Development of a Thermal Barrier Coating Life Model

Thermal barrier coatings (TBCs) are widely used on the first stage turbine buckets and vanes of land-based (F and G class) gas turbine machines. These coatings normally fail by spallation due to delamination of the ceramic layer along the vicinity of the thermally grown oxide (TGO)/TBC interface. The failure processes involve several mechanisms including oxidation of the bond coat, thermomechanical fatigue, sintering, and spallation of the TBC. This paper describes the development of an analytical tool for predicting the useful life of TBCs for land-based gas turbine applications. The analytical model, called TBCLIFE, has been developed to treat bond coat oxidation, sintering and spallation of the TBC, as well as effects of coating thickness and substrate curvature on TBC spallation. In addition, a parallel experimental program has also been initiated to evaluate the durability of a plasma-sprayed TBC under isothermal and thermal cycling exposures. These results will be used to determine the kinetics of TGO scale growth and the material constants for the TBC life model. The TBC life model will be applied to predicting TBC life as a function of cycle time and the results will be presented as coating life diagrams. The utility of a coating life diagram for estimating the remaining life of TBC will be illustrated and discussed.