Evaluation of interface degradation during cyclic oxidation of EB-PVD thermal barrier coatings and correlation with TGO luminescence

The typical degradation mechanism of EB-PVD thermal barrier coatings (TBCs) that occurs during cyclic furnace testing of a commercial 7YSZ coating on a single-crystal nickel-base superalloy is analyzed. The mechanism includes local separation of the ceramic top coat from the thermally grown oxide (TGO), growth of the separated regions along the coating–substrate interface, and, upon reaching a critical size, large-scale TBC spallation. The extent of interface separation was evaluated by analyzing cross-section microstructures of the tested samples at different stages of TBC cyclic life. The morphological changes of the TGO as a result of roughening of the (Ni,Pt)Al bond coat surface were also quantified. Concurrently, the evolution of the TGO luminescence during cyclic oxidation and the development of TBC undulations, associated with large TBC–TGO interface separations, were monitored. It is demonstrated that there is a correlation between the extent of interface damage and the shift in TGO luminescence frequency, which can be evaluated nondestructively.