Formation Mechanism And Discoloration Behavior Of Zirconia Ceramic Active Brazing

ZrO₂ ceramic,as an important engineering and structural ceramic,has high hardness,good high temperature performance and wear resistance.Niobium,as a kind of refractory metal,has excellent ductility,low ductile-brittle transition temperature and good corrosion resistance.The reliable connection between ZrO₂ ceramic and Nb has been commonly applied in the high-temperature and corrosion-resistant space propulsion systems.Active brazing,as a kind of connection method,has been widely used in metallurgical joining of ceramic and metal.In this paper,our research into adding Mo particles to Ag-Cu-Ti filler metal to improve the bonding properties of Nb and ZrO₂ ceramic brazed joints.In this paper,the microstructure evolution,as well as the mechanism for the formation of joints brazed with composite fillers of differing Mo content,is discussed based on the scanning electron microscopy,X-ray diffraction and transmission electron microscopy.The results show that the TiO phase adjacent to the ZrO₂ceramics occupied the preponderance over Ti₃Cu₃O to form a constant reaction layer,and the further growth of the Ti₃Cu₃O layer comes through consumption of the TiO layer.The final reaction layer nearby the ZrO₂ ceramic was consisted of Ti₃Cu₃O layer and remaining TiO layer,which was affected by the calculated Ti activity and the available interfacial Ti concentration adjacent to the ZrO₂ ceramic,can be adjusted by altering the Mo content of the composite filler.The effects of Mo content on the microstructure and mechanical properties of the ZrO₂ ceramic/Nb joint brazed with Ag-Cu-Ti+Mo composite filler were studied.The mechanical properties of the joint were affected by the thickness of the ZrO₂ ceramic reaction layer,the interfacial microstructure and the internal residual stress of the brazed joint.The shear strength of the brazed joint was clearly improved when a suitable amount of Mo added to the Ag-Cu-Ti filler.The maximum shear strength reached 370MPa for the joints brazed with 5wt.%Mo additive,which was 150%higher than that of joints brazed with single Ag-Cu-Ti filler metalAfter analyzing the surface of discolored ZrO₂ ceramic matrix and the brazing interface of ZrO₂ ceramic/filler by XRD,XPS and ESR.The discoloration mechanism of ZrO₂ ceramic during the active brazing was clarified,which is attributed to the formation of oxygen vacancy,also provides evidence for the reaction between ZrO₂and Ti.With the increase of brazing temperature and holding time,the thickness of titanium oxide layer nearby the ZrO₂ ceramic increased.It means that the contents of Ti and O involved in the interfacial reaction increase continuously,the corresponding ceramic discoloration was also more obvious.Moreover,various mechanic properties of ZrO₂ ceramic matrix before and after brazing compared remarkably.