Using focused ion beam (FIB) milling and digital image correlation (DIC), residual stress analysis has become possible for a wide variety of coatings and bulk materials regardless of whether they are crystalline or amorphous. The residual stress is relaxed by FIB milling of certain geometries and the resulting displacement field is tracked by scanning electron microscopy and DIC. Finally, the residual stress state can be calculated based on the determined deformation either by analytical solutions or finite element analysis (FEA).
However, the exact correlation between the local displacement gradients determined by DIC and the internal stress relief of the material for different milling geometries isn’t fully clear yet. In this work, as a part of the European collaborative research project iSTRESS, diamond coatings and Raman spectroscopy as well as bending experiments on a single crystalline Ni-based superalloy and electron backscatter diffraction (EBSD) have been used as reference methods for residual stress analysis by FIB-DIC.
Using a new in-situ µ-Raman spectrometer installed in a FIB, the Raman peak shift during FIB milling of diamond coating was analyzed and the results were compared to FEA. It was found that the combination of FIB-DIC and in situ µ-Raman allows deeper and direct insights in the relaxation of residual stress of thin films and coatings. Moreover, the defined stress state generated by bending experiments on an anisotropic Ni-based superalloy in combination with EBSD allows for quantitative cross check of the FIB-DIC method and estimation of its resolution limit. The results are important for a further improvement of the FIB-DIC method used for determination of residual stress on thin films and anisotropic bulk materials.