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

Thermal barrier coating (TBC) specimens have been prepared by plasma sprayi ng. A vacuum plasma spray (VPS) MCrAlY bond coat and atmospheric plasma spr ay (APS) zirconia top coat were deposited onto a nickel superalloy substrat e. The stiffness of detached top coats was measured by cantilever bending a nd also by nanoindentation procedures. Measurements were made on specimens in the as-sprayed state and after various heat treatments. Significant chan ges 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 i n the coating. A previously-developed numerical model has been modified in order to incorporate the effects of top-coal 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 drivi ng force for debonding at the interface between the top coat and the bond c oat. This effect may be at least partly responsible for the spallation of t op coats which commonly afflicts TBCs after periods under service condition s. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All lig hts reserved.