Fall 1996

Dr. Carl C. Koch: It's a Fine Line - Working in the Realm of the Nanoscale

To the average person, it may seem irrational to make an object like a sword stronger by breaking down the very material that composes it. But to Dr. Carl C. Koch, professor of materials science and engineering, doing so makes perfect sense.

Koch works in the realm of the nanoscale, producing minute crystalline structures from larger, coarse-grained materials (a grain is a single crystal with all the atoms lined up in the same direction). He explains his research by comparing it to that of the medieval artisans who crafted tough swords from Damascus steel:

"Damascus swords were also made by mechanically deforming the iron and the iron carbide by beating on it and refining the grain size of the carbide particles, making them very small. In a way, they were making materials approaching nanocrystalline grain structure," Koch said. "They weren't down to the nanoscale, but they were very fine grains, and that's what made these Damascus swords very hard and strong."

In 1983, Koch was the first scientist to make an amorphous metallic structure from two separate elements by mechanical alloying. Back then, he accomplished this feat with ball milling. These days, he's applying his expertise with this technique to nanocrystalline materials.

The grains in these materials are only about ten nanometers large-compare that to a red blood cell, which is about 5,000 nanometers large-but nanocrystalline materials can be five to ten times stronger than materials with conventional grain sizes. They also may have the ability to resist fracturing and may have better electrical, optical, chemical and magnetic properties.

One of the many ways to make nanocrystalline materials is to condense small clusters of atoms and consolidate them into a solid. Koch, however, takes the opposite approach, starting with a solid crystal and breaking it up to make large grains smaller. He literally beats a material until it is nanocrystalline.

Koch loads a powder along with ball bearings (or sometimes specially made balls of tungsten carbide or ceramics) between 1/4 and 1/2 inch in diameter into a small mill, which vibrates at about 1200 rpms. On a microscopic scale, powder particles get caught between two balls as they collide at very high velocities, repeatedly merging and breaking apart until the internal grain structure becomes very fine.

Koch's present National Science Foundation contract was awarded to him for examining the ductility of nanocrystalline materials. Koch said that current studies show mixed results in this area. His goal, then, is to draw more definitive conclusions about the brittleness of these materials.

"Brittleness is the main fault, if you will, of some materials. If one could make them nanocrystalline and increase the ductility, they might be useful," Koch said. "These are materials like intermetallic compounds for potential use in jet engines. The conventional superalloys have just about reached their limit of temperature use, so we look at other materials like ceramics, but they are very brittle."

Other potential uses for these materials include transformer cores, catalysts, electronics, high efficiency gas turbines, aerospace and automotive components, wear resistant coatings, and cutting tools for steel and other hard materials.

Though many scientists around the world are researching nanocrystalline materials, Koch's work is unique because he is using several synthesis methods to determine intrinsic ductility. In addition to ball milling, he uses two rapid solidification techniques-melt spinning and the arc/hammer-to first create an amorphous alloy and then heat the material to between 800 and 1000 degrees Fahrenheit to form the nanocrystals.

Once he and his team of graduate students get the results from tests on the materials produced by the various methods, he will compare them for commonalties that will help him determine the intrinsic ductility.

Other researchers have theorized an optimum grain size for ductility, but no one has yet provided data to back it up. Koch is performing the research necessary to test the theory. In particular, he is examining the properties as a function of grain size.

"We don't know yet how practical these stronger and harder materials are going to end up being," Koch said. "Right now most of it is still basic research."

Even so, the huge possibilities of these materials with almost inconceivably tiny grain sizes inspire Koch and his research team to continue beating up their materials.

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