ABRASIVES. Abrasive is a substance used to wear down ( cut / polish / grind /sharp) the surface of the materials with which it is in contact

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1 ABRASIVES Topics discussed Definition Classification with examples Mohs Scale Applications Natural abrasives Synthetic abrasives Throughout history, humans have used materials ranging from beach sand to walnut shells to paper bags as abrasives. Widely-used naturally occurring abrasives include garnet, cerium oxide, flint, emery, corundum (aluminum oxide), and diamond. These materials may have varying characteristics and chemical compositions depending on the specific geological source. Manufactured versions of these materials are usually more consistent in chemical composition and other characteristics. Abrasives can be distinguished in a variety of ways based on their hardness / color/ chemical composition / crystal shape etc to name a few. Since the chemical composition- that is, the type of material- determines the abrasiveness, the identification of abrasives based on composition is popular Definition Abrasive is a substance used to wear down ( cut / polish / grind /sharp) the surface of the materials with which it is in contact Abrasion is the ability of a substance to wear or tear the surface of other substance Abrasives should possess good hardness, toughness and refractoriness Hardness: Is defined as the ability of a substance to resist scratching Toughness: Is defined as the ability of a substance to cause indentation or to scratch other material Refractoriness: Is defined as the ability of a substance to withstand high temperature (Frictional Heat) Dr.V.S.Gayathri and Dr.K.Yamuna Page 1 11/27/2008

2 Abrasives are classified Based On Occurrence : Natural and Synthetic Based on Hardness: Hard,Moderate and Soft Occurrence Natural Talc Synthetic SiC, B 4 C Non siliceous Diamond, Corundum Siliceous Quartz, Garnet Hardness Hard (M.S >7) Diamond,, SiC, Moderate (M.S. 3 7) Quartz Soft (M.S. <3) Talc Mohs Scale - Mineral Hardness The Mohs scale of mineral hardness was devised by Frederich Mohs, a German mineralogist in To devise the scale, he selected ten minerals as a basis because they were common or readily available, thus the scale is not linear, but is a bit arbitrary. It is a measure of the relative hardness and resistance to scratching between minerals (tested by Pin on Disc method) Though there are other scales like Rockwell, Vickers and Brinell, Mohs scale is still the most common scale of mineral hardness. Dr.V.S.Gayathri and Dr.K.Yamuna Page 2 11/27/2008

3 Moh s scale Mineral Moh s scale composition Talc 1 Mg 3 Si 4 O 10 (OH) 2 Gypsum 2 CaSO 4.2H 2 O Calcite 3 CaCO 3 Fluorite 4 CaF 2 Apatite 5 CaF 2.3Ca 3 (PO 4 ) 2 Feldspar 6 K 2 O.Al 2 O 3.6H 2 O Quartz 7 SiO 2 Topaz 8 AlF 3.SiO 2 Corundum 9 Al 2 O 3 Diamond 10 C Mnemonics to remember the order of Mhos scale To Get Candy From Aunt Fanny Quit Teasing Cousin Danny Dr.V.S.Gayathri and Dr.K.Yamuna Page 3 11/27/2008

4 Mohs scale of common household substances (Just for information do not memorise) Material Hardness Plastic 1 Salt 2.3 Fingernail 2.5 Gold, Silver Copper coin 3.5 Platinum Window glass 5.5 Steel File 6.5 Ceramic tile 7.0 Dr.V.S.Gayathri and Dr.K.Yamuna Page 4 11/27/2008

5 Natural Abrasives Corundum (Al 2 O 3 ) [Alundum] It is Crystalline Very Hard Moh s scale-9 brown to grey in colour Corundum is used in In grinding wheels To grind glass/lens/metals, Ruby lasers Diamond (C) Diamond exist in three major forms Diamond (gem grade) Borts These are diamonds that are off colour or faulty Carbonado These are black diamonds mined from Brazil. They have good hardness, but due to lack luster, do not find application as jewelry. They are commonly used as abrasives Abrasive grade diamond has the following properties crystalline Chemically inactive Moh s scale-10 Uses In bits of drilling points Saw teeth for cutting rocks In grinding wheels In engraving tools Dr.V.S.Gayathri and Dr.K.Yamuna Page 5 11/27/2008

6 Emery Composition Alumina (crystalline) 50-75% Magnetite 20-40% Other minerals 12 % Dark grey to black Hardness depends upon alumina content Moh s scale - 8 Uses In emery paper and cloth used for polishing. In bits of cutting and drilling tools Garnets Tri silicates of alumina/ magnesia/ ferrous oxide. Common garnet- Calcium alminium iron silicate Moh s scale Uses To paper/cloth to polish wood /metals, As bearing pivots in watches In glass grinding Quartz (SiO 2 ) SiO 2 (pure crystalline) Moh s scale 7 Hydrated form of SiO 2 is called Flint Dr.V.S.Gayathri and Dr.K.Yamuna Page 6 11/27/2008

7 Uses Impure grey quartz used in sand paper As granules in grinding Machines used to grind flour, pigments & ores. Synthetic Abrasives Silicon Carbide / Carborundum / Crystolon (SiC) Preparation Raw materials are i. Petroleum Coke (source of carbon) ii. Sand (source of Si) iii. Saw Dust ( To provide hardness) All raw materials are sized, dried and mixed along with old charge and fed into the Acheson furnace with little amount of NaCl (flux) o C (36 hrs) 3C + SiO 2 SiC + 2CO M.Pt. is 2700 o C Mohs scale hardness is 9.3 Chemically Inert High Thermal Stability Brittle hence strength is less USES Cutting tools Grinding of cast iron, brass, bronze, porcelain marble Polishing leather, lenses (Abrasive paper and Cloth) Refractory in furnace Dr.V.S.Gayathri and Dr.K.Yamuna Page 7 11/27/2008

8 Boron Carbide / Norbide (B 4 C) Preparation i. Petroleum Coke (source of carbon) ii. Boron Oxide iii. Saw Dust (To provide hardness) All raw materials sized, dried and mixed along with old charge and fed into the Acheson furnace with little amount of NaCl (flux) o C 7C+2B 2 O 3 B 4 C + 6CO Uses Moh s scale hardness Chemically inert Resist oxidation more than diamond. Grinding dies Cutting Sharpening hard high speed tools Advantages of Synthetic Abrasives Unlike natural abrasive synthetic abrasives possess uniform chemical composition Due to uniformity in the composition the hardness at various points also remain uniform Dr.V.S.Gayathri and Dr.K.Yamuna Page 8 11/27/2008

9 Important General Applications of Abrasives In grinding wheels (Bonded abrasives) abrasives are used to grind and polish the surface of metals and composites,sharpen tools like knife,saw tooth etc., In the form of paper / cloth (abrasive coated on paper / cloth with the help of glue) abrasives are used for cleaning and polishing soft and precious metals (Au, Ag, W etc.,).they are also used to shine leather, clean lens, windscreen etc., In few places abrasives are used in the form of loose powder-for polishing hard metal and wood Cleaning products also contain abrasives suspended in a paste or cream. For example tooth paste contains calcium carbonate / silica /mica as a "polishing agent" to remove plaque and other matter from teeth as the hardness of the abrasive used is less than that of tooth enamel but more than that of the contaminating agent(plaque material). Abrasives may also be used to prepare surfaces for application of paint and varnish Pumice stone ( abrasive) is used to remove dead cells from skin (usually to keep the feet crack free) Soft abrasives are used in cosmetic industries in various products to rejunivate skin and maintain a smooth and silky texture Dr.V.S.Gayathri and Dr.K.Yamuna Page 9 11/27/2008