High temperature bending strength and fracture energies of a tape-cast silicon nitride with and without rod-like beta-Si_3N_4 seed addition were measured.The experiment results showed that Lu_2O_3-SiO_2 additives were useful to improve the high temperature bending strength.This was attributable to the formation of a higher melting point grain boundary phase,which was extensively crystallized during sintering.Since the seeded and tape-cast silicon nitride has an anisotropic microstructure,where the elongated grains grown from seeds were preferentially oriented parallel to the casting direction.The strength and fracture energies of the Si_3N_4 were strongly dependent on the tape casting direction.When a stress was applied along with the grain alignment,the bending strength of the samples parallel to tape casting direction measured at 1500 deg C was 738 MPa,which was almost the same as the room temperature bending strength 739 MPa,and the fracture energy of the seeded and tape-cast Si_3N_4 was improved from 301 J/m~2 at room temperature to 781 J/m~2 at 1500 deg C.The large high temperature bending strength and fracture energies were attributable primarily to the aligned fibrous Si_3N_4 grains and sintering additives.
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