Adhesion of thermal barrier coatings - the development of metastable alumina

The adhesion of thermal hairier coatings (TBCs) is dependent upon the chara cteristics of the thermally grown oxide (TGO) that forms between the TBC an d the corrosion resistant bond coat. Work has been carried out to investiga te the propel-ties of the TGO as a function of ageing treatments using piez ospectroscopy. Residual stress maps were generated for an electron beam phy sical vapour deposited (EB-PVD) TBC which showed a large variation in resid ual stress over the surface of a coated sample. The two peaks generally ass ociated with alpha alumina (R1 and R2) frequently appear as doublers with a high and low stress component. In addition, the presence of a metastable t heta alumina was detected in aged samples. It is believed that these observ ations can be related to incipient spallation of the TBC. The development o f residual stress and the metastable oxide have been studied and correlated with the spallation behaviour of the TBC.