Ceramics, polymers, composites and metals have typical properties
Ceramic
A hard, brittle, non-metallic material made by heating substances such as clay or sand to a high temperature, including pottery, bricks and glass.
Composite material
A material made by combining two or more different materials, usually fibres or particles held in a matrix, with properties different from those of each material alone.
- Glass is a ceramic made by melting sand with other substances, and it is hard, transparent and brittle.
- Clay ceramics, such as bricks, tiles and china, are made by shaping wet clay and heating it in a kiln, and they are hard, brittle and strong when squashed.
- Polymers, such as poly(ethene), have low densities, are often flexible and are good electrical and thermal insulators.
- Composites combine two materials, with fibres or particles of one set in a matrix of the other, to get properties that neither has alone.
- Metals are strong, dense, malleable and good conductors of heat and electricity, but many corrode.
Each class of material has typical properties, but the values for particular materials within a class can differ widely.
Quantitative data show how much materials differ
Density
The mass of a material per unit volume, usually measured in kg/m³ or g/cm³.
Tensile strength
The largest pulling force per unit area that a material can withstand before it breaks.
- Density data show that steel, at about 7800 kg/m37800\,\text{kg/m}^37800kg/m3, is roughly eight times as dense as poly(ethene), at about 950 kg/m3950\,\text{kg/m}^3950kg/m3.
- A carbon-fibre composite can have a tensile strength of about 600 MPa600\,\text{MPa}600MPa, compared with about 400 MPa400\,\text{MPa}400MPa for steel and about 30 MPa30\,\text{MPa}30MPa for poly(ethene).
- Clay ceramics are strong when compressed but weak when stretched, with a tensile strength often below 10 MPa10\,\text{MPa}10MPa.
- Melting points range from about 110 ∘C110\ ^{\circ}\text{C}110 ∘C for poly(ethene) to about 1500 ∘C1500\ ^{\circ}\text{C}1500 ∘C for steel.
- Aluminium conducts heat about 200 times better than glass, which is why saucepans are made of metal but have polymer handles.
Mass of a steel and a composite car panel
- A car panel needs 0.0020 m30.0020\,\text{m}^30.0020m3 of material; compare the mass of a steel panel (density 7800 kg/m37800\,\text{kg/m}^37800kg/m3) with a carbon-fibre composite panel (density 1600 kg/m31600\,\text{kg/m}^31600kg/m3).
- Mass of steel panel =7800×0.0020=15.6 kg= 7800 \times 0.0020 = 15.6\,\text{kg}=7800×0.0020=15.6kg.
- Mass of composite panel =1600×0.0020=3.2 kg= 1600 \times 0.0020 = 3.2\,\text{kg}=1600×0.0020=3.2kg.
- Divide the masses: 15.63.2=4.9\dfrac{15.6}{3.2} = 4.93.215.6=4.9, so the steel panel has about five times the mass of the composite panel.
Choosing a material by matching properties to use
Brittle
Likely to crack or shatter when hit or bent, rather than changing shape.
- List the properties the use needs, such as strength, density, heat resistance or resistance to corrosion.
- Then find the material whose values meet every requirement, not the one that is best at only one of them.
- Window glass is chosen because it is transparent and hard to scratch, even though it is brittle.
- Bricks suit walls because a wall pushes down on them, and clay ceramics are strong when compressed.
- Poly(ethene) suits carrier bags and bottles because it has a low density, is flexible and does not react with water.
- Steel suits bridges and building frames because it has a high tensile strength, even though it is dense and needs protecting from rust.
Do not describe a polymer as light, because the property to quote is its low density.
Composites combine materials to offset each other's weaknesses
- Reinforced concrete sets steel rods inside concrete, because concrete is strong when compressed and steel is strong when stretched.
- Fibreglass sets glass fibres in a polymer resin, giving a strong, low-density material for boat hulls and canoes.
- Carbon-fibre composite sets carbon fibres in a polymer resin, giving very high strength with low density for racing bikes and aircraft parts.
- Composites are often expensive to make and hard to recycle, because their materials cannot easily be separated.
- Wood is a natural composite of cellulose fibres held in a matrix of lignin.
When a question gives a table and asks you to select a material, quote the values that meet each requirement and reject every other material with the value that rules it out.
- Roughly how many times denser is steel than poly(ethene)?
- Why are bricks suitable for walls but not for parts that are stretched?
- Why are saucepan handles often made of a polymer?
- Why is reinforced concrete stronger than concrete alone?
- Why might carbon-fibre composite be chosen instead of steel for a racing bike frame?
