Hardness is not a relative measurement. It is defined the resistance to the friction, drawing, cutting and plastic deformation.
The resistance to plastic deformation of the material.
Hardness is usually consists of the measuring of the material's resistance despite of the a tapered, global and standart edge's immersion of the material.
When the selected edge immersed the material under the applied load, it leaves a trail on the material. Hardness of the material is inversely proportional to the size of this trail.
In addition, the hardness gives information about the property of the material's workability. There is an inverse relationship between hardness and workability. Processing the hard materials are difficult.
Generally metal industry is used Rocwell, Brinell and Vikers hardness tests.
These tests are made of the measuring of the depth of immersion in the metal with the spheres or cones which are not deformed under the applied load. There are different tests for different materials. And also these results are different too. Therefore when talking about of the materials hardness, the test method must be determined.