Engineering Materials

Abrasives

Abrasives

� Abrasives are those materials characterized by its hardness

� which are used to wear down softer surfaces in operations such as

� grinding,

� polishing,

� cutting,

� sharpening,

� lapping,

� honing, pressure blasting or other similar process.

Different forms of abrasives used in Industries:

� 1. Polishing wheel - For polishing Stainless steel, Steel, Non-metal materials, Stone, Wood

� 2. Grinding wheel, Cutting wheel and grinding head – in granite quarry- cutting and polishing the rocks

� 3. Cup brush - Used in rust removing, dirt removing, polishing, etc

� 4. Abrasive paper/cloth /belts – in fine finishing / polishing operations

� Common uses for abrasives include grinding, polishing, buffing, honing, cutting, drilling, sharpening, lapping, and sanding

Norton R228 Abrasive cloth

� Sandpaper or glasspaper is a form of paperwhere an abrasive material has been fixed to its surface.

General Characteristics:

� - Possess high hardness

� - Possess moderate to high fracture toughness

� - Resistant to mechanical shock

� - Not affected by frictional heat

Mohs scale

� The Mohs scale of mineral hardness characterizes

the scratch resistance of various minerals.

� The Mohs scale of mineral hardness is based on the

ability of one natural sample of matter to scratch

another.

� It was created in 1812 by the German mineralogist

Friedrich Mohs and is one of several definitions of

hardness in materials science.

� Mohs hardness Mineral

� 1 - Talc (Mg3Si4O10(OH)2)

� 2 - Gypsum (CaSO4·2H2O)

� 3 - Calcite (CaCO3)

� 4 - Fluorite (CaF2)

� 5 - Apatite (Ca5(PO4)3(OH-,Cl-,F-)

� 6 - Orthoclase Feldspar (KAlSi3O8)

� 7 - Quartz (SiO2)

� 8 - Topaz (Al2SiO4(OH-,F-)2)

� 9- Corundum (Al2O3)

� 10 - Diamond (C)

� On the Mohs scale, a pencil "lead" (graphite) has a

hardness of 1.

� a fingernail, 2.2-2.5;

� a copper penny, 3.2-3.5;

� a pocketknife 5.1;

� a knife blade, 5.5;

� window glass plate, 5.5;

� and a steel file, 6.5.

� Diamond, a common abrasive occurs both naturally and is industrially produced.

� Corundum which occurs naturally but which is nowadays more commonly manufactured from bauxite as Alundum.

� The most usual abrasive used is aluminium oxide.

� Also common are silicon carbide, tungsten carbide and garnet.

� Softer minerals like calcium carbonate are used as abrasives, such as "polishing agents" in toothpaste.

Classification

� i. Natural abresives

� siliceous – quartz , garnet.

� Non-siliceous -- diamond, corundum, emery

� ii Synthetic / artificial abresives–

carborundum, boron carbide, alundum

Natural abrasives

Diamond

� Crystalline form of C, ranks I in hardness

� Moh’s scale hardness -10, Chemically

inactive,.

� Exist in three major forms – gem diamond,

borts –offcolor diamond, carbonado- black

diamond.

� Used in Jewellery,in bits of drilling

points,saw teeth for cutting rocks,in grinding

wheels,in engraving tools.

� Diamond wheels are grinding wheels with

industrial diamonds bonded to the periphery.

� Diamond abrasive belt developed for grinding

glass and ceramics materials.

Corundum

� Naturally occuring cryst. form of alumina.

� ranks next to diamond.

� Moh’s scale hardness -9.

� not of gem qlty,brown to grey (red-ruby, other than red or dark pink –sapphire("blue stone" ))

� Used in grinding wheels to grind lens/metals

Emery

� It is a hard , fine grained,opaque and dark grey

to black.

� Composition:crystalline alumina (50-5%)

magnetite (20 -40%) other minerals(12 %)

hardness depends upon alumina content –

Moh’s scale hardness -8

� Used in emery paper/cloth – for polishing,

cutting and drilling tools

Garnet

� It is a trisilicates of alumina/ magnesia/ ferrous oxide.

� Common garnet- a complex Calcium alminium iron silicate.Ca3Al2(SiO4)3. Ca3Fe2(SiO4)3.

Fe3Al2(SiO4)3.

� Moh’s scale hardness 6 -7.5.

� Used in paper/cloth to polish wood and metals, in bearing pivots in watches and in glass grinding.

Quartz

� SiO2 (crystalline),

� Moh’s scale hardness – 7.

� Impure grey quartz used in sand paper as granules

� Used in grinding machines used to grind flour,

pigments, ores.

Synthetic abrasives:

� SiC & B4C –Commonly used forms of synthetic

abrasives in abrasive paper/ cloth, grinding wheel.

Silicon carbide/carborundum

� 2000o C

� SiO2 + 3C -------→ SiC + 2CO

� Characteristics : hard synthetic abrasive (Moh’s scale

hardness 9.3),chemically inert, high refractoriness

� Used in grinding of low tensile strengths like cast

iron, bronze, porcelain/marble, glass/ lens

� And in cutting wheels, polishing machines

boron carbide / norbide(B4C)

� 2700oC

� B2O3 + 7C → B4C + 6CO

� It resist oxidation more than diamond.

� Moh’s scale hardness 9.7

� Used in making of hard materials for making grinding

dies, for cutting & sharpening high speed tools

� Cubic Boron Nitride (CBN) Abrasive Wheels are

used for grinding dies, for cutting & sharpening high

speed tools.

Bonded abrasives

� A bonded abrasive is composed of an abrasive material contained within a matrix. This matrix is called a binder and is often a clay, a resin, a glass or a rubber.

Coated abrasives

� A coated abrasivecomprises an abrasive fixed to a backing material such as paper, cloth, rubber, resin, polyester or even metal many of which are flexible. Sandpaper is a very common coated abrasive.

A German sandpaper