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Knoop hardness test

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 pyramidal 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:

HK={{load(\mbox{kgf})} \over {impression area (\mbox{mm}^2)}}={P \over {C_pL^2}}


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

HK values are typically in the range from 100 to 1000, when specified in the conventional units of kgf·mm-2. SI units (pascals) are sometimes used instead: 1 kgf·mm-2 = 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

Gold foil69
Silicon carbide2480

See also

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Knoop_hardness_test". A list of authors is available in Wikipedia.
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