Mollusk teaches ceramics to scientists.
Red abalones fashion calcium carbonate--the stuff of chalk--into complex arrangements that yield rugged shells up to 40 times more fracture resistant than the blackboard implement. Scientists at the University of Washington in Seattle aim to mimic the mollusks' material-making approach for designing synthetic ceramics that can take a beating.
A close look at an abalone shell reveals two layers, both made of calcium carbonate but organized into different microstructures with different properties. The rough outside layer derives from a mineral form known as calcite. But it is the aragonite form found in the inner, nacreous layer that makes the shell remarkably resistant to breakage.
An even closer look at this layer's architecture shows why. The nacreous layer has a laminated, brick-and-mortar structure, says Mehmet Sarikaya, who is conducting the work with Katie E. Gunnison and Ilhan A. Aksay. The micron-wide bricks are made of tiny, high-quality calcium carbonate crystals. For mortar, the abalone secretes its own version of Superglue made of a concoction of organic compounds whose formula the researchers now seek to unravel.
By deliberately stressing abalone shells and then examining the resulting microcracks with electron microscopes, the scientists have observed at least five possible toughening mechanisms. The most important of these, suggesting ways of toughening synthetic materials, are the sliding of adjacent calcium carbonate layers and the formation of crack-bridging "ligaments" in the organic mortar layers, Sarikaya says.
With abalone shell as their model, Aksay and colleague Mehrdad Yasrebi have assembled a synthetic, multilayered structure using the tough ceramic material boron carbide for the bricks and aluminum as the mortar. Scientists term such ceramic-metal composites "cermets." Preliminary results indicate that this cermet is up to 40 percent more fracture resistant than conventional, less structured arrangements of the same materials, Sarikaya says.
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|Date:||Dec 9, 1989|
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