Mechanisms of near-surface structural evolution in nanocrystalline materials during sliding contact

TL;DR

This study uses molecular dynamics simulations to investigate how nanocrystalline copper's surface structure evolves during sliding contact, revealing grain growth, saturation effects, and impacts on hardness and friction.

Contribution

It introduces a methodology for atomistic simulation of cyclic wear damage and elucidates the mechanisms of near-surface structural evolution in nanocrystalline materials.

Findings

Grain growth occurs near contact surface during sliding.

Structural evolution saturates after multiple cycles.

Hardness increases and friction decreases with sliding.

Abstract

The wear-driven structural evolution of nanocrystalline Cu was simulated with molecular dynamics under constant normal loads, followed by a quantitative analysis. While the microstructure far away from the sliding contact remains unchanged, grain growth accompanied by partial dislocations and twin formation was observed near the contact surface, with more rapid coarsening promoted by higher applied normal loads. The structural evolution continues with increasing number of sliding cycles and eventually saturates to a stable distinct layer of coarsened grains, separated from the finer matrix by a steep gradient in grain size. The coarsening process is balanced by the rate of material removal when the normal load is high enough. The observed structural evolution leads to an increase in hardness and decrease in friction coefficient, which also saturate after a number of sliding cycles. This work provides important mechanistic understanding of nanocrystalline wear, while also introducing a methodology for atomistic simulations of cyclic wear damage under constant applied normal loads.