4.9 - Comparing Materials & Choosing One
Comparing the classes of material
The four classes of material are glass and clay ceramics, polymers, composites, and metals.
Properties of metals
- Shiny and good conductors of heat and electricity.
- Malleable - Can be hammered or pressed into shape.
- Ductile - Can be drawn out into a wire.
- Stiff, with generally high densities and high melting points.
- Not brittle - Under a large force a metal bends or dents rather than shattering.
- Some metals corrode in air and water.
The properties materials are compared on
- Melting point - The temperature at which a solid melts into a liquid.
- Softening temperature - The temperature at which a polymer starts to soften below its melting point.
- Electrical conductivity - How well a material conducts electricity.
- Strength - How well a material resists an applied force. Tensile strength is resistance to being pulled apart; compressive strength is resistance to being squashed.
- Stiffness - How well a material resists bending.
- Flexibility - How far a material can bend without breaking.
- Brittleness - How easily a material breaks under force, snapping rather than bending.
- Hardness - How well a material resists being scratched or dented.
- Density - The mass of a given volume.
- Ease of reshaping - How readily a material can be formed into a new shape.
The four classes compared
| Glass and clay ceramics | Polymers | Composites | Metals | |
|---|---|---|---|---|
| Melting point | very high | low | depends on materials combined | high |
| Softening temperature | not applicable | well below melting point | depends on matrix | not applicable |
| Electrical conductivity | insulator | insulator | insulator unless reinforcement conducts | good conductor |
| Strength in tension | low | low to moderate | can be very high | high |
| Strength in compression | high | moderate | high | high |
| Stiffness | high | low for most | depends on reinforcement | high |
| Flexibility | very low | high for many | low | moderate |
| Brittleness | high | low | low | low |
| Hardness | high | low | moderate to high | moderate to high |
| Density | moderate to high | low | low to moderate | high |
| Ease of reshaping | only while hot, not once fired | easily moulded | fixed once matrix has set | malleable and ductile |
- A choice of material is usually a set of compromises: a ceramic is hard and stiff but shatters, a polymer is light and cheap but weak, a metal is strong but heavy, and a composite has tailored properties but costs more.
- A property that's an advantage for one use can be a disadvantage for another: metal is ideal for a saucepan because it conducts heat well, but poor for the handle for the same reason.
Worked example - Choosing a material for a bike frame
A company is choosing a material for a bicycle frame. The frame has to be as light as possible, take a large pulling force without breaking, and not shatter if the bicycle is dropped. Using the data below, decide which material to choose and give a reason for rejecting each of the others.
| Material | Density (g/cm3) | Tensile strength (MPa) | Brittleness |
|---|---|---|---|
| A clay ceramic | 2.4 | 30 | high |
| A rigid polymer | 0.95 | 40 | low |
| A carbon fibre composite | 1.6 | 1,800 | low |
| Aluminium | 2.7 | 300 | low |
Step 1 - Turn each requirement into a property
- Light → low density
- Takes a large pulling force → high tensile strength
- Survives a drop → low brittleness
Step 2 - Rule out the clay ceramic
- Highly brittle, so the frame would shatter if dropped.
- At 30 MPa it also has the lowest tensile strength - it fails two requirements.
Step 3 - Rule out the rigid polymer
Lightest and not brittle, but at 40 MPa its tensile strength is far too low for a frame carrying a rider.
Step 4 - Separate the last two on the numbers
- Aluminium and the composite are both strong enough and neither is brittle.
- The composite has the higher tensile strength (1,800 MPa vs 300 MPa) and the lower density (1.6 g/cm3 vs 2.7 g/cm3).
Step 5 - Answer
- Choose the carbon fibre composite - it meets all three requirements and beats aluminium on strength and weight.
- The clay ceramic is rejected because it's brittle and weak in tension, the rigid polymer because it's far too weak, and aluminium because the composite is both stronger and lighter.
TermDefinition
MalleableAble to be hammered or pressed into shape without breaking.
DuctileAble to be drawn into wires.
DensityThe mass per unit volume of a substance.
Key points to remember
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