Abrasives

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.