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Ceramics That Won't Shatter

A biomimetic ceramic that is strong and tough could be used to make lightweight vehicles.

Ceramics are lightweight and hard, but you can’t make jet engines out of them because they’d shatter like dinner plates. So, materials scientists have been trying to mimic natural materials that combine strength (a measure of resistance to deformation) with toughness (a measure of resistance to fracture). In particular, they’ve looked to the porous but resilient material called nacre that lines abalone shells. Now researchers have developed a method for manufacturing nacre-like materials in the lab. These new materials have mechanical properties similar to metal alloys and are the toughest ceramics ever made. The new method could lead the way to ceramic structural materials for energy-efficient buildings and lightweight but resilient automobile frames.

Biomimicry in bulk: The Berkeley researchers made large pieces of the tough ceramic, while other scientists mimicking tough natural materials have been able to make only thin films. A tough ceramic’s structure mimics that of abalone shells. This scanning electron microscope image (bottom), taken during a stress test, shows one source of the material’s toughness: damage is widely distributed in small, contained cracks.

Nacre, also known as mother-of-pearl, combines plates of stong but brittle calcium carbonate with a soft protein glue in a brick-and-mortar structure that’s 3,000 times tougher than either constituent. Ordinarily, when scientists make composites in the lab, the resulting materials’ properties average out those of their constituents. “When nature makes composites, the properties are better,” says Robert Ritchie, chair of the Department of Materials Science and Engineering at the University of California, Berkeley, who co-led the ceramics research. That’s because nature’s composites have complex structures that are difficult to mimic. “People have tried, but can’t get that fineness of structure,” says Ritchie.

For years, scientists have been trying to design new materials based on tough natural materials like nacre and bone. The Berkeley ceramic “really shows that drawing our inspiration from nature in order to synthesize better materials can be very successful,” says Julia Greer, a materials scientist at CalTech.

To shape their ceramics into nacre-like structures, the Berkeley researchers first create a water suspension of the material to be patterned–in this case, aluminum oxide. Then they chill it in a very controlled way. “You take the heat out at one end,” explains Ritchie. This leads to long, thin structures that the researchers press into microscale, brick-like structures after heating them to evaporate the water. When this process is repeated, it creates a layered, porous structure of aluminum oxide bricks connected to one another by column-like structures–the same shapes found in natural nacre. Then, to mimic the protein glue in the abalone shell, the researchers fill the spaces with a polymer. This process is described online in the journal Science this week. Other groups have made thin films of biomimetic materials; the Berkeley group has succeeded in making large pieces.

Uncushioned by the polymer, the bricks would be brittle like most ceramics. But the polymer permits the brick-like layers to slide over one another when stressed, making the material resistant to fractures. Indeed, this brick-and-mortar structure is tougher than any ceramic ever made in the lab. “High toughness and high strength are usually incompatible” in a ceramic, says Eric Stach, a materials engineer at Purdue University who was not involved with the Berkeley work. But the ceramics created at Berkeley have as much strength and toughness as aluminum alloys, which “you can fly planes with,” says Stach.

Though they caution that the nacre-like ceramics are in their early stages of development, the Berkeley researchers say the materials should make possible applications of ceramics that have seemed unattainable. “You could use ceramics to make the frame of a car instead of steel, and save fuel,” says Ritchie. Antoni Tomsia, a materials scientist at the Lawrence Berkeley Laboratory who co-led the research, says that tough ceramics, which are good insulators, could do double duty as structural elements in energy-efficient buildings. And they might also be used in lightweight bulletproof vests and vehicle armor for the military.

The new work, say materials scientists, shows the way forward for tough biomimetic materials. Paul Hansma, professor of physics at the University of California at Santa Barbara, calls the work “astonishing” and says the performance of the new ceramic “raises the bar in this important field.”

Ritchie and Tomsia are confident they can make the material even better. Natural nacre has ceramic structures an order of magnitude smaller than those in the Berkeley material, as well as a higher ratio of brick to mortar. Ritchie says the group is working on making the ceramic bricks smaller and closer together, and decreasing the polymer content. They’re also experimenting with different mortars. Because the newly developed ceramic contains a gluey polymer, it would fail in high-temperature environments like the inside of an engine. So the Berkeley researchers are experimenting with metal fillers, which can withstand higher temperatures.

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