Control of calcium hexaluminate grain morphology in in-situ toughened ceramic composites

The influence of processing conditions on the morphology of calcium hexaluminate (CA(6)) grains in Al2O3:30 vol % CaO . 6Al(2)O(3) (CA(6)) ceram ic composites was investigated. Specimens were prepared by in-situ reaction sintering using precursor powders of alumina, and either calcium carbonate or calcium oxide. In some samples, 1 vol % anorthite glass was added as a sintering aid. X-ray diffraction was used to study the phase development in the as-calcined and sintered states. The resultant microstructures were characterized using both scanning electron microscopy (SEM), and imaging secondary ion mass spectrometry (SIMS). It was found that the CA(6) grains developed a platelike morphology when CaCO3 was used as the starting calcium-rich powder. In contrast, samples prepared using CaO resulted in equiaxed CA(6) grains. This result was observed to be independent of the anorthite glass addition. The findings are rationalized in terms of distinct CA(6) reaction mechanisms, resulting from differences in the reactivity of the powders during the early stages of calcining.