Microstructure and Cracking Behavior of a Four-Layer Thermal Barrier Coating After Thermal Cycle Test

Microstructure evolution and cracking behavior of a four-layer thermal barrier coating (TBC) with double YSZ layers during thermal cycle tests were studied in the current work. The temperature range of the thermal cycle test ranged from room temperature to 1100 °C under atmospheric conditions. The TBC consisted of tetragonal t′ and t phases as well as monoclinic yttrium oxide. After 500 thermal cycles, the m-ZrO₂ phase was formed through the phase transformation from t′-ZrO₂ to m-ZrO₂ and c-ZrO₂. A large number of bulk thermally grown oxides (TGO), including chromium, spinel, and yttrium aluminates, were formed around pores in the transition layer (TL). Furthermore, the thickness of the TGO layer increased with a relatively low increase rate during the test (where k_p was about 0.17 μm²/h). This may be attributed to the formation of bulk TGO around pores within the TL, which could consume some of the oxygen. The results show that large horizontal cracks are likely to form at the TSL/TIL and TIL/TL interfaces, while vertical cracks tend to occur near the surface of the TSL, and the propagation rate is relatively low. The propagation of horizontal cracks is the primary cause of failure in this four-layer structure. After the thermal cycle test, the porosity of TSL decreased significantly, from 7.17% to 0.76%. The results in this study may help optimize the design and preparation of TBCs with double YSZ layers.