Characterisation of residual stress and interface degradation in TBCs by photo-luminescence piezo-spectroscopy

Residual stress in the TGO in two different TBC systems, one with a Pt aluminide (β structure) bond coat and another with a Pt diffusion (γγ′ structure) bond coat, were studied using photo-luminescence piezo-spectroscopy (PLPS). The luminescence spectra and TGO morphology were investigated progressively with thermal cycling at 1135 °C. The two TBC systems were found to have distinctly different TGO residual stresses and different failure modes. Several stress relaxation mechanisms were found to be operative in the Pt aluminide system, while no stress relaxation was evident in the Pt diffusion system until close to the end of life (spallation). Luminescence spectral shape evolution has been quantitatively analysed and correlated with TBC system degradation processes. Both TBC systems showed clear spectral shape changes as a result of the formation of interface cracks when they reached approximately 75% lifetime. Characteristic spectral shape changes in response to different types of interface crack were demonstrated experimentally. The correlation between spectral shape evolution and interface degradation opens a new avenue for studies of degradation and lifetime assessment of TBCs.