Numerical analysis of the influence of coating porosity and substrate elastic properties on the residual stresses in high temperature graded coatings

The functionality and reliability of coated devices are strongly related to residual stresses of coatings. The major problem in thermal barrier coatings (TBCs) applied to gas turbine components is the failure by spallation of ceramic coating under thermal cycling processes. In order to prevent spallation and to improve the thermo-mechanical behaviour of the TBC the interfacial stresses in the coating system should be reduced. To overcome this problem it is desirable to introduce a graded layer between the metallic bond coat and the zirconia top coating. Therefore, a detailed study of the optimisation of the gradient profile is necessary in respect to thermal stress relief. In this paper a numerical model of thermal stress distribution within a multilayered system which consists of a functionally gradient material (FGM) is presented. The structure of the graded coating system is made of a ceramic layer and a metallic layer, where between them there is an interlayer which is a graded composite made of the metal (NiCr-alloy) and the ceramic (ZrO2Y2O3). The effects on residual stress distribution of elastic properties of the alloy substrate, the graded interlayer thickness and ceramic layer porosity were analysed for the case of a fully graded TBC using a linear compositional profile for the FGM. This model will provide some insights regarding the development of a methodology for designing fail-safe graded coating systems used in high temperature applications.