Mechanistic Understanding for Degraded Thermal Barrier Coatings

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 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 closer to MCrAlY and the gray one was closer to YSZ. The thickness of both layers increased with aging. Countless porosities at the gray layer and microcrack at YSZ were also observed. Most of the macrocrack grew through the porosities. From this point of view one may say that the mechanism of macrocrack formation is a deterioration of adhesion accompanied by an increase of number of the porosities or the microcrack.