title: Plasticity, healing and shakedown in sharp-asperity nanoindentation.
abstract: 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.
authors.affiliation: SFI Trinity Nanoscience Laboratory, Department of Physics, Trinity College, Dublin 2, Ireland. graham.cross@tcd.ie
authors.forename: Graham L W
authors.initials: GL
authors.lastname: Cross
authors.forename: André
authors.initials: A
authors.lastname: Schirmeisen
authors.forename: Peter
authors.initials: P
authors.lastname: Grütter
authors.forename: Urs T
authors.initials: UT
authors.lastname: Dürig