In the second educational article on what is abrasive, we explore the types of materials that make up the abrasives world will be covered. Abrasives fall into two major categories – natural and synthetic.
The main types of naturally occurring abrasives are quartz, garnet, corundum, emery and diamond. Quartz was once a major abrasive material but is use has suffered because of the health risks associated with free silica dust and has been replaced to a large extent by other abrasive types. However, historically quartz has been used in sandpaper for finishing leather and felt, in sandblasting, lapping of soft materials, for write sawing of soft stone and in scouring compounds.
Garnet is an abrasive material found extensively in the water jet cutting and blast cleaning sectors. It is non-toxic and eco-friendly as it can be recycled plus it offers low material consumption rate, purity, and high productivity rates. In addition, garnet produces much less dust than other abrasive materials, and spills are relatively benign and easy to clean up. Outside its two main areas of use, garnet coated papers and cloths are mainly used for sanding wood and for the finishing of plastics, glass and softer metals. Other natural and manmade materials compete with garnet in nearly all of the applications for which garnet can be used and garnet is gradually being replaced by synthetic abrasives such as fused aluminium oxide and silicon carbide for those abrasive applications outside water jet cutting and sand blasting.
Corundum and Emery are essentially the same chemical composition. What makes the difference between these abrasive materials is purity. Corundum is a naturally occurring form of aluminium oxide of high purity while emery contains varying quantities of other minerals, such as magnetite and hematite. Uses of corundum include loose grain polishing and grinding of precision optical components and emery, which is in decline as an abrasive, has been used in coated products to polish metals.
Diamonds of the natural kind have largely been overshadowed by its synthetic counterpart in their use as an abrasive. What is abrasive about diamond is its extreme hardness, making it useful for cutting and grinding other hard materials. Industrial diamond, dubbed boart, of a quality unsuitable for gem making, began to come to prominence during the first half of the 20th century. As diamond mining in South Africa grew, so did the quantities of boart. Eventually, its industrial potential was recognised and demand grew to such an extent that its price was a premium. All that changed with the advent of synthetic production of diamond in the 1950s and by the mid 1980s, the synthetic production of diamond was pre-eminent.
One of the reasons behind the development of manufactured or synthetic abrasives has been to overcome the variability in consistency and size of abrasives. Such variability gives rise to dramatic difference in the behaviour and performance of abrasives in a particular application. This meant that sorting, grading and analysing abrasive grains to ensure they met a particular application criteria was and remains important. This is why it has become the subject of so many standards and classifications and adds to the complexity of choosing an abrasive for a particular application, particularly a superabrasive like diamond.
So, the development of synthetic types of abrasives was to remove some of the variability in material characteristics with a more consistent product. When it comes to the question, “What type of synthetic abrasives are there?” Well, the answer is quite a few.
Some synthetic abrasives include fused aluminium oxides, fused zirconia-aluminas, sintered aluminas, silicon carbides, boron carbides, diamond and cubic boron nitride
Today there are variety of fused aluminium oxides usually derived from bauxite and product by one of two furnace types. The most common fused alumina is known as regular aluminium oxide, a brown material with a chemical composition of about 95% aluminium oxide. It is a coarse material with crystals normally between 10-15 mm in diameter. A second brown variety is semi friable fused alumina and has a slightly higher purity. Other versions are generally very high purity materials such as white fused alumina and monocrystalline aluminium oxide. Derivatives of alumina include fused zirconia-aluminas developed in the 1980s, sintered aluminas derived from bauxite or alumina gels.
Silicon carbide discovered in 1891 was the first synthetic abrasive commercialised and spawned a company called Carbordundum which still exists today. The starting materials for silicon carbide are silicon sand and carbon in the form of coke. Produced by an electric arc heating process, today several varieties are available in colours ranging from black, grey to green. Produced in an electric resistance furnace is the abrasive boron carbide which is used as a loose abrasive.
The development of synthetic diamond in the 1950s is a dramatic tale and at least one book has been written on the subject. Using a high temperature and high pressure process, one form of carbon, graphite, is transformed into another, diamond. When the process was developed it overcame the shortages in natural diamond grain and enabled a variety of different types, sizes and shapes of diamond material to be produced. This is increased the availability and reduced the cost of diamond for abrasive applications. As well as the conventional high pressure, high temperature manufacturing method, diamond for specialist applications can be made using a variety of chemical vapour deposition and detonation technologies.
Essentially the same production process is used for the manufacture of cubic boron nitride. The material was first produced in 1969 on a commercial basis. By using different raw materials and production conditions, a variety of different cubic boron nitride products with a range of friabilities can be made and with colours varying from almost colourless through to yellows, red and black.
In Part 3 of our educational series on what is abrasive, we will outline in more depth the different application areas of each class of abrasives.