TGO Evolution and Coating Property Changes for EB-PVD TBC Coatings Under Cyclic Oxidation Condition

Abstract

The life of a thermal barrier coating (TBC) system is governed by the microstructural evolution of the thermally grown oxide (TGO) layer between the ceramic top layer and the bond coat. While the TGO provides a barrier to the oxygen diffusion, its continuous growth imposes stresses on the TGO/bond coat and TGO/topcoat interfaces that will eventually lead to crack linkage and propagation followed by failure of the TBC system. In addition to TGO growth during thermal exposure, coating properties such as hardness, Young’s modulus and fracture toughness will also change. This study is undertaken to investigate the oxidation behaviour of an electron beam physical vapour deposition (EB-PVD) YSZ/PtAl coating system. Cyclic oxidation tests were carried out with each cycle consisting of 5 hr holding time at 1150°C followed by air cooling to room temperature. TGO evolution, coating property and maximum crack length as functions of total thermal exposure time were investigated in this study. Microstructural analyses of the coating were carried out using scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). Both hardness and Young’s modulus of the bond coat were measured using micro-hardness tester. The results from this study showed a parabolic TGO growth rate as a function of thermal exposure time. While no distinct mathematical relationship was found between the TGO thickness and maximum crack length, a general trend of increasing TGO thickness and maximum crack length was found with respect to exposure time. The mechanical properties of the bond coat were also found to be influenced by the thermal exposure.