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Hardness: the resistance of a smooth surface to scratching
or abrassion. A hard material may still be broken or cracked. For example,
if you take a hammer to a diamond or sapphire watch crystal, it will break
or shatter. Therefore, hardness is the measure of the material's ability
to resist scratches or abrassions.
Moh's Hardness
Vicker's Hardness
Knoop's Hardness
rough measure of the resistance of a smooth surface
to scratching or abrasion, expressed in terms of a scale devised (1812)
by the German mineralogist Friedrich Mohs. The Mohs hardness of a mineral
is determined by observing whether its surface is scratched by a substance
of known or defined hardness. To give numerical values to this physical
property, minerals are ranked along the Mohs scale, which is composed
of 10 minerals that have been given arbitrary hardness values. The minerals
contained in the scale are shown in the Table
below; also shown are other materials that approximate the hardness of
some of the minerals. As is indicated by the ranking in the scale, if
a mineral is scratched by orthoclase but not by apatite, its Mohs hardness
is between 5 and 6. In the determination procedure it is necessary to
be certain that a scratch is actually made and not just a "chalk" mark
that will rub off. If the species being tested is fine-grained, friable,
or pulverulent, the test may only loosen grains without testing individual
mineral surfaces; thus certain textures or aggregate forms may hinder
or prevent a true hardness determination. For this reason the Mohs test,
while greatly facilitating the identification of minerals in the field,
is not suitable for accurately gauging the hardness of industrial materials
such as steel or ceramics. (For these materials a more precise measure
is to be found in the Vickers hardness
or Knoop hardness) Another disadvantage
of the Mohs scale is that it is not linear; that is, each increment of
one in the scale does not indicate a proportional increase in hardness.
For instance, the progression from calcite to fluorite (from 3 to 4 on
the Mohs scale) reflects an increase in hardness of approximately 25 percent;
the progression from corundum to diamond, on the other hand (9 to 10 on
the Mohs scale), reflects a hardness increase of more than 300 percent.
| Table: Mohs Hardness
Scale and Observations on Hardness of Some Additional Materials |
| mineral | Mohs hardness | other materials | observations on the minerals |
| Talc | 1 | | very easily scratched by the fingernail; has a greasy feel |
| Gypsum | 2 | ~2.2 fingernail | can be scratched by the fingernail |
| Calcite | 3 | ~3.2 copper penny | very easily scratched with a knife and just scratched with a copper coin |
| Fluorite | 4 | | very easily scratched with a knife but not as easily as calcite |
| Apatite | 5 | ~5.1 pocketknife | scratched with a knife with difficulty |
| | | ~5.5 glass plate | |
| Orthoclase | 6 | ~6.5 steel needle | cannot be scratched with a knife, but scratches glass with difficulty |
| Quartz | 7 | ~7.0 streak plate | scratches glass easily |
| Topaz | 8 | | scratches glass very easily |
| Corundum | 9 | | cuts glass |
| Diamond | 10 | | used as a glass cutter |
| Source: Modified from C. Klein, Minerals and Rocks: Exercises in Crystallography, |
| Mineralogy, and Hand Specimen Petrology. Copyright l989 John Wiley & Sons. |
| Reprinted by permission of John Wiley & Sons, Inc. |
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a measure of the hardness of a material, calculated
from the size of an impression produced under load by a pyramid-shaped
diamond indenter. Devised in the 1920s by engineers at Vickers, Ltd.,
in the United Kingdom, the diamond pyramid hardness test, as it also became
known, permitted the establishment of a continuous scale of comparable
numbers that accurately reflected the wide range of hardnesses found in
steels.
The indenter employed in the Vickers test is a square-based
pyramid whose opposite sides meet at the apex at an angle of 136°.
The diamond is pressed into the surface of the material at loads ranging
up to approximately 120 kilograms-force, and the size of the impression
(usually no more than 0.5 mm) is measured with the aid of a calibrated
microscope. The Vickers number (HV) is calculated using the following
formula:
HV = 1.854(F/D2),
with F being the applied load (measured in kilograms-force)
and D2 the area of the indentation (measured in square millimetres).
The applied load is usually specified when HV is cited.
The Vickers test is reliable for measuring the hardness
of metals, and it is also used on ceramic materials.
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a measure of the hardness of a material, calculated
by measuring the indentation produced by a diamond tip that is pressed
onto the surface of a sample. The test was devised in 1939 by F. Knoop
and colleagues at the National Bureau of Standards in the United States.
By using lower indentation pressures than the Vickers hardness test, which
had been designed for measuring metals, the Knoop test allowed the hardness
testing of brittle materials such as glass and ceramics.
The diamond indenter employed in the Knoop test is in
the shape of an elongated four-sided pyramid, with the angle between two
of the opposite faces being approximately 170° and the angle between
the other two being 130°. Pressed into the material under loads that
are often less than one kilogram-force, the indenter leaves a four-sided
impression about 0.01 to 0.1 mm in size. The length of the impression
is approximately seven times the width, and the depth is 1/30 the length.
Given such dimensions, the area of the impression under load can be calculated
after measuring only the length of the longest side with the aid of a
calibrated microscope. The final Knoop hardness (HK) is derived from the
following formula:
HK = 14.229(F/D2),
with F being the applied load (measured in kilograms-force)
and D2 the area of the indentation (measured in square millimetres).
Knoop hardness numbers are often cited in conjunction with specific load
values.
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Source: http://www.britannica.com
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