In the present investigation, we studied the interface morphology and grain growth behavior of SiC ceramics, under various experimental conditions, sintering atmosphere, sintering temperature, powder bed, and seed crystal addition.
In chapter 3, the effect of sinetring atmosphere on the interface morphology and grain growth behavior of 10 vol%-YAG added SiC has been investigated during liquid phase sinetring. When α-SiC was sintered in an Ar atmosphere, the grain surface was faceted and the abnormal grain growth occurred regardless of whethere the large seed grains were added or not. In contrast, when sintered in $N_2$ or with addition of $Si_3N_4$, the grain surface became defaceted(rough) and the abnormal grain growth did not occur even with addition of the large seed α-SiC gains. This result suggests that the nitrogen dissolved in the liquid made the grain surface rough and induced the normal grain growth by α → β reverse phase transformation. The conditions for normal and abnormal growth depend on the grain surface structure: abnormal growth for the faceted surface and normal growth for the rough surface.
In chapter 4, the effect of powder bed compositions on the grain shape and grain growth behavior has been investigated. When a BN-containing powder bed was used to suppress the thermal decomposition of α-SiC powder compacts during sintering in Ar atmosphere, a considerable microstructural difference between surface and inside of the samples resulted. The surface microstructure consisted only of fine grains, and the shape of the grains was faceted with rounded edges. In contrast, the inside microstructure consisted of fine matrix grains and abnormally large grains, and the shape of the grains was well faceted. XRD and XPS results revealed that boron and nitrogen which were decomposed from BN powder were dissolved in the samples. Effect of boron on grain shape and grain growth behavior was examined using a boron containing powder bed. Boron accelerated the growth of faceted grains. In contrast, as described earlier, nitrogen induced grain surface defaceting and suppressed the abnormal grain growth. These results suggest that the suppressed growth of α-SiC grains in the surface was due to the nitrogen dissolved in the liquid matrix. The faceted grain shape with rounded edges in the surface region can be attributed to a mixed effect of nitrogen and boron, leading to the normal grain growth.
Chapter 5 concerns the effects of α-seed crystals and sintering temperature on the abnormal grain growth of SiC. Faceted α-SiC grains in an $Al_2O_3-Y_2O_3-based$ liquid matrix grew abnormally regardless of whether the large seed grains were added or not, resulting in a duplex structure with a bimodal grain size distribution. The number density of the abnormal grains in the samples increased with an increase of the volume fraction of the seed crystals, and also an increase in sintering temperature. The observation is consistent with the theoretical prediction that the abnormal growth of faceted grains in a liquid matrix occurs when the size difference among grains is large and its propensity increases with a heat-treatment temperature increase.
