Gradient thermal cycling behavior of a thermal barrier coating system constituted by NiCoCrAlY bond coat and pure metastable tetragonal nano-4YSZ top coat

Pure metastable tetragonal (t’) phase 4YSZ top coats with thickness of 100 and 200 μm were deposited on NiCoCrAlY-coated second generation single crystal superalloy by air plasma spray (APS). The two thermal barrier coatings were evaluated under gradient thermal cycling test using gas mixture of propane and oxygen. After flame shock test, the values of Young's modulus, hardness and degree of densification all exhibited a gradient distribution across YSZ thickness. In contrast to intensive sintering at surface of 200 μm 4YSZ coating, the TBC sample with 4YSZ layer of 100 μm underwent poor oxidation at interface of YSZ and bond coat, forming a duplex oxide scale: (Ni,Co)(Cr,Al)2O4 spinel over Al2O3, which promoted the delamination at the top-coat/bond-coat interface. The resistance against gradient thermal cycling, the phase stability of 4YSZ and the failure mechanism of the TBCs, were discussed correlating to the effects of YSZ thickness.