Research On Mechanism Of SiO_2f/SiO₂ Composite Surface Modificating And Its Brazing With Nb

SiO_2f/SiO₂ composite is the preferred material for radome,due to its light weight,low dielectric constant,good wave transmission and excellent mechanical properties.In the process of the fabrication and the application of radome,the radome composite materials needed to be jointed with the matrix components through a metal ring.Nb was optimiazed material for the joining ring,because of its high specific strength,low density and good corrosion resistance.Therefore,it is scientific and practical to achieve high quality and reliable joining between SiO_2f/SiO₂ composite and Nb.Moreover,during brazing SiO_2f/SiO₂ composite and Nb,two main problems existed:The poor wettability of the brazing alloy on the surface of SiO_2f/SiO₂ composite,and the high residual stress in the joint.Thus,in the study,the wettability mechanism of SiO_2f/SiO₂ composite is firstly studied and a novel approach to improve the wettability of the brazing alloy on SiO_2f/SiO₂ composite by surface selective etching treatment is developed.And then,the relationship among joint structure,residual stress and mechanical property is revealed through the combination of theory and experiments.In addition,the residual stress is further reduced and the joint mechanical property was improved by introducing a 3D quartz-fiber braided interlayer with a loose,porous and flexible structure.SiO_2f/SiO₂ composite is composed of SiO₂ with two different crystalline states.The crystallized SiO₂ consisted of fused silica and amorphous SiO₂ formed SiO₂fibers.AgCuTi active brazing alloy was adopted to wet on the surface of different components of SiO_2f/SiO₂ composite.The wetting results showed that the wetting angle of AgCuTi-crystallized SiO₂ system decreased from 137 degree to 87 degree,which mean the wettability was poor;The wetting angle of AgCuTi-amorphous SiO₂system decreased from 110 degree to 43 degree,which mean the wettability was well.Then,by analyzing the wetting interface in detail,the wetting mechanism of SiO_2f/SiO₂ was revealed:in wetting reaction between AgCuTi active brazing alloy and fused silica,Ti₃O₅ and Ti₂O₃ particle phases were preferentially formed and Cu₃Ti₃O continuous reaction layer was impeded growth.Ti-O compound was not easy to be wetted,which hindered the wetting of AgCuTi active brazing alloy on the surface of SiO_2f/SiO₂.Based on the wetting mechanism of SiO_2f/SiO₂,a method of surface selective etching treatment was proposed.When etching the surface of SiO_2f/SiO₂,the fused silica in SiO_2f/SiO₂ surface layer was consumed,while the quartz fibers braided in three dimensions were left,and SiO_2f/SiO₂ composite with etching treatment(abbreviated as E-SiO_2f/SiO₂ composite)was achieved.The wetting results show that the wettability of AgCuTi active brazing alloy on the surface of E-SiO_2f/SiO₂composite was improved.With the increasing of the etching depth,the surface structure of E-SiO_2f/SiO₂ composite was not changed,despite of the increasing only the consumption of fused silica,and then wetting angle decreased accordingly.It was revealed that the fused silica was the essential reason for poor wettability of SiO_2f/SiO₂ composite.SiO_2f/SiO₂ composite and E-SiO_2f/SiO₂ composite were brazed with metal Nb using AgCuTi active brazing alloy under the same brazing condition,respectively.E-SiO_2f/SiO₂-Nb brazing joint was successfully achieved and no defects,such as cracks and pores were found.AgCuTi active brazing alloy was able to infiltrate into the surface of E-SiO_2f/SiO₂ composite and formed a 3D brazing alloy-SiO_2f/SiO₂composite transition zone?brazing alloy infiltrated buffer layer?.Then the buffer layer can effectively reduce the mismatch of coefficient of thermal expansion?CTE?between SiO_2f/SiO₂ composite and AgCuTi active brazing alloy or metal Nb,forming a good CTE gradient transition.Thus,the residual stress in E-SiO_2f/SiO₂-Nb brazing joint was reduced and shear strength of the joint was reached 52.9MPa,which was approximately 11 times of that in SiO_2f/SiO₂-Nb brazing joint.It was beneficial to form a good CTE gradient transition and enlarge the contact surface between brazing alloy and composite by the buffer layer.However,it make the distribution of residual stress in the joint more complex.Theoretical simulation analysis results show that the shear stress?_xy increased with the increasing etching depth,which led to the residual stress in the joint rised undiminished with the etching depth further increasing.The relation among residual stress,interfacial structure and joint property was revealed through the method of finite element simulation method.According to the simulation analysis of the relationship between interfacial structure and residual stress in E-SiO_2f/SiO₂-Nb brazing joint,it can be knowable that the residual stress in the joint descended first?from 0 to 100?m?and then ascended?from100 to 150?m?.Based on above analysis,the optimized etching depth of E-SiO_2f/SiO₂-Nb brazing joint with 100?m etching depth was best and shear strength reached 61.9MPa.Undoubtedly,regulating the surface structure of SiO_2f/SiO₂composite was an easy and effective method to decreased residual stress and improve mechanical property of brazing joint.On the basis of above research,a 3D quartz fiber braided interlayer with loose,porous and flexible structure(3D-SiO_2f)was introduced to assist brazing composite and metal.The experimental results show that E-SiO_2f/SiO₂-Nb/3D-SiO_2f+Ti brazing joint formed well without defect,and considerable?-Ti,TiSi and Ti₂Cu compounded particles were uniformly distributed in the joint.In addition,the shear strength of the joint ascended first and then descended with the content of Ti element in active brazing alloy increasing.When the content of Ti was 16.0wt.%,the shear strength of E-SiO_2f/SiO₂-Nb/3D-SiO_2f+Ti brazing joint reached the maximium,approximately65MPa.Therefore,it can be realized that 3D-SiO_2f interlayer with loose,porous and flexible structure,was contributed to forming a good CTE gradient transition,significantly reduced residual stress and strengthened the joint.