<VV> Metals Technology (No Corvair)

[Jay Pitchford]

Materials science: Researchers have devised an ingenious way for the
damaged surfaces of metals to repair themselves when they come to
harm.

SADLY for engineers, inanimate objects cannot yet repair themselves.
But work by Claudia dos Santos at the Fraunhofer Institute for
Manufacturing Engineering and Automation, and Christian Mayer at
Duisburg-Essen University in Stuttgart, has brought the day when they
will be able to do so a little nearer. They and their colleagues have
invented a way for damaged metals to heal themselves.

The surfaces of many metal objects are coated with other metals for
protection. Iron, for instance, is frequently galvanised with zinc.
The basic idea of the new technology is to infiltrate this coating
with tiny, fluid-filled capsules. When the metal coating is punctured
or scratched, the capsules in the damaged area burst and ooze
restorative liquids, in the form of compounds called trivalent
chromates. These react with nearby metal atoms and form tough,
protective films a few molecules thick to ameliorate the damage.

The idea of doing this has been around for years, but it has proved
difficult in practice because the capsules used were too big. Surface
coatings tend to be about 20 microns thick. The capsules were 10-15
microns across—large enough to disrupt the coatings, and thus do more
harm than good. The trick worked out by Dr dos Santos and Dr Mayer is
how to create capsules a few hundredths of this size.

The capsules the researchers have come up with are made by mixing
butylcyanoacrylate, a chemical found in superglue, with an oil
carrying the healing compounds. This mixture is then mixed with dilute
hydrochloric acid. The result is an emulsion of droplets between 100
and 300 nanometres across. Each droplet has an oil core surrounded by
a thin layer of butylcyanoacrylate. To make the droplets stable,
phosphate is added to the emulsion. This triggers the polymerisation
of the butylcyanoacrylate into a tough plastic, which forms the
outside of the capsule.

The greatest challenge for the team, however, was not making the
capsules in the first place, but stabilising them during the plating
process. Though galvanisation is often done by dipping steel in liquid
zinc, it is sometimes done by electrolysis—nickel and copper plating
are normally done this way. The capsules, though, tend to stick
together in the liquids used as electrolytes during electroplating,
and are also destroyed by the extreme acidity or alkalinity that is
often involved in the process. To overcome these problems, Dr dos
Santos and Dr Mayer used special detergents that stick to the
polymerised butylcyanoacrylate shell around each capsule, which stops
them sticking together and protects them from the electrolytes.

The researchers have now proved their techniques in electroplated
layers of copper, nickel and zinc, and believe that self-repairing
metals should commonly be available in the years ahead. Moreover,
their nanocapsules may have other applications. Lubricants such as
silicone oils can be included in them, to make the damaged surfaces of
ball-bearings that have run out of oil more slippery, so that they are
not scratched too rapidly. Anti-fouling compounds can be placed in
capsules on the surfaces of metals intended for use in marine
environments. And, in a nod to butylcyanoacrylate’s origins in
superglue, capsules containing chemicals that will react to form
adhesives when two surfaces are put together are also on the horizon.

The Economist Technology Quarterly