Development of gradient thermal barrier coatings and their hot-fatigue behavior

In this paper, a new type of gradient thermal barrier coating was produced by the co-deposition of a tablet of Al-Al2O3-YSZ (Yttria Stabilized Zirconia, ZrO2 + 8 wt.% Y2O3), or Al2O3-YSZ and YSZ onto a NiCoCrAlY bond coat by EB-PVD. The analysis of SEM with EDS showed that the composition and microstructure changed continuously across the thickness of the gradient coating. Due to interdiffusion of elements between the bond coat and the gradient zone, a thin layer of a gamma' phase (Ni,Al) formed at the surface of the bond coat. The coatings were subjected to a series of tests and characterization studies, including thermal cycling, isothermal oxidation, hot-corrosion and microanalyses. Results showed that the substrate temperature during deposition had great effect on the properties of the coatings. The higher the temperature of the substrate, the lower the microhardness of the gradient coating, which can be considered as the formation of microporosity in the two-phase ZrO2 + Al2O3 region in the gradient coating, and its epitaxy into the YSZ topcoat. The thermal cyclic tests of specimens were performed by exposure to air at 1323 K for 0.5 h, and then cooled to room temperature within 5 min by forced air cooling, with a lifetime of more than 500 h. In addition, compared with a conventional two-layered coating, this gradient coating exhibited a better resistance to hot-corrosion than a conventional two-layered coating. (C) 2000 Elsevier Science S.A. All rights reserved.