Research On Reinforcement And Mechanism Of Al₂O₃ Joints Brazed By Borosilicate Glass

Al₂O₃ ceramic is a kind of high temperature structural material with exc ellent performance,widely used in aerospace,defense military,electronics,machinery,etc.Currently,for the manufacture of Al₂O₃ components with complex shapes or large-size,the most effective way is to connect Al₂O₃.To meet the requirements of high strength and low cost for the Al₂O₃ ceramic joining simultaneously,this paper brazed the Al₂O₃ ceramic using the borosilicate glass as the solder.The properties of joints were characterized by Scanning Electron Microscope?SEM?and X-Ray Diffractometer?XRD?,thermal analyzer,and high temperature wetting angle measuring instrument.The microstructure and interface products of the joints were characterized by Electron Probe?EMPA?and X-Ray Photoelectron Spectrum Analyzer?XPS?.The mechanical properties of the joints were tested by Shimadzu electronic universal testing machine and micro-hardness tester and the influence of the process parameters were investigated.Besides,the formation mechanism of the interface products and its reinforcing mechanism for the joints were researched.The 32.3Si O₂-64.7B₂O₃-3Na₂O?mol.%?glass,the 48.5Si O₂-48.5B₂O₃-3Na₂O?mol.%?glass and the 64.7Si O₂-32.3B₂O₃-3Na₂O?mol.%?glass were prepared by high-temperature melting method,The transition temperatures are 349°C,599°C,and 765°C,respectively.The three glasses are all amorphou containing[Si O₄],[BO₃],and[BO₄]network structures.All of them perform a perfect wettability on the surface of Al₂O₃ ceramics with small contact angles less than 25o.The hardness,characteristic temperature,and viscosity of the glass increase with the increasing of n?Si O₂:B₂O₃?.The 32.3Si O₂-64.7B₂O₃-3Na₂O?mol.%?glass was well-bonded to the Al₂O₃ceramics and the aluminum borate whiskers which the main component is Al₄B₂O₉are formed in the interface.The whisker length and diameter increased with the higher bonding temperature and the longer hoiding time.Consequently,the joint strength reaches a maximum of 60 MPa at the brazing temperature of 1050°C for 60 minutes.The mechanical properties of the intermediate layer are improved by the diffusion of Al element into the glass solder,which can repair the network structure of the glass.In addition,the whiskers generated at the joints act as a stitch on the interface and the reinforcement mechanism of it is mainly the pull-out effect.The 48.5Si O₂-48.5B₂O₃-3Na₂O?mol.%?glass was also connected perfectly to the Al₂O₃ ceramics.A large amount of aluminum borate whiskers are generated both in the interface and the interlayer resulting in a whisker reinforced borosilicate glass composite joint under the brazing temperature of 1250°C.The main component of the whisker is Al₁₈B₄O₃₃.The maximum joint strength is 136 MPa with the holding time of 60 minutes.The strengthening mechanism of the whiskers in the glass matrix is pull-out effect and crack bridging.And the reinforment mechanism at the interface is pull-out effect and absorption effect.The whiskers play the role of pinning and stitching on the interface to reinforce the joints.The joints between64.7Si O₂-32.3B₂O₃-3Na₂O?mol.%?glass and Al₂O₃ ceramics appeared cracks.Meantime,a large amount of thick whiskers generated at the interface reduced the joint strength.In general,the high strength connection between Al₂O₃ caremics has been achieved by 48.5Si O₂-48.5B₂O₃-3Na₂O?mol.%?glass at a low cast.