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Analytical and high‐resolution transmission electron microscopies were used to study the structure and chemistry of two‐grain boundaries and three‐grain junctions in polycrystalline alumina sintered with additions of between 0 and 10 wt% calcium silicate. Addition of calcium silicate greatly aided full densification and resulted in the presence of a continuous, amorphous grain‐boundary film at the majority of the two‐grain boundaries, the thickness of which was independent of the bulk level of additive. The chemistry of the glass at the grain boundaries and the three‐grain junctions was notably different. The grain boundaries showed strong segregation of calcium, whereas both silicon and calcium appeared to favor triple pockets and larger‐volume facets at the grain boundaries. Grain‐triple‐pocket interfaces also showed segregation of calcium. The overall extent of segregation appeared to be independent of the additive level. The amorphous grain‐boundary film was of nominal composition CaO6Al2O3 and contained predominantly octahedrally coordinated aluminum within the glass. The triple pockets were generally of a composition within the primary‐phase field of anorthite and contained tetrahedrally coordinated aluminum and silicon.

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