A new technique to determine the elastoplastic properties of thin metallic films using sharp indenters

In recent years, nanoindentation has established itself as one of the most convenient techniques to assess the mechanical properties of thin films by measuring the force-penetration (F-h) curve during loading and unloading. However, the mechanical understanding of the indentation process itself, which involves several non-linearities (inelastic material behaviour, large and non-homogeneous deformations), is very intricate and finite-element analysis is thus combined with experiments in order to develop methods allowing the determination of the flow stress. In this spirit, Dao et al. (Acta Materialia, 49, 2001, 3899) and Bucaille et al. (Acta Materialia, 51, 2003, 1663) conducted finite-element analysis using different sharp indenters and proposed functions relating F-h curves to elastoplastic parameters of metals. In the present work, these methods are applied on galvanically grown nickel films. Samples for both nanoindentation and microtensile tests were grown in the same batch and, therefore, allow their properties to be studied using two different techniques on identical materials. Nanoindentation tests were conducted with the Berkovich and cube corner pyramids at constant strain rate. Young's modulus and stresses corresponding to representative strains, imposed by these indenters, of 3.3 and 12.6% were determined. These values are in good agreement with the stress-strain values measured in tension. We also showed the importance of taking into account the indentation size effect, the friction between the indenter and the material and the strain rate dependence of metal deformation behaviour. The application to thin metallic films is discussed. (C) 2003 Elsevier B.V. All rights reserved.