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·m1/2, and a dielectric constant of 5.896 and tan δ of 0.002 at 17 GHz.