How is the hardness of a material determined by a hardness testing machine?

The hardness of materials is a function of their elastic modulus. As such a number of tests are used to measure hardness.

Relative hardness can be assessed by the scratch test where one material or mineral is used to scratch another. As such, the material that manages to scratch the other is harder, while the one that is scratched is softer. This comparative or relative hardness method is the basis of the Mohs hardness scale.

More quantitative methods of assessing the hardness of materials are based on the height of rebound of a hammer of known mass allowed to fall from a fixed height (which imparts a known energy into the material), where the height to which it rebounds is dependant on the properties of the material. As such, the higher the rebound, the larger the elastic modulus of the material and the harder the material. Two examples of tests which use this methodology are the Schmidt hammer test and the Shore scleroscope test.

Another common method used to measure the hardness of materials is to assess the depth of indentation of a tool of fixed dimensions at a specified applied load into the material of interest, where the larger the size of indentation, the softer the material. This methodology is commonly used for testing metals and forms the basis of the Vickers, Brinell and Rockwell hardness tests.

Please see the related links for more information.