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Diamond is the hardest material known, because its
carbon atoms form very short covalent bonds, according to co-author
Richard B. Kaner, UCLA professor of inorganic chemistry and materials
science and engineering. Most of the diamond used in the world is
actually synthetic and very expensive. Diamond powder is used for oil
drills and machines that build roads and cut holes in mountains.
Diamond cannot be used, however, to cut steel without ruining the
diamond blade.
Cubic boron nitride is a diamond substitute used to
cut steel; it is made synthetically under very high-temperature,
high-pressure conditions, and is even more expensive than diamond,
Kaner said.
There are two ways to make super-hard materials
that are "ultra-incompressible," meaning they are resistant to shape
deformation, which is a necessary condition for hardness: One is to
imitate diamond by using carbon and combining it with boron or
nitrogen to maintain short bonds; the other is to look for metals that
are already incompressible and try to make them hard, said Kaner. He
and his colleagues are developing the second approach.
"Our idea is to combine an incompressible metal,
which happens to be soft, with short covalent bonds to make it hard,"
said Kaner, who is a member of the California NanoSystems Institute (CNSI)
at UCLA, which encourages cross-disciplinary collaboration to solve
problems in nanoscience and nanotechnology.
In 2005, Kaner's research team combined the
relatively soft element osmium, the most incompressible metal known,
with small covalent-bond forming atoms to make a material that is
almost as incompressible as diamond, yet is so hard that it scratches
sapphire, which is ranked 9 on a hardness scale of 1 to 10).
"We found that if we combine boron with osmium, we
push the osmium atoms apart by only 10 percent from where they were in
osmium metal, which is very good; you want to push them apart as
little as possible," Kaner said. "Then we searched through the
transition metals to see if we could do better than osmium, to get an
expansion of less than 10 percent. The only metal we could find that
had the potential for doing this is rhenium; hence, we made rhenium
diboride.
Rhenium is a fairly dense, soft metal, which is
next to osmium on the periodic table of chemical elements.
"We formed short covalent bonds, pushing the
rheniums apart by just 5 percent from where they were in rhenium
metal, making it both incompressible and very hard. The
rhenium-rhenium distance expanded by only 5 percent from the metal -
that's the key to this Science paper. Rhenium diboride is as
incompressible as diamond in one direction, and in the other direction,
just slightly more compressible."
At low applied forces, the hardness of rhenium
diboride is equivalent to cubic boron nitride, the second-hardest
material known, Kaner said. At higher applied forces, rhenium diboride
is a little bit below that.
"Our material is hard enough to scratch diamond,
and much harder than osmium diboride," he said.
While other super-hard materials, including diamond
and cubic boron nitride, are made under expensive, high-pressure
conditions, "our material is made in a simple process without applying
pressure," Kaner said.
Speaking of the collaboration, Kaner said, "The
reason I came to UCLA, and a reason I love this place, is because
whatever you do - in my own case, whenever you make a new material —
you often need equipment and expertise that you don't have. At UCLA,
there will be an expert in that area who has the equipment, and every
time I've asked, everybody is happy to help you do experiments and
excited to collaborate with you."
Despite the potential of new super-hard materials,
they are not likely to replace diamond any time soon, Kaner said. |
