Hardness
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{{This|mechanical properties of materials|Hard}}
'''Hardness''' refers to various properties of [[matter]] in the [[solid]] [[Phase (matter)|phase]] that give it high resistance to various kinds of shape change when [[force]] is applied. '''Hard matter''' is contrasted with [[soft matter]].
Macroscopic hardness is generally characterized by strong [[intermolecular bond]]s. 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 definition]]s of hardness:
* '''[[#Scratch hardness|Scratch hardness]]''': Resistance to [[fracture]] or plastic (permanent) [[deformation]] due to friction from a sharp object
* '''[[#Indentation hardness|Indentation hardness]]''': Resistance to plastic (permanent) [[deformation]] due to a constant load from a sharp object
* '''[[#Rebound hardness|Rebound hardness]]''': Height of the bounce of an object dropped on the material, related to [[elasticity (physics)|elasticity]].
In physics, hardness encompasses:
* [[Elasticity (physics)|Elasticity]], [[plasticity (physics)|plasticity]], [[viscosity]], and [[viscoelasticity]]
* [[Strength of materials|Strength]] and [[Strain (materials science)|strain]]
* [[Brittleness]]/[[ductility]] and [[toughness]]
The equation based definition of hardness is the pressure applied over the projected contact area between the indenter and the material being tested. As a result hardness values are typically reported in units of pressure (i.e. GPa or MPa), although this is only a "true" pressure if indenter / surface interface is perfectly flat.
==Materials science==
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 of mineral hardness|Mohs scale]] or various other scales. Some of the other scales used for indentation hardness in engineering—[[Rockwell scale|Rockwell]], [[Vickers hardness test|Vickers]], and [[Brinell hardness test|Brinell]]—can be compared using [[hardness comparison|practical conversion tables]].
===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 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 12 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]].
===Indentation hardness===
{{sync|Indentation hardness}}
[[Image:Vickers-tester.png|right|thumb|200px|A Vickers hardness tester]]
{{main|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 [[plasticity (physics)|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 scale|Rockwell hardness test]] (HR), principally used in the [[United States|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 [[hardness comparison|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 relationship|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 [http://jpost.com/servlet/Satellite?cid=1132475677365&pagename=JPost/JPArticle/ShowFull 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<ref>"[http://physicsweb.org/articles/news/9/8/16/1?rss=2.0 Diamonds are not forever]": "The hardness of a material is measured by its isothermal bulk modulus." (2005).</ref><ref>[http://focus.aps.org/story/v4/st31 "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).</ref>, and spuriously 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 [[spall]]ing 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.
===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]].
<ref>[http://www.articlestree.com/science/a-guide-to-rebound-hardness-and-scleroscope-test-tx301428.html A Guide To Rebound Hardness And Scleroscope Test]</ref>
One scale that measures rebound hardness is the [[Bennett Hardness Scale]].
==Physics==
[[Image:stress-strain1.svg|thumb|right|400px|Diagram of a [[Stress-strain curve]], showing the relationship between [[Stress (physics)|stress]] (force applied per unit area) and [[Strain (materials science)|strain]] or [[deformation]] of a ductile metal.]]
In [[solid mechanics]], solids generally have three responses to [[force]], depending on the amount of force and the type of material:
* They exhibit [[Elasticity (physics)|elasticity]]—the ability to temporarily change shape, but return to the original shape when the pressure is removed. "Hardness" in the elastic range—a small temporary change in shape for a given force—is known as [[stiffness]] in the case of a given object, or a high [[elastic modulus]] in the case of a material.
* They exhibit [[Plasticity (physics)|plasticity]]—the ability to permanently change shape in response to the force, but remain in one piece. The [[Yield (engineering)|yield strength]] is the point at which elastic deformation gives way to plastic deformation. Deformation in the plastic range is non-linear, and is described by the [[stress-strain curve]]. This response produces the observed properties of scratch and indentation hardness, as described and measured in materials science. Some materials exhibit both [[elasticity (physics)|elasticity]] and [[viscosity]] when undergoing plastic deformation; this is called [[viscoelasticity]].
* They [[fracture]]—split into two or more pieces. The "ultimate strength" or [[toughness]] of an object is the point at which fracture occurs.
[[Strength of materials|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 (materials science)|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 ==
* [[Ceramic]]s
* [[Composites]]
* [[Metals]]
* [[Semiconductors]]
==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]].
==References==
<references />
{{cleanup-references}}
Materials science:
*{{cite book
|last = Dieter,
|first = George E.
|title = Mechanical Metallurgy
|edition = SI Metric Adaptation
|location = Maidenhead, UK
|publisher = McGraw-Hill Education
|year = 1989
|isbn = ISBN 0-07-100406-8
}}
*{{cite journal
|last = Malzbender
|first = J
|title = Comment on hardness definitions
|journal = Journal of the European Ceramics Society
|volume = 23
|year = 2003
|pages = 1355
}}
==See also==
* [[Ceramography#Microindention_Hardness_and_Toughness | Hardness of ceramics]]
==External links==
* [http://www.calce.umd.edu/general/Facilities/Hardness_ad_.htm An introduction to materials hardness]
* [http://www.virginia.edu/bohr/mse209/chapter7.htm Dislocations and Strengthening Mechanisms]
[[Category:Condensed matter physics]]
[[Category:Matter]]
[[Category:Mineralogy]]
[[Category:Materials science]]
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