hardness

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hardness,

property of matter commonly described as the resistance of a substance to being scratched by another substance. The degree of hardness is relative, different substances being compared with one another. Mohs's scale of hardness (named for Friedrich Mohs), used commonly in mineralogy, lists certain minerals in order of hardness: talc, 1; gypsum, 2; calcite, 3; fluorite, 4; apatite, 5; orthoclase, 6; quartz, 7; topaz, 8; corundum, 9; diamond, 10. The listing indicates merely that gypsum (hardness=2) is harder than—i.e., capable of scratching—talc (hardness=1). The listing does not indicate that gypsum (2) is twice as hard as talc (1). The hardness of many minerals falls between those included in the list. For example, the hardness of barite is 3.3. Hardness may differ with the direction of the scratch made on the substance. Thus the mineral kyanite has a hardness of 5 parallel to the length of its crystals and of 7 at right angles to this direction. There are several other methods based on the resistance to indentation for testing engineering materials. The solid elements have been thus classified: diamond (an allotropic form of carbon) is hardest and listed as 10, with cesium the softest, rated as 0.2, the same degree of hardness as wax (hardness=0.2 at 0°C;). The hardness numeral of the Brinell scale is based upon the indentation produced when pressure is exerted by a sphere on the substance. The value thus obtained has a direct relation to the tensile strength of the substance. The hardness of a material may be modified by the presence of small quantities of another substance, as in metallic alloys, or by impurities in minerals.

hardness

[′härd·nəs]
(chemistry)
The amount of calcium carbonate dissolved in water, usually expressed as parts of calcium carbonate per million parts of water.
(electromagnetism)
That quality which determines the penetrating ability of x-rays; the shorter the wavelength, the harder and more penetrating the rays.
(engineering)
Property of an installation, facility, transmission link, or equipment that will prevent an unacceptable level of damage.
(materials)
Resistance of a metal or other material to indentation, scratching, abrasion, or cutting.

hardness

1. The resistance of wood, rubber, sealant, plastic, or metal to plastic deformation by compression or indentation; in wood, hardness is generally related to density. Common methods of measurement include the Rockwell, Brinell, Scleroscope, and Vickers tests.
2. A property of a paint or varnish film that is a measure of its ability to withstand damage from marring, abrasion, etc.
3. The degree of hardness, applied to water, based on the amount of calcium and magnesium salts in the water, expressed as grains per gallon or parts per million of calcium carbonate.