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Classification and
Selection of Materials
1. INTRODUCTION
Materials science and engineering plays a vital role in this modern age of science and technology. Various
kinds of materials are used in industry, housing, agriculture, transportation, etc., to meet the plant and
individual requirements. The rapid developments in the field of quantum theory of solids have opened vast
opportunities for better understanding and utilisation of various materials. The spectacular success in the
field of space is primarily due to the rapid advances in high-temperature and high-strength materials.
The term materials science and engineering combines both materials science and materials engineering. Materials science is at the basic knowledge end of the materials knowledge spectrum and materials
engineering is at the applied knowledge end, and there is no demarcation line between the two (Fig. 1.1).
Materials may be defined as substances of which something is composed or made. Mankind, materials and
engineering have evolved over the passage of time and are continuing to do so.
Figure 1.2 shows a three-stage diagram that indicates the relationship among the basic sciences
(and mathematics), materials science and engineering, and the other engineering disciplines. The basic
sciences are located within the first stage or core of the diagram, while the various engineering disciplines (mechanical, electrical, civil, chemical, etc.) are located in the outermost third stage. The applied
sciences, metallurgy, ceramics and polymer science are located in the middle or second stage. Materials
Materials science
Materials science and
engineering
Materials engineering
Basic knowledge
of materials
Resultant knowledge
of the structure,
properties, processing,
and performance of
engineering materials
Applied knowledge
of materials
Fig. 1.1 Materials knowledge spectrum. Using the combined knowledge of materials from materials
science and engineering engineers convert materials into the useful products
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Engineering
Applied
sciences
Medicine
Basic
sciences
Life
sciences
Mechanics
Physics
Chemistry
Math
Earth
sciences
Mechanical
Chemical
Civil
Metallurgy
ceramics
Materials science
and engineering
Polymers
Electrical
Mining,
mineral and
geological
engineering
Nuclear
Aerospace
Fig. 1.2 Materials science and engineering form a bridge of knowledge between the basic sciences and
engineering disciplines
science and engineering is shown to form a bridge of materials knowledge from the basic sciences (and
mathematics) to the engineering disciplines.
The selection of a specific material for a particular use is a very complex process. However, one
can simplify the choice if the details about (i) operating parameters, (ii) manufacturing processes,
(iii) functional requirements and (iv) cost considerations are known. Factors affecting the selection
of materials are given in Table 1.1.
Table 1.1 Factors affecting the selection of materials
Manufacturing processes
l
l
l
l
l
l
l
l
l
(i)
Plasticity
Malleability
Ductility
Machinability
Casting properties
Weldability
Heat
Tooling
Surface finish
Functional requirements
Cost considerations
(ii)
l
l
l
l
l
l
l
l
l
Strength
Hardness
Rigidity
Toughness
Thermal conductivity
Fatigue
Electrical treatment
Creep
Aesthetic look
l
l
l
l
l
l
l
l
l
(iii)
Raw material
Processing
Storage
Manpower
Special treatment
Inspection
Packaging properties
Inventory
Taxes and custom duty
Operating parameters
l
l
l
l
l
l
l
l
l
(iv)
Pressure
Temperature
Flow
Type of material
Corrosion requirements
Environment
Protection from fire
Weathering
Biological effects
There are thousands and thousands of materials available and it is very difficult for an engineer
to possess a detailed knowledge of all the materials. However, a good grasp of the fundamental
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principles which control the properties of various materials help one to make the optimum selection of
material. In this respect, materials science and engineering draw heavily from the engineering branches,
e.g., metallurgy, ceramics and polymer science.
The subject of material science is very vast and unlimited. Broadly speaking, one can sub-divide the
field of study into following four branches: (i) Science of metals, (ii) Mechanical behaviour of metals
(iii) Engineering metallurgy and (iv) Engineering materials. We shall discuss them in subsequent chapters.
2. ENGINEERING REQUIREMENTS
While selecting materials for engineering purposes, properties such as impact strength, tensile strength,
hardness indicate the suitability for selection but the design engineer will have to make sure that the
radiography and other properties of the material are as per the specifications. One can dictate the method
of production of the component, service life, cost etc. However, due to the varied demands made metallic
materials, one may require special surface treatment, e.g., hardening, normalising to cope with the service
requires. Besides, chemical properties of materials, e.g., structure, bonding energy, resistance to environmental degradation also effect the selection of materials for engineering purposes.
In recent years polymeric materials or plastics have gained considerable popularity as engineering
materials. Though inferior to most metallic materials in strength and temperature resistance, these are
being used not only in corrosive environment but also in the places where minimum wear is required, e.g.,
small gear wheels, originally produced from hardened steels, are now manufactured from nylon or teflon.
These materials perform satisfactorily, are quiet and do not require lubrication.
Thus, before selecting a material or designing a component, it is essential for one to understand the
requirements of the process thoroughly, operating limitations like hazardous or non-hazardous conditions, continuous or non-continuous operation, availability of raw materials as well as spares, availability
of alternate materials vis-a-vis life span of the instrument/equipment, cost etc. Different materials possess different properties to meet the various requirement for engineering purposes. The properties of
materials which dictate the selection are as follows:
2.1 Mechanical Properties
The important mechanical properties affecting the selection of a material are:
(i) Tensile Strength: This enables the material to resist the application of a tensile force. To
withstand the tensile force, the internal structure of the material provides the internal resistance.
(ii) Hardness: It is the degree of resistance to indentation or scratching, abrasion and wear. Alloying
techniques and heat treatment help to achieve the same.
(iii) Ductility: This is the property of a metal by virtue of which it can be drawn into wires or
elongated before rupture takes place. It depends upon the grain size of the metal crystals.
(iv) Impact Strength: It is the energy required per unit cross-sectional area to fracture a specimen,
i.e., it is a measure of the response of a material to shock loading.
(v) Wear Resistance: The ability of a material to resist friction wear under particular conditions, i.e.,
to maintain its physical dimensions when in sliding or rolling contact with a second member.
(vi) Corrosion Resistance: Those metals and alloys which can withstand the corrosive action of a
medium, i.e., corrosion processes proceed in them at a relatively low rate are termed corrosionresistant.
(vii) Density: This is an important factor of a material where weight and thus the mass is critical, i.e.,
aircraft components.
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2.2 Thermal Properties
The characteristics of a material, which are functions of the temperature, are termed its thermal properties.
One can predict the performance of machine components during normal operation, if he has the knowledge
of thermal properties. Specific heat, latent heat, thermal conductivity, thermal expansion, thermal stresses,
thermal fatigue, etc., are few important thermal properties of materials. These properties play a vital role
in selection of material for engineering applications, e.g., when materials are considered for high temperature
service. Now, we briefly discuss few of these properties:
(i) Specific Heat (c): It is the heat capacity of a unit mass of a homogeneous substance. For a
homogeneous body, c = C/M, where C is the heat capacity and M is the mass of the body. One can
also define it as the quantity of heat required to raise the temperature of a unit mass of the
substance through 1°C. Its units are cal/g/°C.
(ii) Thermal Conductivity (K): This represents the amount of heat conducted per unit time through a
unit area perpendicular to the direction of heat conduction when the temperature gradient across
the heat conducting element is one unit. Truely speaking the capability of the material to transmit
heat through it is termed as the thermal conductivity. Higher the value of thermal conductivity, the
greater is the rate at which heat will be transferred through a piece of given size. Copper and
aluminium are good conductors of heat and therefore extensively used whenever transfer of heat
is desired. Bakelite is a poor conductor of heat and hence used as heat insulator.
The heat flow through an area A which is perpendicular to the direction of flow is directly proportional
to the area (A) and thermal gradient (dt/dx). Thermal conductivity (K) is given by
K=
Qx
A ( T1 T2 ) t
kcal/m/°C/s or J/m/s/k or
W/m/k
...(1.1)
where Q ® flow of heat (kcal), A ® face area (m2), t ® time (second), q1 and q2 are temperatures
of hot and cold side of the material (°C) and x is the distance between the two faces (m).
The thermal conductivity of a metal can be expressed as
2
K=
ne2 O
S2 È k Ø
T
3 Ê eÚ
2 m v0
...(1.2)
where l ® mean free path, k ® Boltzmann Constant, m ® electron mass, e ® electronic charge, v0
® initial velocity of the electron. We must note that similar expression is used for electrical
conductivity. The ratio of heat and electrical conductivity (k and s respectively) is given by
2
K S2 È k Ø
T
...(1.3)
V
3 Ê eÚ
Obviously, the thermal conductivity (K) and electrical conductivity (s) vary in the same fashion
2
from one material to another. The ratio
S2 È k Ø
is known as Wridemann—Franz ratio. Thermal
3 Ê eÚ
conductivity for some of the materials is given in Table 1.1.
(iii) Thermal Expansion: All solids expand on heating and contract on cooling. Thermal expansion may
take place either as linear, circumferential or cubical. A solid which expands equally in three
mutually orthogonal directions is termed as thermally isotropic. The increase in any linear dimension
of a solid, e.g., length, width, height on heating is termed as linear expansion. The coefficient of
linear expansion is the increase in length per unit length per degree rise in temperature. The
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increase in volume of a solid on heating is called cubical expansion. The thermal expansion of
solids has its origin in the lattice vibration and lattice vibrations increases with the rise in temperature.
Table 1.2 Thermal conductivity for some materials
Type of the material
Material
(i) Metals
Thermal conductivity (K) (W/m/k)
Copper
Aluminium
Cast iron
Mild steel
Stainless steel
Alumina
Titanium carbide
Glass
Bakelite
(ii) Ceramics
(iii) Polymers
(iv) Composites
Concrete
Wood
380
230
52
54
16
2.0
3.0
1.0
0.23
0.0019
1.4
0.14
(iv) Thermal Resistance (RT): It is the resistance offered by the conductor when heat flow due to temperature difference between two points of a conductor. It is given by
T1 T2
second – °C/Kcal
RT =
H
where H ® rate of heat flow and q1 and q2 are temperatures at two points (°C).
(v) Thermal Diffusivity (h): It is given by
Thermal conductivity ( K )
cm 3 /s
h =
Heat capacity ( C p ) – density ( U )
=
K
represent heat requirement per unit volume
Cp U
A material having high heat requirement per unit volume possesses a low thermal diffusivity
because more heat must be added to or removed from the material for effecting a temperature
change.
(vi) Thermal Fatigue: This is the mechanical effect of repeated thermal stresses caused by repeated
heating and cooling.
The thermal stresses can be very large, involving considerable plastic flow. We can see that fatigue
failures can occur after relatively few cycles. The effect of the high part of the temperature cycle on the
strength of material plays an important factor in reducing its life under thermal fatigue.
2.3 Electrical Properties
Conductivity, resistivity, dielectric strength are few important electrical properties of a material. A material
which offers little resistance to the passage of an electric current is said to be a good conductor of
electricity.
The electrical resistance of a material depends on its dimensions and is given by
Resistance = Resistivity ´
Length
Cross-section area
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Usually resistivity of a material is quoted in the literature. Unit of resistivity is Ohm-metre.
On the basis of electrical resistivity materials are divided as: (i) Conductors (ii) Semiconductors and
(iii) Insulators. In general metals are good conductors. Insulators have very high resistivity. Ceramic
insulators are most common examples and are used on automobile spark plugs. Bakelite handles for
electric iron, plastic coverings on cables in domestic wiring.
When a large number of metals and alloys are sufficiently cooled below transition temperature,
Tc, enter the state of superconductivity in which the dc resistivity goes to zero. The estimates of the
resistivity in the super-conducting phase place it at less than 4 ´ 10–25 W-m, which is essentially zero
for all practical purposes. The highest value of Tc up to 133 K has been reached for mercury cuprate.
2.4 Magnetic Properties
Materials in which a state of magnetism can be induced are termed magnetic materials. There are five
classes into which magnetic materials may be grouped: (i) diamagnetic (ii) paramagnetic (iii) ferromagnetic (iv) antiferromagnetic and (v) ferrimagnetic. Iron, Cobalt, Nickel and some of their alloys and
compounds possess spontaneous magnetization. Magnetic oxides like ferrites and garnets could be used
at high frequencies. Because of their excellent magnetic properties along with their high electrical
resistivity these materials today find use in a variety of applications like magnetic recording tapes,
inductors and transformers, memory elements, microwave devices, bubble domain devices, recording hard
cores, etc. Hysteresis, permeability and coercive forces are some of the magnetic properties of magnetic
substances which are to be considered for the manufacture of transformers and other electronic components.
2.5 Chemical Properties
These properties includes atomic weight, molecular weight, atomic number, valency, chemical composition,
acidity, alkalinity, etc. These properties govern the selection of materials particularly in chemical plant.
2.6 Optical Properties
The optical properties of materials, e.g., refractive index, reflectivity and absorption coefficient etc., affect
the light reflection and transmission.
2.7 Structure of Materials
The properties of engineering materials mainly depends on the internal arrangement of the atoms on
molecules. We must note that in the selection of materials, the awareness regarding differences and similarities
between materials is extremely important.
Metals of a single type atom are named pure metals. Metals in actual commercial use are almost
exclusively alloys, and not pure metals, since it is possible for the designer to realize an infinite variety
of physical properties in the product by varying the metallic composition of the alloy. Alloys are prepared
from mixed types of atoms. Alloys are classified as binary alloys, composed of two components, as
ternary alloys, composed of three components or as multi component alloys. Most commercial alloys are
multicomponent. The composition of an alloy is described by giving the percentage (either by weight or
by atoms) of each element in it.
The basic atomic arrangement or pattern is not apparent in the final component, e.g., a shaft or a
pulley but the properties of the individual crystals within the metallic component, which are controlled
by the atomic arrangement, are mainly responsible for their application in industry.
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One can determine the strength of a piece of metal by its ability to withstand external loading. The
structure of metal or alloy responds internally to the applied load by trying to counteract the magnitude
of the applied load and thus tries to keep the constituent atoms in their ordered positions if however the
load is higher than the force which holds the atoms in place, the metallic bond becomes ineffective and
atoms in the metal are then forced into new displaced positions. The movement of atoms from their
original positions in the metal is termed as slip. The ease with which atoms move or slip in a metal is
an indication of hardness. We must note that the relative movement of atoms or slip within a material
has a direct bearing on the mechanical properties of the material.
3. CLASSIFICATION OF ENGINEERING MATERIALS
The factors which form the basis of various systems of classifications of materials in material science
and engineering are: (i) the chemical composition of the material, (ii) the mode of the occurrence of the
material in the nature, (iii) the refining and the manufacturing process to which the material is subjected
prior it acquires the required properties, (iv) the atomic and crystalline structure of material and (v) the
industrial and technical use of the material.
Common engineering materials that falls within the scope of material science and engineering may
be classified into one of the following seven groups:
(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
Metals (ferrous and non-ferrous) and alloys
Ceramics
Polymeric (plastic) materials
Composites
Electronic materials (Semiconductors)
Biomaterials
Advanced materials.
3.1 Metals
All the elements are broadly divided into metals and non-metals according to their properties. Metals are
element substances which readily give up electrons to form metallic bonds and conduct electricity. Some
of the important basic properties of metals are: (a) metals are usually good electrical and thermal
conductors, (b) at ordinary temperature metals are usually solid, (c) to some extent metals are malleable
and ductile, (d) the freshly cut surfaces of metals are lustrous, (e) when struck metal produce typical
sound and (f) most of the metals form alloys. When two or more pure metals are melted together to form
a new metal whose properties are quite different from those of original metals, it is called an alloy.
Metallic materials possess specific properties like plasticity and strength. Few favourable
characteristics of metallic materials are high lustre, hardness, resistance to corrosion, good thermal and
electrical conductivity, malleability, stiffness, the property of magnetism, etc. Metals may be magnetic,
non-magnetic in nature. These properties of metallic materials are due to: (i) the atoms of which these
metallic materials are composed and (ii) the way in which these atoms are arranged in the space lattice.
Metallic materials are typically classified according to their use in engineering as under:
(i) Pure Metals: Generally it is very difficult to obtain pure metal. Usually, they are obtained by refining
the ore. Mostly, pure metals are not of any use to the engineers. However, by specialised and very expensive
techniques, one can obtain pure metals (purity ~ 99.99%), e.g., aluminium, copper etc.
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(ii) Alloyed Metals: Alloys can be formed by blending two or more metals or at least one being metal.
The properties of an alloy can be totally different from its constituent substances, e.g., 18-8 stainless steel,
which contains 18% chromium and 8% nickle, in low carbon steel, carbon is less than 0.15% and this is
extremely tough, exceedingly ductile and highly resistant to corrosion. We must note that these properties
are quite different from the behaviour of original carbon steel.
(iii) Ferrous Metals: Iron is the principal constituent of these ferrous metals. Ferrous alloys contain
significant amount of non-ferrous metals. Ferrous alloys are extremely important for engineering purposes.
On the basis of the percentage of carbon and their alloying elements present, these can be classified into
following groups:
(a) Mild Steels: The percentage of carbon in these materials range from 0.15% to 0.25%. These are
moderately strong and have good weldability. The production cost of these materials is also low.
(b) Medium Carbon Steels: These contains carbon between 0.3% to 0.6%. The strength of these
materials is high but their weldability is comparatively less.
(c) High Carbon Steels: These contains carbon varying from 0.65% to 1.5%. These materials get
hard and tough by heat treatment and their weldability is poor.
The steel formed in which carbon content is up to 1.5%, silica up to 0.5%, and manganese
up to 1.5% along with traces of other elements is called plain carbon steel.
(d) Cast Irons: The carbon content in these substances vary between 2% to 4%. The cost of
production of these substances is quite low and these are used as ferrous casting alloys.
(iv) Non-Ferrous Metals: These substances are composed of metals other than iron. However, these
may contain iron in small proportion. Out of several non-ferrous metals only seven are available in
sufficient quantity reasonably at low cost and used as common engineering metals. These are aluminium, tin, copper, nickle, zinc and magnesium. Some other non-ferrous metals, about fourteen in
number, are produced in relatively small quantities but these are of vital importance in modern industry.
These includes, chromium, mercury, cobalt, tungsten, vanadium, molybdenum, antimony, cadmium,
zirconium, beryllium, niobium, titanium, tantalum and manganese.
(v) Sintered Metals: These materials possess very different properties and structures as compared
to the metals from which these substances have been cast. Powder metallurgy technique is used to
produce sintered metals. The metals to be sintered are first obtained in powered form and then mixed
in right calculated proportions. After mixing properly, they are put in the die of desired shape and then
processed with certain pressure. Finally, one gets them sintered in the furnace. We must note that the
mixture so produced is not the true alloy but it possesses some of the properties of typical alloys.
(vi) Clad Metals: A ‘sandwich’ of two materials is prepared in order to avail the advantage of the
properties of both the materials. This technique is termed as cladding. Using this technique stainless
steel is mostly embedded with a thick layer of mild steel, by rolling the two metals together while they
are red hot. This technique will not allow corrosion of one surface. Another example of the use of this
technique is cladding of duralium with thin sheets of pure aluminium. The surface layers, i.e., outside
layers of aluminium resist corrosion, whereas inner layer of duralumin imparts high strength. This
technique is relatively cheap to manufacture.
4. POLYMERIC (PLASTIC) MATERIALS
Most polymeric materials consist of organic (carbon-containing) long molecular chains or networks.
Structurally, most polymeric materials are noncrystalline, but some consist of mixtures of crystalline and
noncrystalline regions. The strength and ductility of polymeric materials vary greatly. Because of the
nature of their internal structure, most polymeric materials are poor conductors of electricity. Some of
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these materials are good insulators and are used for electrical insulative applications. One of the more
recent applications of polymeric materials has been in manufacture of digital video disks. In general,
polymeric materials have low densities and relatively low softening or decomposition temperatures.
4.1 Organic, Inorganic and Biological Materials
Organic materials are carbon compounds and their derivatives. They are solids composed of long molecular chains. The study of organic compounds is very important because all biological systems are composed of carbon compounds. There are also some materials of biological origin which do not possess
organic composition, e.g., limestone.
4.2 Organic Materials
These materials are carbon compounds in which carbon is chemically bonded with hydrogen, oxygen and
other non-metallic substances. The structure of these compounds is complex. Common organic materials
are plastics and synthetic rubbers which are termed as organic polymers. Other examples of organic
materials are wood, many types of waxes and petroleum derivatives. Organic polymers are prepared by
polymerisation reactions, in which simple molecules are chemically combined into long chain molecules
or three-dimensional structures. Organic polymers are solids composed of long molecular chains. These
materials have low specific gravity and good strength. The two important classes of organic polymers are:
(a) Thermoplastics: On heating, these materials become soft and hardened again upon cooling, e.g.,
nylon, polythene, etc.
(b) Thermosetting plastics: These materials cannot be resoftened after polymerisation, e.g., ureaformaldehyde, phenol formaldehyde, etc. Due to cross-linking, these materials are hard, tough, nonswelling and brittle. These materials are ideal for moulding and casting into components. They have
good corrosion resistance.
The excellent resistance to corrosion, ease of fabrication into desired shape and size, fine lusture,
light weight, strength, rigidity have established the polymeric materials and these materials are fast
replacing many metallic components. PVC (Polyvinyl Chloride) and polycarbonate polymers are widely
used for glazing, roofing and cladding of buildings. Plastics are also used for reducing weight of mobile
objects, e.g., cars, aircrafts and rockets. Polypropylenes and polyethylene are used in pipes and manufacturing of tanks. Thermoplastic films are widely used as lining to avoid seepage of water in canals and
lagoons.
To protect metal structure from corrosion, plastics are used as surface coatings. Plastics are also
used as main ingredients of adhesives. The lower hardness of plastic materials compared with other
materials makes them subjective to attack by insects and rodents.
Because of the presence of carbon, plastics are combustible. The maximum service temperature is
of the order of 100°C. These materials are used as thermal insulators because of lower thermal conductivity. Plastic materials have low modulus of rigidity, which can be improved by addition of filters, e.g.,
glass fibres.
Natural rubber, which is an organic material of biological origin, is an thermoplastic material. It is
prepared from a fluid, provided by the rubber trees. Rubber materials are widely used for tyres of
automobiles, insulation of metal components, toys and other rubber products.
4.3 Inorganic Materials
These materials include metals, clays, sand rocks, gravels, minerals and ceramics and have mineral origin.
These materials are formed due to natural growth and development of living organisms and are not
biological materials.
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Rocks are the units which form the crust of the earth. The three major groups of rocks are:
(i) Igneous Rocks: These rocks are formed by the consolidation of semi-liquid of liquid material
(magma) and are called Plutonic if their consolidation takes place deep within the earth and volcanic if lava
or magma solidifies on the earth’s surface. Basalt is igneous volcanic where as granite is igneous plutonic.
(ii) Sedimentary Rocks: When broken down remains of existing rocks are consolidated under pressure,
then the rocks so formed are named as sedimentary rocks, e.g., shale and sandstone rocks. The required
pressure for the formation of sedimentary rocks is supplied by the overlying rocky material.
(iii) Metamorphic Rocks: These rocks are basically sedimentary rocks which are changed into new
rocks by intense heat and pressure, e.g., marble and slates. The structure of these rocks is in between
igneous rocks and sedimentary rocks.
Rock materials are widely used for the construction of buildings, houses, bridges, monuments,
arches, tombs, etc. The slate, which has got great hardness is still used as roofing material. Basalt,
dolerite and rhyolite are crushed into stones and used as concrete aggregate and road construction
material.
Another type of materials, i.e., Pozzolanics, are of particular interest to engineers because they
are naturally occurring or synthetic silicious materials which hydrate to form cement. Volcanic ash,
blast furnace slag, some shales and fly ash are examples of pozzolanic materials. When the cement
contains 10-20% ground blast furnace slag, then it is called pozzolans-portland cement, which sets
more slowly than ordinary portland cement and has greater resistance to sulphate solutions and sea
water.
Rocks, stone, wood, copper, silver, gold etc., are the naturally occurring materials exist in nature
in the form in which they are to be used. However, naturally occurring materials are not many in
number. Nowadays, most of the materials are manufactured as per requirements. Obviously, the study
of engineering materials is also related with the manufacturing process by which the materials are
produced to acquire the properties as per requirement.
Copper, silver, gold, etc., metals, which occur in nature, in their free state are mostly chemically
inert and highly malleable and ductile as well as extremely corrosion resistant. Alloys of these metals
are harder than the basic metals. Carbonates, sulphates and sulphide ores are more reactive metals.
4.4 Biological Materials
Leather, limestone, bone, horn, wax, wood etc., are biological materials. Wood is fibrous composition of
hydrocarbon, cellulose and lignin and is used for many purposes. Apart from these components a small
amount of gum, starch, resins, wax and organic acids are also present in wood. One can classify wood as
soft wood and hard wood. Fresh wood contains high percentage of water and to dry out it, seasoning is
done. If proper seasoning is not done, defects such as cracks, twist, wrap etc., may occur.
Leather is obtained from the skin of animals after cleaning and tanning operations. Nowadays, it is
used for making belts, boxes, shoes, purses etc. To preserve the leather, tanning is used. Following two
tanning techniques are widely used:
(a) Vegetable Tanning: It consists of soaking the skin in tanning liquor for several days and then dried
to optimum conditions of leather.
(b) Chrome Tanning: This technique involves pickling the skin in acid solution and then revolving in a
drum which contains chromium salt solution. After that the leather is dried and rolled.
Limestone is an important material which is not organic but has biological origin. It mainly
consist of calcium carbonate and limestone. It is widely used to manufacture cement. In Iron and Steel
Industries, limestone in pure form is used as flux.