Nanoparticle detachment:: a possible link between macro- and nanotribology?

Although particle detachment is a common phenomenon associated with most tribological processes, it seldom occurs that each piece of elemental debris can be considered as the result of a single event. Such an association has been revealed by the systematic study of a specific system, where a pin of graphite is made to rub against thoroughly polished steel. While the discontinuous nature of the transfer film allows a quantitative assessment of the volume of transfer h(e) to be made by 3D optical-profilometry, the linear dependence of the rate of particle detachment dh(e)/dn (n = number of rubbing cycles) with the logarithm of sliding speed upsilon strongly suggests the existence of a particular type of stick-slip, where each stick may lead to the detachment of a debris particle. The variations in size of these debris with environment as revealed by AFM, further suggest that the global rate of particle detachment is of the form: dh(e)/dn = N x epsilon (i), where N is the number of stick-slip events per rubbing cycle, x the proportion of stick events leading to a cohesive rupture, and epsilon (i) the mean volume of an elemental particle. While this relation is apparently supported by most experimental results, its actual validation can only be made by experiments at the level of single (nanoscale) asperities, carried out under well-controlled experimental conditions.