DID YOU KNOW:Classification of Materials

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