An abrasive can be any material that is used to abrade or change the surface or structure of another material. Over its long history abrasives have come from minerals and other materials covering the entire Mohs hardness scale from talc to diamond
A LONG HISTORY
Mankind has known about abrasives for millennia using stones and sand to shape and finish knives, spear and arrow points, and fishhooks. The first sharpening stones were made from natural sandstone where small grains of silica sand are embedded in a stone matrix. Abrasives grew in importance after the discovery of metals because they were among the few tools available to work with these hard materials, one of the reasons most abrasives are minerals on the top end of the Mohs hardness scale of 5 and greater, from quartz to diamond. But soft materials can be abrasives too. Materials such as sponges, baking soda, and fruit pits also have uses as abrasives for cleaning that won't scratch.
ABRASIVES USED IN VARIOUS PROCESSES
Exactly what material can be called an abrasive depends upon the context in which it used. A group of processes used known as blast cleaning
has the greatest variety of materials called abrasives. Blast cleaning, which propels abrasive particles with air or water for cleaning and finishing surfaces, uses relatively soft as well as hard materials including corn cobs and waltnut shells, steel shot, slag, glass, baking soda, and solid carbon dioxide. Silica sand is the most popular abrasive for blast cleaning to remove paint from bridges and other steel structures.
Peening, a subclass of blast cleaning, uses cast metal pellets and cut wire as abrasives. Density is important in peening abrasives, because impact force is needed to alter the metallurgical structure
of surfaces in peening, which is used to increase the service life of structural components such as aircraft landing gear.
, which used fine abrasives entrained in a high pressure waterjet to cut materials, uses primarily natural garnet as well as lesser amounts of materials such as coal slag.
Polishing and buffing
use a wide variety of materials as abrasives. These processes combine powdered abrasives with waxes, petroleum products or water and apply them using a buff or polishing pad to create extremely fine finishes. Cerium oxide, diamond, silica, iron oxide are among the many the minerals used in polishing compounds. Natural minerals such as Tripoli or novaculite are common. Polishing of glass is one of the oldest applications for polishing, and cerium oxide was and remains the abrasive of choice, though silicon carbide and diamond are used in some applications.
Magnetic finishing, rheological finishing and abrasive honing
are processes that use a unique group of abrasives. The latter process identified by the trade name Extrude Hone blends abrasives with a polymer paste that is forced through internal orifices to debur and polish internal surfaces of metal castings. Magnetic finishing uses iron-based abrasives, aluminum oxide or other minerals bonded to a magnetic material, and rheological finishing blends abrasives with fluids whose characteristics can be changed by electric fields. None of these applications is a very large consumer of abrasives.
generally uses loose abrasives mixed with carrier fluids. The abrasives in lapping are typically those commonly found in grinding wheels. Recent advances in lapping use bonded pellets in a process called fixed abrasives machining.
uses abrasives in variety of ways. Some saws use thin grinding wheels typically made with aluminum oxide, reinforcing materials and phenolic resins. Others have abrasive bonded to a metal bonded. Wire saws used to slice silicon wafers in the semiconductor industry and by stone quarries to cut marble, or other stone use powdered abrasives carried by fluids and taunt wires. Abrasive choices are silicon carbide, diamond, boron carbide, aluminum oxide or silicon carbide. In some applications the wires have the abrasives bonded to the surface of the wire.
Many other uses are tools made with abrasive grits bonded to the surface of a base material or mixed with a bonding agent to create a specific form such as a wheel, stone, or other shape. Tools used for deburring or mass finishing
apply abrasives in a variety of formats including bonded and coated abrasives, wire brushes and abrasive brushes. The abrasives are typically aluminum oxide and silicon carbide. Mass finishing usually requires special vibratory or tumbling equipment with specially designed media contains a range of materials similar to blast cleaning: steel and stainless steel, corn cobs, and wood, as well as small bonded shapes made with aluminum oxide in a resin or plastic bond.
The more common grinding, superfinishing, finishing, sanding, and honing
processes use primarily synthetic minerals grouped as conventional abrasives (aluminum oxide, silicon carbide, zirconia) and superabrasives (diamond, or cubic boron nitride). There are a few coated abrasive products made with natural emery and corundum, but their uses are very limited. Natural diamond still appears in bonded products, but the amount is declining as prices of synthetic diamond continue to fall. The history of industrial abrasives for grinding wheels began with natural minerals, initially quartz and flint and later garnet, diamond and a form of carborundum called emery. The use of natural minerals for wheels declined sharply during the first half of the twentieth century, and natural minerals were completely replaced by synthetic minerals by the century's end. Synthetic minerals are preferred because they have consistent quality and mineral properties can be tailored to fit the needs of specific applications. Blocky shaped grits are better for grinding, thin sliver-shaped grits are better for coated abrasives, and aluminum oxide with lower levels of titanium oxide impurities is better for grinding tool steels.
Coated abrasives, the more proper name for sandpaper, utilize abrasives bonded to a flexible backing such as paper, mylar film, or fabric. Usually included in this grouping are products with the abrasive impregnated into a nonwoven materials, foam, or sponges. Although there are thousands of different products clustered under the name of coated abrasives, nearly all use conventional abrasives or superabrasives.
Bonded products in the shapes of wheels or stones represent a vast number of applications largely unknown to the public. While most people are familiar shop grinding wheels and sharpening stones for knives, most known little about the thousands of specialized wheel formulations, sizes and shapes used in industrial applications for making bearings, auto and aircraft engines and a myriad of other products. Most bonded products are made with conventional and superabrasive abrasives.
The contemporary picture of the abrasives industry, specifically the segment concerned with grinding wheels and coated abrasives, is complex because of the host of different tradenames and product types developed over the last century. Landmark events include discovery in the late 1800s of silicon carbide, a mineral not found in nature, followed by synthetic aluminum oxide in the 1890s. By the late 1920s, synthetic aluminum oxide, silicon carbide and natural emery, garnet, and corundum became staples of the manufacturing, giving rise to the designation conventional abrasives. A new process for making aluminum oxide discovered in 1938 introduced high purity aluminum oxide that proved superior for precision machining of high speed steel. Mixing of zirconia with aluminum oxide made an abrasive that could survive abusive handling. Subsequent discoveries introduced a sintered process for aluminum oxide that preserved the original micro-crystalline structure of bauxite source materials leading the way to the manufacture of alumina abrasives using a sol gel process and new Cubitron™ or SolGel™ ceramic abrasives.
Natural diamond became a popular abrasive in 1930 with the development of new bonding materials and grew with a search for better
ways to machine tungsten carbide. By the 1960s research by General Electric produced synthetic diamond, which eventually lead to the discovery of cubic boron nitride, CBN. The latter is a material with hardness near diamond and good for grinding steel, which reacts chemically with diamond.
Though all of these developments lead to new product names, the range of minerals has remained within conventional and superabrasive groups. With the exception of Abral, a new aluminum-oxy-nitride minerals developed by Europe-based Pechiney, efforts to introduce new groups of abrasive minerals into this segment of the abrasives industry have been unsuccessful.
Advances in abrasives were pushed by developments of bonds that improved wheel quality and performance. The development of resin bonds, first known as Bakelite, in the 1930s was perhaps the most significant advancement in bonds greatly extending the variety of applications for grinding wheels. Also important are many vitrified bonds with porosity to carry metalworking fluids and metal chips away from the point of contact between the grinding wheel and workpiece to provide better cooling and cutting.
Although such developments produced reliable high quality abrasive minerals, abrasives would have limited use without tools designed for their use. Hand-held tools such as belt sanders have greatly increased uses for coated abrasives. New diamond, CBN, and ceramic premium abrasives become economical only after machines became available with the stiffness and precision controls necessary for their use.
With their many uses, abrasives are an essential part of modern day manufacturing being used directly or indirectly in every manufactured product made today. Knowledge about how they work has grown from an art understood by a few experienced artisans to a science that can be applied in tightly controlled processes using computers and advanced machine tools. Disguised in today's lowly grinding wheel or sandpaper are advancements based on research using the greatest minds that academia could offer and the highest levels of knowledge in material science, polymer science, mechanical engineering, and chemical engineering. Companies manufacturing abrasives grains, grinding wheels, and coated abrasives use theoretical knowledge drawn from many fields including refractory and ceramics, chemistry, physics, and metallurgy. Abrasives will always be useful because like the past, they are key enabling technologies for manufacturing.