Evolution of Stresses and Microstructure in Thermal Barrier Coatings: A Multiscale Modeling Framework

Abstract

The development of modern gas turbine (GT) engines has been the result of continual improvements in a wide variety of engineering disciplines including turbine design, combustion, and materials. Hot section components of the GT engine are required to operate in harsh environments significantly in excess of their melting temperatures, requiring extensive cooling coupled with heat resistant and protective coatings. Thermal barrier coatings (TBCs) comprise several layers and are designed to simultaneously provide thermal and oxidation protection. Development of high performance TBCs require a better understanding of the complex changes in their structures and properties that occur under operating conditions that lead to their failure. In this study, initially, multiscale analytical models were developed based on a thermodynamic variational principle to model sintering and microstructure evolution in thermal barrier top coat made using the electron beam physical vapour deposition route. It is assumed that the sintering occurs by interfacial diffusion at local contacts between columnar grains and driven by changes in interface free energy and elastic stored energy of the coating. The models link diffusional processes at the scale of contacting feathery columns with the macroscopic deformation and sintering response. Each member of the multi-layered TBC system is dynamic and all interact to control the performance and durability. Therefore, continuum level analytical and computational models were developed to determine the stresses generated in the coating system upon cool down to room temperature from maximum operating temperature accounting for temperature dependent material properties and spatial variation of TBCs properties due to sintering. The effect of sintering on the stresses in the thermally grown oxide layer (TGO) was also examined. Furthermore, the effect of stress dependent creep both in the bond coat and TGO during service at high temperature on the stress evolution was studied. In addition, rumpling and growth of TGO due to oxidation were also analyzed to gain insight into the failure behavior of the TBC system.

Date of Award	May 2015
Original language	American English
Supervisor	Kumar Shanmugam (Supervisor)