Research On The Reaction Mechanism Of Ni-based Brazing Filler Metal And SiC Ceramics And Optimization Of Brazing Filler Metal Compositio

Silicon carbide（SiC）is a kind of ceramic material with a high melting point,high specific strength,good thermal and electrical conductivity,excellent high-temperature performance,strong corrosion-resistance,strong radiation resistance,so in all walks of life,especially in the aerospace field and nuclear industry,SiC has a broad application prospect.However,it is challenging to synthesize and process SiC components with irregular shapes or complex structures.Vacuum brazing with Ni-based brazing filler is considered the most promising brazing system to be used in the brazing of future SiC ceramic served in extreme conditions,such as irradiation and high temperature.However,amounts of graphite phase form in the brazed seam when Ni-based brazing alloys are used to braze SiC.The joint mechanical property tests show that the SiC joint always fractures in a region where graphite exists,and the joint’s strength is proved low.The properties of the SiC joints brazed by current Ni-based brazing alloys need to be improved,and the key to this problem is to suppress the formation of graphite in the brazed seam.Aiming that target,We select a Ni-based alloy with the major constituent elements of Ni,Cr to brazed SiC,and research on the joint’s microstructure,reaction mechanism between the brazing alloy and SiC,the formation of graphite and its effects on the joint’s performances,and the methods of suppressing the graphite formation are carried out.Here are the research results.（1）The essence of the reactions between Ni-based brazing alloy and SiC ceramic is all about the reactions between the element Ni in the alloy and SiC ceramic.During the heating stage when brazing,the Ni element of the brazing alloy can react with SiC to form Ni-Si silicides and release C atoms.Due to the low solubility of C in the products,C finally precipitates as graphite.As the reactions between Ni and SiC continue,the reaction zone thickens,the graphite phase grows and connects,and forms a network structure.Meanwhile,the interfacial reactions release amounts of Si into the liquid brazing alloy.With the increment of Si in the liquid,the liquid’s Gibbs energy increases,weakening the reaction activity between the liquid and SiC.Once the liquid reaches a composition of 33.8 at%Si,49.1 at%Ni,17.1 at%Cr,a balance is reached,and the liquid stops reacting with SiC.In the cooling process,a mixture of Ni-Si silicides and graphite phase forms on both sides of the brazed seam,andδ-Ni₂Si+Cr₃Si eutectic structure and Cr₆Ni₁₆Si₇（G phase）form in the center.（2）The graphite in the brazed seam has poor oxidation resistance to high-temperature air,which seriously deteriorates the joint’s high-temperature resistance and mechanical properties.The oxidation resistance in air at 800 ^oC of the SiC joints brazed by Ni-Cr alloy is evaluated in this paper.It is found that the graphite can get easily oxidized in high-temperature air,leaving voids and cracks form in the reaction zone.The oxygen continues to penetrate into the inner part of the joint,the entire brazed seam gets oxidized within only 1 hour.With the extension of oxidization time,the graphite’s volatilization is intensified.Ni-Si compounds fall off in the reaction zone once graphite is volatilized and severely deteriorates the brazed seam.The maximum average shear strength of the joint before oxidation is 28.2 MPa.However,after 185 h oxidation,the shear strength drops to 5.5 MPa,the fracture occurs in the reaction zone.In conclusion,the critical factor in improving the joint’s properties is to suppress the formation of graphite as a product of the brazing alloy and SiC ceramic reactions.（3）It is predicted according to thermodynamic calculations and proved that adding elements such as Si and Zr can suppress graphite formation in the brazed seam,the joint’s oxidization resistance and mechanical properties are improved.It is found through thermodynamic calculations that the higher the Si content in the liquid brazing alloy,the lower the Gibbs free energy of the liquid phase and the lower the driving force of the reaction with SiC ceramic.Therefore,increasing the Si content in the liquid should be helpful to suppress the liquid and SiC reactions and avoid graphite formation.In addition,it is also found that there is a competitive relationship between Zr and Ni since Zr has a strong reaction tendency to SiC as well.It is calculated that when Zr content surpasses a critical value,the Ni/SiC reactions are drastically suppressed,and Zr/SiC reactions take place,Zr C forms instead of graphite.Based on all these calculations above,23.1 wt%Si and 40 wt%Zr are added into the Ni-based brazing alloy,respectively.The experimental results show that the graphite phase formation is totally suppressed when 23.1 wt%Si or 40 wt%Zr is introduced into the Ni-based brazing alloy.The mechanical properties and high-temperature oxidation resistance in the air of SiC joints are significantly improved once the graphite formation is avoided.（4）Brazing SiC_f/SiC composite ceramic with Ni-based filler metal will cause fiber damage,the quality of SiC_f/SiC composite ceramics can be guaranteed by adding 23.1wt%Si into the brazing alloy.Due to the defects of SiCf/SiC composite ceramic,such as cracks,voids,and other defects,the liquid brazing alloy penetrates into the composite ceramic under the action of capillary and reacts with SiC fiber during the brazing process,resulting in fiber damage.When 23.1 wt%Si is added to the brazing alloy,the reaction tendency of the liquid phase and SiCf/SiC composite ceramic is significantly inhibited.Neither SiC matrix nor SiC fiber reacts with the liquid alloy,the SiC fiber remains intact after brazing,and the integrity of SiCf/SiC composite ceramic is guaranteed.