New process creates lighter, tougher ceramic parts

Researchers have created a new way to make dense ceramics in complex shapes, a development that could lead to light, tough, and hard ceramic parts at lower cost.

The recently patented technique, called 'displacive compensation of porosity,' (DCP), uses a chemical reaction between molten metal and a porous ceramic to generate a new composite material. The technique is said to fill the tiny pores inside the ceramic with additional ceramic material. The resulting super-dense part retains the shape of the original ceramic.

The technology could be used to produce rocket nozzles, body armour, and manufacturing tools, explained inventor Ken Sandhage, professor of materials science and engineering at Ohio State University.

Manufacturers could make hard heat-resistant ceramics cheaper and easier with DCP, since it works at lower temperatures than conventional methods and eliminates the need for post-process machining, Sandhage said. The first step of the process - creating a porous ceramic shape, or preform - is well known in industry.

'The same way you form a teacup, you can make one of our preforms,' Sandhage said.

Today's strongest body armour relies on ceramics, because these materials are lighter and harder than metal. For instance, both military armour and commercially available bulletproof vests can contain ceramic plates wedged between layers of fabric.

Sandhage said manufacturers could create thinner, lighter, and stronger body armour if they used very hard ceramics, such as boron carbide, but such materials are difficult to mould into body-friendly shapes.

With DCP, Sandhage and his students were able to create composites containing some of the world's hardest materials, including boron carbide, zirconium carbide, hafnium carbide, titanium carbide, and zirconium diboride.

In tests, the Ohio State engineers moulded a curved object out of tungsten carbide, a fine grey ceramic powder used in machine tools and abrasives. Then they melted a zirconium-copper alloy and let the molten metal seep into the powder.

'The tungsten carbide sucked up the liquid metal like a sponge sucks up water,' Sandhage said.

At temperatures of 1,200 C to 1,300 C (2,190 F to 2,370 F), the metal and ceramic reacted with each other chemically inside the porous object, producing a zirconium carbide - tungsten composite. Normally, this composite material is created at temperatures closer to 2,000 C (3,630 F), and at very high pressures.

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

'The composite is very light, too,' Sandhage continued. 'We've made tungsten-bearing composite materials that are 40 percent lighter than plain tungsten.'

In another test Sandhage and his partner on the patent, former student Pragati Kumar, formed a composite of magnesium oxide and plain magnesium at 900 C (1,650 F). Other reactions have taken place at temperatures as low as 750 C (1,382 F), Sandhage said.

A start-up company is currently negotiating a license for the process, to further develop it for commercial use.