Science Pictures

Ceramics for Next-Generation Blades

Aubes de nouvelle génération en céramique eutectique
copyright © ONERA 1996-2005

Microstructure of a ceramic made of a mixture of
aluminia (in black) and gadolinium perovskite (in white) - Al₂O₃, GdAlO₃
The material, obtained through directed solidification at high temperature (1800°C), is a three-dimensional lattice of these two single-crystal phases.

(N. Piquet DMSC, M. Parlier DMSC, L. Mazerolles CNRS/CECM)

For next-generation turbomachines (1) to operate at gas temperatures up to 1600°C, the solution consists in cooling nickel-based superalloy (2) turbine blades, which also have to be protected with a thermal barrier coating (3) made from zirconia for example (4).

For higher operating temperatures and without resorting to cooling, you have to produce oxide-based ceramic composites in-situ by directed solidification (6) of melts with eutectic (5) compositions. The resulting structure is a single-crystal three-dimensional phases lattice with no grain boundary - i.e. a material with coherent interfaces and no vitreous phase.

Its mechanical strength is almost constant up to melting temperature, whereas polycrystalline ceramics' strength decreases gradually. Moreover, the rupture stress can be increased by reducing the size of both phases - without reducing creep (7) strength.

At stake is ceramics' inherent brittleness, against which the right phases associations are still to be found. This is the case of perovskite structure oxides, which have high temperature plasticity.

Onera investigates such compositions, in collaboration with the Centre d'Etudes de Chimie Métallurgique (CECM). They focus on eutectic systems (melting between 1700 and 1900°C) which combine aluminia with rare earth oxides with perovskite (XAlO₃ ; X : Gd, Eu) or garnet (Y₃Al₅O₁₂ ; Y : Er, Dy, Yb) structures. Ternary combinations, adding zirconia to the mixture, are also investigated, for their better stiffness.

Current studies focus on the relationship between the microstructure and the mechanical properties. Soon, they will deal with the exhaustive determination of properties at high temperatures and later on the processing of refractory shape to test the compositions in turbomachine background.

(1) Turbomachine: Heat engines with a turbine as driving element.

(2) Superalloy : Alloy made of about ten component metals resisting oxidation at high temperatures. Its mechanical properties are much better than the best steels'.

(3) Thermal Barrier Coating: Low-conductivity coating protecting a substrate from high temperatures.

(4) Zirconia: Acid refractory (zirconium oxide).

(5) Eutectic: Relative to a mixture of substances whose solid and liquid phases in equilibrium have identical composition.

(6) Directional Solidification: Solidification taking place very gradually from one part to another of the casting in a given direction, usually lengthwise.

(7) Creep: Deformation of a material in ruthless conditions.

Microstructure of system Al₂O₃/Y₃Al₅O₁₂ /ZrO₂ (Y₂O₃)