Abstract: SiC_f/Si₃N₄ composites are thermal structural materials for structural-functional-integrated requirements. In order to fully utilize the advantages of continuous SiC fibers as semiconductors in the regulation of dielectric properties, the whole weaving technology based on the mesoscopic scale of the fiber preform structure was proposed. In this work, three SiC_f/Si₃N₄ composites were prepared by the CVI process using two-dimensional half (2.5D), three-dimensional four-direction (3D4X), or three-dimensional five-direction (3D5X) braided structures as preforms. First, the tensile strength of the composites was tested. By comparing with the theoretical value, it was found that there were large deviations in the three composites, which originated in the thermal mismatch between SiC fiber and Si₃N₄ matrix. By analyzing the microstructure, pore distribution, and interfacial shear strength of the three SiC_f/Si₃N₄ composites, the interphase zone of 3D4X composites is more effectively protected compared to 2.5D and 3D5X composites after the CVI Si₃N₄ process. Surprisingly, the disruption of the interphase to form the SiC_f/Si₃N₄ hetero-interface gives 2.5D and 3D5X composites a boosted interfacial polarization, and thus dielectric loss capability and high-temperature insensitivity of electromagnetic wave (EMW) absorption. The results of the high-temperature absorption tests showed that 3D5X composites achieve good absorbing properties in a wide temperature range of 25-900 °C.

Key words: SiC_f/Si₃N₄ composites, Fiber preform, Electromagnetic wave absorption, Interphase, Interfacial polarization