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chapter 1 General introduction
General Material Classifications
There are thousands of materials available for use in engineering
applications. Most materials fall into one of three classes that are based
on the atomic bonding forces of a particular material. These three
classifications are metallic, ceramic and polymeric. Additionally,
different materials can be combined to create a composite material.
Within each of these classifications, materials are often further organized
into groups based on their chemical composition or certain physical or
mechanical properties. Composite materials are often grouped by the
types of materials combined or the way the materials are arranged
together. Below is a list of some of the commonly classification of
materials within these four general groups of materials.
Metals


Ferrous
metals
and
alloys (irons, carbon
steels,
alloy
steels,
stainless steels, tool and
die steels)
Nonferrous metals and
alloys
(aluminum,
copper,
magnesium,
nickel, titanium, precious
metals, refractory metals,
superalloys)
Ceramics

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Glasses
Glass ceramics
Graphite
Diamond
Polymeric

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Thermoplastics
plastics
Thermoset plastics
Elastomers
Composites

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Reinforced plastics
Metal-matrix
composites
Ceramic-matrix
composites
Sandwich structures
Concrete
Each of these general groups will be discussed in more detail in the
following pages.
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HOW CAN WE COMPARE MATERIALS?
- Performance Physical
- Chemical Properties
- Composition & Structure
-Processing & Synthesis
These four categories are useful ways to sort different materials.
Metals, polymers and ceramics tend to have great differences in these
categories.
PROPERTIES
To choose a material with the best performance for a particular
application, we will need to consider the properties of the available
materials. Properties are the observed characteristics of a sample.
Physical properties
Some physical properties describe how an object responds to mechanical
forces. Hardness is one example of a mechanical property. If you drag a
steel knife blade across a hard object, such as a plate, the hard surface is
unchanged; if you drag the blade across a soft object,
such as a piece of chalk, the soft surface will be scratched. An object is
tough if force is unable to break or tear it. The response to force depends
on the material's structure, and also on its shape and size. A piece of
notebook paper can be torn easily, but a telephone book requires much
greater force.
We can easily bend a flexible object such as a nylon jacket, but more
force is required to bend a stiff object like a polyethylene milk jug. If an
object returns to its original shape and size when the force is removed, we
call it elastic. If the deformation remains, it is plastic. An object that
breaks rather than bending is brittle.
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An object is strong if an applied force is unable to deform or break it. A
nylon
Windbreaker is strong, since pulling on it does not change its length.
Sometimes the manner of applying a force makes a difference to the
strength of an object. Ceramics can bear a lot of weight, but will break if
stretched or bent. Nylon survives compression, pulling and twisting.
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Color, texture, and reflectivity can be observed by shining light on a
sample. Mirrors are colorless, smooth, and shiny. Electrical conductivity
is detected by applying a voltage across an object. Applying heat to a
sample reveals its ability to conduct heat, its melting point (temperature
at which a solid changes to liquid), and its boiling point (temperature at
which a liquid changes to gas). Some properties are independent of the
amount of sample. Melting point does not
change if a sample is divided in half. Other properties, including mass
and volume, increase with the amount of sample being studied.
When observing properties in a laboratory, scientists use the Metric
System of
measurements. The basic units of the Metric System are presented in
Metals
Metals account for about two thirds of all the elements and about 24% of
the mass of the planet. Metals have useful properties including strength,
ductility, high melting points, thermal and electrical conductivity, and
toughness. From the periodic table, it can be seen that a large number of
the elements are classified as being a metal. A few of the common metals
and their typical uses are presented below.
Common Metallic Materials

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Iron/Steel - Steel alloys are used for strength critical applications
Aluminum - Aluminum and its alloys are used because they are
easy to form, readily available, inexpensive, and recyclable.
Copper - Copper and copper alloys have a number of properties
that make them useful, including high electrical and thermal
conductivity, high ductility, and good corrosion resistance.
Titanium - Titanium alloys are used for strength in higher
temperature (~1000° F) application, when component weight is a
concern, or when good corrosion resistance is required
Nickel - Nickel alloys are used for still higher temperatures
(~1500-2000° F) applications or when good corrosion resistance is
required.
Refractory materials are used for the highest temperature (> 2000°
F) applications.
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The key feature that distinguishes metals from non-metals is their
bonding. Metallic materials have free electrons that are free to move
easily from one atom to the next. The existence of these free electrons has
a number of profound consequences for the properties of metallic
materials. For example, metallic materials tend to be good electrical
conductors because the free electrons can move around within the metal
so freely. More on the structure of metals will be discussed later.
Ceramics
A ceramic has traditionally been defined as “an inorganic, nonmetallic
solid that is prepared from powdered materials, is fabricated into products
through the application of heat, and displays such characteristic properties
as hardness, strength, low electrical conductivity, and brittleness." The
word ceramic comes the from Greek word "keramikos", which means
"pottery." They are typically crystalline in nature and are compounds
formed between metallic and nonmetallic elements such as aluminum and
oxygen (alumina-Al2O3), calcium and oxygen (calcia - CaO), and silicon
and nitrogen (silicon nitride-Si3N4).
Depending on their method of formation, ceramics can be dense or
lightweight. Typically, they will demonstrate excellent strength and
hardness properties; however, they are often brittle in nature. Ceramics
can also be formed to serve as electrically conductive materials or
insulators. Some ceramics, like superconductors, also display magnetic
properties. They are also more resistant to high temperatures and harsh
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environments than metals and polymers. Due to ceramic materials wide
range of properties, they are used for a multitude of applications.
The broad categories or segments that make up the ceramic industry can
be classified as:

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
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

Structural clay products (brick, sewer pipe, roofing and wall tile,
flue linings, etc.)
Whitewares (dinnerware, floor and wall tile, electrical porcelain,
etc.)
Refractories (brick and monolithic products used in metal, glass,
cements, ceramics, energy conversion, petroleum, and chemicals
industries)
Glasses (flat glass (windows), container glass (bottles), pressed and
blown glass (dinnerware), glass fibers (home insulation), and
advanced/specialty glass (optical fibers))
Abrasives (natural (garnet, diamond, etc.) and synthetic (silicon
carbide, diamond, fused alumina, etc.) abrasives are used for
grinding, cutting, polishing, lapping, or pressure blasting of
materials)
Cements (for roads, bridges, buildings, dams, and etc.)
Advanced ceramics
o Structural (wear parts, bioceramics, cutting tools, and engine
components)
o Electrical (capacitors, insulators, substrates, integrated
circuit
packages,
piezoelectrics,
magnets
and
superconductors)
o Coatings (engine components, cutting tools, and industrial
wear parts)
o Chemical and environmental (filters, membranes, catalysts,
and catalyst supports)
The atoms in ceramic materials are held together by a chemical bond
which will be discussed a bit later. Briefly though, the two most common
chemical bonds for ceramic materials are covalent and ionic. Covalent
and ionic bonds are much stronger than in metallic bonds and, generally
speaking, this is why ceramics are brittle and metals are ductile.
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Polymers
A polymeric solid can be thought of as a material that contains many
chemically bonded parts or units which themselves are bonded together to
form a solid. The word polymer literally means "many parts." Two
industrially important polymeric materials are plastics and elastomers.
Plastics are a large and varied group of synthetic materials which are
processed by forming or molding into shape. Just as there are many types
of metals such as aluminum and copper, there are many types of plastics,
such as polyethylene and nylon. Elastomers or rubbers can be elastically
deformed a large amount when a force is applied to them and can return
to their original shape (or almost) when the force is released.
Polymers have many properties that make them attractive to use in certain
conditions. Many polymers:

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
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are less dense than metals or ceramics,
resist atmospheric and other forms of corrosion,
offer good compatibility with human tissue, or
exhibit excellent resistance to the conduction of electrical current.
The polymer plastics can be divided into two classes, thermoplastics
and thermosetting plastics, depending on how they are structurally and
chemically bonded. Thermoplastic polymers comprise the four most
important commodity materials – polyethylene, polypropylene,
polystyrene and polyvinyl chloride. There are also a number of
specialized engineering polymers. The term ‘thermoplastic’ indicates that
these materials melt on heating and may be processed by a variety of
molding and extrusion techniques. Alternately, ‘thermosetting’ polymers
can not be melted or remelted. Thermosetting polymers include alkyds,
amino and phenolic resins, epoxies, polyurethanes, and unsaturated
polyesters.
Rubber is a natural occurring polymer. However, most polymers are
created by engineering the combination of hydrogen and carbon atoms
and the arrangement of the chains they form. The polymer molecule is a
long chain of covalent-bonded atoms and secondary bonds then hold
groups of polymer chains together to form the polymeric material.
Polymers are primarily produced from petroleum or natural gas raw
products but the use of organic substances is growing. The super-material
known as Kevlar is a man-made polymer. Kevlar is used in bullet-proof
vests, strong/lightweight frames, and underwater cables that are 20 times
stronger than steel.
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General properties of materials
Characteristic Properties of Major Classes
metals
polymers
-hard but malleable
-shiny
-little color
-intermediate
-conduct electricity
-high density
-difficult to burn
ceramics
stiff or flexible
hard but brittle
dull
shiny if glazed
colorless
many colors
low melting highest melting -point
nonconductive
non-conductive
low density intermediate density
flammable
not flammable
Composites
A composite is commonly defined as a combination of two or more
distinct materials, each of which retains its own distinctive properties, to
create a new material with properties that cannot be achieved by any of
the components acting alone. Using this definition, it can be determined
that a wide range of engineering materials fall into this category. For
example, concrete is a composite because it is a mixture of Portland
cement and aggregate. Fiberglass sheet is a composite since it is made of
glass fibers imbedded in a polymer.
Composite materials are said to have two phases. The reinforcing phase is
the fibers, sheets, or particles that are embedded in the matrix phase. The
reinforcing material and the matrix material can be metal, ceramic, or
polymer. Typically, reinforcing materials are strong with low densities
while the matrix is usually a ductile, or tough, material.
Some of the common classifications of composites are:





Reinforced plastics
Metal-matrix composites
Ceramic-matrix composites
Sandwich structures
Concrete
If the composite is designed and fabricated correctly, it combines the
strength of the reinforcement with the toughness of the matrix to achieve
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a combination of desirable properties not available in any single
conventional material. Some composites also offer the advantage of being
tailor able so that properties, such as strength and stiffness, can easily be
changed by changing amount or orientation of the reinforcement material.
The downside is that such composites are often more expensive than
conventional materials.
Basic Properties: Density, viscosity and buoyancy
Density : (weight per unit volume) is one of the most important physical
properties of coating materials and other liquids and semi liquids. Density
is important because it is a first-line indication of the ratios of
components that make up a material.
D = M/V
As you heat a substance its molecules move farther apart and the
substance becomes less dense.
Solid
liquid
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Example: What is the mass of a 500.00 mL sample of seawater with a
density of 1.025 g/mL?
Buoyancy and Archimedes low :
Any object, wholy or partially immersed in a fluid, is buoyed up by a
force equal to the weight of the fluid displaced by the object.
The upward force on a body immersed or partly immersed in a fluid.
This low used for measuring of density of the solid materials
For a fully submerged object, Archimedes' principle can be reformulated
as follows,
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What is Viscosity?
A measure of a material’s resistance to flow, and it is a measure of the
resistance that one layer of liquid experiences when flowing over
another layer. Substances with higher viscosity take longer to flow
down the ramp.
Application of Viscosity:
*Viscosity is the most important property of motor oil.
**Viscosity depends on temperature – Increase temperature will
decrease the viscosity of the material.
In the winter you need lower viscosity oil so the oil will flow better.
In the summer, you need higher viscosity oil so during the higher
summer temperatures it will not flow too fast. (If oil is too thin it does
not adhere to the surfaces and lubricate the moving parts.)
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