Predicting failure within TBC system: Finite element simulation of stress within TBC system as affected by sintering of APS TBC, geometry of substrate and creep of TGO

Abstract

Demand for economically efficient and environmental friendly gas turbine engines leads to the usage of a thermal barrier coating (TBC) system, which is usually sprayed on the top of a superalloy substrate. The system includes a ceramic TBC, a bond coat (BC) and a thermally grown oxide (TGO) layer. Thermo-mechanical mismatch stresses created within the coating at the end of a thermal cycle lead to spallation of the ceramic coating and a rapid increase in the temperature of the substrate. The thickness of the oxide layer and the amount of aluminium depleted during high temperature operation also affect the lifetime of the TBC. As a first step to the prediction of the failure mechanisms and the lifetimes of TBCs, a preliminary study of how the stress distribution within the TBC system is affected by different factors is required. This paper investigates the effects of the sintering of the ceramic layer, of the geometry of the substrate and of the creep of the TGO, on the stresses built up in the TBC system. Three different TBC system geometries were modelled using plane strain FE models with three different sets of TGO creep properties. An Arrhenius equation was fitted to the temperature dependent modulus of the sintered TBC using results published in the open literature. The equation was later implemented within the FE model. It is concluded that the TBC on the top of flatter regions of substrate produces smaller tensile residual stresses compared to sharp corners of the substrate. It was also found that the initiation and propagation of cracks within a TBC, during steady state operation depends on the choice of the creep parameters of the TGO. At the cooling stage, increase in the modulus of the TBC, due to sintering, has been shown to produce stresses within the TBC near the TGO interface that are as large as twice the value that is predicted using a model without sintering.