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·m1/2, 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.