UC Irvine

Flash sintering directly applies an electric field to ceramic green bodies in order to quickly densify at low furnace temperatures, a process that has great potential for saving energy and reducing production costs during manufacturing. This novel manufacturing method has been proven effective for a number of ceramics, especially 8 mol % yttria-stabilized zirconia (8YSZ). In the past, primarily single-phase materials or composites that include 8YSZ have been the focus of flash sintering studies, while research on other composites has been lacking. This study investigated the phenomena of flash sintering applied to a novel monazite (LaPO4) single phase system and the LaPO4-Al2O3 system with comparisons to the LaPO4-8YSZ system. The processing-microstructure-properties relationship between composition/time/furnace temperature/applied voltage/current limit and the resultant microstructures are investigated. While LaPO4 and Al2O3 could not flash sinter under the highest voltage and currents used, the composite LaPO4-Al2O3 system experienced flash sintering at the highest applied voltages, and as expected, the composite LaPO4-8YSZ easily flashed due to presence of 8YSZ. Eutectic microstructures were prominent in the LaPO4-Al2O3 system, and abnormal grain growth of both phases could be produced under suitable flash sintering conditions. Large facetted sapphire crystals formed with unique wetting characteristics with respect to monazite. The formation of an intertwined eutectic microstructure was attributed to the elevated local temperatures during flash and demonstrated the potential of flash sintering for high temperatureprocessing without high temperature furnaces and crucibles. This study proved LaPO4- Al2O3 to be a eutectic-forming system, and the solid solubility, eutectic temperature, and eutectic composition were explored to construct a eutectic phase diagram for LaPO4-Al2O3. In addition, the mechanical properties of the eutectic microstructure were also characterized and found superior compared to polycrystalline microstructures with the two phases. In summary, this work shows that flash sintering can be manipulated to produce unconventional microstructures with improved mechanical properties in certain binary systems.