How do you make extremely dense but ultra-light ceramics? For starters, you immerse them in molten metal and then let the metal seep in. In this new process called DCP, the result is a new extra-hard composite.

Manufacturing light, tough and hard ceramic parts may soon be easier and less costly—thanks to a recently patented technique called “displacive compensation of porosity,” or DCP.

This new method relies on a chemical reaction between molten metal and a porous ceramic, which is soaked in the metal. Once the reaction has taken place, the pores inside the ceramic get filled up with extra ceramic material. The result: an extremely dense ceramic part. And since the original ceramic shape is maintained, producing complex shapes is a snap.

Since the technique creates new, carbide-rich composite materials in complicated shapes, it could be used to make rocket nozzles, body armor and machine tools, says inventor Kenneth Sandhage, professor of materials science and engineering at Ohio State University.

“There are several advantages of our method,” says Sandhage. For starters, DCP requires lower temperatures than conventional methods and doesn’t need high pressures or post-process ceramic machining. Also, it averts the excessive shrinkage that can occur in the processing of dense ceramics.

Thus, with this technique, materials manufacturers can look forward to a cheaper and easier ceramic production process. And the result would be superior ceramics—hard and heat-resistant.

The first part of the process involves making a porous ceramic shape or preform—with which many in the industry are already familiar.
“The same way you form a teacup, you can make one of our preforms,” says Sandhage.

Then, researchers immerse the preform in a liquid metal alloy bath. “The preform absorbs the liquid metal like a sponge, and the liquid metal then reacts with the ceramic powder to form a new ceramic compound that fills in pore spaces,” explains Sandhage. The part’s internal solid volume shoots up, but the external features—the shape and dimensions—of the original preform are maintained.

“When the reaction is complete, we can have twice as much solid material as we started with,” says Sandhage. “That extra material has to go somewhere, so it fills in the pores of the ceramic, creating a very dense material.”

The method’s reaction temperature range of 1,200-1,300˚C is far lower than the 2,000˚C required by traditional methods to create heat-resistant, covalently-bonded ceramics. “The DCP-derived composites are very light, too,” says Sandhage.

DCP could help make thinner, lighter and stronger ballistic protection gear. While today’s toughest body armor already uses ceramics because they are lighter and harder than metal, the new method could take on even extremely hard ceramics, such as boron carbide, and help produce even more effective body protection.

With the new technique, Sandhage and his students have already been able to generate composites made up of some of the world’s hardest materials, such as boron carbide, zirconium carbide, hafnium carbide, titanium carbide and zirconium diboride.

In trials, the Ohio State researchers made a curved preform out of tungsten carbide, a fine gray ceramic powder found in machine tools and abrasives. Next, they melted a zirconium-copper alloy and soaked the preform in the molten metal. “The tungsten carbide sucked up the liquid metal,” says Sandhage.

At temperatures of 1,200-1,300˚C, the metal and ceramic chemically reacted inside the porous preform and produced a zirconium carbide-tungsten composite—a material that is usually produced at much greater temperatures and at extremely high pressures.

“We’ve made tungsten-bearing composite materials that are 40% lighter than plain tungsten,” says Sandhage.

An immediate application for the process could be rocket nozzles, whose liners currently use plain tungsten because it has the highest melting point out of all the metals and resists oxidization in severe solid fuel rocket surroundings. A tungsten composite would be even more suitable for rocket nozzles, says Sandhage, because it is much lighter than pure tungsten.

In fact, two of Sandhage’s former undergraduate students won the 2000 National Collegiate Inventors Award for showing that DCP can be used to make composites with extremely high melting points for such applications as rocket nozzles.

Such dense composites could also be used to fabricate high quality machine tools and parts for aerospace, automotive and manufacturing. Because the final part maintains the shape of the original porous ceramic, post-process reshaping is eliminated. This translates to potential cost savings for manufacturers, which have to use pricey diamond tools to shape such parts.

Because of DCP’s relatively low temperature requirements, manufacturers could also save on electricity and can use less costly furnaces, says Sandhage. The fact that the process doesn’t need high pressures is another budget-friendly feature.

The technique can work at even lower temperatures. In another trial, the engineers created a composite of magnesium oxide and plain magnesium at only 900˚C. In fact, other reactions have occurred at temperatures as low as 750˚C.

Sandhage is collaborating with Ohio-based MetaMateria Partners, which will soon offer prototypes and seek licensing deals with other companies.

Sources: Better Ceramics for Less
Laurie Ann Toupin
Design News, Nov. 18, 2002
http://www.manufacturing.net/DN

New Way to Make Dense Complex-Shaped Ceramics at Lower Cost
Press Release
Ohio State Research, Aug. 2002
http://www.osu.edu/units/research/archive/porefill.htm