The structure and composition of sapphire bicrystal boundaries, produced by liquid-phase sintering, were investigated. Basal 15° or 30° twist boundaries were not wetted by glass, but contained significant amounts of Ca and Si at the boundary. In the case of 7° or 12° tilt boundaries, the glass wetted segments of boundary that contained the basal plane of either crystal. Boundary segments with deviations of 40° or more from the basal plane were de-wetted. Boundary segments which were oriented less than about 40° from the basal orientation were partially wetted, and consisted of segments of wetted and de-wetted grain boundary. In the case of the 12° tilt boundary, Ca and Si could be detected on portions of the boundary that contained no glass. In bicrystal boundaries having tilts of up to 4°, de-wetting occurred for all observed boundary orientations. Basal-oriented segments in these small-angle tilt boundaries contained noticeable concentrations of adsorbed Ca and Si, while non-basal segments were apparently free of Ca and Si. Most of the results could be explained by a combined Wulff-plot construction, which predicted partially-wetted grain boundaries and so-called missing angles for unwetted grain boundaries. Some results could not be explained in this way. These included growth step ledges bounded by non-equilibrium facet planes.

Structure of Sapphire Bicrystal Boundaries Produced by Liquid-Phase Sintering. B.J.Hockey, S.M.Wiederhorn, J.E.Blendell, J.S.Lee, M.K.Kang: Journal of the American Ceramic Society, 2003, 86[4], 612-22