3-D Raman spectroscopy measurements of the symmetry of residual stress fields in plastically deformed sapphire crystals

The development of methods to characterize materials in three dimensions, such as tomography by X-rays, focused ion beam and electrons, has led to progress in the understanding of materials properties. Recently, even stress and deformation tensors could be measured in three dimensions. Specifically the stress fields around indents in metals were studied by three-dimensional (3-D) X-ray stress microscopy. In this paper, we investigate the 3-D residual stress field around a microindent using confocal Raman microscopy with a lateral resolution of 300 nm and a depth resolution of 600 nm. The model system investigated was single crystalline sapphire, which was indented normal to its basal c(0 0 0 1) plane. A cross-section of the indent was studied by transmission electron microscopy to visualize the deformed microstructure. The major result is that the geometry of the indenter has no direct influence on the symmetry of the resulting residual stress field. Residual stresses directly depend on the crystal symmetry and the defect structures formed during indentation. Confocal Raman microscopy is a powerful method for analyzing 3-D stress fields and the corresponding defect structures (by peak width analysis) with a resolution in the submicron range.