Abrasives

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As abrasives , abrasive or abrasive materials , ( English abrasive ) grains of hard material are referred to that for grinding , then, for removing material by machining with bonded grain are used. A distinction is made between natural grain materials ( flint , quartz , corundum , emery , garnet , natural diamond ) and synthetic grain materials (corundum, silicon carbide , chromium oxides , cubic boron nitride , diamonds ).

Coated abrasives are applied to backing materials. The abrasive then sits on the surface of the sandpaper , abrasive cloth, abrasive fiber, abrasive fleece and abrasive sponges, which are also processed into grinding tools such as grinding wheels .

Alternatively, abrasives are formed into abrasive articles by embedding them as a composite material in a matrix .
The following matrix is ​​used as a shaping and connecting matrix:

Among the abrasives are about separation , roughing and diamond grinding wheels, honing stones , special eraser to clean surfaces as well as brushes with abrasive filament .

Abrasives in unbound form are used, for example, as powders or pastes for polishing and vibratory grinding , as blasting agents for blasting and as lapping agents for lapping .

The general usage also includes steel wool and steel brushes among the abrasives, although these do not contain abrasive grains, but have a chip-removing effect due to the sharp edges of the metal threads or the metal wire.

Basics

The grain material of abrasives should

  • be as hard and tough as possible so that material is removed from the workpiece and abrasive grains remain sharp for a long time;
  • be resistant to thermal (alternating) loads in order to withstand high processing temperatures and rapid temperature changes that occur during grinding due to friction ;
  • Be chemically resistant in order not to enter into any undesired chemical compounds at high pressures and temperatures in contact with air , cooling lubricant and the material of the workpiece .

Types of abrasives

corundum

Corundum is the most commonly used abrasive. Its hardness increases with the degree of purity , which can be recognized by the color. In contrast, the toughness can be increased by adding metal oxides and increasing the cooling rate during the manufacture of the abrasive. According to its composition, corundum is divided into:

  • Brown normal corundum with over 94% Al 2 O 3 ( aluminum oxide ) is used for machining unalloyed and low-alloy steels as well as steel and gray cast iron . Its toughness allows higher contact forces.
  • Semi-precious corundum is a mixture of normal corundum and white fine corundum with which steels of high hardness and strength that are not sensitive to heat are ground.
  • White corundum consists of over 99.9% Al 2 O 3 . Due to its hardness and heat resistance up to 2000 ° C, corundum is suitable for tough hard steels over 60 HRC ( tool steel ), for grinding and polishing glass and all steels that require cool grinding.
  • Fine corundum pink has small amounts of foreign matter, which give it a somewhat higher grain toughness and, due to its higher edge strength, enable it to be used for shape and profile grinding. Otherwise it corresponds to white corundum.
  • Ruby corundum has other admixtures of soluble metal oxides , in particular Cr 2 O 3 ( chromium (III) oxide ). Highest toughness allows the grinding of high-alloy steels.

Zirconium corundum

Zirconium corundum contains 10–40% zirconium oxide . Zirconium corundum is processed as a mixture with normal corundum into grinding wheels for high pressure grinding, with which high metal removal rates are possible.

Silicon carbide

Silicon carbide is heat-resistant up to approx. 1600 ° C and is characterized by hard, sharp-edged crystals. An abrasive grain usually consists of one or only a few crystals. It is harder and more brittle than corundum. The area of ​​application includes non-ferrous metals , stainless steels , ceramic and mineral materials, high-carbon steels and the dressing of grinding wheels. At high temperatures, silicon carbide tends to give off carbon atoms to carbon- affine substances such as iron. There is also the higher-quality green silicon carbide for processing glass, porcelain, marble, precious stone, artificial stone, for the fine processing of light and non-ferrous metals and leather.

Boron nitride

Boron nitride is the hardest known substance after diamond. It is only suitable as an abrasive in the cubic crystal form ( borazon ) and must therefore be produced from hexagonal crystalline boron nitride using high pressure synthesis at 1600 ° C and 70,000 bar. The cubic crystalline boron nitride can be used both as monocrystalline and as polycrystalline abrasive grain, with polycrystalline having a higher toughness. Sintered bronze , synthetic resin and ceramic can be used as binders . The porosity of the ceramic bond is advantageous for the transport of coolant and chips. The grinding wheels are glued or soldered to a steel or ceramic base body so that little of the expensive boron nitride has to be used. The thermal resistance is based on the formation of a layer of boron trioxide which, however, is water-soluble. The coolants should therefore be mineral or low in water.

Boron nitride is resistant up to around 1300 ° C. From around 730 ° C, its hardness even exceeds that of diamond. Boron nitride is therefore suitable for the precision grinding of diamond at high temperatures and the grinding of tough steels such as HSS steel , hot and cold work steel . Boron nitride is unsuitable for soft steels, hard metals , non-ferrous metals , coatings made of chrome and nickel as well as non-metals .

diamond

Diamond is the hardest naturally occurring substance, but only resistant up to 800 ° C. Natural diamonds are usually a bit harder than man-made ones. Artificial diamonds have a low color, which is caused by metallic impurities in the manufacturing process. Three different types of diamond abrasive grain are used:

  • monocrystalline grain with a variety of cutting edges,
  • elongated crystallized grains, which allow good utilization of the abrasive when properly integrated into the abrasive
  • Sintered grain, which is much tougher and allows high surface quality.

Depending on the intended use, industrial diamond is embedded in nickel , copper , synthetic resin or a special alloy or fixed galvanically on a steel body. Abrasives made of diamond are used for the precision grinding of hard metal , gray cast iron , glass, ceramics, porcelain, refractory stones, germanium , graphite , cutting ceramics , silicon , rubber , non-ferrous metal , iron carbide alloys , nickel and chrome alloys , tool steels with high carbon and low vanadium content as well as hardened steels like roller bearing steel .

Classification of the grain size according to the mesh fineness of the sieve

P-grit according to FEPA standard

Abrasives are divided into coarse, medium, fine or very fine grains according to their grain size. The European Association of Abrasives Manufacturers (FEPA, Federation of European Producers of Abrasives ) has set values ​​for grit classification. The characteristic value of the grain is called mesh and corresponds to the number of threads per inch of the screen with which the grain was separated. The higher the number of threads or stitches, the finer the grain.

The FEPA differentiates between rigid abrasives, such as grinding discs, and flexible abrasives, such as sandpaper and sanding belts. Depending on the type, the abrasives are given a letter suffix , where P denotes the flexible abrasives (e.g. P120) and F the rigid abrasives (e.g. F180). It should be noted, however, that these letters are only valid for corundum and silicon carbide. If the abrasive is diamond, it is identified by the letter D , while the letter B is used for boron nitride .

The figures are only roughly comparable. For example, P100 according to FEPA for sanding paper corresponds to a grain size of 0.162 mm, while F100 according to FEPA for grinding bodies corresponds to a grain size of 0.129 mm. J100 according to the Japanese standard JIS R6001 corresponds to a grain size of 0.125 mm.

The grain size information for diamond abrasives contradict the usual system in that a higher numerical value indicates a coarser grain size.

Individual evidence

  1. ↑ Wilfried König , Fritz Klocke : Manufacturing processes. Volume 2: grinding, honing, lapping. 3rd, fundamentally revised and expanded edition. VDI et al., Düsseldorf et al. 1996, ISBN 3-540-62349-3 .
  2. ↑ Otto-Albrecht Neumüller (Ed.): Römpps Chemie-Lexikon. Volume 1: A-Cl. 8th revised and expanded edition. Franckh'sche Verlagshandlung, Stuttgart 1979, ISBN 3-440-04511-0 , p. 496.
  3. ↑ FEPA standards 43-1: 2006.
  4. ↑ FEPA Standard 43-2: 2006.

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