Wettability Of SiO_2f/SiO₂ Composites And Its Brazing Mechanism With TC4 Titanium Alloy

Silica fiber reinforced silica(SiO_2f/SiO₂)composites is an advanced fiber-reinforced ceramic matrix composites with great physical properties,especially with great wavetransimissivity,and extreme application potential.In practice,we often need to choose TC4 as a connecting ring and braze it with SiO_2f/SiO₂ composite together.However,the soldering of Si O_2f/Si O₂ composites and TC4 is difficult because of poor wettibility on the surface of composites,excessive dissolution of Ti atoms and residual stress after welding,which limits the application of SiO_2f/SiO₂ composites.Based on this,this paper solved the above problems by means of plasma surface modification,graphene surface coating and foam-like intermediate layer brazing to achieve high quality connection of SiO_2f/SiO₂ composites and TC4.The results showed that Silica fiber has good wetting to Ag-Cu-Ti solder while fused silica does not wet Ag-Cu-Ti solder.The mechanism of the wetting of the SiO_2f/SiO₂ composite was revealed.The SiO_2f/SiO₂ composite material is not wetted because fused silica doesn't wet the active solder.Based on this,two new methods of plasma surface modification and graphene surface coating were developed to improve the surface wettability of SiO_2f/SiO₂ composites.Firstly,the use of plasma surface modification changes the Si-O bond into Si-C bond on the surface of SiO_2f/SiO₂ composite,hardwetting fused silica on the outer surface of the material being changed into a wetting SiC and amorphous Carbon,so that the wetting angle of Ag-Cu-Ti solder on the surface of SiO_2f/SiO₂ composite decreased from 131° to 27°.Second,the surface coating with high active redox graphene,through Ag-Cu-Ti solder wetting greatly on the surface of graphene,significantly improved wetting ability of SiO_2f/SiO₂ composite,so that the wetting angle decreased from 131° to 42.8°,and the method is simple.After the plasma surface modification,the SiO_2f/Si O₂ composites and TC4 were brazed under the conditions of 900 ?/10 min.The results were analyzed by SEM,EDS,Gibbs free energy and XRD patterns.The typical interface organization of the brazed joints are SiO_2f/SiO₂ composites/ TiSi2/ Cu3Ti3O/ Ag?s,s?+Cu?s,s?/ TiCu2/ Ti Cu/ Ti2Cu/ ?Ti+?Ti/ TC4.In order to solve the problem of the formation of continuous brittle compounds caused by the excessive dissolution and diffusion of Ti atoms,foam Cu was used as the intermediate layer to assist brazing and the mechanism of enhancing the performance of the joints is revealed.Foam Cu can not only consume Ti,which is diffused to the Si O_2f/SiO₂ composite side to form a continuous brittle compound,but also form a fine dispersion of titanium-copper compounds.The formation of a good stress transition enhanced the mechanical properties of joints,and shear strength was up to 59.6 MPa.After the surface coating of graphene,the SiO_2f/SiO₂ composites and TC4 were brazed at 850 ?/5 min.The microstructures of SiO_2f/SiO₂ composites were analyzed by SEM,EDS and XRD patterns.The typical interface organization of the brazed joints are SiO_2f/SiO₂ composite/ TiSi2+TiO₂/ Cu3Ti3 O /TiCu+Ag?s,s?+Cu?s,s?/ Ag-Cu eutectic structure/ Cu?s,s?/ Ti2Cu/ Ti?s,s?/ TC4.The mechanism of graphene surface coating to enhance the brazing joint was revealed by strength test and fracture analysis.The use of graphene surface coating,because of its high activity,can significantly improve the surface wetting of SiO_2f/SiO₂ composites.At tha same time it can consume diffusion of Ti atoms witch comes from TC4,inhibiting the formation of continuous brittle compounds.Surface coated process,greatly enhanced the brazing joint strength to 39.2MPa,8.7 times as large as no-surface-coating composites by single step.And rGO surface coating process is simple,easy,and low cost,witch makes it a very promising composite material surface treatment methods.