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 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.