On the origins and mechanisms of the indentation size effect

It is often observed that the hardness of a ceramic or metal changes as the size of the indentation is reduced, the so-called indentation size effect. There are a number of explanations for the observed behaviour including the effects of sample preparation and test method, the increasing perfection of materials as the volume is reduced and the scale of microstructural features compared to the indentation volume. However, there are two theories that offer the best explanation for the observed behaviour in the majority of situations; geometrically necessary dislocations (strain gradient plasticity) and non-uniformity in the elastic/plastic transition with contact scale. In this paper, the main causes of the indentation size effect and the models for describing the variation of indentation load and hardness with contact scale are reviewed. It is shown that the quality of the fit alone is not sufficient for identifying the operating indentation size effect mechanism and that multiple mechanisms are likely to operate in most situations.