B₂O₃-induced free-carbon interface for synergistic enhancement of EMW absorption and mechanical properties in porous mullite fiber-reinforced SiC composites

The advancement of lightweight, high-performance ceramic matrix composites (CMCs) is of crucial significance for advanced aerospace hot-section components. Within these CMCs, the boron nitride (BN) interface assumes a crucial function in governing both microstructure and comprehensive performance. This research centers on porous mullite fiber-reinforced SiC (P-Mu/SiC) composites featuring a BN interface. It systematically explores how the oxidized BN interface regulates their microstructure, electromagnetic wave (EMW) absorption, and mechanical properties. The thickness and morphology of the B₂O₃-induced free-carbon layer within P-Mu/SiC composites can be regulated by manipulating the oxidation duration of the hexagonal BN interface. Additionally, the B₂O₃ phase was employed to further verify the induced mechanism of the free-carbon layer. When the oxidation duration is relatively short (t ≤ 4 h), a double-layer BN-C interface forms in the P-Mu/SiC composite, which is beneficial to optimizing both the EMW absorption and mechanical properties. When the oxidation duration is 4 h, a moderate BN-C interface enables the P-Mu/SiC composite to achieve effective EMW absorption (reflection loss ≤ −10 dB) in X-band at a thinner thickness. Meanwhile, the double interface can promote crack deflection and energy dissipation, resulting in an improved flexural strength of 32 MPa. These findings demonstrate that a B₂O₃-induced free-carbon interface enables synergistic enhancement of both EMW absorption and mechanical performance, providing a feasible design strategy for high-performance CMCs.