Martin Hindley/LONDON

APPLICATIONS FOR lightweight, toughened ceramics could be found, in the jet engines of the future, US researchers claim.

Materials scientists at Cornell University in New York have developed a technique for "tempering" ceramics - improving their crack resistance at temperatures of up to 1,100°C.

It is possible that tempered ceramic jet-turbine blades could withstand higher temperatures and be lighter than nickel-based alloy blades, according to Stephen Sass, professor of materials science and engineering at the University.

While ceramics are lightweight, hard and able to withstand extremely high temperatures, their brittleness makes them susceptible to cracking under stress.

The tempering technique, which is similar to that used in the production of bulletproof glass, aims to make the ceramic less brittle and thus more crack-resistant.

Sass believes that the process could eventually lead to the use of ceramic components in high-temperature and stress environments such as in jet engines, as replacements for metal parts.

Ceramic components would have the advantage of being considerably lighter and more efficient than metal parts at higher temperatures. They would also be resistant to attack from combustion gases, the professor claims.

Toughening the ceramic, is achieved by compressing its outer surface so that microscopic fractures in the material, are prevented from developing into full-blown cracks.

The Cornell scientists, led by Sass, bonded a ring of alumina to a nickel-aluminate core and heated the composite to 1,200°C, in a low-oxygen-content atmosphere.

The process caused the core to shrink as oxygen was driven away, pulling the ring inward and applying a "compressive hoop stress" of 3,800bar (55,000lb/in2) to the ceramic.

The key to achieving a uniform core shrinkage, is controlling the reaction which drives off the oxygen, a process that Sass admits, may take several years to perfect, for complex geometry components.

Sass predicts take-up of the technique in "low-end" applications, possibly followed by use in "high-end" aero-engine components.

He believes that the concept could be suitable for high-speed civil-transport programmes, in which engines would be operated at temperatures some 300¡C higher than conventional jet turbines, and require higher thrust-to-weight ratios.

Source: Flight International