Takaaki Nagaoka

Densification process of α′-sialon ceramics

The liquid-phase sintering process of α′-sialon ceramics has been investigated by high-temperature dilatometry and microstructural observation. In addition, isothermal shrinkage measurements have been performed to examine the densification kinetic parameter. It has been confirmed that densification kinetic parameters in the solution-reprecipitation stage are much larger than the rate exponent predicted for the classic liquid-phase sintering model, and are slightly smaller than that for the viscous flow process. Rapid shrinkage was observed in the solution-reprecipitation stage from the results of shrinkage rate, and corresponds to pore elimination by particle rearrangement and cooperative flow of particle/liquid mixture. These processes provide the major contribution to shrinkage. In addition, the liquid flow process occurs when the silica content in the raw powder increases, but it is retarded due to the formation of α′-sialon. It is anticipated that particle rearrangement and cooperative flow, as well as liquid-flow processes, take place in the solution-reprecipitation stage of sintering of Si3N4-based materials, and cause a large amount of shrinkage.

Microstructure control of silicon nitride by seeding rod-like β-Si3N4 particles

ABSTRACT Rod-like β-Si3N4 single crystal particles with a diameter of about 1μm and length of 5μm were grown from a mixture of β-Si3N4 and oxides, followed by acid rinse treatment to remove residual glassy phase. These β-Si3N4 particles were used as seed crystals to control microstructure of sintered silicon nitride under a low nitrogen gas pressure. The effect of seeding on the microstructure development and mechanical properties of silicon nitride were investigated.