It is believed that the peeling of the ceramic layer is not the sudden cracking at the interface, but the residual stress induced the cracking of the coating layer along the thickness direction. The residual stress in the thermal barrier coating mainly comes from three aspects: the thermal stress generated during the molten droplet cooling during the plasma spraying; the thermal stress caused by the thermal barrier coating; the high temperature growth stress of the bonding layer after the thermal cycle oxidation. For the first type of stress, it has been described above that using the preheated matrix and optimizing the spraying parameters can reduce the residual stress in the spraying process. For the latter two types of residual stress, the double ceramic layer structure can be solved. MaW et al. prepared La2CeO7 / 8 YSZ double-layer ceramic thermal barrier coating. Compared with La2CeO7 and 8 YSZ coatings alone, this design of double-layer ceramic coating effectively overcame the thermal expansion mismatch between La2CeO7 coating and binding layer, and its life was significantly higher than that of La2CeO7 and 8 YSZ coatings alone under high temperature cycle conditions. Similarly, Liu Z G et al. prepared the bilayer (La0.8Eu0.2) 2 Zr 2 O 7 / YSZ, peeling off parts of the coating after the thermal cycle test. And peeling area microscopic morphology as shown in figure 6, long strip cracks mostly occur between two kinds of ceramic layer and YSZ coating, coating damage mainly occurs on the outer surface of the ceramic layer, and combined layer thermal growth oxide layer is not obvious, shows that the structure of the coating can effectively delay the damage of the coating. In the double layer ceramic structure, the upper ceramic layer has low thermal conductivity and high phase stability, which can play the role of heat preservation and protecting the inner layer. This double layer ceramic structure shows good development potential in the research of new thermal barrier coating.

Thermal insulation effect is a basic performance requirement for the ceramic layer, which is not only related to the physical performance parameters of the coating material itself, but also affected by the internal structure of the coating and the phase composition of the coating. It is generally believed that increasing the number of cracks inside the coating can improve the thermal insulation performance of the coating. Liu Yang et al. simulated the relationship between the porosity content and composition and the thermal insulation of the coating. The results showed that the ceramic thermal insulation is enhanced with the increase of the porosity and the pore transverse and vertical ratio, but the excessive pore content in the coating will reduce the binding strength of the coating. Li Haoyu under different spraying power such as Gd PO4 / YSZ the double ceramic structure of ultra-high temperature TBCs, low spraying power due to insufficient powder melting, coating appear certain cracks, these cracks can improve the toughness and thermal insulation, but the high spraying power causes GdPO 4 phase decomposition into GdPO 7 phase, adversely affect the thermal conductivity, thermal expansion coefficient and mechanical properties of the coating.