Fabrication of dense SiBCN monolith at a lower temperature and its high-temperature performance
Zibo Niu, Daxin Li, Dechang Jia, Zhihua Yang, Kunpeng Lin, Yan Wang, Paolo Colombo, Ralf Riedel, Yu Zhou
Abstract
In this study, a crack-free pyrolysis process of partially cured precursor powder compacts was developed to prepare dense SiBCN monoliths at much lower temperatures (1300℃), thereby circumventing the challenges of sintering densification (>1800℃). Unlike the elastic fracture in over-cured precursors or the viscoelastic deformation in under-cured ones, 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<sub>IC</sub>=1.9 MPa·m<sup>1/2</sup>, HV=7.8 GPa, E=122 GPa), the resulting SiBCN monolith exhibits significantly improved mechanical properties (σ=~304 MPa, K<sub>IC</sub>=3.7 MPa·m<sup>1/2</sup>, HV=10.6 GPa, E=161 GPa) and oxidation resistance. Besides, this study delves into the high-temperature performance of the SiBCN monolith including crystallization and oxidation and determines the oxidation kinetics law transition induced by the 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 oxide layer of SiBCN ceramic exhibits exceptional self-healing effects 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.