Gordon England Surface Engineering Forum

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
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

Force (kgf) Enter value
Long diagonal length (mm) Enter value
KHN Result



Click here for Vickers Hardness Number Calculator
Vickers Pyramid Diamond Indentation
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)
Vickers Diamond Pyramid Hardness Number Formula

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:


Arc spray 13% chromium steel photomicrograph showing microhardness indentations Microhardness indentations in tungsten carbide coating


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

Microhardness indentation in steel coating






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

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 and Ultimate 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

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