Failure Process and Mechanism of Multi-rare Earth Oxide Co-doped ZrO₂ Thermal Barrier Coatings Under High Temperature and High Temperature Gradient

Thermal barrier coatings serve as a prevalent advanced heat protection method in aviation engines. The working environment for the coatings becomes increasingly challenging with a rise in engine operating temperatures. Investigating the failure modes of the coatings under high-temperature and high-temperature gradient conditions is essential to improve the operational lifespan of the coatings. A ceramic coating with a thickness of 0.12 mm, comprised of Gd₂O₃-Yb₂O₃-Y₂O₃ co-doped ZrO₂, was produced on the GH3536 substrate using the atmospheric plasma spraying technique. A burner rig test device has been designed to simulate the service environment of coatings. This generates a super high-temperature flame by burning a mixture of aviation kerosene and oxygen, ensuring high temperatures on the sample surface. The sample is of a hollow structure with high-pressure cooling water flowing inside, which ensures low temperatures on the back of the sample, thus generating a severe temperature gradient. The use of various characterization methods allowed for an analysis of the microstructural changes in the coating, leading to a discussion of the failure mechanisms of the coating under high temperature and high temperature gradient conditions. The results show that after burner rig test with surface temperature of about 2350 ℃, the coating life of single long-time test is greater than 1200 s, and the coating life of multiple short-time test is 3 times. The coating showed obvious gradient sintering along the thickness direction. The top area of the coating is heavily sintered, which is called the sintered zone, and the porosity and grain size are distributed in gradient along the thickness direction. After multiple 25 s tests, there was an observed increase in the depth of the sintered zone, a higher quantity of vertical cracks, and an expansion in both the width and length of transverse cracks as the number of tests increased. Furthermore, the thermal growth oxide (TGO) transitioned gradually from alumina to spinel, and there was an expansion of the micro-transverse cracks formed by TGO. After the 1200 s test, the coating maintained a singular cubic phase and demonstrated excellent stability at high temperatures. In contrast to the single 25 s test, the sintering depth increased, leading to a higher number of vertical cracks. However, the quantity of transverse cracks remained consistent, confirming that transverse cracks arise from thermal-mismatch stress during repetitive thermal shock processes. In summary, the failure of the coating under high temperature and high temperature gradient conditions can be attributed to a combination of high-temperature sintering, thermal-mismatch stress, and TGO. The failure process can be summarized as the rapid propagation of early-stage vertical cracks caused by high-temperature sintering, the generation of intermittent transverse cracks at the interface due to thermal mismatch stress. The micro-transverse cracks produced by the thermal growth oxide connect the intermittent transverse cracks at the interface, and the connection through the vertical cracks and the continuous transverse cracks causes the coating to finally fall off. Failures occur earlier and the failure mechanism is more complex under high temperature and high-temperature gradient conditions compared to that in conventional thermal shock tests. The research results provide some support for the development of new thermal barrier coatings. The premature failure of coatings can be alleviated by improving the sintering resistance of coatings, increasing the thickness of coatings appropriately and designing multilayer structures.