Oxidation and Spallation Mechanisms of Thermal Barrier Coating at 1100 OC

Abstract

This article concerns the study of oxidation kinetics of a thermal barrier coating (TBC). The study employed thermal gravimetric analysis and scanning electron microscopy as the essential techniques for observation of the oxidation mechanism and resultant structural damage of the coating. For the study of the oxidation and crack growth behaviour, the coating microstructures after oxidation test at 1100 ^OC were investigated. Attention was paid particularly at the crack initiation sites, which play important role in the study of spallation—type failure mechanisms of TBC. The TBC system in this study consists of a Ni—superalloy substrate, a NiCrAIY bondcoat adjacent to the substrate and a ZrO—8% Y₂O₃ ceramic topcoat The coatings were applied onto the substrate using an air plasma spraying technique [1]. The study shows that failure in TBC occurs as a result of crack propagation at the bondcoat/topcoat interface or within the topcoat

itself. This is because the plasma sprayed coating and the thermally grown oxide (TGO) contain a large number of defects in the forms of microcracks and pores. The defects in TGO and ceramic coating can act as initiation sites for crack. If these cracks propagate, they may result in spallation of the coating materials and, hence, failure of the coating.

Keywords : TBC / Spallation / TGO