  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.
