The abrasive materials can be classified into two groups, natural and artificial (or synthetic) ones. The natural abrasives are generally referred to as those that can be found in nature and because of that, they can contain many impurities and vary in quality. Emery, corundum, quartz, flint, garnet, diamond, tripoli, diatomaceous earth, sandstone, pumice, and natural sharpening stones are some of them. On the other hand, artificial abrasives were first developed in the late nineteenth century and overcame the problems of impurities and inconsistencies, since their manufacture could be carefully controlled. Some manufactured abrasives are silicon carbide, aluminum oxide, glass, and the metallic abrasives such as steel wool and steel shot and grit. Both natural and artificial abrasives are still in use depending on the application. Nevertheless, most natural abrasives have been replaced by artificial ones because nearly all industrial applications demand consistent properties. With the exception of natural diamond, most of nature’s abrasives are too variable in their properties.
The abrasive materials are used in the form of grinding wheels, sandpapers, honing stones, polishes, cutoff wheels, vibratory mass-finishing media, sandblasting, pulp stones, ball mills, and still other tools and products.
In general, an abrasive should possess five properties in order to cut efficiently:
Hardness: The ability of a material to scratch or penetrate another material. Hardness can be measured in a number of scales including the Mohs hardness test, the Knoop hardness test, and the Vickers hardness test.
Toughness: The ability of the abrasive grain to resist impact and pressures created during the grinding operation. It is, in other words, the ability to absorb energy and plastically deform without fracturing. For natural abrasives, toughness is affected by the concentration of impurities in the material. In artificial abrasives, toughness could be controlled by controlling parameters such as the grain shape during the crushing or sizing operation, the purity of the abrasive, the additions of alloying substances, and by controlling the crystal structure within abrasive grains. Toughness can be quantified though a number of metrics such as the modulus of rapture, the fracture toughness parameters, or indirectly through the grinding ratio. Industrially toughness is measured through the “toughness index” (TI). TI consists of measuring the percentage of abrasive remaining on a breakdown sieve upon fixed comminution time. The mechanics of abrasive fracture is affected by the temperature of the abrasive, thus “thermal toughness index” (TTI) is also measured, which is identical to TI with the sample having been annealed before testing.
Heat resistance: The ability of the abrasive grain to withstand the heat of grinding without becoming dull. In general, synthetic abrasives are more stable at higher temperatures than natural ones.
Friability: The ability to fracture under stress along certain cleavage lines so that as the cutting edges become dull, part of the grain brakes off and presents new cutting edges. Friability is directly related to the materials toughness. A material with higher toughness is less likely to fracture when engaging the workpiece. Friability is defined as the inverse term of fracture toughness. Practically it is measured from the loss of abrasive material by splintering. In the relationship between friability and knop hardness is qualitatively shown for two different types of abrasives.
Attrition resistance: Another critical property of any abrasive material is the attrition resistance, as it limits the use of abrasives with other materials. The interaction between the abrasive and the processed material prevents the use of one abrasive as a universal medium. In most of the cases this is attributed to chemical affinity between the abrasive and the other material.
Ranges of Knoop hardness for various materials