The Knoop hardness number KHN is the ratio of the load applied to the indenter, P (kgf) to the unrecovered projected area A (mm

Where:

F = applied load in kgf

A = the unrecovered projected area of the indentation in mm

L = measured length of long diagonal of indentation in mm

C = 0.07028 = Constant of indenter relating projected area of the indentation to the square of the length of the long diagonal.

The Knoop indenter is a diamond ground to pyramidal form that produces a diamond shaped indentation having approximate ratio between long and short diagonals of 7:1. The depth of indentation is about 1/30 of its length. When measuring the Knoop hardness, only the longest diagonal of the indentation is measured and this is used in the above formula with the load used to calculate KHN. Tables of these values are usually a more convenient way to look-up KHN values from the measurements.

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The Vickers Diamond Pyramid harness number is the applied load (kgf) divided by the surface area of the indentation (mm

Where:

HV = Vickers hardness

The Vickers Diamond Pyramid indenter is ground in the form of a squared pyramid with an angle of 136

- Vickers indenter penetrates about twice as deep as Knoop indenter
- Vickers indentation diagonal about 1/3 of the length of Knoop major diagonal
- Vickers test is less sensitive to surface conditions than Knoop test
- Vickers test is more sensitive to measurement errors than knoop test
- Vickers test best for small rounded areas
- Knoop test best for small elongated areas
- Knoop test good for very hard brittle materials and very thin sections

There is now a trend towards reporting Vickers and Knoop hardness in SI units (MPa or GPa) particularly in academic papers. Unfortunately, this can cause confusion. Vickers hardness (e.g. HV/30) value should normally be expressed as a number only (without the units kgf/mm

To convert HV to MPa multiply by 9.807

To convert HV to GPa multiply by 0.009807

Or use conversion calculator

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