Study on Microstructure and Mechanical Properties of the Sapphire Joints Bonded with Bismuth Glass at Low Temperature

Author:Guo Wei

Supervisor:he peng


Degree Year:2018





Sapphire,a single crystal form of pure alumina ceramics,has been widely used as large surveillance windows and high speed missile nose cones due to excellent mechanical and optical properties.However,such applications are often constrained by their physical size,because big size sapphire could not be obtained by avaliable manufacturing technology in the present.Thus,joining is a technology of choice used to mitigate the size limits,expanding its practical uses.In the present,the most commonly used method is active metal brazing.However,the joining process needs vacuum and high temperature,besid es,the CTE mismatch between metal braze and sapphire could lead to the innerstress in the joint.Realizing brazing sapphire at low temperature with a more compatible braze could save cost and release the interness in the joint.In our work,bismuth glass with low melting point was designed and used to join sapphire.Based on reaction characteristic of the bismuth glass/sapphire interfce,we aim to control the interfacial reaction through alting joining parameters and composition of the glass.Firstly,bismuth glasses with chemical composition of 50Bi2O3-30B2O3-20 Zn O,50Bi2O3-40B2O3-10 Zn O,50Bi2O3-50B2O3and40Bi2O3-60B2O3 were prepared by trational melting-quenching method.Thermal and mechanical properties of prepared bismuth glasses were characterized.The results showed that with ratio of B2O3/Bi2O3 increasing,the glass transition temperature(Tg),softening temperature(Tf),hardness(Hv)and three bending strength(σ)increased,the thermal expansion coefficient(α)and crystrallization trendency of the glass decreased.Based on the properties and wetting experiment analysis,sapphire joining process was determined,thus,the prepared glasses were employed to join sapphire.The results showed that when joining sapphire with 50Bi2O3-40B2O3-10 Zn O glass,the phases in the sapphire joints bonded at 600℃,625℃and 635℃were Bi4B2O9,Bi24B2O39and ZnAl2O4,respectively.With increasing of joining temperature,the size of Zn Al2O4 increased,and particle aggreration occurred when the temperature was≥675℃,and the aggreration was becoming more and more serious.With the joining temperature increasing,the shear strength of the joint first increased and then decreased,the maximum value of 85 MPa was achieved at 650℃.With holding time increasing at joining temperature of 650℃,the shear strength decreased.The in-stitu formation of dispersed Zn Al2O4 particles enhance the joint,could induced crack propagation and increase propagation path.When joining polycrystalline Al2O3 ceramics,the infiltration of glass into polycrystalline Al2O3 ceramics was observed.With the alumina purities decreasing,the infiltration was more and more serious,the joint shear strength decreased.The influence of joining temperature on microstructure and shear strength of the 95Al2O3 joint was then studied.The results showed that with the temperature increasing,the infiltration of the glass into95Al2O3 was becoming more and more serious,the joint strength first increased and then decreased,maximum value of 58 MPa was obtained when the joining temperature was 625℃.Through alting glass composition,bismuth borate glass,whose CTE is more compatible with sapphire was also used to as a joining media.In our work,we mainly studied joining characteristic of 50Bi2O3-50B2O3 glass/sapphire system.The results showed that when joining sapphire with 50Bi2O3-50B2O3 glass at tempreture of 600-750℃,Al4B2O9 whiskers were formed in the joint due to a reaction of sapphire and the glass.With joining temperature and holding time increasing,the size of the Al4B2O9 whisker increased,the joint strength first increased and then decreased.The in-stitu formation of Al4B2O9 whiskers in the joint enhanced the CTE match between the seam and sapphire,and exhibited effective resistance to the cracks growth.The maximum shear strength of 95 MPa was obtained then.When bonding sapphire with 40Bi2O3-60B2O3 glass at 700℃for 20 min,compared with the sapphire joint bonded with 50Bi2O3-50B2O3 glass at the same temperature,the size of Al4B2O9 whisker was larger,and the joint shear strength was higher.The influence of Al2O3 crystal form on microstructure and mechanical properties of the joint was studied.The infiltration of glass into polycrystalline Al2O3 could be observed,and with Al2O3 purities decreased,the infiltration was more and more serious and the joint shear strength decreased.