Introduction SiC-based ceramic matrix composites (SiC-CMCs) can meet both of these conditions for excellent high-temperature strength and oxidation resistance simultaneously, making them highly promising lightweight structural materials for future aerospace engine applications. Si dioxide generates volatile Si(OH)4 compounds when reacting with water molecules in the high-temperature environment of an engine in which there exists water and oxygen corrosion. With the progress of the reaction, the SiC substrate is continuously lost, resulting in a sharp decline in the physical and chemical properties of CMCs. Therefore, environmental barrier coatings (EBCs) are needed to be coated on the CMCs surface to prevent the materials from being damaged by corrosive media such as water vapor and improve the service life of SiC-CMCs.Si/Yb2Si2O7 EBCs is formed by a relatively mature controllable preparation method, and its resistance to water vapor corrosion is a key to the related research. Jian et al. found thatYb2Si2O7 coating had an excellent phase stability under the water vapor corrosion at 1 300 ℃. The growth of thermally grown oxides (TGO) was effectively controlled. The oxidation weight gain rate of the coated sample was lower than that of the uncoated sample, thus indicating that Yb2Si2O7 topcoat could have an excellent corrosion resistance. Zhang et al. observed a “self-healing” reaction of Yb2Si2O7 topcoat in the water vapor corrosion at 1 350 ℃, and the growth of the TGO layer showed a positive linear relationship with time, and the overall structure of the coating was basically intact. Ridley et al. reported that after Yb2Si2O7 topcoat was subjected to water vapor corrosion at 1 400 ℃, a porous Yb2SiO5 layer was formed, the thickness and pore size increased with time, and a distinct layered structure appeared in the reaction layers in the high-speed area, i.e., a porous Yb2SiO5 layer, a dense Yb2SiO5 layer, and a highly porous Yb2O3 layer. The existence of the dense layer hindered the inward diffusion of water vapor and oxygen, reducing the total thickness of the reaction layer and effectively protecting SiC substrate. In this paper, a gradient structure yttrium silicate-based environmental barrier coating was prepared for the improvement of high-performance aero-engine thermal protection coating material. In addition, the continuous water vapor damage behavior and corrosion mechanism at 1 350-1 500 ℃ were also investigated.Methods SiC composite samples were cut to a cube of 10 mm × 10 mm × 10 mm. Si bond coat (~50 μm) and Yb2Si2O7 topcoat (~200 μm) both were deposited by an air plasma spray technique in an air plasma spray system. These samples were annealed in vacuum at 1 300 ℃ for 2 h before the corrosion test. The equipment used for continuous water vapor exposure was a self-designed experimental platform. And the coating samples were placed in a flowing 90% (in volume) H2O-10% O2 under an atmospheric pressure. For the experiments, the furnace temperature was set at different temperatures (i.e., 1 350, 1 400, 1 450 ℃ and 1 450 ℃), respectively. The ten samples were prepared in a zirconia crucible. Finally, the sample was removed from the furnace tube at each temperature for different durations (i.e., 50, 100, 150 h and 200 h).The composition phases on the surface of the samples were analyzed by a model XD-3 X-ray diffractomter (XRD, Aeris Co., the Netherlands). The microstructure of the sample cross section and the thickness of the TGO layer in secondary electron (SE) and backscattered electron (BSE) images were determined by a model MIRA 3 emission-scanning electron microscopy (SEM, Tescan Co., Czech Republic). The element distribution and proportion were characterized by an energy dispersive X-ray spectroscope (EDS).Results and discussion After 200 h water vapor corrosion, the coatings at different temperatures show different results. At 1 350 ℃, the coating shows a transformation from Yb2Si2O7 phase to Yb2SiO5 phase, as well as some holes appear due to the volatilization of Si(OH)4. The coefficient of thermal expansion (CTE) mismatch results in a “fragmented” layer on the top of the coating. At 1 400 ℃, a “self-healing” reaction occurs in the coatings. The proportion of Yb2Si2O7 phase increases slightly, and some transverse cracks heal. The “self-healing” reaction is related to Si(OH)4 concentration. Also, Yb2O3 phase appears, and some cracks occur due to the CTE mismatch. At 1 450 ℃, a black phase (i.e., Yb3Al5O12) appears on the top of the coatings. Gaseous Al(OH)3 diffuses into the coating, and a dark mullite phase forms. The “self-healing” reaction still exists at 1 450 ℃. At 1 500 ℃, many transverse cracks appear on the top of the coatings, the “self-healing” reaction is inhibited, Yb2O3 phase appears at the coating interface, and many cracks and holes occur inside the coating. The coating mass gain and TGO growth are linearly related to the corrosion time at <1 500 ℃, but they are not related to the corrosion time at 1 500 ℃.Conclusions After 200 h continuous water vapor corrosion at 1 350-1 500 ℃, Yb2Si2O7 topcoat exhibited an excellent phase stability, and TGO layer grew at a slow rate. This indicated that Si/Yb2Si2O7 bi-layer EBCs could have a good corrosion resistance, and the preparation process of the fully covered sample could reflect the protective effect of the coating on the SiC substrate. The corrosion mechanisms were different at different starting temperatures. At 1 350 ℃, the reaction volume of Yb2Si2O7 and water vapor decreased to generate cracks, and the CTE mismatch of the product Yb2SiO5 led to crack expansion. At 1 400 ℃ and 1 450 ℃, the “self-healing” reaction gradually played a dominant role, and the new phase generated by the penetration of Al(OH)3 led to some holes in the coating. At 1 500 ℃, water vapor diffused to Si/Yb2Si2O7 coat interface in the later stage of corrosion and reacted to produce Yb2O3 phase, resulting in channel cracks at the TGO layer interface.