|
  
Nature Gives a Lesson in Armor Design
|
Professor of Materials Science Christine Ortiz,
left, examines a seashell with graduate student Benjamin
Bruet. Samples of the shell’s makeup appear on
the screen. Ortiz and Bruet are researching the natural
armor system used by shells. (Photo by Donna Coveney.)
|
CAMBRIDGE, Mass., September 21, 2005 – The ocean is
a perilous environment for a soft-bodied creature like a sea
snail, so nature gives it an advanced nanostructured armor
system that is stiff and strong yet lightweight. It's called
a shell.
Understanding the fundamental design principles of natural
armor systems like shells may help engineers design improved
body armor systems for humans in perilous situations, like
soldiers and police officers. At MIT's Institute
for Soldier Nanotechnologies, researchers are studying
the structure and mechanics of the tough inner layer of mollusc
shells, called "nacre" or mother-of-pearl, at extremely
small, nanometer-length scales (a nanometer is a billionth
of a meter).
In the September
issue of the Journal of Materials Research, Professor
Christine Ortiz of the Department of Materials Science
and Engineering, Professor
Mary Boyce of the Department of Mechanical Engineering
and doctoral student Benjamin Bruet of materials science report
their results. They show that nature is indeed an expert nanoengineer.
"The complexity we have observed in nacre at the nanoscale
is quite amazing and seems likely to be a critical determinant
of the toughness of the material," said Ortiz.
Nacre is composed of two relatively weak materials: 95 percent
calcium carbonate, a brittle ceramic, and 5 percent flexible
biopolymer. These materials are organized into a "brick-and-mortar"
structure with millions of ceramic plates, each a few thousand
nanometers in size, that are stacked on top of each other
like rolls of coins. Each layer of plates is glued together
by thin layers of the biopolymer. The MIT team has focused
its studies on small nanometer-sized regions of the individual
tiny plates.
"Even though the calcium carbonate is very weak and
brittle on its own, one can get enormous increases in toughness
through design at multiple-length scales," said Ortiz.
"Understanding how the material is designed and functions
at the smallest-length scales will be critical to learning
how to create tough biomimetic synthetic composites."
Replacing the weak building blocks of nacre with stronger
materials—in a similar design—has the potential
to yield much tougher composites for use in armor systems
or structural applications like automobile panels or plane
wings.
The MIT team began its experimental studies by imaging the
tiny plates cut from the nacre of Trochus niloticus,
a sea snail, using a powerful instrument called an atomic
force microscope. They found that each individual plate also
had its own complicated nanostructure and was divided like
a pie into separate sectors, with cylindrical beams running
through the thickness of the plates, a fine surface of nanosized
bumps, called nanoasperities, which were further organized
into groups, and biopolymer molecules, only about 1 nanometer
in height, traversing over and bound to the mountainous array
of nanobumps.
They then used a diamond-probe tip only a few hundred nanometers
in size in the Department of Materials Science and Engineering's
Nanomechanical Technology
Laboratory to push into the surface of an individual plate
(a technique called nanoindentation) while "feeling"
the force that resulted. "I was surprised to find that
the tablets were both extremely stiff and strong at these
length scales and that they resisted brittle crack formation
and propagation even at exceedingly high forces," said
Bruet.
Although scientists have studied the properties of nacre
at the macroscale and microscale, Ortiz says that very little
is known about its behavior at the nanoscale, which is where
structure and properties set the foundation for the material's
overall behavior.
The team is currently studying the nanoscale adhesion forces
that exist between the ceramic plates and flexible biopolymer
in the nacre, as well as the single molecule nanomechanical
properties of the biopolymer. This research may shed light
on the longstanding question of how to create durable interfaces
in synthetic composites that can withstand high forces in
water environments. Ortiz's group is also studying the nanostructure
and nanomechanical properties of other natural materials,
such as bone and cartilage.
"Nature uses nanoscale structural design principles
to produce materials with superior mechanical properties,"
said Ortiz. "In many aspects, human engineers have yet
to achieve the same skill. However, as nanotechnology methods
advance, the creation of artificial nacre—and other
kinds of high-performance armors—is becoming a more
and more realistic goal."
Contact:
Franklin Hadley
617-324-6413
fhadley@mit.edu
Back to Research News
|
