Superlubricity controlled by the multiatomic nature of nanocontacts

Within the context of dry friction, the ultralow friction regime has been reported at various atomically small contacts. Realization of large superlubric contacts under ordinary environmental conditions would greatly impact daily life and technology. Here, we focus on the multiatomic nature of a finite-size nanoparticle sliding on an atomically clean surface. The particle is subject to an effective field propagated by the surface and intermediated by the contact layer of the particle. The structural parameters, including the size, rigidity, and atomic configuration of the contact layer are taken into account to study the friction of the sliding nanoparticle. Several collective features show up once the particle contact layer is incommensurate with the surface potential, and the intralayer atomic coupling is strong. For a rigid layer, a considerable reduction of the friction is predicted at some particular sizes. In addition to these superlubric sizes that are determined by the lattice mismatch ratio only, enhancing the multiatomic feature by increasing the layer size and/or the intralayer coupling strength results in a friction reduction, which is essentially exponential for different values of the normal load (represented by the interaction amplitude). The latter kind of superlubricity is attributed to the increase of the number and synchronization of the intermediate slips of individual atoms in the contact layer that prevents the formation of high potential barriers. The edge atoms, on the other hand, are shown to be determinant in increasing the friction when they refuse to contribute to the collective slips.