High-temperature mechanical properties of sinter-forged silicon nitride with ytterbia additive

Anisotropic silicon nitride with ytterbia additive was successfully fabricated by using a sinter-forging technique. The sinter-forged specimen had a strongly anisotropic microstructure where rod-like silicon nitride grains preferentially aligned perpendicular to the forging direction. The specimen exhibited higher strength and higher fracture energy compared to the conventionally hot-pressed specimen. These superior mechanical properties of the sinter-forged silicon nitride were attributed to grain bridging and pullout enhanced by grain alignment. At elevated temperatures, softening of the grain boundary glassy phase and melting of the secondary crystalline phase should lead to degradation of strength and increment of fracture energy.