Optical Spectroscopy & Synchrotron X-ray Diffraction for Stress Measurements in Aerospace Materials

Optical Spectroscopy such as photo-stimulated luminescence spectroscopy (PSLS) for stress measurement in chromium-doped alumina has been motivated by the need for a non-destructive technique to establish the integrity of the thermally grown oxide (TGO) layer and life of thermal barrier coatings (TBC) on turbine blades of jet engines. Applications of PSLS have been based on the well-known R-line peak positions and their stress dependence has been fully exploited for planar measurements. New findings on the piezospectroscopic nature of vibronic bands in the optical spectrum, pave the way for the measurement of the complete stress state using the PSLS technique. The development of a novel spectral analysis methodology based on genetic algorithms for optimization of the curve fitting was instrumental in detecting and monitoring the shifts of the peaks within the vibronic bands with stress. Another key feature of this work is the combination of synchrotron techniques with PSLS measurements. By comparing strains as determined from x-ray methods with PSLS-derived peak-shifts ex-situ, the piezospectroscopic coefficients were uniquely calibrated and determined. Developments made through this research are currently being used to initiate Raman and PSLS studies on carbon nanotubes and Alumina-Epoxy (ALOX) composites. Ongoing research aims to build on our methodologies and expand the versatility of PSLS and Raman techniques to formulate the stress-spectral model for CNT and to study interface stresses in Alumina nanoparticles in an epoxy matrix.