Study On Microstructure And Properties Of SiC Ceramic Joint With CaO-Al₂O₃ Based Solder

In the energy field,nuclear energy with clean,green,renewable characteristics has become the focus of future development.However,safety is the most critical issue in the development of nuclear energy.Especially,the nuclear fuel cladding,as a structural component to protect nuclear fuel from leakage,is the most important issue in nuclear energy safety.SiC ceramic has become one of the candidate materials for the new generation of nuclear reactor core bacause it possesses high strength,high melting point,excellent irradiation dimensional stability and good corrosion resistance.Realing the radiation resistant joining of SiC ceramics can effectively overcome the shortcomings of ceramic brittleness and poor machinability,which has broad research and application prospects.Based on this background,some novel glass solders were designed and prepared to join SiC ceramics.By simulating irradiation damage of possible phases in the SiC joint,the CaO-Al₂O₃（CA）based solder was designed to join SiC ceramics.Explore the properties of solder itself and the influence of different solder components and different joint processes on the structure and performance of the joint.In addition,the microstructural evolution,joining mechanism and residual stress distribution of the joint were revealed.In this paper,CaO-Al₂O₃（CA）glass solder,CA+SiCp composite solder and CaO-Al₂O₃-SiO₂（CAS）glass-ceramics were designed for joining SiC ceramic under the background of nuclear irradiation.The possible phases in the joints were2CaO·Al₂O₃·SiO₂,CaAl₂Si₂O₈,SiO₂and 12CaO·7Al₂O₃,which were simulated by the SRIM program.The results showed that the value of radiation damage peak and He concentration peak of each crystal phase are smaller,which was close to SiC ceramic.The results of research on CA solder showed that the melting point of CA glass solder was 1437℃.The activation energy of crystallization was 274.07 kJ/mol and the precipitated crystal was 12CaO·7Al₂O₃.The crystallization activation energy of CAS glass is 483.7 kJ/mol.The crystallization activation energy of CAS glass with the nucleating agent at different crystallization peaks was 373.3 kJ/mol and 104.6 kJ/mol,respectively.Furthermore,the precipitated CAS glasscrystal was anorthite（CaAl₂Si₂O₈）.The thermal expansion coefficient of CAS glass is 5×10^-6/℃ amd the crystallized glass-ceramics decreases to 4.86×10^-6/℃,which matched that of SiC ceramics(4×10^-6/℃).By studying the wetting behavior of CA based solders with different compositions on SiC ceramics,it was found that the formation of a layer of SiO₂ film on the surface of SiC ceramics by preoxidation would improve the wettability.The minimum wetting angle of CA solder and CAS solder on pre-oxidized SiC ceramic was 23°and 28°.Using CA solder to join pre-oxidized SiC ceramics,the joint interface was well bonded,and two kinds of crystal phases 2CaO·Al₂O₃·SiO₂ and 12CaO·7Al₂O₃were formed in the joint.However,there are cracks and holes in the joint resulting the maximum shear strength was 32.5 MPa.The CA+SiCp composite solder was used to join the pre-oxidized SiC ceramic.The SiC ceramic particles were evenly distributed in the joint,which can effectively avoid the occurrence of cracks in the joint.The optimal joint strength can be obtained by using CA+20wt.%SiCp composite solder,which was 42MPa.When CAS based solders were used to join pre-oxidized SiC ceramics,only anorthite was precipitated in the joint.By adjusting the bonding temperature and adding heat treatment process parameters,the shear strength of the CAS glass joint was 51MPa under the process of at 1450℃ holding for 10min,while after holding at 1050℃ for 120min,the shear strength of the joint is increased to 86MPa.Raman laser was used to measure the joint residual stress,and it was found that the residual stress in the joint perpendicular to the seam presented a trend of first compressive stress,then tensile stress.The residual compressive stress in the joint had a great influence on the mechanical properties of the joint.The residual stress of CAS joint was larger than that of the joint treated by CAS crystallization.