Plasticity, healing and shakedown in sharp-asperity nanoindentation

Spatially localized stress fields produced by instrumented, sharp indentation probes are a route to testing the mechanical properties of materials at the smallest length scales. Here we provide direct experimental measurement of indentation plasticity with contact strain fields involving up to a few thousand atoms. We observe two types of nanoscale plasticity: on the pristine surface, high-resolution sensing shows an overall smooth, remarkably reversible indentation response interjected by sudden discrete drops in indenter load. The jumps often occur in pairs with pop-in motion during loading healed by a corresponding pop-out motion on the unload stroke to define a compact hysteresis loop. Despite the general reversibility, cyclic indentation at a single sample position leads to a subtle plastic ratchet and shakedown behaviour with displacements correlated to the underlying gold lattice constant. Our results concur with a previously established picture of thermally activated atomistic plasticity, but suggest a new mechanism at reduced scales that suppresses permanent mass transport.