Material as Tough as Steel? The Abalone Fits the Bill

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Posted by on March 22, 2005 at 00:36:06:

When Kenneth Vecchio was a boy in the 70's, racing around Atlantic Beach on Long Island, he could not believe how hard it was to break seashells. "They looked fragile, but I'd have to hit some of them with a hammer to get a piece off," he recalled.

Now a mechanical and aerospace engineer at the University of California, San Diego, Dr. Vecchio still marvels at seashells, but he now uses them for practical inspiration. His aim is to create synthetic materials that match what nature has cranked out in stupendous quantities since hard-shelled marine life appeared 600 million years ago.

He and others in his line are not there yet, but they are getting better.

Dr. Kenneth Vecchio holds abalone shells, which are thickly lined with the iridescent material nacre. He created a hard synthetic material inspired by nacre.

Top, a marble-sized BB did not fully penetrate an abalone shell. Above, Dr. Vecchio's new material's bullet-stopping ability was tested when a tungsten alloy rod was fired into it.

Dr. Vecchio's group has been getting calls from aerospace companies and other businesses fascinated by his reports on a new, extraordinarily hard, strong and tough material whose raw ingredients are aluminum and titanium. When his team gets done with it, the stuff is stiff as steel at half the weight. It is harder than brick. If it cracks, the crack splits into ever smaller cracks that wander about and often fade away, with no shattering.

Dr. Vecchio reports in this month's Journal of the Minerals, Metals and Materials Society that it not only makes a good lightweight structural material, it performs "spectacularly" on depth-of-penetration tests - another way to say it stops bullets. (In the lab, a tungsten rod fired at 2,000 miles per hour penetrated only halfway through a three-quarter-inch-thick sheet of Dr. Vecchio's material.)

It also seems to possess qualities needed for lightweight armor and for aerospace applications where strength, low weight and good heat conductivity are at a premium.

Dr. Vecchio is a devotee of biomimetics, or the imitation of life, a growing movement in engineering. Its inspirations to human engineers include bone; porcupine quills; the dry, sticky pads of gecko feet; and the aerodynamics of flies. And seashells. Each has qualities that engineered materials cannot yet equal.

Dr. Vecchio credits his new "metallic-intermetallic laminate" composite to the example provided by abalone nacre, or mother of pearl. Nacre is the iridescent material thickly lining the shells of abalone, shellfish native to the Pacific and resembling flattened snails up to a foot wide and prized for their tasty flesh.

Forms of nacre also strengthen the shells of many other mollusks, bivalves and cephalopods like the chambered nautilus. Pearls are pathological growths of nacre formed by oysters irritated by grit wedged between their living tissue and the shells they are making. For at least two decades, nacre has also been a prime focus of biomimetics.

Under a microscope, nacre is astonishingly orderly. Its layers of complexity, from large to small scale, give it what engineers call a hierarchical structure. A microscopic cross section looks like brickwork, with flat, hexagonal tablets of a crystalline, calcium carbonate mineral stacked in neat layers. Mortaring them is a flexible protein-rich gum originally secreted by the shellfish.

The thickness of each stratum is about one-hundredth the diameter of a human hair. These finer layers are themselves organized into thicker layers separated by extra-broad bands of protein. The shells are so tough that driving over one with a truck is unlikely to crack it.

But here is where the genius of living architecture comes in. "When you think about it, an abalone shell is just chalk," Dr. Vecchio said.

It is 95 percent calcium carbonate, one of the most abundant, and weakest, minerals on earth. It is the main ingredient of limestone. Nacre's organic component is similarly fragile. But in a case of the whole outdoing the sum of its parts, nacre multiplies its components' strength dozens of times.

Dr. Vecchio's team set out to make such a composite, alternately stacking thin sheets of aluminum and titanium and pressing them together at about 1,300 degrees Fahrenheit. The metals react to form a hard, brittle substance resembling a ceramic, called titanium-aluminate. Separating them are thinner layers of flexible titanium.

Under a microscope its layering resembles nacre. It is hard, and cracks are dissipated by the ductile titanium. They are also working with other combinations.

Other teams are performing similar work. Last week, at the American Chemical Society meeting in San Diego, a University of Michigan group led by Dr. Nicholas Kotov, an associate professor of engineering, reported a process of dipping panes of glass alternately into solutions of ordinary clay and specialized organic molecules.

The result is a hard film of nacrelike composite. The team pushes the material's strength beyond natural nacre by adding chitosan, extracted from lobster and crab shells. Dr. Kotov said he expected it to interest makers of military aircraft: it is strong and does not reflect radar. "It's great for stealth," he said.

For all the progress, naturally evolved nacre seems still to have an edge on human cleverness. Dr. Mehmet Sarikaya, a materials scientist at the University of Washington, has been studying it since the 80's.

"People have pieces here, and pieces there. Nobody knows all the details," he says. "Nature has designed an interface of organic and inorganic materials interpenetrating in ways that we cannot yet equal."

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