MICROHARDNESS TEST –

It is frequently important, particularly in research or development work, to test the hardness of material that is very thin or very small in area. A number of special machines have been developed for determining “microhardness”.

One of the more commonly used pieces of equipment of this type is the Tukon microhardness tester. Normally, the machine is fitted with an elongated diamond-shaped penetrator. Microscopic measurement of the impression provides information that can be converted to Knoop numbers. Knoop hardness measurement often cannot be compared directly with Brinell or Vickers hardness measurement because the elongated impression is rather strongly affected by the directional properties of the material being tested. The use of a symmetrical, square-based, pyramid shaped indentor will provide hardness data comparable with that of the other systems.

In microhardness checks, in which the impression may be only a few thousandths of an inch long, small scratches and surface imperfections may contribute large errors.

Microhardness testing is usually performed on a highly polished surface and in many cases, to obtain reproducibility, it is necessary to etch the surface to reveal the constituent structure in order to locate the impression properly.