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
pyramidA pyramid is a building where the outer surfaces are triangular and converge at a point. The base of a pyramid can be trilateral, quadrilateral, or any polygon shape, meaning that a pyramid has at least four faces...
al
diamondIn mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is the second most stable form of carbon, after graphite; however, the conversion rate from diamond to graphite is...
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
microscopeA microscope is an instrument to see objects too tiny for the naked eye. The science of investigating small objects using such an instrument is called microscopy. Microscopic means invisible to the eye unless aided by a microscope.-History:An early microscope was made in 1590 in Middelburg, The...
. 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.
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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
pyramidA pyramid is a building where the outer surfaces are triangular and converge at a point. The base of a pyramid can be trilateral, quadrilateral, or any polygon shape, meaning that a pyramid has at least four faces...
al
diamondIn mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is the second most stable form of carbon, after graphite; however, the conversion rate from diamond to graphite is...
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
microscopeA microscope is an instrument to see objects too tiny for the naked eye. The science of investigating small objects using such an instrument is called microscopy. Microscopic means invisible to the eye unless aided by a microscope.-History:An early microscope was made in 1590 in Middelburg, The...
. 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:
where:
- 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 kg
f·mm
-2. The SI unit,
pascalThe pascal is the SI derived unit of pressure, stress, Young's modulus and tensile strength. It is a measure of force per unit area, defined as one newton per square metre...
s, 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) of the USA in 1939, and is defined by the ASTM D1474 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
micrometreA micrometre or micron is one millionth of a metre,or equivalently one thousandth of a millimetre....
), and the time needed to prepare the sample and apply the indenter.
Sample values
| Material | HK |
|---|
| Gold Gold is a chemical element with the symbol Au and an atomic number of 79. It has been a highly sought-after precious metal for coinage, jewelry, and other arts since the beginning of recorded history. The metal occurs as nuggets or grains in rocks, in veins and in alluvial deposits. Gold is...
foil |
69 |
QuartzQuartz is the second most abundant mineral in the Earth's continental crust . It is made up of a continuous framework of SiO 4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO 2.There are many different varieties of...
|
820 |
Silicon carbideSilicon carbide , also known as carborundum, is a compound of silicon and carbon with a chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
|
2480 |
DiamondIn mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is the second most stable form of carbon, after graphite; however, the conversion rate from diamond to graphite is...
|
8000 |
See also
- Vickers hardness test
The Vickers hardness test was developed in 1924 by Smith and Sandland at Vickers Ltd as an alternative to the Brinell method to measure the hardness of materials. The Vickers test is often easier to use than other hardness tests since the required calculations are independent of the size of the...
- Knoop hardness of ceramics
- Leeb Rebound Hardness Test
The Leeb rebound hardness test is one of the four most used method for testing metal hardness. This portable method is mainly used for testing sufficiently large workpieces .-History:...
External links