Classes of materials

The four traditional classes are not an arbitrary filing system — they are sorted by bonding, and everything else follows. Metals share a sea of delocalised electrons, which is simultaneously why they conduct heat and electricity, why they are opaque and shiny, and why they bend rather than snap: the bonds are non-directional, so planes of atoms can slide past each other without the structure coming apart.

Ceramics bond ionically or covalently, with strong and directional bonds. That makes them hard, chemically stable and able to survive high temperatures, and it also makes them brittle — there is no easy way for atoms to slide, so a crack has nothing to blunt it and runs. The important correction is that brittle does not mean weak. Ceramics are outstanding in compression, which is why concrete, brick and stone hold up buildings; they are unreliable in tension, where a single flaw decides the outcome.

Polymers are long covalent chains held to each other by weak forces, so they are light, flexible, insulating and cheap to form. "Polymer means soft" is wrong, though — Kevlar and UHMWPE are polymers with strength-to-weight ratios beyond steel. Composites then combine classes on purpose so that each phase does what it is good at: stiff brittle fibres carry the load, a tough matrix transfers stress between them and stops cracks from running. The result is not an average of the constituents but better than either, which is precisely why composites exist.