Investigation of TBCs on turbine blades by photoluminescence piezospectroscopy

Thermal barrier coating (TBC) blade specimens with Pt diffusion bond coats were subjected to thermal cycling with periodic measurements of the residual stress in the thermally grown oxide (TGO) using photoluminescence piezospectroscopy. Two distinct stress levels were generally found to coexist in the probed volume, i.e. a high stress (∼4 GPa) and a low stress (∼500 MPa) level. Both the high and low stress levels were independent of the curvature of the blade surface, in agreement with numerical modelling based on a composite cylinder stress model. The relative contributions of the two stress levels appear to be correlated with the θ-Al2O3 content of the TGO, which was dependent on the position on the blade. The TBCs tended to fail along the TGO/bond coat interface in thermal cycling. This was modified by the presence of mixed transition metal oxides in the TGO. The results are interpreted in terms of a likely failure mechanism.