Diffusion cells and chemical failure of MCrAlY bond coats in thermal-barrier coating systems

It is proposed that bond coats in thermal-barrier coating (TBC) systems, pa rticularly those deposited by plasma spraying, can contain regions which ar e diffusionally isolated from the bulk of the coating. This can arise throu gh the internal formation of alumina layers as a consequence of the ingress of molecular oxygen into the relatively porous structure. Such isolated re gions, termed diffusion cells, will experience enhanced depletion of alumin um as a result of the continued thickening of the alumina layer at their su rface. This process has been demonstrated for a CoNiCrAlY bond coat after o xidation in air at 1100 degreesC. A consequence of this enhanced depletion is that chemical failure will occur sooner in diffusion cells and voluminou s breakaway oxides,will form above them at the interface of the bond coat a nd the ceramic top coat. The associated spatial variation in oxidation and displacement rates across the surface of the bond coat are expected to aid delamination of the outer ceramic layer.