Knoop hardness test 1887779 207127478 2008-04-21T15:06:02Z 195.195.235.11 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 [[pyramid]]al [[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: :<math>HK={{load(\mbox{kgf})} \over {impression area (\mbox{mm}^2)}}={P \over {C_pL^2}}</math> where: :''L'' = length of indentation along its long axis :''C''<sub>p</sub> = correction factor related to the shape of the indenter, ideally 0.070279 :''P'' = load :''D'' = dynamics HK values are typically in the range from 100 to 1000, when specified in the conventional units of [[kgf]]·mm<sup>-2</sup>. SI units ([[pascal (unit)|pascal]]s) are sometimes used instead: 1 kgf·mm<sup>-2</sup> = 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 == {| class="wikitable" !Material!!HK |- |[[Gold]] foil||69 |- |[[Quartz]]||820 |- |[[Silicon carbide]]||2480 |- |[[Diamond]]||8000 |} == See also == * [[Vickers hardness test]], another microhardness test * [[Ceramography#Microindention_Hardness_and_Toughness | Knoop hardness of ceramics]] == External links == * [http://www.efunda.com/units/hardness/convert_hardness.cfm?cat=Steel&HD=HK efunda] * [http://www.lib.umich.edu/dentlib/Dental_tables/Knoophard.html Dental hardness tables] [[Category:Hardness tests]] [[de:Härte#H.C3.A4rtepr.C3.BCfung nach Knoop]] [[fr:Échelle de Knoop]] [[pt:Dureza Knoop]]