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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 www.amnh.org/exhibitions/diamonds www.chm.bris.acuk/pt/diamond/jamesthesis/chapter1.html www.astro.uiuc.edu/~akspeck/witch-stuff/Research/chapter3/node6.html 4. S.-T. Lee, Z. Lin and X. Jiang, Mater. Sci. Eng. Reports, 1999, 25:123. 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. 9. J.G.Buijnsters, P.Shankar, W.Fleischer, W.J.P.van Enckevort, J.J.Schermer and J.J.ter Meulen, Diamond Relat. Mater., 11 (2002) 1760. 10. J.G.Buijnsters, P.Shankar, P.Gopalakrishnan, W.J.P.van Enckevort, J.J.Schermer,S.S.Ramakrishnan and J.J.ter Meulen, Thin Solid films, 426 (2003) 85.