In recent years, considerable efforts have been directed towards the clarification of the atomic origins of friction, largely spurred by the miniaturization of moving components in technological devices. By analyzing the friction between an atomic force microscopy (AFM) cantilever tip and the sample surface, friction force microscopy (FFM) has proven to be a powerful tool for nanotribology. Unfortunately, FFM has some limitations inherent to the experimental configuration. Apart from the limited variety of material combinations, the fixed tip radius makes it difficult to analyze effects as a function of the contact area. However, the contact area dependence is one of the most fundamental yet unsolved current questions in nanotribology.
In order to achieve significant progress by overcoming the limitations of FFM, the friction between two objects in relative motion with a well-defined contact area should be measured. One possible approach is to measure the lateral force between nanoscale islands supported by flat substrates while being pushed by an AFM tip. Thus, experiments have been conducted where the frictional resistance of antimony nanoparticles featuring contact areas from 10000 nm² to 310,000 nm² is measured.
In this configuration, if the interface is atomically clean, a state of virtually frictionless sliding is anticipated, often referred to as ‘structural lubricity’. Here, the lattice mismatch at the interface causes a decrease of the potential barrier between stable states with increasing contact size that ultimately leads to vanishing friction. However, experiments performed under UHV and ambient conditions showed two distinct frictional states: While some particles assume a state of frictionless sliding, other particles show finite friction increasing linearly with contact area, thus resembling Amonton’s law at the nanoscale. This unexpected duality of friction states might be related to contamination of the interface altering the frictional properties.

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