Formation Mechanism of High Thermal Resistance Oxide Eutectic Film Prepared by High Speed Solidification Process

This chapter summarized the authors' efforts in developing a process for the fabrication of highly heat-resistant oxide eutectic films for Al2O3/HfO2 and CaZrO3/ZrO2 eutectic system with fine microstructures. In these processes, light focusing floating zone apparatus and/or high-power laser equipment were used. Formation mechanism of the oxide eutectic films and their properties were described. Solidified Al2O3/HfO2 eutectic film was prepared using optical zone melting method on the silicon carbide substrate. At high temperatures, a small amount of silicon carbide decomposed into silicon and carbon. The components of Al2O3 and HfO2 in molten phase also react with the free carbon on silicon carbide substrate. The Al2O3 phase reacts with free carbon and vapor specie of AlO phase is formed. The composition of the molten phase becomes HfO2 rich from the eutectic composition and HfO2 phase solidifies as primary phase on the substrate. The solidified HfO2 phase also reacts with the free carbon and HfC phase is formed, then a high-density intermediate layer was formed on the surface of silicon carbide substrate. The adhesion between the intermediate layer and the substrate is excellent by an anchor effect and chemical bonding. When the solidification process finished before all of solidified HfO2 primary phase is reduced to HfC phase, HfC-HfO2 functionally graded layer is formed on the silicon carbide substrate and the Al2O3/HfO2 eutectic structure grow from the top of the intermediate layer. As the intermediate layer is formed by the chemical reaction between the oxide melt and the silicon carbide substrate, the thickness of the intermediate layer decreased with increasing the speed of the transference velocity of the molten zone. On the other hand, the authors' experiments showed that phase separation from ZrO2 results in the formation of a fine CaZrO3/ZrO2 eutectic structure even during rapid solidification. When CaZrO3/ZrO2 eutectic was formed on a ZrO2 substrate by the same solidification method as above, it was found that when the transference velocity below 3000 mm/h, the 200 peak for cubic ZrO2 phase strongly oriented. At fast transference velocity of the melting zone, rapid crystallization from supercooling melt occurs. In such cases, crystals grow in the (100) direction. Rapid crystal growth results in the formation of several peculiar film structures. Columnar crystals, cauliflower-like columnar crystals and eutectic microstructures filling the gaps in the columnar crystals are the main microstructures.