A small 4 hp recuperated gas turbine engine is being developed at the Naval Research Laboratory. The high turbine inlet temperature of 1225°C on a small-sized engine along with a design life of 1000 hours places stringent conditions on what materials can be used. Mullite appears to be an excellent candidate due to its low density, low thermal conductivity, relatively low coefficient of thermal expansion, and decent thermal shock resistance. However, mullite has low fracture toughness. To improve the toughness six different zirconia-toughened mullite systems were tested using commercially available powders. Two pressureless sintering processes were explored: in-situ reaction sintering between zircon, alumina and various dopants and conventional solid state reaction sintering of mullite with various zirconia powders. Dopants included ceria, yttria, magnesia, and dysprosia. Fracture toughness was measured by an indentation method. From these measurements, X-ray diffraction results, and density measurements two systems were downselected for further study and optimization: mullite doped with 18 vol% yttria-doped ZrO_2 or doped with 18 vol% ceria-doped ZrO_2. Detailed microstructural and mechanical characterization of these two systems are presented along with the fracture toughness and X-ray diffraction characterization of the other unselected systems.
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