Hardness test is used in industry; hardness may be defined as the ability of a material to resist permanent indentation or deformation when in contact with an indenter under load. Generally a hardness test consists of pressing an indenter of known geometry and mechanical properties into the test material. The hardness of the material is quantified using one of a variety of scales that directly or indirectly indicate the contact pressure involved in deforming the test surface. Since the indenter is pressed into the material in hardness test, hardness is also viewed as the ability of a material to resist compressive loads. The indenter may be spherical (Brinell hardness test), pyramidal (Vickers and Knoop hardness tests), or conical (Rockwell hardness test). In the Brinell, Vickers, and Knoop tests, hardness value is the load supported by unit area of the indentation, expressed in kilograms per square millimeter (kgf/mm2). In the Rockwell hardness test, the depth of indentation at a prescribed load is determined and converted to a hardness number (without measurement units), which is inversely related to the depth. Hardness test is no longer limited to metals, and the currently available tools and procedures cover a vast range of materials including polymers, elastomers, thin films, semiconductors, and ceramics. Hardness measurements as applied to specific classes of materials convey different fundamental aspects of the material. Thus, for metals, hardness is directly proportional to the uniaxial yield stress at the strain imposed by the indentation. This statement, however, may not apply in the case of polymers, since their yield stress is illdefined. Yet hardness measurement may be a useful characterization technique for different properties of polymers, such as storage and loss modulus. Similarly, the measured hardness of ceramics and glasses may relate to their fracture toughness, and there appears to be some correlation between microhardness and compressive strength.
The consequence of material hardness also depends on its application in industry. For example, a fracture mechanics engineer may consider a hard material as brittle and less reliable under impact loads; a tribologist may consider high hardness as desirable to reduce plastic deformation and wear in bearing applications. A metallurgist would like to have lower hardness for cold rolling of metals, and a manufacturing engineer would prefer less hard materials for easy and faster machining and increased production. These considerations lead, during component design, to the selection of different types of materials and manufacturing processes to obtain the required material properties of the final product, which are, in many cases, estimated by measuring the hardness of the material.