An indigenous technique of manufacturing carbon-carbon composites -- state-of-the-art materials of immense commercial and military importance.The manufacture of these composite has been till now a monopoly of the industrialed countries.
THE TECHNOLOGY for manufacturing carbon fibre reinforced composites -- state-of-the-art materials used in making products as diverse as missiles, spaceships, top-quality sports goods, and prosthetics -- have long been the monopoly of the industrialised nations. But not any longer, for National Physical Laboratory (NPL) scientists have indigenously developed the technology for these composites, also known as carbon-carbon (C-C) composites.
"This is an important breakthrough as there is an embargo by the industrialised nations on the transfer of this technology, chiefly because of its strategic importance. Even the availability of technical reports on this material is restricted," says O P Bahl, who led the NPL team.
The C-C composite technology is quite complex as it involves a "technology package consisting of many important subsystems", Bahl said. And, it is vital to master each of these subsystems for developing composites, he added.
Some of the NPL-developed subsystems are simpler, such as, for example, the process for making pitch, a sticky resinous black substance obtained by distilling tar or turpentine, which is used to make the composite denser and harder. Says T L Dhami, a member of the NPL team, "In contrast to the conventional processes, ours does not involve cumbersome techniques such as filtration or centrifugation and is capable of producing pitch with less quinoline insolubles (QI). QIs, he explained, are organic compounds that make the pitch hard, but this is undesirable because pitch should be soft enough to penetrate deep into the structure woven from carbon fibres.
NPL scientist G Bhatia, who specialises in pitches, says their new process is not only cheaper, but it can also produce special low-QI pitches that are essential for making graphite electrodes used in batteries as well as a high density-high strength graphite used in arc furnaces in steel plants.
NPL has also evolved a novel technique for producing C-C composites of high density, used in the manufacture of rockets and missiles. In the West, such high density composites are made using a different technology. Dhami said, "It involves pressures of the order of 1,000 bars (unit of pressure) and is not only complicated and expensive, but also not accessible to us in this country. Our new technique is much cheaper than that used in other countries". The NPL technique involves heating the composite to a temperature of 2,700 degrees Celsius to open up pores that can then be filled with pitch to further increase the density of the composite. The cycle can then be repeated until the desired density is achieved. Using this technology, a scaled-down, rocket nose-tip was made at NPL and tested successfully at the Defence Research Development Laboratory (DRDL) in Hyderabad.
The strength of a C-C composite depends to a great extent on the quality of the woven structure, which, in turn, is governed by the weaving pattern. The fibres can be woven in one or more directions, depending on the application of the composite. Development of an indigenous system, which has since been patented, for manually weaving the carbon fibres in three dimensions is a salient feature of NPL's carbon-carbon composites technology package, says Dhami.
Composites are created by joining two or more naturally occurring, dissimilar materials together to achieve functional capabilities superior to either individual constituent. Among carbon composites, C-C composites are par excellence because they are light, strong and high-temperature resistant. As such, they are perfect material for re-entry vehicles, such as the AGNI missile, light combat aircraft (LCA), and the space shuttles, which encounter temperatures of about 3,000 * C during re-entry into the atmosphere.
Nevertheless, these space-age materials are not without fault. One shortcoming of C-C composites is that they are brittle and, unlike plastics, break easily under stress. How brittle the C-C composite is depends on the bonding between the fibres and the matrix. "If the bonding is too strong, the composite will become very brittle, and if it's too weak, it will lose its strength," Dhami explained.
NPL studies indicate an optimum bonding strength can be achieved by proper choice of carbon fibres, size and weave pattern and top-quality C-C composites are best made with high stress-bearing fibres of a small size.
C-C composites are excellent high-temperature materials, but only if they are used in vacuum or inert atmosphere. Like other materials, they react with atmospheric oxygen to form oxides at temperatures as low as 500 * C. It is crucial, therefore, to protect them against oxidation upto 2000 * C if they are to be used in missiles and spaceships and this is done by coating the composite with ceramic materials such as carbides and nitrides.
NPL studied composite samples with silicon carbide (SiC) coatings and Dhami says the scanning electron microscope revealed cracks containing oxygen atoms that could oxidise carbon atoms. The NPL team developed a technology to fill these cracks with boron oxide.
The NPL technology, however, cannot be commercially exploited unless carbon fibres can be manufactured indigenously. For one thing, imported carbon fibres are very expensive; for another, the industrialised nations have put an embargo on their sale. Confronted by this grim scenario, Indian Petrochemicals Corp Ltd (IPCL) bought from a British firm the technology to manufacture carbon fibres at a cost of Rs 50 crore and set up a plant in Baroda in 1990. "Commercial production of carbon fibres hasn't begun yet, as it is still in the experimental stage. The annual turnover is about 12 metric tonnes," Dhami disclosed.
The government is also beginning to appreciate the commercial and military significance of carbon composites. A plan is underway to set up a joint-sector production unit involving Defence Research and Development Organisation (DRDO) and a private firm, to manage the Composite Production Centre (COMPROC) at Hyderabad. Furthermore, Technology Information Forecasting and Assessment Council (TIFAC) is spearheading a national programme for composite technology with a view to promote the use of composites in the civilian sector. A joint programme involving the Aeronautical Development Agency (ADA), DRDL and Defence Materials Research Laboratory (DMRL) is under way to explore the use of C-C composites as brake-pad materials for both civilian and military aircraft.
The modern age of composites was heralded by the invention of carbon fibres in the 1940s and the discovery by aircraft designers that they could produce a light, strong and stiff material with which to make airplanes. It spurred scientists to look for similar materials.
Carbon composites, or for that matter all composites of fibres of glass and kevlar, a strong synthetic fibre used in the manufacture of bullet-proof jackets, are ecology friendly.
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