To the scientist, the diamond
is impressive because of its wide range
of extreme properties. It is the hardest
known material, has the lowest coefficient
of thermal expansion, is chemically inert
and wear resistant, offers low friction,
has high thermal conductivity, is electrically
insulating and optically transparent from
the ultra-violet (UV) to the far infrared
(IR). Given these many notable properties,
it should come as no surprise to learn that
diamond already finds use in many diverse
applications including, of course, its use
as a precious gem, but also as a heat sink,
as an abrasive, and as inserts and/or wear-resistant
coatings for cutting tools.
Obviously, given its many
unique properties it is possible to envisage
many other potential applications for diamond
as an engineering material, but progress
in implementing many such ideas has been
hampered by the comparative scarcity of
natural diamond. Hence the long running
quest for routes to synthesise diamond in
the laboratory. So called 'industrial diamond'
has been synthesised commercially for over
30 years using >high-pressure high-temperature
(HPHT) techniques, in which diamond is crystallised
from metal solvated carbon at P~50-100 kbar
and T~1800-2300 K.
Some of the outstanding
properties of diamond
- Extreme mechanical hardness
Strongest known material, highest bulk
modulus (1.2 x 1012 N/m2),
- Lowest compressibility
(8.3 x 10-13 m2/ N)
- Highest known value of
thermal conductivity at room temperature
(2 x 103 W / m / K).
- Thermal expansion coefficient
at room temperature (0.8 x 10-6 K) is
comparable with that of Invar.
- Broad optical transparency
from the deep UV to the far IR region
of the electromagnetic spectrum.
- Good electrical insulator
(room temperature resistivity is ~1016
- Diamond can be doped
to change its resistivity over the range
10-106 O cm, so becoming a semiconductor
with a wide bad gap of 5.4 eV.
- Very resistant to chemical
- Biologically compatible.
- Exhibits low or 'negative'
World interest in diamond
has been further increased by the much more
recent discovery that it is possible to
produce polycrystalline diamond films, or
coatings, by a wide variety of chemical
vapor deposition (CVD) techniques using,
as process gases, nothing more exotic than
a hydrocarbon gas (typically methane) in
an excess of hydrogen. This CVD diamond
can show mechanical, tribological, and even
electronic properties comparable to those
of natural diamond. There is currently much
optimism that it will prove possible to
scale CVD methods to the extent that they
will provide an economically viable alternative
to the traditional HPHT methods for producing
diamond abrasives and heat sinks, whilst
the possibility of coating large surface
areas with a continuous film of diamond
will open up whole new ranges of potential
application for the CVD methods.