A phonon-based approach to describe and harness nanoscale friction

Abstract

In this habilitation thesis, a novel description of nanoscale friction is presented. In order to develop an experimental model of the nanoscale friction, it is common practice to obtain the relevant parameters from a trial and error approach at varying tribological conditions; similarly, conventional atomistic simulations usually describe the friction response in terms of sequences of model geometries aimed to represent the sliding events in working regime. However, the attempt to distinguish the frictional forces from the remaining ones responsible for the integrity of the material is not general and is system-dependent: the nanoscale friction is the response of the system as a whole and the proper description of it requires a holistic approach. To this aim, the phonon theory and the quantum mechanics are exploited, which provide a universal system-independent framework. The friction force is then recast in terms of phonon eigendisplacements, eigenfrequencies and phonon-scattering tensor elements calculated at the quantum-mechanical level. This framework allows to tune the friction and energy dissipation response of any system by guiding the selection of suitable atomic types and inert intercalant species, and enables the on-demand friction control by means of external electric fields. Moreover, the presented framework avoids the use of challenging dynamics simulations, while providing guidelines on how to design tribological materials with targeted tribological response. Finally, the work opens the way towards a universal friction theory which will allow the calculation of the friction coefficient of two surfaces in contact based on the sole knowledge of the atom types and their geometric arrangement.