Multiscale approach to predict the lifetime of EB-PVD thermal barrier coatings

Abstract : Thermal barrier coatings (TBCs) are used to protect hot components from combustion gases in gas turbines. One of the most widely used TBC systems is that applied by an electron beam-physical vapour deposition (EB-PVD) onto a Ni-base intermediate or bond coat. The resulting top zirconia based thermal insulator exhibits a characteristic columnar morphology. During service, the combination of severe thermal loads and high temperatures leads to the selective oxidation of the intermediate metallic coating, to TBC degradation and, eventually, to the development of microcracks. This may, in turn, be followed by spalling of the top coating, which constitutes the life limiting event for the component. Different approaches have been proposed to predict these phenomena, generally based on macroscopic TBC stresses as the driving force for TBC failure or on fracture mechanics approaches to predict interfacial or cohesive failure. However, no previous work integrates local interface damage and macroscopic stresses or stored strain energy in the prediction of TBC spallation. The objective of this thesis is to develop a multi-scale life predictive approach for TBC life which accounts for the evolution of local interface damage, and its effect on the fracture resistance relevant to the dominant failure mode, such as oxide interface spallation. Even though the study focuses on an EB-PVD TBC system, the proposed approach is generic and can be adapted to other types of TBCs. The lifetime assessment and the modelling of TBCs require an understanding of individual material properties and interface morphologies, and their in-service evolution. In this thesis, the evolution of each TBCs constituent microstructure has been investigated using scanning electron micrograph, energy-dispersive spectroscopy techniques and imageprocessing analyses. Based on the understanding gained from the experimental study, a multi-scale and multi-physics approach is proposed which incorporates (i) the kinetics of oxide growth, (ii) the growth strains associated with bond coat oxidation, (iii) realistic (2D and 3D) oxide morphologies, and (iv) the morphological evolution of the oxide and top coat. The approach has been implemented into the finite element method and used to predict the local stress and strain fields driving the evolution of observed interfacial local damage (i.e. porosities, microcracks) through a local continuous damage variable. Through this numerical approach, it is also possible to take into account the time-evolution of the TBCs morphology (sintering of columnar top coat layer, oxide thickness and roughness), and microcracking under both constant and cyclic temperature histories. The proposed approach relies on the value of the interface fracture resistance, linked to the current level of interface damage, and on the global stored elastic strain energy to account for the evolution towards a critical state. The latter is assumed to be attained when the stored energy reaches the realevant fracture resistance. The time evolutions of the stored energies and the fracture resistance are inferred from simulations results and TBC life data. The approach can be easily adapted to predict TBC lifetime for long in-service conditions.
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  • HAL Id : pastel-00869360, version 1

Citation

Julien Frachon. Multiscale approach to predict the lifetime of EB-PVD thermal barrier coatings. Materials. École Nationale Supérieure des Mines de Paris, 2009. English. ⟨NNT : 2009ENMP0019⟩. ⟨pastel-00869360⟩

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