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, and E = 122 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 E = 161 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.

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.

Bulk Si₂BC₃N ceramics were reinforced with SiC coated multi-walled carbon nanotubes (MWCNTs). The phase compositions, mechanical properties, and thermal shock resistance, as well as the oxidation resistance of the designed Si₂BC₃N ceramics were comparatively investigated. The results show that nano SiC coating can be formed on MWCNTs through pyrolyzing polysilazane, which improves the oxidation resistance of MWCNTs. A stronger chemical bonding is formed between the SiC coated MWCNTs and SiC particles, contributing to improved flexural strength (532.1 MPa) and fracture toughness (6.66 MPa·m^1/2). Besides, the 2 vol% SiC coated MWCNTs reinforced Si₂BC₃N ceramics maintains much higher residual strength (193.0 MPa) after thermal shock test at 1000 ℃. The enhanced properties should be attributed to: (1) the breaking of MWCNTs and the debonding between MWCNTs and SiC interfaces, which leads to more energy dissipation; (2) the rough surfaces of SiC coated MWCNTs increase the adhesion strength during the "pull out" of MWCNTs.

Multi-walled carbon nanotubes (MWCNTs) reinforced Si₂BC₃N ceramics were prepared through mechanical alloying (MA) and following spark plasma sintering (SPS). The thermal shock resistance of Si₂BC₃N ceramics was evaluated comparatively through ice water quenching test and theoretical prediction. Furthermore, the oxidation resistance of MWCNTs incorporated Si₂BC₃N ceramics was evaluated under high temperature. The results show that the calculated parameters such as the critical thermal shock temperature (R) and the thermal stresses resistance (R_st), as well as the toughness (R′′′′) are improved with addition of 1 vol% MWCNTs. In addition, the crack propagation resistance of 1 vol% MWCNTs incorporated Si₂BC₃N ceramics is obviously improved through generating more tortuous crack propagation paths attributing to the “crack bridging”, “pull-out”, and “crack deflection” mechanisms of MWCNTs. Therefore, the residual strengths of 1 vol% MWCNTs containing specimens remained the highest after the thermal shock tests. Besides, the present work also reveals that the oxidation resistance is more sensitive to relative density than MWCNTs addition.

SiC fiber reinforced SiBCN ceramic matrix composites (CMCs) have been prepared by mechanical alloying and consolidated by hot pressing. During the sintering process, amorphous SiC fibers crystallized seriously and transformed into β-SiC. Meanwhile, the interfacial carbothermal reactions caused the strong bonding between the matrix and fibers. As a result, SiC_f/SiBCN fractured in a typical catastrophic manner. Room-temperature mechanical properties reached the maximums for the CMC samples sintered at 1900 ℃/60 MPa/30 min. The density, flexural strength, Young’s modulus and fracture toughness are 2.56±0.02 g/cm³, 284.3±17.9 MPa, 183.5±11.1 GPa and 2.78±0.14 MPa·m^1/2, respectively.