Microstructure and boundary phases of Lu–Al-doped silicon nitride by pressureless sintering

Various silicon nitride materials were fabricated by pressureless sintering using lutetia and alumina as sintering additives. Densification behavior, microstructure, strength and formation of secondary crystalline phases were investigated. The combination of Lu2O3/Al2O3 sintering aids can promote the densification and evolution of a fine grain microstructure of Lu–Al-doped silicon nitride because of the low viscosity of the liquid. The J′ phase given by Lu4Si2−xAlxO7+xN2−x was considered to be secondary crystalline phase at the grain pockets. The composition with a Lu2O3/Al2O3 weight ratio 10/10 had highest strength of 690 ± 50 MPa.