One of the most fundamental yet unresolved questions in tribology concerns the area dependence of frictional forces at the nanoscale. Theory predicts two contrasting scenarios for finite, atomically flat sliding contacts, depending on the exact condition of the interface. The first scenario depicts contacts featuring adsorbed, but mobile molecules that are trapped between the sliding surfaces. These ‘dirt’ molecules prevent a direct interaction of the surface potentials of the sliding interfaces by acting as spacers. Since their mobility allows them to simultaneously lock at surface potential minima for both sliders, an area independent friction coefficient is obtained for any surface geometry, in direct analogy to Amontons’ law valid for dry sliding friction of macroscopic contacts. In contrast, if the interface is atomically clean, a state of virtually frictionless sliding is anticipated, often referred to as ‘superlubricity’ or ‘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.
Direct verification of these scenarios has been difficult in the past since established experimental procedures are severely limited by a size gap between the small contact areas of scanning probe microscopes and the contact areas offered by the surface force apparatus of some ten thousands of µm2. To overcome this gap, we performed experiments where the frictional resistance of thermally evaporated antimony nanoparticles featuring contact areas between 25,000nm2 to 310,000nm2 is measured in ultrahigh vacuum while they are pushed by the tip of an atomic force microscope (AFM) operated in contact mode. Our experiments show that the two scenarios can in fact coexist. We find 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. These results validate both allegedly conflicting theories while simultaneously highlighting the sensitivity of frictional properties on the atomic-scale details of the interface structure.
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