Process And Mechanism Of Al₂O₃ Ceramic Reactive Air Brazing Joint Using Ag-CuO Nano Brazes

Alumina ceramics have a series of excellent properties such as high mechanical strength,corrosion resistance,high temperature resistance and good electrical insulation properties.It is one of the most widely used engineering ceramics.At present,active brazing is still the main method for joining alumina ceramics.Although high joint strength was able to obtain by active brazing,there are still the following three problems:（1）the joints exhibit poor performance in high temperature and oxidation atmosphere;（2）the differences between the physical properties of brazes and ceramics produce the high residual stress in joints;（3）The micro-component of brazes segregate and purity of brazes is low.Therefore,the research on the high-quality brazing joint of ceramic materials under non-vacuum is of great significance.Under the air atmosphere,the molten oxide and precious metal can be used as the brazing filler metal to join the ceramic materials.For the joining of materials,the commonly used braze is Ag-CuO.The joints obtained by AgCuO brazes have the advantages of excellent plasticity,high temperature resistance and oxidation resistance.However,there are some problems found in the joining ceramics using precious metal-oxide brazes by reactive air braze such as component segregation,relatively low joint strength and large residual stress.In order to solve the problem of the above-mentioned micron Ag-CuO brazes,in this study,the nano-composite brazes are prepared by chemical method and then low thermal expansion coefficent（CTE）were added.Finally,the degree of segregation of the brazes is reducedand and the residual stress in joints was alleviated which enhance the strength.In order to alleviate the residual thermal stress of the joint and improve the joint strength,the Ag-CuO-Pt composite brazes were used to realize the reactive air brazing（RAB）of the alumina ceramics.The results show that the addition of Pt particles can enhance the joint,when the amount of nano Pt is 9.0wt.% under the brazing parameter which is 1050°C/30 min,the highest joint shear strength is 46 MPa,which is increased by 17%.Pt have a high solubility in Ag and exhibits a dispersion distribution in the brazing seam.The joint interfacial microstructure was Al₂O₃/CuAl₂O₄/CuO/Ag（s,s）/ CuO/CuAl₂O₄/Al₂O₃.Two kinds of nano brazes,including Ag-CuO and Ag-CuO-Pt,were synthesized.The microstructure and composition of nano brazes were characterized.The melting characteristics of synthesized nano brazes were studied.The results show that the melting point of Ag-CuO nano brazes is reduced by 4°C.The melting point of Ag-CuO-Pt nano brazes was improved by 21°C.The RAB bonding of alumina ceramics was realized by using prepared Ag-CuO nano brazes.It is found that the joint shear strength increases with the increase of the brazing temperature.When the joining process is 980°C/10 min,the joint strength of up to 41 MPa can be obtained which is 2% higher than that of the Ag-CuO micron brazes.RAB joining of alumina ceramics was realized using Ag-CuO-9.0wt.%Pt nano composite brazes.The experimental results show that when the joining process is 980°C/10 min,uniformly distributed CuO particles and Al₂O₃ particles from base material were observed in joints.Attributing to the particle-solid solution composite strengthening effect,the highest shear strength can reach 47 MPa and it is increased by 19.6% compared to joints using the Ag-CuO micron brazes.