Research On Process And Bonding Mechanism For Joining Bn-Si₃N₄ And AlON With Ag-Cu-Ti Filler

Si₃N₄ ceramic with outstanding mechanical properties and dielectric properties is an ideal wave-transmitting material applied on fabricating missile radome.The adulteration of h-BN in Si₃N₄ ceramic has some damage to the mechanical properties of Si₃N₄ ceramic,while it can improve the dielectric properties significantly.Therefore,the wave-transmitting ability of BN-Si₃N₄ composite ceramic is better than Si₃N₄ ceramic.Meanwhile,aluminum oxynitride?AlON?ceramic has high strength and high hardness,excellent resistance to rain and sand damage and desirable visible-MWIR ability,which is good for a infrared dome used in missile radomes.Therefore,the reliable joining of BNSi₃N₄/AlON is crucial to fabricate infrared guided missiles.In the paper,AlON ceramic was prepared by SPS technique firstly.The wetting and spreading behavior of Ag-Cu-Ti filler alloy on BN-Si₃N₄ ceramic and AlON ceramic was studied.Then Ag-Cu-Ti filler foil was used to braze BN-Si₃N₄ ceramic and AlON ceramic.The effect of brazing temperature,holding time and the thickness of filler foil on the microstructure and mechanical properties was studied.Finally,thermodynamic analysis was used to explain the mechanism of brazing.AlON ceramic was prepared by SPS technique using Al₂O₃ powder and Al N powder under vacuum condition.The results show that the amount of AlON in the sample increases with increasing the sintering temperature and holding time.Pure AlON ceramic was successfully prepared at 1650? for 15 min when the molar ratio of Al₂O₃ and Al N is 62 : 38.The wetting and spreading behavior of Ag-Cu-Ti filler on BN-Si₃N₄ ceramic and AlON ceramic is reactive wetting,which can be divided into four stages: initial melting stage of filler alloy,slow change of contact angle,rapid change of contact angle and equilibrium stage.The typical microstructure of BN-Si₃N₄/BN-Si₃N₄ joint is: BN-Si₃N₄ ceramic/Ti N+Ti B2+Ti5Si3 layer/Ag-Cu eutectic/Ti N+Ti B2+Ti5Si3 layer/BN-Si₃N₄ ceramic.The typical microstructure of the BN-Si₃N₄/AlON joint is: BN-Si₃N₄ ceramic/Ti N+Ti B2+Ti5Si3 layer/Ag-Cu eutectic+block Al Cu2 Ti phase+ Cu?s,s?/?Cu,Al?3Ti3O reaction layer/AlON ceramic.With rising the brazing temperature,prolonging the holding time or increasing the thickness of the filler alloy,the shear strength of the joints first increased and then dropped.The joint with maximum strength of 104 MPa has been obtained when it was brazed at 850 °C for 15 min and the thickness of the filler foil was 150?m.The brazing mechanism of BN-Si₃N₄/AlON joint can be divided into three stages,a: Ag-Cu-Ti filler foil melt and converted to liquid state,then active element Ti diffused to substrates;b: Ti reacted with BN-Si₃N₄ ceramic and AlON ceramic to form Ti N+Ti B2+Ti5Si3 reaction layer and?Cu,Al?3Ti3O reaction layer respectively.c: in the cooling process,liquid phase solidified to form room temperature microstructure.