Microstructure And Mechanical Properties Of SiC Joints Brazed With SiTi-x?x=SiC,CNT,B₄C? Composite Fillers

Silicon carbide?SiC?ceramic has high strength,high melting point,high Vickers harderness,good neutron irradiation as well as good chemical stability.Due to their excellent physical and chemical properties,SiC ceramics have been regarded as an ideal structural materials for extremely rigorous fields such as aerospace,electronics and nuclear industry.However,it is difficult to fabricate SiC components with large dimensions and complex shapes due to the limitation of technology and equipment.Therefore,joining technology for SiC ceramic is a critical factor of restriction for its applications.In order to satisfy the requirements for SiC brazed joint in high-temperature field,in this study,the brazing process parameters of SiC ceramic brazed with Si-24Ti?wt.%?eutectic alloy were optimized,and the braze exhibits good wettability for SiC ceramic.On this basis,the composite brazes prepared by the mix of Si-24Ti?wt.%?alloy powders and nano SiC particulates or carbon nano tubes or B₄C powders,and the influence of formation and distribution for reinforcement particles on microstructures and mechanical properties of SiC joints were systematically investigated.In addition,the strengthening mechanisms of different reinforcement phases were clarified.This work have important practical application value and theoretical significance for engineering applications of SiC ceramics.The main research consequences are:?1?The SiC joints were obtained using Si-24Ti eutectic alloy consist of TiSi₂ and Si phases as braze at 1360-1400?with the holding time of 10-30 min.The important influence of brazing temperature and holding time on microstructures and mechanical properties of SiC joints were investigated.SiC joint with the shear strength of 88.5 MPa was brazed at 1380?with the holding time of 20 min.?2?With the addition of SiC particles??1 wt.%?with the size of 60 nm,the particle sizes of Si and TiSi₂ in SiC joint were refined,and the volume fraction of Si+TiSi₂ eutectic zone was increased.When SiC joint brazed with SiTi-1SiC,the refinement efficiency of grain size was optimized,and the shear strength of SiTi-1SiC joint could reach 106.3 MPa,which was 20.1%higher than SiTi-0SiC joint.For SiTi-1.5SiC joint,the agglomeration of SiC reforcement particles was observed.?3?Brazing of SiC ceramics with carbon nano tubes reinforced Si-24Ti eutectic braze was feasibly.With addition of CNT,the dispersed distribution of SiC particles formed by in situ reaction of Si-Ti eutectic brazes and CNT,and the microstructure of brzing seam and the strength of joint were improved.When SiC joint brazed with SiTi-3CNT,its shear strength could reach 108.1 MPa, which was 22.1%higher than without CNT.However,with the addition of 5 wt.%CNT,the shear strength?55.1 MPa?of SiC joint was decreased dramatically,due to the cavities in brazing seam of SiTi-5CNT joint.?4?The dispersed distribution of SiC and TiB₂ particles with grain size of the micron and submicron formed by in situ reaction of Si-Ti eutectic brazes and B₄C.With the increase of B₄C content,the volume fraction of SiC and TiB₂ phases in brazing seam,the Vickers hardness of brazing seam,and the shear strength of joints was increased gradually,until the maximum value of shear strength and hardness in brazing seam were obtained for the joint brazed with SiTi-5B₄C filler,which was 114.5 MPa and 1139.7 Hv_0.05,respectively.However,with the addition of 7 wt.%B₄C,mechanical propersities of SiC joint was decreased dramatically due to appearance of a large number of cavities.?5?Due to the enormous mismatch of CTE between TiSi₂ and SiC or Si_ss,the residual stresses are easily formed during cooling after brazing,and the interfacial faces of TiSi₂/SiC and TiSi₂/Si_sss are incompatibility,which leads to the formation of micro-cracks in coarse TiSi₂ phase.Ceramic reinforcement phases in brazing seam formed multiple nucleation points,which restrict the growing of Si and TiSi₂ phases,and the grain sizes of Si and TiSi₂ phases could be refined.The fine size of TiSi₂ phase could improve the generation of micro-cracks,and then the mechanical propersities of SiC joint could be increased.Additionally,the fine size of SiC and TiB₂ ceramic phases acted as an enhancement for the strength of brazing seam.