Pressureless Sintered Nitride Composites in the AlN-Al2O3System

Particulate nitride composites have been fabricated by sintering the compacted powder of AlN and 5 - 64.3 mol% Al₂O₃, with a small addition of Y₂O₃ (Y₂O₃ /AlN, 1 wt%), in 1-atm nitrogen gas at 1650 - 1900℃. The composites were characterized in terms of sintering behavior, phase relations, microstructure and thermal shock resistance. AlN, 27R AlN pseudopolytype, and alminium oxynitride (AlON, 5AlN·9Al₂O₃) were found to existin the sintered material. Regardless of batch composition, the AlN-Al₂O₃ powder compacts exhibited similar sintering behavior; however, the degree of shrinkage commonly increased with increasing Al₂O₃ content, consequently giving high sintered bulk density. By increasing the Al₂O₃ addition up to ≥ 50 mol%, the matrix phase in the sintered material was converted from AlN or 27R to AlON. Above 1850℃, a liquid phase was formed by the reaction of Al₂O₃ with AlN, aided by Y₂O₃ and mainly existed at the grain boundaries of AlON. Thermal shock resistance was superior in the sintered composite consisting of AlON with dispersed AlN or AlN matrix phase.