What is Abrasive? A General Introduction
28 September 2011
In the first of a series of educational articles, the Abrasives Hub takes a look at what an abrasive is and the main types and uses of abrasives. This could be a useful set of guides for students, people new to the abrasives industry etc.
One of the broadest definitions of the world abrasive can be found in Webster’s Dictionary. This states that an abrasive is “any of a wide variety of natural or manufactured substances used to grin, wear down, rub away, smooth scour clean or polish, often combined with a binder to make grinding wheels or affixed to the surface of paper or cloth.”
Abrasives are used in a huge variety of applications throughout industry and in most manufacturing process, an abrasive will be employed at some point. The main uses remain grinding, cutting, sawing, honing, polishing, lapping, sanding, barrelling, drilling and blasting.
What are the types of abrasives?
Generally abrasives fall into four main categories – abrasive grains, bonded abrasives, coated abrasives and superabrasives.
Abrasive grains are usually used in polishing, lapping, barrelling and blasting applications. They are usually supplied as powders, pastes, slurries or grains. Grains are normally classified by particle or grit size. Macrogrits are generally larger than 50 microns in size, while finer particles or microgrits are below that figure. Standards exist for sizing of abrasive particles and these are different depending on whether they are use for coated and bonded applications or are superabrasives. However these standards are generally based on the mean size and the proportion of coarser and finer particles present. For some applications, the particle shape is important and this is also reflected in grain classification.
Bonded abrasives are usually held in a matrix or bond. Bond types are may be glass (vitrified bonds), metal or organic such as polymers, rubber and shellac Diamond, cubic boron nitride, silicon carbide and fused alumina are common grains used in bonded abrasive products. For bonded abrasives, normally segments of the abrasive materials are manufactured and these are then further attached to a supporting body. Most grinding wheels employ bonded abrasives. As the permutations of abrasive type, particle size, bond, wheel body size and type are vast, there is an enormous range of grinding wheels produced by manufacturers.
The term, Coated Abrasive, is normally given to those abrasives products in which the abrasive particles or grains are applied to the surface of a flexible backing material by a bond or adhesive. Their main use is for mass production grinding and finishing or light to medium parts, plastics and other material. Most of us are aware of sandpaper which is an essential part of any DIY kit. Normally, the coated material is produced in large rolls and then cut or finished into various forms – belts, bands, cones, discs, rolls or sheets for grinding, sanding or finishing operations. The grains are normally deposited using an electrostatic process for all but the coarsest materials.The three main components of a coated abrasives are the particle, the backing and the bond. Common grain materials are aluminium oxide and silicon carbide, garnet and emery. Backing materials are either paper, cloth or fibre.
Figure 1: Coated Abrasive Example
The fourth type are called superabrasives and normally the abrasive is either diamond or cubric boron nitride. Large individual diamonds for example are used in bits for oil-well drilling, for example. However for the majority of granular abrasives the sizes of particles or grits range from about 5mm down to microns and even nanometres in diameters for specialist polishing applications.
What types of Abrasive Materials are there?
A broad range of natural and synthetic materials can be used as an abrasive. This includes naturally occurring materials such as quartz, garnet, corundum, emery and diamond and, manufactured materials such as fused aluminium oxides, fused zirconia- aluminas, sintered aluminas, silicon carbide, silicon carbide, boron carbide, and, the superabrasives – synthetic diamond and cubic boron nitride. In the next article, the Abrasives Hub will look at abrasive materials in more depth.