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 us ed to measure the residual displacement d(r). of a spherical indenter, foll owing loading-unloading cycles at increasing maximal force F-max. First, th e direct problem, i.e. simulating by the finite element method the nano-ind entation 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. Se cond, the inverse problem, i.e. obtaining the strain-hardening law from the nano-indentation response, is studied on carburised and nitrided layers. T o 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 cha racteristic parameter depending on the carbon or nitrogen concentration. He nce, the plastic behaviour for small strains is obtained through the identi fication of this characteristic nano-indentation parameter. Finally, the va lidity of the method is discussed in terms of the volume of material invest igated, the strain range and the influence of residual stresses. (C) 2001 E lsevier Science B.V. All rights reserved.