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, particularly those deposited by plasma spraying, can contain regions which are diffusionally isolated from the bulk of the coating. This can arise through the internal formation of alumina layers as a consequence of the ingress of molecular oxygen into the relatively porous structure. Such isolated regions, termed diffusion cells, will experience enhanced depletion of aluminum as a result of the continued thickening of the alumina layer at their surface. This process has been demonstrated for a CoNiCrAlY bond coat after oxidation in air at 1100 degreesC. A consequence of this enhanced depletion is that chemical failure will occur sooner in diffusion cells and voluminous breakaway oxides,will form above them at the interface of the bond coat and 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.