Friction on the atomic scale: An ultrahigh vacuum atomic force microscopy study on ionic crystals

We performed atomic force microscopy in ultrahigh vacuum on the ionic crystal of KBr(001), The morphology and the tribological properties of this cleavage face are characterized and discussed, The local friction coefficient was extracted by means of the two-dimensional histogram technique. For loads below 3 nN a linear behavior was found between normal and lateral forces yielding a friction coefficient of less than 0.04, Ln this load regime, wearless friction is observed. For higher lends, the friction coefficient increases to a value of about 0.7-1.2, The corresponding topography images reveal the typical onset of wear, On the atomic scale a periodicity of 4.7 Angstrom was found which corresponds to the distance of equally charged ions on the KBr(001) surface. On this scale, the lateral force map exhibits the typical stick-slip phenomenon which is discussed in terms of a novel theoretical approach. (C) 1996 American Vacuum Society.