The growth characteristics of thermally grown oxides (TGO) and their influence on microcracking in an air-plasma sprayed (APS) thermal barrier coating (TBC) were studied. The TBC samples were prepared in either as-received condition or with a pre-heat treatment. In the as-received TBC, TGO that formed upon thermal exposure predominantly consisted of layered and clustered chromia, spinels and nickel oxide, whereas in pre-heat treated samples the TGO was predominantly alumina. The growth characteristics of TGO was found to exhibit a three-stage behavior that was most pronounced in the as-received TBC. Micro-cracks were found to nucleate in clustered oxides, these cracks would grow in association with thickening of the TGO layer. Eventually, oxide-induced cracking and cracking along pre-existing discontinuities near the ceramic/bond coat interface led to spallation of the topcoat. A relationship between the maximum crack size and TGO thickness was established based on fracture mechanics considerations. This relationship is shown to be useful for TBC life prediction.

This content is only available as a PDF.
You do not currently have access to this content.