Interface Structure And Formation Mechanism Of Brazed SiO_2f/SiO₂ Composite And Invar Alloy Joint

Based on ther properties of well wave transmissivity and heat protection, Quartz fibers reinforced silica composites (QFSC, tell SiO_2f/SiO₂ for short) received wide application in aerospace industry. At present, the bad high temperature resistence and heat ageing resistance of adhesive-banded joint is the technical bottleneck in the application. Therefore, brazing of SiO_2f/SiO₂ and Invar alloy is expected in the present work to realize reliable joining. Direct and indirect brazing of SiO_2f/SiO₂ and Invar were experimented, one kind of new process named as epoxy resin assisted brazing (ERAB) was introduced under the base of above. High quality of SiO_2f/SiO₂ and Invar joint is successfully realized by ERAB. Microstructure, bonding mechanism and reaction kinetic equation of joint were identified. In addition, ERAB process can be applied in bonding of Al2O3 and Cf/SiC, respectively.SiO_2f/SiO₂ composite was brazed to Invar using active interlayer composing epoxy resin and TiH2 powder, Ag-Cu eutectic and pure copper as interlayer. After interfacial analysis through SEM, EDS and XRD, the typical interface structure was identified as SiO_2f/SiO₂ composite /TiO+Si+Cu(s,s)+Ag(s,s)+TiC/Cu(s,s)+Ag(s,s)+ Fe2Ti/Invar. When the brazing temperature is 850℃and holding time is 15min, the largest shear strength 44MPa of joint was reached, which is about 4 times the shear strength of SiO_2f/SiO₂. The spreading and wetting of solder reinforced the base of SiO_2f/SiO₂ near junction. The fracture of joint occurred at the base of SiO_2f/SiO₂ without solder coated.From the brazing experiments with various parameters, it can be identified that the kind of interfacial phases in the SiO_2f/SiO₂ and Invar joint was stable, amount and distribution of Cu(s,s) and Fe2Ti, thickness of reaction layer TiOx+Si were influenced obviously by the brazing parameters. With rising of brazing temperature and increasing of holding time, amount of Fe2Ti and thickness of reaction layer TiOx+Si increased, the diffuse spots of Fe2Ti aggregated gradually from Invar to the center of junction. The shape of Cu(s,s) changed into small and thin strip when brazing temperature and holding time increased at first, and aggregated into massive structure when brazing temperature reached 950℃or holding time was over 35min.Based on the thermal analysis and interface structure observation, formation of SiO_2f/SiO₂ and Invar joint was divided into six stages as follows: adhesive bonding, physical contact after thermal decomposition of active interlayer and induced decomposition of TiH2; decomposition of TiH2 powders and initial reaction of pure Ti in the interface; melting of Ag-Cu alloy, diffusion and reaction at the interfaces of both sides; increase of thickness of reaction layer near SiO_2f/SiO₂ and formation of Fe2Ti; formation of solid solution and stop of interfacial reactions; freezing of solid solutions, intermetallic componds and formation of joint. Under the base of thermal decomposition kinetics of active interlayer and reaction layer growth kinetics during the formation of joint, the kinetic behavior of interface in the joining formation was analysed. Thermal decomposition mechanism function, kinetics equation and activation energy of active interlayer in the adhesive bonding stage, growth kinetics equation and corresponding parameters of reaction layer TiOx+Si were calculated.Thinning and dividation of Invar ring can effective reduce residual stress in the sleeving structure of SiO_2f/SiO₂ and Invar. Joined component of SiO_2f/SiO₂ and Invar can be obtained by ERAB in appropriate structure.