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: In materials science, hardness is the characteristic of a solid material expressing its resistance to permanent deformation. ... Geometric shape created by connecting a polygonal base to an apex A pyramid is a geometric shape formed by connecting a polygonal base and a point, called the apex, by triangular faces. ... A scattering of round-brilliant cut diamonds shows off the many reflecting facets. ... A microscope (Greek: micron = small and scopos = aim) is an instrument for viewing objects that are too small to be seen by the naked or unaided eye. ...

where:

L = length of indentation along its long axis

C_p = correction factor related to the shape of the indenter, usually about 0.07

P = load

HK values are typically in the range from 100 to 1000, when specified in the conventional units of kg·mm^-2. SI units (pascals) are sometimes used instead. The pascal (symbol Pa) is the SI unit of pressure. ...

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. As a non-regulatory agency of the United States Department of Commerce’s Technology Administration, the National Institute of Standards (NIST) develops and promotes measurement, standards, and technology to enhance productivity, facilitate trade, and improve the quality of life. ... 1939 was a common year starting on Sunday (link will take you to calendar). ...

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 micrometres), and the time needed to prepare the sample and apply the indenter.

Sample values

General Name, Symbol, Number Gold, Au, 79 Chemical series transition metals Group, Period, Block 11 (IB), 6, d Density, Hardness 19. ... For other uses of this word, see Quartz (disambiguation). ... Silicon carbide (SiC) or moissanite is a ceramic compound of silicon and carbon. ... A scattering of round-brilliant cut diamonds shows off the many reflecting facets. ...

See also

• Vickers hardness test, another microhardness test

External links

• efunda (http://www.efunda.com/units/hardness/convert_hardness.cfm?cat=Steel&HD=HK)
• Dental hardness tables (http://www.lib.umich.edu/dentlib/Dental_tables/Knoophard.html)