Compare metals, ceramics, polymers and composites and link their properties to bonding.
Why it matters
Every engineered object is made of something, and choosing that something well is often the difference between a product that lasts decades and one that cracks, corrodes or costs too much. Materials science explains why materials behave as they do, by linking four things that engineers often draw as a chain:
processing → structure → properties → performance
How a material is made (cast, rolled, sintered, moulded, heat treated) sets its internal structure (atoms, crystals, grains, defects). Structure sets properties (stiffness, strength, conductivity). Properties decide how the part performs in service. Change one link and the others change too. The rest of this course follows that chain.
The four main classes
Most engineering materials fall into four groups, largely because of how their atoms are bonded.
Metals and alloys. Atoms share a "sea" of free electrons (metallic bonding). That makes metals good conductors of heat and electricity, opaque and shiny, and, importantly, ductile: they can deform a lot before breaking, so they bend or dent instead of shattering. Examples: steel, aluminium alloys, copper, titanium. Typical densities range from about 2.7 g/cm³ (aluminium) through 4.5 (titanium) to 7.8 (steel) and 8.9 (copper).
Ceramics and glasses. Compounds of metals with non-metals (oxides, nitrides, carbides) held by strong ionic and covalent bonds. They are hard, stiff, resistant to heat and corrosion, and usually electrical insulators, but brittle: they break suddenly with little warning. Examples: alumina, silicon carbide, porcelain, window glass, cement and concrete.
Polymers. Long chain molecules of carbon and hydrogen (often with oxygen, nitrogen or chlorine), with strong covalent bonds along each chain and weak secondary bonds between chains. They have low density (around 0.9 to 1.5 g/cm³), are easy to shape, resist many chemicals, and insulate well, but have low stiffness and soften at modest temperatures. Three families:
- thermoplastics soften on heating and can be remoulded (polyethylene, PVC, nylon, PET);
- thermosets cross-link permanently when cured and do not melt (epoxy, phenolics);
- elastomers are lightly cross-linked and stretch a lot elastically (natural and synthetic rubbers).
Composites. Two or more materials combined so the result outperforms either alone, usually strong fibres in a matrix. Glass-fibre and carbon-fibre reinforced polymers are stiff and light; reinforced concrete puts steel where concrete is weak (in tension); wood is a natural composite of cellulose fibres in a lignin matrix. Composites are often anisotropic: much stronger and stiffer along the fibres than across them.
Two more groups are often listed separately: semiconductors such as silicon, whose conductivity can be controlled precisely by tiny additions of impurities, and biomaterials, chosen to work safely inside the body.
A rough comparison
| property | metals | ceramics | polymers | composites (CFRP) |
|---|---|---|---|---|
| density | medium to high | medium | low | low |
| stiffness (E) | high | very high | low | high along fibres |
| toughness | high | low | medium | medium |
| max service temperature | medium to high | very high | low | low to medium (set by the matrix) |
| electrical conductivity | high | very low | very low | varies |
These are generalisations. Within each class the range is enormous: a soft lead alloy and a high-strength tool steel are both metals.
Worked example: picking a class
A designer needs three parts.
- A kitchen knife blade. Needs hardness to hold an edge and enough toughness not to chip when dropped. A hardened steel is the usual answer; ceramic (zirconia) blades are harder and stay sharp longer but chip more easily.
- A furnace lining. Must survive 1,400 °C and insulate. Only ceramics (refractory bricks, alumina fibre) work.
- A bicycle frame. Needs stiffness and strength at low weight. Steel, aluminium, titanium and carbon-fibre composite are all used; the choice depends on cost, ride feel and how damage-tolerant it must be. The last lesson of this course turns this kind of reasoning into numbers.
Common mistakes
- Equating strength with hardness or stiffness. Glass is stiff and hard but weak in tension because it breaks at tiny flaws; rubber is soft but very tough.
- Assuming every plastic is weak. Engineering polymers and fibre composites carry serious loads in cars and aircraft.
- Forgetting processing. The same steel can be soft and ductile or hard and brittle depending on heat treatment.
Recap
- Processing sets structure, structure sets properties, properties set performance.
- Metals: ductile and conductive. Ceramics: hard, stiff, heat resistant, brittle. Polymers: light, easy to shape, low stiffness and temperature limit. Composites: tailored combinations, often anisotropic.
- Bonding explains most of the differences between the classes.