Hardness refers to various properties of matter in the solid phase that give it high resistance to various kinds of shape change when force is applied. Hard matter is contrasted with soft matter.
Macroscopic hardness is generally characterized by strong intermolecular bonds. However, the behavior of solid materials under force is complex, resulting in several different scientific definitions of what might be called "hardness" in everyday usage.
In materials science, there are three principal operational definitions of hardness:
- Scratch hardness: Resistance to fracture or plastic (permanent) deformation due to friction from a sharp object
- Indentation hardness: Resistance to plastic (permanent) deformation due to a constant load from a sharp object
- Rebound hardness: Height of the bounce of an object dropped on the material, related to elasticity.
In physics, hardness encompasses:
- Elasticity, plasticity, viscosity, and viscoelasticity
- Strength and strain
- Brittleness/ductility and toughness
The equation based definition of hardness is the pressure applied over the projected contact area between the indenter and the material being tested. As a result hardness values are typically reported in units of pressure, although this is only a "true" pressure if the indenter and surface interface is perfectly flat.
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