Legendary Swords' Sharpness, Strength From Nanotubes, Study Says

Mason Inman
for National Geographic News
November 16, 2006
New studies of Damascus swords are revealing that the legendary blades contain nanowires, carbon nanotubes, and other extremely small, intricate structures that might explain their unique features.

Damascus swords, first made in the eighth century A.D., are renowned for their complex surface patterns and sharpness. According to legend, the blades can cut a piece of silk in half as it falls to the ground and maintain their edge after cleaving through stone, metal, or even other swords.

But since the techniques for making these swords have been lost for hundreds of years, no one is sure exactly why these swords are so exceptional.

Now studies of the swords' molecular structure are uncovering the tiny structures that may explain these properties.

Peter Paufler, a crystallographer at Technical University in Dresden, Germany, and his colleagues had previously found tiny nanowires and nanotubes when they used an electron microscope to examine samples from a Damascus blade made in the 17th century.

Today in the journal Nature, the teams reports that it has also discovered carbon nanotubes in the sword—the first nanotubes ever found in steel, Paufler says.

The nanotubes, which are remarkably strong, run through the blade's softer steel, likely making it more resilient. (Related: "Nano-Switches Could Yield Even Smaller Gadgets" [August 16, 2005].)

"It is a general principle of nature," Paufler said. "Materials that are softer, you can strengthen by including harder wires."

Secret Techniques

Some of the nanowires Paufler and his team had previously found were made of an extremely hard iron-based mineral called cementite.

In the new research, the team discovered that carbon nanotubes encase some cementite nanowires, protecting them.

These nanotube-nanowire bundles may give the swords their special properties, Paufler says.

The bundles run parallel to the blade's surface and may help larger particles of cementite arrange in layers. These hard layers, which have softer steel in between, could help explain how the steel remains strong yet flexible.

This combination of strength and flexibility makes the steel ideal for forging swords.

The blades were generally made from metal ingots prepared in India using special recipes, which probably put just the right amount of carbon and other impurities into the iron (India map).

By following these recipes and following specific forging techniques, "craftsmen ended up making nanotubes more than 400 years ago," Paufler and his colleagues write.

When these blades were nearly finished, blacksmiths would etch them with acid. This brought out the wavy light and dark lines that make Damascus swords easy to recognize.

But it could also give the swords their sharpness, Paufler says. Because carbon nanotubes are resistant to acid, they would protect the nanowires, he theorizes.

After etching, many of these nanostructures could stick out from the blade's edge, giving it tiny saw-like teeth.

Skeptical Smiths

The techniques for making the steel were lost around A.D. 1700. But many researchers are studying how to recreate the blades—even though metallurgical experts warn that the blades, though exceptional for their time, are far outperformed by modern steels.

While some scientists have claimed success, others dispute that the reproductions are truly the same as the originals.

And many experts doubt that the new findings will clear things up.

John Verhoeven, a metallurgist at Iowa State University at Ames who has worked on reproducing the Damascus sword-making techniques, is skeptical that Paufler and his colleagues have cracked the secret of Damascus blades.

"I don't think that [the nanowires] are anything unusual," Verhoeven said. "I think those structures would be found in normal steels."

The Damascus sword is also an example of how unexpected nanosize structures can show up in materials—and sometimes give them surprising properties, experts say.

But not all these nanoproperties are good. Asbestos, for example, comes in needle-like particles that cause severe lung disease. Break these particles into shorter pieces, and they much less harmful.

Because of nanomaterials' unpredictable behavior, several researchers asked in an article published today in Nature for more studies of these materials and their potential side effects.

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