Linking microstructural evolution and macro-scale friction behavior in metals

TL;DR

This paper establishes a link between microstructural evolution and macro-scale friction behavior in metals, showing how grain size changes influence friction regimes through atomistic mechanisms, validated by experiments and simulations.

Contribution

It introduces a simplified, predictive model connecting grain size evolution with friction regimes, advancing understanding of deformation mechanisms in metals.

Findings

Low friction linked to ultra-nanocrystalline surface films

High friction associated with dislocation-dominated plasticity

Model validated for copper and gold

Abstract

A correlation is established between the macro-scale friction regimes of metals and a transition between two dominant atomistic mechanisms of deformation. Metals tend to exhibit bi-stable friction behavior -- low and converging or high and diverging. These general trends in behavior are shown to be largely explained using a simplified model based on grain size evolution, as a function of contact stress and temperature, and are demonstrated for pure copper and gold. Specifically, the low friction regime is linked to the formation of ultra-nanocrystalline surface films (10 to 20 nm), driving toward shear accommodation by grain boundary sliding. Above a critical combination of stress and temperature -- demonstrated to be a material property -- shear accommodation transitions to dislocation dominated plasticity and high friction. We utilize a combination of experimental and computational methods to develop and validate the proposed structure-property relationship. This quantitative framework provides a shift from phenomenological to mechanistic and predictive fundamental understanding of friction for crystalline materials, including engineering alloys.