An aqueous gelcasting route to dense β-Si₄Al₂O₂N₆-0.5SiO₂ ceramics

Abstract

Aqueous particulate slurries containing 45-50 vol% α-Si₃N₄, α-Al₂O₃, and Y₂O₃ powder mixtures were gelcast and sintered for 2-4 h at 1675°-1800°C to obtain dense β-Si₄Al₂O₂N₆-0.5SiO₂ ceramics. For comparison, dense ceramics of the same composition, together with a stoichiometric β-Si₄Al₂O₂N₆, were also prepared following a conventional dry-powder pressing route. The sintered materials were thoroughly characterized for bulk density, apparent porosity, water-absorption capacity, X-ray diffraction phase, microstructure, hardness, fracture toughness, coefficient of thermal expansion (CTE), and dielectric constant at 16-18 GHz frequency. The characterization results suggest that the sintered properties of β-Si₄Al₂O₂N₆-0.5SiO₂ are little influenced by the processing route, and the in situ generated SiO₂ was found to reduce the CTE and dielectric constant of β-Si₄Al₂O₂N₆ considerably. An aqueous particulate slurry containing a 48 vol% β-Si₄Al₂O₂N₆-0.5SiO₂ precursor mixture was successfully gelcast in an indigenously designed and fabricated aluminum mold to fabricate defect-free crucibles of 500 mL volume. The gelcast β-Si₄Al₂O₂N₆-0.5SiO₂ obtained from a slurry containing 48 vol% solids exhibited a bulk density of 3.13 g/cm³, a β-SiAlON phase of ~90%, a CTE of 3.197 × 10⁻⁶ °C⁻¹ (between 30° and 1000°C), a fracture toughness of ~3.42 MPa·m^1/2, a three-point bend strength of ~199 MPa, and a dielectric constant of ~6.32 at 17 GHz frequency after sintering for 4 h at 1750°C with 7 wt% Y₂O₃.