Microplasticity characteristics obtained through nano-indentation measurements: application to surface hardened steels

A method for obtaining the local plastic properties of materials from nano-indentation measurements is described. The nano-indentation technique is used to measure the residual displacement d(r). of a spherical indenter, following loading-unloading cycles at increasing maximal force F-max. First, the direct problem, i.e. simulating by the finite element method the nano-indentation response, d(r) (F-max), from the knowledge of the strain-hardening law, sigma (epsilon (p)), is studied. It is validated on steel specimens, which are carburised homogeneously with different carbon concentrations. Second, the inverse problem, i.e. obtaining the strain-hardening law from the nano-indentation response, is studied on carburised and nitrided layers. To solve this problem, it is shown that both d(r) (F-max) and sigma (epsilon (p)) responses can be fitted with simple analytical laws with a single characteristic parameter depending on the carbon or nitrogen concentration. Hence, the plastic behaviour for small strains is obtained through the identification of this characteristic nano-indentation parameter. Finally, the validity of the method is discussed in terms of the volume of material investigated, the strain range and the influence of residual stresses. (C) 2001 Elsevier Science B.V. All rights reserved.