Origin and Growth Kinetics of Platelike Abnormal Grains in Liquid‐Phase‐Sintered Alumina

Abstract

The grain‐growth behavior of Al 2 O 3 compacts with small contents (≤10 wt%) of various liquid‐forming dopants was studied. Equiaxed and/or elongated grains were observed for the following dopants: MgO, CaO, SiO 2 , or CaO + TiO 2 . The platelike grains, defined as the abnormal grains larger than 100 μm with an aspect ratio ≥5 and with flat boundaries along the long axis, were observed when the boundaries were wet with the liquid phase and the codoping satisfied two conditions of size and valence. These dopings were Na 2 O + SiO 2 , CaO + SiO 2 , SrO + SiO 2 , or BaO + SiO 2 . However, an addition of MgO to the Al 2 O 3 doped with CaO + SiO 2 resulted in the change of grain shape from platelike to equiaxial. Equiaxed grains were also observed for the MgO + SiO 2 doping, indicating that two conditions were necessary but not sufficient to develop the platelike grains. The fast growth rate of the platelike grains was explained by an increased interfacial reaction rate due to the codopants. AT the same time the codopants made the basal plane, which appeared as the flat boundaries, the lowest energy plane. The appearance of the platelike grains was favored in compacts with a small grain size and with a narrow size distribution at the onset of abnormal grain growth. Accordingly, the use of starting powders with a small particle size and narrow size distribution, smaller amounts of dopings, and high sintering temperature resulted in an increased number of the platelike grains.