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A story of diamonds
P.Shankar and Baldev Raj
Metallurgy and Materials Group
IGCAR, Kalpakkam 603 102
1.0 �Diamonds� � The Indian Pride
For centuries diamonds have captured the hearts and minds of millions, including
scientists. For most, the word diamond immediately relates to a brilliant gem, wealth, status and/or
prosperity. For scientists, diamond is known as one of the strongest and most chemically inert
material. The very word diamond is derived from the Greek word adamos, meaning the
unconquerable. However, diamonds have been known to India even much before it derived this
Greek name!!
The story of diamonds transcends many cultures and eras and is about three billion years
old, about two-thirds the age of earth. Hence, for some it is just a matter of pride to own an object
as old as three billion years, and that which has seen the rise and fall of many civilizations and
kingdoms!! However, the first gems of diamond were found in India as alluvial deposits only about
2800 years ago. When the first diamond stones were found, it was not particularly regarded for its
brilliance. However, it was found to be extremely hard and impossible to break with known tools of
that generation. It was also found to withstand attack of most chemicals, arising the curiosity and
popular faith attributing these mysterical stones to manifestation of GOD with magical powers. The
earliest known reference to diamond is in a Sanskrit manuscript, the Arthasastra (the lesson of
profit) by Kautiliya, a minister to chandragupta of the mauryan dynasty in northern India, written
about 2300 years ago [1]. Diamond has been referred to as Vajra or Indrayudha in sanskrit.
Diamond's supreme hardness was recognized by Indians In a 6th century text on gems called
�Ratnapariksa� the hardness of diamond has been realized and quoted as "The gems and the
metals that exist on earth are all scratched by the diamond: the diamond is not (scratched) by
them. A noble substance scratches that which is noble and that which is not; the diamond scratches
even the ruby. The diamond scratches all and is not scratched by any."
The Indian alluvial deposits were virtually the only supplies of diamond in the world for
many centuries, contributing a few world famous gems like �The Great Mogul� and the
�Kohinoor�. In a second century B.C. manuscript it has been reported that the Chinese had
realized diamond�s �industrial� use stemming from its unequalled hardness and were importing
diamonds from India [2]. It has been estimated that India's entire production, during a period of
about 2500 years, was mere 10 million carats or an average of 4000 carats per annum, while
diamond sales now exceed 10 million carats per annum. The earliest record of diamond-polishing
(with diamond powder) is also Indian, and probably dates from the fourteenth century. There are
also contemporary references to the practice of diamond polishing in Venice. Infact till about the
18th century diamond was not popular for jewellery. Only after the discovery of cutting diamonds
into facets in about the 15th century and with development of skills in making jewellery with these
gems, did it become much sought after by the public since 18th century. With dwindling of diamond
sources in India particularly from 18th century, there had been a search for other sources and led to
discovery of diamonds in Brazil, South Africa, western Canada, Australia etc.
Recent investigations show evidences of nano diamonds in meteorites of pre-solar origin.
Diamond particles, though of less than 1% volume fraction, were reported in 1969 by Saslaw and
Gaustad [3]. These diamond particles and the kind of impurities and structure, can provide valuable
information and as validation to the many theories on interstellar origin. Some of these stellar
diamonds are therefore even older than our planet Earth. Diamond particles have also been found to
form deep under the earth�s crust by continental collisions and plate techtonics, for example in
Kazaksthan [1].
2.0 Virtues Of Diamond
Apart from being the hardest known naturally occurring material till presently, diamond also
enjoys several other superior host of properties. Diamond has very high wear resistance and a low
coefficient of friction. Diamond is also inert to most chemical environments and hence diamond
coatings can be used for corrosive and/or corrosion-erosion applications as well. It has the highest
know bulk modulus (~1200 GPa) and the highest thermal conductivity at room temperature. It has
optical transparency over large range of spectrum, from deep UV to far IR region of electromagnetic
spectrum. It has very high room temperature electrical resistivity of about 1016 Wcm, but can be
made a semi-conductor by doping with elements like boron. It has very low thermal expansion
coefficient at room temperature. Diamond also has low or negative electron affinity. It is also a
bio-compatible material. By virtue of these commendable properties, diamonds have a potential for
wide ranging applications for wear and corrosion resistant applications, field emission and electronic
devices, heat sinks, optical windows etc [4]. However the high costs of natural diamonds severely
restricts their application potential.
3.0 Synthetic Diamonds
The origin of the science of synthetic diamonds perhaps owes to a startling discovery by
Smithson Tennant in 1766. He proved that diamond is also another pure form of carbon, by
discovering that combustion of diamond produced the same volume of carbon dioxide as the
expected from the same mass of carbon. It was a costly experiment indeed !!, but with equally
profiting outcomes in the future !!! It should be recognized that this discovery by Tennant was truly
remarkable, considering the fact that at that period of time, it was almost impossible for people to
believe that the hardest material on earth like diamond is made of the same carbon that constitutes
the soft graphite. This caught the imagination and curiosity of scientists to synthesis solid carbon
with diamond structure.
3.1 High Pressure High Temperature (HPHT) Synthesis of Synthetic Diamonds
With the realization in the nineteenth century that natural diamonds are produced deep
under the earths crust under conditions of high temperature and high pressure, scientists started
experiments to imitate this condition in laboratories. The first success in synthesising diamond
particles by HPHT probably belongs to Swedish in about 1953. However these results were not
published. The technique was pioneered and patented in 1955 by General Electric, USA for
producing a few small crystallites of diamond of about few millimetres in diameter by HPHT.
It can be seen from the carbon phase diagram (Fig.1) that the diamond structure is stable
only at very high temperatures and pressures. For conversion of graphite into diamond structure, a
pressure of about 15-20 GPa and temperatures of about 3000 K would be normally required.
However by catalytic methods involving liquid metal solvents, HPHT synthesis of diamond has been
achieved at pressures of about 7-10 GPa and at temperatures of less than 2000K . This pressure can
be equated to a Eiffel tower (7000 metric tonnes of steel) resting on a 5 inch plate!! .
Such high pressures and temperatures are
needed for conversion of graphite into diamond
because of the high activation barrier involved.
Although at room temperature, the free energy of
graphite is lower than that of diamond by only about
2.9 kJ/mol, the high activation energy (~ 728
kJ/mol) is mainly because there are no other
metastable structures to aid the transformation of
graphitic to diamond structure. Conversion of the
graphitic to diamond structure, would require
breakdown of significant number of the covalent
bonds in graphite necessary to destabilise the
structure and rebonding into sp3 hybridisation. Due
to the high binding energy of the carbon covalent
bonds, the activation energy associated is also high.
For the same reason, once diamond is formed,
though metastable from thermodynamic
considerations, it is kinetically stabilised by the high
activation barrier. At room temperature and
atmospheric pressure, it requires about 1090 years
for spontaneous transformation of diamond into
graphite.
Fig.1: Phase diagram of carbon
3.2 The CVD Process
In contrast to the HPHT technique, diamond films are synthesized at near atmospheric pressures
and at temperatures below 1000 oC by chemical vapor deposition [6]. This is because unlike HPHT
route, there is no conversion of graphite to diamond in CVD process. CVD involves direct deposition
of a carbon film from an activated carbon radical source and optimizing the parameters to stabilize
the growing carbon film with a diamond structure. Ever since the first discovery of CVD techniques
to synthesise diamond in mid 1950�s by Russians, there has been a spurt of activities in Japan and
US. In 1962, Eversole improvised the CVD method to obtain good quality diamond with low graphitic
phases. Angus et al., further made significant contributions to the CVD diamond technology and also
elaborated the role of hydrogen in preferential etching of graphite in-situ during the deposition
process. However it was only in 1981 that the first success of depositing diamond on non-diamond
substrates was reported by Spitsyn et al. Ever since, there has been a remarkable boost in
deposition of diamond over several substrates in the last two decades and also in improvising the
deposition techniques for obtaining better quality diamonds films with higher deposition rates [7,8].
Fig.2: A schematic of a typical hot filament CVD diamond reactor
Chemical vapor deposition of diamond requires
significant levels of gas phase activation to facilitate
deposition of diamond on the solid substrate. Gas
phase activation can be achieved by several
different routes like I) thermal (hot filament CVD),
ii) plasma (DC, RF, microwave, etc) or iii)
combustion CVD using oxyacetylene flame. A
typical schematic of a hot filament CVD reactor
(HFCVD) is shown in Fig.2. The resultant diamond
films are normally micro-cystalline or
nano-crystalline depending on the deposition
conditions. The most important deposition
parameters influencing the diamond film quality
(ratio of sp3 to sp2 bonded carbon in film),
microstructure and defect state are the filament
temperature, substrate temperature, distance
between filament and substrate surface, gas phase
composition, gas pressure and flow rate. Apart from
grain boundaries, the other most significant types of
defects in CVD diamond films are vacancies,
impurities like nitrogen and hydrogen, micro twins
and multiply twinned particles (MTP) with 5-fold
symmetry as can be seen from Fig.3.
The film deposition rate is normally in the
range of 0.1 �10 microns/hour in HF CVD
to about 100-500 microns/hour in flame
combustion technique. The diamond films
are generally predominant with {111}
facets, since this is the slowest growing
plane. However, by suitable modification of
the deposition parameters, square {100}
facet or rectangular {110} facet can be
achieved, and can result in smoother films.
Factors such as increasing methane
concentration in gas phase, higher substrate
deposition temperatures, higher chamber
pressure and doping with nitrogen, have
found to promote {100} facets in
preference to {111} facets.
be seen.
Fig.3: Typical microstructure of a CVD diamond film
deposited over a molybdenum substrate. The presence
of microtwins and multiply twinned particles (MTP) can
4.0 Application Prospects of CVD Diamond Films
As has been discussed in the earlier section, diamond has many unique combinations of
properties. With the discovery of processes for depositing synthetic diamond films by means of
chemical vapour deposition (CVD), it has become possible to commercially utilise many of the
advantageous and unique properties of diamond for engineering applications[8]. Diamond has a
very high thermal conductivity, roughly four times superior to that of copper, and it is an electrical
insulator: It is therefore being developed for applications in laser diodes and VLSI coatings.
Particularly for higher speed operations, coatings of diamond serving as heat sinks can be very
essential for long life and reliability of the integrated circuits. By virtue of their very high hardness
and wear resistance, they are employed as coatings on cutting tools for machining non-ferrous
metals, plastics and composite materials. CVD diamond coated drill bits result in significant
extension of service life, better surface finish and faster cutting operation. Diamond films also have
extreme chemical resistance in many acid solutions and exhibit electrochemical stability over a wide
range of potentials. Hence diamond film coatings can find potential applications for corrosion and
erosion-corrosion protection. By virtue of their good electrochemical properties CVD diamond coated
electrodes are being used for many applications in electroanalytical, electrowinning and other
electrochemical processes. Diamond coated electrodes have for example been used in synthesis of
ammonia from a nitrate solution. CVD diamond coated electrodes are also used for many other
applications like detection of several toxic gases, pollution control by oxidation of organic
compounds into CO2 at diamond surface in treatment of waste effluent from industries, bio sensors
etc. CVD diamond coatings are also extremely biocompatible and hence diamond film coated bio
implants are expected to result in improved performance. CVD diamond coatings are also used as
protective coatings for IR windows. Free standing diamond films are being considered for future
applications of IR windows. When hot, corrosive and abrasive environments make standard crystals
unacceptable, CVD diamond windows have a long useful life. CVD diamond's high thermal
conductivity minimizes distortion of windows in high power applications such as CO2 laser output
windows.
By virtue of the negative electronic affinity of diamond, they are also very good materials
for field emission applications like flat panel displays. Unlike liquid crystal displays, diamond cold
cathode emission displays would have high brightness, have a large viewing angle, and most
importantly, have the ability to be scaled up to large dimensions. Miniature x-ray sources for
medical and space applications have been developed based on CVD diamond films. CVD diamond
films are being employed for several electronic device applications, like piezoelectric effect devices,
radiation detectors and field effect transistors.
5.0 Hot Filament CVD diamond deposition on Steel
Steels are one of the most widely used engineering materials of today. Hence coating with
ultrahard and ultra-chemical resistant coatings like diamond can help to widen the application
potential for tribological, corrosion and erosion-corrosion resistant application as well. The main
hindrances to deposition of uniform and adherent coating of diamond onto steels results from (i) the
catalytic nucleation of graphite instead of diamond and (ii) very large differences in thermal
expansion coefficient between diamond and steel, resulting in high thermal stresses and higher
propensity for delamination of films during cooling. Three different type of interlayers, namely CrN,
Si and borided steel have been used by the present authors to achieve an uniform coating of
diamond over tool steel and AISI type 316 stainless steel [9,10]. Uniform and adherent coatings
could be achieved of ferrritic tool steel with CrN and borided steels. However, with austenitic
stainless steel continuous film could only be achieved with a borided surface layer. Use of a silicon
interlayer resulted in a composite surface film consisting of diamond and carbides on both tool steel
and 316 stainless steel. The type of interlayer used was found to have a strong influence not only
on the continuity of the diamond films (Fig.4), but was also found to influence the thermal stress,
diamond crystallite size and defect density, adhesion and corrosion resistant properties.
(A)
(B)
Fig.4: A typical micrograph of a diamond film over a tool steel with (a) CrN interface, showing
continuous and adherent diamond film and (b) Si interface, revealing a diamond/carbide composite
surface film.
At present, there is also an active project at the Indira Gandhi center for Atomic Research,
Kalpakkam, to develop particle and radiation sensors with free standing diamond films for
applications in future Fast Breeder Reactors and reprocessing facilities.
6.0 Nanocrystalline Diamond coatings
By suitable modification of the deposition parameters nanocrystalline diamond films with
crystallites in the size range of about 100nm have been deposited. Nanocrystalline diamond films
(NCD) also have very low coefficient of friction compared to microcrystalline films. Attempts are
being made to develop NCD films for MEMS (microelectromechanical systems). In comparison to
silicon films, diamond films are expected to have much improved wear resistance and hence
improved service life of the MEMS and microelectromechanical moving assemblies. It is therefore
expected to find wide range of applications if developed successfully. NCD is also a candidate
material for surface acoustic wave devices used for high-frequency telecommunications and are
expected to increase the speed of signal transmissions.
6.1 Summary
The birth of the science of synthetic diamond technology truly owes to the startling
discovery in 1776 by Tennant that diamond is solely a form of carbon and the discovery in 19th
century that diamond is formed deep in the earths crust. The first attempts were to imitate the
natures technology of synthesizing diamonds by HPHT methods. Although early successes in both
HPHT and CVD methods have been reported since mid 1950�s, there have been only limited
applications of these for industrial needs. However, in the last two decades there has been an
exponential increase in the research of CVD diamond films, with success in depositing diamond films
onto non-diamond substrates as well. This has led to opening up a wide market potential for this
technology. CVD diamond films are being considered for applications in multitude of fields, like
tribology, corrosion resistant coatings, electroanalytical and electrosynthesis electrodes,
bio-compatible coatings, optical windows, heat sinks, gas and particle sensors, field emission
devices, MEMS, etc.
There are perhaps more fascinating aspects yet to be realized from this amazing member of the
carbon family. A commercial firm by name �Life gem� can convert the carbon remains from the
ashes of the dead into precious diamond by HPHT technique for an affordable price of about 3000 �
4000 dollars. A novel way to remain materially immortal and to remain precious for the generations
following for a long time, indeed!!
6.2
1.
2.
3.
References
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www.astro.uiuc.edu/~akspeck/witch-stuff/Research/chapter3/node6.html
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5. S.Tennant, Phil. Trans. R. Soc. Lond., 87 (1797) 123.
6. Synthetic Diamond: Emerging CVD Science and Technology, Edited by K.E. Spear and J.P.
Dismukes (Wiley, 1994).
7. P.W.May, Endeavour Magazine 19(3), (1995) pp101-106.
8. P.W. May, Phil Trans. R. Soc. Lond. A, 358 (2000) 473.
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Diamond Relat. Mater., 11 (2002) 1760.
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J.J.Schermer,S.S.Ramakrishnan and J.J.ter Meulen, Thin Solid films, 426 (2003) 85.