Abrasives

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Abrasives

Abrasive materials are hard crystals that occur naturally or are 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, emery, pumice, and even walnut shells, are used for special applications.

Abrasives have several general uses. The first application is the cleaning of surfaces and the coarse removal of excess material, such as rough off-hand grinding in foundries. Secondly, abrasives are valuable in shaping other materials, as in form grinding and tool sharpening. Abrasives are also used to alter the size of materials, primarily in precision grinding. Finally, abrasives are important in separating sections of material, such as in cut-off or slicing operations.

Abrasives are used in metalworking because their grains can penetrate even the hardest metals and alloys. However, their hardness also makes them suitable for working with such other hard materials as stone, glass, and certain types of plastics. Abrasives are also used with relatively soft materials including wood and rubber, because their use permits high stock

Table 1. Common Industrial Abrasives. (Thomson Gale.)
Common industrial abrasives
Abrasive	Used for
aluminum oxide	grinding plain and alloyed steel in a soft or hardened condition
silicon carbide	cast iron, nonferrous metals, and nonmetallic materials
diamond	grinding cemented carbides, and for grinding glass, ceramics, and hardened tool steel
cubic boron nitrate	grinding hardened steels and wear-resistant superalloys

Table 2. Mohs Hardnesses of Selected Materials. (Thomson Gale.)
Mohs hardnesses of selected materials
Abrasive	Mohs hardness
wax (0 deg C)	0.2
graphite	0.5 to 1
talc	1
copper	2.5 to 3
gypsum	2
aluminum	2 to 2.9
gold	2.5 to 3
silver	2.5 to 4
calcite	3
brass	3 to 4
fluorite	4
glass	4.5 to 6.5
asbestos	5
apatite	5
steel	5 to 8.5
cerium oxide	6
orthoclase	6
vitreous silica	7
beryl	7.8
quartz	8
topaz	9
aluminum oxide	9
silicon carbide (beta type)	9.2
boron carbide	9.3
boron	9.5
diamond	10

removal, long-lasting cutting ability, good form control, and fine finishing.

Manufactured abrasives such as silicon carbide and aluminum oxide have largely replaced natural abrasives. Synthetic diamonds have nearly supplanted by natural 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.

Industrial abrasives are used in three basic forms. They can be bonded to form solid tools such as grinding wheels, cylinders, rings, cups, segments, or sticks. Secondly, they can be applied as a coating on backings made of paper or cloth in the form of sheets (such as sandpaper), strips, or belts. Finally, abrasives can be unfixed 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).

One industrial use of abrasives is in construction. The ability to cut through material such as concrete requires abrasives such as synthetic diamond, which is incorporated into a cutting wheel.

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 in which materials are ranked according to their ability to scratch materials of lesser hardness.

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. 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.

See also Crystal.