SiBCN based metastable ceramics and their composites possess unique microstructure and excellent high-temperature performance, and exhibit significant application potential under harsh conditions such as high-temperature oxidation, severe thermal shock, and gas flow ablation. This review focused on the SiBCN based metastable ceramics and their composites by mechanical alloying, summarized the research progress on the microstructure characteristics and evolution, mechanical properties, oxidation resistance, thermal shock resistance, and ablation resistance of SiBCN based amorphous ceramic powders and bulk ceramics based on mechanical alloying technology in recent years, also in comparison with the polymer derived counterparts. The future research focus and development trend of SiBCN based metastable ceramics with higher performance were finally pointed out.

In this study, a crack-free pyrolysis process of partially cured precursor powder compacts was developed to prepare dense silicon boron carbonitride (SiBCN) monoliths at much lower temperatures (1300 °C), thereby circumventing the challenges of sintering densification (> 1800 °C). Unlike the elastic fracture in over-cured precursors or the viscoelastic deformation in under-cured precursors, the partially cured precursor, exhibiting elastic‒plastic deformation behavior, facilitates limited nanoscale pore formation in a dense structure, achieving a balance between crack-free pyrolysis and densification. Compared to SiBCN derived from the over-cured precursor (σ = ~159 MPa, K_IC = 1.9 MPa·m^1/2, Vickers hardness (HV) = 7.8 GPa), the resulting SiBCN monolith exhibited significantly improved mechanical properties (σ = ~304 MPa, K_IC = 3.7 MPa·m^1/2, HV = 10.6 GPa) and oxidation resistance. In addition, this study investigated the high-temperature performance of SiBCN monoliths, including crystallization and oxidation, and determined the oxidation kinetics induced by pore structure healing and the different oxidation mechanisms of Si–C–N and B–C–N clusters in the amorphous structure. Due to its unique composition and structure, the SiBCN ceramic oxide layer exhibits exceptional self-healing effects on repairing the nanoporous system in the initial stage and shows outstanding high-temperature stability during prolonged oxidation, mitigating adverse effects from bubble formation and crystallization. Due to the nanoporous structure, the oxidation rate is initially controlled by gas diffusion following a linear law before transitioning to oxide layer diffusion characterized by a parabolic law. Finally, due to different valence bond configurations, Si–C–N transforms into an amorphous SiCNO structure after phase separation, unlike the nucleation and growth of residual B–N–C.

To improve the oxidation resistance of short carbon fiber (C_sf)-reinforced mechanically alloyed SiBCN (MA-SiBCN) (C_sf/MA-SiBCN) composites, dense amorphous C_sf/SiBCN composites containing both MA-SiBCN and polymer-derived ceramics SiBCN (PDCs-SiBCN) were prepared by repeated polymer infiltration and pyrolysis (PIP) of layered C_sf/MA-SiBCN composites at 1100 °C, and the oxidation behavior and damage mechanism of the as-prepared C_sf/SiBCN at 1300–1600 °C were compared and discussed with those of C_sf/MA-SiBCN. The C_sf/MA-SiBCN composites resist oxidation attack up to 1400 °C but fail at 1500 °C due to the collapse of the porous framework, while the PIP-densified C_sf/SiBCN composites are resistant to static air up to 1600 °C. During oxidation, oxygen diffuses through preexisting pores and the pores left by oxidation of carbon fibers and pyrolytic carbon (PyC) to the interior of the matrix. Owing to the oxidative coupling effect of the MA-SiBCN and PDCs-SiBCN matrices, a relatively continuous and dense oxide layer is formed on the sample surface, and the interfacial region between the oxide layer and the matrix of the as-prepared composite contains an amorphous glassy structure mainly consisting of Si and O and an incompletely oxidized but partially crystallized matrix, which is primarily responsible for improving the oxidation resistance.

The atomic structural features and the mechanical properties of amorphous silicoboron carbonitride ceramics with 13 different compositions in the Si–BN–C phase diagram are investigated employing ab-initio calculations. Both chemical bonds and local structures within the amorphous network relate to the elemental composition. The distribution of nine types of chemical bonds is composition-dependent, where the B–C, Si–N, Si–C, and B–N bonds hold a large proportion for all compositions. Si prefers to be tetrahedrally coordinated, while B and N prefer sp²-like trigonal coordination. In the case of C, the tetrahedral coordination is predominant at relatively low C contents, while the trigonal coordination is found to be the main feature with the increasing C content. Such local structural characteristics greatly influence the mechanical properties of SiBCN ceramics. Among the studied amorphous ceramics, SiB₂C₃N₂ and SiB₃C₂N₃ with low Si contents and moderate C and/or BN contents have high elastic moduli, high tensile/shear strengths, and good debonding capability. The increment of Si, C, and BN contents on this basis results in the decrease of mechanical properties. The increasing Si content leads to the increment of Si-contained bonds that reduce the bond strength of SiBCN ceramics, while the latter two cases are attributed to the raise of sp²-like trigonal configuration of C and BN. These discoveries are expected to guide the composition-tailored optimization of SiBCN ceramics.

Geopolymers have attracted recent attention due to their wide source of raw materials, environmental protection, low-carbon technology and unique properties. However, the geopolymerization behavior of geopolymers is susceptible to the reaction reactivity of raw materials, leading to large fluctuations in the properties such as mechanical properties and durability. The issues mentioned above are directly related to the geopolymerization mechanism of geopolymers, geopolymerization kinetics, and influencing factors and regulation approach of the properties of geopolymers. Therefore, this review represented recent work and gave some future research aspects in this field to promote the utilization of low-quality aluminosilicate minerals or aluminosilicate industrial waste for the possible purpose of ‘peak carbon dioxide emissions’ and ‘carbon neutrality’.

Textured hexagonal boron nitride (h-BN) matrix composite ceramics were prepared by hot- pressing using different contents of 3Y₂O₃-5Al₂O₃ (molar ratio of 3:5) as the sintering additive. During hot-pressing, the liquid Y₃Al₅O₁₂ (YAG) phase showing good wettability to h-BN grains was in situ formed through the reaction between Y₂O₃ and Al₂O₃, and a coherent relationship between h-BN and YAG was observed with [010]_h-BN// $[\overline{1}11]$_YAG and (002)_h-BN//(321)_YAG. In the YAG liquid phase environment formed during hot-pressing, plate-like h-BN grains were rotated under the uniaxial sintering pressure and preferentially oriented with their basal surfaces perpendicular to the sintering pressure direction, forming textured microstructures with the c-axis of h-BN grains oriented parallel to the sintering pressure direction, which give these composite ceramics anisotropy in their mechanical and thermal properties. The highest texture degree was found in the specimen with 30 wt% YAG, which also possesses the highest anisotropy degree in thermal conductivity. The aggregation of YAG phase was observed in the specimen with 40 wt% YAG, which resulted in the buckling of h-BN plates and significantly reduced the texture degree.

BN/La–Al–Si–O composite ceramics were fabricated by hot-pressed sintering using hexagonal boron nitride (h-BN), lanthanum oxide (La₂O₃), aluminia (Al₂O₃), and amorphous silica (SiO₂) as the raw materials. The effects of sintering temperature on microstructural evolution, bulk density, apparent porosity, and mechanical properties of the h-BN composite ceramics were investigated. The results indicated that La–Al–Si–O liquid phase was formed during sintering process, which provided an environment for the growth of h-BN grains. With increasing sintering temperature, the cristobalite phase precipitation and h-BN grain growth occurred at the same time, which had a significant influence on the densification and mechanical properties of h-BN composite ceramics. The best mechanical properties of BN/La–Al–Si–O composite ceramics were obtained under the sintering temperature of 1700 ℃. The elastic modulus, flexural strength, and fracture toughness were 80.5 GPa, 266.4 MPa, and 3.25 MPa·m^1/2, respectively.

The in situ nano Ta₄HfC₅ reinforced SiBCN-Ta₄HfC₅ composite ceramics were prepared by a combination of two-step mechanical alloying and reactive hot-pressing sintering. The microstructural evolution and mechanical properties of the resulting SiBCN-Ta₄HfC₅ were studied. After the first-step milling of 30 h, the raw materials of TaC and HfC underwent crushing, cold sintering, and short-range interdiffusion to finally obtain the high pure nano Ta₄HfC₅. A hybrid structure of amorphous SiBCN and nano Ta₄HfC₅ was obtained by adopting a second-step ball-milling. After reactive hot-pressing sintering, amorphous SiBCN has crystallized to 3C-SiC, 6H-SiC, and turbostratic BN(C) phases and Ta₄HfC₅ retained the form of the nanostructure. With the in situ generations of 2.5 wt% Ta₄HfC₅, Ta₄HfC₅ is preferentially distributed within the turbostratic BN(C); however, as Ta₄HfC₅ content further raised to 10 wt%, it mainly distributed in the grain-boundary of BN(C) and SiC. The introduction of Ta₄HfC₅ nanocrystals can effectively improve the flexural strength and fracture toughness of SiBCN ceramics, reaching to 344.1 MPa and 4.52 MPa·m^1/2, respectively. This work has solved the problems of uneven distribution of ultra-high temperature phases in the ceramic matrix, which is beneficial to the real applications of SiBCN ceramics.

Ceramics are usually composed of randomly oriented grains and intergranular phases, so their properties are the statistical average along each direction and show isotropy corresponding to the uniform microstructures. Some methods have been developed to achieve directional grain arrangement and preferred orientation growth during ceramic preparation, and then textured ceramics with anisotropic properties are obtained. Texture microstructures give particular properties to ceramics along specific directions, which can effectively expand their application fields. In this review, typical texturing techniques suitable for ceramic materials, such as hot working, magnetic alignment, and templated grain growth (TGG), are discussed. Several typical textured structural ceramics including α-Al₂O₃ and related nacre bioinspired ceramics, Si₃N₄ and SiAlON, h-BN, MB₂ matrix ultra-high temperature ceramics, MAX phases and their anisotropic properties are presented.