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·m1/2), 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·m1/2 and 3.532 × 10−6/°C between 30° and 700°C.