The effects of microstructure design on the lifetime performance of lanthanum zirconate (La2Zr2O7; LZO) based thermal barrier coatings (TBCs) were investigated through various thermal exposure tests, such as furnace cyclic thermal fatigue, thermal shock, and jet engine thermal shock. To improve the thermal durability of LZO-based TBCs, composite top coats using two feedstock powders of LZO and 8 wt.% yttria doped stabilized zirconia (8YSZ) were prepared by mixing at different volume ratios (50:50 and 25:75, respectively). Also, buffer layers were introduced in layered LZO-based TBCs deposited by air plasma spray method. The TBCs with two buffer layers showed the best thermal shock resistance and thermal cyclic performance among all samples in all tests. For applications with mildly slow cooling rates, the thermal durability in single-layer TBCs is more effectively enhanced by controlling composite ratio in the blended powder, better than introducing a single buffer layer. For applications with fast cooling rates, the thermal durability can be effectively improved by introducing buffer layer than composite top coat, since buffer layer provides fast localized stress relief due to its high strain compliance. These research findings allow us to control the TBC structure and the buffer layer is efficient in improving thermal durability in the cyclic thermal exposure and thermal shock environments.
Crack-growth behavior in yttria-stabilized zirconia-based thermal barrier coatings (TBCs) is investigated through a cyclic thermal fatigue (CTF) test to understand TBCs' failure mechanisms. Initial cracks were introduced on the coatings' top surface and cross section using the micro-indentation technique. The results show that crack length in the surface-cracked TBCs grew parabolically with the number of cycles in the CTF test. Failure in the surface-cracked TBC was dependent on the initial crack length formed with different loading levels, suggesting the existence of a threshold surface crack length. For the cross section, the horizontal crack length increased in a similar manner as observed in the surface. By contrast, in the vertical direction, the crack did not grow very much with CTF testing. An analytical model is proposed to explain the experimentally-observed crack-growth behavior.
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