This document discusses superhard materials, which are defined as having a Vickers hardness over 40 gigapascals. Diamond is currently the hardest known material, with a hardness of 115 GPa. Other superhard materials discussed include cubic boron nitride, carbon nitride, metal borides, and nanostructured forms. These materials have properties like high compressibility, elasticity, and electron density that give them extreme hardness. They have applications as abrasives, polishing tools, and protective coatings due to their unique mechanical properties.

1. SUPER HARD MATERIALS

2. INTRODUCTION
 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.

3. 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, 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 superhard
materials, but recent superhard material syntheses aim at
using less energy and lower cost materials.

4. Vickers hardness of selected hard
materials
Material Vickers hardness (GPa)
Diamond 115
c-BC2N 76
c-BN 48
OsB2 37
B4C 30
ReB2 ~20

5. DIAMOND
 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-centered
cubic (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.

6. SYNTHETIC DIAMOND
 The high-pressure synthesis of diamond in 1953 in Sweden
became a milestone in synthesis of artificial superhard
materials.
 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

7. RECENT STUDIES
 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.

8. CUBIC BORON NITRIDE
 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.

9. STRUCTURE AND PROPERTIES
OF C-BN
 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.

10. CARBON NITRIDE
 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.

11. BORON 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.

12. METAL BORIDES
 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

13. 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
 The elimination of microcracks can strengthen the material by 3
to 7 times its original strength.

14. RECENT DEVELOPMENTS
 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.

15.  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