Fabrication of dense SiBCN monolith at a lower temperature and its high-temperature performance

Abstract

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, KIC = 1.9 MPa·m1/2, Vickers hardness (HV) = 7.8 GPa, and E = 122 GPa), the resulting SiBCN monolith exhibited significantly improved mechanical properties (σ = ~304 MPa, KIC = 3.7 MPa·m1/2, 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.

Keywords

• silicon boron carbonitride (sibcn) ceramics
• crack-free pyrolysis
• amorphous structure
• oxidation mechanisms
• kinetic law