The main classes of material
Definition
Composite material
A material made from two or more materials combined, which has properties that neither has on its own.
- Metals conduct heat and electricity, are malleable, and are usually strong.
- Glass and clay ceramics are hard, brittle and resist heat and chemical attack.
- Polymers are light, easily shaped, and are electrical insulators.
- Composites combine two materials so that the result outperforms either alone.
- Each class has a characteristic set of properties that suits it to particular jobs.
Key Idea
Selecting a material means matching its properties to the demands of the job.
Comparing the classes using data
- Density decides whether an object will be heavy for its size.
- Strength decides what load the material can carry before it fails.
- Hardness decides how well it resists scratching and denting.
- Melting point decides whether it can be used somewhere hot.
- Electrical and thermal conductivity decide whether it conducts or insulates.
Example
- Metal: high density, high strength, conducts, high melting point.
- Polymer: low density, lower strength, insulates, low melting point.
- Ceramic: hard and heat-resistant but brittle, so it cracks rather than bends.
Composites combine the best of two materials
- A composite has a matrix holding a reinforcement material within it.
- The reinforcement supplies strength and the matrix holds it in shape.
- Reinforced concrete is concrete with steel bars inside it.
- Concrete is strong when squashed but weak when stretched, and the steel covers that weakness.
- Fibreglass combines glass fibres with a polymer, giving strength at low density.
Note
A composite is a mixture, so each part keeps its own properties within the whole.
Choosing a material for a job
- List what the job demands, such as strength, low mass or resistance to corrosion.
- Compare the data for each candidate against those demands.
- Rule out any material that fails a demand the job cannot compromise on.
- Among those left, weigh cost and availability.
- State the choice with the property that decided it, using the figures given.
Exam technique
- A selection answer quotes the numbers from the table rather than describing them in words.
- Naming the demand the material fails is what rules a candidate out.
- The final choice is stated with the property that decided it.
Self review
- Give two properties typical of a ceramic.
- Why is a polymer often chosen where low mass matters?
- What is a composite material?
- Why is steel put inside concrete?
- How would you choose between two materials given a table of data?
