Coveted nanocrystals found in shavings from machine shops

INDIANAPOLIS -- Scientists who have worked for years to find a way to mass-produce tiny, unusually hard crystals called nanocrystals should look no farther than their local machine shop, new research suggests.

Purdue University researchers say they found that metal shavings produced by lathes, drills and milling machines are filled with nanocrystals, which hold the promise of superstrong mechanical parts and composite materials.

The researchers said these shavings, now routinely discarded or melted down for reuse, could be a source of the crystals at a hundredth of the cost of producing them using current methods.

"We were surprised by what we found and the more we thought about it we wondered why no one had looked there before," said W. Dale Compton, an industrial engineering professor.

"But as a rule, people just aren't interested in these shavings."

Intrigued by research which found that causing intense strains in metals can create nanocrystalline structure, the Purdue team examined the shavings sheared away from metals by industrial cutting tools.

They found that shavings of a dozen metals, including copper, tool steel, stainless steel and iron, were entirely or primarily nanocrystals and up to three times as hard as the original metal.

The findings appear in the October issue of the Journal of Materials Research.

Shrinking crystals

When viewed under a microscope, metals have a grainy appearance because of granules of tiny crystals of various shapes.

Stretching or otherwise deforming a metal with intense strains breaks up this crystal structure, shrinking some crystals and making them stronger and harder than normal-size crystals.

Such strains occur when a hardened metal die is forced into a spinning metal rod at a 90-degree angle during the lathing process, engineering professor Srinivasan Chandrasekar said.

"It's like you're cutting cheese, you're literally forcing that tool into the material and it's being forced up in a highly compressed state," he said.

The same approach might work with silicon, yielding silicon nanocrystals for tiny transistors or other computer components, he said.

For the present, however, nanocrystals are not yet used to make products, and creating them is costly and time-consuming. One process involves heating metal until it vaporizes and then collecting the nanocrystals as they condense.

Although the Purdue team created their own crystal-laden metal shavings by using a variety of metals, the next step -- actually extracting the nanocrystals for industrial uses -- is far more daunting.

That's because to use them in products such as ball bearings or gears for cars and aircraft, the shavings would likely have to be pulverized and heated, a process that might well destroy the crystals' unique qualities.

Stephen Pennycook, a senior scientist at the Oak Ridge National Laboratory, said the Purdue team's research was intriguing and could prove important in the ongoing pursuit of nanocrystals.

"It's an interesting finding. Now they need to put some work into finding a way to extract the nanocrystals and to preserve their useful properties," he said.

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On the Net:

Journal of Materials Research: www.mrs.org/publications/jmr/