ABNORMAL GRAIN-GROWTH IN ALUMINA-DOPED HAFNIA CERAMICS

Hafnia (HfO2) ceramics containing 0.0, 5.0, and 10.0 vol % Al2O3, respectively, were sintered, at 1600-degrees-C for various periods from 2-24 h. Abnormal grain growth was found to occur in the Al2O3-containing compositions. Hafnia containing 5.0 vol % Al2O3 exhibits an average grain size of almost double that of the Al2O3-free hafnia matrix, coupled with a much wider grain-size distribution. The material containing 10.0 vol % Al2O3 shows a smaller average grain size than the composition containing 5.0 vol % Al2O3. However, its average grain size is still larger than that of the Al2O3-free hafnia on sintering at 1600-degrees-C for more than 8 h. Microstructural characterization, carried out using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with an energy dispersive analysis facility (EDX), indicated that there existed a continuous segregant layer at the grain boundaries and grain junctions in the Al2O3-free hafnia. Hafnia exhibits a low solubility in the segregant layer phase which inhibits the growth of the hafnia grains. The Al2O3 particles act as a scavenger for the silicon-rich glassy phase, damaging the continuous nature of the boundary segregant layer and promoting grain growth in the Al2O3-doped hafnia ceramics. The microstructural development at the sintering temperature is an overall result of the concurrent scavenger effect and grain pinning by the Al2O3 particles.