Fast mobility induced self-lubrication at metallic glass surface

The enhanced surface mobility in metallic glasses (MGs) has been a constant source of fascination due to its unique mechanical properties. We show experimentally that the mobile surface layer of MGs functions as a lubricating layer in friction experiments, which is evidenced by a reduction of a friction coefficient of 50% or less and suppression of dissipative stick-slip behavior with decreasing scratch depth down to nanoscale in the various MGs. The lubrication mechanism could be attributed to easier shearing of the mobile surface layer induced by homogeneous plastic flow. Importantly, the thickness of the self-lubricating layer is inversely proportional to glass transition temperature with a higher homologous temperature yielding a larger thickness. These results extend the fundamental understanding of the ubiquitous MG surface and present a path for the rational design of self-lubricating materials.