Thermal barrier coatings have been widely used to provide thermal protection to components in the hot section of gas turbines. This research focuses on two influencing factors on coating behavior: thermal gradient and cyclic oxidation. The delamination mechanics under thermal gradient is analyzed, taking thermally grown oxide into consideration. Coatings experience thermal gradients at different stages during actual service flight. One is due to engine power shut down when landing and the other due to internal cooling of the substrate. Thermally grown oxide (TGO) also acts as a critical factor in delamination mechanics. The induced stress gradient and corresponding energy release rate for interface delamination and shallower delamination are presented. Mechanism maps that explain the criteria for preventing delamination from developing and propagating are established. Three cooling trajectories are envisaged to analyze the variation in the possibility of delamination. Multilayer coatings used in components of the hot section of aero turbine engines also experience cyclic temperature variation during flight cycles. As experiment conditions vary and coating performance is improved, the time required to run through the test of coating failure can be both time-consuming and prohibitive. Therefore, protocols providing prediction of quantified coating behavior are in demand to shorten life-time tests. Curves of mass change are obtained from quantifying scale growth and loss by different models such as Cyclic Oxidation Spall Program (COSP). A modification is made by combining COSP and a mechanic based model to obtain critical parameters for lifetime prediction from short time experiment. The time for coatings to reach peak temperature during cycling is discovered to influence prominently on modeling results. Predictions for several coating compositions and cycling conditions are consistent with the data from the existing experiments of the coating system.

Atmospheric plasma sprayed (APS) thermal barrier coatings (TBCs) are commonly used for thermal protection of components in modern gas turbine application such as power generation, marine and aero engines. TBC is a duplex material system consisting of an insulating ceramic topcoat layer and an intermetallic bondcoat layer. TBC microstructures are highly heterogeneous, consisting of defects such as pores and cracks of different sizes which determine the coating's final thermal and mechanical properties, and the service lives of the coatings. Failure in APS TBCs is mainly associated with the thermo-mechanical stresses developing due to the thermally grown oxide (TGO) layer growth at the topcoat-bondcoat interface and thermal expansion mismatch during thermal cycling. The interface roughness has been shown to play a major role in the development of these induced stresses and lifetime of TBCs.The objective of this thesis work was two-fold for one purpose: to design an optimised TBC to be used for next generation gas turbines. The first objective was to investigate the relationships between coating microstructure and thermal-mechanical properties of topcoats, and to utilise these relationships to design an optimised morphology of the topcoat microstructure. The second objective was to investigate the relationships between topcoat-bondcoat interface roughness, TGO growth and lifetime of TBCs, and to utilise these relationships to design an optimal interface. Simulation technique was used to achieve these objectives. Important microstructural parameters influencing the performance of topcoats were identified and coatings with the feasible identified microstructural parameters were designed, modelled and experimentally verified. It was shown that large globular pores with connected cracks inherited within the topcoat microstructure significantly enhanced TBC performance. Real topcoat-bondcoat interface topographies were used to calculate the induced stresses and a diffusion based TGO growth model was developed to assess the lifetime. The modelling results were compared with existing theories published in previous works and experiments. It was shown that the modelling approach developed in this work could be used as a powerful tool to design new coatings and interfaces as well as to achieve high performance optimised morphologies.

The durability of Thermal Barrier Coatings (TBCs) used on the turbine blades of aircraft and power generation engines has been known to be affected by sand particle ingression comprised of Calcium-Magnesium-Alumina-Silicate (CMAS). Previous studies have shown that these effects present themselves through variations in the thermomechanical and thermochemical properties of the coating. This study investigated the impact of CMAS ingression on the Yttria Stabilized Zirconia Topcoat (YSZ) and Thermally Grown Oxide (TGO) strain in sprayed Thermal Barrier Coating (TBC) samples of varying porosity with and without CMAS ingression. In-Situ Synchrotron X-ray Diffraction measurements were taken on the sample under thermal loading conditions from which the YSZ and TGO peaks were identified and biaxial strain calculations were determined at high temperature. Quantitative strain results are presented for the YSZ and TGO during a thermal cycle. In-plane strain results for YSZ near the TGO interface for a complete thermal cycle are presented, for a 6% porous superdense sample with CMAS infiltration. The outcomes from this study can be used to understand the role of CMAS on the strain tolerance of the TBC coating. It is well known that under engine operational conditions the development of the TGO layer, with large critical stresses, has been linked to failure of the coating. The growth of the TGO manifests as undulations in a series of peaks and troughs. Understanding the mechanics of the oxide layer at these locations provides significant information with respect to the failure mechanisms of the TBC coating. This study investigated the stress at the peak and trough of a TGO undulation for a cycled Dense Vertically Cracked (DVC) plasma sprayed TBC sample through photo-luminescence (PL) spectroscopy. High resolution nanoscale stress maps were taken nondestructively in the undulation of the TGO. Preliminary results from first line mapping of TGO peak and trough scan, at a resolution of 200 nm, have shown a non-uniform TGO stress variation. The results obtained from this study can be used to understand the stress variation in the peak and trough of a DVC sample's TGO undulation and how it contributes to the life of the TBC coating.