Research On Brazing Process And Mechanism Of Si₃N₄ Ceramic To(TiB+Y₂O₃)/Titanium Matrix Composite

Silicon nitride(Si₃N₄)ceramics exhibit excellent wave-transparent properties,corrosion and oxidation resistance,but their intrinsic brittleness makes them difficult to be machined.(Ti B+Y₂O₃)/Ti matrix composites possess high specific strength and stiffness,and excellent high-temperature mechanical properties.The joining of Si₃N₄ ceramics with(Ti B+Y₂O₃)/Ti matrix composites can realize the complementary advantages of the two materials and expand the range of material applications.Among many material joining methods,brazing is one of the most commonly used methods for joining ceramics to metals because of its simple process and high adaptability of joint size and shape.In order to achieve a high-quality joint between Si₃N₄ ceramics and(Ti B+Y₂O₃)/Ti matrix composites,a brazing joint between the two materials was performed in this work using Ag Cu/Cu foam/Ag Cu composite brazing filler in which porous metal foam Cu was sandwiched in the brazing filler,and the interfacial reactions and the composition of the physical phases in the joint were analyzed with the help of SEM,EDS and XRD,the brazing process parameters were optimized,the formation mechanism of the joint was investigated,and the residual stress distribution in the joint was analyzed by means of finite element simulation.A well connected TMC/Si₃N₄ joint was obtained after brazing with Ag Cu/Cu foam/Ag Cu composite brazing material with the interfacial structure of TMC/Ti Cu+Ti Cu₂/Ag(s,s)+Cu(s,s)/Ti Cu+Ti₅Si₃+Ti N/Si₃N₄ at the process parameters of 800°C/20min.It was found that the thickness of the Ti N+Ti₅Si₃ reaction layer formed on the Si₃N₄ ceramic side increased when the brazing temperature was increased or the brazing time was prolonged,and it was in accordance with the parabolic law controlled by diffusion,and the growth kinetics of the reaction layer was obtained by thermodynamic and kinetic calculations.The growth kinetics of this reaction layer was obtained by thermodynamic and kinetic calculations.However,the excessive brazing temperature and brazing time lead to excessive dissolution of the base material,more Ti dissolved in the liquid brazing material and increased metallurgical reactions in the joint,resulting in an increase in the number of formed Ti-Cu intermetallic compounds,which adversely affects the mechanical properties of the joint.To further improve the mechanical properties of the joints,the brazing parameters were optimized.The results showed that when the brazing time was fixed at 20 min,the shear strength of the TMC/Si₃N₄ joints could reach 165.0 MPa after brazing at a brazing temperature of 820°C.After optimizing the brazing time on this basis,the optimal brazing process was obtained as 820°C/10 min,and the shear strength of the TMC/Si₃N₄ joints reached about 180.4MPa.The shear strength of the TMC/Si₃N₄ joint reached the highest level of about 180.4 MPa.The mechanism of joining Si₃N₄ceramic to TMC is as follows:during the brazing of TMC to Si₃N₄,the Ag Cu filler melted first,subsequently,the liquid filler filled the pores of the foam Cu and wetted the foam Cu and the substrate.The Ti in the TMC dissolved into the liquid filler then diffused and reacted to the Si₃N₄,forming a uniform and continuous Ti N reaction layer,and the released Si reacted with Ti to form granular Ti₅Si_3,which acted as a nucleation site to promote the formation of Ti Cu on the surface of the Ti N layer.The Cu in the liquid filler also reacted with Ti to form minor brittle Ti-Cu intermetallic compounds.On the TMC side,the content of Ti was relatively high and reacted with Cu in the liquid filler to form a Ti-Cu compound layer,thus a joining of the Si₃N₄ ceramic to the titanium matrix composite was achieved.Due to the large difference in coefficient of thermal expansion between ceramics and metals,residual stresses are induced in the joints,which is an important reason for the low quality of ceramic/metal brazed joints.The residual stresses in the TMC/Si₃N₄ joints were analyzed by means of finite element simulation,and the results showed that the stresses were mainly concentrated in the Si₃N₄ ceramics and the stress levels were higher in the region near the brazing seam.The addition of Cu foam allowed a stronger plastic deformation of the brazing seam,which effectively relieved the residual stress in the joint and shifted the peak residual stress toward the brazing seam,reducing the risk of failure of the Si₃N₄ ceramic at the stress concentration.