Knoop hardness test
The Knoop hardness test /kəˈnuːp/ 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. The Knoop hardness HK or KHN is then given by the formula:
where:
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 and colleagues at the National Bureau of Standards (now NIST) of the USA in 1939, and is defined by the ASTM E384 standard.
Indentation hardness
Indentation hardness tests are used in mechanical engineering to determine the hardness of a material to deformation. Several such tests exist, wherein the examined material is indented until an impression is formed; these tests can be performed on a macroscopic or microscopic scale.
When testing metals, indentation hardness correlates linearly with tensile strength. This relation permits economically important nondestructive testing of bulk metal deliveries with lightweight, even portable equipment, such as hand-held Rockwell hardness testers.
Material hardness
As of the direction of materials science continues towards studying the basis of properties on smaller and smaller scales, different techniques are used to quantify material characteristics and tendencies. Measuring mechanical properties for materials on smaller scales, like thin films, can not be done using conventional uniaxial tensile testing. As a result, techniques testing material "hardness" by indenting a material with an impression have been developed to determine such properties.
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