Knoop hardness test
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The '''Knoop hardness test''' is a microhardness test - a test for mechanical [[hardness]] used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. A [[pyramid]]al [[diamond]] point is pressed into the polished surface of the test material with a known force, for a specified dwell time, and the resulting indentation is measured using a [[microscope]]. The Knoop hardness ''HK'' or ''KHN'' is then given by the formula:
:<math>HK={{load(\mbox{kgf})} \over {impression area (\mbox{mm}^2)}}={P \over {C_pL^2}}</math>
where:
:''L'' = length of indentation along its long axis
:''C''<sub>p</sub> = correction factor related to the shape of the indenter, ideally 0.070279
:''P'' = load
:''D'' = dynamics
HK values are typically in the range from 100 to 1000, when specified in the conventional units of [[kgf]]·mm<sup>-2</sup>. SI units ([[pascal (unit)|pascal]]s) are sometimes used instead: 1 kgf·mm<sup>-2</sup> = 9.80665 MPa.
The test was developed by F. Knoop and colleagues at the National Bureau of Standards (now [[NIST]]) of the [[USA]] in [[1939]], and is defined by the ASTM D-1474 standard.
The advantages of the test are that only a very small sample of material is required, and that it is valid for a wide range of test forces. The main disadvantages are the difficulty of using a microscope to measure the indentation (with an accuracy of 0.5 [[micrometre]]), and the time needed to prepare the sample and apply the indenter.
== Sample values ==
{| class="wikitable"
!Material!!HK
|-
|[[Gold]] foil||69
|-
|[[Quartz]]||820
|-
|[[Silicon carbide]]||2480
|-
|[[Diamond]]||8000
|}
== See also ==
* [[Vickers hardness test]], another microhardness test
* [[Ceramography#Microindention_Hardness_and_Toughness | Knoop hardness of ceramics]]
== External links ==
* [http://www.efunda.com/units/hardness/convert_hardness.cfm?cat=Steel&HD=HK efunda]
* [http://www.lib.umich.edu/dentlib/Dental_tables/Knoophard.html Dental hardness tables]
[[Category:Hardness tests]]
[[de:Härte#H.C3.A4rtepr.C3.BCfung nach Knoop]]
[[fr:Échelle de Knoop]]
[[pt:Dureza Knoop]]