SiC/Si-B-C composite materials, with their excellent properties including low density, high specific modulus and strength, high toughness, and oxidation resistance, have become highly promising key materials for hotsection components in aviation and aerospace propulsion system. High-temperature water-oxygen environments constitute their typical service conditions. This paper systematically investigates the performance evolution and damage mechanism of boron-modified 2.5D SiC/Si-B-C composites exposed to an extreme water-oxygen environment (90 kPa H₂O-10 kPa O₂) at 1100 ℃ for corrosion durations of 50, 100, 150, 200, 250, and 300 hours. By analyzing the density, apparent porosity, room-temperature flexural properties and microstructure of the materials before and after corrosion, the following findings are obtained. In the initial corrosion stage (≤200 h), the material properties remain stable, with flexural strength retention exceeding 97% and a slight decrease in apparent porosity. Boron achieves dynamic self-healing through the formation of a borosilicate glass phase, effectively delaying the invasion of the oxidizing media. However, after long-term (300 h) corrosion, the material properties undergo significant degradation, with the flexural strength retention rate sharply dropping to 40% and apparent porosity increasing to 5.2%. The fiber pull-out length at the fracture surface is markedly reduced, presenting brittle fracture characteristics. Microscopic analysis reveals that the continuous volatilization of boron leads to matrix loosening and causes the surface CVD SiC coating to transform from a cauliflower-like structure into a molten glassy state with macroscopic cracks thereby losing its barrier function. This study reveals the possible performance degradation mechanism of boron-modified SiC/SiC composites in long-term extreme water-oxygen environments, providing a theoretical basis for their engineering applications

with the advantages of low textile processing cost, short period and excellent in-plane mechanical properties, the 2D SiC/SiC composites has become one of the most potential structure in engineering application. In this article, base on automatic CNC sewing system and lock stitch method, the SiC fiber preform is sewed by SiC fibers with different sewing density. The sewing 2D SiC fiber preform with PyC interface is densified by polymer infiltration and pyrolysis (PIP) method. The microstructure of 2D SiC/SiC composites is characterized by micro-CT and scanning electron microscope (SEM). Specifically, for the sewing densities of 5mm, 7.5mm, 10mm, 12.5mm, and 15mm, the average flexural strength at room temperature was 378.8 MPa, 447.1 MPa, 485.6 MPa, 504.0 MPa, and 521.5 MPa, and the average tensile strength at room temperature was 261.6 MPa, 286.4 MPa, 293.8 MPa, 300.5 MPa, and 306.5 MPa, respectively. The average interlayer shear strength at room temperature was 84.7 MPa, 76 MPa, 70.2 MPa, 63 MPa and 54.4 MPa, respectively. The results show that with the increase of sewing density, the densification process is affected by the bending of the preform, which leads to the decrease of the in-plane tensile and flexural strength of the composites at room temperature, but the increase of interlaminar shear bearing fibers increases the interlaminar shear strength of the composites at room temperature.