Hardness

The technological use of material has made the term hardness very common but it still remains a poorly defined material property. Even a layman can differentiate between a harder and softer material, but it may be difficult to designate a material by its quantitative hardness because it is difficult to correlate the property with stress or strain. In general hardness may be understood as a property which provides resistance to permanent plastic deformation. Such a deformation may leave a permanent impression on the material surface. For engineering purpose the property of hardness is defined as resistance to indention or scratching. Depends on these two methods of hardness measurement, such as

a.    scratch hardness, and
b.    indentation hardness are used.

Ability of material to resist abrasion, cutting or penetration is also attributed to hardness.