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Macroscopic hardness is generally characterized by strong intermolecular bonds. However, the behavior of solid materials under force is complex, resulting in several different scientific definitions of what might be called "hardness" in everyday usage.
In materials science, there are three principal operational definitions of hardness:
In physics, hardness encompasses:
Additional recommended knowledge
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. Some of the other scales used for indentation hardness in engineering—Rockwell, Vickers, and Brinell—can be compared using practical conversion tables.
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 natural 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 over 2 of and a stiffness 1.11 of 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.
Other materials which can scratch diamond include boron suboxide and rhenium diboride.
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
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. The Vickers and Brinell hardness scales correlate well over a wide range, however, with Brinell only producing overestimated values at high loads.
Hardness increases with decreasing particle 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.
In the December 4, 2005 issue of The Jerusalem Post, Professors Eli Altus, Harold Basch and Shmaryahu Hoz, with doctoral student Lior Itzhaki reported the discovery of a polyyne that is 40 times harder than diamond. It is a "superhard" molecular rod, comprised of acetylene units.
It is important to note that hardness of a material to deformation is dependent to its microdurability or small-scale shear modulus in any direction, not to any rigidity or stiffness properties such as its bulk modulus or Young's modulus. Scientists and journalists often confuse stiffness for hardness, and spuriosely report materials that are not actually harder than diamond because the anisotropy of their solid cells compromise hardness in other dimensions, resulting in a material prone to spalling and flaking in squamose or acicular habits in that dimension. E.g., osmium is stiffer than diamond but is as hard as quartz. In other words, a claimed hard material should have similar hardness characteristics at any location on its surface.
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. 
One scale that measures rebound hardness is the Bennett Hardness Scale.
In solid mechanics, solids generally have three responses to force, depending on the amount of force and the type of material:
Strength is a measure of the extent of a material's elastic range, or elastic and plastic ranges together. This is quantified as compressive strength, shear strength, tensile strength depending on the direction of the forces involved. Ultimate strength is measure of the maximum strain a material can withstand.
Brittleness, in technical usage, is the tendency of a material to fracture with very little or no detectable deformation beforehand. Thus in technical terms, a material can be both brittle and strong. In everyday usage "brittleness" usually refers to the tendency to fracture under a small amount of force, which exhibits both brittleness and a lack of strength (in the technical sense). For brittle materials, yield strength and ultimate strength are the same, because they do not experience detectable plastic deformation. The opposite of brittleness is ductility.
The toughness of a material is the maximum amount of energy it can absorb before fracturing, which is different than the amount of force that can be applied. Toughness tends to be small for brittle materials, because it is elastic and plastic deformations that allow materials to absorb large amounts of energy.
Materials whose properties are different in different directions (because of an asymmetrical crystal structure) are referred to as anisotropic.
Examples of hard matter
Mechanisms for Strength Hardening
It is no surprise that materials can be manipulated so that they may exhibit properties such as higher yield strength. Some mechanisms that can be used are Work Hardening, solid solution strengthening, precipitation hardening, and grain boundary strengthening.
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|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Hardness". A list of authors is available in Wikipedia.