Failure analysis of plasma-sprayed thermal barrier coatings

Abstract

In this work thermally induced failure processes of plasma-sprayed thermal barrier coatings were examined. Cracking processes give rise to noise which was monitored by acoustic emission techniques. The sequential failure of coatings was examined from samples which were thermally cycled. Coatings of yttria-stabilized zirconia with and without an Ni-Cr-Al-Zr bond coat were plasma sprayed onto a U 700 alloy rod. In some cases the substrate was intentionally overheated prior to deposition of the thermal protection system. In this way a qualitative appraisal of how process variables affect coating integrity could be made in terms of cracking behavior. Results from up to seven consecutive thermal cycles are reported here. Coating failure was observed in all cases. Failure of the thermal protection system is progressive, since cracking and crack growth have been observed prior to ultimate failure, and thus catastrophic failure occurs at some stage when there is a transformation from the microcrack to a macrocrack network.