Knoop hardness test
The Knoop hardness test (English pronunciation: /kǝˈnuːp/) 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 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:
- 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 gf·mm−2. The SI unit, pascals, are sometimes used instead: 1 kgf·mm−2 = 9.80665 MPa.
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.
See also 
- "Microhardness Test", Surface Engineering Forum
- F. Knoop, C.G. Peters and W.B. Emerson, “A Sensitive Pyramidal-Diamond Tool for Indentation Measurements,” Journal of Research of the National Bureau of Standards, V23 #1, July 1939, Research Paper RP1220, p 39–61.