Structure of Sapphire Bicrystal Boundaries Produced by Liquid-Phase Sintering

The structure and composition of sapphire bicrystal boundaries produced by liquid-phase sintering depend on the crystallographic misorientation of the crystals across the boundary and on the orientation of the boundary. Basal twist boundaries of 15 or 30 degrees were not wetted by glass, but contained significant amounts of Ca and Si at the boundary. For tilt boundaries of 7 or 12 degrees, the glass wetted segments of boundaries that contained the basal plane of either crystal. Boundary segments with orientations of 40 degrees or more from the basal plane, however, were dewetted i.e., dry. Boundary segments oriented less than 40 degrees from the basal orientation were partially wetted, consisting of segments of wetted and dry grain boundaries. For the 12 degree tilt boundary, Ca and Si could be detected on portions of the boundary that contained no glass. For bicrystal boundaries having tilts of equal-to-or-less-than 4 degrees, dewetting occurred for all observed boundary orientations. Basal-oriented segments in these small angle tilt boundaries contained noticeable concentrations of adsorbed Ca and Si, while nonbasal segments were apparently free of Ca and Si.