Metals and materials · Contrast
Ceramics, polymers and composites
A china plate shatters on a tiled floor and a plastic beaker bounces. But you could stand on the plate and it would hold you. So which of the two is the stronger material?
Start here
A china dinner plate and a plastic beaker are knocked off the same table onto the same tiled floor.
The plate breaks into eleven pieces. The beaker bounces twice and rolls under a chair. Yet you could stand on the plate and it would hold you, and standing on the beaker would flatten it.
So which of the two is the stronger material?
The plate is the stronger one — it takes far more force to break. It is also brittle: it cannot bend even slightly, so when it does fail it fails all at once. The beaker is weaker and tough: it gives, absorbs the knock and comes back. Two different properties, and “which is better” is not a question until you know what the material has to survive.
Metals are not the only useful materials, and most things around you are not metal. Three other families do most of the work: ceramics, polymers and composites.
Each family behaves in a characteristic way, because of what it is made of. Knowing the family tells you roughly what a material will do before you have ever handled it.
Reference · keep this one open
Three families
Ceramics
Clay or minerals, fired hard. Hard and stiff, stand high temperatures, do not conduct electricity, and brittle — they crack rather than bend. Brick, porcelain, glass, the tile on a bathroom floor.
Polymers
Very long chains of atoms, mostly from crude oil. Light, easily shaped into anything, usually flexible and tough, and they soften or burn when heated. Polythene, PET, nylon, the case of the device you are reading this on.
Composites
Two materials built into one, so the result does what neither could alone: stiff fibres take the pull, and the material around them holds the fibres in place and spreads the load. Reinforced concrete, carbon fibre, plywood, bone.
Every one of those properties is a tendency, not a law. There are polymers that will sit in an oven and ceramics you can see through, and the bench below has both.
Your turn · four jobs
Four jobs, six materials on the shelf. One material fits each job.
0 of 4 matched
Pick a job, then pick a material.
Firebrick for the Lining for a pizza oven floor
That one fits.
A firebrick was made for exactly this. It takes the heat, it does not mind being heated and cooled every day, it holds the weight of the fire, and it is cheap enough to lay a whole floor. That it is heavy and would shatter if you dropped it does not matter — nobody is going to drop the oven floor.
Heat-proof glass-ceramic for the Lining for a pizza oven floor
Not this one.
- cheap by the square metrea sheet of it costs many times what the same area of firebrick does
Polythene for the Lining for a pizza oven floor
Not this one.
- stands red heatit softens in hot water and melts not far above that
- survives daily heating and coolingthe first heating is the last one — there is nothing left to cycle
- stiff under loada sheet of it bends between your fingers
PET for the Lining for a pizza oven floor
Not this one.
- stands red heatit buckles in an oven and softens in boiling water
- survives daily heating and coolingit deforms on the first strong heating, so there is no second one
- stiff under loadan empty bottle crumples in one hand — it is the pressure inside that keeps a full one firm
Carbon-fibre composite for the Lining for a pizza oven floor
Not this one.
- stands red heatthe resin holding the fibres together chars long before anything glows
- survives daily heating and coolingevery strong heating weakens the resin a little more until the fibres let go
- cheap by the square metreit is the most expensive material on the shelf by a wide margin
Reinforced concrete for the Lining for a pizza oven floor
Not this one.
- survives daily heating and coolingwater trapped inside turns to steam and blows flakes off the surface
Firebrick for the A bottle for a fizzy drink
Not this one.
- lighta brick weighs what a brick weighs, and a frame made of them would need a lorry
- holds pressurised gas infired clay is full of tiny holes — that porosity is part of why it insulates so well
- does not shatter when droppeddropped on a hard floor it cracks straight through
- see-throughit is opaque, and firing clay will never make it otherwise
Heat-proof glass-ceramic for the A bottle for a fizzy drink
Not this one.
- lighta pane thick enough to be safe is heavy
- does not shatter when droppeda sharp knock on a corner and it goes — it is still a ceramic
Polythene for the A bottle for a fizzy drink
Not this one.
- holds pressurised gas incarbon dioxide seeps out through it, and a fizzy drink stored in it goes flat in days
- see-throughit comes out milky rather than clear, like a supermarket milk bottle
PET for the A bottle for a fizzy drink
That one fits.
PET holds the gas in, weighs almost nothing, bounces when it is dropped, and you can see the drink through it. It softens in an oven, and no fizzy drink has ever needed to go in one.
Carbon-fibre composite for the A bottle for a fizzy drink
Not this one.
- holds pressurised gas inwound fibre and resin does not seal on its own — composite gas cylinders have a liner inside them
- see-throughit is black, and the weave shows
Reinforced concrete for the A bottle for a fizzy drink
Not this one.
- lightit is the heaviest thing on the shelf
- holds pressurised gas inconcrete is porous — gas works its way through it
- see-throughit is opaque
Firebrick for the A racing bike frame
Not this one.
- lighta brick weighs what a brick weighs, and a frame made of them would need a lorry
- takes repeated flexing and knocksit does not flex at all; the first bend is a crack
Heat-proof glass-ceramic for the A racing bike frame
Not this one.
- lighta pane thick enough to be safe is heavy
- takes repeated flexing and knocksit has no give, so repeated flexing finds the smallest scratch and runs a crack out of it
Polythene for the A racing bike frame
Not this one.
- stiff under loada sheet of it bends between your fingers
PET for the A racing bike frame
Not this one.
- stiff under loadan empty bottle crumples in one hand — it is the pressure inside that keeps a full one firm
Carbon-fibre composite for the A racing bike frame
That one fits.
Carbon fibre is the one that gets all three at once: stiff so the pedalling is not wasted, light enough to argue about grams, and tough enough to take years of flexing. It costs a great deal, and on a racing frame that is the trade being made deliberately.
Reinforced concrete for the A racing bike frame
Not this one.
- lightit is the heaviest thing on the shelf
- takes repeated flexing and knocksit cracks instead of flexing; the steel inside stops the crack running, which is not the same as bending
Firebrick for the The window in a stove door
Not this one.
- see-throughit is opaque, and firing clay will never make it otherwise
Heat-proof glass-ceramic for the The window in a stove door
That one fits.
Heat-proof glass-ceramic is the only material here that is transparent and unbothered by the heat. It is expensive and heavy, and a stove door is small — so those are the right costs to accept for the one property nothing else offers.
Polythene for the The window in a stove door
Not this one.
- see-throughit comes out milky rather than clear, like a supermarket milk bottle
- stands red heatit softens in hot water and melts not far above that
- survives daily heating and coolingthe first heating is the last one — there is nothing left to cycle
- stiff under loada sheet of it bends between your fingers
PET for the The window in a stove door
Not this one.
- stands red heatit buckles in an oven and softens in boiling water
- survives daily heating and coolingit deforms on the first strong heating, so there is no second one
- stiff under loadan empty bottle crumples in one hand — it is the pressure inside that keeps a full one firm
Carbon-fibre composite for the The window in a stove door
Not this one.
- see-throughit is black, and the weave shows
- stands red heatthe resin holding the fibres together chars long before anything glows
- survives daily heating and coolingevery strong heating weakens the resin a little more until the fibres let go
Reinforced concrete for the The window in a stove door
Not this one.
- see-throughit is opaque
- survives daily heating and coolingwater trapped inside turns to steam and blows flakes off the surface
Four jobs, and the winners came from three different families.
Two of the winners were ceramics and they lost each other's jobs — one is opaque and cheap, the other clear and expensive. The family tells you what to expect. The particular material has to be checked against the particular job.
Notice what never decided a job: how STRONG a material is on its own. Every rejection above named a specific thing the material could not do — passes gas, cracks when cycled, will not stand red heat. Strong and tough are two properties, and neither of them is a score out of ten.
Key fact
A composite is two materials built into one, so that the result does something neither could do alone: strong fibres take the pull, and the material around them holds the fibres in place and spreads the load.
Six words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“Plastic is one material.”
One word covering a great many substances. Commit before you read on.
“Plastic” is a family, not a substance — like “metal”. A carrier bag goes soft in hot water. A kettle body holds boiling water all day and stays rigid. A saucepan handle sits above a gas flame for years. A bulletproof vest is woven from polymer fibre. Those are four polymers with almost nothing in common except the long chains they are built from.
Some polymers soften whenever they are heated, and can be melted and reshaped over and over. Others are set hard as they are made and will char rather than melt. Asking “will plastic do?” is like asking “will metal do?” — the answer depends entirely on which one.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Which of these is a composite?
Rung 2 · The one that catches people
A ceramic tile and a polythene sheet are each pressed with a rising load. The tile takes far more force and then breaks suddenly. The sheet bends early and never quite breaks. Which statement describes them?
Rung 3 · Explain
Explain why reinforced concrete is used for bridges when neither concrete nor steel alone would do, using the word composite in your answer.
Rung 4 · Take it somewhere new
Choose a material for the see-through door of a wood-fired oven that is lit every evening and left to go cold overnight. Justify your choice and say why an ordinary glass pane would fail.
Key note
Ceramics are made by firing clay or other minerals. They are hard, stiff and stand high temperatures, they do not conduct electricity, and they are brittle — strong under a steady load, but they crack rather than bend. Polymers are built from very long chains of atoms. They are light, easily shaped, usually flexible and tough, and they soften or burn when heated. A composite is two or more materials combined so the result has properties neither had alone: fibres for strength, and a surrounding material to hold them and spread the load.
Going further
Concrete does not fail in a fire the way people expect. It holds water in tiny pores, and when the surface is heated hard that water turns to steam faster than it can escape. The pressure blows flakes off the face — spalling — and each flake exposes fresh concrete to the heat. That is why reinforced concrete lost the pizza-oven job above: the problem is not the temperature, it is the same surface being taken up and down through it every day.
A composite is hard to take apart again, which is the price of being two materials in one. A cracked carbon-fibre bicycle frame is currently shredded, burned for its energy, or buried; a steel one goes back into a furnace and comes out as steel. Separating fibres from the resin around them is an active engineering problem rather than a settled one, and it is the strongest argument anyone makes for building things out of one material where one will do.
Before this lesson
Next in this unit
- Metals and non-metals
- polymers-and-plastics (Metals and materials — coming soon)
At GCSE this becomes
- Polymer chains and cross-linking, thermosoftening against thermosetting, and choosing materials against a full specification including cost and life cycle.
Where to next
- Next: Inside the Earth
The Earth and its atmosphere
- Previous: Getting metals out of rocks
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