Hydrolysis-Induced Aqueous Gelcasting of β-SiAlON-SiO2 Ceramic Composites: The Effect of AlN Additive

Dense beta-SiAlON-SiO2 (SiO2=20, 40, 50, 60, and 80 wt%) ceramic composites have been prepared from beta-Si4Al2O2N6 and fused silica by sintering at 1500 degrees-1750 degrees C for 3-4 h. For comparison purposes, a powder mixture consisting 60 wt% beta-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 degrees 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% beta-Si4Al2O2N6 and 40 wt% fused silica at 1750 degrees C for 3 h exhibited a flexural strength of similar to 140 MPa, Young's modulus of 214 GPa, coefficient of thermal expansion of 3.5 x 10-6 degrees C-1, hardness of 1390 kg/mm2, fracture toughness of 4.2 MPa center dot m1/2, and a dielectric constant of 5.896 and tan delta of 0.002 at 17 GHz.