Hardness

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For the mathematical concept of the difficulty of proving a conjecture, solving an equation, etc., see computational complexity theory. For water hardness, see hard water.

In materials science, hardness is the characteristic of a solid material expressing its resistance to permanent deformation. Hardness can be measured on the Mohs scale or various other scales.

There are three principal operational definitions of hardness:

Scratch hardness
Indentation hardness
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 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 aggregated diamond nanorods, with a hardness 1.11 times diamond.

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.

In the December 4 2005 issue of The Jerusalem Post, Professors Eli Altus, Harold Basch and Shmaryahu Hoz, with doctoral student Lior Itzhaki report the discovery of a polyyne that is 40 times harder than diamond. It is a superhard molecular rod, comprised of acetylene units.

Indentation hardness

Primarily used in engineering and metallurgy, indentation hardness seeks to characterise a material's hardness, i.e. its 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 (HB)
Janka Wood Hardness Rating
Knoop hardness test (HK) or microhardness test, for measurement over small areas
Meyer hardness test
Rockwell hardness test (HR), principally used in the USA
Shore durometer hardness, used for polymers
Vickers hardness test (HV), has one of the widest scales
Barcol hardness test, for composite materials, scale from 0 to 100

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.

Hardness increases with decreasing grain size. This is known as the Hall-Petch effect. However, below a critical grain-size, hardness decreases with decreases grain size. This is known as the inverse Hall-Petch effect.

For measuring hardness of nanograined materials, nanoindentation is used.

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. One scale that measures rebound hardness is the Bennett Hardness Scale.

References

Dieter, George E. Mechanical Metallurgy (SI Metric Adaptation). Maidenhead, UK: McGraw-Hill Education, 1989. ISBN 0071004068.

External links

Mineralogy | Materials science

This article is licensed under the GNU Free Documentation License. It uses material from the "Hardness".

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