# Knoop hardness test

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 (often 100g) load, 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.[1] The Knoop hardness HK or KHN is then given by the formula:

$HK={{\textrm{load}(\mbox{kgf})} \over {\textrm{impression\ area} (\mbox{mm}^2)}}={P \over {C_pL^2}}$

where:

L = length of indentation along its long axis
Cp = correction factor related to the shape of the indenter, ideally 0.070279

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 test was developed by Frederick Knoop[2] and colleagues at the National Bureau of Standards (now NIST) of the USA in 1939, and is defined by the ASTM 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), and the time needed to prepare the sample and apply the indenter.

Variables such as load, temperature, and environment, may effect this procedure, which have been examined in detail.[3]

Comparison between the Mohs and the Knoop scales.
Sample values
Material HK
Dentin 68
Gold foil 69
Tooth enamel 343
Quartz 820
Silicon carbide 2480
Diamond 7000