What is Gradient Coating？

       In thermal barrier coatings, due to the large difference in the thermal expansion coefficients of the bond layer metal and zirconia ceramics, this difference will lead to excessive stress in the coating, and early failure of the ceramic coating often occurs under thermal cycling conditions. In order to reduce the internal stress and improve the bonding strength between the coating and the substrate, material scientists have begun to introduce the preparation technology of functionally graded materials in conventional thermal barrier coatings.



Japanese scholars Shinno Masatori, Hirai Toshio and Watanabe Ryuzo first proposed the concept of FGM. At the same time, Chinese scholars Yuan Runzhang and others also proposed the concept of FGM, and took the lead in conducting research in this area in China. The design idea of FGM is for two or more materials with different properties, by continuously changing their composition, structure, structure, pores and other elements, so that the internal interface disappears, and a new type of heterogeneous composite material with continuous and stable performance is obtained. . With the help of the concept of functionally graded materials, the structure of the thermal barrier coating is graded, and accordingly, the thermal expansion coefficient will gradually change along the thickness of the coating, so as to alleviate the thermal stress generated during the coating preparation process and the thermal cycling process.



Gradient functional materials provide an effective method for the thermal stress relaxation problem that cannot be solved by metal/ceramic coating materials, which brings exciting prospects for the application of thermal barrier coatings, so it is greatly appreciated by the material community all over the world. of attention. After Japan, Germany and the United States have also started large-scale research and development, and my country has also included this research in the "863" plan. In just ten years, rapid development has achieved remarkable achievements. Aerospace, aviation, aircraft, satellites, launch vehicles and other thermal barrier materials that require ultra-high temperature resistance, heat-resistant materials for nuclear reactors and engines, and thermal shielding materials can greatly improve thermal efficiency after using FGM thermal barrier coatings.



The thermal shock resistance of functionally graded thermal barrier coatings has been studied in China. Wang Fuqi et al. studied the failure mechanism of ZrO2-NiCrAl gradient thermal barrier coatings prepared by plasma spraying under transient thermal load, and pointed out that the radial tensile force of the ceramic surface layer in addition to the cooling process exceeds that of the ceramic material. In addition to the mode of coating failure, during the heating process, a large axial tensile stress appears at the interface between the ceramic layers, which can eventually lead to the coating spalling. Zhu Jingchuan et al. studied the thermal shock and thermal fatigue behavior of Zr02-Ni system gradient thermal barrier coatings. The results show that the thermal shock resistance parameters of Zr02-Ni system gradient thermal barrier coatings are distributed in a gradient, and thermal shock damage is consistent with thermal Fatigue damage mechanism, the quasi-static propagation of cracks is the controlling factor; thermal fatigue cracks are initiated and propagated in the way of micropore aggregation and connection in the gradient layer, and there are no transverse penetration cracks between the gradient layers, which overcomes the thermal stress of traditional coatings Peeling problem. Huang Weigang studied the gradient thermal barrier coating of Z-Q2-NiCoCrAlY system, and believed that stress relief annealing could further improve the thermal shock resistance of the coating.