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You might have seen our recent Nanowerk Spotlight on modern military nanotechnology (Military nanotechnology - how worried should we be?) and read about the hundreds of millions of dollars that the U.S. military pours into nanotech research every year. Well, it turns out that metalsmiths in India perhaps as early as 300 AD, and presumably with a much lower budget, developed a new technique known as wootz steel that produced a high-carbon steel of unusually high purity. Wootz, which are small steel ingots, was widely exported and became particularly famous in the Middle East, where it became known as Damascus steel. This steel had extraordinary mechanical properties and an exceptionally sharp cutting edge. The original Damascus steel swords were made possibly as early as 500 AD to as late as 1750 AD. What's so interesting about this? It turns out that the secret of Damascus steel is carbon nanotubes. Recently discovered in the nanostructure of a 17th century Damascus saber, the nanotubes could have encapsulated iron-carbide (cementite) nanowires that might give clues to the mechanical strength and sharpness of these swords.
The critical characteristic of wootz steel is the abundant ultrahard metallic carbides in the steel matrix precipitating out in bands, making wootz steel display a characteristic banding on its surface. Wootz swords were renowned for their sharpness and toughness and the medieval crusaders, with inferior swords, learned this the hard way when they battled Muslim armies.
During steel making, impurities are added to control the properties of the resulting alloy. In general, notably during the era of Damascus steel, one could produce an alloy that was hard and brittle at one extreme by adding up to 2% carbon, or soft and malleable at the other, with about 0.5% carbon. The problem for a swordsmith is that the best steel should be both hard and malleable – hard to hold an edge once sharpened, but malleable so it would not break when hitting other metal in combat. This was not possible with normal processes. However, Damascus steel possessed these properties and it has become clear now that they are due to carbon nanotubes. European blacksmiths never managed to learn the secrets and copy the process.
It is still not quite clear how the carbon nanotubes formed inside the steel blades. Professor Peter Paufler at the Institute for Structural Physics at the Technical University of Dresden in Germany thinks it is a combination of the forging process and the impurities contained in the wootz steel. At high temperatures the impurities in the Indian ores could have catalyzed the growth of nanotubes from carbon.
Read the full article on the Nanowerk website.
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