Sliding friction under a negative load

Microfriction experiments in a scanning electron microscope have shown that continuous sliding under a tensile force (negative load) is possible between a soft metal stylus and a hard smooth surface of graphite or diamond. The stylus was first deformed plastically with a positive load to produce a relatively large apparent contact area; the load could then be reduced and made negative under either static or sliding conditions and the static adhesion or negative friction measured. The friction force was found to decrease with increasing negative load, and the interpolated load for zero friction was found to correspond closely to the pull-off load for the static adhesion experiments. The apparent contact area was typically 5 μm2 and the maximum adhesive stresses were about 5 × 107 N m−2 (5 kg mm−2) on diamond and 3 × 106 N m−2 on the cleaved basal plane of a graphite single crystal. Possible mechanisms of attraction are considered in relation to the magnitude of the attractive stres. It is concluded that van der Waals forces are sufficient to explain the observed adhesion, but stronger short-range forces may also be playing a significant role in the case of diamond. The effect of interfacial geometry is discussed and possible reasons are considered for the absence of previous observations of attractive stresses of this magnitude in larger-scale experiments.