Adhesion of thermal barrier coatings–the development of metastable alumina

The adhesion of thermal barrier coatings (TBCs) is dependent upon the characteristics of the thermally grown oxide (TGO) that forms between the TBC and the corrosion resistant bond coat. Work has been carried out to investigate the properties of the TGO as a function of ageing treatments using piezo-spectroscopy. Residual stress maps were generated for an electron beam physical vapour deposited (EB-PVD) TBC which showed a large variation in residual stress over the surface of a coated sample. The two peaks generally associated with ? alumina (R1 and R2) frequently appear as doublets with a high and low stress component. In addition, the presence of a metastable ? alumina was detected in aged samples. It is believed that these observations can be related to incipient spallation of the TBC. The development of residual stress and the metastable oxide have been studied and correlated with the spallation behaviour of the TBC.