Failure behaviors of EB-PVD thermal barrier coatings under isothermal oxidation and thermal cycling conditions

Abstract

The thermal growth oxide (TGO) layer formed by oxidation of the bond coat (BC) layer is an important factor affecting the durability of thermal barrier coatings (TBCs). In this study, a series of isothermal oxidation and thermal cycling experiments were performed on samples containing TBCs at different temperatures. The initiation, propagation and connection process of cracks in the cross-section of the samples were studied, and the oxidation failure mechanism of the samples was analyzed. The results demonstrate that the diffusion of the Al element in NiCoCrAlYHf coating to the top coat (TC) layer promotes the formation and rapid thickening of TGO at TC/BC interface. With the consumption of Al, two sublayers of mixed oxide layer and Al2O3 layer are gradually formed in the TGO layer. The TC/TGO interface is the main initiation location of interface crack. In addition, the types and initiation locations of cracks caused by isothermal oxidation are different from those caused by thermal cycling. Under isothermal oxidation, the TC layer near the TC/TGO interface and the TC/TGO interface are prone to induce horizontal cracks, which finally lead to the delamination and spalling of TBCs. Under the thermal cycling, vertical cracks occur in the TC layer, extend towards the TC/TGO interface, and merge with the interface cracks, eventually resulting in the delamination failure of the TBCs.