In this dissertation, finite element models are used to investigate catastrophic failure of thermal barrier coatings (TBCs) due to delaminations along susceptible interfaces of thermally grown oxide (TGO) with the ceramic top coat and the inter-metallic bond coat. The materials and geometries in the studies are chosen to be representative of TBC materials in real applications.
The characteristics of the failure modes along the TGO and bond coat interface (e.g. buckling instability and strain energy driven delamination propagation) are investigated using thermo-elastic finite element models. The solution of a linear elastic eigen-value problem determines the onset of the buckling instability with a pre-existing delamination between bond coat and the TGO. The virtual crack extension method is employed to study strain energy release rate driven interfacial delamination at wavy interfaces. The materials and geometries in the study are chosen to be representative of TBC materials in real applications. Extensive sensitivity analyses are conducted to identify the critical design parameters affecting the onset of buckling and extension of interfacial delamination, as well as to develop parametric relations that enhance the understanding of these mechanisms. Finally, a numerical exercise demonstrates that the buckling instability is the leading failure mechanism at flat interfaces or at the locations of minimum cross-section in a wavy interface. However, in the vicinity of waviness, crack extension becomes a dominant mode of failure.
The top coat crack initiation and propagation is investigated using a thermo-elastic finite element model with bond coat creep. Cracks are assumed to initiate when the maximum principal stress exceeds rupture stress of the top coat. A sensitivity analysis estimates the contribution of geometric and material parameters and forms a basis to develop parametric relation to estimate maximum principal stress. Subsequently, crack propagation simulations using a hysteretic cohesive zone model are performed for parametric combinations which initiate cracks away from the interface. These analyses conclude that parametric combinations initiating top coat cracks also assist in propagation and eventual delamination of TGO and top coat interface.
A homogenization based continuum damage mechanics (HCDM) modeling framework is proposed for TBC failure effects of top coat microstructural defects. An extended Voronoi cell finite element (X-VCFEM)is employed to perform the micro-mechanical analysis of RVE and the results show that HCDM model has limited validity due to loss of material stability with significant damage. A sensitivity analysis reveals that the range of HCDM validity is dependent on top coat cohesive energy.