THE MICROHARDNESS OF [[sigma]]-Al₂O₃ GROWN BY THE SKULL-MELTING TECHNIQUE. By W. Guse, C. Lathe, M. Kriens, H. Saalfeld, Mineralogisch-Petrographisches Institut der Universitat Hamburg, 20146 Hamburg, Germany

Single crystals of [[sigma]]-Al₂O₃ with spinel-type structure were grown by a modified skull-melting technique and investigated for microhardness. Due to their different hardness they have to be divided into two groups: 1. H_K ~ 21 GPa, 2. H_K ~ 30 GPa. SAALFELD, H. & GUSE, W. (1991: "The mysterious [[sigma]]-Al₂O₃".-N.Jb., Miner. Abh. 163; 159-167) reported that [[sigma]]-Al₂O₃ single crystals are strongly disordered. Wei[[beta]]enberg photographs of the plane hkl revealed that there are at least two types of disorder. The first is represented by photographs showing strong spinel reflections (111, 311, 511) surrounded by weak satellites, the other by having no spinel reflections any more, and the whole intensity is distributed into the satellites. The X-ray analysis of crystals type I revealed that they all belong to the less disordered spinel-structure type; i.e. all spinel reflections clearly exist. The crystals of type II all produce X-ray-photographs having no spinel reflection hkl odd. These crystals are more disordered and therefore more strained than those of type I and exhibit the greater hardness with maximum values up to H_K = 30,9 + 1,5 GPa. The new alumina phase [[sigma]]-Al₂O₃ is not showing any hardness anisotropy and exhibits an extremely high microhardness (type II). Compared to [[sigma]]-Al₂O₃, the hardness properties of [[sigma]]-Al₂O₃ are dominant and therefore [[sigma]]-Al₂O₃ might be an interesting material for the industrial application.