SUPER HARD MATERIALS

A superhard material is a material with a hardness value exceeding 40 gigapascals (GPa) when measured by the Vickers hardness test.

Highly 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, abrasives, polishing and cutting tools and wear-resistant and protective coatings.

INTRODUCTION

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, high bulk modulus and does not deform plastically.

Should have a defect-free, isotropic lattice. This greatly reduces structural deformations that can lower the strength of the material.

Traditionally, high-pressure and high-temperature (HPHT) conditions have been used to synthesize superhardmaterials, but recent superhard material syntheses aim at using less energy and lower cost materials.

DEFINITION AND MECHANICS OF HARDNESS

Material Vickers hardness (GPa)

Diamond 115

c-BC2N 76

c-BN 48

OsB2 37

B4C 30

ReB2 ~20

Vickers hardness of selected hard materials

Hardest known material to date, with a Vickers hardness in the range of 70–150 Gpa

Diamond demonstrates both high thermal conductivity and electrically insulating properties.

Diamond is an allotrope of carbon where the atoms are arranged in a modified version of face-centeredcubic (fcc) structure known as "diamond lattice".

Diamond has several limitations for mass industrial application, including its high cost and oxidation at temperatures above 800 °C.

Diamond dissolves in iron and forms iron carbides at high temperatures and therefore is inefficient in cutting ferrous materials including steel.

DIAMOND

The high-pressure synthesis of diamond in 1953 in Swedenbecame a milestone in synthesis of artificial superhardmaterials.

Four years after the first synthesis of artificial diamond, cubic boron nitride c-BN was obtained and found to be the second hardest solid.

Synthetic diamond can exist as a single, continuous crystal or as small polycrystals interconnected through the grain boundaries.

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

SYNTHETIC DIAMOND

It has recently been reported that HPHT single crystals and nanocrystalline diamond aggregates (aggregated diamond nanorods) can be harder than natural diamond.

Nitrogen doping can enhance mechanical strength of diamond,[22] and heavy doping with boron (several atomic percent) makes it a superconductor.

RECENT STUDIES

First synthesized in 1957 by Robert H. Wentorf.

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 metals or their nitrides, followed by spontaneous nucleation of c-BN under high pressure, high temperature (HPHT) conditions.

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.

CUBIC BORON NITRIDE

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.

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

The thermal conductivity of BN is among the highest of all electric insulators.

STRUCTURE AND PROPERTIESOF C-BN

The structure of carbon nitride (C3N4) was proposed in 1985.

Predicted to be harder than diamond and less hard than c-BN

Shear modulus is 60% of that of diamond

Difficult to synthesis and unstable.

Carbon nitride is only stable at a pressure that is higher than that of the graphite-to-diamond transformation.

C3N4 would pose problems of carbide formation if they were to be used to machine ferrous metals.

CARBON NITRIDE

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.

also possible to make compounds containing B-C-O, B-O-N, or B-C-O-N under high pressure

They are expected to be thermally and chemically more stable than diamond, and harder than c-BN

Used for high speed cutting and polishing of ferrous alloys.

BORON CARBON NITRIDE

Metal borides can be easily synthesized in large quantities under ambient conditions, which is an important technological advantage.

Examples:RuB2, OsB2 and ReB2

The extensive covalent B-B and M-B bonding (M = metal) leads to high hardness

Metals such as osmium, rhenium, tungsten, etc. are desirable due to the high electron density, small atomic radius, high bulk modulus, and highly controlled directional bonding with boron.

The M-B bond contributes to this due to the overlapping of the transition metal d states and boron p states

METAL BORIDES

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 superhardmaterials

The elimination of microcracks can strengthen the material by 3 to 7 times its original strength.

NANOSTRUCTURED SUPERHARD MATERIALS

Ultra Strength: Scientists Develop Method to Produce Material Twice as Hard as Diamond.

Researchers have developed a new method to synthesise fullerite, the hardest material on earth.

RECENT DEVELOPMENTS

SAPPHIRE GLASS

Just like diamonds, sapphire can be

made synthetically — in fact, the first synthetic

sapphires were made back in 1902. The natural compound aluminum oxide is ground into

a powder, then heated to at least 3,600 °F.

The iPhone 5, in fact, makes use of sapphire glass in its camera lens, which makes it virtually unscratchable.

sapphire glass is more expensive than

Gorilla Glass