The shapes and structures of grain boundaries, formed between the basal (00•1) surface of large alumina grains and randomly oriented small alumina grains, were shown to depend upon additions of SiO2, CaO and MgO. When a sapphire crystal was sintered at 1620C, in contact with high-purity alumina powder, the grain boundaries which formed between the (00•1) sapphire surface and the alumina grains were curved and did not exhibit any hill-and-valley structure when observed using transmission electron microscopy. The observations indicated that the grain boundaries were atomically rough. When 100ppm[mol] of SiO2 and 50ppm[mol] of CaO were added, the (00•1) surfaces of the single crystal and the elongated abnormal grains formed flat grain boundaries with most of the fine matrix grains. These grain boundaries, which maintained their flat shape even at triple junctions, were possible only if they were singular and corresponded to cusps in the polar plots of the grain-boundary energy as a function of the grain-boundary normal. When MgO was added to specimens containing SiO2 and CaO, the flat (00•1) grain boundaries became curved at all scales of observation; thus indicating that they were atomically rough. The grain boundaries between small matrix grains also became atomically rough.

Singular Grain Boundaries in Alumina and Their Roughening Transition. C.W.Park, D.Y.Yoon, J.E.Blendell, C.A.Handwerker: Journal of the American Ceramic Society, 2003, 86[4], 603-11