Hardness is not a relative measure, but is defined as resistance to friction, scratching, cutting and plastic deformation. It is the resistance of the material against plastic deformation.

Hardness measurement generally consists of measuring the resistance of the material when a conical spherical standard tip is dipped into the material. A properly selected hard tip will leave a mark on the material when dipped into the material under the applied load. The hardness of the material is inversely proportional to the size of this scar.

In addition, Hardness gives information about the machinability of the material. There is an inverse proportion between hardness and traceability. Hard materials are difficult to machine.

Rocwell, Brinell and Vikers hardness tests are generally used in the metal industry.

These tests are done by measuring the immersion depth of a sphere or cone that does not deform under a given load. There are different test methods for different materials, and their results differ. Therefore, when talking about the hardness of a material, the test method must also be determined.