A superhard material is a material with a hardness value exceeding 40 gigapascals (
GPa
Grading in education is the process of applying standardized measurements for varying levels of achievements in a course. Grades can be assigned as letters (usually A through F), as a range (for example, 1 to 6), as a percentage, or as a numbe ...
) when measured by the
Vickers hardness test
The Vickers hardness test was developed in 1921 by Robert L. Smith and George E. Sandland at Vickers Ltd as an alternative to the Brinell method to measure the hardness of materials. The Vickers test is often easier to use than other hardness t ...
.
They are virtually incompressible solids with high electron density and high
bond covalency. As a result of their unique properties, these materials are of great interest in many industrial areas including, but not limited to,
abrasive
An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away by friction. While finishing a material often means polishing it to gain a smooth, reflec ...
s, polishing and
cutting tools
In the context of machining, a cutting tool or cutter is typically a hardened metal tool that is used to cut, shape, and remove material from a workpiece by means of machining tools as well as abrasive tools by way of shear deformation. The majori ...
,
disc brake
A disc brake is a type of brake that uses the calipers to squeeze pairs of pads against a disc or a "rotor" to create friction. This action slows the rotation of a shaft, such as a vehicle axle, either to reduce its rotational speed or to hol ...
s, and
wear
Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical (e.g., erosion) or chemical (e.g., corrosion). The study of wear and related processes is referred to as tribology.
Wear in m ...
-resistant and protective coatings.
Diamond
Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the Chemical stability, chemically stable form of car ...
is the hardest known material to date, with a Vickers hardness in the range of 70–150 GPa. Diamond demonstrates both high
thermal conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
and
electrically insulating properties, and much attention has been put into finding practical applications of this material. However, diamond has several limitations for mass industrial application, including its high cost and oxidation at temperatures above 800 °C. In addition, diamond dissolves in
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
and forms
iron carbides at high temperatures and therefore is inefficient in cutting ferrous materials including
steel
Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
. Therefore, recent research of superhard materials has been focusing on compounds which would be thermally and chemically more stable than pure diamond.
The search for new superhard materials has generally taken two paths. In the first approach, researchers emulate the short, directional covalent carbon bonds of diamond by combining light elements like boron, carbon, nitrogen, and oxygen. This approach became popular in the late 1980s with the exploration of C
3N
4 and B-C-N ternary compounds. The second approach towards designing superhard materials incorporates these lighter elements (B, C, N, and O), but also introduces transition metals with high valence electron densities to provide high incompressibility. In this way, metals with high
bulk moduli but low hardness are coordinated with small covalent-forming atoms to produce superhard materials. Tungsten carbide is an industrially-relevant manifestation of this approach, although it is not considered superhard. Alternatively, borides combined with transition metals have become a rich area of superhard research and have led to discoveries such as
ReB2,
OsB2, and
WB4.
Superhard materials can be generally classified into two categories: intrinsic compounds and extrinsic compounds. The intrinsic group includes
diamond
Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the Chemical stability, chemically stable form of car ...
,
cubic boron nitride
Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagonal ...
(c-BN),
carbon nitrides, and ternary compounds such as B-N-C, which possess an innate hardness. Conversely, extrinsic materials are those that have superhardness and other mechanical properties that are determined by their microstructure rather than composition.
An example of extrinsic superhard material is nanocrystalline diamond known as
aggregated diamond nanorod
Aggregated diamond nanorods, or ADNRs, are a nanocrystalline form of diamond, also known as nanodiamond or hyperdiamond.
Discovery
Nanodiamond or hyperdiamond was produced by compression of graphite in 2003 by a group of researchers in Japa ...
s.
Definition and mechanics of hardness
The hardness of a material is directly related to its incompressibility, elasticity and resistance to change in shape. A superhard material has high
shear modulus
In materials science, shear modulus or modulus of rigidity, denoted by ''G'', or sometimes ''S'' or ''μ'', is a measure of the elastic shear stiffness of a material and is defined as the ratio of shear stress to the shear strain:
:G \ \stackrel ...
, high
bulk modulus
The bulk modulus (K or B) of a substance is a measure of how resistant to compression the substance is. It is defined as the ratio of the infinitesimal pressure increase to the resulting ''relative'' decrease of the volume.
Other moduli describe ...
, and does not deform
plastically. Ideally superhard materials should have a defect-free, isotropic lattice. This greatly reduces structural deformations that can lower the strength of the material. However, defects can actually strengthen some covalent structures. Traditionally, high-pressure and high-temperature (
HPHT
Lab-grown diamond (LGD; also called laboratory-grown, laboratory-created, man-made, artisan-created, artificial, synthetic, or cultured diamond) is diamond that is produced in a controlled technological process (in contrast to naturally formed ...
) conditions have been used to synthesize superhard materials, but recent superhard material syntheses aim at using less energy and lower cost materials.
[
Historically, hardness was first defined as the ability of one material to scratch another and quantified by an integer (sometimes half-integer) from 0 to 10 on the ]Mohs scale
The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material.
The scale was introduced in 1812 by th ...
. This scale was however quickly found too discrete and non-linear. Measuring the mechanical hardness of materials changed to using a nanoindenter
A nanoindenter is the main component for indentation hardness tests used in nanoindentation. Since the mid-1970s nanoindentation has become the primary method for measuring and testing very small volumes of mechanical properties. Nanoindentation, ...
(usually made of diamond) and evaluating bulk moduli, and the Brinell, Rockwell, Knoop, and Vickers
Vickers was a British engineering company that existed from 1828 until 1999. It was formed in Sheffield as a steel foundry by Edward Vickers and his father-in-law, and soon became famous for casting church bells. The company went public in 18 ...
scales have been developed. Whereas the Vickers scale is widely accepted as a most common test,[ there remain controversies on the weight load to be applied during the test. This is because Vickers hardness values are load-dependent. An indent made with 0.5N will indicate a higher hardness value than an indent made with 50N. This phenomenon is known as the indentation size effect (ISE). Thus, hardness values are not meaningful unless the load is also reported. Some argue that hardness values should consistently be reported in the asymptotic (high-load region), as this is a more standardized representation of a material's hardness.
Bulk moduli, shear moduli, and elasticity are the key factors in the superhard classification process. The incompressibility of a material is quantified by the ]bulk modulus
The bulk modulus (K or B) of a substance is a measure of how resistant to compression the substance is. It is defined as the ratio of the infinitesimal pressure increase to the resulting ''relative'' decrease of the volume.
Other moduli describe ...
B, which measures the resistance of a solid to volume compression under hydrostatic stress In continuum mechanics, hydrostatic stress, also known as volumetric stress, is a component of stress which contains uniaxial stresses, but not shear stresses. A specialized case of hydrostatic stress, contains isotropic compressive stress, which ch ...
as B = −Vdp/dV. Here V is the volume, p is pressure, and dp/dV is the partial derivative of pressure with respect to the volume. The bulk modulus test
Bulk can refer to:
Industry
* Bulk cargo
* Bulk liquids
* Bulk mail
* Bulk material handling
* Bulk pack, packaged bulk materials/products
* Bulk purchasing
*
Baking
* Bulk fermentation, the period after mixing when dough is left alone to ferme ...
uses an indenter tool to form a permanent deformation in a material. The size of the deformation depends on the material's resistance to the volume compression made by the tool. Elements with small molar volumes and strong interatomic forces usually have high bulk moduli. Bulk moduli was the first major test of hardness and originally shown to be correlated with the molar volume (Vm) and cohesive energy
Cohesion may refer to:
* Cohesion (chemistry), the intermolecular attraction between like-molecules
* Cohesion (computer science), a measure of how well the lines of source code within a module work together
* Cohesion (geology), the part of shea ...
(Ec) as B ~ Ec/Vm.
Bulk modulus was believed to be a direct measure of a material's hardness but this no longer remains the dominant school of thought. For example, some alkali
In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...
and noble metal
A noble metal is ordinarily regarded as a metallic chemical element that is generally resistant to corrosion and is usually found in nature in its raw form. Gold, platinum, and the other platinum group metals (ruthenium, rhodium, palladium, o ...
s (Pd, Ag) have anomalously high ratio of the bulk modulus to the Vickers of Brinell hardness. In the early 2000s, a direct relationship between bulk modulus and valence electron density was found as the more electrons were present the greater the repulsions within the structure were.[ Bulk modulus is still used as a preliminary measure of a material as superhard but it is now known that other properties must be taken into account.][
In contrast to bulk modulus, ]shear modulus
In materials science, shear modulus or modulus of rigidity, denoted by ''G'', or sometimes ''S'' or ''μ'', is a measure of the elastic shear stiffness of a material and is defined as the ratio of shear stress to the shear strain:
:G \ \stackrel ...
measures the resistance to shape change at a constant volume, taking into account the crystalline plane and direction of shear. The shear modulus G is defined as ratio of shear stress to shear strain: G = stress/strain = F·L/(A·dx), where F is the applied force, A is the area upon which the force acts, dx is the resulting displacement and L is the initial length. The larger the shear modulus, the greater the ability for a material to resist shearing forces. Therefore, the shear modulus is a measure of rigidity. Shear modulus is related to bulk modulus as , where v is the Poisson's ratio, which is typically ~0.1 in covalent materials. If a material contains highly directional bonds, the shear modulus will increase and give a low Poisson ratio.
A material is also considered hard if it resists plastic deformation
In engineering, deformation refers to the change in size or shape of an object. ''Displacements'' are the ''absolute'' change in position of a point on the object. Deflection is the relative change in external displacements on an object. Strain ...
. If a material has short covalent bonds, atomic dislocations that lead to plastic deformation are less likely to occur than in materials with longer, delocalized bonds. If a material contains many delocalized bonds it is likely to be soft.[ Somewhat related to hardness is another mechanical property ]fracture toughness
In materials science, fracture toughness is the critical stress intensity factor of a sharp crack where propagation of the crack suddenly becomes rapid and unlimited. A component's thickness affects the constraint conditions at the tip of a c ...
, which is a material's ability to resist breakage from forceful impact (note that this concept is distinct from the notion of toughness
In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing.[MPa
MPA or mPa may refer to:
Academia
Academic degrees
* Master of Performing Arts
* Master of Professional Accountancy
* Master of Public Administration
* Master of Public Affairs
Schools
* Mesa Preparatory Academy
* Morgan Park Academy
* Mound ...]
·m1/2, which is high compared to other gemstones and ceramic materials, but poor compared to many metals and alloys – common steels and aluminium alloys have the toughness values at least 5 times higher.
Several properties must be taken into account when evaluating a material as (super)hard. While hard materials have high bulk moduli, a high bulk modulus does not mean a material is hard. Inelastic characteristics must be considered as well, and shear modulus might even provide a better correlation with hardness than bulk modulus. Covalent materials generally have high bond-bending force constants and high shear moduli and are more likely to give superhard structures than, for example, ionic solids.[
]
Diamond
Diamond
Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the Chemical stability, chemically stable form of car ...
is an allotrope of carbon where the atoms are arranged in a modified version of face-centered cubic
In crystallography, the cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals.
There are three main varieties of ...
(''fcc'') structure known as "diamond cubic
The diamond cubic crystal structure is a repeating pattern of 8 atoms that certain materials may adopt as they solidify. While the first known example was diamond, other elements in group 14 also adopt this structure, including α-tin, the sem ...
". It is known for its hardness (see table above) and incompressibility and is targeted for some potential optical and electrical applications. The properties of individual natural diamonds or carbonado
Carbonado, commonly known as black diamond, is one of the toughest forms of natural diamond. It is an impure, high-density, micro-porous form of polycrystalline diamond consisting of diamond, graphite, and amorphous carbon, with minor crystall ...
vary too widely for industrial purposes, and therefore synthetic diamond
Lab-grown diamond (LGD; also called laboratory-grown, laboratory-created, man-made, artisan-created, artificial, synthetic, or cultured diamond) is diamond that is produced in a controlled technological process (in contrast to naturally formed ...
became a major research focus.
Synthetic diamond
The high-pressure synthesis of diamond in 1953 in Sweden and in 1954 in the US, made possible by the development of new apparatus and techniques, became a milestone in synthesis of artificial superhard materials. The synthesis clearly showed the potential of high-pressure applications for industrial purposes and stimulated growing interest in the field. Four years after the first synthesis of artificial diamond, cubic boron nitride
Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagonal ...
c-BN was obtained and found to be the second hardest solid.
Synthetic diamond
Lab-grown diamond (LGD; also called laboratory-grown, laboratory-created, man-made, artisan-created, artificial, synthetic, or cultured diamond) is diamond that is produced in a controlled technological process (in contrast to naturally formed ...
can exist as a single, continuous crystal or as small polycrystals interconnected through the grain boundaries. The inherent spatial separation of these subunits causes the formation of grains, which are visible by the unaided eye due to the light absorption and scattering properties of the material.
The hardness of synthetic diamond (70–150 GPa) is very dependent on the relative purity of the crystal itself. The more perfect the crystal structure, the harder the diamond becomes. It has been reported that HPHT single crystals and nanocrystalline diamond aggregates (aggregated diamond nanorod
Aggregated diamond nanorods, or ADNRs, are a nanocrystalline form of diamond, also known as nanodiamond or hyperdiamond.
Discovery
Nanodiamond or hyperdiamond was produced by compression of graphite in 2003 by a group of researchers in Japa ...
s) can be harder than natural diamond.[
Historically, it was thought that synthetic diamond should be structurally perfect to be useful. This is because diamond was mainly preferred for its aesthetic qualities, and small flaws in structure and composition were visible by naked eye. Although this is true, the properties associated with these small changes has led to interesting new potential applications of synthetic diamond. For example, nitrogen doping can enhance mechanical strength of diamond, and heavy doping with ]boron
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
(several atomic percent) makes it a superconductor.
In 2014, researchers reported on the synthesis of nano-twinned diamond with Vickers hardness values up to 200 GPa. The authors attribute the unprecedented hardness to the Hall-Petch effect
In materials science, grain-boundary strengthening (or Hall–Petch strengthening) is a method of strengthening materials by changing their average crystallite (grain) size. It is based on the observation that grain boundaries are insurmountabl ...
, which predicts that smaller microstructural features can lead to enhanced hardness due to higher density of boundaries that stop dislocations. They achieve twins with an average thickness of 5 nm using a precursor of onion carbon nanoparticles subjected to high temperature and pressure. They also simultaneously achieve an oxidation temperature that is 200 °C higher than that of natural diamond. Higher thermal stability is relevant to industrial applications such as cutting tools, where high temperatures can lead to rapid diamond degradation.
Dense Amorphous Carbon
A dense AM-III form of transparent amorphous carbon Amorphous carbon is free, reactive carbon that has no crystalline structure. Amorphous carbon materials may be stabilized by terminating dangling-π bonds with hydrogen. As with other amorphous solids, some short-range order can be observed. Amorp ...
has a Vickers hardness of 113 GPa. This heat-treated fullerene
A fullerene is an allotrope of carbon whose molecule consists of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to seven atoms. The molecule may be a hollow sphere, ...
is currently the hardest amorphous material.
Cubic boron nitride
History
Cubic boron nitride
Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagonal ...
or c-BN was first synthesized in 1957 by Robert H. Wentorf at General Electric, shortly after the synthesis of diamond. The general process for c-BN synthesis is the dissolution of hexagonal boron nitride (h-BN) in a solvent-catalyst, usually alkali or alkaline earth metal
The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar properties: they are al ...
s or their nitride
In chemistry, a nitride is an inorganic compound of nitrogen. The "nitride" anion, N3- ion, is very elusive but compounds of nitride are numerous, although rarely naturally occuring. Some nitrides have a find applications, such as wear-resistant ...
s, followed by spontaneous nucleation of c-BN under high pressure, high temperature (HPHT) conditions.[ The yield of c-BN is lower and substantially slower compared to diamond's synthetic route due to the complicated intermediate steps. Its insolubility in iron and other metal alloys makes it more useful for some industrial applications than diamond.]
Pure cubic boron nitride is transparent or slightly amber. Different colors can be produced depending on defects or an excess of boron (less than 1%).[ Defects can be produced by doping solvent-catalysts (i.e. Li, Ca, or Mg nitrides) with Al, B, Ti, or Si. This induces a change in the morphology and color of c-BN crystals.]
The result is darker and larger (500 μm) crystals with better shapes and a higher yield.
Structure and properties
Cubic boron nitride adopts a sphalerite crystal structure, which can be constructed by replacing every two carbon atoms in diamond with one boron atom and one nitrogen atom. The short B-N (1.57 Å) bond is close to the diamond C-C bond length (1.54 Å), that results in strong covalent bonding between atoms in the same fashion as in diamond. The slight decrease in covalency for B-N bonds compared to C-C bonds reduces the hardness from ~100 GPa for diamond down to 48 GPa in c-BN. As diamond is less stable than graphite, c-BN is less stable than h-BN, but the conversion rate between those forms is negligible at room temperature.[
Cubic boron nitride is insoluble in iron, nickel, and related alloys at high temperatures, but it binds well with metals due to formation of interlayers of metal borides and nitrides. It is also insoluble in most acids, but is soluble in alkaline molten salts and nitrides, such as LiOH, KOH, NaOH/Na2CO3, NaNO3 which are used to etch c-BN.] Because of its stability with heat and metals, c-BN surpasses diamond in mechanical applications. The thermal conductivity of BN is among the highest of all electric insulators. In addition, c-BN consists of only light elements and has low X-ray absorptivity, capable of reducing the X-ray absorption background.
Research and development
Due to its great chemical and mechanical robustness, c-BN has widespread application as an abrasive, such as on cutting tools and scratch resistant surfaces. Cubic boron nitride is also highly transparent to X-rays. This, along with its high strength, makes it possible to have very thin coatings of c-BN on structures that can be inspected using X-rays. Several hundred tonne
The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton ( United State ...
s of c-BN are produced worldwide each year.[Wilke, K.T. and Bohm, J. (1988) ''Kristallzüchtung'', ]Verlag Harri Deutsch
The (VHD, HD) with headquarters in Frankfurt am Main, Germany, as well as in Zürich and Thun, Switzerland, was a German publishing house founded in 1961 and closed in 2013.
Overview
The ' with headquarters in Frankfurt am Main, Germany, wa ...
, Frankfurt. By modification, Borazon, a US brand name of c-BN, is used in industrial applications to shape tools, as it can withstand temperatures greater than 2,000 °C. Cubic boron nitride-coated grinding wheels, referred to as Borazon wheels, are routinely used in the machining of hard ferrous metals, cast irons, and nickel-base and cobalt-base superalloy
A superalloy, or high-performance alloy, is an alloy with the ability to operate at a high fraction of its melting point. Several key characteristics of a superalloy are excellent mechanical strength, resistance to thermal creep deformation, g ...
s. Other brand names, such as Elbor and Cubonite, are marketed by Russian vendors.[
New approaches in research focus on improving c-BN pressure capabilities of the devices used for c-BN synthesis.][ At present, the capabilities for the production of c-BN are restricted to pressures of about 6 GPa. Increasing the pressure limit will permit synthesis of larger single crystals than from the present catalytic synthesis. However, the use of solvents under supercritical conditions for c-BN synthesis has been shown to reduce pressure requirements.][ The high cost of c-BN still limits its application, which motivates the search for other superhard materials.
]
Carbon nitride
The structure of beta carbon nitride
Beta (, ; uppercase , lowercase , or cursive ; grc, βῆτα, bē̂ta or ell, βήτα, víta) is the second letter of the Greek alphabet. In the system of Greek numerals, it has a value of 2. In Modern Greek, it represents the voiced labiod ...
(β-C3N4) was first proposed by Amy Liu and Marvin Cohen in 1989. It is isostructural Isostructural chemical compounds have similar chemical structures. "Isomorphous" when used in the relation to crystal structures is not synonymous: in addition to the same atomic connectivity that characterises isostructural compounds, isomorphous s ...
with Si3N4 and was predicted to be harder than diamond. The calculated bond length was 1.47 Å, 5% shorter than the C-C bond length in diamond. Later calculations indicated that the shear modulus is 60% of that of diamond, and carbon nitride is less hard than c-BN.
Despite two decades of pursuit of this compound, no synthetic sample of C3N4 has validated the hardness predictions; this has been attributed to the difficulty in synthesis and the instability of C3N4. Carbon nitride is only stable at a pressure that is higher than that of the graphite-to-diamond transformation. The synthesis conditions would require extremely high pressures because carbon is four- and sixfold coordinated.[ In addition, C3N4 would pose problems of carbide formation if they were to be used to machine ferrous metals. Although publications have reported preparation of C3N4 at lower pressures than stated, synthetic C3N4 was not proved superhard.
]
Boron carbon nitride
The similar atomic sizes of boron, carbon and nitrogen, as well as the similar structures of carbon and boron nitride polymorphs, suggest that it might be possible to synthesize diamond-like phase containing all three elements. It is also possible to make compounds containing B-C-O, B-O-N, or B-C-O-N under high pressure, but their synthesis would expect to require a complex chemistry and in addition, their elastic properties would be inferior to that of diamond.
Beginning in 1990, a great interest has been put in studying the possibility to synthesize dense B-C-N phases. They are expected to be thermally and chemically more stable than diamond, and harder than c-BN, and would therefore be excellent materials for high speed cutting and polishing of ferrous alloys. These characteristic properties are attributed to the diamond-like structure combined with the sp3 σ-bonds among carbon and the heteroatoms. BCxNy thin films were synthesized by chemical vapor deposition
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (substra ...
in 1972. However, data on the attempted synthesis of B-C-N dense phases reported by different authors have been contradictory. It is unclear whether the synthesis products are diamond-like solid solutions between carbon and boron nitride or just mechanical mixtures of highly dispersed diamond and c-BN. In 2001, a diamond-like-structured c-BC2N was synthesized at pressures >18 GPa and temperatures >2,200 K by a direct solid-state phase transition of graphite-like (BN)0.48C0.52. The reported Vickers and Knoop hardnesses were intermediate between diamond and c-BN, making the new phase the second hardest known material. Ternary B–C–N phases can also be made using shock-compression synthesis. It was further suggested to extend the B–C–N system to quaternary compounds with silicon included.[
]
Metal borides
Unlike carbon-based systems, metal borides can be easily synthesized in large quantities under ambient conditions, an important technological advantage.[ Most metal borides are hard; however, a few stand out among them for their particularly high hardnesses (for example, WB4, RuB2, OsB2 and ReB2).
These metal borides are still metals and not semiconductors or insulators (as indicated by their high electronic ]density of states
In solid state physics and condensed matter physics, the density of states (DOS) of a system describes the number of modes per unit frequency range. The density of states is defined as D(E) = N(E)/V , where N(E)\delta E is the number of states i ...
at the Fermi Level
The Fermi level of a solid-state body is the thermodynamic work required to add one electron to the body. It is a thermodynamic quantity usually denoted by ''µ'' or ''E''F
for brevity. The Fermi level does not include the work required to remove ...
); however, the additional covalent B-B and M-B bonding (M = metal) lead to high hardness. Dense heavy metals
upright=1.2, Crystals of osmium, a heavy metal nearly twice as dense as lead">lead.html" ;"title="osmium, a heavy metal nearly twice as dense as lead">osmium, a heavy metal nearly twice as dense as lead
Heavy metals are generally defined as ...
, such as osmium
Osmium (from Greek grc, ὀσμή, osme, smell, label=none) is a chemical element with the symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mos ...
, rhenium
Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-gray, heavy, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion (ppb), rhenium is one ...
, tungsten
Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isolat ...
etc., are particularly apt at forming hard borides because of their high electron densities, small atomic radii, high bulk moduli, and ability to bond strongly with boron.
Osmium diboride
Osmium diboride
Osmium borides are compounds of osmium and boron. Their most remarkable property is potentially high hardness. It is thought that a combination of high electron density of osmium with the strength of boron-osmium covalent bonds will make osmium ...
(OsB2) has a high bulk modulus of 395 GPa and therefore is considered as a candidate superhard material, but the maximum achieved Vickers hardness is 37 GPa, slightly below the 40 GPa limit of superhardness. A common way to synthesize OsB2 is by a solid-state metathesis reaction containing a 2:3 mixture of OsCl3: MgB2.[ After the MgCl2 product is washed away, ]X-ray diffraction
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
indicates products of OsB2, OsB and Os. Heating this product at 1,000 °C for three days produces pure OsB2 crystalline product. OsB2 has an orthorhombic structure (space group
In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it unchan ...
P''mmn'') with two planes of osmium atoms separated by a non-planar layer of hexagonally coordinated boron atoms; the lattice parameters are ''a'' = 4.684 Å, ''b'' = 2.872 Å and ''c'' = 4.096 Å.[ The ''b'' direction of the crystal is the most compressible and the ''c'' direction is the least compressible. This can be explained by the orthorhombic structure. When looking at the boron and osmium atoms in the ''a'' and ''b'' directions, they are arranged in a way that is offset from one another. Therefore, when they are compressed they are not pushed right up against one another. Electrostatic repulsion is the force that maximizes the materials incompressibility and so in this case the electrostatic repulsion is not taken full advantage of. When compressed in the ''c'' direction, the osmium and boron atoms are almost directly in line with one another and the electrostatic repulsion is therefore high, causing direction ''c'' to be the least compressible. This model implies that if boron is more evenly distributed throughout the lattice then incompressibility could be higher. Electron backscatter diffraction coupled with hardness measurements reveals that in the (010) plane, the crystal is 54% harder in the <100> than <001> direction. This is seen by looking at how long the indentation is along a certain direction (related to the indentations made with a Vickers hardness test). Along with the alignment of the atoms, this is also due to the short covalent B-B (1.80 Å) bonds in the <100> direction, which are absent in the <001> direction (B-B = 4.10 Å).][
]
Rhenium borides
Rhenium was targeted as a candidate for superhard metal borides because of its desirable physical and chemical characteristics. It has a high electron density, a small atomic radius and a high bulk modulus. When combined with boron, it makes a crystal with highly covalent bonding allowing it to be incompressible and potentially very hard. A wide array of rhenium borides have been investigated including Re3B, Re7B3, Re2B, ReB, Re2B3, Re3B7, Re2B5, ReB3 and ReB2. Each of these materials has their own set of properties and characteristics. Some show promise as superconductors and some have unique elastic and electronic properties, but the most relevant to superhard materials is ReB2.[
]Rhenium diboride
Rhenium diboride (ReB2) is a synthetic superhard material. It was first synthesized in 1962 and re-emerged recently due to hopes of achieving high hardness comparable to that of diamond. The reported ultrahigh hardness has been questioned, althou ...
(ReB2) is a refractory compound which was first synthesized in the 1960s, using arc melting, zone melting
Zone melting (or zone refining, or floating-zone method, or floating-zone technique) is a group of similar methods of purifying crystals, in which a narrow region of a crystal is melted, and this molten zone is moved along the crystal. The molte ...
, or optical floating zone furnaces. An example synthesis of this material is the flux method, which is conducted by placing rhenium metal and amorphous boron in an alumina crucible with excess aluminium. This can be run with a ratio of 1:2:50 for Re:B:Al, with the excess aluminium as a growth medium. The crucible is placed in an alumina tube, inserted into a resistively heated furnace with flowing argon
Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
gas and sintered
Clinker nodules produced by sintering
Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction.
Sintering happens as part of a manufacturing ...
at 1,400 °C for several hours. After cooling, the aluminium is dissolved in NaOH. Each ReB2 synthesis route has its own drawbacks, and this one gives small inclusions of aluminium incorporated into the crystal lattice.[
Rhenium diboride has a very high melting point approaching 2,400 °C and a highly anisotropic, layered crystal structure.] Its symmetry is either hexagonal (space group P63''mc'') or orthorhombic (C''mcm'') depending on the phase. There, close-packed Re layers alternate with puckered triangular boron layers along the (001) plane. This can be seen above on the example of osmium diboride. The density of states for ReB2 has one of the lowest values among the metal borides, indicating strong covalent bonding and high hardness.[
Owing to the anisotropic nature of this material, the hardness depends on the crystal orientation. The (002) plane contains the most covalent character and exhibits a maximum Vickers hardness value of 40.5 GPa, while the perpendicular planes were 6% lower at 38.1 GPa. These values decrease with increased load, settling at around 28 GPa each. The ]nanoindentation
Nanoindentation, also called instrumented indentation testing, is a variety of indentation hardness tests applied to small volumes. Indentation is perhaps the most commonly applied means of testing the mechanical properties of materials. The nanoi ...
values were found to be 36.4 GPa and 34.0 GPa for the (002) and perpendicular planes respectively. The hardness values depend on the material purity and composition – the more boron the harder the boride – and the above values are for a Re:B ratio of approximately 1.00:1.85. Rhenium diboride also has a reported bulk modulus of 383 GPa and a shear modulus of 273 GPa. The hardness of rhenium diboride, and most other materials also depends on the load during the test. The above values of about 40 GPa were all measured with an effective load of 0.5–1 N. At such low load, the hardness values are also overestimated for other materials, for example it exceeds 100 GPa for c-BN.[ Other researchers, while having reproduced the high ReB2 hardness at low load, reported much lower values of 19–17 GPa at a more conventional load of 3–49 N, that makes ReB2 a hard, but not a superhard material.]
Rhenium diboride exhibits metallic conductivity which increases as temperature decreases and can be explained by a nonzero density of states due to the d and p overlap of rhenium and boron respectively. At this point, it is the only superhard material with metallic behavior. The material also exhibits relatively high thermal stability. Depending on the heating method, it will maintain its mass up to temperatures of 600–800 °C, with any drop being due to loss of absorbed water. A small loss of mass can then be seen at temperatures approaching 1,000 °C. It performs better when a slower heat ramp is utilized. Part of this small drop at around 1,000 °C was explained by the formation of a dull B2O3 coating on the surface as boron is leached out of the solid, which serves as a protective coating, thereby reducing additional boron loss. This can be easily dissolved by methanol to restore the material to it native shiny state.[
]
Tungsten borides
The discovery of superhard tungsten tetraboride is further evidence for the promising design approach of covalently bonding incompressible transition metals with boron. While WB4 was first synthesized and identified as the highest boride of tungsten in 1966, it was only recognized as an inexpensive superhard material in 2011.
Interestingly, lower borides of tungsten such as tungsten diboride are not superhard. Higher boron content leads to higher hardness because of the increased density of short, covalent boron-boron and boron-metal bonds. However, researchers have been able to push WB2 into the superhard regime through minority additions of other transition metals such as niobium and tantalum in the crystal structure. This mechanism of hardness enhancement is called solid solution strengthening
In metallurgy, solid solution strengthening is a type of alloying that can be used to improve the strength of a pure metal. The technique works by adding atoms of one element (the alloying element) to the crystalline lattice of another element (t ...
and arises because atoms of different sizes are incorporated into the parent lattice to impede dislocation motion.
Aluminium magnesium boride
Aluminium magnesium boride Aluminum magnesium boride or Al3Mg3B56 colliqually known as BAM is a chemical compound of aluminium, magnesium and boron. Whereas its nominal formula is AlMgB14, the chemical composition is closer to Al0.75Mg0.75B14. It is a ceramic alloy that is h ...
or BAM is a chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
of aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
, magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
and boron
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
. Whereas its nominal formula is AlMgB14, the chemical composition is closer to Al0.75Mg0.75B14. It is a ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
alloy
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
that is highly resistive to wear and has a low coefficient of sliding friction.
Other boron-rich superhard materials
Other hard boron-rich compounds include B4C and B6O. Amorphous a-B4C has a hardness of about 50 GPa, which is in the range of superhardness. It can be looked at as consisting of boron icosahedra-like crystals embedded in an amorphous medium. However, when studying the crystalline form of B4C, the hardness is only about 30 GPa. This crystalline form has the same stoichiometry as B13C3, which consists of boron icosahedra connected by boron and carbon atoms.[ Boron suboxide (B6O) has a hardness of about 35 GPa. Its structure contains eight B12 icosahedra units, which are sitting at the vertices of a rhombohedral unit cell. There are two oxygen atoms located along the (111) rhombohedral direction.]
Nanostructured superhard materials
Nanosuperhard materials fall into the extrinsic category of superhard materials. Because molecular defects affect the superhard properties of bulk materials it is obvious that the microstructure of superhard materials give the materials their unique properties. Focus on synthesizing nano superhard materials is around minimizing microcracks occurring within the structure through grain boundary hardening. The elimination of microcracks can strengthen the material by 3 to 7 times its original strength. Grain boundary strengthening
In materials science, grain-boundary strengthening (or Hall–Petch strengthening) is a method of strengthening materials by changing their average crystallite (grain) size. It is based on the observation that grain boundaries are insurmountabl ...
is described by the Hall-Petch
In materials science, grain-boundary strengthening (or Hall–Petch strengthening) is a method of strengthening materials by changing their average crystallite (grain) size. It is based on the observation that grain boundaries are insurmountable ...
equation[Bouchaud, Elisabeth; Jeulin, Dominique and Prioul, Claude (2001]
''Physical aspects of fracture''
Springer. . p. 23
:
Here σc is the critical fracture stress, d the crystallite size and σ0 and kgb are constants.
If a material is brittle its strength depends mainly on the resistance to forming microcracks. The critical stress which causes the growth of a microcrack of size a0 is given by a general formula[
:
Here E is the ]Young's modulus
Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied leng ...
, kcrack is a constant dependent on the nature and shape of the microcrack and the stress applied and γs the surface cohesive energy.
The average hardness of a material decreases with d (crystallite size) decreasing below 10 nm. There have been many mechanisms proposed for grain boundary sliding and hence material softening, but the details are still not understood. Besides grain boundary strengthening, much attention has been put into building microheterostructures, or nanostructures of two materials with very large differences in elastic moduli. Heterostructures were first proposed in 1970 and contained such highly ordered thin layers that they could not theoretically be separated by mechanical means. These highly ordered heterostructures were believed to be stronger than simple mixtures. This theory was confirmed with Al/Cu and Al/Ag structures. After the formation of Al/Cu and Al/Ag, the research was extended to multilayer systems including Cu/Ni, TiN/VN, W/WN, Hf/HfN and more. In all cases, decreasing the lattice period increased the hardness.[ One common form of a nanostructured material is ]aggregated diamond nanorod
Aggregated diamond nanorods, or ADNRs, are a nanocrystalline form of diamond, also known as nanodiamond or hyperdiamond.
Discovery
Nanodiamond or hyperdiamond was produced by compression of graphite in 2003 by a group of researchers in Japa ...
s, which is harder than bulk diamond and is currently the hardest (~150 GPa) material known.
See also
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References
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