In 1939, Frederick Knoop and his colleagues at The National Bureau of Standards submitted an alternatively designed indenter based on a rhombohedra-shaped diamond whose long diagonal was approximately seven times as long as the short diagonal. Figure 1 displays the shape of the indenter.
The Knoop indenter is used in the same machine as the Vickers indenter, and the test is conducted in exactly the same manner. The difference is that the Knoop hardness (HK) is based on the measurement of the long diagonal only and calculation of the projected area of the indent rather than the surface area of the indented impression.
The Knoop indentation is measured simply by microscopically measuring the length of the vertical axis. The result is determined by the load applied (based on the selected load for the indentation). One then refers to the appropriate set of hardness values in relation to the axis measurement and the load mass.
The same principle is applied to the Vickers hardness test, with the exception that both the vertical and horizontal axis are measured. Using the same test load as on Vickers, Knoop indentations may be more closely spaced than Vickers indentations, making case-hardness cross traverses easier to apply, particularly for case-hardened components.
The Knoop indenter is a much better choice for hard materials where it may be possible the indenter could cause crack initiation. The Knoop indent makes a shallower indented depth along the long diagonal than the Vickers indentation. Consequently, the Knoop test is also better suited for testing thin coatings. On the negative side, Knoop hardness varies with test load, and results are more difficult to convert to other test scales.