Modelling of TBC system failure: Stress distribution as a function of TGO thickness and thermal expansion mismatch

Advances in gas turbine technology place an increasing demand on thermal protection systems of nickel-base superalloys in turbine blades. Current strategies for performance improvements are focused on thermal barrier coatings (TBC). Typical current TBC system are composed of: top coat (TC), an yttria stabilised zirconia outer layer that provides thermal insulation; a bond coat (BC) layer, aluminium rich, supplying oxidation resistance and adhesion of TC to the metal; a thermally grown oxide (TGO) scale, predominantly alumina, that is a reaction product formed between TC and BC as a consequence of BC oxidation at high temperatures. At present, the capabilities of TBCs cannot be fully exploited due to the lack of a reliable lifetime prediction model of the coating. Hence, continuous efforts are made by materials scientists in this direction and this is the purpose of our work. To achieve this objective, a preliminary activity is necessary to determine stress distribution in the system as a function of each factor affecting TBC behaviour. First, we have developed a model of BC oxidation, based on Wagners theory, which predicts a parabolic law for the growth of TGO scale. Then, using finite element method, we performed an analysis of stress distribution in the system because of TGO thickening and thermal expansion mismatch. This is a prerequisite to understand failure mechanism that are different depending on processing mode of TBC, either plasma spray (PS) or electron beam physical vapour deposition (EB-PVD).