Research On The Formation Mechanism Of Al₂O₃-YAG Eutectic Ceramic With Applied Electric Field

Oxide eutectic ceramics have excellent high temperature creep resistance,which are the candidate materials for high temperature structural parts.However,there are still faces many challenges for making oxide eutectic ceramic parts with excellent performance.Generally,oxide eutectic ceramics were prepared by directional solidification technology.It realized that eutectic growth of oxide ceramics was controlled by adjusting the degree of undercooling.However,it is hard to obtain a high degree of undercooling during traditional directional solidification technology due to the low thermal conductivity of ceramic materials,so it is difficult to produce a large-sized components with fine eutectic structure.Laser zone melting technology can obtain a high degree of undercooling,but solidification efficiency is very low due to the small beam spot.Therefore,it is still a challenge to make a large-sized oxide eutectic ceramics with fine eutectic structure via solidification technology.It was found that the critical electric field induced ultrafast mass transfer at low temperature during the flash sintering of oxide ceramics,which caused extremely rapid densification and phase transition.Hence,it is possible to prepare oxide eutectic ceramic at low temperature with the applied electric field.The new method will break through the challenge of making large-sized oxide eutectic ceramics.This thesis focused on the Al₂O₃-YAG eutectic ceramics,and the eutectic transformation process under electric field was investigated carefully.The main research work was as follows:（1）Using the constant heating flash sintering method,the onset temperature of flash sintering of Al₂O₃-YAG green body was determined to be 1500℃,and the onset electric field strength of flash sintering was determined to be 600 V/cm.Al₂O₃-YAG ceramic composite were prepared by direct flash sintering of Al₂O₃-YAG green body at a furnace temperature of1350℃.The ceramic composite was consisted by eutectic structure,polycrystalline region,segregation structure and pores.The hardness of eutectic structure was 18.5±0.3 GPa,the fracture toughness of 4.3±0.3 MPa·m^1/2,the mechanical properties close to eutectic ceramic prepared by directional solidification.（2）Using the constant heating flash sintering method,The onset temperature for flash sintering of Al₂O₃-Y₂O₃ green body was determined to be 600℃,and the onset electric field strength for flash sintering was determined to be 550 V/cm.It showed that solid-state reaction significantly reduced the onset conditions of flash sintering.At a furnace temperature of800℃,reaction assisted flash sintering was achieved using Al₂O₃ and Y₂O₃ as raw materials at low-temperature,and the Al₂O₃-YAG ceramic composite were prepared.The ceramic composite also contained eutectic structure,composite ceramic,segregated structure and pores.（3）The surface temperature distribution of the Al₂O₃-YAG green body was monitored online during the flash sintering,it was found that the peak temperature of flash sintering was1770℃,which was close to the eutectic temperature of 1820℃,but the steady-state temperature was only 1550℃during the flash sintering,which preliminarily proved the eutectic structure was formed via a solid-state processing.The peak temperature of Al₂O₃-Y₂O₃ flash sintering was 1538℃,and the steady-state temperature was only 1350℃during the flash sintering,which is far below than the eutectic temperature.It proved that the eutectic structure was formed by an electric field drove rapid diffusion process at solid-state.（4）The effect of the post-annealing time of the Al₂O₃-YAG green body was investigated carefully during the steady stage of flash sintering.It was found the size of the Al₂O₃ layer was refined from 620 nm to about 140 nm when the post-annealing time was extended from 0s to 240 s.A similar structural evolution of the eutectic structure was also observed at lower temperature when the post-annealing time of Al₂O₃-Y₂O₃ green body was adjusted during the steady stage of reaction assisted flash sintering,it proved that the electric field triggered the microstructure evolution at solid state during flash sintering.The dense Al₂O₃-YAG ceramic composite also underwent eutectic transformation with applied electric field,which proved that the densification and f eutectic transistion were two separated processes,the formation of eutectic occurred after densification.（5）Base on the results form temperature testing and microstructure characterization,it believed that Joule heating was not the dominant factor for the formation of eutectic.The formation of eutectic was related to the migration of Frenkel defects/oxygen vacancy under the applied electric field.