Dense β-SiAlONs consolidated by a modified hydrolysis-assisted solidification route

Abstract

Dense β-Si₄Al₂O₂N₆ materials were fabricated by a modified hydrolysis-assisted solidification (HAS) route from aqueous slurries containing 48-50 vol.% solids, in which 5-22 wt.% of the required α-Al₂O₃ was replaced by equivalent amounts of unprotected aluminium nitride (AlN) powder to promote consolidation via AlN hydrolysis. A fixed amount (9.37 wt.%) of AlN passivated against hydrolysis with a coating phosphate layer was also added to all the samples consolidated by the modified HAS method. The aqueous slurries were cast in non-porous moulds, allowed to set and dried before sintering at 1675 °C for 4 h. For comparison purposes, ceramics with the same predicted final composition (having 64.33% α-Si₃N₄, 23.36% α-Al₂O₃, 9.37% AlN and 7% Y₂O₃ as starting materials) were also consolidated by a conventional dry-powder pressing (CDPP). The β-Si₄Al₂O₂N₆ ceramics consolidated by the modified HAS route exhibited superior outstanding properties (bulk density, apparent porosity, water absorption capacity, hardness and fracture toughness) in comparison to the traditional dry-powder pressing route.