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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