Mimicking Mother-Of-Pearl

MOTHER NATURE has mastered how to make tough hybrid materials that resist fracture, such as bone and shell, using comparatively weak starting materials. Now, researchers at Lawrence Berkeley National Laboratory have made a composite that is structurally similar to mother-of-pearl and exhibits resistance to fracturing that rivals that of aluminum alloys such as those used in aerospace engineering (Science 2008, 322, 1516).

Natural mother-of-pearl, also known as nacre, has a brick-and-mortar structure: Layers of "bricks" made from a calcium carbonate mineral are held together by thin films of a biopolymer "mortar" such as chitin.

To make the nacrelike structure, the Berkeley team used ice templates to create layers of the ceramic aluminum oxide, pressed and sintered the layers to make the bricks, and filled the voids with polymethyl methacrylate mortar.

"We used the freezing properties of ice to specifically control the ceramic layer thickness and surface roughness," says team leader Robert O. Ritchie, who is also a professor of materials science at the University of California, Berkeley. Those characteristics proved problematic in previous attempts by other researchers to make bulk materials, he explains.

The team's mechanical tests show that the composite is 300 times more resistant to cracking than its constituent polymer and ceramic materials. The new composite is likely the most fracture-resistant ceramic-based material ever made, Ritchie adds.

David Dunand, a professor of materials science at Northwestern University, calls the brick-and-mortar composite a "breakthrough." In its ability to resist the growth of existing cracks perpendicular to the bricks, the composite "achieves a combination of strength and toughness equal to that of aluminum but has much higher stiffness, hardness, and abrasion resistance," he says. "This is a truly remarkable result."

Compared with natural nacre, however, the composite contains too much polymer by volume, and its ceramic layers are 10 times too thick, Ritchie notes. Reducing the amount of polymer will further improve the composite's strength, as will replacing the polymer with metals, he adds.