Research On Brazing Process And Mechanism Of YAG Ceramic And Kovar Alloy

Due to its high mechanical strength and stable physical and chemical properties,especially its outstanding properties,such as high transmittance to ultraviolet,visible and infrared light,Y₃Al₅O₁₂（Yttrium-Aluminium garnet-YAG）is regarded as a prominent crystal material for use in optical substrates,ultraviolet,bonding crystal materials and infrared windows.Nevertheless,its inherent brittleness makes it difficult to be processed into large-size or complex parts,and it is necessarily attached or joined to metals to realize its application.Kovar alloy has good ductility,low thermal expansion and excellent processing performance,and can be used as a sealing material between ceramic and metal parts.Therefore,YAG and Kovar assembly hybrid parts can be brazing to form benign complement in performance to meet the needs of engineering applications.In this paper,the wetting and spreading behavior of AgCu-based filler metal on YAG ceramics and Kovar alloy under different protective atmospheres（vacuum,argon）was investigated,which provides a theoretical basis for selecting appropriate brazing processes for YAG/Kovar joints.The YAG/Ag Cu Ti/Kovar brazing joint was obtained under a high vacuum of 6～8×10^-4Pa.The effects of different brazing processes（brazing temperature,holding time）on the microstructure（interface layer,intermediate phase）and mechanical properties（shear strength,nanoindentation）of YAG/Ag Cu Ti/Kovar brazed joints were investigated.And the microstructure evolution and fracture mechanism of the joint were elaborated for alleviating the relief of residual stress of YAG/Kovar joints.The results show that a good contact angle（θ=10.7°）was obtained in vacuum by wetting Ag Cu Ti filler metal on the YAG ceramic substrate,while the wetting spreading effect in Ar is poor and there are long cracks at the interface,and the higher oxygen content in the interface layer hinders the formation of Ti Y₂O₅reaction layer,which is not conducive to the wetting of Ag Cu Ti and YAG.The Ag Cu Ti/Kovar system also has a good wetting angle（θ=16.6°）under vacuum,while the wetting process in Ar is slow and the spreading effect is not significant.Therefore,Ag Cu Ti has better wettability with YAG and Kovar in vacuum than Ar.YAG/Ag Cu Ti/Kovar brazing joint is well bonded and there are no defects,and its microstructure is YAG/Y₂O₃/（Fe,Ni）Ti/Ag（s,s）+Cu（s,s）/Fe₂Ti+Ni₃Ti/Fe₂Ti/Kovar.YAG/Kovar joints have certain differences in the microstructure of different brazing temperature and holding time.With the increase of brazing temperature,the interface layer of（Fe,Ni）Ti gradually widens,and the content of Fe₂Ti+Ni₃Ti brittle phase gradually increases,which is most obvious at 950°C.When holding at 870°C for 1 minute,no（Fe,Ni）Ti layer was found on the YAG side and less band-like Fe₂Ti+Ni₃Ti structure was formed,and there are still some cracks on the YAG side.The joint structure holding for 30 min is similar to that for 10 min,except that a thicker（Fe,Ni）Ti layer（4.13μm）is formed.When the holding time was 60 minutes,no reaction layer was found on the YAG ceramic side and Ag-Cu eutectic diffusion into Kovar damaged the joint strength.In addition,the shear strength of the joint first increases and then decreases as the increasing brazing temperature.A joint with an optimal shear strength of 51.3 MPa was obtained at 870°C for 10 min.The nanoindentation reveals a clear difference in hardness between the phases.The hardness difference between YAG and（Fe,Ni）Ti is small,which reduces the residual stress between YAG ceramics and filler.