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, which has small grains of silica sand embedded in a stone matrix. Abrasives became important following the discovery of metals, because abrasives were the only ways to shape and finish the new hard materials. Generally abrasives are very hard minerals found at the top end of the Mohs hardness scale --from quartz to diamond. Soft materials can be abrasives, too, depending upon the applications. Materials such as sponges, baking soda, and fruit pits can rightfully be called abrasives for use in blast cleaning.
ABRASIVES USED IN VARIOUS PROCESSES
Whether a material can be called an abrasive or not depends upon the context in which the material is used. Generally there are several classes of processes. A class of processes known as blast cleaning
uses the greatest variety of materials as an abrasive. Blast cleaning, which propels abrasive particles with air or water for cleaning and finishing surfaces, uses materials ranging from corn cobs and waltnut shells, to steel shot, slag, glass, baking soda, and solid carbon dioxide. Silica sand is the most popular abrasive for blast cleaning of bridges and other steel structures. Peening
is a subclass of blast cleaning that uses have metals such as cast metal pellets and cut wire as abrasives. Peening requires abrasives with high density to alter subsurface metallurgical structure and increase services life of structural components such as aircraft landing gear.
is a process similar to blast cleaning that uses fine abrasives entrained in a high pressure waterjet to cut materials. The abrasive allied in this group of processes are primarily natural garnet and to a lesser degree coal slag.
Polishing and buffing
is an important class of processes using abrasives. These processes combine powdered abrasives with waxes, petroleum derivatives or water and apply them using a buff or polishing pad. Cerium oxide, diamond, silica, iron oxide are among the minerals used in polishing compounds. Natural Tripoli or novaculite are common polishing abrasives. Cerium oxide is the most common mineral used for polishing of glass,one of the oldest polishing applications. In recent years,however,silicon carbide and diamond are used for glass.
Magnetic finishing, rheological finishing and abrasive honing
are relatively high-tech processes that use a unique group of materials as abrasives. One 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 an abrasives made by bonding aluminum oxide or other minerals to a magnetic material. 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.
is a class of processes that uses loose abrasives mixed with a carrier fluid to form a slurry that is rubbed against a workpiece. The abrasive materials use in lapping are typically those commonly found in grinding wheels. Recent advances in lapping technology replace the slurry with small bonded pellets in a process called fixed abrasives machining.
is class that uses abrasives in variety of ways. Some saws are thin grinding wheels typically made with aluminum oxide, reinforcing materials and phenolic resins. Other saws bond abrasives to a metal disk. Wire saws for slicing silicon wafers in the semiconductor industry and extracting marble from stone quarries use powdered abrasives carried by fluids and taunt wires. The abrasives used in wire saws are typically aluminum oxide, silicon carbide, diamond, and boron carbide. Some applications bond abrasives to the metal wire.
Bonded products form one of the largest segments of the abrasives industry. These products use abrasive materials bonded to a surface or combined with a bonding agent to create a rigid form such as a wheel, stone, or other shape.
The more common classes of abrasives in bonded format are grinding, superfinishing, finishing, sanding, and honing
. Contemporary applications of bonded products use primarily synthetic minerals, which are grouped in the industry as conventional abrasives (aluminum oxide, silicon carbide, zirconia) and superabrasives (diamond, or cubic boron nitride). Exceptions are natural emery and corundum, which were historically important but have very limited use in modern manufacturing. Also natural diamond still appears in bonded products, but declining prices shifts used to synthetic diamond. The history of industrial abrasives for grinding wheels began with natural minerals, initially quartz and flint and later garnet, followed by diamond and a form of carborundum called emery. The shift away from of natural minerals for wheels began in first half of the twentieth century and was complete by end of the century. Synthetic minerals are preferred because they have consistent quality and properties can be tailored to fit the needs of a specific application. Blocky shaped grits, for example, can be created for grinding wheels and thin sliver-shaped grits manufactured for coated abrasives. Also, altering the minerals to reduce levels of titanium oxide impurities in aluminum oxide produces an premioum abrasive good for grinding tool steels.
Coated abrasives, the more proper name for sandpaper, bond minerals 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 products largely unknown to the public. While most people are familiar shop grinding wheels and sharpening stones for knives, most are aware of the thousands of specialized wheel formulations, sizes and shapes used by makers of bearings, auto and aircraft engines and a myriad of other products. Most bonded products are made with conventional and superabrasive abrasives.
Tools used for deburring or mass finishing
apply abrasives in a wide variety of shapes a including bonded and coated abrasives, wire brushes and abrasive brushes. The abrasives in deburring 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 contemporary picture of the abrasives industry concerned with grinding wheels and coated abrasives is complex because of the host of different tradenames and product types. Landmark events in these segments include discovery 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 were staples of the manufacturing, giving rise to the contemporary designation conventional abrasives. A new process for making nearly pure aluminum oxide in 1938 introduced white friable aluminum oxide that proved superior for precision machining of high speed steel. Later, a discovery that a blend of zirconia and aluminum oxide yielded an abrasive that is suitable for 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 a synthetic diamond, which eventually lead to the discovery of cubic boron nitride, CBN. CBN has a hardness near diamond and is good for grinding steel because it doesn't react iron.
Despite all these developments, the range of minerals for the bonded segment of the industry remained within conventional and superabrasive groups. A recent exception is Abral, a new aluminum-oxy-nitride minerals developed by Europe-based Pechiney.
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 greatly extending the variety of applications for grinding wheels. New vitrified bonds increase wheel porosity to carry metalworking fluids and metal chips away from the grinding wheel and workpiece to provide better cooling and cutting.
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.