Comparison of sgregation behaviors for special and general boundaries in polycrystalline Al₂O₃ with SiO₂-TiO₂ impurities

A systematic study of grain boundaries with an elongated grain morphology in TiO₂doped polycrystalline Al₂O₃ materials with SiO₂ impurities is reported in this paper. Over 20 grain boundaries, all having common (0001) basal plane surface on one side, were investigated by using HRTEM and analytical TEM. A few of them were basal plane twin boundaries or other non-wetting boundaries, and the majority of grain boundaries were covered by amorphous phases, either as continuous films or with small pockets bounded by surface facets. Si and Ti were segregated to all boundaries, however, two categories of segregation were observed. Excesses of both segregants were between 1.0-4.0 atom/nm² at special and non-wetting boundaries, while they were in the range of 7.0-30 atom/nm ² at boundaries with amorphous phases. A variation of amorphous film thickness was also observed, which has a clear relation with Si excess level while Ti excess remained at the same level. This observation suggests that the amorphous phases were primarily made of SiO₂ but Ti segregants were attached to grain surfaces. The variation in thickness of SiO₂-based film is strongly related to the surface structure of anisotropic Al ₂O₃ grains.