Aqueous gelcasting process for β-Si₄Al₂O₂N₆ ceramics

Abstract

Dense β-Si₄Al₂O₂N₆ ceramics were obtained from α-Si₃N₄, α-Al₂O₃, AlN, and Y₂O₃ upon sintering (for 4 h at 1675°C) green bodies consolidated by an aqueous gelcasting and a conventional dry-powder processing route. The as-purchased AlN powder was treated with H₃PO₄ and Al(H₂PO₄)₃ and dispersed in aqueous-organic premix solution along with α-Si₃N₄, α-Al₂O₃, and Y₂O₃ powder with the help of a commercial amino alcohol-based cationic (Dolapix A88) dispersing agent to obtain suspensions with 45-50 vol% solids loading for gelcasting purpose. The gelcat β-Si₄Al₂O₂N₆ exhibited superior hardness (1423±6 Hv), fracture toughness (3.95±0.3 MPa·m^1/2), and coefficient of thermal expansion (3.798 × 10⁻⁶/°C between 30° and 1000°C) in comparison with the ceramic consolidated by conventional dry-powder pressing route, which exhibited only 1317±5 Hv, 3.30±0.2 MPa·m^1/2 and 3.532 × 10⁻⁶/°C between 30° and 700°C.