3.3.1 Importance of Crystal Size
A limitation of the electrofusion route is that the resulting abrasive crystal structure is very large; an abrasive grain may consist of only one to three crystals. Consequently, when grain fracture occurs, the resulting particle loss may be a large proportion of the whole grain. This results in inefficient grit use. One way to avoid this is to dramatically reduce the crystallite size.
3.3.2 Microcrystalline Grits
The earliest grades of microcrystalline grits were produced in 1963 (U. S. Patent 3,079,243) by compacting a fine-grain bauxite slurry, granulating to the desired grit size, and sintering at 1,500°C. The grain shape and aspect ratio could even be controlled by extruding the slurry.
3.3.3 Seeded Gel Abrasive
The most significant development, however, probably since the invention of the Higgins furnace, was the release in 1986 of SG (seeded gel) abrasive by The Norton Company (U. S. Patents 4,312,827 1982; 4,623,364 1986). This abrasive was a natural outcome of the wave of technology sweeping the ceramics industry at that time to develop high strength engineering ceramics using chemical precipitation methods. In fact, this class of abrasives is commonly termed “ceramic.” SG is produced by a chemical process whereby MgO is first precipitated to create 50-nm-sized alumina-magnesia spinel seed crystals in a precursor of boehmite. The resulting gel is dried, granulated to size, and sintered at 1,200°C. The grains produced are composed of a single-phase a-alumina structure with a crystallite size of about 0.2 pm. Again, defects from crushing are avoided; the resulting abrasive is unusually tough but self-sharpening because fracture now occurs at the micron level.