Abrasives

Abrasive materials are hard crystals that are either found in nature or manufactured. The most commonly used of such materials are aluminum oxide, silicon carbide, cubic boron nitride, and diamond.

Other materials such as garnet, zirconia, glass, and even walnut shells are used for special applications.

Abrasives are primarily used in metalworking because their grains can penetrate even the hardest metals and alloys. However, their great hardness also makes them suitable for working with such other hard materials as stones, glass, and certain types of plastics. Abrasives are also used with relatively soft materials, including wood and rubber, because their use permits high stock removal, long-lasting cutting ability, good form control, and fine finishing.

Applications for abrasives generally fall in the following categories: 1) cleaning of surfaces and the coarse removal of excess material, such as rough off-hand grinding in foundries; 2) shaping, as in form grinding and tool sharpening; 3) sizing, primarily in precision grinding; and 4) separating, as in cut-off or slicing operations.

For the past 100 years or so, manufactured abrasives such as silicon carbide and aluminum oxide have largely replaced natural abrasives-even natural diamonds have nearly been supplanted by synthetic diamonds. The success of manufactured abrasives arises from their superior, controllable properties as well as their dependable uniformity.

Both silicon carbide and aluminum oxide abrasives are very hard and brittle, and as a result they tend to form sharp edges. These edges help the abrasive to penetrate the work material and reduce the amount of heat generated during the abrasion. This type of abrasive is used in precision and finish grinding. Tough abrasives, which resist fracture and last longer, are used for rough grinding.

Industry uses abrasives in three basic forms: 1) bonded to form solid tools such as grinding wheels, cylinders, rings, cups, segments, or sticks; 2) coated on backings made of paper or cloth in the form of sheets (such as sandpaper), strips or belts; 3) loose, held in some liquid or solid carrier as for polishing or tumbling, or propelled by force of air or water pressure against a work surface (such as sandblasting for buildings).

Abrasion most frequently results from scratching a surface. As a general rule, a substance is only seriously scratched by a material that is harder than itself. This is the basis for the Mohs scale of hardness (see Table 2) in which materials are ranked according to their ability to scratch materials of lesser hardness.

Abrasives are therefore usually considered to be refractory materials with hardness values ranging from 6 to 10 on the Mohs scale that can be used to reduce, smooth, clean, or polish the surfaces of other, less hard substances such as metal, glass, plastic, stone, or wood.

During abrasion, abrasive particles first penetrate the abraded material and then cause a tearing off of particles from the abraded surface. The ease with which the abrasive particles dig into the surface depends on the hardness of the abraded surface; the ease with which the deformed surface is torn off depends on the strength and, in some cases, on the toughness of the material. Between hardness, strength, and toughness, hardness is usually the most important factor determining a material's resistance to abrasion.

When two surfaces move across each other, peaks of microscopic irregularities must either shift position, increase in hardness, or break. If local stresses are sufficiently great, failure of a tiny volume of abraded material will result, and a small particle will be detached. This type of abrasion occurs regardless of whether contact of the two surfaces is due to sliding, rolling, or impact.

Some forms of abrasion involve little or no impact, but in others the energy of impact is a deciding factor in determining the effectiveness of the abrasive. Brittle materials, for example, tend to shatter when impacted, and their abrasion may resemble erosion more than fracture.