New microstructures in ceramic materials from the melt for high temperature applications

The development of new ultra high temperature structural materials in the aerospace field and in particular for gas turbine applications is a real challenge nowadays. In fact, the use of super-alloys at temperatures beyond 1150 °C will be difficult despite the different studies performed in order to increase their heat-resistance. For higher temperatures, ceramic oxides offer many advantages compared to nickel-based super-alloys: resistance to oxidation and abrasion, lower density. Unfortunately, sintered ceramics are brittle and their failure strength decreases when the temperature increases. Ceramic composites prepared by unidirectional solidification from the melt add new potentialities to the advantages of sintered ceramics: a higher strength almost constant up to temperatures close to the melting point (no secondary phase at the grain boundaries), good creep resistance, stability of the microstructure and no chemical reaction between the constituent phases. Synthesis at a eutectic composition usually gives rise to oriented microstructures. Recently, studies on binary eutectics between alumina and rare-earth oxides led to microstructures consisting of two entangled phases in a three-dimensional and continuous network. After solidification, the eutectic phases are alumina and either a perovskite phase LnAlO3 (Ln: Gd, Eu) or a garnet phase Ln3Al5O12 (Ln: Y, Yb, Er, Dy). In the case of ternary systems, zirconia was added as a third element. Mechanical properties at room temperature were studied in relation with microstructural features. In particular, it was established that fracture toughness of ternary systems is higher than that of binary systems.