Carbon fiber composites hold significant promise as electromagnetic wave (EMW)-absorbing materials. However, balancing lightweight materials with excellent mechanical properties, low thermal conductivity, and EMW absorption for multifunctional applications remains challenging. Herein, a novel hydrothermal carbon (HC)-coated three-dimensional (3D) needled carbon fiber-reinforced silicon–boron carbonitride (C_f/HC–SiBCN) composite was developed via an optimized precursor infiltration and pyrolysis (PIP) process combined with impregnation–filtration. By adjusting the precursor concentration and number of impregnation‒filtration cycles, a hierarchical C_f/HC–SiBCN composite with the density of 0.32 g·cm⁻³ was obtained, which exhibited 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. Owing to their combination of lightweight, enhanced mechanical properties, low thermal conductivity, and superior EMW absorption capabilities, C_f/HC–SiBCN composites are 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.