· ~

First published 1982 Reprinted 1984

Second edition published 1988 by Robert Brown & Associates (Aust) Pty. Ltd.

Third Edition published 1994 by Technical Secretarial Service P.O. Box l88 Forestville, NSW 2087

Reprinted 1998, 1999,2000,2002,2004, 2006, 2008

Copyright © P.A. Sheedy, 1988

National Library of Australia Card Number and ISBN 978-0-9592907-1-4

Distributed by Technical Secretarial Service P.O. Box l88, Forestville, N.S.W. 2087 Australia

Printed in Australia by Ligare Ply Ltd, Riverwood, N.S.W. 22LO

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system , or transmitted in any fann or by any means, electTonic, mechanical, photocopying, recording, or otherwise, without the prior permission of the publisher.

TABLE OF CONTENTS

INTRODUCTION ....................................................................................................................... 1

CHAPTER 1 CLASSIFICATION OF MATERIALS .............................................................. 2 SOLIDS, LIQUIDS AND GASES ............ ..... ....... ... .. .. .. .... ... ................. .... .... .................. 2 ELEMENTS, COMPOUNDS AND MIXTURES ................... ... .. ...... ... ............. .. ........... .. 2 CHANGES IN MA TIER ...................... ... .. .. .... ........... ... .... ..... ..... .. ................................ . 4 METALS, NON METALS, COMPOSITES AND METALLOIDS ..... .. ... ... ... .. .... .. .. ... ..... 5

CHAPTER 2 NATURE OF MATERIALS ............................................................................... 13 THE BOHR ATOM ..... ... ............................. ... .... ... .. .. ...... ... ..... ........................................ 15 PERIODIC TABLE OF ELEMENTS ... ... .. .. ....... ........... ..... ... ..... ..... ......... .. .. ... ................ 15 BONDING ........... .......... ... ... ... ... .. .. .................................................................. ............... 16 RELATIONSHIP BETWEEN BONDING AND PROPERTIES ....................................... 20

CHAPTER 3 PHYSICAL PROPERTIES OF MA TERIALS .................................................. 24 THERMAL PROPERTIES .... .... ... ...... .. .. .......... ........ .... ... .......... ... ... ........... .......... ...... ..... 24 ELECTRICAL PROPERTIES ... ... .... .... .. .... ....... ........... ........ ..... ...................................... 27 MAGNETIC PROPERTIES ........................ ......................................... .... ... .. ...... .. ...... .... 29 DENSITY ... ... ..... ... ...... ..... .. .... .. .. .. ... .... .. .. .. .. .......... .... ...... ...... ..... ........ ... ... ....................... 33 REFRACTIVE INDEX .. ........................................................................... .... ... ... ..... ........ 33

CHAPTER 4 PHYSICAL TESTING ........................................................................................ 36 THERMAL PROPERTIES ... ... ... .......... .... .... ....... ....... ... ............................. ....... .............. 36 THERMAL CONDUCTIVITY .... .. ..... ..... .. ... ... ............ ..... .. ..... .... .... ................................ 36 ELECTRICAL PROPERTIES .......................................................... ...... ....... ... ..... ...... ... . 38 MAGNETIC PROPERTIES ........................................ ..................... .. ....... .. .. .. ...... ....... ... 39 DENSITY ...... ..... ........ ..... ... .... .. .... ... ... ... .. .. .... ... .... .... ... ... ... ... ....... .................................... 40 REFRACTIVE lNDEX ................................................... ... ... ..... ..... ... ...... ... .... ........ .... ..... 40

CHAPTER 5 MECHANICAL PROPERTIES OF MATERIALS ........................................... 42 STRENGTII ............ ....... .. ............................................... ................ ..... ... .... ... ....... ..... ..... 42 HARDNESS ........................... ... ...................................... ........ ........ .... .... ............. ........... 43 TOUGHNESS ... .......... ..... ...... .... ... ... ... .... .. .. ... .... ... .... ................................... ................... 44 ELASTICITY .. ..................... ... ..... ... .. ... ... ....... .... ....... .... ............. ... ........ ....... ................... 44 PLASTICITY .............. .................. ....... ... .. ..... .... ................................ .... .. .. .. .... ... ...... ...... 45 DUCTILITY .... ............ ...... ... ... ... .. ... .. ... ... .. ... ...... .... ... ....... .......... ..................................... 45 MALLEABILITY ........ ........................................... ..... .......... ..... ...... ........ .. .. ..... .. .... ........ 45 FATIGUE .... .. .... .... .. .... .... ... .. ... ... .. ... .. ... ... ..... ...... .. ... .. .. .. ............................................. .... . 46 CREEP STRENGTII .. .... ... ... ... ... .. ..... ... .... .... ...... .. .. .. ..... ... ..... ..... ...... ... ............................ 46

CHAPTER 6 MECHANICAL TESTING ................................................................................ 48 TENSILE TEST .................................................................... ......... ........ .......... ..... .......... 48 HARDNESS TESTING ..... ... ....... ... ....... ... .. .. ... ........... .. .... .. .......... ................................... 5S IMPACT TESTS .. .......... ... ... ... .... ... .. ..... .... ... ... ....... ........ .... ... ......... ..... .... ...... .................. 62

CHAPTER 7 NON·DESTRUCTIVE TESTS ........................................................................... 69 LIQUID PENETRANT TESTS .. .. ... .. .... ... .. .. ... .... .... ................. .............. ....... ... ........ ....... 69 MAGNETIC PARTICLE TESTS ................. ...................................... ..................... ..... ... 70 EDDY CURRENT INSPECTION .......... .... .................................................... ... .......... .. ... 73 RADIOGRAPHIC INSPECTION ... ....... ..... .. ................................................. ... ............ ... 74 ULTRASONIC TESTING ........... ... ............... ... ... .... ............... ......................................... 77 STRAIN GAUGING .... ............ ... .. .. ............................ ... ... .. ........... ......... .. ....... .. ....... ....... 78 COATING THICKNESS TESTING .. ....... .... ... ... ................... ..... ... ... ............................... 80

CHAPTER 8 CORROSION ...............................................•.•.................................................... 83 ELECTROLYTES .......... ...... ... ... ... ...... .... ..... .. ...... ...................... ............. ..... ................. .. 84 FORMATION OF ANODE AND CATHODE ... ...................................... .... ...... .............. 85 CHEMICAL REACTIONS AT THE ANODE AND CATHODE ....................... .............. 92 FACTORS AFFECTING CORROSION .... ........ ... ...... ... ........... ... ........ .. ...................... .... 95 CORROSION SYSTEMS .. ............. .... ...... ............ ................................... ... ....... .............. 97 CORROSION PROTECTION .. ............ .... .. .. . ; ............ .............. ... .. ... ... .. ................ ... ... .... 102 DEGRADATION OF PLASTICS ..... ................................... ..................... ..... ......... ......... 106 CORROSION TESTING OF METALS .... .. .. ... .... ... ... ....................... ............. .................. 110 TESTING OF PLASTICS ............... ....................... ... ......... ....................... ... .... ..... ....... .... 112

CHAPTER9 WATER .......•....................................................•.....•....................................•....... 114 NATURAL WATER ........ ...... ...... .... .. ... .... ............ ............................ ....... ..... ......... ......... 114 PURIFICATION OF WATER .......................... ... ... .. ..... .. .. ..... ....... .............................. ... . 116 BOILER WATER CONDITIONING ................ ............................................. ......... ......... 117

CHAPTER 10 BITUMINOUS MA TERIALS .......................................................... ... ............... 120 TESTING OF BITUMINOUS MATERlALS ................................................... ............ .... 121

CHAPTER 11 FERROUS METALS .......................................................................................... 123 CAST IRONS ..... ........................... ............. ............ .............. ........................... ..... ..... ...... 126 STEELS - CAST AND WROUGHT ............ ... ........ ......... ..... .... ..... .. .. .. .. .......................... 129 HEAT TREATMENT OF STEELS .. .. ... ............... ........... ................... ... ... .. ..... ..... ........... 130 SURFACE HARDENING OF STEELS ....... ...... ... ..... ...... ........ ... ... .. ... .... ......................... 133 EFFECTS OF PROCESS HEAT ON STEELS .................................... .... .. .... .......... ......... 135

CHAPTER 12 NON-FERROUS METALS - NON METALS ................................................... 137 NON-FERROUS METALS ......... ............. ....... ............................................. ..... .... ..... .... . 137 SURFACE TREATMENT OF ALUMINTIJM .. ......... ........ ... ..... ... .... ...... ... ......... .............. 142 NON-METALS ........ ................. ............................ ............ ....... ................................... .... 144 GLASSES .............. ...... ... .... .... .. ... ...................... ....... ....... ... ..... ... .... .. .. .. .. .. ........ .............. 147

CHAPTER 13 PROCESSING OF MATERIALS .......................................................... ......... ... 150 SAND CASTING ... ........ ..... .. .. ....... ... ... ...... ... ........... ........ .. ..... .... ..... .......... ....... ....... ... .... 150 SHELL MOULDING ... ........................................................... ............. .. .. ...... .............. .... 151 INVESTMENT CASTING .. .......... ... ... .... ... .............. ... ...... ..... .......... ........ ....... ......... ... ... . 152 PERMANENT MOULD CASTING ................ ....... ......... ...................... .......... .......... ...... 152 DIE CASTING .. ................ ... ... ... .. .. ... .............. .................. .. ......... ..... .... .... ... ................... 153 HOT WORKING OF METALS ... ...... ... .................. ... ... ... ..... ... .............. .... ...... ... ....... ...... 153 COLD WORKING OF METALS ....... .. ............. ...................... ... .... .. ... .. ... .... ...... ....... ...... 157 NEWER FORMING TECHNIQUES ........... ... ... ........... ... ........ .... .. ..... ... .......................... 160 POWDER METALLURGY ............ .... ... ... .. ................ ..... ............................... .... ..... ..... ... 162 PROCESSING OF PLASTICS ... ... ..... ... ............ ... ... .............. ..... .. .. .. .. .. ... .. ... ................... 164

CHAPTER 14 JOINING OF MATERIALS ............................................................................... 166 FASTENERS ............ ........................... ... .. .. ... ........... ... .. .. ......... ........... .. .. .......... .............. 166 SOFT SOLDERING ...................... ... ... ........... ........ ........ ... ...... .. .... .... .. .. .. ... ....... .......... .... 166 BRAZING .. ........ ..... .. ................. .. ...... .............. .... .. ........ ............. ......... ...... ..................... 168 WELDING ... ........... ..... ..... ................. .. ..................... ........ ........ ............................... .... ... 168 ADHESIVES ........... ..... ..... ................... ... .... ......................... ..... ....... ........ ..... ..... ...... ... ... . 176

CHAPTER 15 FINISHING AND COATING ................•..•...............•..•.................. .•.................. 181 ELECTROPLATING .. ..... ... ... ... .. ....... .. ... .. .. ... .. ..... .... ... ...................... .. .. ..... .. ............. ... ... 182 IMMERSION PLATING .... ... .. .... ... ... .. .. ... ... .................... ... ............... .. .. .. ........... ....... ... ... 185 ELECTROLESS PLATING (AUTOCATALYTIC PLATING) ...... .. ................................ 185 VACUUM DEPOSITION ..................... .. .. .. ..... ..... ................. .................... ... .... .. ..... ..... ... 185 OXIDE COATINGS ... ..... .... ... ..... .. ............................. ........... ..... ..................................... 185 PHOSPHATE COATINGS ................. ... ... .. .. ... ..... ........ ................. .. .. .. .. .................. ........ 186

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CHROMA 1E COATINGS ... ... ... ................ ..... ........ .. .......... ................ ....... ............... ...... 186 ANODISED COATINGS .................. ....................................... ...... ..... ............ ... ......... .... 186 NON-METALLIC COATINGS ......... ... ............................................................ ..... .... ...... 186 PAINT SYS1EMS ..... .. ... .... ... ... ..... .............. ........ ... ..... ...... .............................................. 188 COATING SELECTION ... .......... ........... ..... .. ... ...... ........... ..... ..... ...... ............ ... ............... 189

CHAPTER 16 SPECIAL MANUFACTURING TECHNIQUES ........ u ••••••••••••••••••••••••••••••••••••• 191 PRIN1ED CIRCUIT BOARDS .. ..... ............... .......... .................................... ... ................ . 191 ENCAPSULATION MATERIALS ........ ..... ..................................................................... 192 SEMICONDUCTOR MATERIALS .................................................... ............................. 194

CHAPTER 17 THE MATERIAL SELECTION PROCESS ................. u •••••••••• u •••••••••••••••••••••• 197 ANALYSIS ... ................................. .......... ...... .................. .... .... ....................................... 197 AL1ERNATNE SOLUTIONS ................... ........ ................................... ....... ..... ....... ....... 197 EVALUATION ............................................. .. ........... ...... ........ ..... .................................. 197 THE FINAL SELECTION .......... ................ ..... ...... ........ ...... ........... ....... .......................... 198 THE LAST STEP .......... .... ........................................ ..................... ..... ....... ..................... 198

INDEX ..................................... u •••••• • ••••••••• uu •••••••••• • •• uuu ••••••••••• •• • • • •••• u ••••••••••••••••••••• uu •••••••••••• 199

iii

INTRODUCTION

Man, today, has available to him an infinite variety of materials and tools to produce almost any object he desires.

Yet if we look briefly back into history, we fmd that these materials have evolved over some 5000 years.

One can consider mans' first encounter with 'technology' began in the Stone Age with the discovery of the powers of fire, not only for cooking, but as a means of hardening wood for weapons and tools.

Man developed an awareness of the property we call hardness, in the use of hard rocks for grinding spears and other implements, of flint for sharp edged knives and axes.

The first metals known to be used were gold and silver, probably because they were found in nature in the "free" or metallic state. These metals have always been considered to be precious and so have received considerable attention, though their industrial use represents only an infmitesimally small percentage of the tonnage of all metals used today.

It is believed that copper was first extracted from its ores, by accident, about 4000 Be. Some 2000-3000 years later, iron was being smelted by the Egyptians and Syrians, for the manufacture of armour, swords and knives, as well as for ancient agricultural implements.

Later still, aluminium and the more exotic metals were discovered.

Of the other materials, concrete had its beginnings in the Roman Period where it was known as Pozzolana

Plastics were not exploited until the mid nineteenth century with the discovery of celluloid, and then in the early 20th century, bakelite was discovered. Since then, a wide range of plastics, each with specialised properties, has been developed.

Perhaps the most dramatic development in materials in recent years has resulted from research into the properties of the semiconductor materials such as silicon, germaniwn and arsenic. This science is advancing so rapidly that one cannot predict just where it will lead in the future.

We live today in a period where science and technology are advancing at an ever increasing rate.

This text reflects the knowledge of materials as we know them, and it is hoped, will provide the student with some insight into the science on which Materials, Their Properties, Selection and Testing is based.

CHAPTER 1 CLASSIFICATION OF MATERIALS

Everything we touch, everything we see, everything we use in our world is made from some combination of elements selected from a total of 105 elements known to man, of which 92 occur m nature.

Most of these elements are of little use in their 'pure' fonn. Rather they are used in combination with other elements to produce materials with specific properties.

SOLIDS, LIQUIDS AND GASES

There are numerous ways in which we may classify these materials - I have chosen to commence at the beginning - all materials can be basically classified according to their state: • Solid • Liquid • Gaseous.

A solid we defme as having a defmite shape and occupying a fixed space or volume. In the solid state, the atomic particles that make up the solid are packed closely together, generally in an ordered three­dimensional network. There are defmite bonds holding the atoms together in a pattern that is repeated throughout the whole of the solid. These solids have a crystalline structure - they can be broken down to individual crystals or grains. There are some solids that do not have this repetitive three­dimensional pattern and are not crystalline in nature - these are known as amorphous solids they are without fonn or structure.

Most metals and inorganic solids are crystalline in nature, whereas glasses and plastic solids are generally amorphous, that is they have no crystalline structure.

A liquid is a substance that flows. or that confonns to the shape of the vessel in which it is contained. The particles in a liquid are mobile, and can move freely past each other. Thus a liquid does not exhibit any structure, although the particles do have a definite chemical composition.

A gas is a substance that diffuses and fills the vessel in which it is contained - its particles are more mobile than those in a liquid, and are spaced further apart than they are in a liquid. The actual volume of the particles in a gas is much less than the volume occupied by the gas. Hence gases can readily be compressed.

It is important to realise that the state of matter can be changed by changing the conditions of its envirorunent.

Thus liquid water can be changed to solid ice by cooling or to steam by heating.

Likewise steel may be liquefied by heating to over 1500°C, and air may be liquefied by cooling to about -200°C.

ELEMENTS, COMPOUNDS AND MIXTURES

All materials, irrespective of their state, can also be classified as: • elements • compounds • mixtures.

An element is a simple subst.'lIlce that cannot be broken down by chemical or physical means into chemically simpler substances.

2

CHAPTER I CLASSlFICA TION OF MA 1ERIALS

There are only 105 elements known to man . The most common method of classifying elements is the Periodic Table, and one of the features of an element is that its propenies give it a defmite place in the Periodic Table.

Of the 105 elements listed in the Periodic Table, only 92 are stable and are known to OCCur naturally, and of these: • 79 are solid • 2 are liquid • 11 are gaseous in their natural state.

A compound is formed when TWO or more elements combine together chemically in fIxed proponions by weight. The properties of the compound in no way resemble the properties of the elements from which it is formed.

For example: Sodium (Na) is a very reactive metal Chlorine (CI) is a very noxious gas. Yet when these TWO elements combine, we have sodium chloride (NaCI) or as it is better known - common salt.

Compounds are homogeneous, and have a defmite composition. Compounds can be broken down by chemical or physico/chemical means into their component elements. However the procedures for recovering the individual elements are often very complex and require a good deal of energy.

A mixture is formed when TWO or more elements or compounds are mechanically mixed together in any proportion. In a mixture there is no chemical reaction or bond beTWeen the components of the mixture, and they can be separated by physical or mechanical means. Thus, we may dissolve salt in water to form a mixture - salt water. The two may be subsequently separated by distilling off the water to leave a salt residue. This is a physical separation, and does not involve any form of chemical reaction, or chemical change.

Most metals used in engineering applications are alloys (or mixtures) of metals which appear homogeneous, but when examined microscopically, their individual components may be identified. Likewise, many composite materials such as reinforced concrete, and fIbre reinforced plastics are mixtures in the chemical sense.

Most natural forms of matter, as they OCCur are mixtures of TWO or more elements or, more commonly, TWO or more compounds, held together by physical rather than chemical means. For instance iron OCCurS in nature as a compound, iron oxide, which is mixed with a large amount of earthy waste (called gangue) containing silica (quanz), manganese oxides, and other compounds. Thus iron ore is mined as a mixture that must fIrst be separated before the iron can be extracted from its oxide. Mixtures can vary widely in composition, and are often heterogeneous.

TO SUMMARISE

An element • is a pure substance • is homogeneous - it has the sarne composition throughout • cannot be chemically decomposed or changed • has properties distinct from all other elements • has a defmite fIxed place in the periodic table.

3

CHAYfER I CLASSIFICATION OF MA1ERIALS

A compound • is a pure substance • is homogeneous - it has the same composition throughout • cannot be physically separated, but can be chemically separated into its constituent elements • has properties unlike those of the constituent elements • is composed of elements combined together in fixed proportions by weight.

A mixture • may be homogeneous or heterogeneous. It has no fixed composition • can be physically separated into its component parts • the ingredients maintain their individual properties.

CHANGES IN MATTER

Malter may undergo both physical change and chemical change.

A physical change is one that does not result in a change in the chemical composition of the material -for example, ice can be melted to water, but both ice and water have the same chemical composition. Furthermore, a physical change may be reversed - the water can be cooled again to ice.

Other physical changes include: • distilling of salt water to separate salt and water • magnetising a piece of iron • melting a piece of copper.

A chemical change is one in which one or more entirely new substances are formed. These substances may be elements or compounds, and they have their own peculiar properties. A chemical change may be accompanied by a physical change.

For example, when iron rusts (or corrodes), a new chemical compound is formed from the chemical reaction between iron (Fe) and its environment (0,) to produce iron oxide FeO. Or when hydrogen gas (fI,) and oxygen (0,) are mixed in the presence of a spark or flame, a violent explosion results, and the product of this chemical reaction is water (HP). These reactions:

2Fe + 0, -7 2FeO and 2H, + 0, -7 2Hp

can be reversed, but they require a considerable amount of energy - the production of iron from iron oxide requires a blast furnace; and oxygen and hydrogen can be produced from water by the use of electrical energy. Thus, for a chemical change to reverse, the change is generally difficult.

TO SUMMARISE

In a physical change: • No compound is formed or destroyed • It is generally easy to reverse the change • Generally only a small amount of energy is involved.

In a chemical change: • New elements or compounds are formed • It is often difficult to reverse the change requiring a large amount of energy • It may occur spontaneously and give off a large amount of energy.

4

»

C HAPTER 1 CLASSrncAllONOF MAlERlALS

METALS, NON METALS, COMPOSITES AND METALLOIDS

The 105 elements in the periodic table can be divided, roughly, into metals, non-metals and metalloids. Mixtures and compounds containing these elements can also be divided along the same lines.

However, scientists and engineers have developed systems that make use of the favourable features of each system to create materials with extraordinary combinations of physical and mechanical properties.

These materials are composites - a combination of two or more materials that has properties that none of the individual components have by themselves.

METALS

A metal is a material in which the atoms are held together by a matrix of electrons. Some of the electrons associated with each atom are, to a certain extent, free to move between the atoms that constitute the crystal structure of a metal . Because electrons carry an electric charge, they are involved in the passage of an electric current. The free movement of electrons in a metallic structure is responsible for the good electrical conductiviry of most metals.

~

/ / / / / /

II / 1/ / / /

V II II / / /

Fig 1.1 A simple cell (left) and an arrangement of cubic ceUs (above). Atoms in a solid metal arrange themselves in this and other types of patterns. One crystal contains many such cells.

In their pure form , most metals are normally soft and malleable. They are composed of just one kind of atom, and in the solid state, these atoms crystallise in a regular pattern or structure that is characteristic of that me~1.l .

5

CHAPTER I CLASSIFICA nON OF MA 1ERIALS

-- -Body-centred Cubic

Face-centred Cubic ---Close-packed Hexagonal

Fig 1.2 Typical crystal structures

If one was to prepare a large single cryst:lI of a metal, then this pattern or arrangement of atoms would be continuous throughout the whole crystal. However, most metals crystallise as an agglomeration of numerous small crystals, each oriented in different directions, and each separated from the other by a surface that we call a grain boundary.

Characteristic properties of a metal include: • Usually solid at room temperature (except mercury which is liquid at room temperature) • Exhibit a sheen or lustre, particularly on a freshly cut surface • Are good electrical conductors • Are good thermal (heat) conductors • Are malleable - they can be plastically deformed • Are opaque - light cannot pass through them • They can generally be strengthened by mixing with other metals or alloys, or by thermal

treatment • Chemically speaking, metals have, for their valence a small positive number, usually 1,2, or 3.

Metals are generally categorised as being ferrous (composed ptincipally of iron) or non ferrous (composed prinCipally of elements other than iron).

The main ferrous metals are: • Carbon steels • Alloy steels • Tool steels

• Stainless steels • Cast irons.

All steels are alloys of iron with small amounts - up to 2% - of carbon, although most steels contain less than I % carbon. In addition, other elements such as manganese. nickel and chromium are added to enhance the properties of the steel.

Cast Irons are alloys of iron with 24% carbon plus other elements such as manganese, silicon and nickel which, once again, are used to enhance the properties of iron. The high carbon content of cast irons results in a very low malleability. Hence they are supplied only as castings, produced by pouring hot liquid iron into a prepared mound.

6

CHAPTER I CLASSIFICATION OF MA1ERlALS

Non-ferrous metals include all the metal and alloy systems whose principal constituent is a metal other than iron. Thus aluminium, copper, lead, zinc, tin , nickel, silver, gold, magnesium, chromium, molybdenum, and tungsten and all of their alloys fonn just some of the non-ferrous metal systems.

NON· METALS

Whilst non-metallic elements make up the minority group in the periodic table, the number of non­metallic compounds is inflnite.

The main non-metallic groups of interest to engineers are: • Ceramics • Plastics (or polymers) • Rubbers • Wood.

Typical propenies of non-meL'lls include: • May be solid, liquid, or gaseous • Poor electrical conductivity • Poor thermal conductivity • In solid state, they have a dull appearance • They are generally brittle • Generally have a low speciflc gravity • Chemically, they are often based upon elements having a valence of 5, 6, 7 or 8. (Elements with

valence 8 are men. They have a very low tendency to react or to combine with other elements to fonn compounds).

A ceramic is a combination of one or more metallic elements with a non-metallic element. The propenies of a ceramic resemble those of a non-metal. The propeny that distinguishes a ceramic from other non-metals is the nature of the bond that holds the atoms together. Ceramics usually have a vety rigid ionic/covalent bond, and tend to be very hard and brittle, chemically inen or unreactive, and good electrical insulators. This latter propeny results because the electrons that provide electrical conductivity are held fmnly in place by the covalent bonding.

Aluminium oxide (AlP,) is a typical ceramic material; it is fonned by two aluminium atoms combining with three oxygen atoms. It is a good insulating material, both electrical and thennal, it is highly refractory, and extremely hard. It is used for insulated mountings in switchgear and high tension lines. for refractory brick linings in metal melting furnaces and as an abrasive in grinding wheels.

Plaslies , or more correctly 'polymers' are a large group of materials which consist of, or conL'lin, as an essential ingredient, a subsL'lnce of high molecular weight whiCh, while solid in the finished SL'lte, is at some stage of its manufacture soft enough to be fonned into various shapes, mostly through the application of heat and/or pressure.

Polymers are composed of many long chain molecules, each molecule conL'lining a large number of atoms. In most polymers, the elements carbon and hydrogen fonn the major constituents.

Polyethylene is a typical polymer, that is produced by "polymerisation" of many ethylene molecules to fonn a long chain molecule, in which the individual atoms are held together by covalent bonds so that each atom shares its electrons with adjacent atoms,

7

CHAPTER I CLASSIFICA nON OF MA TERlALS

Fig 1.3 Polyethylene Molecule (C,H,)n

The bonding between the molecular chains in polymer materials is by weak electrostatic forces called Van der Waals bonds. Thus polymers are relatively low in strength and high in plasticity. Strengthening can be accomplished by techniques that restrict the movement of the chains, including the use of fibres, and fillers, and by cross-chain linking.

Plastics are often classified as thermosetting and thermosoftening, sometimes called thermoplastic. All plastics soften when initially heated to allow the plastic to be shaped or fabricated. However, after cooling from the initial heating, their subsequent behaviour upon re-heating determines their classification.

Thermosetting plastics are those which are hardened permanently by the action of heat. This means that after final curing, the plastic cannot be softened or reshaped in any way by heating. Further heating may eventually cause decomposition.

Thermoplastic (thermosoftening) plastics are those which may be softened and reshaped when reheated, and when cooled, they become hard again. This cycle may be repeated several times providing the maximum temperature is kept within limits, and is not high enough to cause decomposition. Thermosoftening plastics may also be fabricated by the combined action of heat 'Uld pressure. Typical thermosetting and thermosoftening plastics include:

THERMOSETTING PLASTIC THERMOSOFTENING PLASTIC

A1kvds Acrvlics (Perspex) Epoxies ABS Phenol formaldehyde Cellulose Urea formaldehyde P.T.F.E. (Teflon) Melamine Polyamides (Nylon) Polyesters Polycarbonates Silicones Pol ystyrenes

Polyethylene (Polythene) Polyvinyls Polypropylenes Fluorocarbons

Natural rubber is extracted from latex obtained from the tree "Hevea Braziliensis" . The latex contains 30% to 45% rubber, and the balance is mainly moisture. In its crude form, rubber is a fairly useless tacky compound. It is combined chemically with additives and subsequently vulcanised (or heated) to convert it into a useable product.

8

-

CHAPTER I CLASSIFICA TION OF MA 1ERlALS

The additives are blended by masticating the crude rubber in a powerful blade type mixing machine in which colouring agents, curing agents and inert fillers may also be added. Sulphur is also added to aid the vulcanising process. Vulcanising involves heating the material to about 150°C, generally in a metal mould of the desired shape. This provides the energy for the chemical reaction that creates the chain bonds responsible for the elastomeric properties of rubber.

One of the limitations of natural rubbers is their low resistance to deterioration due to the effects of ozone in the atmosphere. A large range of synthetic rubbers have been developed which have good ozone and weather resistance, or have resistance to heat or to particular chemicals. The principal synthetic rubbers include butadiene, polychloroprene (neoprene), polyisobutylene, nitrile, polysulphide and silicone. These are used in seals , gaskets, O-rings and similar parts in situations where natural rubber would have a restricted life.

Wood is of biological origin, and is one of the earliest materials used by man. It is composed principally of fibres of the complex hydrocarbon, cellulose, which are surrounded by a thin k~yer of lignin which 'cements' the fibres together. Wood also contains small amounts of a number of other compounds starch, resin, gum, wax, tannins etc. The different arrangements of the cell structure in wood give each wood its particular properties. Woods are generally classified as "hardwood" or "softwood".

Hardwoods contain large cells known as "vessels", whereas the cells in softwood are small. Also hardwoods contain pores whereas softwood is non-pored. These tenns can be misleading because some "hardwoods" can, in fact, be quite soft - the classic example is Balsa wood - a pored (yet very soft) hardwood.

Wood often requires seasoning before use - this is the drying out of the wood - because freshly felled timber contains a high percentage of water, both free water in the cell cavities and chemically combined water within the structure of the cell walls. If wood is not correctly seasoned, it can warp, crack and even collapse.

COMPOSITE MATERIALS

A composite is a combination of two or more materials that has the properties that neither material has by itself. Wood is really a composite - cellulose fibres held together by a glue or lignin. In plywood, the directional properties of wood - it is stronger in the longitudinal direction than in the transverse direction - are overcome by alternating the grain direction in successive sheets.

At the present time the more important composites include: • reinforced concrete • fibre reinforced polymers • reinforced metals • laminated metal systems

Reinforced concrete is probably the most widely used of all constructional materials; it has many advantages - a relatively low cost, comparatively easy to position, and it is maintenance free. Concrete is strong in compression, but relatively weak in tension and bending. By carefully locating steel reinforcement in the concrete, the tensile load can be carried by the steel. In some cases the concrete may be prestressed to counteract an initial deflection in a concrete member. The steel reinforcement is loaded in tension whilst the concrete is poured around it. After the concrete has cured, the tensile force is removed with a resulkwt contraction in the reinforcement. The effect is to apply a residual compressive force to the concrete. Subsequent tensile loading of the concrete must overcome this compressive force before any tensile force is felt by the concrete member.

9

CHAPTER I CLASSlFICA TION OF MA lER.IALS

Concrete in tension fails

CCIn.crete in compression Load

l

Steel reinforcement takes tensile load

Concrete in compression Load

l

Steel reinforcement concrete in compression

Fig 1.4 Effects of reinforcement in concrete

Just as the tensile properties of concrete may be improved by steel reinforcement, so the properties of polymers can be improved by a similar procedure. Reinforcement materials used in polymers include fibreglass, graphite, and boron.

Glass fibres are very strong. but if they are notched. they fracture easily. By coating the fibre in a polyester resin, it can be protected from damage so that its stiffness and strength can be utilised. Fibreglass reinforced plastics are utilised in marine, automotive and aircraft applications where its lighmess, strength and corrosion resistance properties are important However the glass quickly loses its strength once the temperature is raised. Fibres of asbestos. graphite and boron have been employed successfully as reinforcement materials in plastics because of the improved temperature resistance and greater strength and stiffness over glass fibre.

More recently, very strong (and often brittle) whiskers or fibres have been used to further improve the physical and mechanical properties of metals.

A whisker is a single crystal of a material that has been prepared by "growing" in a preferred direction. It measures about 0.5 to 2.0 microns in diameter, and up to 20mm long. Because of their small size, and their high degree of crystal perfection, whiskers consistently exhibit properties approaching those calculated for perfect materials with strengths up to 35000 MPa (the strength of steels range up to about 2000 MPa).

Ceramic whiskers of alumina have been embedded in cobalt to produce a strong and heat resistant composite. Boron and rungsten have been embedded in aluminium and copper to improve the stiffness of these metals.

An important group of composite materials is the clad metals group. Different metals may be bonded together for a variety of reasons, including strength, corrosion resistance or differential expansion.

Thermostatic controls, or bi-metallic strips, are produced by roll-bonding a high thermal expansion alloy such as copper to a low expansion alloy such as steel. Heating the composite causes a differential expansion of the two metals with the result that the bi-metallic strip bends or deflects towards the low expansion alloy.

10

CHAPTER 1 CLASSIFICA nON OF MA 1ERIALS

Austenitic stainless steel is roll bonded to mild steel to provide a corrosion resistant lining on a steel shell. This provides a cost saving and also, the !tigher thermal conductivity of the mild steel gives a more uniform heating of a heated vessel.

METALLOIDS

Metalloids are those elements intermediate between the non metals and the metals, and whose properties sometimes resemble the non-metals. The metalloid elements include silicon, germanium, arsenic and boron. These materials are fmding their major demand as semiconductor materials in electronic devices.

11

CHAPTER 1 CLASSlFICA nON OF MA TERlALS

GLOSSARY Metal Chemical change

Non-metal Homogeneous

Metalloid Heterogeneous

Plastic Composition

Polymer Electron

Solid Malleable

Liquid Ceramic

Gas Thennosetting Crystalline Thennosoftening

Element Rubber

Mixrure Wood

Compound Composite

Physical change Reinforcement

Crystal S nucrure Laminate

Amorphous Polymer Whisker

QUESTIONS

1. Distinguish between elements. compounds and mixrures.

2. What is the purpose of reinforcement a) in concrete? b) in plastics?

3. What distinguishes a ceramic material from a metal?

4. What is a composite material? Name two types of composite materials.

5. Distinguish between metals and non-metals.

6. Discuss the important fearures of a) a physical change b) a chemical change

12

CHAPTER 2 NATURE OF MATERIALS

Before we embark on a study of the properties of materials, it is important to understand the fundamentals of the chemistry that controls their properties.

Pltilosophers and scientists have, for centuries, speculated as to the nature of matter. However the beginning of the atomic theory as we understand it today can be traced to a Scottish Chemist, John Dalton who, in 1808 set forth ltis atomic model of matter - Dalton considered that: • Elements are made up of minute individual particles called atoms (a word derived from the Greek

word 'Atomos' meaning indivisible) • All atoms of anyone element have the same properties and mass.

Subsequent research scientists such as Thomson, Rutherford and Bohr have shown that the Dalton concept of an indivisible atom was false, and that the atom is a complex structure built up from still smaller particles wltich have been given the names: • Neutrons • Protons • Electrons.

Furthermore many of the properties of materials are best understood in terms of the structure of the atom, and the manner in wltich atoms join together to form molecules. The atom is basically a spherical structure whose overall diameter is about 10.7 nun. It contains a core or nucleus composed of protons and neutrons wltich is surrounded by electrons moving in ·'shells" or "orbitals" at various distances from the nucleus.

A proton is a particle weighing about 1.67 x 10.27 kg and possessing a single positive electrical charge.

A neutron is a particle weighing 1.008 times the mass of the proton but with zero electrical charge.

The nucleus of the atom comprises protons and neutrons (except the hydrogen atom wltich contains just one proton).

The electron exists in orbits surrounding the nucleus. Its mass is only 1/1836 that of the proton or neutron, and each electron has a single negative electrical charge.

Since the atom is electrically neutral , the number of electrons orbiting an atom is equal to the nwnber of protons in its nucleus.

Fig 2.1 Structure oj an atom

• neutron

o proton

e electron

Because the electron has a very low mass when compared with the proton and neutron the mass of an atom is contained primarily in its nucleus. However the diameter of the atom is several thousand times the diameter of the nucleus. Thus much of the atom is, in fact, open space. It has been estimated that,

13

CHAPTER 2 N A TIlRE OF MATERIALS

if the density of a child's marble was equal to the density of an atomic nucleus, then the marble would weigh over 30 million tonnes.

Each energy level or shell containing electrons can contain up to a given maximum number of electrons given by the general fonnula:

maximum no. of electrons = 2n2

where n is the shell number, counted as number I nearest to the nucleus. The shells are generally represented by the letters k to q.

The simplest, and the lightest, atom is hydrogen, which we picture as:

e

(i) (ii)

Fig 2.2 (i) Orbital view of hydrogen atom and (ii) A simplified 2-dimensional representation

A more complex atom is aluminium containing 13 protons and 14 neutrons in its nucleus and 13 electrons SUITOWlding the nucleus.

• electron

Fig 2.3 A simplified 2 dimensional representation of an atom of aluminium

It is the electrons, particularly those in the outermost shell or shells, that determine many of the chemical, physical, and engineering properties of materials. Because of the special nature of these outer electrons they are given a special name of valence electrons.

The basic structure of the atom of any particular element may be defmed by its: • Atomic number • Mass number • Atomic weight.

The atomic number (Z) of an element is equal to the number of protons in the nucleus of its atom.

14

CHAPTER 2 NATURE OF MATERIALS

Since all atoms have no electrical charge, then the atomic number also signifies the number of electrons revolving around the nucleus. And the atomic number also provides us with the position of the element in the Periodic Table of elements.

The mass /lumber (A) of an element is equal to the sum of the protons and neutrons in the nucleus of the atom. Mass numbers tell how much lighter or heavier an atom of one element is relative to an atom of another element.

The atomic weight of an element is the average mass of the element compared to the mass of a carbon atom which is taken as twelve atomic mass units. Atomic weight is often slightly different to the mass number because many elements contain atoms with different mass numbers called isotopes.

An isotope is an atom of an element having the same atomic number (number of protons) as ,mother atom of the same element, but a different mass number, hence a different number of neutrons.

Chlorine, atomic number 17, has two isotopes, one with mass number 37 and one with mass number 35. The atomic structures of these two isotopes are:

Mass number 37 - 17 protons, 20 neutrons, 17 electrons Mass number 35 - 17 protons, 18 neutrons, 17 electrons

Chlorine contains 23% mass number 37 and 77% mass number 35 and so has an atomic weight equal to 35.46.

THE BOHR ATOM

The classic Bohr theory depicts the atom as a nucleus surrounded by orbiting electrons (in much the same way that our solar system is centred on the sun with the planets orbiting around it). Furthermore each electron remains in its particular shell unless excited by an external force which may cause it to "jump" into another shell, thereby gaining or losing a certain amount of energy.

This concept of the atom is adequate for the purposes of this course. However, it should be realised that this is a somewhat simplified explanation. Current atomic theory states that the spatial arrangement of electrons in an atom is described by a "quantum number", and requires a study of the quantum theory and wave mechanics, a study that is beyond the scope of this text. Whatever theory one studies, it is accepted that chemical interactions between elements and compounds involve the interaction of electrons, particularly those that occupy the outer shells of the atom.

The valence number of the atom is a function of the number of electrons in the outer shell. Elements with up to 4 outer electrons have valence number equal to this number of outer electrons. Elements with 5-8 outer shell electrons have a valence equal to (8-N) where N = number of outer shell electrons. An outer electron shell is considered to be full if it has 8 outer shell electrons (or 2 in the case of Helium). This is in spite of the fact that, shells m - q, can theoretically contain more than 8 electrons.

Elements with 8 outer electrons are generally unreactive - they do not tend to combine with other elements to form compounds. When an element has less than 8 outer shell electrons, it will tend to combine with other elements in a marmer that will fill the outer shells of the reacting elements. We call this process bonding.

PERIODIC TABLE OF ELEMENTS

Scientists have, for several hundred years, recognised that a certain pattern or periodicity exists in the chemical and physical properties of elements. In 1869 Dimitri Mendeleef, a Russian scientist, was able to show that, when elements are arranged in order of increasing atomic weight, a striking periodicity, or repetition, of physical and chemical properties becomes evident. Subsequently, the scientist Moseley, in 1912. showed that the periodic properties of the elements are a function of the

15

CHAPTER 2 NATURE OF MA1ERIALS

atomic numbers, and not atomic weights as was postulated by Mendeleef. (In all but a few of the elements. atomic weight increases with increasing atomic nwnber; however there are three instances of inversion of order of atomic weights in the periodic table). It is interesting to note that in early versions of the periodic table there were several blank spaces, representing elements that had not then been found in nature, yet the properties of these unknown elements were able to be predicted with great accuracy.

There are several features of the periodic table which deserve attention: • There are 105 elements, from Hydrogen number I - the lightest element, to Hahnium - nwnber

105 - the heaviest. • Of these 105 elements, 92 are known to occur naturally, the remaining 13 have been

manufactured in laboratories or reactors. • The table is divided into 8 vertical "groups" and seven horizontal "periods" . • The table can be divided, for convenience, into 5 distinct zones:

a) Light metals, which occupy the two groups IA and IlA on the left hand side of the table. b) Transition metals, which occupy groups III B, IV B, V B, VI B, Vil B, VIIl B, I B and Il B in

the central portion of the table - these are all high melting point metals (except mercury (Hg», they usually fonn coloured compounds when they react to fonn salts, and many of them are capable of forming compounds in more than one valence.

c) Soft metals, which fall between the transition metals and the metalloids -these include aluminium (AI), tin (Sn), lead (Pb), bismuth (Bi), all of which are relatively soft, have low melting points and are fairly unreactive.

d) Metalloids, which include elements boron (B), carbon (C), silicon (Si), antimony (Sb) and tellurium (Te), are elements which may behave as metals or as non-metals. Hence they are the elements that feature in the semiconductor industry.

e) Non-metals, which occupy the extreme right hand side of the table, and include the inert gases of group VIIl A (sometimes called group D).

In summary, the 105 known elements may be depicted as:

1 05 Elements

I I I 4 Unstable 88 Stable 13 Trans-Uranium

I I I 64 Metals 7 Metalloids 17 Non-Metals

I I I I I 63 Solid 1 Liquid 5 Solid 11 Gaseous 1 Liquid

I I 5 Reactive 6 Inert

BONDING

In any solid material the atoms that comprise the elements, and the elements themselves, are held together by chemical bonds - if they were not everything we use would be dust, and a very fme dust at that. The basic premise in all chemical bonds is that atoms combine with each other fonn complete outer electron shells. They do this by gaining, losing or sharing electrons from their outer shell so that they are left with outer shells containing eight electrons (or two electrons in the case of hydrogen).

IONIC (ELECTRO VALENT) BOND

In the ionic bond, two or more atoms gain or lose electrons. Thus an atom of oxygen, whose electron structure is (2.6) tends to gain two electrons to make its structure (2.8). Likewise chlorine (2.8.7) gains one electron to become (2.8.8). Sodium (2.8.1) lose one electron to become (2.8), and potassium (2.8.8.1 ) loses an electron to become (2.8.8). These electron arrangements resemble those of the inert gases neon (2.8) and argon (2.8.8). The electrons gained or lost are referred to as the valence electrons.

16

-CHAPTER 2 NATURE OF MA 1ERlALS

Since there is no change in the nucleus of the atom, this gain or loss of one or more electrons results in a nett negative (if electrons are gained) or positive (if electrons are lost) charge on the atom because the atom now contains an excess or a deficit of electrons (negative charges) compared with their complement of protons (positive charged particles). These charged atoms are known as ions.

An atom that loses electrons to become a positively charged ion is known as a cation. These lost electrons are transferred to the non-metallic atom to produce an anion. Metallic elements tend to form cations. These cations and anions are held together by the electrostatic attraction that oppositely charged particles have for each other. The unit formed by this ionic bond between two or more oppositely charged ions is called a molecule.

In each molecule the number of electrons lost by the cations must equal the number of electrons gained by the anions, so that the molecule, as a whole, is electrically neutral. We may represent the chemical reaction between magnesium metal and chlorine gas to fann magnesium chloride as follows:

Mg + CI, --> MgCl,

Ionically this is written: Mg' + CI,' --> Mg2. + 2CI-

The atomic strucrures of the participants in this reaction are:

Mg + 2CI Fig 2.4 Atomic representation of reaction between Magnesium and

Chlorine

This electrostatic attraction is the only force of attraction that exists between the ions in an Ionically bonded solid or crystalline material. These materials are sometimes called 'inorganic' materials. If the material is dissolved in water, the ions become detached and are mobile or free to move.

The presence of an electric field is sufficient to overcome the forces between the ions and to attract the charged ions towards oppositely charged electrodes inunersed in the aqueous solution. Thus aqueous solutions of ionised salts will conduct an electric current. This property enables us to electroplate metals such as nickel, copper and chromium from aqueous solutions containing ions of the particular metal .

Ionised salts also become conductors if they are heated until they are molten, or fused , for in this state, the energy provided by the heat is sufficient to overcome the electrostatic attraction between charged

17

CHAPTER 2 NATIJRE OF MATERIALS

ions. The imposition of an electric field again results in attraction of the ions towards the oppositely charged electrodes.

(i) (ii)

• Mg-ion o cf ion

Fig2.5 i) ii)

iii)

Magnesium Chloride crystal Aqueous Magnesium Chloride, in which the salt is ionised with ions randomly distributed. Aqueous Magnesium Chloride with an electric potential applied through the electrodes - charged inns are attracted to the oppositely charged electrode.

COVALENT BONDS

+

(iii)

The majority of chemical compounds are not ionic - they are not salts - but rather are bonded by different types of forces. In a covalently bonded molecule, there is no transfer of electrons between atoms. Rather the bond results from the sharing of pairs of electrons between atoms. In this manner, atoms are seen to again develop a full outer shell of electrons.

There is almost an inImite munber of organic compounds based principally upon the elements carbon and hydrogen that are bonded by covalent bonds. For instance methane (CH.) may be represented as:

Hydrogen atoms

Ic~~~n \ , ~

• Electrons contributed by carbon atom o Electrons contributed by hydrogen atoms

Fig 2.6 Methane Molecule

Four hydrogen atoms, each share a pair of electrons with the carbon atom. In each pair of electrons, one is donated by each of the sharing atoms. It may be seen that carbon has developed an outer shell

18

CHAPTER 2 NAnJRE OFMATERlALS

of 8 electrons and each hydrogen atom has 2 electrons. Each shared electron passes from an orbil'll controlled by one atom to an orbital controlled by two atoms, and it is this control which constitutes the covalent bond. Chemists represent the structuraJ formula of a covalent bond by a dash ".".

Thus methane is represented structurally as:

H I

H-C-H I H

Fig 2.7 Structural representation of Methane molecule

One "." represents one pair of shared electrons. For example acetylene C,l!, in which three pairs of electrons are shared between the two carbon atoms, and one pair of electrons is shared between each of the hydrogen and carbon atoms, is represented structuraJly as:

H-C:EC-H Fig 2.8 Acetylene Molecule

Organic molecules are not the only molecules that are bonded by covalent bonds· water (HP) and many of the gases including oxygen (0,), nitrogen (N,), chlorine (Cl,), and hydrogen (H,) also contain covalent bonds.

Perhaps one of the more imPOl1ant aspects of the covalently bonded compounds is that, because they do not contain charged particles or ions, they do not conduct an electric current.

The development of our plastics industry is based upon the properties of the covalent bond. Plastic materials are produced by chemical processes called "polymerisation" in which covalent compounds can be made to join together to form long chain type molecules in which the bond strengths are very high. For instance polythene (or polyethylene) is made by a polymerisation reaction involving the gas ethylene. Structurally we may represent this:

n (C,H.,) --7 (C,H.,),

H H H H H H I I I I I I C=C +C=C +C=C+ ... > I I I I I I H H H H H H

Figure 2.9 Polymerisation of Ethylene

The name given to the family of plastics is "polymer" , derived from the Latin "poly" = "many" and "mer" = "units". A polymer is a long chain organic molecule produced by the chemical joining of many units of one or more individual molecules, which we call "monomers". As the molecule size of covalent compounds increases, so the bond strength increases. Rubber, which consists of large interlocked covalent molecules, has great strength and elasticity because of the strength contained in the covalent bonds.

19

II

CHAPTER 2 NA TIlRE OF MA 1ERIALS

It must be realised that, in any compound, it is the bond strength that is responsible for strength and the elastic properties of the material, When a rubber band is stretched, it is the atom to atom bond that is being deformed - not the atom itself.

METALLIC BOND

The metallic bond is more complex than either the ionic or covalent bond, The absence of oppositely charged ions in the structure of a pure metal removes any possibility of ionic attraction, Also, since most metals contain only I to 3 valence electrons, there are insufficient electrons to form covalent bonds, The metallic bond results from the sharing of the outer valence electrons by two or more atoms, not by a fixed overlap of outer electron shells, but by the electrons becoming "free" to fonn a negatively charged "electron cloud" around the positively charged metallic ions,

Because the positive metal ions tend to repel each other, they take up positions in their crystal lattice according to some form of geometrical pattern, with a space between each atom, What holds these positive metal ions together is their mutual attraction for the negatively charged cloud of electrons, In this arrangement, these valence electrons are not held fmnly to one atom or pair of atoms, Rather they are relatively free to travel which explains why metals are such good electrical conductors,

The opaque lustre of metals is brought about by the vibration of surface electrons when struck by a ray of light A light wave is a form of energy - this energy is absorbed by the vibration of surface electrons when the light ray strikes the surface, thus stopping the wave from being transmitted any further. The vibrating electron may itself become the source of a wave, resulting in the reflective property of metals,

e e ® ® ® ® ®

e e e e e e ® e ® ® ® e e e e

® ® ® ® ® e

Fig 2,IOMetallic Bond

MOLECULAR BONDS

The fourth type of bond that may exist between atoms is the molecular bond. This bond occurs between the noble gas atoms whose outer shell contains 8 electrons and so is, effectively, full (except heliwn which contains 2 electrons).

The forces of attraction between noble gas atoms are very small, and are known as Van der Waa!'s forces. These forces are the result of unbalanced distribution of electrons around the atoms. These forces also act to supplement the normal electron bonds (or primary bonds) in many non-metallic materials.

RELATIONSHIP BETWEEN BONDING AND PROPERTIES

A simple method of classification of materials is related to the type of bonds that bind the atoms or molecules together.

20

CHAPTER 2 NA nJRE OF MATERlALS

Ionic compounds are non-conductors in their solid state, but become conductors in aqueous solution. or when molten.

Covalent compounds are non-conductors because their electrons are bound tightly to the molecule. Metals, however are excellent conductors because the electrons forming the "elecrron cloud" are relatively free to move and so rransrnit an electric charge through the conductor.

Ionically bonded materials generally exhibit moderately high melting points and moderate hardness, whereas those materials which are bonded by covalent bonds vary from very low in hardness, and low melting point to extremely high hardness and melting point. Diamond, which is the hardest of all materials and melts at over 6000'C is a covalently bonded molecule of carbon.

An important property of most metals is the fact that they can undergo considerable plastic deformation - they can be stretched and deformed without fracture. This is due to the nature of the metallic bond. Under the action of suitable forces , layers of atoms (or ions) in a metal can slide or slip over each other without significantly altering their relationship with the electron cloud. These may be contrasted with ionic materials that show negligible ductility since deformation results in ruprure of the bond.

Table 2.1 Summary of the Effect of Bond on the Properties of Materials

Type of Binding Optical Electrical Thermal Mechanical Examples Bond Forces Properties Properties Properties Properties

(Solids) Ionic Electrostatic Transparent Insulators Medium to Generally hard NaCI

attraction of or coloured when solid high melting and brittle MgSO, ions of conductors point opposite when aqueous charge or molten

Covalent Sharing of Maybe Insulators Solids have a Solids are hard Diamond electron pairs transparent non- high melting and brittle. to plastic

to opaque conductors point. Plastics Plastics are soft have a low and ductile meltin~ point

Metallic Attraction Opaque and Conductors High melting Tough and Cu between reflecting point ductile Fe positive 'ions' and 'free' electrons

Molecular Polar Transparent Insulators Low melting Soft and plastic Ar attraction of point unbalanced charoes

21

CHAIYI"ER 2 NATURE OF MA l"ERIALS

PERIODIC TABLE Key

I Atom ic Number 79 2

H Au S)111bol of cicment He 1.008 Atomic mass 197.0 4.003

lI)u,,,£,,,, 0," name of clement IIda,m

3 4 5 6 7 8 9 10

Li Be B C N 0 F Ne 6.94 1 9.012 10.8 1 12.0 1 14.01 16.00 19.00 20. 18 UIII,.." Be "II,,.., BOlon CJloon Nill rn O~, n Fluorine N'M

II 12 13 14 15 16 17 18

Na Mg AI Si P S CI Ar 22.99 2UI 26.98 28.09 30.97 32.06 35.45 39.95 Sodit,,,, "bf."".hrn Akmini .. " Silicon """'M SUIfUf ChlorirK "M

19 20 2 1 22 23 24 25 26 27 28 29 30 3 1 32 33 34 35 36

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 39. 10 40.08 H .96 47 .90 50.94 52.00 54.94 55.85 58.93 58.7 1 63.55 65.38 69.72 72.59 74.92 78.96 79.9 83.8

Pou'iSium C.k i.Jm S ..... H .. " TilJnl,." "andi.." Ch,,:mium Mw.III"IIs.e hM Cobol! Nkt.1 c , n" G.ILiJm Ocmllllium " .. "rue Scl,nl .. " Ilromisw: K 'M 37 38 39 40 41 42 43 H 45 46 47 48 49 50 5 1 52 53 54

Rb Sr Y ZI' Nb Mo Tc Ru Rh Pel Ag Cd In Sn Sb Te I Xc 85.47 87.62 88.9 1 91.22 92.9 1 95.94 98.91 10 1.07 102.91 106.4 107. 87 11 2..1 I IH2 11 8.69 121.75 127.6 126.9 131.3 R"bid i" ", Smmill'" Yllri",,, 7.1rc<~'i"'n Nio\~"m Mo hkrom T«hn<ti'''' Rmh<u ollll Rh"Hnm Pll lorljlH" SUI'" Codml ..... hH,." To """"". TeUuri m' Iodine Xo"o"

55 56 57-7 1 72 73 74 75 76 77 78 79 80 8 1 82 83 84 85 86

Cs Ba Lwl","o:lu Hf Ta W Re Os II' Pt Au Hg TI Pb Bi Po At Rn 132.91 137.34 178.·19 180.95 183.85 186.2 190.2 192.22 195.09 196.97 200.59 204.37 207.2 208.98 209 2 10 222

CHi"", Ihri,,,, II>&l b . .. T,"ul,m T ~." Rt.:nl .... O!:rni,m lridi,." Pl.!! .... '" Gold Mc",u . Thilll .. n ... ~ Di ...... th Polonium "<t.cine .-87 88 89- 103

Fr Ra Aclinidoel

223 226 I'nnciClm Rldi ....

57 58 59 60 6 1 62 63 64 65 66 67 68 69 70 7 1

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 138.9 1 140. 12 1·10.91 IH.24 145 150.4 151.96 157.25 158.93 162.5 164.93 167.26 168.93 173.04 174.97 1. •• 111 .. , .. " Ceriu", P'.n»t", ... ., Ne"" mhm P.ml<thi",,, Samll;"" I:ur limn O.da1!nillm T. ,!>;,m 1>\" toosh.n Hob",,,m flbill m n",lillm rU.lbill," Lu\clillm

89 90 9 1 92 93 94 95 96 97 98 99 100 101 102 103

Ac Th Pa U Np Pu Am em Bk Cf Es Fm Mel No Lr 227 232.0 231.0 238.0 237.0 244 243 247 247 25 1 254 257 258 255 256

Actini,." lh~rilllt1 P,cl>ctlnl"", Ur ... r,m N. IUl;"m Plu\OIulIIlI Arnoricinm en rinm D.rkol"'m C.li!cmi ,m E;n~.inlum P.n,,",", M .... 101.Ii'un Not-"li.,m \. ...... r~I"n

CHAPTER 2 NATURE OF MA 1ERIALS

GLOSSARY Atom Covalent Bond Proton Metallic Bond Neutron Molecule Electron Anion Atomic Number Cation Atomic Weight Inorganic Mass Number Polymer Isotope Monomer Ionic Bond

QUESTIONS

I. Sketch the atomic structure of the element oxygen, which has atomic number 8 and mass number 16.

2. Why are ionically bonded salts non-conductors in their solid state. yet become conductors when dissolved in water?

3. Is pure water a conductor or a non-conductor?

4. Why is normal tap water a conductor?

23

CHAPTER 3 PHYSICAL PROPERTIES OF MATERIALS

The physical properties of a material may be considered to be those properties that are determined by nature. They can often be measured without destroying the material. There are so many physical properties that it is not possible to review them all in this course. However certain of the physical properties are important:

THERMAL PROPERTIES

The thermal properties must be considered for any material that is to be fabricated or used at a temperature other than ambient, or if it is expected to perform some type of heat transfer function.

THERMAL CONDUCTIVITY

Thermal conductivity, which is the rate of heat flow in a homogeneous material under steady state conditions, is measured as Watt/metre Kelvin (W.m·'K'). (The Watt is the power used when work is done at the rate of I joule/second (I J-S"') - the Kelvin is the unit of thermodynamic temperature, and is equal to t'Celsius + 273.15).

It can be shown that the quantity of heat (Q) that flows through a material per unit time is equal to:

where

K.A.t.T

X

Q = quantity of heat conducted through the material per second (Watts) K = thermal conductivity (W.m·'K·') A = area of the conducting surface (metre') X = thicmess of the material (metre) t. T = temperature difference at opposite surfaces

Extracted •• __

heat •

energy •

T (cold)

thickness Fig 3.1 Steady state heat flow through a homogeneous solid

material.

Incident

heat

energy

A high value of thermal conductivity indicates that the material is a good conductor of heat. A degree of caution is needed in determining thermal conductivities since surface effects (surface fIlms of liquid and gas and surface oxides on a metal) can have a major influence on the overall transfer of heat through a material. For example, aluminium has a thermal conductivity of 218 W.m·'K'. However

24

CHAPTER 3 PHYSICAL PROPERTIES OF MATERIALS

under most normal conditions, aluminium is coated with a film of aluminium oxide (Alp,) on all of its exposed surfaces. The thermal conductivity of AlP, is less than I W.m·' K·' so that this creates a banier to the transfer of heat.

T (hot)

........... _- T,

T

Fig 3.2 Thermal gradient across oxidised aluminium.

Thus the thermal gradient through an oxidised aluminium section may be represented as shown in figure 3.2.

Calculations for heat transfer through such a section may sometimes be simplified by the use of an overall heat transfer coefficient, or thermal conductance for the section.

THERMAL CONDUCTANCE

Thermal conductance is measured in watt/square metre Kelvin (W.m·' .K'), and is a unit by which the heat transfer for different materials may be directly compared.

THERMAL EXPANSION

Thermal expansion is also important when materials are to be heated or cooled. Almost all materials expand when heated and contract when cooled. If a material that is being heated is restrained in such a manner that it is unable to expand, or if it is fixed to a different material with a different thermal expansion, then stresses must inevitably develop within the system. Because materials become weaker as temperarures increase, such stresses often lead to failure by fracture, or, more commonly, distortion or buckling of the components.

This effect is used to advantage in bi-metallic temperature sensing switches. Two snips of metal, one with a high thermal expansion. and the other wi th a low thermal expansion, are firmly bonded together. As the temperarure is increased , the snip bends towards the metal of lower thermal expansion. If the temperarure is decreased, the strip bends in the opposite direction. This type of device is widely used as the sensing and switching element in many types of thermal switches.

The thermal expansion coefficient of a material may be expressed in millimetres/millimetre Kelvin (mm.mm·'K·' ) or more simply 11K or K" since mm.mm·' is nothing more than a ratio and has no 'dimensions',

The important point to remember about the use of the thermal expansion coefficient is that it may be used with any linear unit. Thus if the coefficient of thermal expansion for a material is O.OO IK' then on heating through 10K (or I'C since the temperature interval is the same on both Celsius and Kelvin scales), then I mm will expand to 1.001 mm, 1m will become I.oolm and so on.

25

II

II I

CHAPTER 3 PHYSICAL PRoPERTIES OF MA 1ERlALS

HEAT CAPACITY

Heat capacity (specific heat) is the amount of heat energy required to heat a material through I K (or l aC), and is expressed in the units joules/Kelvin (J.K·! ). Heat capacity is generally determined in terms of the volume of a material (J.m·' K·! ), or in terms of the mass of a material (J.kgK'! ). Heat capacity is an important consideration when estimating the requirements for electronic heat sinks where a high heat capacity combined with high thermal conductivity is desirable to conduct heat from electronic devices.

MAXIMUM USE TEMPERATURE

A number of polymer materials become soft and of little use at temperatures above about 1000C - the temperature of boiling water. Maximum use temperature for a polymer is that temperature, above which, the material loses its useful engineering properties. Typical maximum use temperatures are shown in table 3.1, which is a summary of data published in British Standard BS2757: 1956 (Australian Standard AS.C320:1958). This standard defines 7 classes of insulating materials and assigns maximum use temperatures to each class.

Class Maximum Use Examples Temp. 'C

Y 90 Untreated or unimpregnated cellulose, e.g. paper, cotton, silk, fibre, and plastics.

A 105 Treated or impregnated coltons, paper etc. otherwise similar to Y.

E 120 Cellulose cotton and paper bonded with phenol-formaldehyde. Polyester and epoxy resins. Alkyd, vinyl formaldehyde and urethane resins, as coatings on wire.

B 130 Combinations of material such as mica, glass fibre, asbestos, etc. with suitable bonding, impregnating or coating substances such as epoxy resins, phenol-formaldehyde and polyester resins.

F 155 Materials similar to Class B but with higher temperature bondino and impregnatino substances.

H 180 Silicones and materials such as mica, glass, asbestos, teflon.

C 220+ Inorganic insulants such as ceramics, glass, mica, quartz and P.T.FE.

MELTING TEMPERATURE

Melting temperature is the temperature at which a solid changes to a liquid state. For some materials, this change occurs abruptly at a particular temperature, whereas for others it may occur gradually over a range of temperatures.

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CHAPTER 3 PHYSICAL PROPERTIES OF MA 1ERIALS

Table 3.2 Typical Melting Temperatures

Material Melting Temperature DC

Mercury -39 Water 0 Lead 327 Aluminium 660 Copper 1083 Iron 1535 Titaniwn 1812 Silica brick 1700 Magnesite brick 1900 Graphite brick 3500

HEAT RESISTANCE

Heat resistance is the maximwn useful temperature for metallic and refractory materials. A refractory is a material that retains its mechanical and chemical stability at elevated temperature. Heat resistance is partly determined by the nature of the environment. Most metals suffer. rapid oxidation at elevated temperatures so that this factor controls their useful upper temperature limit. However in a non­oxidising or reducing atmosphere. this upper temperature limit may be considerably extended.

ELECTRICAL PROPERTIES

Whenever a design requires that a pan act as an electrical conductor or an electrical insulator, then the electrical properties of the material become important. The principal electrical properties are resistivity and conductivity.

RESISTIVITY

Resistivity, is the rate at which electrical current will flow through a given cross section or along a given length. The unit used is the ohm metre (Qrn) or a multiple (MQrn or GQrn) for insulating

materials or micro ohm metre (Ilflm) for conducting materials.

The resistivity due to solid solution (one metal dissolved in the other) may be expressed as: aX=YxX(I-X)

where X = the mole fraction of solute Y = the solid solution resistivity coefficient. Y is larger for greater differences in valence and

atom size between solute and solvent atoms

For dilute solid solutions aX = YxX

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CHAPTER 3 PHYSICAL PROPERTIES OF MA lERlALS

Table 33 Effect of an addition of 0.5% alloying element on conductivity of copper

Solute Conductivity %IACS

Silver 98

Zinc 95 Nickel 75

AJuminiwn 56

Berylliwn 44

Silicon 28

Phosphorous 15

For multi-phase alloys, the relationship becomes more complex.

CONDUCTIVITY

Conductivity is simply the reciprocal of resistivity. However the unit for conductivity is siemen/metre (S.m']) where one siemen is the conductance of a conductor that has an electric resistance of one ohm. In chapter 2 we spoke of the structure of atoms, and of valence being the nwnber of outer electrons in an atom, or the nwnber of vacant electron sites. To understand conductivity we must look a little closer at these electrons, because electrical conductivity results from drift of electrons in a direction opposite to the electric field, and a corresponding drift of "electron holes" in the direction of the field.

Variations in conductivity (8) with temperature and composition depends upon the nwnber of carriers of electric charge (n), their charge (q). and their mobility (!!).

This may be equated as: 8 = nQ!!

In insulators, the electron valence bonds are completely filled. Thus there are no "free" electrons or electron holes. rEO, hence 8=0.

In a metallic conductor, the valence electrons are not bound to any particular atom, but they are free to move among the atoms in all directions through the crystal structure. If an electric field is superimposed on the conductor, the negatively charged ~ctrons will be attracted towards the positive pole. As the electron moves in this direction, it will soon encounter either the electric field of another atom, or even the solid nucleus of an atom and so may be deflected or refl ected. Whilst the nett effecl is a movement of electrons towards the positive pole, .the limiting factor on the conductivity of a metal is the mobility of the electron, or, as it is often referred to, the mean free path of the electron. The longer mean free path provides for a greater electrical conductivity. Mean free path is affected by atomic and crystal structure (which are fixed for any particular metal), and by composition, temperature and cold work.

Increasing alloy or impurity content decreases electrical conductivity as noted above. Increasing temperature also decreases electrical conductivity because increasing temperature causes greater agitation of atoms in a metal. It has been shown that, except for very low temperatures, electrical resistivity varies linearly with temperature in accordance with the relationship

crt= £ 273 {I + Yt(T- 273)1

where £ m = resistivity at 273' K (O'C) Yt = temperature resistivity coefficient which has a value of approximately O.005/K in pure

metals

At absolute zero (O'K or -273'C) many metals approach a condition of super conductivity (zero resistivity). This property is being exploited in some high technology areas.

28

CHAPTER 3 PHYSICAL PROPERTIES OF MATERIALS

Plastic defonnation or cold working of a metal by rolling or drawing or shaping the metal in a cold condition also decreases the electrical conductivity, because it defonns the crystalline lattice.

Table 3,4 Electrical Resistivity of Metals

Metal Conductivity (% lACS)* Resistivity (Microhm-cm)

Copper 100 1.7 Aluminium (SiMg alloy) 42 4.1 Aluminium (Zn-CuMg alloy) 32 5.3 Magnesium 37 4.6 Lead 7.8 22 Type 304 stainless steel 2.5 70 High alloy steel 2.9 60 Cast steel 10.7 16 * Conductivity of a metal is often quoted as %IACS or percentage of the International Annealed Copper Standard

For any metal, the total resistivity due to (i) alloying elements or impurities, (ii) temperature. and (iii) cold work is the sum of the resistivities for each factor. This may be expressed as

0' = ax + crt + as

where ax = resistivity due to presence of impurity atoms crt = resistivity due to temperature as = resistivity due to cold work

Resistivities of some common metals are shown in table 3.4.

For electrical insulating materials - dielectrics - there are other properties that must be considered. These include: • DIELECTRIC STRENGTH, a measure of the insulation breakdown voltage, expressed as

volts/metre (V.m·l ) or kV.mm·l . Typical values range from about 20 to 100 kV.mm·l for insulating materials. Dielectric strength is sensitive to temperature, and decreases with increasing temperature.

• ARC RESISTANCE is the resistance of a material to deterioration by burning and carbonisation when an arc is struck across its surface.

• WATER ABSORPTION, particularly in polymeric materials, where absorption can result in swelling of the material and in a significant lowering of its resistivity and its dielectric strength.

MAGNETIC PROPERTIES

The magnetic properties of a material can be described by a number of factors. Perhaps the most important of these is FERROMAGNETISM - that property that detennines whether a material will be attracted by a magnetic field. There are only four common elements that exhibit ferromagnetism -iron, nickel, cobalt and gadolinium. However there are a number of alloys that exhibit varying degrees of ferromagnetism, although these are based mainly on the above elements. There are also some alloys, such as Mn-Bi, that contain none of the above elements but are ferromagnetic .

Magnetic properties arise from the fact that electrons, as they orbit around a nucleus, also spin on an axis, in much the same way that the earth and other planets spin as they orbit the sun. If two electrons within the one energy level (or sub-orbital) are paired - rotating in opposite directions - their magnetic effects are cancelled out. however, when the electrons are unpaired and rotate in like direction, the elements can develop magnetic properties. Under suitable conditions, ferromagnetic materials may

29

CHAPTER 3 PHYSICAL PROPERTIES OF MA lERIALS

become permanent magnets, with large numbers of unpaired electrons in the outer electron orbitals. These unpaired electron spins give rise to a great number of magnetic domains within each crystal ill the metal or alloy.

In an unmagnetised ferromagnetic material , these domains are randomly oriented, so that their effects cancel out. However. if th~ domains are aligned by a magnetic field. the material becomes magnetic.

A material is saturated with magnetism when all the domains are aligned in the one direction. Whether or not a material retains the magnetism is determined by its magnetic hardness. A magnetically hard metal is one in which magnetism is retained, and a magnetically soft melltl is one in which magnetism is lost at some time after the removal of the magnetic field , or source of magnetisation.

Most materials are not visibly affected by even the most powerful magnets. However, in a strong magnetic field, the majority of metals do exltibit a very weak attraction, and are called paramagnetic materials. Other materials tend to be repulsed by a strong magnetic field. These are mostly the non­metallic materials such as glass, and even water. These are called diamagnetic materials. It takes very sensitive instruments to detect these effects. Since the magnetic characteristics of these paramagnetic and diamagnetic materials are less than one millionth as effective as those of the iron - cobalt - nickel alloys, there is little engineering interest in them for their magnetic properties.

It is well known that the passage of an electric current (electrons) through a conductor produces a magnetic field . This principle has been used extensively in a range of devices from delicate galvanometers to massive electromagnets. This magnetic field produces a magnetic flux density which is measured in Tes1a (T). Within a vacuum, the magnetic flux density (B) is related to the magnetic field strength (H) through the magnetic permeabili ty of a vacuum (J.lol by:

B !!o= -

H

The units for these quantities are: Magnetic permeability (!!.,) - Henry/metre (H.m·') Magnetic flux density (B) - Tesla (T) Magnetic field strength (H) - Amp/metre (A.m·' )

The value of!!o - magnetic penneability of a vacuum is 4" x 10' Henry/metre.

If a material is present to replace the vacuum in this situation, the magnetic flux density, which is also known as magnetic induction, changes because the material has a different magnetic permeability. When comparing permeabilities, it is convenient to use a relative value called relative permeability (!!,). Thus for a material

B !!o!!, = H or

For a vacuum !!, = 1.0. For paramagnetic materials, !!, is slightly greater than one, and for diamagnetic materials, it is slightly less than one.

e.g. for Aluminium !!, = 1.000022, and for Bismuth !!, = 0.9998

Ferromagnetic materials have very high valves of !!" ranging up to about one hundred thousand (10'). The magnetic flux density (magnetic induction) (B) that develops in the presence of a material is defmed by the relationship

B =!!o (H + M) = !!oH + !!oM where M is the magnetisation, measured in amp/metre, which is the same unit as is used for magnetic field strength. Thus M = (!!, -I)H

(!!, - I) is called the magnetic susceptibility (Xm)

i . e . Xm = !!, -I and Xm = ~

30

C HAPTER 3 P HYSICAL PROPERTIES OF MA TERlALS

Magnetic susceptibilities for the three types of materials are: paramagnetic: +0.0001 to 0.01 diamagnetic: -0.00005 to 0 ferromagnetic: + 102 to 106

HYSTERESIS

The magnetic characteristics of ferromagnetic materials are generally induced by passing a current through a coil that surrounds the material . By staning with zero current, hence zero magnetic field and zero flux density, the nett magnetisation can be increased by steadily increasing the current. This produces an alignment of the magnetic domains within the material so that there is a rapid increase in flux density until most of the domains are aligned in the same direction, and the material reaches its saturation magnetisation. If the magnetic field is then removed (by stopping the current flow through the coil), the induction (flux density) does not disappear inunediately. In a magnetically hard alloy. it may remain indefmitely. In a magnetically soft alloy, it may disappear over a reasonably short period of time. In all cases, there is a residual or remanent magnetisation or remanent induction (B,) that persists.

If an opposing magnetic field is produced (by reversing the direction of current flow through the conductor), the magnetic induction may be balanced or offse~ so that the nett effect is zero. Increasing this opposing field (or coercive field) will ultimately result in a reversal of the magnetic induction. A plot of magnetic field (H) against magnetic induction (B), is called a hysteresis loop, and the energy consumed per cycle is equal to the BH area within the loop.

remanent magnetisation

Induction (8) (fiux density)

-----~~=v=tn_-t----~~Magnetic field (H)

Magnetic Domains

Saturation full alignment

• • t •

~]-

Fig 3.3 Typical hysteresis loop.

Original non-aligned domains

Saturation reverse full alignment

In a soft magnet, remanent induction (remanence) is low, and in a hard magnet, it is high. Remanence is expressed in webers/m'.

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CHAPTER 3 PHYSICAL PROPERTIES OF MA 1ERIALS

The properties of interest in pennanent magnet materials are: • remanence • coercive field • maximum demagnetising product

Table 3.5 Magnetic properties of hard ferromagnetic alloys.

Material Remanence Coercive Field Max Demagnetising product (B,) Webers/m' (·Hc)amp/m (BH max.) joules/m'

Carbon steel 1 0.4 x 104 0.1 x 104 Alnico V 1.2 5.5 x 104 3.4 x 104 Ferroxdur 0.4 15 x 104 2.0 x 104

Note: Alnico is an aluminium - nickel - cobalt permanent magnet alloy. Ferroxdur is a compound BaFeoO.

For soft magnetic materials, the propenies of main interest are: • coercive field • saturation induction • max. relative permeability

Table 3.6 Magnetic properties of some soft ferromagnetic alloys.

Material Saturation Induction Coercive Field Maximwn Relati ve Permeabilitv

(Bs) Webers/m' (-He) amp/m (Il,(max.)) Pure Iron 2.2 80 5000 Silica ferrite transformed sheet 2 40 15000 Ni-Fe permalloy 1.6 10 2000 Ni-Fe-Mo Superpermalloy 0.2 0.2 100000 Cobalt 1.7 Nickel 0.6

Maximum relative permeability (11, Max.) represents the maximum slope of the B-H curve. It is useful because it indicates the field strength necessary to provide high induction values.

B

~B

I ~H

Fig 3.4 Maximum Relative Permeability

~B Slope = tan 8 = ~H

=11, max

H

CURIE POINT (Curie temperature). All ferromagnetic materials gradually lose their magnetic properties as temperature is increased, and at a cenain temperature their ferromagnetic propenies

32

CHAPTER 3 PHYSICAL PROPERTIES OF MATERIALS

disappear altogether. This temperature is known as Curie Point or Curie Temperature. Above this temperature ferromagnetic materials become paramagnetic. When cooled, they again become ferromagnetic.

Curie temperatures for the three basic ferromagnetic metals are:

Metal Curie Point (DC)

Iron 768

Cobalt 1120

Nickel 358

These values show the limit of the useful ferromagnetic temperature range for the material.

DENSITY

The density of a material is its mass per unit volume, and is related directly to the atomic weight of the atoms that constitute the material and to the number of each type of atom present in unit volume of the material. Thus high atomic weight atoms packed closely together will produce a high density material.

Another way of expressing density is in terms of its specific gravity. Specific gravity is a ratio - the ratio of the density of the material to the density of water. The specific gravity of water is 1.000.

Density, or specific gravity of a material is used when it becomes necessary to detennine the mass of a component or structure; knowing the volume of material in the structure and its density, the total mass is simply:

M;oxV

where 0; density V; volume

The standard unit of density is kg.m·3 (or g.L·'). Specific gravity, being a ratio, has no units.

REFRACTIVE INDEX

When light strikes a material it may be totally reflected as occurs with most solid materials, it may be transmitted, as with clear glass, or it may be partly absorbed, the colour of the transmitted light depending upon the nature of the material. A material that reflects all light is referred to as OPAQUE, one that transmits all (or most) of the incident light is TRANSPARENT. and one that transmits only some of the incident light is TRANSLUCENT.

If a beam of light strikes a transparent material at an oblique angle, it is bent or REFRACTED at the interface, in such a way that the angle of incidence (i) and angle of refraction (r) are related to each other by Snell's Law which states:

where and

Sin(i) ;~ Sin(r) V,

V, = velocity of light in medium I V, = velocity of light in medium 2

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CHAPTER 3 PHYSICAL PROPERTIES OF MATERIALS

. I I

I r

Fig 3.5 Refraction oj Light

Medium 1 e.g. air

Medium 2 e.g. water

The ratio of velocity of light in a vacuum (II) to that in a medium (11m) is the refractive index of the medium.

V,S -.:::in:o.( i",-, '-) Thus n=-= -Vm Sin(rm)

Typical values of refractive index are:

Medium Refractive Index

Air 1.0002926 Water 1.333 Crown Glass 1.5171 Fused Quartz 1.4584 Diamond 2.42

34

CHAYrER 3 PHYSICAL PROPERTIES OF MA 1ERIALS

GLOSSARY Thermal conductivity Thermal expansion Bi-metallic Heal resistance Coefficient Oxidation Resistivity Mean free path Solute Solvent Solid solution Plastic deformation Strain hardening

QUESTIONS

Dielectric Ferromagnetism Magnetic domain Paramagnetism Diamagnetism Magnetic fl ux Magnetic permeability Hysteresis Remanence Coercive field Density Refractive index

I . Discuss the factors that control the transfer of heat through a solid material.

2. Why are long steam supply lines built with a loop at regular intervals along their length?

3. In selecting a material for a furnace lining. would you use one with a high heat capacity or a low heat capacity? Why?

4. What is a refractory material?

5. What is the effect of temperature on the resistivity of a conductor?

6. In selecting an insulating oil for a high voltage transformer, would you specify a high or low dielectric strength? Why?

7. Discuss a practical application of ferromagnetism.

35