WHAT material would you trust to haul an elevator into space? Such a capsule
shuttles back and forth between the Earth and a satellite in geostationary orbit
in Arthur C. Clarke鈥檚 book The Fountains of Paradise. Clarke鈥檚
futuristic space engineers rely on robust cables made from diamond, the
strongest of all known materials. But how do you spin a diamond fibre?
Researchers at the University of Bristol have the answer.
Since 1993, Peter Partridge of the university鈥檚 Interface Analysis Centre and
Paul May and his colleagues in the chemistry department have used a technique
called chemical vapour deposition (CVD) to form diamond films on metal wires.
They鈥檝e produced a fantastic range of fibre-like structures, from coaxial cables
with metal cores and diamond cladding to hollow diamond tubes (like the one
below) and crisscross lattices.
Diamond wires carrying passengers and equipment 36 000 kilometres up to an
orbiting space station are still in the realms of imagination, but Partridge
believes his creations could have a host of more down-to-earth applications. The
diamond wires could act as stiff reinforcement to produce the ultimate in
composite materials for aircraft, cars and sports equipment. Because diamond is
an excellent conductor of heat, the new composites should perform better at high
temperatures by rapidly smoothing away local concentrations of heat. Hollow
diamond fibres could even carry the sensory 鈥渘erves鈥� of smart materials that
respond to their environment.
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Diamond is a form of pure carbon, and for more than forty years people have
been making synthetic diamond by subjecting carbon-rich compounds to high
pressures and temperatures, mimicking the conditions under which diamond forms
naturally deep in the Earth. Diamond made this way is used on an industrial
scale, for example in saw blades and drill bits.
Radio exposure
By contrast, the CVD process that lies at the heart of the Bristol chemists鈥�
work takes place at low pressure, and involves heating a mixture of hydrogen and
carbon-rich gases such as methane and other hydrocarbons (see 鈥淟ittle Gems鈥�,
快猫短视频, 28 October 1995, p 36). The carbon atoms join up to form
diamond films on the hot surface of the material being coated. To speed up the
growth of films for commercial applications, the gases must be 鈥渁ctivated鈥� by
passing them over a hot filament or exposing them to powerful radio waves. This
breaks the molecules into small, highly reactive fragments that serve as
building blocks for the diamond films.
It might sound like a trick to rival turning base metals into gold, but
diamond wires and tubes made by the Bristol team look very different from the
sparkling gemstones that the word 鈥渄iamond鈥� normally brings to mind. They
consist of thousands of tiny diamond crystallites鈥攅ach one a miniature
gemstone about a micrometre across鈥攕tuck together in a haphazard array to
form a surface like sandpaper. Despite this disorder, the strong, short bonds
between carbon atoms both within and between the crystallites make these films
extremely hard.
Diamond films make good protective coatings for engineering components
because they are highly resistant to mechanical wear and chemical corrosion.
Another big attraction for engineers is that diamond is a lightweight material.
One of the great things about the CVD process is that the films do not have to
be flat, but instead follow the contours of the material on which they are
grown. Partridge and May have taken this characteristic to its limit to produce
their stiff, light wires and tubes.
To make diamond wires or fibres, you need a suitable material鈥攖he
substrate鈥攐n which to grow them. Partridge and May mainly use tungsten
wire, because it is stable even at very high temperatures, and forms a strong
bond with the diamond coating.
Not all materials will passively accept a diamond coat, though. Hot iron and
steel dissolve and react with carbon to form iron carbide. So if you try to grow
diamond on steel, the carbon is soaked up like ink on blotting paper. 鈥淭he
diamond diffuses straight away through the steel,鈥� says May. This is a major
limitation because many engineering components are made of steel and can鈥檛
benefit from a wear-resistant diamond film. To get round this, it may be
possible to coat steel with a protective coating of another substance before
laying down the diamond on top.
The Bristol team also use other metals, including titanium and molybdenum, as
the substrate for their wires and tubes, as well as nonmetals such as silicon.
These materials also form carbides when a diamond film is deposited, but the
carbon does not diffuse into the material as it does with iron and steel.
Instead, it produces a very thin carbide layer on the surface. Once this layer
has formed, the diamond film happily grows on top. The carbide layer acts as a
kind of buffer that prevents the diamond on top from reacting with the substrate
below. This difference is partly due to the different solubilities of carbon in
the metals, and partly because the carbon atoms are less mobile in many other
metals than they are in steel.
The diamond-coated wires made by Partridge and May might find a major
engineering use as reinforcing fibres in structural materials that need to be
both strong and stiff. Metals can be very strong but are not very
stiff鈥攖hey tend to bend rather easily. One way to improve this is to embed
stiff, whisker-like fibres inside the metal to give it a 鈥渂ackbone鈥�. Partridge
sees this as ideal territory for the diamond鈥搕ungsten fibres.
For applications like these, diamond fibres will be competing with silicon
carbide fibres, which are already used to stiffen metal鈥揻ibre composites.
These too are grown by CVD鈥攊n this case using silicon carbide鈥攐nto
thin tungsten wires. Metals and alloys stiffened with these fibres are used in
the aerospace industry to make lightweight components that need to stay strong
at high temperatures. Similarly, materials made by embedding stiff fibres of
graphite, another form of carbon, in a plastic matrix are widely used in racing
cars and in the shafts of tennis rackets.
Impressive though their properties are, silicon carbide fibres could be
eclipsed by diamond鈥搕ungsten fibres. Partridge says that his fibres are
about twice as stiff as silicon carbide fibres. And as the diamond coating grows
thicker, the stiffness of the fibres gets more impressive, eventually
approaching that of pure CVD diamond鈥攚hich is itself just a little lower
than that of natural diamond, the stiffest of all known materials.
The fibres themselves might be very stiff, but do they make better
composites? Partridge and his colleagues have put this to the test. They
reinforced a titanium alloy with diamond鈥搕ungsten fibres by first coating
the individual fibres with a thick layer of the alloy. The coated fibres were
then stacked side-by-side and compressed at high temperature, fusing the alloy
coatings into a continuous matrix.
Partridge predicts that this material will be more than 50 per cent stiffer
than a commercial composite of the same alloy laced with silicon carbide fibres,
and preliminary tests conducted in the past few months in a collaboration with
Britain鈥檚 Defence Research Agency at Farnborough, Hampshire, show that this
prediction may be borne out.
And as for the long-term future of the materials, Andr茅 Bormanis, a
scientific adviser to the makers of Star Trek, has suggested that a
diamond fibre鈥搕itanium alloy composite might be just what is needed to
build the starship Enterprise. 鈥淐onceivably, it might even meet the demands of
24th-century spacecraft designers鈥�, he says.
Meanwhile, the fibres might benefit 21st-century aircraft manufacturers.
Partridge says it is possible to make microheaters by passing an electric
current through the metal cores of the fibres鈥攄iamond is able to withstand
temperatures of up to 1500 掳C if not exposed to air. This approach could be
used to warm up localised regions of a material reinforced with the fibres
without passing an electric current through the matrix metal. Such a system
might prove invaluable in preventing ice forming on aircraft wings.
Partridge and May have not limited themselves just to making fibres. They
have also produced stiff sheets of the material, in which the diamond fibres
form a woven, trellis-like mesh. They do this by weaving individual tungsten
wires so they pass above and below each other as if in a fabric. The diamond
film produced by CVD then fuses the crossing points to make a rigid,
two-dimensional framework.
Self-healing materials
CVD diamond may be stiff, but it can also be brittle. 鈥淭he only drawback
[with these materials]鈥�, says Partridge, 鈥渋s that the diamond fibres snap quite
easily.鈥� The problem is that the surfaces of the polycrystalline films are
jagged, with lots of notches where cracks can start. Silicon carbide fibres, on
the other hand, can be grown with very smooth surfaces. The Bristol team is
exploring the possibility of smoothing the surface of its diamond fibres by
shaving the bumps off with lasers. 鈥淲e can produce very smooth surfaces this
way,鈥� says Partridge.
While this approach is effective, it may cost too much to be of commercial
use, so the brittleness of the fibres may continue to limit the strength of
diamond鈥揳lloy composites. Max Yoder of the Office of Naval Research in
Washington DC says that in any case standard graphite fibres have greater
tensile strength than diamond fibres鈥攖hey can withstand heavier loads. But
for stiffness, diamond should win out, he says.
One product of Partridge and May鈥檚 research, however, is completely without
parallel in other materials. By removing the substrate wire after CVD diamond
has been deposited on its surface, they can make hollow tubes of diamond. These
pipes, about as wide as a human hair, are incredibly robust. The method is
possible only because diamond is so resistant to corrosive chemicals. This
allows the researchers to use acids or other caustic chemicals to etch away the
tungsten core, leaving the cylindrical diamond coatings.
But there are limitations. Tubes much longer than a centimetre cannot be made
this way, says May, because the etching agent cannot penetrate far down the
diamond鈥搕ungsten wire to scour out the metal core. So he and his team have
devised a different approach for making long tubes. They make a coil of fine
tungsten wire by wrapping it around a straight central core, which they then
pull out. The resulting spring-like coil of tungsten is coated with diamond, and
as the diamond films grow on adjacent turns of the coil, they merge to create a
continuous hollow tube of diamond. May thinks it should be possible to make
tubes tens of centimetres long this way.
So what might these microtubes be used for? One attraction is that they are
about as stiff as the metal-filled fibres, but much lighter because the metal
has gone. More interestingly, the space left by the metals can be put to use.
The tubes might serve as ducts through a material, or could provide a pathway
for sensory information in the smart materials of the future.
May speculates that the tubes could be joined together to provide plumbing
systems for composite materials. For example, a coolant fluid or gas might be
pumped through the diamond channels to regulate the temperature of the
composite. Because diamond conducts heat extremely well, the diamond tubes would
be highly efficient at passing heat from the matrix material to the coolant.
Another idea is to fill the tubes with air-curing glue. So if the composite
fractured, and the tubes cracked open, the glue would spill out and heal the
rift. Researchers hope that self-healing materials will one day help to avert
accidents such as structural failures in aircraft and bridge collapses.
鈥淵ou could even put tiny devices up the tubes鈥�, says May, producing stiff
composite materials that could be controlled or monitored by a network of
microscopic instruments. Measuring temperature is one possibility: May points
out that thermocouples can easily be made small enough to fit down the centre of
the tubes. Strain gauges might fit too, allowing any shape changes in regions
where the material is under a lot of stress to be monitored continuously.
For now, the Bristol team seems to have the potential market in these
materials to itself. According to Yoder, the US Army had a programme five years
ago to make wear-resistant optical fibres by coating them with diamond, but 鈥渋t
never succeeded, it seems鈥�.
But the Bristol team can only take advantage of their monopoly if industry
shows an interest. May says that some British aerospace companies have chosen
not to use even well-developed silicon carbide fibre technology because it costs
too much. By comparison, diamond fibres could cost about three times as much.
For some tasks, however, nothing else will do. 鈥淭hese materials have fantastic
properties,鈥� May argues, 鈥渋f people are prepared to pay for it.鈥� Perhaps it will
take a starship Enterprise to get diamond fibres off the ground.