Hardness (materials science)

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For other uses, see Hardness.

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 or dynamic hardness

In everyday usage, "hardness" may refer to various other properties; see hard matter.

1 Scratch hardness
2 Indentation hardness
3 Rebound hardness
4 References
5 See also
6 External links

[edit] 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. One tool to make this measurement is the sclerometer.

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.

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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. However, hardness to yield has to do with the microdurability or small-scale shear modulus in any direction, and not to any rigid or stiff properties such as the bulk modulus or Young's modulus. Scientists and journalists often confuse stiffness for hardness^[1]^[2], and spuriously report materials that are not actually harder than diamond because the greater anisotropy of their solid cells compromise hardness in other dimensions to end in a material prone to spalling and flaking in squamose or acicular habits. Osmium, e.g., is stiffer than diamond but is as hard as quartz.

[edit] Indentation hardness

A Vickers hardness tester

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 decreasing grain size. This is known as the inverse Hall-Petch effect.

For measuring hardness of nanograined materials, nanoindentation is used.

[edit] Rebound hardness

Also known as dynamic hardness, rebound hardness measures the height of the "bounce" of a diamond-tipped hammer dropped from a fixed height onto a material. The device used to take this measurement is known as a scleroscope. ^[3]

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

[edit] References

Dieter,, George E. (1989). Mechanical Metallurgy, SI Metric Adaptation, Maidenhead, UK: McGraw-Hill Education. ISBN ISBN 0-07-100406-8.
Malzbender, J (2003). "Comment on hardness definitions". Journal of the European Ceramics Society 23: 1355.

^ "Diamonds are not forever": "The hardness of a material is measured by its isothermal bulk modulus." (2005).
^ "Hard as a Diamond?": "..bulk modulus would be surpassed only by diamond; and if combined with some impurity atoms to fill in the voids, it might be even harder than diamond." (1999).
^ [1]