Translated Abstract
Thermal cyclic lifetime and thermal insulation performance are two essential performances for thermal barrier coatings (TBCs). Therefore, the above two indexes must be both taken into consideration during the innovative design of advanced thermal barrier coatings. In order to develop TBCs with high insulation and long lifetime, a functional-layered structure based on revealing the peeling failure mechanism of TBCs was developed. First, gradient thermal cyclic tests were performed to simulate the service conditions of thermal barrier coatings, revealed the ceramic layer peeling failure mechanism of TBCs caused by gradient sintering stiffness of ceramics. Second, the equivalent thermal insulation design conception was proposed. Through the lifetime comparison between vertically crack structure and layered structure TBCs, the dominant role of stress accumulation in the TBCs failure mechanism was clarified. Finally, the functional-layered structure design covering layer structure and sub-layer structure on the basis of putting forward the design of decreasing the coating thickness combine reducing stress accumulation. Since the ceramic layer peeling is inhibited, high insulation and long lifetime design TBCs and realized.
Ceramic peeling mechanism induced by gradient sintering was clarified by the gradient thermal cyclic tests under different surface temperatures. Under the condition of gradient thermal cyclic tests with the surface temperature of 1250~1350 oC，the thickness of thermally grown oxide (TGO) is thin, and is far less than the critical thickness of causing the whole ceramic peeling. YSZ shows no significant phase change after failure. The sintering of ceramics also presents a gradient distribution characteristic because of the temperature gradient in the coating thickness direction. With the increase of surface temperature, the lifetime of the TBCs decreases significantly, with the sintering degree of ceramic surface reaches the same level. The increase of the surface temperature significantly accelerates the sintering of ceramics. After the ceramic layer sintered to the critical level, ceramic delamination failure happens. The sintering degree of ceramic layer can be used as a reference for the evaluation of TBCs lifetime.
The thermal insulation performance conception was proposed for the design of TBCs. By comparing the lifetime of dense vertically crack (DVC) and layered structure TBCs, the leading role of stress accumulation in delamination failure is clarified. The gradient thermal cyclic experimental results with different ceramic structures showed that, the lifetime expectancy contribution of coating thickness reducing design is more efficient to the DVC structure. After more than 4000 cycles thermal cyclic tests, coating thickness reducing TBCs show no obvious damage, but the DVC structure with same thermal insulation failed at about 2000 cycles. DVC structured TBCs can improve the lifetime of TBCs under the condition of the same thickness. The TGO thickness is very thin, and YSZ shows no obvious phase change, thus these two factors are not the dominate failure inducements of TBCs. Sintering induced competition between cracking driving force and cracking resistance determine the lifetime of TBCs. The thickness reduction of ceramic layer under the guidance of equivalent thermal insulation performance can significantly decrease the stress accumulation in TBCs system and then prolong the lifetime.
The thermal cyclic tests by structure design between LZO/YSZ ceramic layers, double lifetime expectancy higher than lameller pure YSZ was realized. With the of increases LZO substutional thickness ratio, the lifetime of LZO/YSZ decreases; Numerical simulation analysis shows that, with the decrease of elastic modulus and the thickness of the as sprayed ceramics, the energy release rate at ceramic layer interface crack tips significantly decreases after the strain applied. Different levels of elastic modulus were obtained by the precise control of LZO spraying parameters. LZO sub-layer functional-layered structure was developed. By decreasing the overall modulus of LZO and modulus transition through the LZO layer, the lifetime of LZO double ceramic thermal barrier coating with large thickness is significantly been greatly improved. Finally, the design and preparation technology of the stable and efficient thermal barrier system is established.
Translated Keyword
[Hydrothermal method, Transition metal selenides, Micro/nanostructure, Electro-catalytic HER, Catalytic activity]
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