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Chapter 17: D. Testing Methods 195
Operating
position
h
136°
136°
d
Figure 17.16 The Vickers hardness test.
area normal to the force, and is therefore not a pressure. This test, the same as the
other tests, uses an indenter to test the metal but in the shape of diamond with 136
degrees between opposite faces (see Figure 17.16).
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.
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
(Surface Engineering Forum 2008) (CALCE 2001).
Part III.D.4
Microhardness Test
Microhardness testing of metals, ceramics, and composites is useful for a variety
of applications where other test methods are not useful, such as testing very thin
materials like foils, measuring individual microstructures within a larger matrix,
or measuring the hardness gradients of a part along the cross section. Microhardness
testing gives an allowable range of loads for testing with a diamond indenter;
the resulting indentation is measured and converted to a hardness value (Surface
Engineering Forum 2008) (CALCE 2001).
The term microhardness test usually refers to static indentations made with
loads not exceeding one 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