Enhanced thermal properties of silica-based ceramic cores prepared by coating alumina/mullite on the surface of fused silica powders

In this study, an alumina/mullite coating was synthesized on the surface of fused silica powders to form an alumina/mullite-silica core-shell structure. The effects of the alumina/mullite coating on the cristobalite crystallization, thermal properties, and leachability of the silica-based ceramic cores were investigated using the simulated casting process. The X-ray diffraction results indicated that the crystallization of cristobalite was significant at the simulated casting temperature of approximately 1400 degrees C. An increase in the cristobalite content during this stage resulted in a large thermal expansion because of its higher coefficient of thermal expansion compared with that for fused silica. The addition of optimum amounts of the alumina/mullite powders resulted in an increase in the initial shrinkage temperature and a decrease in the shrinkage of the specimens. When the coating powders were added at 43 wt%, the initial shrinkage temperature increased from 1092 degrees C to 1200 degrees C and the shrinkage decreased sharply. Leaching tests showed that the silica-based ceramic cores were removed in the form of stripped layers. The washing and shaking process accelerated the disintegration of the ceramic core and improved its leachability.