From Malcolm Hunter, Leicester, UK
I read your article about biological materials inspiring attempts to produce metals incorporating layers, or gradients in their composition or structure, in order to combine hardness and toughness (29 September, p 40).
Surely there is a long history of layering steel or iron to make weapons such as swords – with a hard, high-carbon edge, but a tougher, less brittle, lower-carbon core. Similarly, “case hardening” weapons and armour, using techniques to infuse steel surfaces with extra carbon, or with nitrogen to create surface nitride layers, also has a long history.
