Testing is made by pressing or indenting one material into another with a known amount of mechanical force. Since de ability of the material to resist deformation is related to the yield point and the material’s capacity for work-hardening, the result is actually a measurement of relative hardness.

Indenters are produced from the hardest materials available, such as diamond, and the deformation is limited to the testing material. The shape of indenters is defined by the respective standard of hardness testing and can be very different, depending on whether the indenter is a cone, pyramid or sphere. At the point of contact between the indenter and tested material, the stress easily exceeds the yield strength of the tested material, which is plastically deformed as the indenter moves into the material. All hardness tests are based on the same principle.

There are a number of tests available for hardness testing, all standardised by different standardisation organisations (ISO, EN, ASTM). The applicable standards are listed below:

• The Brinell test is a simple indentation test involving a spherical indenter. Testing is done on the flat surface of a workpiece, the Brinell hardness number is load divided by the curved surface area of the indentation. After removal the load, the diameter of the round indentation is measured in millimetres.
• The Rockwell test is based on the difference of indenter depths from two load applications, first a minor load and then a mayor one.  The difference in depths is the Rockwell hardness number.  The initial application of the minor load increases the accuracy of the testing, since it eliminates the effect of surface layers.
• The Vickers hardness test indenter is a diamond pyramid with an inclination of 136º. The load is normally applied for 10-15 seconds and then removed.  Vickers hardness testing is universal and it is often used for very hard steels.

Complete info here