High temperature strength and oxidation behaviour of hot-pressed silicon nitride-disilicate ceramics

Twelve different silicon nitride-disilicate ceramics have been fabricated by hot-pressing Si₃N₄ with the oxides of Y, Yb, Ho, Dy, Er, Sm, Ce, Lu, La, Pr, Gd, and Sc that are used as sintering additives. The high temperature strength and oxidation behaviour of the hot-pressed ceramics were investigated and correlated with the cationic radii of the oxide additives. The flexural strength at 1200°C increased, from 666 MPa for Si₃N₄-La₂Si₂O₇ to 965 M Pa for Si₃N₄-Sc₂Si₂O₇ which is correlated with a decreasing cationic radius of the oxide additive. The weight gain during oxidation at 1400°C for 192 h in air decreased, from 0.8732mgcm^-2 for a Si₃N₄-Sm₂Si₂O₇ ceramic to 0.1089mgcm^-2 for a Si₃N₄-Sc₂Si₂O₇ ceramic, which is a function of the decreasing cationic radius of the oxide additive.