Failure characteristics during cyclic oxidation of yttria stabilized zirconia thermal barrier coatings deposited via electron beam physical vapor deposition on platinum aluminide and on NiCoCrAlY bond coats with processing modifications for improved performances

Abstract

In this study, the cyclic oxidation lives of the current state-of-the-art thermal barrier coating (TBC) systems (heavy grit-blasted Pt aluminide and NiCoCrAlY bond coats with EBPVD TBCs) were investigated first, followed by TBC systems that were modified based on the results obtained on the failure of the state-of-the-art TBC systems. The specimens were subjected to cyclic oxidation testing, mostly at 1100 °C, in a bottom-loading furnace in laboratory air. Optical and scanning electron microscopy techniques were used to characterize the as-processed and failed specimens. The state-of-the-art TBC systems with NiCoCrAlY bond coats failed as the result of defects that were identified as TBC defects, transient oxides, surface defects, and reactive element-rich oxide protrusions. On the other hand, the failures of the state-of-the-art TBC systems with Pt aluminide bond coats were due to deformation of the bond coat by a mechanism known as ratcheting. The stored strain energy in the thermally grown oxide (TGO) was also a factor that contributed to the failure of both systems. Most of the modifications performed on the state-of-the-art TBC systems improved their lives to some extent. In the case of NiCoCrAlY systems, elimination or at least minimization of the identified defects was responsible for the improvements, whereas the prevention of the ratcheting type of failure was the main reason for the improvement in lives in the case of Pt aluminide systems. On the other hand, other issues, such as slower growth of the TGO as well as improved TGO/bond coat interfacial toughnesses with some of the modifications, were observed to be contributing factors in the improved lives. Based on the observations on the failure of both the state-of-the-art as well as the modified TBC systems, the surface condition of the bond coats and the morphology of the TBCs close to the TGO were found to have a first-order effect on the failure of TBC systems. The characteristics of the TGO, such as composition, growth rate, and adherence both to the bond coat and the TBC, as well as the characteristics of the bond coats were also observed to have an effect on the failures. Recommendations for future work that should be pursued to better define the conditions necessary for optimized TBC performances are given.