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Chapter VIII Micro-hardness studies…
swung over the specimen to measure the square indentation to a tolerance of
plus or minus 1/1000 of a millimeter. The measurements taken across the
diagonals to determine the area and are averaged. The correct Vickers
hardness is designated as the number followed Hv (Hardness Vickers). The
Vickers hardness test provides extremely accurate readings and just one type
of indenter is used for all types of metals and surfaces. This method is
adaptable and very precise for testing the softness and hardness of all the
materials, under the varying loads. The Vickers machine is a floor standing
unit that is rather more expensive than the Brinell or Rockwell machines. The
present author has used Vaiseshikha Vickers micro-hardness tester.
The geometry of the indentation made with Vickers pyramid is
independent of the depth and hence the hardness should be independent of
the load. This is not the case as in the Brinell test.
4. Knoop Test
Knoop et al [21] have modified the Vickers test. In this test, a diamond
indenter is used but its shape is such that it forms an elongated pyramidal
impression, with the angles between the long and short edges being 172 0 30'
and 130 0 respectively. The shape of the impression is that of a parallelogram
for which one diagonal is about seven times the length of the other. The
knoop test is very useful in studying the effect of crystal orientation on
hardness.
The Knoop hardness (Hk) is obtained from the following formula:
Hk = 14.229 (P/d 2 ) (8.4)
Where, P is the applied load (measured in N) and d 2 is the area of the
indentation (measured in square meter). The Knoop hardness numbers are
often cited in conjunction with specific load variation. Aside from a minor
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