Hot-pressed silicon nitride ceramics with Lu2O3 additives: elastic moduli and fracture toughness

The elastic moduli and fracture toughness of hot-pressed Si3N4 ceramics, using 1.68, 3.33, 6.52 and 12.51 wt.% of Lu2O3 additives, were studied by means of ultrasonic measurements and Vickers indentation crack size measurements, respectively. The elastic moduli were found to increase with the amount of Lu2O3 additive for silicon nitride which was not completely densified. However, the elastic moduli remained nearly constant independent on the amount of additive for completely dense silicon nitride. The elastic moduli of completely dense silicon nitride should reach the following average values: shear modulus G=132 GPa, Youngʹs modulus E=338 GPa, bulk modulus B=255 GPa and Poissonʹs ratio ν=0.28. On the other hand, the fracture toughness showed a clear dependence of additive amount for both incompletely and completely dense silicon nitride ceramics. The fracture toughness increased with additive amount over the range 1.68 to 12.51 wt.% Lu2O3. The improvement in the fracture toughness with the amount of additive was mainly attributed to elongated grain growth during the sintering process.