Atomic-scale simulations on the sliding of incommensurate surfaces: The breakdown of superlubricity

TL;DR

This study uses molecular dynamics simulations to explore how superlubricity between incommensurate surfaces depends on tip stiffness and normal force, revealing breakdown conditions and complex metastable state behaviors.

Contribution

It extends the Frenkel-Kontorova-Tomlinson model to realistic 3-D AFM simulations, analyzing superlubricity breakdown mechanisms under various conditions.

Findings

Superlubricity vanishes with softer tips and higher normal force.

FKT scaling applies to 3-D models except at very low stiffness and high load.

Meta-stable states can reduce friction and cause non-monotonic force dependence.

Abstract

Molecular dynamics simulations of frictional sliding in an Atomic Force Microscope (AFM) show a clear dependence of superlubricity between incommensurate surfaces on tip compliance and applied normal force. While the kinetic friction vanishes for rigid tips and low normal force, superlubric behavior breaks down for softer tips and high normal force. The simulations provide evidence that the Frenkel-Kontorova-Tomlinson (FKT) scaling applies equally to a more realistic 3-D incommensurate AFM model except in the limit of very low stiffness and high normal load limit. Unlike the FKT model in which the breakdown of superlubricity coincides to the emergence of the meta-stable states, in the 3-D model some meta-stable states appear to reduce frictional force leading to non-monotonic dependence of force on normal load and tip compliance. Meta-stable states vary with the slider positions, and the relative stabilities of these meta-stable states result in varying transition mechanisms depending on sliding velocity.