The effect of heat treatment on the stiffness of zirconia top coats in plasma-sprayed TBCs

Abstract

Thermal barrier coating (TBC) specimens have been prepared by plasma spraying. A vacuum plasma spray (VPS) MCrAlY bond coat and atmospheric plasma spray (APS) zirconia top coat were deposited onto a nickel superalloy substrate. The stiffness of detached top coats was measured by cantilever bending and also by nanoindentation procedures. Measurements were made on specimens in the as-sprayed state and after various heat treatments. Significant changes were detected in the Young's modulus of the top coat as a result of the heat treatments. The rate of sintering was found to be a function not only of the temperature but also of whether or not the coating was attached to the substrate during the heat treatment. This influences the stress state in the coating. A previously-developed numerical model has been modified in order to incorporate the effects of top-coat stiffening on the development of stress within the TBC system during exposure to high temperature. It is expected that sintering of the top coat will lead to increases in the driving force for debonding at the interface between the top coat and the bond coat. This effect may be at least partly responsible for the spallation of top coats which commonly afflicts TBCs after periods under service conditions.