Carbon fiber composites hold significant promise as electromagnetic wave (EMW) absorbing materials, yet balancing lightweight with excellent mechanical properties, low thermal conductivity, and EMW absorption for multifunctional applications remains challenging. Herein, a novel hydrothermal carbon coated three-dimensional (3D) needled carbon fiber reinforced silicon-boron carbonitride (C_f/HC-SiBCN) composite was developed using an optimized precursor infiltration and pyrolysis (PIP) process combined with impregnation-filtration. By adjusting the precursor concentration and impregnation-filtration cycles, a hierarchical (C_f)/(HC)/(SiBCN) composite with a density of 0.32 g·cm⁻³ was achieved, exhibiting remarkable mechanical properties, including flexural strengths of 14.75 ± 0.43 MPa (xy-direction) and 14.45 ± 0.66 MPa (z-direction), along with a compressive strength of 9.36 ± 0.20 MPa (z-direction). It also demonstrated low thermal conductivity (0.145 W·m⁻¹·K⁻¹) and exceptional EMW absorption, with a minimum reflection loss (RL_min) of −58.13 dB and an effective absorption bandwidth (EAB) of 7.38 GHz. The combination of lightweight, enhanced mechanical properties, low thermal conductivity, and superior EMW absorption capabilities makes C_f/HC-SiBCN composites highly suitable for multifunctional applications.

As a new category of ultra-high-temperature ceramics (UHTCs), multi-anionic high-entropy (HE) carbonitride UHTCs are expected to have better comprehensive performance than conventional UHTCs. However, how to realize the green and low-cost synthesis of high-quality multi-anionic HE carbonitride UHTC powders and prepare bulk ceramics with excellent mechanical properties still faces great challenges. In this work, a green, low-cost, and controllable preparation process of (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C_xN_1−x powders is achieved by sol–gel combined with the carbothermal reduction/nitridation method for the first time. The as-synthesized (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C_xN_1−x powders possess high compositional uniformity and controllable particle size. In addition, the obtained bulk ceramics prepared at 1800 ℃ exhibit superior fracture toughness (K_IC) of 5.39± 0.16 MPa·m^1/2 and high nanohardness of 35.75±1.23 GPa, elastic modulus (E) of 566.70±8.68 GPa, and flexural strength of 487±41 MPa. This study provides a feasible strategy for preparing the high-performance HE carbonitride ceramics in a more environmentally friendly and economical manner.