Joining Of Zirconia And Ni Using Sn-Ag-Cu Fillers Under Direct Current

Zirconia ceramics doped3%molY2O3are widely used in various fields of engineeringtechnology because of its high strength and good ionic conductivity. However, it is brittleand shows a poor machinability, which makes it difficult to fabricate complex-shaped andlarge-sized components. Joining of ceramic/metal can overcome these drawbacks to a largedegree. There are many methods for joining ceramics and metals, among which brazing hasbeen widely used and investigated in terms of its structural reliability and economicalindustrialization. Most filler metals can not wet on the zirconia ceramics, in order toovercome this problem traditional method is using active brazing which the mostrepresentative solder is eutectic Ag-Cu filler metals adding2–5wt.%active element of Ti.However, this solder contains60–70wt.%noble metal Ag which increases the cost ofengineering application and limits its application.In our thesis we used Sn-Ag-Cu filler metals which have low melting point and notcontaining active elements. The wettability of molten Sn-Ag-Cu solders on the zirconiaceramics was improved by applying a direct current on the Sn-Ag-Cu/zirconia system. Aswell as using this method we brazed the zirconia and nickel successfully and obtained brazedjoints with good mechanical properties. Through the research on mechanical properties,microstructures and brazing parameters, the connection of the three factors were established.The major results of this paper are as follows:(1) The wettability of molten Sn-Ag-Cu on the zirconia creamics can be improvedsignificantly by applying a direct current which the current flowing from zirconia ceramicsto Sn-Ag-Cu solders. Temperature and current intensity have significant effect on thewettability of Sn-Ag-Cu/zirconia system. Reveals the wetting mechanism of moltenSn-Ag-Cu on the zirconia ceramics is caused by the transformation of stoichiometric forzirconia ceramics at the interface of Sn-Ag-Cu/ZrO2system. Wettability of theSn-Ag-Cu/ZrO2-3%molY2O3system is significantly dependent on the temperature. The molten Sn-Ag-Cu can not wet the zirconia ceramics until temperature is higher than723Kwhich we think this is related to conductivity of zirconia ceramics.(2) Brazing of zirconia and nickel using Sn-Ag-Cu filler metals which contain no activecomponents can be achieved with applying a direct current which the current flowing fromzirconia ceramics to nickel. Good mechanical properties’ joints of Ni/Sn-Ag-Cu/ZrO2can beobtained using this method. Temperature is the most significant influence factor on the shearstrength for brazed joints of Ni/Sn-3.0Ag-0.5Cu/ZrO2and holding time for brazed joints ofNi/Sn-49.9Ag-19.6Cu/ZrO2. Through the observation of microstructures we revealed that thekey factor determining the shear strength of the brazed joints was whether there was thediffusion of Zr on the interface of Sn-Ag-Cu/ZrO2. The shear strength of the brazed jointscan be improved significantly when there is the diffusion of Zr into the solders but it is notsignificant for excess Zr into the solders.(3) The shear strength of the brazed joints is also influlenced by the strength of thesolder. The shear strength for the brazed joints obtained by Sn-49.9Ag-19.6Cu was themaximum for Sn-3.0Ag-0.5Cu, Sn-22.1Ag-7.1Cu and Sn-49.9Ag-19.6Cu solders. Theoptimal parameters for Ni/Sn-3.0Ag-0.5Cu/ZrO2system was that the temperature was1073K for10min under a10mA current and the maximum shear strength was43±4MPa. Theoptimal parameters for Ni/Sn-49.9Ag-19.6Cu/ZrO2system was that the temperature was973K for30min under a10mA current with the cooling rate in5K/min, which the maximumshear strength was147±9MPa.