Interfacial Wetting And Joining Of Metal And Zirconia Under The Application Of A Micro-current

Zirconia ceramic?ZrO₂?has a wide range of applications in the fields of sensors,electronic packaging,fuel cells and so on due to its great advantages such as high hardness,low thermal conductivity,high thermal shock resistance and high temperature ion conductivity.The connection between metal and zirconia ceramic is often involved during the process of application,and the wetting between them is the premise condition to achieve the connection.However,the huge difference in physical and chemical properties between zirconia ceramics and metal results in difficulty for their wetting.Conventional approaches for improving wettability include adding active elements to the metal or depositing the wettable metal layer on the surface of the ZrO₂,but neither of these methods is feasible under any conditions.Therefore,to explore a more feasible and effective way to improve the wettability between the metal and ZrO₂ is the goal that the researchers are striving for.In this thesis,based on the oxygen ionic conductivity of zirconia ceramics at high temperature,we innovatively utilized the current coupled seat drop devices to realize the wetting and joining between the metals without active elements?Al,Sn–Ag–Cu and Ag–Cu?and ZrO₂ by applying a direct current.The process parameters,microstructure and mechanical properties were investigated in detail to establish the native relationship between them.The main results obtained in this thesis are listed as follows:?1?The wetting of metal?Al,Sn–Ag–Cu and Ag–Cu?on ZrO₂ could be obviously promoted by applying a negative current?i.e.the current flows from ZrO₂ to the metal?.The electrochemical reduction behavior of ZrO₂ ceramics to substoichiometric ZrO₂-x and formation of Zr atoms on the interface were proved to be the main mechanism for improving the wetting properties.?2?It has been clarified that applying a negative currents could promote the formationof Zr-containing intermetallic compounds at the solid-liquid interface and the thickness of the intermetallic compounds can be controlled by adjusting the technique parameters.The appropriate thickness of intermetallic compounds could greatly improve the interface bonding strength of metal-ZrO₂ ceramic,thereby enhance the performance of brazed joints.But the formation of intermetallic compounds on the interface exhibited limited effect on the wetting improvement.?3?The effects of current polarity,temperature,current intensity and ZrO₂ substrate on the wetting of Al/ZrO₂ system were revealed.Applying a positive current?the current flowing from the metal to ZrO₂?led to the oxidation of the molten Al,which results in non-wetting of the system.On the contrary,applying a negative current greatly improved wetting.The increase of current intensity accelerated the formation of substoichiometric ZrO₂-x to prompt the rapid wetting of the system.The temperature and the content/type variation of stabilizer in ZrO₂ substrate affected the self-oxygenation of ZrO₂ ceramic.When the amount of self-oxygenation increased,the oxide film of Al turned thicker,which is difficult to be broken so that the wetting rate of the system reduced.?4?The interface bonding strength of Al/ZrO₂ system was influenced by the current,the duration of applying current,the temperature and the content/type of stabilizer of Zr O₂ substrate.The thickness of ZrAl3 intermetallic compound at interface determined the interfacial joint strength.?5?Taking Sn–Ag–Cu/ZrO₂ as an example,the wetting rule of alloy and ZrO₂ under current and the mechanism in connecting Ni and ZrO₂ were revealed.Since the ZrO₂ ceramics had poor conductivity at low temperature,the formation rate of substoichiometric ZrO₂-x under current determined the spread speed of metal on the ZrO₂ ceramics.Both increasing the current intensity and enhancing the temperature could improve the wetting of the system.The Sn–Ag–Cu filler could be used to join the Ni and ZrO₂ with applying negative current.The formation of Zr–Sn–Ni phase in the brazing seam was the key factor to improve the shear strength of the joint.?6?Taking Ag–Cu/ZrO₂ as an example,the wetting rule for non-oxidized alloy and ZrO₂ and the mechanism for connecting 304 S–S and ZrO₂ were revealed.Both positive andnegative current application could improve the wetting of system.The application of positive current resulted in the oxygen enrichment at the interface and reducing the interface energy to promote the wetting of system.But too much oxygen on the interface would lead to none connection of alloy and zirconia ceramic.In the case of negative current,the application of Ag–Cu filler could achieve the joining between 304 S–S and ZrO₂ ceramic.The AgCu4 Zr phase formed at the interface increased the shear strength of the joints.When the current increased,the joint fracture locates at the Ag enrichment layer with 72Ag–28Cu as filler,but the joint fracture locates at 304 S-S side with 60Cu–40Ag as filler.In conclusion,this thesis presents a new approach to achieve wetting and connecting of metal and ZrO₂ ceramics,and reveals the controllable wetting mechanism,the law of interface evolution and the intrinsic relationship of technique factors-interface structure-joint performance.The obtained results not only help to enrich the theory of interface wetting with applying current,but also provide important guidance and reference for the control of material service behavior under the condition of electricity.