A Critical Comparison Among High-Resolution Methods for Spatially Resolved Nano-Scale Residual Stress Analysis in Nanostructured Coatings

Residual stresses in multilayer coatings are indeed a complex phenomenon due to intricate interaction between multiple factors such as layer number and thickness, layer and substrate materials, coefficient of thermal expansion mismatch (CTE), deposition technique, process parameters and deposition environment. The measurement of residual stresses is crucial and for that reason a series of AlGaN/GaN multilayer coatings with different layer structure was produced by metalorganic vapor phase epitaxy (MOVPE) on sapphire substrate. These produced coatings were characterized by high-resolution x-ray diffraction to verify their epitaxial growth and structural properties. The coating structure and layer thicknesses were obtained by performing the FIB cross-section and TEM lamella preparation. The residual stresses within the AlGaN/GaN multilayer coatings were evaluated by two different techniques with different spatial resolution from sub-nano scale to micron level: high-resolution scanning transmission electron microscopy-geometrical phase analysis (HRSTEM-GPA) and focused ion beam-digital image correlation (FIB-DIC). The residual stresses resulting from these techniques (non-destructive and semi-destructive) were critically compared based on variable spatial resolution ranging from sub-micron to sub-nano level, in order to check the reliability and compatibility of both techniques in terms of accuracy and efficiency. This study observes the residual stresses calculated by different spatial resolution techniques are reliable and comparable at required coating depths and locations.