Indentation across size scales and disciplines: Recent developments in experimentation and modeling

Indentation is a remarkably flexible mechanical test due to its relative experimental simplicity. Coupled with advances in instrument development, ease of implementation has made indentation a ubiquitous research tool for a number of different systems across size scales (nano to macro) and scientific/engineering disciplines. However, the exploration of different materials systems and the potential usage of indentation as a precise and quantitative method beyond the research laboratory have prompted intense modeling and interpretation efforts for robust analysis of experimental results. In this review, we describe progress in a number of different aspects of this method, including continuum-based modeling of homogeneous and heterogeneous systems, microstructural size effects and atomic modeling of nanoindentation experiments, in situ transmission electron microscopy observations of nanoscale contact, and novel and emerging uses for indentation. A recurring theme is the consideration of what is meant by 'hardness' in different physical scenarios