On the degradation modes and oxidation behavior of platinum aluminide bond coats in thermal barrier coating used as surface protection system for a turbine blade superalloy

Adhesion of thermally grown oxide (TGO) to the bond coat is known to limit the useful life of thermal barrier coatings used in gas turbine blade applications. This is determined by the structure and composition of the bond coat as well as its thermal stability and in turn, its ability to develop and maintain a protective oxide. In this study, the degradation modes of platinum aluminides of the β-(Ni,Pt)Al- and PtAl₂ + β-(Ni,Pt)Al- types used as bond coats in thermal barrier coatings deposited on Ni-base superalloy and utilizing zirconia-7 wt% yttria and as top coat have been examined. Thermal exposure tests have been carried out at 1150 C with cycling to room temperature every 24 h. Various electron-optical techniques have been used to characterize the microstructures of the bond coats and TGO. Particular emphasis has been placed upon the susceptibility of the bond coat to degradation by interdiffusion, oxidation, rumpling and formation of internal cavities. It is shown that the oxidation behavior and thermal stability characteristics are functions of the exact distribution of Pt in the bond coats. The β-(Ni,Pt)Al-type bond coat is found to have higher thermal stability and oxidize at a slower rate in comparison with the PtAl₂ + β-(Ni,Pt)Al₂-type. However, both bond coats are observed to exhibit a similar behavior in that the Al-rich and Pt-modified β-phase is progressively transformed into the Al-depleted γ′- and γ-phases with continued thermal exposure but at a slower rate in the β-(Ni,Pt)Al bond coat. Under the test conditions used in the study, there has been no evidence for rumpling, however, internal cavities are observed near the surface of each bond coat during the later stages of thermal exposure showing that rumpling is not necessarily a prerequisite. Failure of the respective thermal barrier coating systems is found to occur by loss of adhesion between the TGO and bond coat whose composition has approached that of the superalloy substrate by interdiffusion.