Mechanistic understanding for degraded thermal barrier coatings

A two-layer plasma sprayed thermal barrier coating on Ni base superalloy su bstrate was characterized. The coating was comprised of an inner layer of M CrAlY bond coating and an outer layer of 8 wt% yttria stabilized zirconia ( YSZ) thermal barrier coating (TBC). After aging, the thermally grown oxide (TGO) layer at interface between YSZ and MCrAlY was observed. The TGO layer had two different contrast layers in the SEM images. The black one was clo ser to MCrAlY and the gray one was closer to YSZ. The thickness of both lay ers increased with aging. Countless porosities at the gray layer and microc rack at YSZ were also observed. Most of the macrocrack grew through the por osities. From this point of view one may say that the mechanism of macrocra ck formation is a deterioration of adhesion accompanied by an increase of n umber of the porosities or the microcrack.