A two-layer plasma sprayed thermal barrier coating on Ni base superalloy substrate was characterized. The coating was comprised of an inner layer of MCrAlY bond coating and an outer layer of 8wt% yttria stabilized zirconia (YSZ) thermal barrier coating (TBC). After aging, the reaction layer at interface between YSZ and MCrAlY was observed. The reaction layer had two different contrast layers in the SEM images. One was black which was closer to MCrAlY and the other was gray which was closer to YSZ. The thickness of both layers increased with aging. Also countless porosities at the gray layer and microcrack at YSZ were observed. Most of the macrocrack grew through the porosities. From this viewpoint one may say that the mechanism of macrocrack formation is a deterioration of adhesion which was accompanied by an increase of the porosities or the microcrack.
