Abstract: Abrasive grain with significantly improved toughness is obtained by pressureless sintering of .alpha.-silicon carbide powder with oxidic sinter additives, especially aluminum oxide/yttrium oxide, which is also suitable for those applications where an ordinary silicon carbide abrasive grain is too brittle.

Abstract: Aluminum oxide powders having a high .alpha. proportion, crystallite sizes of approximately 0.1 .mu.m and narrow particle size distribution are prepared from customary Bayer aluminum hydroxide (gibbsite) or from aluminum oxide calcined at a low temperature. The starting material is wet-disagglomerated, mixed with .alpha.-aluminum oxide nuclei, dried and calcined. The calcination produces agglomerates which already have the crystallite size required, and which only need to be disagglomerated to the particle size required. The .alpha.-aluminum oxide powders prepared by such process are particularly suitable as a starting material for the production of high-performance ceramics, for example, of blanking dies.

Abstract: A sintered material, especially usuable as an abrasive, based on .alpha.-aluminum oxide is produced from Al(OH).sub.3 or aluminum oxide calcined at a lower temperature. Aluminum hydroxide resulting in the Bayer process can be used as the Al(OH).sub.3. The sintered material is distinguished by a crystallite size of standard <0.5 .mu.m, high density and great hardness.