Nonepitaxial heterogeneous nucleation of alpha-sialon in the Ca-doped system

Ca-alpha -sialon compacts pressureless-sintered to intermediate temperature s, which consisted of both alpha -sialon and unreacted alpha -Si3N4 grains, were investigated with transmission electron microscopy for an overall com position Ca1.8Si6.6Al5.4O1.8N14.2. Special attention was paid to identifica tion of the possible crystallographic orientation between alpha -sialon and the alpha -Si3N4 particles. In contrast to the frequently occurring hetero epitaxial nucleation of alpha -sialon in rare-earth-doped samples with low x values, this study showed that most of the newly formed alpha -sialon gra ins had no epitaxial orientation relationship with the alpha -Si3N4 particl es, suggesting nonepitaxial heterogeneous nucleation to be a more probable mechanism for the Ca-alpha -sialon phase with high Ca concentrations.