The optimization of impedance matching by the absorbing structure can significantly broaden the absorption bandwidth. However, the high dielectric properties of SiC_f/Si₃N₄ composites at high temperatures resulted in the attenuation of absorption performance. Herein, the strategy of regulating the high-temperature dielectric properties of SiC_f/Si₃N₄ was proposed to enhance the high-temperature absorption performance of corrugated structure SiC_f/Si₃N₄. The boron nitride (BN) interphase and Si₃N₄ matrix were deposited on the corrugated structure fiber preform by chemical vapor infiltration (CVI) to obtain corrugated structure SiC_f/Si₃N₄. Through the fiber pretreatment process and SiC matrix deposition, the three samples show different dielectric properties. The real part (ε′) and image part (ε′′) of permittivity at 10 GHz at 600 °C are 14.4, 11.3, and 13.7 and 26.1, 6.4, and 17.4, respectively. The regulation of high-temperature dielectric properties enables the corrugated structure SiC_f/Si₃N₄ to exhibit an effective absorption bandwidth (EAB) of 7.6 GHz at 1000 °C. In addition, the EAB covers the 4–6 GHz frequency range, while the average reflection loss in the low-frequency region (4–8 GHz) is less than −13 dB. Excellent broadband and low-frequency absorption performance depends on a good match between the corrugation structure and dielectric properties. This work provides a new method for improving the absorption performance at high temperatures.

The new generation of weapons and equipment urgently requires high-temperature stealth performance, and it is essential to develop high-temperature microwave absorbing materials. SiCN ceramics have attracted much attention as high-temperature microwave absorbing materials due to advantages such as low density, high-temperature resistance, and adjustable dielectric properties. This work reviews the research progress of SiCN ceramics as microwave absorbing materials. Two kinds of processes, chemical vapor deposition (CVD) and polymer-derived ceramics (PDCs), can be used to synthesize the SiCN ceramic. The first one, CVD, requires lower temperatures, and the dense SiCN phase can be obtained, which can be used for the preparation of SiCN thin films. The latter one, PDCs-SiCN has the advantage of good formability and machinability, and most of the research in recent years has mainly been based on this method. Polysilazanes are usually used to prepare different forms of SiCN ceramics. SiCN aerogel contains a large number of pores that allow for multiple electromagnetic wave reflections and scatterings, effectively expanding the electromagnetic wave's propagation route, and thus the impedance matching characteristics can be modified to obtain the excellent microwave absorption performance. After high-temperature heat treatment, free carbon and SiC nanocrystals can be precipitated from PDCs-SiCN, leading to the polarization loss. However, the single loss mechanism limits the microwave absorbing performance, and the recent research tried to modify the structure of SiCN ceramics from both micro and macro scales to enrich the loss mechanism to enhance the microwave absorbing performance.

Microscopic scale modification can be divided into two categories. The first approach is trying to introduce the magnetic phase, and the magnetic loss mechanism can be introduced, which results in the couple with polarization loss. Usually, Fe, Co and Ni can be introduced into the polysilazane precursor, and the magnetic phase can be in-situ formed into the PDC-SiCN ceramics to adjust impedance matching and improve microwave absorption performance. Another approach is to introduce conductive or dielectric loss phases into SiCN ceramics, such as carbon nanotubes, graphene, and SiC fibers. In these phases, the introduction of the carbon phase leads to an increase in the conductivity loss of SiCN ceramics, and SiC fibers increase the number of interfaces in SiCN ceramics, thereby increasing polarization loss.

Macroscale modifications can be achieved by designing sub-wavelength structural units of SiCN metamaterials, such as honeycomb structures and hollow spheres. Through the optimization of structure parameters, the impedance matching can be adjusted. At the same time, the structure can increase the propagation path of electromagnetic waves by multiple reflections to optimize the microwave absorbing properties. Through the above modification methods, the electromagnetic loss mechanism of SiCN ceramics can be enriched, and the microwave absorbing performance can be significantly improved. Additionally, the research on the combination of continuous fiber and SiCN ceramic was also carried out to obtain the microwave-absorbing ceramic matrix composites, which reveal great potential for the application in aero-engine fields. Finally, this work provides an outlook on the future development trend of SiCN ceramics and its composites.

In this work, low-softening-temperature and low-modulus SiO₂ was introduced as an embedded interface between SiC nanowires (SiCnws) and a Ba_xSr_1−xAl₂Si₂O₈ ceramic matrix to enhance strength and toughness of the ceramic. During the sintering process, molten SiO₂ enhances the flowability of the ceramic powders and modifies the dispersion of SiCnws. The strengthening effect of SiCnws was fully realized, and the flexural strength of the optimized ceramics reached 193±16 MPa, which represents an increase of 52.6%. After the formation of the embedded SiO₂ interface with a low modulus, cracks can deflect along the SiCnws surface, which is consistent with the criterion of He and Hutchinson. This can effectively extend the crack propagation path, and the fracture toughness ( $K_{\mathrm{I}\mathrm{C}}$) is thus improved by 94.0%, reaching 3.1±0.5 MPa·m^1/2.

Oxidation behaviors of carbon fiber reinforced SiC matrix composites (C/SiC) are one of the most noteworthy properties. For C/SiC, the oxidation behavior was controlled by matrix microcracks caused by the mismatch of coefficients of thermal expansion (CTEs) and elastic modulus between carbon fiber and SiC matrix. In order to improve the oxidation resistance, multilayer SiC-Si₃N₄ matrices were fabricated by chemical vapor infiltration (CVI) to alleviate the above two kinds of mismatch and change the local stress distribution. For the oxidation of C/SiC with multilayer matrices, matrix microcracks would be deflected at the transition layer between different layers of multilayer SiC-Si₃N₄ matrix to lengthen the oxygen diffusion channels, thereby improving the oxidation resistance of C/SiC, especially at 800 and 1000 ℃. The strength retention ratio was increased from 61.9% (C/SiC-SiC/SiC) to 75.7% (C/SiC-Si₃N₄/SiC/SiC) and 67.8% (C/SiC-SiC/Si₃N₄/SiC) after oxidation at 800 ℃ for 10 h.

In this work, bulk Zr₃Al₃C₅-based ceramics were synthesized by the infiltration of Al–Si melt into zirconium carbide (ZrC) perform. The phase composition, microstructure, and mechanical properties of as-fabricated ceramics were studied. The results demonstrate that Si is more effective to reduce the twin boundary energy of ZrC than Al, and thus promotes the decrease of formation temperature of Zr₃Al₃C₅. With the infiltration temperatures increasing from 1200 to 1500 ℃, the Zr₃Al₃C₅ content increases from 10 to 49 vol%, which is contributed to the increase of flexural strength from 62±9 to 222±10 MPa, and fracture toughness (K_IC) from 2.8±0.2 to 4.1±0.3 MPa·m^1/2. The decrease of mechanical properties for the samples fabricated at 1600 ℃ is ascribed to the abnormal growth of Zr₃Al₃C₅ grains.

The effect of pyrolytic carbon (PyC) thickness on the tensile property of mini T800 carbon fiber reinforced SiC matrix composites (C/SiC) was studied. PyC interphase was prepared by chemical vapor infiltration (CVI) process using C₃H₆-Ar as gas source, the PyC thickness was adjusted from 0 to 400 nm, and then the SiC matrix was prepared by CVI process using methyltrichlorosilane (MTS)-H₂-Ar as precursor and gas source. The results showed that the tensile strength of mini T800-C/SiC increased first and then decreased with the increase of the PyC thickness. When the thickness of PyC was 100 nm, the average strength reached the maximum value of 393 ± 70 MPa. The Weibull modulus increased from 2.0 to 8.06 with the increase of PyC thickness, and the larger the Weibull modulus, the smaller the dispersion, which indicated that the regulation of PyC thickness was conducive to improve tensile properties.