The term microhardness testing usually refers to static indentations
made with loads not exceeding 1 kgf. The indenter is either the Vickers
diamond pyramid or the Knoop elongated diamond pyramid. The procedure
for testing is very similar to that of the standard Vickers hardness
test, except that it is done on a microscopic scale with higher
precision instruments. The surface being tested generally requires a
metallographic finish; the smaller the load used, the higher the
surface finish required. Precision microscopes are used to measure the
indentations; these usually have a magnification of around X500 and
measure to an accuracy of +0.5 micrometres. Also with the same
observer differences of +0.2 micrometres can usually be
resolved. It should, however, be added that considerable care and
experience are necessary to obtain this accuracy.
Knoop Hardness Indenter Indentation
The Knoop hardness number KHN is the ratio of the load applied to the
indenter, P (kgf) to the unrecovered projected area A (mm2)
KHN = F/A = P/CL2
Where:
F = applied load in kgf
A = the unrecovered projected area of the indentation in mm2
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.
The Vickers Diamond Pyramid harness number is the applied load (kgf)
divided by the surface area of the indentation (mm2)
Where: F= Load in kgf d = Arithmetic mean of the two diagonals, d1 and d2
in mm
HV = Vickers hardness
The Vickers Diamond Pyramid indenter is ground in the form of a squared
pyramid with an angle of 136o between faces. The depth of
indentation is about 1/7 of the diagonal length. When calculating the
Vickers Diamond Pyramid hardness number, both diagonals of the
indentation are measured and the mean of these values is used in the
above formula with the load used to determine the value of HV. Tables
of these values are usually a more convenient way to look-up HV values
from the measurements.
Knoop vs. Vickers
Comparing the indentations made with Knoop and Vickers Diamond Pyramid
indenters for a given load and test material:
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/mm2).
Rigorous application of SI is a problem. Most Vickers hardness testing
machines use forces of 1, 2, 5, 10, 30, 50 and 100 kgf and tables for
calculating HV. SI would involve reporting force in newtons (compare
700 HV/30 to HV/294 N = 6.87 GPa) which is practically meaningless and
messy to engineers and technicians. To convert a Vickers hardness
number the force applied needs converting from kgf to newtons and the
area needs converting form mm2 to m2 to give
results in pascals using the formula above.
