The Knoop hardness test uses a rhombohedral-shaped diamond indenter. The long diagonal is seven times (7.114 actually) as long as the short diagonal. With the indenter shape, elastic recovery can be held to a minimum. The Knoop hardness test is conducted in the same mannar, and the the same tester as the Vickers hardness test. However, only the long diagonal is measured. This, of course, saves a little time.
The Knoop hardness is calculated from the following equation:
HK = 14229L / d2
where the load L is in gf and the long diagonal d is in µm. The Knoop hardness is expressed in the same manner as the Vickers hardness; i.e., 375 HK0.3 means that a 300 gf load produced a Knoop hardness of 375. (The kgf/mm2 unit information is no longer reported). Again, the symbol HK was adopted in the early 1960’s while other terms; e.g., HKN or KHN, are obsolete and should not be used.
Knoop hardness tests, like Microindentation Vickers, are typically performed at test forces from 10 - 1000g. The measured area is used in a formula that includes applied force to determine a hardness value. Tables or automatic electronic or imaging measurements are a more common and convenient way to generate Knoop hardness test numbers.
Aside from minor savings of time, one chief merit of the Knoop hardness test is the ability to test thin layers more easily. Around any indent in a material there will be an area of plastic deformation. If this deformed area coincides with an interface (such as the edge of the material) or a similar deformed area from another indent, then the resultant hardness reading will be incorrect. This deformed area is linked to the indent size, and to ensure correct tests it is usually recommended that the indents should be at least
2.5x the diagonal of the indent apart. The elongated form of the Knoop indent means that they can be placed much closer together than a Vickers hardness test using the same load. Furthermore, if the hardness varies strongly with the depth, the Vickers indent can be distorted by this change; that is, the diagonal parallel to the hardness change will be affected by the hardness gradient. Care should also be taken that the thickness of the material is sufficient for testing. For Vickers hardness tests, the thickness should also be at least 2.5x the diagonal measurement. However, because the Knoop indent is shallower than the Vickers at the same load, significantly thinner specimens can be tested.
The down side of the Knoop indent is that the three dimensional indent will change with test load and, consequently, HK varies with load. At higher loads this variation is not substantial. Conversion of HK values to other test scales can only be done reliably for HK values performed at the standard load, generally 500gf, used to develop the correlations. All hardness scale conversions are based on empirical data. Conversions are not precise but are estimates.
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