Hardness test: A test of hardness usually by determining
the resistance of the material to indentation under standard conditions.
The following are established methods of hardness testing :-
Brinell hardness test: A test to determine hardness by pressing
a hard steel ball of known diameter under a standard load into the surface
of the material and measuring the diameter of the indentation produced.
The Brinell hardness number, BHN = Load in kg / Spherical area of impression
in sq. mm.
Rockwell hardness test: Determines hardness by indicating
on a dial the depth of the impression caused by a loaded indenter in the
form of either a diamond cone (Scales A and C) or a hardened steel ball
(Scale B). When the load is removed, the dial gauge recordes the depth
of impression in terms of Rockwell numbers.
Vickers pyramid hardness test: When the Brinell test
is used on very hard materials, low values result owing to the spherical
shape of the indenter and flatening of the ball. These are eliminated
by using a square based diamond pyramid indenter which does not deform
easily and gives geometrically similar impressions under various loads.
The diamond pyramid with an angle between opposite faces of 136 degree
is pressed under a standard load into the surface of the material, and
the diagonal of the indentation produced is measured- The load divided
by the contact area of impression gives the Vickers Pyramid Number. VPN
= Load in kg / Pyramidal area of impression in sq. mm.