It was noted that grain boundaries in pure alumina compacts, sintered at 1400C, were smoothly curved; thus indicating that they had atomically rough structures. When these specimens were heat-treated at 900 and 1100C, a small fraction of the grain boundaries developed hill-and-valley shapes, or kinked shapes with flat segments. Some of the flat boundary segments lay on the {01▪¯2} plane of one of the grain pairs. These grain boundaries therefore appeared to become singular at those temperatures. When a corundum crystal with a basal surface was sintered in alumina powder at 1400C, all of the grain boundaries which formed between the corundum basal surface and small grains, as well as those between small grains, were smoothly curved – thereby indicating their rough structure. When heat-treated (900C, 72h) some 30% of the grain boundaries, between the corundum basal surface and the small grains, developed kinks with flat boundary segments. Some of these flat segments lay on the basal plane of the corundum. When heat-treated again at 1400C, all of the grain boundaries were curved; thus indicating that they had become reversibly rough. It was concluded that at least some of the grain boundaries in alumina underwent roughening-singular transitions at between 900 and 1100C.

Facet-Defacet Transition of Grain Boundaries in Alumina. M.J.Kim, Y.K.Cho, D.Y.Yoon: Journal of the American Ceramic Society, 2004, 87[3], 443-8