CONTROL OF CALCIUM HEXALUMINATE GRAIN MORPHOLOGY IN IN-SITU TOUGHENEDCERAMIC COMPOSITES

The influence of processing conditions on the morphology of calcium he xaluminate (CA(6)) grains in Al2O3:30 vol % CaO . 6Al(2)O(3) (CA(6)) c eram ic composites was investigated. Specimens were prepared by in-sit u reaction sintering using precursor powders of alumina, and either ca lcium carbonate or calcium oxide. In some samples, 1 vol % anorthite g lass was added as a sintering aid. X-ray diffraction was used to study the phase development in the as-calcined and sintered states. The res ultant 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 w hen CaCO3 was used as the starting calcium-rich powder. In contrast, s amples prepared using CaO resulted in equiaxed CA(6) grains. This resu lt was observed to be independent of the anorthite glass addition. The findings are rationalized in terms of distinct CA(6) reaction mechani sms, resulting from differences in the reactivity of the powders durin g the early stages of calcining.