ABRASIVE

sharp, hard material used to wear away the surface of softer, less resistant materials. Included within the term are both natural and synthetic substances, ranging from the relatively soft particles used in household cleansers and jeweler's polish to the hardest known material, the diamond. Abrasives are indispensable to the manufacture of nearly every product made today. Abrasives are used in the form of grinding wheels, sandpapers, honing stones, polishes, cutoff wheels, tumbling and vibratory mass-finishing media, sandblasting, pulpstones, ball mills, and still other tools and products. Only through the use of abrasives is industry able to produce the highly precise components and ultrasmooth surfaces required in the manufacture of automobiles, airplanes and space vehicles, mechanical and electrical appliances, and machine tools. This article surveys the principal materials used in abrasives, the properties of those materials, and their processing into industrial products. Most abrasive products are made of ceramics, which include some of the hardest materials known. The origins of hardness (and other properties) in ceramic materials are described in the article ceramic composition and properties. relatively hard natural or synthetic material that is used to grind or polish materials softer than itself. Grinding wheels, oilstones, and sandpaper are common examples of abrasives. The high-precision components and extremely smooth surfaces essential in modern machine tools, electronic equipment, and spacecraft can be obtained only through abrasive grinding operations. The use of abrasives dates to prehistoric times, when stone tools and weapons were shaped by rubbing them against harder stones. Drawings in ancient Egyptian tombs show the polishing of jewelry and vases with abrasives. In later historical times, craftsmen used natural abrasive stones shaped into wheels, such as grindstones and mill wheels, and into blocks for use by hand in sharpening scythes, knives, axes, and woodworking tools. Powdered abrasives were used for polishing in much the same manner as they were employed by the Egyptians. It was not until late in the 19th century, when it became necessary to work with harder metals and to closer tolerances, that abrasives assumed a major role in the manufacturing industry. Some natural abrasives are mined throughout the world; these include flint, a form of quartz that is the most common material used for sandpaper, and sand, which is naturally granulated quartz, used in sandblasting and other abrasive processes. Another fairly common natural abrasive is pumice, or hardened lava foam, which is widely used in metal polishes and scouring powders. Corundum and emery, which are natural forms of aluminum oxide, occur less commonly, as does garnet. Others occur only in relatively limited areas; diamonds, for example, are mined principally in South Africa and to a much lesser extent in Australia and South America. The diamonds and other precious stones used as abrasives are not of gem quality. In the 20th century various synthetic abrasives, including silicon carbide (produced under such trade names as Carborundum, Crystolon, and Carbolon), synthetic diamonds, boron carbide, and alumina, were developed, and new machines were designed to utilize these abrasives to best advantage. Natural abrasives are still widely used in some applications, but in industrial grinding they have been largely replaced by synthetics. Most abrasives intended for industrial use are first crushed into a powder, which may then be mixed with a liquid or paste, pressed into wheels or blocks, or glued to the surface of paper or cloth. Additional reading Kenneth B. Lewis and William F. Schleicher, The Grinding Wheel, 3rd ed. (1976), contains general information on the various types of grinding operations, presented from a nontechnical point of view. Another nontechnical source is P.S. Houghton, Grinding Wheels and Machines (1963). Additional sources on abrasives and cutting tools include Jzef A. Borkowski and Andrzej M. Szymanski, Uses of Abrasives and Abrasive Tools, trans. from Polish (1992); and two subsections in Theodore J. Reinhart (ed.), Engineered Materials Handbook, vol. 4, Ceramics and Glasses, ed. by Samuel J. Schneider (1991): Ernest Ratterman and Roger Cassidy, "Abrasives," pp. 329-335; and Peter T.B. Shaffer, "Engineering Properties of Carbides," pp. 804-811. Papers presented at conferences of the Industrial Diamond Association and the Abrasive Engineering Society are published as Proceedings; topics vary from general to highly technical. Papers on abrasives appear also in Transactions of the American Foundrymen's Society (annual); and Transactions of the North American Manufacturing Research Institute of SME (annual), published by the Society of Manufacturing Engineers.A good introduction to ceramics in general is provided by David W. Richerson, Modern Ceramic Engineering: Properties, Processing, and Use in Design, 2nd ed., rev. and expanded (1992). The processing of traditional ceramics is described in F.H. Norton, Elements of Ceramics, 2nd ed. (1974); James S. Reed, Introduction to the Principles of Ceramic Processing (1988); George Y. Onoda, Jr., and Larry L. Hench, Ceramic Processing Before Firing (1978); and in four sections of the Reinhart text cited above: "Ceramic Powders and Processing," pp. 41-122; "Forming and Predensification, and Nontraditional Densification Processes," pp. 123-241; "Firing/Sintering: Densification," pp. 242-312; and "Final Shaping and Surface Finishing," pp. 313-376. D. Joseph Bodin Thomas O. Mason The Editors of the Encyclopdia Britannica