Microstructure And Thermal Shock Resistance Of Directionally Solidified Al₂O₃/SmAlO₃ Eutectic Ceramics

In this paper,the preparation process of Al₂O₃/SmAlO₃（Al₂O₃/SAP）eutectic ceramics by Optical Floating Zone（OFZ）method was explored.The directionally solidified Al₂O₃/SAP eutectic ceramics were successfully prepared by changing the length,shape and thickness of the preform,adjusting the seed crystal installation mode,pulling rate and rotation rate.The microstructure,composition,preferred orientation and interface relationship of directionally solidified Al₂O₃/SAP eutectic ceramics were studied.The mechanical properties of directionally solidified Al₂O₃/SAP eutectic ceramics were tested,especially the thermal shock resistance.The results show that:The cross arrangement of precursors during sintering can effectively improve the formability and density of the die,which is helpful to obtain more regular preforms.The macro defects of directional solidification of optical suspension zone melting can be effectively avoided by using 40 mm/h pulling rate and 20 rpm rotation rate.The results show that the transverse section and longitudinal section of directionally solidified Al₂O₃/SAP eutectic ceramics are interlaced and coupled in three-dimensional space,showing an irregular lamellar structure,and the longitudinal section shows an obvious growth trend.The XRD diffraction pattern of the prepared sample powder is completely consistent with the position of the diffraction peak of the standard PDF card.In the directionally solidified Al₂O₃/SAP eutectic ceramics,the preferred orientation of Al₂O₃ phase is<1 0 （?） 0>,while there are two preferred orientations of SAP phase,and the orientation relationship is<1 0 （?） 0>Al₂O₃//<100>SAP-1//<0 1 1>SAP-2.The interface between Al₂O₃ and SAP is smooth,flat and clear,and there is no amorphous phase.The interface matches well.The crystal plane relationship are（0 0 0 3）Al₂O₃//（0 1 1）SAP-1 and（0 0 0 3）Al₂O₃//（0 1 （?））SAP-2.According to the NCSL model,the volume strain of the two phases was calculated as εv1=-0.054 and εv2=-0.037,respectively.The two interfaces have almost the same volume strain,which is the main reason for the coexistence of the two orientations of SAP.The elastic modulus of directionally solidified Al₂O₃/SAP eutectic ceramics of transverse and longitudinal section are 400 ± 13.8 GPa and 378 ± 9.6 GPa,respectively.The average hardness of directionally solidified Al₂O₃/SAP eutectic ceramics is 16.74 ± 0.39 GPa,and the fracture toughness is 8.34 ± 1.11 MPa·m1/2.It is feasible to evaluate directionally solidified Al₂O₃/SAP eutectic ceramics by indentation-quenching method.The existence of critical thermal shock resistance temperature difference makes the eutectic components appear thermal shock instability and the thermal shock resistance shows anisotropy in transverse section and longitudinal section.The anisotropy of thermal shock resistance of directionally solidified Al₂O₃/SAP eutectic ceramics is mainly due to different thermal expansion coefficient and parallel interface during solidification.The flexual strength of directionally solidified Al₂O₃/SAP eutectic ceramics is 144.25 ± 3.41 MPa.The compressive strength of directionally solidified Al₂O₃/SAP eutectic ceramics is anisotropic,and the compressive strength parallel to the growth direction is 1.02 ± 0.16 GPa,and the compressive strength perpendicular to the growth direction is 0.78 ±0.36 GPa.There are cleavage steps in the compression fracture.