BUEHLER®

Knoop Hardness Table
Definition
Knoop Hardness: The value calculated according to the following formula from the test load at the time when the test surface
is indented and the projected area obtained from the length of longer diagonal of the indentation by use of diamond indenter
in the form of a right pyramid with a rhomboid base having angles at the vertex between two opposite edges of 172° 30 ‘
and 130° :
where HK : Knoop hardness
F : test load (N)
A : projected area of indentation ( mm 2
)
c : indenter constant (the constant to obtain A from d, it is taken as 0.07028)
d : length of longer diagonal of indentation (mm)
The numerical value of HK is not attached with unit.
HK = 0.102 F
A =0.102 F =1.451 F
2
cd d 2
Note: In the case where the unit of test load F is kgf, Knoop hardness shall be calculated according to the following formula:
HK = F
cd 2
F
d 2
= 14.23 x . . . . f: kgf, d: mm)
= 14.23 x 10 F
3
x . . . . f: kgf, d: µm)
d 2
F
d 2
= 14.23 x 10 6 x . . . . f: kgf, d: µm)