SiC/SiC composites have emerged as a promising choice and development pathway for the high temperature structural components of next-generation aero engines^[1-2]. This is owing to their outstanding properties, including low density and high-temperature resistance, which facilitate weight reduction and enhance service temperature capability^[3]. However, the high-temperature structural components in service face a challenging environment marked by elevated temperatures and water-oxygen coupling environment, imposing stricter demands on the oxidation resistance of materials^[4]. Presently, efforts to enhance the oxidation resistance of SiC/SiC composites mainly concentrate on the environmental barrier coatings (EBCs)^[5-6]. Nonetheless, the intrinsic oxidation resistance of the composite remains unaddressed, leaving it vulnerable to oxidation erosion after coating degradation. Therefore, enhancing the intrinsic oxidation resistance of SiC/SiC composites is crucial for extending the service life of the materials^[7].

Rare earth silicates, characterized by a low thermal expansion coefficient, low oxygen diffusion coefficient, and low water vapor volatilization rate, are emerging as a novel optimization system for EBCs^[8]. Currently, there are numerous reports on the modification of SiC/SiC composites with rare earth silicates to enhance their oxidation resistance properties^[9⇓-11]. Boakye et al.^[12] fabricated SiC/SiC mini-composites featuring Y₂Si₂O₇ interfaces using an in-situ coating method combined with polymer impregnation pyrolysis technique. Oxidation resistance tests revealed that the Y₂Si₂O₇ coating remained stable and maintained the crucial function of the weak interface between the fiber and matrix after exposure to steam at 1000 ℃ for 100 h^[13]. Wang et al.^[14] prepared layered-Y₂Si₂O₇-modified SiC/SiC composites using the chemical vapor infiltration (CVI) method. Results demonstrate that Y₂Si₂O₇ tends to accumulate on the oxidation surface, forming a protective layer during the oxidation process. Notably, the Yb₂Si₂O₇ exhibits greater structural stability than Y₂Si₂O₇, as it does not undergo the phase change at high temperatures. This offers the advantages of high melting point, low density, and low oxygen diffusion coefficient^[15]. These characteristics are beneficial for the application of SiC/SiC composites.

In this study, SiC/SiC mini-composites modified by Yb₂Si₂O₇ were prepared by Sol-Gel and CVD methods to enhance oxidation resistance. SiC/SiC mini-composites refer to unidirectional composites. Microstructure of the Yb₂Si₂O₇ modified SiC/SiC mini-composite was characterized. Effects of introducing Yb₂Si₂O₇ on the mechanical properties and oxidation resistance of SiC/SiC mini-composites were discussed.

Yb₂Si₂O₇ modified SiC/SiC mini-composites were prepared using a combination method of Sol-Gel and CVD. Ytterbium nitrate pentahydrate ((Yb(NO₃)₃·5H₂O, AR, 99.99%, Shanghai Adamas Reagent Co., Ltd., China), tetraethyl orthosilicate ((C₂H₅)₄SiO₄, TEOS, AR, 99%, Shanghai Adamas Reagent Co., Ltd., China) and hydrochloric acid (HCl, AR, 36.0%-38.0%, Sinopharm Chemical Reagent Co., Ltd., China) were dissolved in ethanol (AR, 99.7%, Shanghai Titan Scientific Co., Ltd., China). In brief, the molar concentration of Yb(NO₃)₃·5H₂O was 0.4 mol/L and the molar ratio of Yb(NO₃)₃·5H₂O to TEOS was 1 : 1.2. The mixture solution was vigorously stirred at room temperature for 2 h, and then catalyst hydrochloric acid was added to promote the formation of sol. After stirring at 70 ℃ for several hours and aging at room temperature for several days, the uniform and transparent sol was obtained.

As depicted in Fig. 1, preparation of Yb₂Si₂O₇ modified SiC/SiC mini-composites includes fiber treatment, interfacial deposition, SiC-Yb₂Si₂O₇ multilayer matrix preparation, and final SiC matrix preparation. The SiC fiber bundle was wound onto a frame graphite mold to maintain its straightened state. After desizing, a BN interface was deposited onto the surface of SiC fibers, succeeded by deposition of SiC as a protective layer using CVD. Subsequently, the preform was immersed in Yb₂Si₂O₇ sol along the fiber direction and left for more than 2 min. Then, using the dip-coating method at a constant speed, Yb₂Si₂O₇ gel film was formed on the fiber preform surface as the solvent evaporated. The Yb₂Si₂O₇ coating was prepared after heat treatment at 1200 ℃ for 1 h under an argon atmosphere. The process of preparing SiC-Yb₂Si₂O₇ multilayer matrix was repeated three times. Finally, SiC matrix was deposited until the material was compacted to obtain the mini-composites (SiC/SiC-Yb₂Si₂O₇).

The polished cross-section morphology and tensile fracture morphology of SiC/SiC mini-composites were observed by field emission scanning electron microscope (SEM, Magellan 400, FEI, USA). The mechanical tests were carried out on MTS servo-hydraulic machine with a load displacement control of 0.2 mm/min. Different mini-composites were placed in a tubular furnace and heated to 1200 and 1400 ℃ at rates of 8 ℃/min below 700 ℃ and 5 ℃/min above 700 ℃. There were more than three parallel samples of mini-composites oxidized at each temperature. The cooling rate was programmed by the furnace to cool slowly to 700 ℃ at a rate of 5 ℃/min, followed by natural cooling to room temperature within the furnace. The total oxidation time duration was 50 h. Throughout the high-temperature air oxidation process, both ends of the tube furnace remained open.

The oxidation kinetics were analyzed using a parabolic power law as follows^[16]:

where $w_{i}$ is the sample weight after oxidation for i h, $w_{0}$is the sample weight before oxidation, t is the oxidation time, and k is the parabolic rate constant. The number of samples used in each test was more than three parallel samples.

Fig. 2 illustrates the morphology and structure of the fibers and mini-composites. As shown in Fig. 2(a), the SiC protective layer is deposited by CVD onto BN layer to protect BN interface during the introduction of Yb₂Si₂O₇via Sol-Gel. The step was taken to prevent the oxidation of BN due to the presence of oxygen-containing functional groups in the precursor sol during the Yb₂Si₂O₇ preparation. The boundary between BN interface and SiC protective layer is clear, and the layer thickness is uniform. The Yb₂Si₂O₇ layer prepared via Sol-Gel is grown in situ on the surface of SiC layer (Fig. 2(b)). Cross-section morphologies of SiC/SiC mini-composites and modified SiC/SiC-Yb₂Si₂O₇ mini-composites are shown in Fig. 2(c-f). The Yb₂Si₂O₇ layer, evident in the white region, distributes outside the BN interface and SiC protective layer, demonstrating a multi-layer matrix morphology in local areas. However, achieving a uniform distribution of the Yb₂Si₂O₇ phase becomes increasingly difficult with increasing preparations of SiC via CVD and Yb₂Si₂O₇ by Sol-Gel. The density and open porosity of SiC/SiC mini-composites are 2.61 g/cm³ and 13.90%, whereas SiC/SiC-Yb₂Si₂O₇ mini-composites are 2.52 g/cm³ and 14.12%, respectively. The content of Yb₂Si₂O₇ is about 5.9% of the total content of SiC matrix. Some Yb₂Si₂O₇ concentrates in the matrix, particularly in the bridge region between the fibers, resulting in the formation of SiC-Yb₂Si₂O₇ complex phase matrix. In comparison to SiC/SiC mini-composites, the internal pores of SiC/SiC-Yb₂Si₂O₇ mini-composites increase, predominantly concentrated within the Yb₂Si₂O₇ layer enrichment. This might be attributed to the introduction of Yb₂Si₂O₇ blocking the channel of reaction gas into the fiber bundle during CVD process. This conclusion is also supported by the findings of Wang et al.^[14], who prepared Y₂O₃-modified SiC/SiC composites via CVI. The results indicate that an increase in the open porosity of the composites could lead to a decrease in their mechanical strength.

The tensile tests of SiC/SiC mini-composites and SiC/SiC-Yb₂Si₂O₇ mini-composites were conducted at room temperature. As shown in Fig. 3, the force- displacement curves for both mini-composites exhibit non-linear behavior. The tensile fracture morphologies are illustrated in Fig. 4. Compared with Fig. 4(a, c), it’s evident that both mini-composites exhibit distinct non-brittle fracture characteristics. Notable fiber pullout is observed, which is unimpeded by the modification of Yb₂Si₂O₇ regarding the length of fiber pullout. Additionally, interface debonding is exclusively observed at BN interface in SiC/SiC mini-composites (Fig. 4(b)), while it occurs at both BN and Yb₂Si₂O₇-SiC interfaces in SiC/SiC-Yb₂Si₂O₇ mini-composites (Fig. 4(d)). Boakye et al.^[12] reported that debonding at the rare earth silicate and SiC interface in SiC/Y₂Si₂O₇/SiC mini-composites was attributed to the weak bond strength between the rare earth silicate and SiC. The fracture morphologies indicate that the addition of Yb₂Si₂O₇ enhances the energy dissipation path of SiC/SiC mini-composites, thereby contributing to the toughening of the material.

SiC/SiC and SiC/SiC-Yb₂Si₂O₇ mini-composites underwent oxidation in air at 1200 and 1400 ℃ for 50 h, and the oxidation kinetics was analyzed as depicted in Fig. 5. The results reveal that the weights of both materials increase with the extended oxidation time, whereas the weight gain rates gradually decrease. The weight change during oxidation at 1400 ℃ is significantly greater than that at 1200 ℃, which indicates that oxidation is intensified with rising temperature. The weight change curves are fitted to the parabolic power law expressed in Eq. (1). The fitting results exhibit high R² (>0.97), and the inset demonstrates that a similar R² is sustained even after 80 h of oxidation at 1200 ℃. This suggests that high accuracy of the fitting results, and the oxidation process of the two mini-composites is governed by a diffusion mechanism. Compared to SiC/SiC mini-composites, the weight gain rate of SiC/SiC-Yb₂Si₂O₇ mini-composites displays a decreasing trend. This phenomenon may be attributed to the presence of Yb₂Si₂O₇ phase, which has a low oxygen diffusion coefficient, thus reducing the overall oxygen diffusion rate.

The tensile properties of different composites after oxidation at 1200 and 1400 ℃ were assessed. Tensile fracture morphologies of SiC/SiC mini-composites in Fig. 6(a, c, e) indicate that the fiber pullouts from the matrix are significantly shorter than those in the as-manufactured samples. The tensile fracture in SiC/SiC mini-composites displays a visible flat area. As shown in Fig. 6(c), pores are present near the BN interface within the flat region. These pores may be caused by the volatilization of gas generated during the oxidation of BN, leading to interface embrittlement and observed flat fractures. The lengths of fiber pullouts in SiC/SiC-Yb₂Si₂O₇ mini-composites are longer than those in SiC/SiC, with the pullout of SiC-Yb₂Si₂O₇ interface still observable. Results obtained by energy dispersive spectrometer (EDS) show that the oxygen atom ratio at BN interface in SiC/SiC-Yb₂Si₂O₇ mini-composites is lower than that in SiC/SiC mini-composites.

Table 1 summarizes the changes in tensile strength of mini-composites before and after oxidation. The room temperature tensile strengths of SiC/SiC and SiC/SiC-Yb₂Si₂O₇ mini-composites are 506 and 484 MPa, respectively, which are relatively high strength values compared with the existing literature reports^[17-18]. Following oxidation for 50 h, the strengths of the mini-composites decrease, and SiC/SiC-Yb₂Si₂O₇ mini-composites maintain retention rates of 88% and 77% after oxidation at 1200 and 1400 ℃, respectively, whereas SiC/SiC mini-composites retain 77% and 69%. The improved strength retention rate in SiC/SiC-Yb₂Si₂O₇ mini-composites is attributed to the reduced oxygen content at BN interface.

SiC/SiC mini-composites modified by Yb₂Si₂O₇ were prepared successfully. Yb₂Si₂O₇, introduced via Sol-Gel, exhibits a partial lamellar distribution in matrix. Interlayer debonding occurs at Yb₂Si₂O₇-SiC interface, contributing to material toughening. During the oxidation process in static air at 1200 and 1400 ℃ for 50 h, the weight change of the material follows the parabolic power law, indicating that the oxidation is predominantly governed by the diffusion mechanism. The incorporation of the Yb₂Si₂O₇, which has a low oxygen diffusion coefficient, decreases the weight change rate of the composite during oxidation. After oxidation, the fracture morphology of SiC/SiC mini-composites displays a flat fracture, and SiC/SiC-Yb₂Si₂O₇ mini-composite maintains a significant fiber pullout. Both interface debonding and fiber pullout are observable at Yb₂Si₂O₇-SiC interface. Moreover, EDS results reveal that the interface oxygen content at the fracture surface of SiC/SiC mini-composites is higher than that of SiC/SiC-Yb₂Si₂O₇ mini-composites. After being exposed to oxidation in air at 1200 and 1400 ℃ for 50 h, the tensile strength retention rates of SiC/SiC-Yb₂Si₂O₇ mini-composites are 88% and 77%, respectively. The findings suggest that modifying by Yb₂Si₂O₇ can enhance the oxidation resistance of the composite.