Ab-initio friction forces on the nanoscale: A DFT study of fcc Cu(111)

TL;DR

This study introduces a parameter-free, quantum mechanical model to predict nanofriction forces on fcc Cu(111) surfaces, revealing exponential load dependence and a non-zero offset at low loads, applicable to arbitrary sliding paths.

Contribution

It presents a novel, all-electron DFT-based quasi-static model for nanofriction that accounts for atomic relaxations and arbitrary sliding directions without path length restrictions.

Findings

Friction force exhibits exponential dependence on load for all directions.

Identified two periodic paths bounding the nanofriction response.

Friction converges for long paths to an intermediate value.

Abstract

While there are a number of models that tackle the problem of calculating friction forces on the atomic level, providing a completely parameter-free approach remains a challenge. Here we present a quasi-static model to obtain an approximation to the nanofrictional response of dry, wearless systems based on quantum mechanical all-electron calculations. We propose a mechanism to allow dissipative sliding, which relies on atomic relaxations. We define two different ways of calculating the mean nanofriction force, both leading to an exponential friction-versus-load behavior for all sliding directions. Since our approach does not impose any limits on lengths and directions of the sliding paths, we investigate arbitrary sliding directions for an fcc Cu(111) interface and detect two periodic paths which form the upper and lower bound of nanofriction. For long aperiodic paths the friction force convergences to a value in between these limits. For low loads we retrieve the Derjaguin generalization of Amontons-Coulomb kinetic friction law which appears to be valid all the way down to the nanoscale. We observe a non-vanishing Derjaguin-offset even for atomically flat surfaces in dry contact.