All Fired Up About Ceramic Technology
In another life, Ken Sandhage might have been a great chef. Sandhage and his graduate students at Ohio State University have come up with a new way to blend components into ceramics that they say could lead to better parts for everything from our cars to our bodies.
For years, scientists have worked toward creating ceramics that would be lighter and more resistant to wear and heat than metal parts, and ceramic engineering has become a hot-button topic. But the technology has proved to be far more difficult than initially thought and progress has been slower than many had hoped.
Yet the stakes are high. According to Rutgers University, one of the leaders in the field, ceramics are expected to capture 5% to 10% of a projected $400-billion market for advanced materials by 2000.
Ceramic engineers use organic binding materials, such as carbon, to glue ceramic powders together. But the carbon eventually has to be burned out of the ceramic, leaving tiny pores in the material. The pores must then be filled and the part refired, but that can cause it to deform.
Sandhage approached that problem from an unusual direction. He earned his doctorate in ceramic engineering at the Massachusetts Institute of Technology, but his faculty advisor specialized in metallurgy. So he came away with an appreciation for both fields, and he concluded that ceramic engineers had made a fundamental mistake.
*
Instead of using carbon as the binding agent, Sandhage decided to use metals. Other engineers balked at the idea, because metals oxidize and expand during the firing process, thus distorting the finished product.
That’s not a big problem for a kitchen saucer, where slight distortions are not even noticeable, but it can prove critical in areas in which tolerances are extremely tight. Traditionally, ceramic parts with minor imperfections are machined into the right shape, but that is a tricky process.
“Ceramic tends to be very brittle,” Sandhage said. “You can get 99% of the way through a process, and at the very end you get a crack and that screws everything.”
The answer, he decided, was to develop a process that would yield a finished part that was exactly the same shape and size as the “precursor,” the unfired ceramic materials.
Sandhage knew from his early years at MIT that not all metals expand upon oxidation. Some metals, known as alkaline earth metals, such as magnesium, calcium, strontium and barium, shrink when they oxidize.
So Sandhage latched onto a device that looks a lot like a paint mixer in a hardware store. He put ceramic powders into the bucket, and he stirred in just the right amounts of metals that shrink and metals that expand to serve as binding agents.
The device blended the elements so uniformly that when he fired the material, expansion was precisely the same as contraction. He got ceramic parts that were exactly the size and shape of the parts he had loaded into the kiln.
Since the raw material contains malleable metal, the researchers can shape it with metallurgical techniques such as rolling, forging, extruding and machining. The researchers were even able to roll it into a sheet about five times thinner than a human hair.
And since it does not deform during the firing process, it has all kinds of potential applications, Sandhage says.
“It sounds like the right thing to try,” said Vijay Gupta, professor of mechanical and aerospace engineering at UCLA’s School of Engineering and Applied Sciences. But Gupta said factors other than distortion are the biggest problems facing engineers. The primary hurdle, Gupta said, is to figure out how to make ceramics tough enough.
A ceramic turbine for an aircraft, for example, could offer substantial advantages over a metal blade in terms of resistance to heat and corrosion, but ceramics are brittle.
“If you have a flying bird hit your blade, then that blade is going to shatter,” Gupta said. An alternative is to make the blade out of metal, and then coat it with a ceramic material, but adhesion of ceramics to metal has also been a difficult problem to overcome.
Sandhage believes his process could have medical applications, such as a substitute for human bone, he said. Surgeons could scan a defective jawbone, for example, and that scan could be used to create a precise replacement very quickly.
“We’re pretty excited about that,” Sandhage said. “In a short period of time, you could have a ceramic part in exactly the shape you want.”
Gupta is not sure that’s a practical application because he doubts the ceramic would be tough enough. Surgeons use titanium implants coated with ceramic to encourage the growth of new bone, which gives the implant the desired strength and lightness.
Sandhage said other applications for the technology could include precise parts for aircraft and automobiles, including ceramic sensors for monitoring exhaust that would be better suited to withstand high temperatures and corrosion.
*
Sandhage published his work in a recent issue of the Journal of Materials Research and has patented the process.
It’s so simple, Sandhage said, that even he is astonished no one had tried it before. The reason, he thinks, is it required the blending of two disciplines. Ceramic engineers wanted to keep the metal out because they thought the metal would expand and distort the product. And metallurgists, who knew that was not always the case, didn’t share their understanding with their colleagues down the hall.
“I saw it from both perspectives and realized these guys ought to talk a lot more often because there’s a lot of cross-fertilization that could really be useful,” Sandhage said.
*
Lee Dye can be reached via e-mail at leedye@compuserve.com.
