Theoretical modelling of fracture of functionally graded thermal barrier coatings at elevated temperatures, including oxidation effects

• Applicant: Professor Dr.-Ing. Stefan Weihe, since 6/2023
• Subject Area: Coating and Surface Technology ; Mechanics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 505664011

Functionally graded thermal barrier coatings (FGCs) are used in different engineering structures, e.g. in gas turbine engine components, where the thermal barrier coatings protect metal parts from overheating and melting. In order to increase the capacity of engines, the operating temperature should be increased, and this relies on further improvements of FGCs with respect to enhancement of thermal and fracture resistance. Therefore, a study of thermal fracture of FGCs on a homogeneous substrate (FGC/H) under the influence of high heat fluxes is of crucial importance.At high temperatures and complex mechanical loads, diffusion and oxidative processes occur between the ceramic coating and the underlying alloys, resulting in the formation of a thermally growing oxide (TGO) layer. Consequently, the problem will be considered for a complex system consisting of an FGC, the underlying metal (homogeneous substrate, H) and the TGO between them, that is, FGC/TGO/H structures.The main goal of the proposal is to develop a semi-analytical model for the thermal fracture analysis of FGC/TGO/H structures that takes into account oxidation effects at the metal/ceramic interface at high heat fluxes and mechanical loads. The main focus will be put on incorporating oxygen diffusion effects in the model. The presence of pre-existing systems of cracks in FGC/TGO will be considered. Among them are edge cracks, cracks in the TGO layer, as well as internal cracks in the FGC. A number of new problems will be considered that arise at elevated temperatures and high heat fluxes, such as, diffusion problems along with thermal conductivity problems for FGC/TGO/H structures. The problem will be formulated by means of singular integral equations. The solution will be obtained numerically, using special quadrature formulae for the integrals. Besides, new approximate analytical solutions will be obtained for some special cases, e.g. for the interaction of thermally permeable cracks with taking into account oxygen diffusion into the crack tips.The thermal, diffusion and elastic properties of FGCs will be modeled from the point of view of their practical application. To simplify the problem, some functional forms will be applied, such as exponential functions or any proper rule of mixture. This semi-analytical approach allows to correlate the structural material parameters (material gradation, crack parameters) and the thermo-mechanical loading parameters with the main fracture characteristics, such as the thermal stress intensity factors, the critical heat fluxes as well as the crack deflection angles. The model can help for a better understanding of the fracture processes at elevated temperatures including oxidation effects, as well as for determining to improve the fracture resistance of FGC/TGO/H structures.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Professor Dr. Siegfried Schmauder, until 6/2023