Life Assessment of Thermal Barrier Coatings for Gas Turbine Applications

Abstract

A typical TBC system consists of a ceramic top coat material, such as 7–8% yttria stabilized zirconia, applied over a metallic bond coat material, such as diffusion aluminide or MCrAIY. The bond coat serves as an oxidation protection layer for the substrate, as well as providing an adherent surface for the ceramic top coat layer. Estimation of coating durability is critical for design of highly reliable TBC coated parts. Laboratory testing and historical engine experience have revealed that TBC can degrade by various mechanisms; such as fatigue, oxidation, erosion, and impact. Mechanism-based life prediction techniques are needed to assess the durability of TBC in a simulated gas turbine service cycle consisting of: start-up to full speed, steady state operation at full load, and shut-down to zero speed. New testing protocols have been developed in order to simulate the TBC loading under these conditions. The lifing methodology has been calibrated using GE Power Systems gas turbine experience. A nondestructive technique was also used for evaluation of pre- and post-tested TBC specimens.