DID YOU KNOW:Classification of Materials
We can generally classify materials
into metallic and non-metallic. We can further classify metallic
materials into ferrous and non-ferrous, while non-metallic materials can be
classified into organic and inorganic. Examples of these materials are given in
Table 2.1.
Table 2.1 Examples of Engineering Materials
Ferrous
metallic
|
Non-ferrous
metallic
|
Organic
|
Inorganic
|
Steel, gray cast iron, wrought iron,
malleable iron
|
Aluminum, copper, magnesium, nickel,
lead, zinc, titanium
|
Leather, wood, rubber, natural
fibers, resin
|
Ceramic, glass, graphite
|
Ferrous materials contain iron as the
base metal and range from plain carbon steel containing more than 98 percent
iron to high alloy steel containing up to 50 percent of a variety of alloying
elements. Non-ferrous metallic materials can be further sub-divided into light
metals, such as aluminum, magnesium and titanium, low-melting point metals
(e.g. lead and tin), refractory metals (molybdenum, tantalum and tungsten), and
precious metals (e.g. gold, silver and platinum).
Within the group of alloyed metallic
materials, we can further classify them according to (a) chemical composition,
e.g. carbon content and alloy content in steels, (b) finishing, e.g. hot rolled
or cold rolled, and (c) product form, e.g. bar, plate, sheet, tubing,
structural shape.
Metallic materials can also be divided
into cast alloys and wrought alloys according to the method of production. Cast
alloys make up about 20 percent of all industrial metallic materials and are
directly cast into the final shape of the product. Wrought alloys are usually
shaped by hot or cold working into semi-finished materials such as plates,
sheets, rods, wires or tubes. These are used as the input material for further
processing into the final product.
Besides cast and wrought alloys,
another type of metallic material that is gaining attention in the industries
is powder metals and alloys. The powders are compacted and sintered (that is,
heated without melting) to produce components that are ready to use and need
very little further processing.
The most common non-metallic materials
are the polymeric materials. They have a wide range of mechanical, physical and
chemical properties. These materials are characterized by their low density,
good thermal and electrical insulation, high resistance to most chemicals and
ability to be process using different colors and opacities. Compared to metals
and alloys, these materials are mechanically weaker and less stiff. These
drawbacks, however, can be overcome by reinforcing the materials with various
types of fibers.
One of the main reasons why polymers,
such as plastics, are increasing used in making products is because they can be
easily processed into complicated shapes in one step with little need for
further processing or surface treatment. Polymers can also be easily machined
and joined. They are also light and need less processing power compared to metals.
Products made from plastics can have high dimensional accuracy and surface finish.
They can be made in attractive colors at high speed and low cost.
Plastics are classified into
thermoplastics and thermosets. Thermoplastics soften when heated and harden
when cooled no matter how often the heating and cooling processes are repeated.
Thermosets, however, does not soften upon heating once they are fully cured. To
a product designer, the difference between thermoplastics and thermosets is
important because they need different manufacturing methods.
Rubbers are similar in structure to
plastics. The difference is based on the degree of stretching. Rubbers can be
stretched to at least twice their original length. Once the stress is released,
they return to their original length.
Ceramics are inorganic compounds of
one or more metals with a non-metallic element. Some examples of ceramics and
their non-metallic elements are shown in Table 2.2. Ceramics have high hardness,
stiffness and stability. They are also generally good insulators to heat and
electricity. They are, however, brittle.
Table 2.2 Ceramics and Their Non-metallic Elements
Ceramic
compounds
|
Non-metallic
elements
|
Aluminum oxide
|
Oxygen
|
Silicon oxide
|
Carbon
|
Silicon nitride
|
Nitrogen
|
