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.