A-level Applied Science/Choosing and Using Materials/Classes of materials

The different classes of materials

Materials can be grouped into classes according to their composition. You should know:

• the basic categories of materials . metals, ceramics, glasses, polymers and a group of combined materials known as composites.

You should be able to:

• give examples of uses of each of these basic categories of materials and be able to offer and justify a range of common uses of these materials in different situations such as construction, domestic goods, industrial plant, medical applications and sports equipment;
• investigate how the properties of the categories of materials influence their use.

For each class of material, you will need to give examples of their uses, with justification:

(“X is used for Y because it is A, B and C.” e.g. 'Copper' is used for 'domestic wiring' because it has a 'low electrical conductivity', is 'flexible' and 'cheap'.)

Consider uses such as

• construction
• domestic goods
• industrial plant
• medical applications
• sports equipment

The basic categories of materials: 1 metals

Metals are crystalline materials with the uniquely fluid metallic bonding. The free electrons of metallic bonding make metals reflective and electrically- and thermally conductive.

The metallic bonding makes the structure able to sustain strain. Metals are can be hard, but are also ductile and tough. They are strong when subjected to all sorts of stress.

Metals can be shaped when cold (they are plastic/ductile) or melted and formed into new shapes by casting (they are thermoplastic).

The basic categories of materials: 2 ceramics

Ceramics are crystalline materials. As such, they tend to be opaque.

The bonding is ionic or covalent, which makes the structure unable to sustain strain. Ceramics are hard, inflexible and brittle. They are strong when subjected to compressive stress, but not to tensile stress or normal stress.

Ceramics are formed by the chemical reaction of plastic precursors e.g. clay. Once formed, they cannot be re-moulded i.e. they are thermoset.

The basic categories of materials: 3 glasses

Glasses are amorphous materials: They have no crystal structure. Glasses are amorphous ceramics but it is possible to make amorphous metals and amorphous polymers.

Being amorphous, glasses are transparent. Their properties are similar to ceramics except that they are thermoplastic.

The basic categories of materials: 4 polymers

Polymers can be crystalline or amorphous. Amorphous polymers tend to be clear whereas crystalline ones are opaque.

Crystalline polymers are 'anisotropic': Their properties vary depending on whether they are tested along their polymer chains (strong, rigid) or across the chains (weak, ductile).

Amorphous polymers have average properties in all directions of testing (they are 'isotropic').

Thermoplastic polymers consist of long chains, which when heated will become fluid and can then be re-moulded to new shapes.

Some thermoplastic polymers have such strong intermolecular bonding that the heat required to melt them is more than the heat required to make them combust. These behave like thermoset polymers.

Thermoset polymers are like ceramics: They are formed from fluid precursors but form a permanent 3D lattice of covalent bonds which is impossible to melt. Like ceramics, thermoset polymers are brittle. Unlike ceramics, thermoset polymers are softer, less dense and much less heat resistant.

The basic categories of materials: 5 composites

Examples

Steel-reinforced concrete

Concrete has excellent resistance to compressive stress, and is cheap and durable. Steel is expensive and corrodes but has excellent resistance to tensile and normal stresses.

Carbon fibre reinforced plastic

Plastics are tough but flexible. Carbon fibre has tremendous tensile strength but little resistance to compressive or normal stress (it buckles). Both materials are very low density.

Combining plastic and carbon fibre gives a tough, strong, lightweight material. By varying the direction of weave of the carbon fibre, the properties can be made anisotropic. If a fibre breaks, the plastic transfers the stress to other fibres.

Plywood

Wood is anisotropic – it is considerable weaker to stresses which are tensile or normal to the grain of the wood.

Plywood has layers of wood at right angles, so that stress in any direction is tensile to the grain of some layers of wood. The glue which binds the plywood also adds to its strength.