Research On Brazing Process And Mechanism Of C/C Composites And Nb With Carbon Nanotubes-assisted

C/C composite is a kind of heterogeneous carbon material with carbon fiber reinforced.It has excellent properties such as low density,good friction performance,high thermal conductivity,excellent high temperature properties,good thermal shock resistance and fatigue resistance.In the application of aerospace industry,it makes a great sense to get a reliable and stable joint between C/C composites and Nb.Brazing has been widely applied for joining dissimilar materials due to the advantages such as convenient assembly and small deformation.However,the poor wettability of filler metal on C/C composites surface and the big residual stress reduce the quality of the brazed joint significantly.In this paper,the method of surface growing carbon nanotubes（CNTs）is adopted.The wettability and interfacial bonding strength of filler metal on C/C composites surface are improved.The interfacial structures and mechanical properties of the brazed joint have been analyzed simultaneously.On this foundation,carbon nanotubes film（CNT_f）is used as an interlayer to assist the brazing.With that,the interfacial structures are regulated and the residual stress of the joint is relieved.The performance of the brazed joint has been optimized ulteriorly.Finally,the reliable brazed joints between C/C composites and Nb have been achieved.CNTs have been synthesized on the surface of C/C composites by plasma enhanced chemical vapor deposition（PECVD）method.The effects of preparation methods of Fe catalyst on the growth of CNTs have been compared and analyzed.The catalyst prepared by evaporation method is conductive to the growth of CNTs with high density and good uniformity.The optimum evaporation time is 5 s,and the thickness of the Fe film is approximately 10 nm.The CNTs has bottom growth model and a large number of defects.The evolution process of Fe catalyst on the carbon material surface has been analyzed.Four stages are composed of the amorphous deposited film,crystalline nanoparticles,active catalysts for CNTs growth and inactive Fe₃C particles.It has been verified that the transformation of the amorphous deposited film to catalyst particles occurs in the heating process and the etching reaction of the catalyst particles to the carbon substrate occurs in the holding process.The key factor to realize the growth of CNTs is that the catalyst forms nanoparticles and is not completely etched into the substrate.The PECVD parameters have been optimized.The optimum growth parameters of CNTs are as follows:the growth temperature is 800℃,the pressure is 800 Pa,and the gas flow ratio is H₂:CH₄=10sccm:40 sccm.CNTs have been grown on the surface of C/C composites（CNTs-C/C composites）according to the above process.Ti27-Cu73 filler metal has been used for brazing CNTs-C/C composites and Nb.The impacts of the brazing parameters on the interfacial microstructure and mechanical properties have been discussed.With the increasing of the brazing temperature or holding time,the reaction layer is thicken and the（Ti,Nb）C phase dispersion zone is widen.When the brazing temperature is too high or the holding time is too long,the thickness of the reaction layer increases significantly and desorbs into the brazing seam.The optimum parameters are brazing at 950℃for holding 10 min.The shear strength of the brazed joints reaches 37 MPa and 77 MPa at room temperature and 600℃respectively,which is 32%and 83%higher than that of direct brazing.The fracture position is adjacent to the interface of CNTs-C/C composite.In order to further improve the performance of the joint,the CNT_f interlayer has been designed and prepared to assist in brazing CNTs-C/C composite and Nb.The brittle Ti Cu phase disappears in the joint interface.The intermetallic compound zone is transformed into CNT_f interlayer zone.The structure of the CNT_f interlayer will be destructed by the excessive brazing temperature or holding time.At this time,the agglomerated（Ti,Nb）C phase and Ti Cu phase are formed in the interface.Under the optimum parameters,the shear strength at room temperature and 600℃reaches 75MPa and 129 MPa respectively,which is improved by 103%and 68%compared with the joints without CNT_f interlayer.The fracture position is adjacent to the interface of CNTs-C/C composite.The interfacial reaction products and microstructure are familiar with double CNT_f interlayer assisted.The brazing seam becomes widened and two CNT_f interlayer zones are formed,between which is the（Ti,Nb）C phase dispersion zone.The shear strength of the brazed joint has been not changed compared to that with a single CNT_f interlayer assisted.The enhancement mechanism of the brazed joints by CNTs have been studied.The essential factor for the good wetting of Ti-Cu filler metal on the CNTs surface is that Ti atoms preferentially react at the defects to form Ti C particles.On the surface of CNTs-C/C composites,the filler metal reats with the CNTs at the wetting frontier area and forms an infiltration zone.The filling and spreading ability of the filler metal has been improved.The interface contact area has been increased.Besides,a small amount of Ti Cu₄ phase is formed at the interface between CNTs-C/C composite and the reaction layer.It is conducive to relieve the interface stress and improve the interface bonding strength.After adding CNT_f interlayer in brazed joints,the physical properties of CNT_f interlayer zone have been calculated.The elastic modulus,initial yield strength and coefficient of thermal expansion（CTE）of the nanoparticles phase in the interlayer zone are reduced significantly compared to the brittle Ti Cu phase.The CTE of the brazing seam decreases from 14.15×10^-6/℃to 12.36×10^-6/℃.Based on energy theory,the elastic strain energy in C/C composite has been calculated.After adding CNT_f interlayer,the local strain has been reduced and the uniformity of stress distribution in has been improved.The elastic strain energy has been reduced from 5.41 m J to 4.33 m J.It indicates that the residual stress on the side of C/C composite has been effectively relieved.Thus,the shear strength of the brazed joint has been enhanced.Ultimately,in order to solve the poor wettability of filler metal on the quartz fiber reinforced composite（QFSC）surface,the method of surface growing CNTs has been applied to Ag Cu Ti/QFSC system.The wettability,spreading and filing ability of Ag Cu Ti are significantly improved.At 870℃for holding 10 min,the contact angle has been reduced by 68%.The shear strength of the brazed joint has been increased from lower than 5 MPa to 17MPa.