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Hardness is a property that defines a materials ability to resist permanent deformation under compressive force. There are three measures of hardness due to the complex nature of a material behavior; scratch hardness, indentation hardness, & rebound hardness.
- Indention hardness determine the ability for a material to resist deformation from a focussed compressive force. Vickers and Rockwell hardness tests are commonly used for the shape of the point used. The surface area deformed is measured after pressure is applied to determine the material hardness
- Scratch hardness is a materials resistance to fracture or permanent deformation from a sharp point. The principle is that a harder material will always scratch a softer one. This was measured using the Mohs Scale from 1-10 with 10 being the hardest as Diamond and Tungston Carbide the highest metal at 8.5 - 9. Recently the Vickers Scale is used
- Rebound Hardness or dynamic hardness is a measure of a metals elasticity. A diamond or tungston carbide tipped hammer is dropped from a set height onto the surface of the material and the height bounced/rebounded is measured. The amount of energy absorbed by the test piece affects the height of bounce. A small magnet follows the movement of the hammer through a coil of wire recording a measurable voltage which is proportional to the speed
Material Properties - Metal Properties