. Several mechanisms have shown to contribute the latter one, such as electromagnetic fluctuations, electronic and phononic excitations. The corresponding formulae are given. For weak macroscopic contact the sliding friction force proves to be proportional to the bulk shear strength (modulus) and surface energy of the contacting metals. Calculations are made of the friction forces for several nanotribosystems, including metals, diamond and silicon. It is shown that the order of magnitude of the dynamical friction is in agreement with experimental data measured with quartz-crystal microbalance technique. (C) 1999 Elsevier Science S.A. All rights reserved.