The design of a simple inexpensive instrument which can be used along with a simple laboratory microscope to measure microhardness of crystals is described. The design is based on the fact that the lengths of arms of indentation dislocation rosette (idr) are related to the hardness. By controlled indentation and subsequent etching of two similar crystals, the microhardness of one can be estimated in terms of that of the other from measurements of the arms ofidr.

The hardness of NaCl and KCl crystals has been estimated from the lengths of dislocation rosette formed around indentation at various temperatures up to 400°C. The hardness decreases with increasing temperature. This is due to the softening of the crystals at elevated temperatures which results in the easy movement of dislocations. The results are discussed using a few available relations which connect hardness to temperature. Arms of indentation dislocation rosette are well defined up to 300°C but around 400°C the rosette pattern is spread over a circular region. A possible mechanism is discussed.

Indentations have been made on the (111) planes of CaF₂, SrF₂ and BaF₂ at various temperatures up to 700°C. By etching the crystals, the indentation dislocation rosettes (IDRs) have been recorded and the relative hardness is estimated from the dimensions of the IDRs. The hardness of all these crystals falls with temperature according to the equationH=A exp(−BT). Dramatic changes are observed in the shape of the IDR at elevated temperatures indicating activation of new slip systems.