Atomic friction

We show that wearless atomic friction, recently observed by AFM techniques, is well desribed by the mechanism of "plucking of atoms" proposed by Tomlinson in 1929. Our results for a single-asperity contact yield a satisfactory fit of the experimental data. For extended contact areas, the relative orientation of the crystal lattices and the resulting misfit becomes important. The misfit gives rise to the formation of domains where the two surface structures are approximately in registry. The domains are separated by two sets of shift lines, crossing each other in topological defects. During quasistatic sliding, the whole domain pattern moves perpendicular to the driving force. Frictional behaviour occurs if the ratio of the coupling strength of the surface atoms to the bulk of the body to the potential barrier for sliding is less than a critical value. Dissipation and friction hysteresis are caused by irreversible jumps of these topological defects. The friction force depends strongly on both the sliding direction and the misfit angle.