Hydrolysis-induced aqueous gelcasting of β-SiAlON-SiO₂ ceramic composites: the effect of AlN additive

Abstract

Dense β-SiAlON-SiO₂ (SiO₂=20, 40, 50, 60, and 80 wt%) ceramic composites have been prepared from β-Si₄Al₂O₂N₆ and fused silica by sintering at 1500°-1750°C for 3-4 h. For comparison purposes, a powder mixture consisting 60 wt%β-Si₄Al₂O₂N₆ and 40 wt% fused silica has been consolidated following a new near-net shape technique based on hydrolysis-induced aqueous gelcasting (GCHAS) and sintered for 3 h at 1750°C. In the GCHAS process, consolidation of suspensions containing 50 vol% solids was achieved by adding a polymerization initiator, a catalyst, and AlN powder equivalent to 1-5 wt% Al₂O₃. Thin-wall radomes consolidated by GCHAS (using AlN equivalent to 5 wt% Al₂O₃ in the suspension) have exhibited green strengths >20 MPa. The sintered materials were characterized for various properties including hardness, fracture toughness, mass loss, shrinkage, coefficient of thermal expansion, and dielectric constant. The Si₂N₂O formed from a powder mixture of 60 wt%β-Si₄Al₂O₂N₆ and 40 wt% fused silica at 1750°C for 3 h exhibited a flexural strength of ~140 MPa, Young's modulus of 214 GPa, coefficient of thermal expansion of 3.5 × 10⁻⁶°C⁻¹, hardness of 1390 kg/mm², fracture toughness of 4.2 MPa·m^1/2, and a dielectric constant of 5.896 and tan δ of 0.002 at 17 GHz.