Non-Destructive Evaluation of Residual Stress in Thermal Barrier Coatings: Raman and Photoluminescence Spectroscopy Techniques

Thermal barrier coatings (TBCs) are critical in enhancing the efficiency and longevity of gas turbine engines by offering thermal insulation to components operating under extreme conditions. This study investigates non-destructive evaluation (NDE) techniques—primarily Raman and photoluminescence (PL) piezo spectroscopies for residual stress measurement and condition monitoring of air plasma sprayed (APS) TBCs. Yttria-stabilized zirconia (YSZ), a standard TBC material, was doped with rare earth cations (Er³⁺, Eu³⁺, Sm³⁺) as chromophores for PL without affecting the phase composition and relevant properties. The shift in PL wavelength was calibrated using in-situ micro-compression testing to convert spectral shifts to stress values. Simultaneous Raman-PL mapping was performed to measure stress in both the YSZ topcoat and the thermally grown oxide (TGO) layer under isothermal and cyclic oxidation conditions. Stress evolution with ageing and spatial mapping revealed localized stress ‘hot spots’ as well as phase transitions. Comparative studies with XRD validated the accuracy of the Raman and PL methods. The results show that Raman-PL techniques provide high repeatability and spatial resolution for stress tracking, aiding the development of lifing models and reliable monitoring frameworks for TBC systems in high-temperature applications.