Effect of exposure conditions on the oxidation of MCrAlY-bondcoats and lifetime of thermal barrier coatings

The influence of testing procedure on the lifetime of electron beam physical vapor deposited (EB-PVD) TBC's with a conventional and a Zr modified NiCoCrAlY bondcoat was investigated. It is shown that the lifetime during discontinuous testing (166 h at 1000 °C hot dwell time) the system with Zr modified bondcoat outperformed the conventional system by a factor of four, while during cyclic temperature exposure (dwell time 2 h) only a minor lifetime improvement was found. The results could be rationalized in terms of differences in the microstructure and associated failure mechanisms of the thermally grown oxide (TGO) formed on the two bondcoats. In the system with the conventional bondcoat a flat, dense TGO formed and delaminations classically propagated at the TGO/bondcoat interface. Therefore the TBC-lifetime could be related to a critical TGO thickness and appeared to be insensitive to the change of the cyclic parameters. In contrast with Zr-modified bondcoat heterogeneous, rapidly inward growing TGOs with zirconia precipitates formed, whereby the failure path was shifted to the TBC/TGO interface. For the latter TBC system the lifetime is mainly determined by the number of temperature cyclic events. By reducing the Zr reservoir in the bondcoat, however, the cyclic lifetime can be significantly increased. This lifetime extension compared to the Zr-free NiCoCrAlY bondcoat requires an exact control of the Zr content, the BC thickness and the bondcoat processing parameters.