Crystallinity dependence of high-temperature oxidation of silicoboron carbonitride monoliths

Comparative investigation of performances of metastable materials in amorphous and crystalline states is pivotal to exploring their applications in engineering. The oxidation behavior and kinetics of the partially crystalline Si₂BC₃N (PCS) monoliths, i.e. amorphous Si₂BC₃N with precipitation of nanocrystallites (10−30 nm) of SiC and BN(C), were investigated at 1500−1700 °C with dwell time ranging from 0.5 h to 16 h. Oxidation leads to the formation of amorphous SiO₂ and cristobalite, and more sufficient crystallization of PCS. Due to the release of gases including CO, CO₂, N₂ and evaporation of B₂O₃, PCS undergoes weight loss at 1500−1700 °C. Loose oxide scales with low adhesion strength are formed at 1500−1600 °C, while the scales are highly dense, adherent at 1700 °C indicating good oxidation resistance of PCS. Compared to completely amorphous Si₂BC₃N (CAS), PCS exhibits lower activation energy of ∼50.0 kJ‧mol⁻¹ for 1500−1600 °C/≤16 h, and higher parabolic rate constants (42.5 μm²‧h⁻¹ for 1500 °C/≤16 h, 172.2 μm²‧h⁻¹ for 1600 °C/≤16 h, and 120.4 μm²‧h⁻¹ for 1700 °C/≤4 h), which suggests the inferior oxidation resistance of PCS. One most possible reason is that the changes in superficial chemical compositions and reactivity during crystallization cause larger oxygen uptake on PCS surfaces.