Research on Microstructure and Properties in Porous Si3N4 Ceramic/Invar Alloy Joint Brazed with Composite Filler

Author:Liu Jia Yin

Supervisor:zhang jie


Degree Year:2018





The porous Si3N4 ceramic has been considered to be an ideal material for the missile radome due to its low density,good dielectric property and proper loading capacity.However,the components cannot be fabricated and assembled directly,attributed to its poor machinability.Therefore,the porous Si3N4 ceramic is generally applied by bonding to metal hoops.Since there exists large difference between these two materials in physical properties,for example,the coefficient of thermal expansion(CTE)and deformation ability,large thermal stress is always unavoidable,which dominates the reliability of the component.In the present work,the Ag-Cu-Ti(+Mop)/Cu/Ag-Cu composite filler has been developed on the basis of the traditional Ag-Cu-Ti filler,in which the Mop refers to the Mo particles,in order to realize the bonding between porous Si3N4 ceramic and Invar alloy.The optimal filler composition has been revealed,and the effect of brazing parameters are studied,thereby illustrating the brazing mechanisms.In addition,the residual stress distribution in the Si3N4/Invar joints is simulated by finite element method.The wetting behaviors of Ag-Cu-Ti filler,as well as the Ag-Cu-Ti+Mop composite filler on both two substrates have been studied.In the case of these two fillers on the porous Si3N4 ceramic,the wetting behavior could be divided into three stages: melting,spreading and steady.In addition,the infiltration of the liquid filler into the ceramic substrate is observed,which could be improved by increasing the Ti content.Therefore,the Ti element is considered to be reactive to the ceramic substrate during the wetting process,which provides the driving force for wetting and infiltration.The introduction of Mo particles is found to have significant effects on the fluidity of the filler: a remarkable increase in the contacting angle between filler and ceramic substrate is observed with the rising content of Mo particles.In the other case of fillers on the Invar alloy,the Ag-Cu filler exhibits excellent wettability,totally spread out on the surface,which is mainly attributed to the solution and diffusion process between the filler and Invar substrate.On the other hand,the Ag-Cu-Ti filler is found to have worse wettability,due to the formation of Ni3 Ti and Fe2 Ti intermetallic compounds with the presence of Ti element.In the brazing process using Ag-Cu-Ti filler,the typical structure of the Si3N4/Invar joint is: porous Si3N4/reaction layer/Ag-Cu eutectic+Ni3Ti/Fe2Ti/Invar.While the Ag-Cu-Ti/Cu/Ag-Cu composite filler is applied,the Cu interlayer acts as an isolation which prevents the formation of intermetallic compounds.In addition,a thin Cu interlayer would completely dissolve into the liquid filler during the brazing process,and a thick one is found to be harmful for the mechanical properties of the joints.There is an optimal content for Ti element as well: when the Ti content is low,it is difficult to obtain a reliable bonding due to the weak reaction;when the content is high,the Cu interlayer would be consumed and more intermetallic compounds are found.The influence of brazing parameters on the microstructure of Si3N4/Invar joint is studied,which turns out to be similar: low brazing temperature and short holding time will result in inadequate reaction,whereas high brazing temperature and long holding time can lead to the deterioration of joint microstructure.The CTE mismatch between porous Si3N4 ceramic/filler can be lowered by introducing Mo particles into the multi-layer filler(Ag-Cu-Ti+Mop/Cu/Ag-Cu).Mo can absorb Ti at high temperature and release it during the cooling process.The released Ti reacts with Cu to promote the dissolution of Cu interlayer.Thus,a large content of Mo particles could destroy Cu interlayer,thereby generating intermetallic compounds.In conclusion,the optimal parameters are: brazing temperature 900 °C,holding time 20 min,with the filler composition Ag-Cu-10 wt.%Ti+5.1wt.%(5vol.%)Mop /Cu/Ag-Cu.The corresponding maximum joint shear strength is 83 MPa,improved by 84% compared with the joint brazed with Ag-Cu-Ti filler,which is close to the shear strength of the porous Si3N4 ceramic substrate.The model of porous Si3N4/Invar system is built by ABAQUS finite element simulation software.The residual stress distribution in the cooling process is simulated.The results show that in the cooling process,the residual stress is concentrated at the ceramic/filler interface.Since the CTE of Invar alloy changes remarkably around the Curie point,the maximum value of the residual stress appears during the cooling state.The effect of filler thickness is investigated as well.It is found that the filler with a proper thickness is beneficial for reducing the internal stress on the ceramic side,but a thick filler has a negative impact on the loading capacity.The introduction of Cu layer can optimize the joint microstructure,improve the plastic deformation ability of the filler,thereby relieving the residual stress.But the addition of Cu interlayer thickness will inevitably increase the whole filler thickness and reduce the joint strength.The addition of Mo particles is helpful to lower the residual stress.However,when the content is high,it is harmful for the wetting process and will deteriorate the microstructure of the joint.A thin reaction layer is not conducive to the combination between ceramic and filler,while the reaction layer is too thick,large residual stress will be introduced on the ceramic side.During the cooling state,the fillet forms and helps to tighten the ceramic substrate,which is considered to capable of bearing load as well in the uploading process.Afterwards,the detailed models of the porous ceramic substrate,the infiltration of reaction layer and the composite filler with Mo particles are built using the sub-model module.Obvious residual stress concentration is observed on the angular position in the porous ceramic.In addition,the reaction layer generates large thermal stress in the sub-models,which contributes to an even stress distribution at the same time.The introduction of Mo particles helps to increase the loading capacity,which is beneficial for the mechanical properties of the joint.In summary,the achieved simulation results provide a reasonable explanation for the influence of the above factors on the joint performance,which are consistent with the experimental results.