Normal and abnormal grain growth in ceramics-NbC-Co and alumina

Normal and abnormal grain growth has been observed in 70NbC-30Co with varying B concentrations at 1450 degrees C and in alumina with varying impurity and additive concentrations at 1600 degrees C -1650 degrees C as typical systems with and without liquid matrix. The grain growth behavior depends on the roughening of the interfaces as indicated by the grain and grain boundary shapes. When 4% B is added to 70NbC-30Co, the NbC grains in Co-rich liquid matrix are spherical and undergo diffusion controlled normal growth, because the grain-liquid interface is rough. As the B concentration is decreased to 3, 2, 1, and 0%, the NbC grains become more cubic and the tendency for abnormal grain growth increases because of the step growth mechanism of the flat singular surface segments. When compacts of high purity alumina, powder are sintered at 1650 degrees C, the grain boundaries are smoothly curved, indicating their atomically rough structures. With increasing impurity content-in particular SiO2-in the alumina powder, abnormal grain growth becomes more pronounced with increasing number of flat grain boundaries. These singular grain boundaries are expected to move by a step mechanism and thus cause the abnormal grain growth. These results show that the interface roughening and hence the grain growth mode changes gradually with the additive or impurity concentrations. Therefore, the abnormal grain growth cannot be sharply distinguished from the normal grain growth as has been previously suggested in general and for alumina in particular.