Study On Process And Strengthening Mechanism Of Electrochemical Corrosion-assisted Brazing Of C/C And Nb

The C/C composite material selects graphite as the matrix,and uses high-strength,high-modulus carbon fiber as the reinforcement.The resulting carbon fiber-reinforced ceramic matrix composite material has the characteristics of light specific gravity,friction resistance,and good thermal conductivity.It can be widely used in aerospace,civil industry and other fields.In practical applications,Nb with higher ductility is often used as a transition material for brazing connection with C/C ceramics.However,the difference in properties between the two leads to high stress concentration in the joint,which severely reduces the performance requirements in production applications.For this reason,this topic chooses to improve the ceramic surface structure,optimize the design of composite solder(BNi2+W),and HEA intermediate layer assisted brazing connection to solve the problems in the joint and realize the effective combination of C/C composite material and Nb.Based on the electrical conductivity of C/C ceramics and the difference in resistivity between carbon fiber and matrix,NaOH solution is used to adjust the surface structure of C/C ceramics by electrochemical corrosion.The improved ceramic structure increases the bonding area with the solder,and both Staggered to form a gradient transition to ease the difference in attributes.The study found that the corrosion effect of the ceramic surface reaches the best state under the corrosion parameters of 3V/30min.The solder penetrates into the gap around the fiber to form a mutual pinning structure.At this time,the depth of the C_f/AgCuTi interlaced zone is 80?m,and the joint strength can be increased to 37.7 MPa.In order to meet the needs of the joints at high temperatures and reduce the excessive brittle compounds in the weld,on the basis of using BNi2 as the brazing filler metal,a high-entropy alloy with excellent high-temperature mechanical properties was selected as the intermediate layer(HEA).HEA itself has high plasticity.While blocking the diffusion of metal Nb to the ceramic side during brazing,it can also absorb brittle compound forming elements in the brazing filler metal.The continuous reaction layer phase on the ceramic side is controlled,and the compound phase in the weld is mostly replaced by solid solution.When the thickness of the HEA is 200?m,the stress will be fully released.The best strength of the joint is 36.8MPa at room temperature and 29.3MPa at 800?.Since the HEA intermediate layer has limited absorption of brazing elements,in order to further adjust the difference in properties between ceramic and HEA,and at the same time improve the high-temperature performance of this area,the optimization is carried out by adding W particles with high melting point and low thermal expansion coefficient to BNi2.The formation of Ni₄W effectively suppresses the number of other Ni-containing brittle phases in the weld.At the same time,the solid solution combination of W on the Cr element in the solder makes the continuous reaction layer on the ceramic side thinner,and the uniform distribution of W and Ni4W in the weld To a certain extent,it also plays a role of dispersion enhancement.When the content of W(200nm)is 30wt%,the maximum strength of the C/C-HEA joint is 37.8MPa at room temperature and 26.4MPa at 1000?high temperature.Integrating the above three methods in the best state for brazing connection,the obtained optimized combination C/C-Nb joint shear strength can be increased to39.5MPa at room temperature and 33.6MPa at 1000?.