Stiffness and atomic-scale friction in superlubricant MoS$_2$ bilayers

TL;DR

This study uses simulations to show how layer stiffness influences atomic-scale friction in superlubricant MoS₂ bilayers, revealing that increased stiffness significantly reduces friction and that sliding velocity affects heat transfer and friction regimes.

Contribution

It demonstrates the impact of layer stiffness on superlubricant friction in MoS₂ bilayers, isolating effects by engineering identical energy surfaces with different rigidity.

Findings

Increasing intra-layer stiffness reduces friction by about six times.

Two sliding regimes are identified based on velocity and heat exchange.

Temperature gradients develop at high velocities, affecting friction behavior.

Abstract

By using ab-initio-accurate force fields and molecular dynamics simulations we demonstrate that the layer stiffness has profound effects on the superlubricant state of two-dimensional van der Waals heterostructures. These are engineered to have identical inter-layer sliding energy surfaces, but layers of different rigidity, so that the effects of the stiffness on the microscopic friction in the superlubricant state can be isolated. A twofold increase in the intra-layer stiffness reduces the friction by approximately a factor six. Most importantly, we find two sliding regimes as a function of the sliding velocity. At low velocity the heat generated by the motion is efficiently exchanged between the layers and the friction is independent on whether the sliding layer is softer or harder than the substrate. In contrast, at high velocity the friction heat flux cannot be exchanged fast enough, and the build up of significant temperature gradients between the layers is observed. In this situation the temperature profile depends on whether the slider is softer than the substrate.