Abrasive Materials

materials of high hardness for machining metals, ceramics, rock, minerals, glass, wood, leather, rubber, and the like. Artificial abrasives (electrocorundum, silicon carbide, boron carbide, monocorun-dum, synthetic diamonds, among others) have been in use since the end of the past century, while natural abrasives (flint, emery, garnet, pumice, corundum, diamonds) have been used from earlier times.

The basic characteristics of abrasives are hardness, strength, and ductility; the shape of the abrasive grain; abrasive power; and grain grit (grain size). The hardness of abrasives is determined (in meganewtons [MN] per m²) by pressing a diamond pyramid into the surface of the material to be tested (for example, 11,000–11,300 MN/m² for quartz, 18,000–24,000 for electrocorundum, 84,250–100,000 for diamond). The hardness is also defined in kilograms-force (kgf) per mm² (lkgf/mm² ≈ 10 MN/m²). As the hardness of abrasives increases, the resistance to cutting forces is improved. The compressive strength of abrasives is several times greater than their bending and tensile strength. The tensile strength and compressive strength of abrasives decline with increasing grinding temperature. The abrasive grain is usually a crystalline fragment (crystallite), less commonly a single crystal or aggregate consisting of a number of fine crystals (polycrystals). The cutting edge of the grain is the side formed by any pair of intersecting crystallographic planes. The grain can have approximately equal height, width, and thickness (isometric shape) or may exhibit a gladiate (swordlike) or tabular shape. The shape is determined by the type of abrasive and by the degree of pulverization of the original grain. The isometric grain shape or some shape close to it is an efficient one, since each grain acts as a cutter. The most highly favored form is acicular (needle-shaped). The grain has several faces forming vertices having angles from 30° to 130°, with radii of curvature of the grains ranging from 200 to 4 microns. The angles and radii of curvature are smaller in synthetic diamond grains than in natural diamond grains, so that it is possible to produce finer chips. The term abrasive power is a measure of the mass of the material removed by grinding before the grains become dull. Abrasives are arranged in the order of their abrasive power: diamond, cubic boron nitride, silicon carbide, monocorundum, electrocorundum, emery, flint. Abrasive power depends on the shape of the materials to be ground, on grinding conditions, and on the ductility and strength of the grains. The fewer the impurities present in the abrasive materials, the greater the abrasive power will be. Graininess, or grit size, describes the size and homogeneity of the abrasive grains; it is determined by classifying the grains in their linear dimensions by the method of screen analysis, by sedimentation in a fluid, or by some other method. The grit size is established in accordance with the linear dimensions of the grain of the basic mesh fraction. The more homogeneous the abrasive grains are in shape and size, the better their working qualities will be. Abrasive grit is governed by standards. Machining with the aid of abrasives is characterized by the simultaneous participation of a large number of randomly located cutting faces of grains in the cutting process. The hardness and strength of the abrasive tool makes it possible to resort to high cutting speeds, which, when combined with a large number of blades operating simultaneously, remove a fairly large volume of material. On the other hand, abrasives can be used in the finest precision machining—for example, finishing.

Abrasives are used in the form of grains bonded together in tools of varying shapes and designs, or applied to a flexible base (fabric, paper, etc.) in the form of an abrasive cloth. They may also be used in the unbonded state as powders, pastes, or suspensions.