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 coupled with an indenter system has given new insights into the elasto-plastic deformation of thin films during indentation with a resolution down of 500 nm. In this work, 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 below ~100 nm.
For this purpose, a 75 µm wide diamond wedge indenter tip with an opening angle of 60 deg and a tip radius of 2 µm, was coated using chemical vapour deposition with a nanocrystalline diamond thin film of 4 µm thickness. In order to test the mechanical response of the indenter-sample system, wedge samples with a thickness of ~70 µm were prepared by means of consecutive mechanical polishing, femtosecond laser ablation and focused ion beam milling steps from nanocomposite AlCrSiN, a biomimetic CuZr-ZrN multilayer and a nanocrystalline diamond thin films.
The samples of highly different elasto-plastic behaviour are loaded to the same indentation depths, which depending on the stiffness yields highly different loads. 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 in 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.