Sliding friction between the tip of a friction force microscope and NaCl(100) was studied to deduce the velocity dependence of friction forces on the atomic scale. A logarithmic dependence of the mean friction force is revealed at low velocities. The experimental data are interpreted in terms of a modified Tomlinson model which is based on reaction rate theory.
In this review, we present various results obtained by friction force
microscopy in the last decade. Starting with material-specific contrast,
commonly observed in friction force maps, we discuss how the load dependence
of friction and the area of contact have been estimated and compared to
elasticity theories. The features observed in a sliding process on the atomic
scale can be interpreted within the Tomlinson model. An extension of the
model, including thermal effects, predicts a smooth velocity dependence of
friction, which recent experiments have confirmed. Other subjects like
anisotropy of friction, role of environment, topographical effects, electronic
friction and tip modifications are also discussed. The growing importance of
molecular dynamics simulations in the study of tribological processes on the
atomic scale is outlined.
Friction force microscopy experiments in ultrahigh vacuum allow the observation of atomic-scale stick-slip processes on Cu͑111͒ surfaces. The lateral stiffness of the contact and the electrical characteristic of the junction are discussed. It is suggested that the tip is covered by copper forming a contact of a few atoms with the Cu͑111͒ surface. The mean friction exhibits a clear dependence on the scan velocity.
We discuss recent results on atomic friction at clean atomically flat surfaces obtained by atomic-force microscopy in terms of a simple two-dimensional model. Instabilities of the tip position give rise to irreversible jumps which are responsible for the occurrence of friction. The model calculations are in good agreement with the experimental observations. We propose a method for the reconstruction of the lateral atomic force field from measured data.
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