Research On Processing And Mechanism Of Brazing SiO₂-BN Ceramics And Nb

Due to the excellent mechanical property, electrically transparent property and thermal shock stability, Si O2-BN ceramics is regarded as an important candidate material for developing advanced thermal protection parts, such as the radome of missile which requires the Si O2-BN ceramics be brazed to Nb. However, the high thermal residual stress in the Si O2-BN ceramics/Nb joints caused by the difference of coefficient of thermal expansion restricted the application of brazing. In this study, the Si O2-BN ceramics was brazed to Nb using Ti Ni Nb filler. Furthermore, in order to improve the mechanical property of the joints, porous Si3N4 was used as interlayer to control the residual stress.In order to investigate the mechanism of the reaction wetting of Ti Ni Nb filler/Si O2-BN ceramics system, the wetting and spreading behavior of Ti Ni Nb filler liquid on the Si O2-BN ceramics was researched. The result showed that the formation of Ti N and Ti B2 could determine the wettability between Ti Ni Nb and Si O2-BN ceramics. When the Ti at the interface of Si O2-BN ceramics was enough to occur reactions, Ti N and Ti B2 were formed at the Si O2-BN ceramics interface. Because of the good affinity of Ti and Ti N, more Ti was carried to the Si O2-BN ceramics interface, and kept the interface reactions running. At the same time, a good wettability of Ti Ni Nb filler on the surface of Si O2-BN ceramics was observed. With the increase of the content of BN in the Si O2-BN ceramics, the wettability of Ti Ni Nb filler on the surface of Si O2-BN ceramics improved. The porous Si3N4 was used as an interlayer between the Si O2-BN ceramics and Nb. Results showed that the porous Si3N4 interlayer could release the residual stress in the joint and improve the shear strength. The shear strength reached 64 MPa when the thickness of porous Si3N4 interlayer was 70μm.The typical interface of Si O2-BN ceramics/Si3N4/Nb joints at 1180℃/10 min was: Nb/Ti2Ni+Ti Ni-(βTi,Nb)+Ni9Ti3Nb8/Si3N4+Ti N+Nb3Ni2Si/Ti Ni+Ni9Ti3Nb8/Ti N+Ti B2+Ti5Si3+Ti3O5/Si O2-BN ceramics. In order to adjust the thermal expansion coefficient of the interlayer and relieve the residual stress of the joints, the double interlayer of the porous Si3N4 was designed and the effect of the interlayer on the joint’s mechanical properties was discussed. The shear strength of the joint increased when the interlayer became thicker, and the shear strength of the joint reached 80 MPa using double interlayers(the thickness of the interlayers were both 80μm). Moreover, with the addition of different thickness of double interlayer(70μm and 90μm, respectively), the shear strength of the joint was increased to 88 MPa. In this study, the reinforced mechanism was also analyzed. Based on the compound material mechanics theory and finite element simulation method, thermal expansion coefficient and elastic modulus of the interlayer of porous Si3N4 and brazing seam were deduced in detail. The result showed that, contrast with direct brazing, the maximum residual stress moved from Si O2-BN ceramics to brazing seam by the addition of the interlayer of porous Si3N4. In the direct brazing joints, the maximum residual stress was 381 MPa. When using the different thickness of double interlayer of porous Si3N4, the maximum residual stress greatly decreased to 163 MPa, which mean that the porous Si3N4 interlayer could release residual stress in the joint of Si O2-BN ceramics and Nb.