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[[File:Knoop_hardness_test_indenter.svg|thumb|300px|Angles of a Knoop hardness test indenter]]
[[File:Knoop_hardness_test_indenter.svg|thumb|300px|Angles of a Knoop hardness test indenter]]
The '''Knoop hardness test''' {{IPAc-en|k|ə|ˈ|n|uː|p}} 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 (often 100g) load, for a specified dwell time, and the resulting indentation is measured using a [[microscope]]. The geometry of this indenter is an extended pyramid with the length to width ratio being 7:1 and respective face angles are 172 degrees for the long edge and 130 degrees for the short edge. The depth of the indentation can be approximated as 1/30 of the long dimension.<ref>[http://www.gordonengland.co.uk/hardness/microhardness.htm "Microhardness Test", Surface Engineering Forum]</ref> The Knoop hardness ''HK'' or ''KHN'' is then given by the formula:
The '''Knoop hardness test''' {{IPAc-en|k|ə|ˈ|n|uː|p}} 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 (often 100&nbsp;g) load, for a specified dwell time, and the resulting indentation is measured using a [[microscope]]. The geometry of this indenter is an extended pyramid with the length to width ratio being 7:1 and respective face angles are 172 degrees for the long edge and 130 degrees for the short edge. The depth of the indentation can be approximated as 1/30 of the long dimension.<ref>[http://www.gordonengland.co.uk/hardness/microhardness.htm "Microhardness Test", Surface Engineering Forum]</ref> The Knoop hardness ''HK'' or '''''KHN''''' is then given by the formula:
[[File:Knoop-and Mohs- scale.svg|thumb|250px|Comparison between the Mohs and the Knoop scales.]]
[[File:Knoop-and Mohs- scale.svg|thumb|250px|Comparison between the Mohs and the Knoop scales]]
{| class="wikitable"
{| class="wikitable"
|+ Sample values
|+ Sample values
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|}
|}


:<math>HK={{\textrm{load}(\mbox{kgf})} \over {\textrm{impression\ area} (\mbox{mm}^2)}}={P \over {C_pL^2}}</math>
: <math>HK={{\text{load}(\mathrm{kgf})} \over {\text{impression area} (\mathrm{mm}^2)}}={P \over {C_\text{p}L^2}}</math>

where:
where:
:''L'' = length of indentation along its long axis
: ''L'' is the length of indentation along its long axis
:''C''<sub>p</sub> = correction factor related to the shape of the indenter, ideally 0.070279
: ''C''<sub>p</sub> is the correction factor related to the shape of the indenter, ideally 0.070279
:''P'' = load
: ''P'' is the load


HK values are typically in the range from 100 to 1000, when specified in the conventional units of kg<sub>f</sub>·mm<sup>−2</sup>. The SI unit, [[pascal (unit)|pascal]]s, are sometimes used instead: 1&nbsp;kg<sub>f</sub>·mm<sup>−2</sup> = 9.80665 MPa.
HK values are typically in the range from 100 to 1000, when specified in the conventional units of kg<sub>f</sub>⋅mm<sup>−2</sup>. The SI unit, [[pascal (unit)|pascal]], is sometimes used instead: 1&nbsp;kg<sub>f</sub>⋅mm<sup>−2</sup> = 9.80665&nbsp;MPa.


The test was developed by [[Frederick Knoop]]<ref>{{cite journal|author=F. Knoop, C.G. Peters and W.B. Emerson|title=A Sensitive Pyramidal-Diamond Tool for Indentation Measurements|journal=Journal of Research of the National Bureau of Standards|volume=23|issue=1|year=1939| pages=39–61 (Research Paper RP1220)|url=https://archive.org/details/jresv23n1p39|doi=10.6028/jres.023.022|doi-access=free}}</ref> and colleagues at the National Bureau of Standards (now [[NIST]]) of the United States in 1939, and is defined by the [[ASTM]] E384 standard.
The test was developed by [[Frederick Knoop]]<ref>{{cite journal|author=F. Knoop, C.G. Peters and W.B. Emerson|title=A Sensitive Pyramidal-Diamond Tool for Indentation Measurements|journal=Journal of Research of the National Bureau of Standards|volume=23|issue=1|year=1939| pages=39–61 (Research Paper RP1220)|url=https://archive.org/details/jresv23n1p39|doi=10.6028/jres.023.022|doi-access=free}}</ref> and colleagues at the National Bureau of Standards (now [[NIST]]) of the United States in 1939, and is defined by the [[ASTM]] E384 standard.
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{{Div col}}
{{Div col}}
* [[Vickers hardness test]]
* [[Vickers hardness test]]
* [[Ceramography#Microindention hardness and toughness|Knoop hardness of ceramics]]
* [[Ceramography#Microindentation hardness and toughness|Knoop hardness of ceramics]]
* [[Leeb Rebound Hardness Test]]
* [[Leeb Rebound Hardness Test]]
* [[Meyer hardness test]]
* [[Meyer hardness test]]

Latest revision as of 08:54, 12 October 2024

Angles of a Knoop hardness test indenter

The Knoop hardness test /kəˈnuːp/ 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 pyramidal diamond point is pressed into the polished surface of the test material with a known (often 100 g) load, for a specified dwell time, and the resulting indentation is measured using a microscope. The geometry of this indenter is an extended pyramid with the length to width ratio being 7:1 and respective face angles are 172 degrees for the long edge and 130 degrees for the short edge. The depth of the indentation can be approximated as 1/30 of the long dimension.[1] The Knoop hardness HK or KHN is then given by the formula:

Comparison between the Mohs and the Knoop scales
Sample values
Material HK
Dentin 68
Gold foil 69
Tooth enamel 343
Quartz 820
Silicon carbide 2480
Diamond 7000

where:

L is the length of indentation along its long axis
Cp is the correction factor related to the shape of the indenter, ideally 0.070279
P is the load

HK values are typically in the range from 100 to 1000, when specified in the conventional units of kgf⋅mm−2. The SI unit, pascal, is sometimes used instead: 1 kgf⋅mm−2 = 9.80665 MPa.

The test was developed by Frederick Knoop[2] and colleagues at the National Bureau of Standards (now NIST) of the United States in 1939, and is defined by the ASTM E384 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.

Variables such as load, temperature, and environment, may affect this procedure, which have been examined in detail.[3]

See also

[edit]

References

[edit]
  1. ^ "Microhardness Test", Surface Engineering Forum
  2. ^ F. Knoop, C.G. Peters and W.B. Emerson (1939). "A Sensitive Pyramidal-Diamond Tool for Indentation Measurements". Journal of Research of the National Bureau of Standards. 23 (1): 39–61 (Research Paper RP1220). doi:10.6028/jres.023.022.
  3. ^ Czemuska, J. T. (1984). Proc. Br. Ceram. Soc. 34: 145–156. {{cite journal}}: Missing or empty |title= (help)
[edit]
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