# On the connection between sliding friction and phonon lifetimes: Thermostat induced thermolubricity effects in molecular dynamics simulations

## TL;DR

This study investigates how thermostat parameters influence phonon behavior and friction in molecular dynamics simulations of nanotribology, revealing the roles of phonon lifetime, substrate temperature, and sliding speed in thermolubricity effects.

## Contribution

The paper clarifies the relationship between thermostat tuning, phonon lifetimes, and friction, providing insights into thermolubricity in molecular dynamics simulations with harmonic and anharmonic interactions.

## Key findings

- Friction maximum occurs at optimal damping parameter GAMMA.
- Substrate temperature TSUB shows a pronounced minimum at high sliding speeds.
- Thermolubricity effects differ between harmonic and anharmonic substrate interactions.

## Abstract

A typical nanotribology simulation setup is the semi-infinite substrate, featuring a sliding bead on top, and with the lower substrate layers thermostatted to control temperature. A challenge is dealing with phonons that backreflect from the substrate lower boundary, as these will artificially reduce the friction FR acting on the sliding bead. One proposed solution is to use a Langevin thermostat, operating at temperature TLAN, and with the corresponding damping parameter, GAMMA, optimally tuned such that FR is maximized [Benassi etal, PHYSREVB, V82, P081401, 2010]. In this paper, the method is revisited, and related to the substrate phonon lifetime, the substrate temperature TSUB, and the sliding speed. At low sliding speed, where the time between stick-slip events is large compared to the phonon lifetime, we do not observe much dependence of FR on GAMMA, and here thermostat tuning is not required. At high sliding speed, upon varying GAMMA, we confirm the aforementioned friction maximum, but also observe a pronounced minimum in TSUB, which here deviates from TLAN. For substrate particle interactions that are strongly anharmonic, the variation of FR with GAMMA can be understood as a manifestation of thermolubricity, backreflections being essentially unimportant. In contrast, for harmonic interactions, where phonon lifetimes become very long, FR is strongly affected by backreflecting phonons, though not enough to overturn thermolubricity.