Relationship between Hardness and Other Properties

The property of hardness is related to the binding forces between atoms and molecules. The hardness like other strength, increase with the binding forces. The molecular solids, like plastics are soft and have low strength while metallic and ionic solids are harder and stronger. The covalent solids are hardest. Diamond, the well known hardest solid has covalent bonding.

   Figure: Approximate Relationship between Tensile Strength and different Hardness Numbers

The dependence of both strength and hardness on the binding forces between atoms will certainly correlate the strength and hardness, a fact which was pointed out in previous Section. However, because the mechanism of indentation is quite complex and involves such factors as triaxial stress state, friction, strand hardening and creep, it is not theoretically possible to correlate hardness with properties of strength, friction and creep etc. However, attempts have been made to produce closely approximated relations and one such between hardness and ultimate tensile strength is that ultimate tensile strength equals 3.4 BHN in N/mm2. Figure 35 describes the relationship between hardness on different scales and ultimate tensile strength of structural steels. Similar relationship exists for different materials but they will be different than the ones shown in Figure .

The techniques used for improving the material strength, such as heat treatment, mechanical treatments or alloying also result in the increase in hardness. Therefore, hardness check of specimens is the quick and easy method of ascertaining the success of such treatment. The hardness test is most widely used for all manufacturing operations as a measure of uniformity and quality.