## 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

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

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:
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

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

Custom Search

Telephone: +44 (0)1252 405186

Thermal Spray Processes:

Combustion Wire Thermal Spray Process

Combustion Powder Thermal Spray Process

Arc Wire Thermal Spray Process

Plasma Thermal Spray Process

HVOF Thermal Spray Process

Detonation Thermal Spray Process

Plasma Flame Theory

Cold Spray Coating Process

Wear and Use of Thermal Spray Coatings

Corrosion and Use of Thermal Spray Coatings

Glossary of Thermal Spray and Surface Engineering Terms

Image Directory for Thermal Spray Coatings

Plasma Gas Flow Information

Plasma Gas Flow Correction Calculator

Contact Form

Links to other interesting sites related to thermal spray and surface engineering

Periodic Table of the Elements

SI Units

Calculators for Conversion between Units of Measurement

Hardness Testing

Surface Engineering Message Board Archive

Surface Engineering Message Board Archive Index

Search This Site

Photography Gallery2

Photography Gallery3