Abrasive

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An abrasive is a material that is used to smooth, machine, or, in some cases, roughen another softer material through extensive rubbing. Abrasives are used to remove surface materials such as metal, ceramics, glass, plastics, and paint and this process is often referred to as cutting. Abrasives come in a variety of sizes (often called grit size) and materials.

Abrasives may be shaped into or used with discs, belts, blast machines and sandblasters, as well as sheets, rolls, and handpads. Some abrasives are designed for use on bench grinders, while others are designed for use on portable or handheld grinders or sanders.

Cutting compound used on automotive paint is an example of an abrasive suspended in a paste, as are some polishing liquids for silverware and optical media. Toothpaste contains calcium carbonate as a polishing agent to remove plaque and other matter from teeth as the hardness of calcium carbonate is less than that of tooth enamel but more than that of the contaminating agent.

A bonded abrasive is composed of an abrasive material contained within a matrix. This matrix is called a binder and is often a clay, a resin, a glass or a rubber. This mixture of binder and abrasive is typically shaped into blocks, sticks, or wheels. The most usual abrasive used is aluminium oxide. Also common are silicon carbide, tungsten carbide and garnet. Artificial sharpening stones are often a bonded abrasive and are readily available as a two sided block, each side being a different grade of grit.

A coated abrasive comprises an abrasive fixed to a backing material such as paper, cloth, rubber, resin or polyester, many of which are flexible. Sandpaper is a very common coated abrasive.

As abrasives remove material care must be taken in selecting the proper substance and size of abrasive. An abrasive which is too hard or too coarse can remove too much material or leave undesired scratch marks. Besides being unsightly, scratching can have other, more serious effects. It may diminish or destroy their usefulness (as in the case of scratched optics and compact discs); trap dirt, water, or other material; increase surface area (permitting greater chemical reactivity such as increased rusting which is also affected by matter caught in scratches); erode or penetrate a coating (such as a paint or a chemical or wear resistant coating); overly quickly cause an object to wear away (such as a blade or a gemstone); increase friction (as in jewelled bearings and pistons).

A finer or softer abrasive will tend to leave much finer scratch marks which may even be invisible to the naked eye; a softer abrasive may not even abrade a certain object. A softer or finer abrasive will take longer to cut as tends to cut less deeply than a coarser, harder material. Also, the softer abrasive may become less effective more quickly as the abrasive is itself abraded. This allows fine abrasives to be used in the polishing of metal and lenses where the series of increasingly fine scratches tends to take on a much more shiny or reflective appearance or greater transparency. Very fine abrasives may be used to coat the strop for a cut-throat razors. The final stage of sharpening Japanese swords called polishing and may be a form of superfinishing.

Scratched compact discs and DVDs may sometimes be repaired through polishing with a very fine compound, the principle being that a multitude of small scratches will be more optically transparent than a single large scratch. However, this does take some skill and will eventually cause the protective coating of the disc to be entirely eroded (especially if the original scratch is deep), after which the data surface will be destroyed if abrasion continues.

Different chemical or structural modifications may be made to alter the cutting properties of the abrasive.^[1]

Some naturally occurring abrasives are:

• Sand
• Ceramic
• Diamond dust (synthetic diamonds are used extensively)
• Aluminium oxide (corundum) or Emery (impure corundum)
• Pumice dust

Some abrasive minerals (such as zirconia alumina) may occur naturally but are sufficiently rare that a synthetic stone is used industrially.

• Glass powder
• Silicon carbide (carborundum)
• Zirconia alumina
• Borazon (cubic boron nitride or CBN)

Various shapes or applications of abrasives include:

• Diamond tools
• Grinding wheel
• Coated abrasive
• Sandpaper
• Bonded abrasive