Science Fair Project Encyclopedia
There are three principal operational definitions of hardness:
- Scratch hardness
- Indentation hardness
- Rebound, dynamic or absolute 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 natural mineral substance and will scratch any other material. Diamond is therefore used to cut other diamonds; in particular, higher-grade diamonds are used to cut lower-grade diamonds.
The hardest substance known today is ultrahard fullerite which has an estimated hardness 1.17-1.52 times that of diamond. However, this material is currently available only in microscopic quantities: its scratch hardness had to be measured under the atomic force microscope.
Estimates from proposed molecular structure indicate the hardness of beta carbon nitride should also be greater than diamond (but less than ultrahard fullerite). This material has not yet been successfully synthesized.
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 definitions of indentation hardness, the most common 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 qualitatively different behaviours under the various measurement methods.
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 .
- Dieter, George E. Mechanical Metallurgy (SI Metric Adaptation). Maidenhead, UK: McGraw-Hill Education, 1989. ISBN 0071004068.
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