Fabrication and properties of porous boron nitride/silicon oxynitride ceramic composites via gas pressure sintering

Highlights: • Pore structure was modified by controlling the degree of Si₂N₂O decomposition. • Li₂O accelerates the Si₂N₂O decomposition while BN alleviates it. • Borosilicate glass seals the surface and protects the crystals from oxidation damage. • One of the samples could resist a thermal shock of 1300 °C in air atmosphere. • The composites exhibit both good mechanical properties and dielectric properties. - Abstract: Silicon oxynitride (Si₂N₂O) based wave transparent composites with different volumes of hexagonal boron nitride (h-BN) were prepared by gas pressure sintering at 1700 °C with 5.0–10.0 mol% Li₂O as additive. The effects of Li₂O and BN contents on phase, microstructure, mechanical and dielectric properties of the composites were investigated. The decomposition of Si₂N₂O and pore structures was influenced by Li₂O content, while the incorporation of h-BN effectively alleviated Si₂N₂O decomposition. The BN/Si₂N₂O composites could withstand a thermal shock of 1100 °C without strength degradation in N₂, while only the samples with adequate porosity exhibited high thermal shock resistance in air. The high thermal shock resistance was attributed to the crack-healing ability of the composites. The as-sintered BN/Si₂N₂O ceramics showed both low dielectric constant (ε 2N₂O composites could be used as promising high temperature wave transparent material.