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Chapter VIII Micro-hardness studies…
to be account for multiple grain structures and any irregularities in the
uniformity of the alloy.
The test has its demerits when it is applied for materials with hardness
greater than 500 kg/mm 2 ; the deformation of the ball and in turn erratic
measurements occur. An ultrasonic hardness tester – a modified version of
the Brinell test has been developed to enable instantaneous and automatic
read out by Szilard [17].
2. Rockwell Test
In this test a spherical or conical diamond indenter with a spherical tip
is used. First a small preliminary load is applied and the dial guage is set to
zero, then the main load is applied, upon removal of the load the depth of the
residual indentation is measured, which is directly read off on the dial guage
quoted as Rockwell Numbers. The advantage of this method lies in the
rapidity with which it can be carried out and also because it does not involve
optical measurements; which makes it very favorable in industries.
The main disadvantage of this test lies with the piling up, sinking in and
the elastic recovery upon removal of the load. A possible solution is due to the
area of the indentation formed under the load is deduced from the acoustic
impedance of the indenter-metal interface [18]. Again the dial reading is
taken for hardness number, but it is based on the area of the indentation not
on its recovered depth.
3. Vickers Test
In the Vickers test, Smith and Sandland [19] have used a diamond
pyramidal indenter. The diamond is the hardest material; it is applicable to
determine the hardness of extremely hard materials. This test has the
advantage of geometrical similarity of impression at different loads.
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