Meddling with Metal: Novel nanocontrol yields chromium rival

A legal battle launched in 1993 over toxic chromium metal became the basis for the movie Erin Brockovich, which featured superstar Julia Roberts. Now, materials scientists have quietly taken aim at one common use of that harmful substance by creating a nontoxic alloy with the potential to replace a coating containing chromium.

AGAINST THE GRAIN. Computer simulations show how extra tungsten in a nickel-tungsten alloy (left) would lead to crystal grains (blue chunks) 2 nanometers across, whereas less tungsten (right) would yield 4-nm grains. Schuh

Costarring in these laboratory developments is a new method for making alloys. With it, scientists can dictate the sizes of nanoscale crystals in an alloy’s structure—and therefore the alloy’s properties—by manipulating its atomic composition. Christopher A. Schuh of the Massachusetts Institute of Technology (MIT) and his colleagues have applied the method to alloys composed mainly of nickel.

The scientists set out to create nontoxic coatings as hard and corrosion resistant as the chromium layer applied to steel. The so-called hexavalent form of chromium—used in those coatings and also in paints and dyes—poses a cancer risk to more than a half-million U.S. workers.

The Occupational Safety and Health Administration on Feb. 28 slashed the permissible concentration of hexavalent chromium in workplace air by a factor of 10.

To make their nontoxic coatings sturdy, Schuh and his coworkers applied a well-known strategy: creating nanoscale crystalline grains in the materials. To do this, other researchers have rolled and pounded metals to reduce grain size (SN: 8/24/02, p. 117: Available to subscribers at A Cut above the Ordinary: Low-tech machining yields coveted nanostructure) or heated nanocrystalline powders to weld together smaller structures. Neither of these methods gives precise size control, Schuh says.

In a new twist, his team devised an electroplating method that finely tunes the mixture of nickel and tungsten atoms as it coats, for example, steel or copper. Tungsten atoms are bigger than nickel ones, so they don’t fit comfortably into nickel’s crystal lattice, Schuh explains. Grains of tungsten-poor alloy become cemented together by tungsten-rich material to form a brick-and-mortar structure.

Upping the alloy’s overall fraction of tungsten forces the brick size to shrink to provide additional room for the big atoms in the mortar, Schuh says. Varying the tungsten fraction from 10.7 percent to 14.5 percent to 17.5 percent yielded grains of 20 nanometers, 10 nm, and 3 nm, respectively, Schuh and Andrew J. Detor, also at MIT, report in an upcoming Materials Research Society Proceedings.

Tests of the 10-nm alloy as a coating for steel have shown it to be as hard as chromium and more resistant to marine corrosion, Schuh says.

That could be good news for coating gun bores and other military items, says materials scientist Deepak Kapoor of the U.S. Army’s Picatinny Arsenal in New Jersey. He adds that he’s even more impressed with the way in which the new coating was created. “Schuh can predict and then, basically at will, produce an average grain size…. That’s the beauty of this,” he says.

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