Microhardness Test
The term microhardness test 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.
Knoop Hardness Number Calculator
Click here for Vickers Hardness Number Calculator
Vickers Pyramid Diamond Indenter Indentation
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.
To convert HV to MPa multiply by 9.807
To convert HV to GPa multiply by 0.009807
Or use conversion calculator
Links To:
Hardness Testing
Rockwell Hardness Test
Rockwell Superficial Hardness Test
Brinell Hardness Test
Vickers Hardness Test
Microhardness Test
Mohs Hardness Test
Scleroscope and other hardness testing methods
Hardness Conversion Tables and Charts:
Hardness Conversion Table(colour version - may take time to load)
Hardness Conversion Table(non-colour version)
Hardness Conversion Chart (1)
Hardness Conversion Chart (2)
Chart of Brinell, Vickers and Ultimate Tensile Strength Equivalents (1)
Chart of Brinell, Vickers andUltimate Tensile Strength Equivalents (2)
Hardness Conversion Table related to Rockwell C Hardness Scale (hard materials) (colour)
Hardness Conversion Table related to Rockwell C Hardness Scale (hard materials) (non-colour)
Hardness Conversion Chart related to Rockwell C Hardness Scales (hard materials)
Estimated Hardness Equivalent Chart related to Rockwell C and Vickers (hard materials)
Hardness Conversion Table related to Rockwell B Hardness Scale (soft metals) (colour)
Hardness Conversion Table related to Rockwell B Hardness Scale (soft metals) (non-colour)
Hardness Conversion Chart related to Rockwell B Hardness Scale (soft metals)
Table of Minimum Test Piece Thickness for Rockwell Hardness Testing using Ball Indenters
Table of Minimum Test Piece Thickness for Rockwell Hardness Testing using Diamond Indenters
HV, MPa and GPa Conversion Calculator
Telephone: +44 (0)1252 405186
Email: tsc@gordonengland.co.uk
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