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.

Crystallographic Observation and Delamination Damage Analyses for Thermal Barrier Coatings Under Thermal Exposure

“…As TBCs are used in very severe conditions, thermally induced damage and material degradation are inevitably induced during service and delamination of coating layer may occur finally as the result of coalescing multiple lateral cracks and some vertical cracks after the evolution of oxide layer between top coat and bond coat schematically shown in Fig.1. However, the mechanism of damage and degradation is still not clear enough because there are so many factors affecting the delamination life [2]. Therefore, one objective of this paper is to focus on how the EBSD observation of thermal exposure samples of TBC top coatings can be applied to identify the particle morphologies after the plasma spraying and another is to evaluate the damage process until the delamination of top coatings.…”

EBSD Observation of Micro Crack Morphologies after Thermal Exposure in Thermal Barrier Coatings

“…As TBCs are used in very severe conditions, thermally induced damage and material degradation are inevitably induced during service and delamination of coating layer could occur finally as the result of damage accumulation. However, there are so many factors affecting damage evolution that it is still not clear how the damage and degradation mechanisms determine the delamination life (2) . Furthermore, the precise microstructural feature of TBC top coat has not been clearly observed by the conventional optical microscope or scanning electron microscope (SEM) because those means could not give the comprehensive crystallographic information of ceramic coatings.…”