Nanoindentation is routinely used to determine local mechanical properties of materials such as hardness and Young’s modulus. Especially for the testing of thin films, the versatile nanoindentation method is used also on materials approaching a stiffness and hardness regime close to diamond, typically the indenter tip's material. Yet, up to now, the stress-strain response in the indenter tip remained unknown during testing of materials of extremely high hardness.
Contrary, in recent years, in situ cross-sectional X-ray nanodiffraction (CSnanoXRD) coupled with an indenter system has given new insights into the elasto-plastic deformation of thin films during indentation with a resolution down of 200 nm. In order to test the mechanical response of the indenter tip using CSnanoXRD, a ~80 µm wide diamond wedge indenter tip with an opening angle of 60 deg and a tip radius of 2 µm, was coated using hot-wall chemical vapour deposition with a nanocrystalline diamond thin film of 4 µm thickness. The nanocrystalline diamond thin film was then removed at the edges of the wedge using focused ion beam (FIB) milling to ensure uniform signal during the CSnanoXRD experiment. In following, the in situ indentation setup developed for the ID13 beamline at the ESRF was used for the first time to determine experimentally the multi-axial stress distributions across both the indenter and the tested material with a resolution of 80×80 nm2.
In order to test the mechanical response of the indenter-sample system, wedge samples were prepared from biomimetic CuZr-ZrN multilayer and nanocrystalline diamond thin films with thicknesses of ~57 and 75 µm by means of consecutive mechanical polishing, femtosecond laser ablation and FIB milling steps.
These two samples of highly different elasto-plastic behaviour were loaded to the same indentation loads, which depending on the stiffness yielded highly different indentation depths and imprint geometries, where a maximum stress of ~-13 GPa was evaluated in the direction of the contact. Therefore, unique multiaxial stress-strain transfer across the indenter tip-sample interface was evaluated for each sample system depending on the Young’s modulus, hardness and the ability for plastic deformation of the indented material. This new kind of indentation experiment allows for tor the first time to directly assess the multi-axial stress distributions across the contact area for both tip and tested volume. The thereby gathered results give unprecedented insights into the deformation of both indenter and tested (thin film) material.