Relating Grain Boundary Sliding to Low Friction in Metals*

Friction in metals is a direct result of the nanoscale deformation mechanisms.  Previous work has demonstrated that low friction is associated with the formation/persistence of a thin layer of ultra-nanocrystalline material (10 nm) at the sliding interface.  At these sizes, shear is no longer accommodated by dislocations but rather through grain boundary sliding (GBS), as in inverse Hall-Petch behavior.  We present a treatment of GBS that provides qualitative and quantitative descriptions of low friction in metals.  This framework -- based on materials properties with no adjustable parameters -- accurately predicts the strength of a variety of metals (FCC, BCC, and HCP), dilute alloys and metallic glasses. By directly connecting bond strengths and deformation mechanisms with the macroscale properties of metals, this work implies opportunities for optimization of alloys for low friction and high strength applications.