Growth of interfacial cracks in a TBC/superalloy system due to oxide volume induced internal pressure and thermal loading

Residual stresses develop in thermal barrier coating during cool down from processing temperature due to the thermal expansion mismatch between the different layers e.g., substrate, bond coat, thermally grown oxide, and TBC. These residual stresses can initiate microcracks at the bond coat/TGO/TBC interfaces and can lead to debonding at the bond coat/TGO/TBC interfaces. Crack like flaws at the interface can be responsible for initiating debonding and accelerating the oxidation process. Effect of oxidation growth between bond coat and ceramic layer (TBC) can be modeled as volume increase. In this part of the investigation, we represent this change in volume as an induced pressure across the crack faces. The energy release rate G, for both Mode I and Mode II cases were evaluated using the virtual crack extension method. The specimen was cooled down from processing temperature of 1025–25°C. The variation of the properties as a function of temperature were used for analysis. A four layer model which includes the TGO layer was analyzed using the finite element method, two cases were considered. Case one is a specimen with a crack at middle of the oxide layer (TGO) while case 2 is a specimen with an interface crack between bond coat and TGO layers. Also, for both cases, a sensitivity study for the effect of variation of materials properties (±25%) was undertaken using the finite element method.