Investigation of the failure mechanism of thermal barrier coatings prepared by electron beam physical vapor deposition

Two-layer structure thermal barrier coatings (TBCs) (NiCoCrAlY [bond coat]+[6–8 wt.%] Y2O3-stabilized ZrO2 [YSZ top coat]) were deposited by electron beam physical vapor deposition (EB-PVD) on a Ni-base superalloy. Pre-treatments were carried out in a vacuum to improve the oxidation resistance of the bond coat, and the thermal cyclic life of the TBC system was investigated through thermal cyclic tests. It was found that the pre-treatments in the vacuum at 1273 K increased the thermal cyclic life of the TBCs by improving the oxidation resistance of the bond coat. The results of thermal cyclic tests indicated that the TBCs prepared by EB-PVD were degraded mainly at the interface between the bond coat and ceramic coat, due to the spallation of the YSZ top coat from the bond coat. According to the results of microstructure observation and composition analysis, the mechanism of failure was proposed as follows: micro-cracks are first initiated in the YSZ top coat along columnar boundaries, and then propagate through the whole top coat. The cracks formed at the thermal growth oxide (TGO) layer, due to the growing micro-cracks, are considered to cause abnormal thermally-grown oxides of the bond coat beneath the cracks, and consequently, the build-up stresses due to the volume increase, which are significant in weakening the interface combination of the top coat and bond coat. During the following cooling process, compressive stresses are introduced, which tend to separate the YSZ top coat from the bond coat, and finally cause the occurrence of the failure of TBC system.