The Knoop Hardness Test Microhardness

The KHT was devised in 1939 by F. Knoop. A Knoop diamond indenter may be used in a Tukon Hardness Tester (or in a Buehler Instrument Model 1600-6400). The indentation produced by the KHT resembles that made by a pyramid-shaped diamond indenter (developed by the U.S. National Bureau of Standards). The apical angles are 130° and 172°. Thus, a narrow, rhombus-shaped indenter is used to produce a rhomboid-shaped impression, as seen in Fig. 1.61. 

The measuring apparatus is preset to apply a 25 g load. The duration of the contact between the indenter and the specimen should be 10-15 s. The length of the long diagonal of the impression is measured with a high-powered microscope. This procedure is repeated until at least five impressions have been made at widely spaced locations. The KHN is then calculated. The test loads are in the range of 10-1000 g. The samples are normally mounted and polished. One scale covers the entire hardness range. 

Projected area =2xthe triangles defining the rhombohedralshape 

Po is related to residual surface stresses in the test specimen. Analyses indicate that the existing PSR model does not provide a satisfactory description for the observed ISE in ceramics and warrants further modification. 

The Knoop hardness for several ceramics, as a function of load, is indicated in Fig. 1.65. The pattern of the lines in Fig. 1.65 is similar to those in Figs. 1.56 and 1.59 in the VHT. The plateaus are not well defined, though there is such a tendency. The load variation with d is illustrated for all the materials shown in Figs. 1.65 and 1.66. Experience with a wide range of ceramics has proven that Knoop indentations are far less likely to crack than Vickers indentations. This explains the development of KHTs as an alternative to VHTs. Indeed, for a wide range of ceramics and other brittle materials, this is justified. 

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