Observations and analysis of the influence of phase transformations on the instability of the thermally grown oxide in a thermal barrier system

Abstract

Observations and measurements have been performed on a thermal barrier system comprising a Ptaluminide bond coat and a thermal barrier coating deposited by electron beam evaporation. Past research has highlighted a displacement instability in the thermally grown oxide (TGO), as it affects the failure mechanism in the thermal barrier coating (TBC). Phase transformations in the bond coat have also been identified, with a proposed role in the TGO instability. The present study assesses this influence by characterizing the transformations as well as their spatial correlation with the instability sites. Both the isothermal transformation from β→γ′ and the martensite transformation in the β have been addressed. Toward the end of life, the instabilities are preferentially located in the β phase, between neighboring domains of γ′. After cycling, the composition of the β grains is spatially uniform. Within the γ′, there are Ni and Al composition gradients in narrow layers near the interfaces with the β phase and the TGO. An evaluation suggests that the primary influence of transformation on the cyclic displacement of the TGO is to create a local misfit between the growing γ′ domains and the volume strain accompanying the martensite transformation in the intervening β phase, upon cooling and reheating.