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Material
science is an interdisciplinary field involving the properties of matter and
its applications to various areas of science and engineering. Material
science also deals with properties and characteristics of materials.
What
is a material?
Material
is the thing of which something is made, or something that consist of
matter. Materials comprises of metals and non-metals which must be
operated upon to form the end product/desired product.
Material
classification
Most
engineering materials are classified into:
Metals – ferrous and
non-ferrous
Organics
Composites
Ceramics
Semi-conductors – metalloids
(a)
Metals
Metals
play a major role in the industries and everyday life of human beings.
Metals from our chemistry are composed of elements which readily give up
electrons to produce a metallic bond and electrical conductivity.
Examples of metals are: iron, copper, aluminum, zinc and magnesium.
Metals
generally possess the following characteristics:
i)
Lustre
ii)
Hardness
iii)
Low specific heat
iv)
Plastic deformability
v)
Good thermal and electrical conductivity
vi)
Relatively high melting point
vii)
Strength
viii) Ductility
ix)
Malleability
x)
Opaquity
xi)
Stiffness
xii)
Rigidity
xiii) Formability
xiv) Machinability
xv)
Weldability
xvi) Castability
xvii) Dimensional stability
(b)
Ceramic materials
Ceramics
usually consist of oxide of nitrides, carbides, silicates or borides of various
metals. Ceramics materials contain compounds of metallic and non metallic
elements such as Mg0, S102, SiC,
glasses etc. Such compounds contain both ionic and covalent bonds.
Example
of ceramic materials are: Refractoriness, insulators, cement, glass,
sand, brick, concrete and silicon carbide.
The
following are the characteristics of ceramics:
(i)
Brittleness
(ii)
Rock-like
(iii)
Resistance to high temperatures
(iv)
Hardness
(v)
Abrasiveness
(vi)
Insulation to flow of electric current
(vii) Corrosion
resistance
(viii) Opaque to light
(ix)
High temperature strength
(c)
Organic materials – Non-metals
Organic
materials from our chemistry means materials that contain carbon
compounds. There are two major groups of organic/polymeric materials
namely, - natural and synthetic. Examples of organic materials are:
i.
Plastics
ii.
Textiles
iii.
Rubber
iv.
Paper
v.
Wood
vi.
Lubricants
vii.
Paints and finishes
viii. Adhesive
Characteristics
of organic materials are as follows:
a. Light weight
b. Combustible
c. Soft
d. Ductile
e. Poor conductor of heat and
electricity
f. Non dimensionally
g. Poor resistance to temperature
(d)
Composition
Composites
materials are materials that contain more than one material type for example fibreglass
in which glass fibres are embedded within a polymeric material. Fibre
glass acquire its strength from the glass and flexibility from the
polymer. It has the characteristics of both materials
(e)
Semiconductors
Semiconductors
materials are referred to as metalloids in chemistry, that is element that are
not metals or non-metals. Semiconductors have electrical properties that
are intermediate between the electrical conductors and insulators.
(f)
Nano materials
Nano
matrials are materials on a nano scale 10-9 that has novel properties.
Nano materials aree formed with the following methods – thin film deposition
chemical film deposition and these materials are formed by refining
conventional materials.
1.3
Engineering Requirements of Materials
Engineering
requirements of a material mean what is expected from the material so that the
same can be successfully used for making Engineering components such as
crankshaft, spanner etc.
When
an Engineer thinks of deciding and fabricating an engineering part, he goes in
search of that material which possesses such properties that will permit the
component part to perform its functions successfully while in use.
The
main engineering requirements of materials fall under three categories
(i)
Fabrication requirements
(ii)
Service requirements
(iii)
Economic requirements
1.3.1
Fabricability requirements -
means that the material should be ale to get shaped (e.g. cast, forged, formed,
machined, sintered etc) and joined (e.g. welded, brazed etc) easily.
Fabrication requirements relate themselves with materials machinability,
ductility, castability, heat-treatability, weldability etc
1.3.2
Service requirements – means that the material selected for the purpose must
stand up to service demands e.g. proper strength wear resistance, corrosion
resistance, etc.
1.3.3
Economic requirements – means
that the engineering part should be made with minimum overall cost.
Minimum overall cost may be achieved by proper selection of both technical and
marketing variables.
2.0
Properties of Engineering Materials
Material
property is a factor that influences quantitatively or qualitatively the
response of a given material to imposed stimuli and constraints for example forces
temperatures and environments. Properties of a material define its
suitability for a particular use in industry.
In
principle all material properties have a statistical behavior. The
following below are the different material properties:
(a) Mechanical properties
(b)
Thermal properties
(c)
Electrical properties
(d) Magnetic properties
(e)
Chemical properties
(f) Optical properties
(g)
Physical
properties
(h) Technological properties
2.2
Mechanical Properties
Mechanical
properties of material is the term that describes the behavior of the material
under the action of external forces that is applied forces and loads.
Material response of materials to applied forces will depend on the type of
bonding, the structural arrangement of atoms or molecules and the type and
number of imperfections. The following are the various mechanical
properties:
2.2.1
Elasticity
The
ability of a loaded material to return to its original shape after
unloading. Loading a solid thus affects its dimensions but the resulting
deformation will disappear upon unloading.
2.2.2
Plasticity
The
property of a material by virtue of which is permanently deformed when it has been
subjected to an externally applied force great enough to exceed the elastic
limit.
3.3.3
Toughness
Toughness
– it is the property of a material to resist fracture due to high impact loads
like hammer blows. It is the ability of a material to resist load beyond
plastic deformation up to fracture.
3.3.4
Resilience
Resilience
is closely related to toughness. It is the ability of a material to
resist shock and impact loads. The ability of a material to deformed
elastically and recovered upon unloading.
3.3.5
Tensile strength
Tensile
strength is a measure of the strength and ductility of a material. It is
a ratio of the maximum load to original cross-sectional area.
3.3.6
Yield strength
The
ability of material to resist plastic deformation is called the yield strength
and is calculated by dividing the force initiating the yield by the original
cross sectional area of the specimen.
U T S
|
Yield point
|
Rapture strength
|
Elastic limit
|
Proportional limit
|
Strain
|
 |
3.3.7
Impact strength
It
us the ability of a material to resist or absorb shock energy before it
fractures. It depends on the structure of the material.
3.3.8
Ductility
Ductility
refers to the ability of a material to undergo deformation under tension
without rupture. Ductility is the ability of a material to be drawn.
3.3.9
Malleability
Malleability
is the ability of a material to withstand deformation under compression without
rupture. The property of a metal which makes it possible for it to be
formed by hammering or rolling. Malleability is compressive property
while ductility is tensile.
3.3.10
Fatigue
Fatigue
is a kind of failure that occurred as a result of fluctuating or dynamic
loading being imposed on a machine element. The said machine element
fails below the tensile strength for that material. The failure is
progressive, beginning as minute cracks that grow under the action of
fluctuating stresses whose maximum value is less than the tensile strength of
the material. It is the most catastrophic failure that materials
experience in operations.
3.3.11
Hardness
Hardness
is the resistance of material to plastic deformation usually by
indentation. However, the term may refer to stiffness or temper or to
resist scratching. The hardness of a material depends on the type of
bonding forces between atoms ions or molecules.
3.3.12
Brittleness
Brittleness
is defined as a tendency to fracture without appreciable deformation and is the
opposite of Ductility or malleability.
3.3.13
Wear Resistance
Wear
is the unintentional removal of solid material from rubbing surfaces. The
ability of a material to resist wear and abrasion. There are two types of
wear namely – Adhesive wear, referred to as galling, scuffing or scoring.
This is characterized by an intensive interaction between two bearing surfaces
resulting from mutual adhesion of the metals at the junction.
Abrasive
wear – it is the removal by plowing or gouging out from the surface of material
by another body much harder than the abraded surface.
3.3.14
Creep
Creep
is the failure that occurs to a material that is used at an elevated
temperature and a constant loading. For metals, creep becomes important
at temperature greater than about 0.4Tm (Tm = absolute
melting temperature) for lead is less than room temperature. It is a
progressive phenomenon.
3.4
Factors affecting Mechanical Properties
Mechanical
properties are those which define the behavior of a material under applied
loads. The following are the factors:
(1) Alloy contents such
as addition of w, cr etc improve hardness and strength of materials.
(2) Grain size – It is
either fine or coarse the fine grams here higher strength.
(3) Crystal
imperfections such as dislocations reduce the strength of the material
(4) Manufacturing
defects – such as cracks, blowholes etc
(5) Excessive cold working
produces strain – hardening and the material may crack.
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