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Ceramics are still surprising us

Ceramics have been a little neglected recently but they still have something to reveal to us: the latest generations of eutectic" ceramics are very promising for engines, while ceramic composites – more traditional – have had a new lease of life with the development of low-cost processes.

18 July 2007

Number 34

Microstructure d'un alliage ternaire (à 3 composants), étudié pour accroître la ténacité, c'est-à-dire s'opposer à la propagation des fissures pour diminuer la fragilité.

The microstructure of a ternary alloy (with 3 components), studied to increase its toughness, i.e. its resistance to the propagation of cracks in order to reduce fragility.

The reduction of consumption and pollution is today a priority for the whole of the aeronautic industry and ceramics may be able to play a role in this race for efficiency. To improve the performance of aircraft and helicopter engines, the temperature needs to be increased. But then we come up against the limits of the materials used to manufacture the engine. Most metal alloys are unable to resist temperatures greater than 1,100° C, and ceramics are fragile. "In the eighties, we had high hopes for ceramic matrix composites; that is, ceramics reinforced with fibers", recalls Michel Parlier, researcher at the Department of composite materials and systems at Onera. "But the results were disappointing: the mechanical characteristics were not as good as we'd have liked, and the cost was too high".

That is why Onera is working on designs for new high performance materials, oxide-based eutectic ceramics. "We are really looking for great things from this family of materials", emphasized Michel Parlier. They are a mixture of metallic oxides which, in the right proportions, melt together at a temperature which is slightly lower than the fusion temperature of each individual oxide. In this way ceramics are obtained in which the metallic oxides are very closely interwoven in the form of monocrystals, which gives them remarkable mechanical properties. "They are stable almost right up to the temperature of fusion, around 1700 °C for materials based on alumina (aluminum oxide), yttrin (yttrium oxide) and zirconia (zirconium oxide)", commented the researcher.

Exemples d'aubes de turbines en céramiques eutectiques élaborées par UBE au Japon.
Examples of turbine blades in eutectic ceramics developed by UBE in Japan.

The objective is to create materials for non-cooled turbine blades. The superalloys currently used, stable up to a maximum 1200 °C at emergency engine speed, need to be air-cooled. This results in loss of performance. Furthermore, the necessary air injection makes the turbine design more complex. "We are looking at small aircraft engines, and also cogeneration turbines, which produce both heat and electricity", continued Michel Parlier.

A first thesis, complete at the end of 2005, aimed to develop eutectic ceramics and study the correlation between the composition of these ceramics and their structure at the microscopic level. A second thesis is in progress to evaluate the mechanical properties of these materials, particularly creep properties (deformations) and toughness (fragility in the presence of a defect).

At the same time, ceramic composites with fiber reinforcements have not been completely abandoned. The objectives are now less ambitious: we are now concentrating on the parts of the engine at temperatures of between 700 and 800 °C, rather where it is hottest. Organic composites can't withstand these temperatures and titanium alloys, which are also expensive, have a limit of 600 °C. "So we are trying to develop low cost ceramic composites", said Michel Parlier. To do this, the manufacturing process has to be simplified. "We start with a silicon compound, which we modify to create bonds between the silicon and carbon", continued the researcher. Then we add the fibers, and we form this material as for an epoxy type resin, like those used for boats. Finally, we pyrolyze everything and we get low cost ceramics thanks to the cheapness of the raw materials and the simplicity of the process".

Image au microscope électronique à transmission d'une fissure déviée par une phase de zircone

Transmission electron microscope image of a crack deviated by a phase of zirconia

Image au microscope électronique à balayage du composite ternaire Al2O3-YAG-ZrO2 solidifié à 10 mm/h. L'alumine apparait en noir, le YAG en gris et la zircone en blanc.
Scanning electron microscope image of the ternary composite Al2O3-YAG-ZrO2, solidified at 10 MM/h. The alumina appears black, the YAG gray and the zirconia white.

Cécile Michaut, scientific reporter.

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