What is “hardness”?

With regard to metals, hardness has always been a subject of much discussion among technical people, resulting in a wide range of definitions.

Hardness properties include such varied attributes as resistance to abrasives, resistance to plastic deformation, high modulus of elasticity, high yield point, high strength, absence of elastic damping, brittleness or lack of ductility.

To a metallurgist, hardness is a material’s resistance to penetration. In general, an indenter is pressed into the surface of the material to be tested under a specific load for a definite time interval, and a measurement is made of the size or depth of the indentation.

Hardness is not a fundamental property of a material, but a response to a particular test method.

Basically hardness values are arbitrary, and there are no absolute standards for hardness.

Hardness has no quantitative value, except in terms of a given load applied in a specific, reproducible manner and with a specified indenter shape.

Static indentation tests in which a ball, cone or pyramid penetrates into the surface of the material being tested are widespread. The relationship of load to the area or depth of indentation is a measure of hardness, such as that found in common bench-top Brinell, Rockwell, Vickers or Knoop hardness testers.

The different methods and differently shaped indenters used by, for example, Brinell (HB) and Rockwell (HRC) produce dissimilar responses of the material under test. Conversion tables relating to e.g. HRC and HB values have to be determined empirically by experimental evaluation of a specific material’s hardness with the different test methods.

There exists no mathematical equation to transfer measurements from one scale to another. To compare the hardness of two different samples, both must be measured using the same hardness scale, or a scale must be developed to convert from one measurement to the other.