Hardness

Hardness is the property of a material that describes its ability to resist plastic (permanent) deformation when a load is applied to it — the harder a material is, the greater the resistance it offers to being permanently deformed. The term is also used loosely to describe a material's resistance to being scratched, abraded, cut, or bent. Materials that are very hard tend to have little ductility and are often, to some extent, brittle as well (section 6.6) — diamond is the most familiar example of an extremely hard material, so hard that scratching it is genuinely difficult, whereas a metal like aluminium has very low hardness by comparison.

Hardness is easy to confuse with two related but distinct mechanical properties:

• Strength describes how large a force is needed to produce deformation in a body at all — the larger this force must be, the stronger the material. Steel, for instance, has high strength, while plasticine clay has very low strength because it deforms readily under even a small applied force.
• Toughness describes a material's ability to resist fracturing (breaking apart) when a force is applied — plasticine clay, despite being weak (easily deformed), is actually relatively tough, since it can be stretched and reshaped extensively by an applied force without ever breaking. …