The Vickers hardness testing method consists of indenting the test
material with a diamond indenter, in the form of a right pyramid with a
square base and an angle of 136 degrees between opposite faces
subjected to a load of 1 to 100 kgf. The full load is normally applied
for10 to 15 seconds. The two diagonals of the indentation left in the
surface of the material after removal of the load are measured using a
microscope and their average calculated. The area of the sloping
surface of the indentation is calculated. The Vickers hardness is the
quotient obtained by dividing the kgf load by the square mm area of
indentation.
F= Load in kgf d = Arithmetic mean of the two diagonals, d1 and d2
in mm
HV = Vickers hardness
When the mean diagonal of the indentation has been determined the
Vickers hardness may be calculated from the formula, but is more
convenient to use conversion tables. The Vickers hardness should be
reported like 800 HV/10, which means a Vickers hardness of 800, was
obtained using a 10 kgf force. Several different loading settings give
practically identical hardness numbers on uniform material, which is
much better than the arbitrary changing of scale with the other
hardness testing methods. The advantages of the Vickers hardness test
are that extremely accurate readings can be taken, and just one type of
indenter is used for all types of metals and surface treatments.
Although thoroughly adaptable and very precise for testing the softest
and hardest of materials, under varying loads, the Vickers machine is a
floor standing unit that is more expensive than the Brinell or Rockwell
machines.
Vickers Hardness Number Calculator
There is now a trend towards reporting Vickers 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.
