Andre Schirmeisen, Center for Nanotechnology (CeNTech), Universität Münster
One of the most fundamental questions in nanotribology is the contact area dependence of friction forces on the nanometer scale. Unfortunately, conventional friction force microscopy techniques are limited for analyzing this problem due to the unknown and ill defined tip-sample-contact. However, this limitation can be circumvented by measuring the lateral force signal during the manipulation of nanoscale particles with a well-defined and clean contact to the substrate. In our study, the samples under investigation were metallic islands with diameters between 50-500 nm grown by thermal evaporation of antimony on highly oriented pyrolytic graphite (HOPG). By pushing a large variety of islands of different sizes we found that the experiments of controlled nanoparticle translation in ultrahigh vacuum show two distinct frictional states during particle sliding . While some particles show finite friction increasing linearly with interface area, thus reinforcing Amonton's law at the nanoscale, other particles assume a state of frictionless or 'superlubric' sliding. This unexpected duality of friction states seems to be related to contamination effects of the interface altering the frictional properties.
 D. Dietzel et al., Phys. Rev. Lett. 101, 125505 (2008)