The Knoop Hardness Test, using an elongated pyramidal indenter, is a micro-indentation method suitable for thin plastic sheets, thin metal sheets, brittle materials, or materials where the applied load must never exceed 3.6 kgf.
In the Knoop Hardness Test, a load (typically in the range of 25 ~ 3600 gf) is applied to the surface of the test specimen via a pyramidal diamond indenting tool. The resulting indentation is shaped like the diamond tool, with a long-to-short axis ratio of 7:1, and point angles of 130º and 172º respectively. Knoop indentations are about 2.8 times longer and shallower than the Vickers indentations made at the same load. Optical microscope resolution may limit the application of the Knoop hardness test due to the resulting shallow indentions.
The Knoop hardness number is the ratio of the applied load to the area of the indentation, in accordance with the formula: