Nanotubes toughen up ceramics. (Fracture Protection).

Ceramics are famous for being hard but easy to break. Now, researchers have demonstrated that adding carbon nanotubes to a ceramic material can nearly triple its resistance to fracturing.

Since carbon nanotubes were discovered a decade ago, ceramics researchers have tried to exploit the tiny tubes' extraordinary strength and flexibility to make much more fracture-resistant materials.

Such durable materials could eventually replace conventional ceramics or even metals in countless products, says Joshua D. Kuntz of the University of California, Davis. For instance, engineers might use the toughened ceramic to make gears, bearings, or other parts for everything from racecars to industrial food-processing equipment.

In the new research, Kuntz, Amiya K. Mukherjee, and their UC-Davis coworkers aimed to toughen a ceramic made of alumina crystals only nanometers wide. Such nanocrystalline ceramics are particularly hard, but they're brittle and fracture easily.

In previous work, other researchers had added carbon nanotubes to alumina during processing. The best of these attempts improved the resulting composite's fracture resistance, or toughness, by only 24 percent. That experiment used multiwall carbon nanotubes, which resemble a set of nested straws.

The UC-Davis researchers suspected that high processing temperatures damaged many of the added nanotubes. They also predicted that single-wall carbon nanotubes would work better than the multiwall kind.

In their recent experiments, the scientists mixed alumina powder with single-wall carbon nanotubes and then forced the particles together with a combination of heat, pressure, and pulses of electric current. Called spark-plasma sintering, the method operates at lower temperatures than the conventional sintering technique used in previous attempts to make nanotube-reinforced composites.

When the researchers made a ceramic with nanotubes as 5.7 percent of its material, the product's fracture toughness was more than twice that of a pure-alumina ceramic. With carbon nanotubes at 10 percent of the volume, the ceramic's toughness nearly tripled. The researchers report their results in the January Nature Materials.

"It's a very nice piece of work," says Richard W. Siegel of Rensselaer Polytechnic Institute in Troy, N.Y., who had worked with other researchers to produce the toughest previous alumina-nanotube ceramic. Says Siegel, "Combining single-wall carbon nanotubes ... with the rapid sintering technique that they used seems to be the key, and it's exciting."

Siegel also points out that single-wall carbon nanotubes aren't cheap. The earliest uses of ceramics made with these materials would probably be applications in which cost is a secondary concern, such as in space vehicles and medical devices, he says.