Knoop Hardness
In the Knoop hardness test [5] a diamond indenter, in the form of a rhombic-based pyramid with angles a (172,5º) and b (130º) between the opposite edges at the vertex, is pressed into the surface of the test piece using a prescribed force F (N) and, after the force has been removed, the long diagonal d (mm) of the resulting indentation is measured. The time for the initial application of the force should not exceed 10 s, and the test force is maintained for 10 s to 15 s. The Knoop hardness value is proportional to the test force divided by the projected area of the indentation:
HK = Constant × Test force / Projected area of indentation
| where: | indenter constant |  |
(0,070 28 for an ideal indenter) |
| Hardness symbol | Test force F / N |
| HK 0,01 | 0,098 07 |
| HK 0,02 | 0,196 1 |
| HK 0,025 | 0,245 2 |
| HK 0,05 | 0,490 3 |
| HK 0,1 | 0,980 7 |
| HK 0,2 | 1,961 |
| HK 0,3 | 2,942 |
| HK 0,5 | 4,903 |
| HK 1 | 9,807 |
| HK 2 | 19,614 |
Last Updated: 25 Mar 2010
Created: 18 Jun 2007