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Numerical modelling of failure development within TBC systems

Final Activity Report Summary - TBC FAILURE (Numerical modelling of failure development within TBC systems)

During the last decade, there has been an enormous effort to introduce single crystal (SC) and thermal barrier coating (TBC) technologies for the manufacture of high temperature components for advanced power generation gas turbines. The main objective of the project was a realistic FEM modelling of all TBC system constituents including the creep response of the Ni-based super alloy CMSX-4. The current research took into account a broad spectrum of materials features, including time dependent behaviour of CMSX-4, the creep of APS TBC, bond coat (BC) and of the thermally grown oxide (TGO). The oxidation process was taken into account by a volumetric swelling of the TGO layer. In order to simulate crack development, cohesive zone elements were placed at the interface between the TGO and BC.

Having achieved that, a numerical tool for a mapping of a broader damage scenarios related to realistic loading profiles in gas turbines has been created. The scientific / technological result of the project is an enhanced, physically based life prediction concept for TBCs, which enables to predict the durability or service life of TBCs under realistic loading programs, so that the material potential of ceramic thermal barrier coatings can be used to a higher extent. Furthermore, knowledge of the quantitative effect of material parameters on the durability of TBCs will enable an improvement of material properties of its constituents.

In order to get an insight into the delamination and spallation of TBC, a mathematical model of a thin film delaminating from a substrate was created. A two dimensional plane stress problem of an elastic film bonded to the rigid substrate and loaded by a concentrated in-plane force was considered.

An analytical solution has been obtained for a specific value of Poisson's ratio assuming the rigid-friction interaction between the film and the substrate. This serves as a basis for a better understanding of the delamination process. It was possible to determine the boundary between the damaged and bonded zones at the interface, shear stress distribution at the interface and the stresses within the thin film.