Structural alumina ceramics used in ballistic armour and wear applications with varying alumina contents and manufactured using both slip casting and dry pressing techniques, have been investigated and characterized in terms of their hardness, elastic modulus, fracture toughness, and microstructural characteristics. For a given alumina content, fracture toughness decreases with increasing hardness. Dry pressed samples show slightly higher hardness, and lower fracture toughness for the same alumina content. The hardness, elastic modulus and fracture toughness are higher for the 98% alumina samples while the differences between the lower alumina samples (95 and 91%) are negligible. The grain sizes are bimodal with the majority less than or equal to3 mum and the size range narrows with decreasing alumina content. The microstructures are composed of a matrix phase, corundum (alpha -Al2O3), grain boundary phases consisting of a glassy phase with varying Al2O3, SiO2, and CaO contents, a crystalline phase, triclinic anorthite (CaAl2Si2O8), and an additional phase, spinel (MgAl2O4), in the lower alumina samples. The proportion of the boundary phase increases with decreasing alumina content and no effect of fabrication method is observed. (C) 2001 Kluwer Academic Publishers.
