Research On The Al₂O₃/Er₃Al₅O₁₂/ZrO₂ Eutectic Ceramic Prepared By High Frequency Induction Zone Melting

The oxide eutectic ceramics prepared by the directional solidification process have excellent mechanical properties,oxidation resistance and ablation resistance in an ultra-high temperature environment（1600?C）.Their microstructure is modulated by the eutectic growth mode,which is affected by the properties of each phases and solidification parameters.Al₂O₃/Er₃Al₅O₁₂（EAG）/ZrO₂ ternary eutectic ceramics were synthesized by the high frequency induction zone melting method.Their phase component,growth plane,mechanical properties and the effect of solidification rate on microstructure were investigated in detail.The main conclusions are drawn below:Firstly,the mixing powders of Al₂O₃,Er₂O₃ and ZrO₂ were pressed as a pre-sintered body.As the sintering temperature increased,the porosity of sample decreased because the eutectic phase gradually changed from a smaller particle to a larger layer structure.When the sintering temperature was lower than 1350?C,the density increased with the sintering temperature;when the sintering temperature was higher than 1350?C,the density gradually decreased as the sintering temperature increased.It is due to the formation of EAG phases with lower density at1350-1500?C;and the higher the sintering temperature,the larger the proportion of EAG phase.A pre-sintered body with certain density can be prepared by holding the rod-shaped preform at 1500?C for 4 h.The directionally solidified eutectic ceramics were composed of three phases of Al₂O₃,EAG and ZrO₂.At low solidification rates,the eutectic phase were intertwined together to present a network-like structure.As the solidification rate increased,the size of the eutectic phase decreased and the microstructure was refined.The relationship between the eutectic spacing and solidification rate was described by the formula of?²?（28）25.91.The hardness and fracture toughness increased slightly with the solidification rate,with the maximum value reaching to 17.62 GPa and 5.03MPa·m^-1/2,respectively.These average values were 1.98 and 2.44 times of those obtained from the pre-sintered ceramics with the same composition.The hardness of Al₂O₃/EAG/ZrO₂ ternary eutectic ceramics was 18.31%lower than that of Al₂O₃/EAG binary eutectic ceramics due to the addition of ZrO₂ phase with lower hardness in the eutectic components.Significantly its fracture toughness was 88.40%higher than that of the binary eutectic ceramic due to the phase transformation toughening of ZrO₂.As the solidification rate increased,the distribution of eutectic phase was varied.The remarkable variation was that the phase distribution of ZrO₂ changed from the original places,such as the internal of Al₂O₃ phase,EAG phase and interface between two-phase to new sites,including the internal of Al₂O₃ phase,the interface of Al₂O₃phase and EAG phase.Finally,the ZrO₂ phase was fully located on the interface of Al₂O₃ phase and EAG phase.The above changes in phase distribution were explained by the perspective of kinetic undercooling and melting entropy.When the solidification rate was 4.16μm/s,unstable regular microstructure appeared in the sample.When the solidification rate increased to 13.89μm/s,the cellular or dendritic microstructures were observed in the sample.During the rapid solidification process,the internal cracks and amorphous phases were formed.Meanwhile,the crystalline phase grew in both transverse and longitudinal directions due to the existence of temperature gradients.