Ceramics stretch for future applications

Everyone with kitchen experience knows how easy it is to break a ceramic plate. Less well known is the capability of some ceramic materials to become soft and pliable when heated. For years, materials scientists have hoped to exploit this unusual property to develop ceramics that can be machined into intricate parts as easily as metals or polymers can.

A team of Japanese researchers has now overcome one obstacle to using so-called superplastic ceramics commercially. It has made a ceramic that can stretch quickly enough to appeal to manufacturers.

Engineers often find ceramics attractive for products such as spacecraft coatings and automobile engine parts because the materials can withstand more extreme conditions than many metals can, says team member Byung-Nam Kim of the National Institute for Materials Science in Tsukuba, Japan. Ceramics are hard, lightweight, chemically resistant, and electrically insulating. On the other hand, it’s much easier to mold a metal or polymer into a desired form than to shape conventional ceramics.

That’s where superplastic ceramics might come in. Researchers can fashion these materials into a desired shape while they’re hot and then allow the material to cool and harden.

Yet the superstretchy ceramics, which were discovered 15 years ago in Japan, haven’t yet taken hold commercially, comments materials scientist T.G. Nieh of Lawrence Livermore (Calif.) National Laboratory.

One reason for this, he says, is that the materials developed so far require slow stretching. For example, one superplastic ceramic created by another research team takes 20 hours to stretch to about 10 times its initial length before breaking. That’s too slow to be commercially viable, says Nieh.

Now, however, Kim’s team has made a ceramic that can undergo an elongation of more than 10-fold in just 25 seconds. “We believe the present ceramic can be deformed more,” Kim adds. The material didn’t break even when the team’s stretching apparatus reached its maximum.

The material is made from certain high purity powders of alumina, magnesia, and zirconia. When heated and stretched like chewing gum pulled into a string, it thins uniformly, Kim’s group reports in the Sept. 20 Nature.

The ceramic can behave this way, says Kim, because the original components create a mix of three types of grain that suppresses the growth of individual grains, and small particle size permits the material to easily stretch.

The increased rate of such stretching constitutes a long-sought “breakthrough,” comments Nieh. Still, like other superplastic ceramics, the new material has an Achilles’ heel–cost. The price of the raw materials must fall before such ceramics will come into widespread use, he says.

If that happens, Nieh suspects that superplastic ceramics’ manufacturing properties could make them especially attractive for creating small, intricate forms, such as parts of implants used in the body, where ceramics’ chemical inertness would be valuable.