Introduction
Hardness is the resistance of a material to a harder body pressing into it. It gives a rough guide to strength and wear resistance. There are two common ways to measure it: by scratching and by indentation.
Scratch methods
- Mohs scale: ten minerals ranked from talc (1) to diamond (10), where each scratches the one before it. It is a rough comparison, used for abrasive dental materials.
- Microcharacter and sclerometer: a diamond pyramid scratches the surface. Hardness is read from the depth or width of the groove.
Indentation methods
A body of a set shape is pressed into a polished surface with a set load. It overcomes the elasticity of the material and leaves a permanent dent. Hardness comes from the size of the dent and the load, in kg/mm² or MPa.
Brinell and Vickers values are very close and can be converted:
For very small objects, Vickers testing uses loads under 9.8 N and a microscope. This is called microhardness.