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ARTICLE
October 2004
U.S. Navy Finding New Applications For Advances In Nanotechnology
by Joe Pappalardo
The Navy is finding a slew of applications for enhanced materials arising from
advances in nanotechnology. Scientists believe these improved composites eventually
will help the Navy lower the cost of procuring and maintaining ships.
“The applications run from the mundane to solutions to intractable problems,”
said Larry Kabacoff, program manager for nano-structured materials for the Office
of Naval Research. “The return on investment of some of these things is
just staggering.”
Many of the Navy projects use new processes to imbue aluminum oxide and titanium
oxide into thin layers of ceramics, which produces substances that are tough,
adhesive and pliable.
“The problem with current ceramic coatings is not that they wear down,
but that they come off,” he said, referring to the tendency of ceramics
to crack and flake when damaged. “What is different is that [the new coatings]
stay where you want them to.”
The nano-ceramic coating is applied with a plasma spray, with miniscule particles
of aluminum dust mixed into the spray and shot at high temperatures onto a surface.
Some of the nano-sized particles group together upon hitting the surface, forming
30-micron sized balls that remain unmelted. These clumps are fixed under pressure
by the contraction of the material when it rapidly cools. The mix of melted
and unmelted portions makes a nano-composite that adds strength while keeping
the chemistry of the original material.
This invisible interaction gives the product its resistance to cracking, Kabacoff
explained. When the ceramic bends or is impacted, small cracks appear, but run
into the islands of unmelted, micron-size lumps. “It does crack, but the
cracks run into regions of micron-scale particles,” Kabacoff said. “Cracking
doesn’t get very far in our material.
“When a crack reaches the interface in our material, the tensile stress
at the crack tip is cancelled by the compressive stress in the material out
in front of the tip,” Kabacoff explained. “This is exactly how pre-stressed
concrete works. Metal rods are compressed while the concrete hardens. When a
load is put on the concrete, this stress just relaxes the pre-existing stress
rather than adding new stress.”
Kabacoff noted that these coatings also adhere to surfaces better than normal
ceramics, and have the ability to deform along with whatever it is covering.
This is a key point in weapons systems that are built to endure the abuses of
the environment and war, such as a submarine under a depth-charge attack.
The first real-world application of nano-enhanced ceramic coatings was as “a
fancy paint” for submarine doors that open underwater to deploy sensors
and arrays, Kabacoff said. The problem resolved the corrosion resulting from
a set of titanium doors that were bolted to steel frames and immersed in saltwater.
“When you put two dissimilar metals in seawater, you call that a battery,”
Kabacoff noted. “They used to paint the metal, but if it gets dinged you’re
in trouble.”
The solution was ONR’s nano-enhanced ceramic coating, which does not
provoke a galvanic reaction, is very adhesive and resistant to damage. It was
an undemanding application for the researchers, but a useful one.
A tougher problem plaguing Navy minesweepers required a more demanding application
for nano-ceramic coatings, a challenge the material appears to be meeting with
high marks, Kabacoff said.
Using iron or steel on minesweepers could be lethal, given the trigger mechanisms
of many underwater mines. To keep the signature low, propeller shafts are crafted
from bronze, which is considerably softer than steel. Problems arise when debris
collects under the vessel. This grit damages the hollow propeller shafts as
friction from the torque of as the shafts’ rotation wears through the
soft metal. Replacing these shafts requires dry-docking the boats.
Metal was out, and polymer coatings were judged to be too soft, Kabacoff said,
and even he doubted his shop’s ceramics would survive in such a dynamic
environment. “I didn’t think our coating would stay on,” he
said, but the shafts of two minesweepers were treated with the nano-ceramics
and headed for the Gulf of Mexico for trials.
The preliminary tests results were positive, and two other test ships were
outfitted for analysis. Underwater divers found no sign of failure—the
grit was no match for the enhanced material, and the adhesiveness withstood
the random, violent churning of the shafts. Once the vessels are taken out of
the water for a more thorough analysis, this technology is likely headed for
the field, Kabacoff predicted.
Other Navy systems with expensive problems have found solutions in nano-enhanced
ceramics. For example, the ball valves that regulate water flow in submarines,
part of the pumps that control diving and resurfacing, were suffering from metal-on-metal
friction. Replacing them required cutting through the submarine’s hull,
a costly proposition.
Researchers with A&A Company Inc., of Plainfield N.J., working with ONR,
decided that the problem could be approached with newly designed materials.
Scientists there took the aluminum-enhanced ceramic coating and added Teflon
into the pores. “The preliminary data is that the ball valves may last
the life of the ship,” Kabacoff said. “As new ones go in the coating
will be put on replacements.”
Better than expected performances come with savings, even on seemingly mundane
applications. Navy ships operate 80-ton air conditioners that are prone to breaking
down when grit or leaking coolants heat up and melt aluminum sleeves that house
vital gears. The aluminum welds to the shaft, and the replacement operation
can be estimated to cost nearly a half a million dollars a year, fleet-wide,
Kabacoff said. When replaced by an enhanced ceramic, the aluminum will not stick
and the units lose their costly flaw.
“There has to be an application pulling these things,” Kabacoff
said.
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