Hardness

There are three principle operational definitions of hardness:

1. Scratch hardness
2. Indentation hardness
3. Rebound, dynamic or absolute hardness

Scratch hardness

In mineralogy, hardness commonly refers to a material's ability to penetrate softer materials. An object made of a hard material will scratch an object made of a softer material. Scratch hardness is usually measured on the Mohs scale of mineral hardness. Pure diamond is the hardest known substance so it will scratch any other material. Diamond, being the hardest material, is the substance used to cut diamond. Higher-grade diamonds are used to cut lower-grade diamonds.

Indentation hardness

Primarily used in engineering and metallurgy, indentation hardness seeks to characterise a material's resistance to permanent, and in particular plastic, deformation. It is usually measured by loading an indenter of specified geometry onto the material and measuring the dimensions of the resulting indentation.

There are several alternative defintions of indentation hardness, the commonest of which are:

• Brinell hardness test
• Knoop hardness test or microhardness test, for measurement over small areas
• Meyer hardness test
• Rockwell hardness test, principally used in the USA
• Vickers hardness test

There is, in general, no simple relationship between the results of different hardness tests. Though there are practical conversion tables for hard steels, for example, some materials show qualitativey different behaviours under the various measurement methods.

Rebound hardness

Also known as dynamic or absolute hardness, rebound hardness measures the height of rebound of an indenter dropped onto a material using an instrument known as a scleroscope.

Bibliography

• Dieter, G E (1988) Mechanical Metallurgy ISBN 0071004068

External links

• An introduction to materials hardness

Referenced By