Abstract
Abstract The grain-boundary microstructure and interface chemistry of Yb2O3/Al2O3-fluxed sintered Si3N4 materials with and without the addition of CaO has been investigated by means of high-resolution and analytical electron microscopy. Each Si3N4 grade examined, exhibited a thin amorphous intergranular layer at both homophase and heterophase boundaries. The presence of calcium within this amorphous layer is confirmed in the CaO-doped materials. Furthermore, no solid solution of CaO within the Si3N4-matrix grains could be detected. The observed variability in the grain-boundary film thickness appears to be closely related to changes in the glass composition, in particular to the Ca content. The addition of CaO resulted in a widening of the intergranular film of up to 0·4 nm. Moreover, postsintering heat-treatment, which induces crystallization of the secondary phase pockets, led to only small changes in the intergranular film thickness of 0·1 nm when compared to the as-sintered specimens, for both CaO-doped and undoped materials.
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