The Knoop hardness test is a microhardness test – a test for mechanical

hardness In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion (mechanical), abrasion. In general, different materials differ in their hardn ...

used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. A

pyramid A pyramid (from el, πυραμίς ') is a structure whose outer surfaces are triangular and converge to a single step at the top, making the shape roughly a pyramid in the geometric sense. The base of a pyramid can be trilateral, quadrila ...

diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...

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 A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisibl ...

. 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. The Knoop hardness ''HK'' or ''KHN'' is then given by the formula: Knoop-and Mohs- scale

HK

where: :''L'' = length of indentation along its long axis :''C''_p = 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 kg_f·mm⁻². The SI unit, pascals, are sometimes used instead: 1 kg_f·mm⁻² = 9.80665 MPa. The test was developed by Frederick Knoop and colleagues at the National Bureau of Standards (now

NIST The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sc ...

) of the United States in 1939, and is defined by the

ASTM ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, a ...

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 The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of ...

), 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.

References

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External links

efunda

Hardness tests de:Härte#Härteprüfung nach Knoop

See also

References

External links