Research On Brazing Process And Bonding Mechanism In The Si₃N₄Ceramic Joint Brazed With Ag-Cu-Ti Based Composite Filler

Silicon nitride ceramics are very common structural ceramic materials. It iscritically important to investigate the Si₃N₄ceramic bonding. When the traditionalmetal brazing alloy was used to join Si₃N₄ceramic, the high residual stresses wereusually induced due to a large coefficient of thermal expansion （CTE） between theceramic substrates and brazing alloy. In this research, in order to decrease the CTEof the brazing alloy, Ag-Cu-Ti brazing alloy incorporated with SiCp or Mo particleswas introduced into joining the Si₃N₄ceramic. On that basis, the formationmechanism in the joint was analyzed. Furthermore, by incorporating a definiteamount of SiCp or Mo in the joint, a high joint strength could be obtained and thestrengthening mechanism was revealed by Digital Image Correlation technique.The Ag-Cu-Ti+SiCp composite filler was introduced into joining the Si₃N₄ceramic. When the brazing temperature was lower or holding time shorter, a lowerjoint bend strength was obtained due to insufficient reaction on the interface. Whilerising the brazing temperature or prolonging the holding time, the incorporatedSiCp in the brazing layer reacted with Ti completely, the microstructure obtainedshowed bad ability in decreasing CTE of the filler alloy, and thus resulting in alower joint strength. In this study, when the content of Ti was4wt.%in the joint, thejoint bend strength increased and then decreased with elevating the SiCp content inthe composite filler. In addition, when the content of SiCp was5vol.%in the joint,the joint bend strength also increased and then decreased with increasing the Ticontent in the composite filler. The highest joint strength （506MPa） was receivedwhile the Si₃N₄ceramic was brazed with (Ag₇₂Cu₂₈)₉₂Ti₈+5vol.%SiCp at900℃for10min.The Ag-Cu-Ti+Mo composite filler was introduced into joining the Si₃N₄ceramic. With elevating brazing temperature, the joint bend strength was increasedand then decreased. However, they decreased with prolonging holding time. Thejoint bend strength increased and then decreased with increasing the Mo particlescontent from0vol.%to20vol.%（Ti:4wt.%）. Lots of Ti-Cu intermetallics occurredin the joint while the higher content of Mo particles was incorporated, impairing theplasticity in the filler alloy and causing the lower joint bend strength. The joint bendstrength increased and then decreased while the content of Ti increased from2wt.% to5wt.%（SiCp:5vol.%）. The highest joint bend strength （429MPa） could beobtained while the Si₃N₄ceramic was brazed with （Ag72Cu28）96Ti4+5vol.%Mocomposite filler at900℃for10min.The formation mechanism in the Si₃N₄ceramic joint brazed withAg-Cu-Ti+SiCp composite filler was as follow: during heating, the brazing alloymelted and the active Ti element diffused towards Si₃N₄ceramic and react withthem. A TiN reaction layer with grain size of30-50nm was formed at the interface.The released Si atoms diffused towards the brazing alloy, a Ti₅Si₃reaction layerwith the grain size of200nm was produced at the TiN/brazing alloy interface.HRTEM analysis revealed that TiN reaction phases were formed along some crystalsurfaces of Si₃N₄ceramic. In the brazing layer, SiCp also reacted with Ti andformed Ti₃SiC₂and Ti₅Si₃. Ti₃SiC₂could be transformed into TiC and Ti₅Si₃owingto the reaction between Ti₃SiC₂and Ti, which was essentially caused by thede-intercalation of Si from Ti₃SiC₂due to the weak bonding between Ti and Si inthe Ti₃SiC₂.The formation mechanism in the Si₃N₄ceramic joint brazed with Ag-Cu-Ti+Mocomposite filler was as follow: the reaction products at the Si₃N₄/brazing alloyinterface were similar to the joint brazed with Ag-Cu-Ti+SiCp composite filler. Thecentral part of joint was mainly composed of Ag and Cu based solid solution, inwhich Mo particles and Ti-Cu intermetallics were dispersed. We observed that manykinds of Ti-Cu intermetallics precipitated in the joint, which was also validated bystandard Gibbs free energy of Ti-Cu intermetallics and TEM analysis in the brazinglayer. In addition, by nanoindentaion technique, the elastic modulus and hardnessfor Ti-Cu intermetallics are higher.In the research, a high quality joint was received while a definite amount ofSiCp or Mo was incorporated （5vol.%）. Here, SiCp were used as an example andstrengthening mechanism was revealed by DIC method. We believed that thefollowing three factors contribute the large joint strength: the lower macroscopicalresidual stresses level in the joint due to SiCp addition, the lower microscopicalresidual stresses due to an earlier and larger plastic deformation in the brazing layer,and the reinforcement effect of SiCp. In addition, the highest joint bend strengthcould be obtained in the joint brazed with Ag-Cu-Ti+SiCp composite filler at thesuitable brazing parameters and compositions, which were higher than the jointstrength brazed with the Ag-Cu-Ti+Mo composite filler. We believed that the optimum structure that relaxed the residual stresses to a large extent could beobtained while the Ag-Cu-Ti+SiCp were introduced by adjusting the content ofSiCp and Ti in the composite filler flexibly. However, the ability in adjusting theCTE mismatch between the joined materials by using Mo particles as theincorporation was limited. In addition, the in-situ formation of Ti-Cu intermetallicsdisplays the poorer mechanical properties than that of TiC and Ti₃SiC₂, so theycould not play the role of reinforcement in the joint and might become the source ofcracks during bending tests.In the research, the Ag-Cu-Ti+Mo composite filler was also used to join Si₃N₄ceramic and42CrMo steel. The miximum joint bend strength （587MPa） while thejoint was brazed with （Ag72Cu28）96Ti4+10vol.%Mo composite filler.