Fifth element makes the ultimate material

Tom Shelley reports on a long sought for material that could make the ultimate bearings and has potential for extraordinary structures and electronics

A fanciful version of the September 1999 Eureka magazine cover produced by a colleague with the author as an astronaut

Russian scientists are making thin films of a ‘fifth form of carbon’ – whose chain length is limited only by the thickness of the material film.

Nearly as hard as diamond, it can be laid down on metals, polymers and elastomers without cracking or exfoliating and appears to be the ultimate material for resisting wear.

It also has unusual electrical properties, with potential as a new semiconducting material. And, if a method can ever be found to make it in bulk without breaking the chains, its tensile strength should be an order of magnitude stronger than nanotubes, which can themselves be six times as strong as the strongest steel.

The new form of carbon has been given the name ‘Tetracarbon’ and consists of linear carbon chains perpendicular to the surface on which they are grown – a true carbon polymer. The linear carbon chains are densely packed and kept together by relatively weak Van der Waals forces. Materials scientists all over the world have long sought such a material, since it has been hoped that its tensile strength would be limited only by covalent bond strength. In metals, strength is limited by the movement of imperfections through the overall crystal structure. In most polymers, strength is limited by Van der Waals forces between long molecules. In DuPont’s ‘Kevlar’ aramid fibre, for example, strength is limited by hydrogen bonding. But if a material could be made that was limited only by the strength of short range covalent bonding, ultimate tensile strength would be much greater.

The best candidate has always been carbon, because of its ability to form four covalent bonds per atom, and the existence of diamond as the hardest substance. Some years ago, it was found possible to make tiny ‘nanotubes’ of rolled up graphite sheet. These use covalent bonds as the source of their strength and are said to have tensile strengths of at least 20GPa, or 20,000 N/mm2. (For comparison, the very strongest steel, patented cold drawn wire, has a maximum tensile strength of only 3,400 to 3,780 N/mm2). Although Tetracarbon has only so far been made as thin films, it could be even stronger than nanotubes along chain length.


Nanotubes

The other remarkable property, which gives it its bearing properties, is that it is highly compliant between the chains, making it a kind of super strength elastic velvet on the atomic scale. Distance between atoms along the length of a chain is 1.19 to 1.38Angstroms. Between chains, the distance is 4.8 to 5.03Angstroms. Distances between atoms in diamond are all 1.54Angstroms whereas in graphite, distances between atoms within each hexagon are 1.42Angstroms and between layers, 3.35Angstroms. Generally speaking, the shorter the bond length, the stronger the bond. The only material with a structure even approaching that of Tetracarbon is the carbyne form of carbon discovered in 1969, which also has chains of carbon atoms, but with vacant atom sites and bendings, greatly reducing its mechanical properties. Distance between chains in this case is 2.97Angstroms. Interest in carbyne is now mainly centred on its chemical properties since it seems to be somewhat reactive.


Typical structures of graphite (left) the most common form of carbon and diamond (right)


The structure of Tetracarbon

The new material was discovered by Professor Malvina Guseva, Dr Nikolay Novikov and Dr Vladimir Babaev. They were studying ion beam assisted plasma deposited carbon coatings with a view to improve adhesion and flexibility. They detected small amounts of the new form in 1983 and larger amounts in 1991. Realising its potential, they began targeted research in 1995.

Hardnesses of films of the new material are in the range 4,000 to 9,000 Hv. This compares with 10,000 Hv for diamond and up to 12,000 Hv for diamond-like carbon (DLC). It is worth remembering at this point, that it was Russian scientists who were the original discoverers of the techniques for vapour depositing diamond and diamond like carbon – technologies that have since been taken up all over the world. Bearing steel typically has a hardness of 650 to 850 Hv. One of the problems with diamond coatings for bearings is that as well as being hard it is also fairly brittle. Tetracarbon, on the other hand, is so easily stretched sideways that it can be laid down on elastomers.

In mechanical tests on silicone rubber, including elongations of 300 per cent and multiple deformations, the coating withstood the tests without visible damage. Neither cracks nor exfoliation could be seen. In adhesion tests, the new material was removed only with parts of the substrate material, so its adhesion to most substrates is higher than the strength of the substrate. The coating process is performed at temperatures of only 20 to 200 degrees C and so is applicable to polymers such as polyethylene and polyurethane as well as semiconductors, metals, glass and other materials.

In biomedical experiments, Tetracarbon shows ‘exceptionally’ low blood coagulation potential, improving the biocompatibility of medical implants. Among its most prospective medical applications are cadiovascular devices, orthopaedic and dental implants, contact lenses, interocular lenses, soft tissue implants, surgical needles and sutures.

It also shows unusual electrical properties. Electrical conductivity along the chains is much higher than in other directions. Using tunnel microscopy, gold particles are clearly visible through 270 Angstroms thick Tetracarbon film, thanks to electronic tunnelling along the vertical carbon chains. Film deposited on a copper plate has a resistance of about 1 Ohm, almost independent of coating thickness whether 160 or 2,000 Angstroms. It is also possible that it may be given different semiconducting properties by doping, and it is worth noting that there is much research in using doped diamond as a semiconductor as an alternative to silicon, because of its higher thermal conductivity and resistance to radiation and other hazards.

But in the very long term, its greatest potential is probably as a material of exceptional tensile strength. It is not so many years ago that the first carbon whiskers showed the promise of carbon fibre. Today, carbon fibre is the material of choice for sports equipment and fast cars both on and off the track. The potential of nanotubes and Tetracarbon is an order of magnitude greater. It then becomes possible to think of concepts such as tethered orbiting satellites, and lifts running up the tethers to carry materials to launch sites above the earth’s atmosphere.

Such artefacts are impossible to construct using conventional materials because they cannot support the weight of the necessary tens of kilometres of their own length. But the latest form of carbon puts such goals within the realms of possibility.

Design Pointers

* Adheres to must substrates including stainless steel, glass, polymers, and silicone rubber. Adhesion is higher than substrate strength

* Coating thickness can be up to 10Ám on hard substrates. Recommended thickness for most applications is 1,000 to 2,000 Angstroms. Coating deposition rate is 1,000 Angstroms per minute. Coated areas using present technology are 150mm x 150mm

* Wear resistance is superior to that of diamond like carbon

For more information on the material: Tetra Consult,or mcgltd@iol.ie

For more information on the space elevator idea: http://highways.net/affordablespaceflight/home.html

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