Martens hardness of Constantan thin films on (100) Si wafer: Improvement in contact area function in nanoindentation

Abstract

This work presents a mechanical study by nanoindentation of Constantan thin films deposited by cathodic sputtering technique with an intermediate thin adhesion layer of titanium on a (100) silicon wafer substrate. A methodology based on a modified contact area function is proposed for a suitable processing of the nanoindentation data in order to extract the Martens hardness both of the substrate, titanium layer and Constantan films. The raw data of the substrate Martens hardness have been studied using the most useful models among them those of Nix & Gao, Li & Bradt and Bull & Page showing a significant indentation size effect. However, when considering the tip defect length in the Martens hardness computation, the corrected values are found constant and no indentation size effect occurs. Within this objective, an accurate determination of the tip defect length is required. Its value has been determined both with a correlation between the contact area function of Oliver & Pharr and the improved model of Chicot et al. and also with the self-calibration method proposed by Chicot et al. The tip defect length is afterwards implemented in the model of Jönsson & Hogmark modified by Rahmoun et al. for the hardness determination of the titanium layer and the Constantan thin films. As a main result, the Martens hardness of the titanium layer and of the substrate are found equal to 8 GPa thus allowing to neglect the influence of the titanium layer in the film hardness determination. As a main result, the hardness of the 4 Constantan films is constant whereas the substrate hardness changes with the film thickness. This unexpected behavior is related to the brittleness of the substrate where cracks are observed at the interface and by the film compacting which is trapped between the rigid indenter and the hard substrate.