S/TEM and TEM investigations of thermal barrier coatings based on Gd₂Zr₂O₇ zirconate

Detailed investigations of thermally grown oxides (TGO) zone and areas adjoining to it were presented in this article. The thermal barrier coating (TBC) based on gadolinium zirconate (Gd₂Ze₂O₇, GZ) and deposited by air plasma spraying (APS) on AMS 5599 superalloy with NiCrAlY type of bond-coat was the analysed material. Analyses were made on TBC system after spraying process and after time to 500 hours of oxidation at temperature 1100°C in atmosphere of laboratory air. Ultrahigh resolution scanning-transmission (S/TEM) electron microscope TITAN 80-300 was used to conduct the tests. Quanta 200i Dual Beam apparatus by FEI for precise and localized focused Ga+ ion beam (FIB) sample preparation was used as well. The basic aim of presented investigations was related with detailed characterization of interface zone between ceramic top-coat (TC) of Gd₂Zr₂O₇ type and NiCrAlY bond-coat after spraying process and after 500 hours of deposition in oxidized atmosphere where the TGO zone was formed with the thickness ca. 5-10 μm. The special emphasis was placed on research of this zone and its morphology due to very strong influence of formed thermally grown oxides on overall life-time and destruction processes which took place on the interface between TGO and ceramic top-coat. Obtained data revealed that the most important factors influenced on destruction processes are related with the thickness of TGO zone (consisted mainly Al₂O₃, NiAl₂O₄ and evetually NiO) rather than formation of undesirable phases such as GdAlO₃ complex oxide.