The kinetics of microstructural change in sialon, a potentially important high-temperature engineering material, has been studied. Firstly, rate controlling mechanism of grain growth and α → β'$ phase transition in β'-sialon has been studied. Two types of experiments have been performed to reveal the mechanism. One is to observe the change of average grain size and fraction of residual phase with matrix phase(liquid) content at β'-sialon with constant substitution ratio, z, and the other is to observe the change with z-value at β'-sialon with a constant matrix content. The grain growth and the phase transition rate increase rapidly with inceasing z-value, but do not change with increasing the matrix content. These results show that the grain growth and the α → β' phase transition are controlled by solid-liquid interface reaction.
Secondly, microstructural change during the densification of β'-sialon has been investigated and compared with the present theories of liquid phase sintering. Liquid coagulation and redistribution occur in this system, similarly to the W-Ni system investigated earlier. The result shows that the liquid phase is connected completely and flows freely during sintering. The specimens densify more rapidly with increasing the matrix content and the z-value. These result suggest that the analysis of Park, Cho, and Yoon could explain more accurately the liquid phase sintering behavior of β'-sialon than the Kingery model.
Thirdly, the effect of phase (α , β) of $Si_3N_4$ raw material on the densification and the grain growth of silicon nitride ceramics has been studied. β-sialon (and β-silicon nitride) prepared from $α-Si_3N_4$ powder shows lower density than that from $β-Si_3N_4$ after α → β (β') phase transition; this result differs from those obtained earlier. The difference may be due to the grain morphology difference. Grains of β -silicon nitride prepared from $β-Si_3N_4$ raw material show equiaxed morphology independently of sintering time. But, when the sintered compact is resintered in a liquid bath, the β grains become rodlike. This result shows that the stable growth shape of β-silicon nitride is rodlike.
Fourthly, the kinetics of α' → β' phase transition and grain morphology of α' and β' phases has been studied by immersing the presintered α'-sialon in a liquid bath and resintering for a long time. The kinetics of α' → β' phase transition show sigmoidal shape, proposed by Avrami. Each grain of α' and β' has been ascertained by EDS. The grain shapes of α' and β', abnormal grain growth of β'-sialon, coalescences of α' and β' , and chemically induced interface migration of reprecipitated β' grains have been observed and are discussed.
