SiC_f/SiC composites are considered as potential materials for hot-section components in advanced aircraft engine due to their outstanding properties, such as low density, superior toughness, high temperature resistance and non-brittle fracture failure^[1,2,3]. However, because SiC reacts with water vapor and oxygen to form volatile silicon hydroxide (Si(OH)₄)^[4,5], the wet oxygen environment in aircraft engine leads to serious erosion or even failure for SiC_f/SiC composites. Environmental barrier coatings (EBCs) were applied on SiC_f/SiC composites to overcome the drawbacks as they could isolate the composites from the corrosion and avoid the reaction between SiC and wet oxygen environment^[6,7].

The rare earth silicate (RE₂Si₂O₇, RE=Y, Sc, Yb, Lu) were reported to show low volatilization rate in water vapor environment and the ability to endure temperature higher than 1482 ℃ for thousands of hours^[8,9]. The multi-layer structure EBCs with rare earth silicate as the outermost layer has good corrosion resistance to water vapor and oxygen, which enabled them to be applied to the new generation of advanced aero-engine. Because of the coefficient of thermal expansion (CTE) mismatch^[10], the peeling-off of EBCs may occur during the long-term service in aircraft engine and lead to the failure of the composites. Therefore, improving the oxidation resistance of SiC matrix to water vapor and oxygen is gradually becoming a research hotspot^{[11,12,13,14]}. Y₂Si₂O₇ has similar CTE with SiC (SiC: ~4.5×10^-6 K^-1, Y₂Si₂O₇: ~4×10^-6 K^-1) and excellent chemical compatibility with SiC^[15,16]. These properties motivate the investigation of Y₂Si₂O₇ to modify the SiC matrix.

The aim of this research is to get a robust matrix with excellent corrosion resistance properties by introducing layered yttrium silicate (Y₂Si₂O₇) in SiC_f/SiC composites. In this study, yttrium oxide (Y₂O₃) was generated by the solution impregnation and pyrolysis method in SiC fiber preform firstly, and layered Y₂Si₂O₇ was obtained from the transformation of Y₂O₃ during chemical vapor infiltration (CVI) process. It is expected that the service reliability of SiC_f/SiC composites increases as long as the composites are modified by layered-Y₂Si₂O₇.

2D stitched fabric of silicon carbide fibers (National University of Defense Technology, China) were used as the preforms. BN interphase was deposited by chemical vapor infiltration (CVI). Boron trichloride (BCl₃) and Ammonia (NH₃) (2 : 1, molar ratio) were carried by hydrogen (H₂) to be deposited at 1073 K under a total pressure of 3.0 kPa. To prevent the erosion of BN interphase, a thin SiC layer was also prepared by isothermal CVI at 1000 ℃ under a total pressure of 5 kPa, in which methyltrichlorosilane (MTS) was used as the precursor of SiC. After the BN interphase and SiC layer were applied on the surface of the fibers, a solution made up of Y(NO₃)₃∙6H₂O (Aladdin, Shanghai, China, 99.99%) and ethanol was introduced into SiC preform by vacuum impregnating process. Then the preform was heated to 1000 ℃ for 0.5 h under argon atmosphere and the related preform was named P₁. SiC matrix was then prepared under the same CVI condition as SiC layer. During the process, layered-Y₂Si₂O₇ structure could be obtained in SiC_f/SiC composites. Solution impregnation pyrolysis process and isothermal CVI were repeated once or three times to obtain the dense composite C₁ and C₂, respectively, in which one layer or three layers of Y₂Si₂O₇ in the matrix could be prepared. A reference sample named C₀ was also prepared by the same method but without the solution impregnation pyrolysis.

Phase composition of the samples was analyzed using a Micro-region X-ray diffraction (XRD, D8 DISCOVER DAVINCI, Germany). The microstructures of the composites before and after oxidation were characterized using scanning electron microscope (SEM, S-4800, Hitachi, Tokyo, Japan). The elemental analysis was conducted by energy dispersive spectroscopy (EDS, Aztec X-Max 20, Oxford, UK).

The oxidation behavior of the composites was investigated in wet oxygen environment at 1400 ℃ with the heating and cooling rates of about 5 ℃/min, and the flow rate of O₂/H₂O was kept constant at 500 mL/min (200 mL O₂ + 300 mL H₂O) by a liquid phase vaporization system (LVD-F1, Hefei Crystal Materials Technology Co., Ltd., Hefei, China). The detail about this system was described elsewhere^[17]. The density and porosity of the composites were measured by Archimedes drainage method. The bending strength of the composites before and after oxidation were tested by three-point-bending test (DDL20, Changchun Research Institute for Mechanical Science Co. Ltd., Changchun, China) at room temperature and a loading rate of 0.5 mm/min. The samples were cut and polished to 40 mm×4 mm×3 mm, each point was averaged by five samples.

Micro-region X-ray diffraction (XRD) patterns of C₀, C₁, C₂ and P₁ are shown in Fig. 1(a). It can be found that the matrix of sample C₀ is mainly made of β-SiC, which is obtained from the pyrolysis of MTS by CVI. It is shown from sample P₁ that Y₂O₃ can be formed in the preform after the pyrolysis of Y(NO₃)₃·6H₂O. From the patterns of sample C₁ and C₂, β-SiC and Y₂Si₂O₇ (z- Y₂Si₂O₇：JCPDS Card 21-1459) are found. In order to understand the formation of Y₂Si₂O₇, XRD of Y₂O₃ before and after CVI is carried out (Fig. 1(b)). It shows that Y₂O₃ can be transformed into Y₂Si₂O₇ completely after CVI. In addition, Energy Disperse Spectroscopy (EDS) of the representative white region in sample P₁ and C₁ shows that the atomic ratio of Y to O changed from approximately 1 : 2 to 1 : 3.5 (Fig. 2(a-b)), and it can be inferred that Y exists in the form of Y₂Si₂O₇ in sample C₁. It demonstrates that the transformation of Y₂Si₂O₇ from Y₂O₃ with CVI occurs.

Scanning electron microscope (SEM) images of the polished cutting surface of these composites are shown in Fig. 2. Y₂Si₂O₇ with layered distribution can be found around the fiber bundles before oxidation. Sample C₁ shows a single-layer distribution (Fig. 3(a)) while sample C₂ shows an obvious three-layer distribution (Fig. 3(b)). As it is not dense enough for Y₂O₃ after pyrolysis, SiC can be deposited in the enrichment area of Y₂Si₂O₇ due to the inner pores working as the deposition channels. Therefore, the white region is the mixing region of Y₂Si₂O₇ and SiC.

Fig. 4 illustrates the oxidation process. The oxidation of SiC matrix in wet oxygen consists of two parts: oxidation of SiC by O₂ to form silica which is a low viscosity melt in wet oxygen environment, and volatilization of the silica in the form of Si(OH)₄ (g) by reaction with H₂O. Both SiC and Y₂Si₂O₇ exist on the surface before oxidation, and Y₂Si₂O₇ remain stable in wet oxygen at high temperature. The volatilization of Si(OH)₄ will promote the redistribution of Y₂Si₂O₇ in the surface layer of silicate melt during the oxidation process. Therefore, a large amount of dispersed Y₂Si₂O₇ can be found on the surface of the composites after oxidation. In sample C₂, Y₂Si₂O₇ was more abundant and distributed more widely before the oxidation, so the degree of dispersion is more pronounced and the surface of Y₂Si₂O₇ is denser than that of sample C₁ after the oxidation (Fig. 3(e-f)). By contrast, for sample C₀, the silica glass phase precipitates into crystalline cristobalite in oxidation environment at 1400 ℃. Due to the mismatch of the coefficients of thermal expansion (SiO₂ glass phase: ~7×10^-7 K^-1, crystalline cristobalite: ~170×10^-7 K^-1), a large number of cracks on the surface are formed during the cooling process (Fig. 3(d)), which provide a diffusion channel for oxidizing atmosphere^[18].

The strength/oxidation time and strength retention rate/oxidation time of the composites are shown in Fig. 5. Before oxidation, the bending strength of C₁ and C₂ is (474.93±24.29) MPa and (383.61±30.29) MPa, respectively, while sample C₀ reaches (532.28±32.28) MPa. As the impregnation and pyrolysis times increases, the porosity of the composites increase gradually (Table 1) since Y₂O₃ can block some pores and hinder the permeation of reactive gases during the CVI process, which may lead to the difference of the mechanical properties. During the oxidation process, Y₂Si₂O₇ can disperse on the surface of sample C₁ and sample C₂ to prevent further diffusion of the oxidizing atmosphere. The mechanical properties of the materials after oxidation are better preserved. After 80 h of oxidation, the strength retention of sample C₀, C₁ and C₂ is about 50.11%, 60.38% and 71.93%, respectively.

Layered Y₂Si₂O₇ was prepared to modify SiC_f/SiC composites by solution impregnation and pyrolysis combining isothermal CVI process. The result shows that Y₂O₃ could be converted into Y₂Si₂O₇ by CVI. The oxidation behavior of SiC_f/SiC composites modified by layered-Y₂Si₂O₇ were tested in the wet oxygen environment at 1400 ℃. It demonstrates that Y₂Si₂O₇ could form a protective layer over the sample surface and block the oxidizing atmosphere. After the oxidation for 80 h, the retention rate of bending strength of the composite with three layers of Y₂Si₂O₇ reaches 71.93%, which is 43.54% higher than SiC_f/SiC without Y₂Si₂O₇. It indicates that the addition of rare earth silicate with layered structure could improve the oxidation resistance of SiC_f/SiC composites in wet oxygen environment, which has beneficial effect on further application in engine components.