Hydrolysis-induced aqueous gelcasting of β-SiAlON-SiO<SUB>2</SUB> ceramic composites: the effect of AlN additive
Abstract
Dense β-SiAlON-SiO<SUB>2</SUB> (SiO<SUB>2</SUB>=20, 40, 50, 60, and 80 wt%) ceramic composites have been prepared from β-Si<SUB>4</SUB>Al<SUB>2</SUB>O<SUB>2</SUB>N<SUB>6</SUB> and fused silica by sintering at 1500°-1750°C for 3-4 h. For comparison purposes, a powder mixture consisting 60 wt%β-Si<SUB>4</SUB>Al<SUB>2</SUB>O<SUB>2</SUB>N<SUB>6</SUB> 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% Al<SUB>2</SUB>O<SUB>3</SUB>. Thin-wall radomes consolidated by GCHAS (using AlN equivalent to 5 wt% Al<SUB>2</SUB>O<SUB>3</SUB> 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 Si<SUB>2</SUB>N<SUB>2</SUB>O formed from a powder mixture of 60 wt%β-Si<SUB>4</SUB>Al<SUB>2</SUB>O<SUB>2</SUB>N<SUB>6</SUB> 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<SUP>−6</SUP>°C<SUP>−1</SUP>, hardness of 1390 kg/mm<SUP>2</SUP>, fracture toughness of 4.2 MPa·m<SUP>1/2</SUP>, and a dielectric constant of 5.896 and tan δ of 0.002 at 17 GHzSimilar works
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