Abrasive s

Engineering MaterialsAbrasives

Dr.V.S.Gayathri & Dr.K.Yamuna

Contents

Definition

Classification with examples

Synthesis of SiC and B4C

Abrasive products

Coated Abrasives


Mohs Scale

Natural Abrasives

Synthetic Abrasives

Coated Abrasives

Grinding wheels

Definition

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


Abrasive power / Abrasion It is the strength of an abrasive to grind away another materials. It depends on hardness, toughness and refractoriness.

Terms and Terminologies

They should possess good hardness, toughness and refractoriness

Hardness: Ability to resist scratching Hardness: Ability to resist scratching

Toughness: Ability to make indentation

Refractoriness: Ability to withstand high temperature (Frictional Heat)

Mohs Scale - Mineral Hardness

The Mohs scale of mineral hardness was devised by Frederich Mohs, a German mineralogist in 1812.

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.

Mineral Mohs scale composition

Talc 1 Mg3Si4O10(OH)2Gypsum 2 CaSO4.2H2OCalcite 3 CaCO3Fluorite 4 CaF2Apatite 5 CaF .3Ca (PO )


Apatite 5 CaF2.3Ca3(PO4)2Feldspar 6 K2O.Al2O3.6H2OQuartz 7 SiO2Topaz 8 AlF3.SiO2Corundum 9 Al2O3Diamond 10 C


Microcline is commonly called Feldspar

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Classification

Based on Occurrence -2 types

Based on Hardness 3 types Based on Hardness 3 types

Note

Files which are used in workshop are not abrasives.

They remove material not by scratching or rubbing, but by


They remove material not by scratching or rubbing, but by the cutting action of sharp teeth which have been cut into the surface of the file, very much like those of a saw.

Natural Abrasives

Corundum

Diamond

EmeryEmery

Garnet

Quartz

Corundum / Alundum - Al2O3PropertiesCrystalline Very HardMohs scale-9brown to grey


Usesin grinding wheelsto grind glass/lens/metals,ruby lasers

Diamond


Diamond (C)Propertiescrystalline and Chemically inactiveMohs scale-10

Exist in three major forms Diamond (gem) Borts (Faulty) Carbonade ( black diamond of Brazil)


Carbonade ( black diamond of Brazil)

UsesGem quality : JewelleryBorts and Carbonade: in bits of drilling pointssaw teeth for cutting rocksin grinding wheelsin engraving tools

EmeryCompositionAlumina (crystalline) 50 - 75%Magnetite 20 - 40%Other minerals 12 %

Properties


Dark grey to blackHardness depends upon alumina contentMohs scale 8

Uses In emery paper and cloth used for polishing. In bits of cutting and drilling tools

Garnets

Like diamond available in two forms : gem and abrasive

Tri silicates of alumina/ magnesia/ ferrousoxide. (Gem quality)

Abrasive garnet- Calcium aluminium iron silicate


silicate

PropertiesModerate HardnessMohs scale 6 -7.5

Usesto paper/cloth to polish wood /metals,as bearing pivots in watchesin glass grinding

Quartz (SiO2)

PropertiesSiO2 (pure crystalline)Moh 7Hydrated form of SiO2 is called Flint

Uses


Uses

Impure grey quartz used in sand paperAs granules in grindingmachines used to grind flour, pigments & ores.

Synthetic Abrasives

SiC :Silicon Carbide / Carborundum / Crystolon

B4C:Boron Carbide / Norbide

Preparation of SiCi. Petroleum Coke (source of carbon)

ii. Sand (source of Si)

iii. Saw Dust

All raw materials sized, dried and mixed along with old charge and fed into the Acheson furnace with little amount of NaCl (flux)

1650 - 2200 C1650 - 2200oC

3C+SiO2SiC+2CO(36 hrs)

Sawdust added to produce porosity and to permit gas to escape duringOperation in addition to providing hardness

Salt added to assist in removing impurities

Properties and uses

Suited for grinding materials that have low tensile strength and high density

Harder and tougher than aluminum oxide Color varies from green to black Green used mainly for grinding cemented carbides and other hard

materials Black used for grinding cast iron and soft nonferrous metals (also

ceramics) M.Pt. is 2700oC


M.Pt. is 2700oC Mohs scale hardness is 9.3 Chemically Inert High Thermal Stability BrittleCutting tools , Grinding of cast iron , brass , bronze , porcelain and marble Polishing leather , lenses ( Abrasive paper and Cloth)Refractory in furnace

Norbide

Preparationi.Petroleum Coke (source of carbon)

ii. Boron Oxide

iii. Saw Dust ( To provide hardness)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).

2700oC 7C+2B2O3B4C+6CO

Properties and UsesMohs scale hardness - 9 . 7Chemically inert

Hardest material manufactured with exception of diamond Not suitable for use in grinding wheels Used only as loose abrasive and as cheap substitute for diamond

dust


dust Manufacture of precision gages and sand blast nozzles Used in ultrasonic machining applications Grinding dies & Cutting and sharpening hard & high speed tools

Advantages of Synthetic Abrasives

Uniform Chemical Composition

Uniform Hardness

Size Purity and Hardness can be controlled

Abrasive Products

After abrasive is synthesised, they are converted into products Grinding wheel ,Paper, Cloth ,sticks etc.,


Supporting base material components and abrasive grain are bond using a suitable adhesive

Bond Types

Function of bond is to hold abrasive grains together in form of wheel / paper / cloth

Six common bond types used in grinding wheel manufacture: Vitrified Resinoid


Resinoid Rubber Shellac Silicate Metal

Coated Abrasives

(paper / cloth)


Common types of coated abrasive (sand paper)

Abrasive Material Use of paper

Garnet Wood Working


Aluminium Oxide Metal Working

Silicon Carbide Wet / dry application

A coated abrasive comprises an abrasive fixed to a backing material such as paper, cloth, rubber, resin, polyester or even metal sheet, which are flexible

Sandpaper is a very commonly used coated abrasive.


Manufacture of coated abrasives

Supporting base material consist of flexible backing (cloth or paper) to which abrasive grains have been bonded

Two purposes for which coated abrasives are made Metal grinding and polishing

Coarse-grit used for rapid removal of metal; fine grits used for polishing


Two types of abrasive materials used Natural: garnet, flint, and emery Manufactured: aluminum oxide, silicon carbide

Scheme


Grinding Wheels

Grinding wheel consists of hard abrasive grains called grits, held in the weak bonding matrix

A grinding wheel is commonly identified by the type of


the abrasive used

Manufacture of Grinding wheels

Manufacture of these wheels is a precise and tightly controlled process

The wheels are generally made from a matrix of coarse particles pressed and bonded together to form a solid,


particles pressed and bonded together to form a solid, circular shape

Alternatively they may also be made from a solid steel or aluminium disc with abrasive particles bonded to the surface

Corundum wheel Carborundum wheel


To use the grinding wheel, it must first be clamped to or mounted on the grinding machine


A grinding wheel with a reservoir to hold water as a lubricant and coolant.

Grinding wheels with diamond or Cubic Boron Nitride (CBN) grains are called super abrasives

Grinding wheels with Aluminum Oxide (corundum), Silicon Carbide or Ceramic grains are called conventional abrasives


conventional abrasives

Basic Functions of Grinding Wheels

Generation of cylindrical, flat and curved surfaces

Removal of stock

Production of highly finished surfaces


Production of highly finished surfaces

Cutting-off operations

Production of sharp edges and points

Documents

Abrasive s