Microstructure And Mechanical Properties Of The Si₃N₄/316L Steel Brazed With Ag Based Composite Filler

Silicon nitride ceramic(Si₃N₄)has been considered one of the promising materials at both room temperature and high temperature due to its excellent mechanical properties and thermal stability.However,because of its brittle fracture characteristics and poor processing performance,it still faces technical challenges in manufacturing Si₃N₄ component with complex structures.Active metal brazing technology is normally used to connect ceramics to metal,which helps address this problem to some extent.However,the great difference in thermophysical properties between Si₃N₄ and metal(especially the coefficient of thermal expansion)causes large residual stresses in the brazed joints,resulting in poor mechanical properties of the joints.Therefore,it is of considerable importance for joint strength to alleviate the residual stress of the ceramic-metal joint.Under the purpose of relieving the stress of the joint so as to achieve better performance,the Si₃N₄ ceramic and 316L stainless steel were brazed by Ag-Cu-Ti alloy with some reinforced particles Mop into it.And the effects of brazing process parameters(including brazing temperature and holding time)and filler metal composition(including the content of Mop and Ti)on the microstructure and mechanical properties of the joint were researched.The results showed that with the increase of the brazing temperature,the holding time and the content of Ti in brazing filler,the thickness of interfacial reaction layer increased,and the shear strength of the joint first increased and then decreased;Moreover,the compensation of Ti contributed to the increase of Cu-Ti phase which was not conducive to the mechanical properties of the joint.When the content of Mop increased,the thickness of the interface reaction layer on Si₃N₄ side became thinner,the Cu-Ti intermetallic compound in the joint increased gradually,and the shear strength of the joint first increased and then decreased.Finally,the Si₃N₄-316L joint,brazed by(Ag₇₂Cu₂₈)₉₇Ti₃+5vol.%Mop composite brazing filler,reached its best mechanical properties with the shear strength of110.8MPa.To further relieve the residual stress of the joint,a buffer layer(Ag or Ni intermediate layer)was additionally used apart from the composite brazing filler.When(Ag₇₂Cu₂₈)₉₇Ti₃+5vol.%/Ag/Ag Cu composite interlayer was used to braze Si₃N₄/316L,the joint strength first increases and then decreases accompanied by corresponding increase in the thickness of Ag foil.The shear strength of the joint reached 205.4MPa with the Ag interlayer of 700?m used.The significant improvement of the joint strength attributed to the fact that the Ag layer can not only relieve the stress of the joint,but also prevent the reaction between Ti and Fe(or Ni)to form brittle intermetallic compounds which had negative effect on the properties of the joint.When(Ag₇₂Cu₂₈)₉₇Ti₃+5vol.%Mop/Ni/Ag Cu composite interlayer was used to braze Si₃N₄/316L,the joint strength first increased slightly and then decreased with the increase of Ni thickness,and the shear strength reached the highest value of 118mpa when the thickness of Ni layer was 200?m.In general,the mechanical properties of the joint with composite solder layer were superior to that without interlayer.The finite element analysis software ABAQUS was applied to calculate the residual stress distribution of the joint,and the influence of the content of Mop in the composite solder,the thickness of the composite solder and the thickness of the additional interlayer on the residual stress distribution of the joint was explored.The results indicated that the residual stress of the joint was mainly concentrated in the region of Si₃N₄ near the brazing seam,and the residual stress first increased and then decreased with the increase of mop content;The edge stress near the seam decreased with the increase of the thickness of the composite solder and the interlayer,which was almost consistent with the shear test results.