Research On Processing And Mechanism Of Brazing SiO₂ Composites And Invar Alloy With VFG/CuTi

SiO_2f/SiO₂ composites show good toughness, excellent resistance to thermal shock which makes them become the ideal materials for the radome. Invar alloys have a low coeffcient of thermal expansion（CTE） at room temperature which makes them become the candidate materials for the metal ring of the radome. It is benificial to exploit their advantages when achieving the reliable joining of the two materials. However, the CTE of the parent materials show big differences at the holding temperature. As a result, a high residual stress will occur because of the different shrinking behaviour of the parent materials, which leads to a low shear strength of the jonit. The graphene processes low density and small size. The edge of the vertial few-layer graphene（VFG） grown on the substrate shows high chemical reactivity. The purpose of this thesis is to develop a new type of brazing filler doped with VFG. The VFG with active chemical reactivity showed the high affinity to the active element in the brazing filler and refined the interfacial microstructure. The massive brittle intermetallics was prohibited and the interfacial microstructure mainly composed of the ductile Cu（s,s） was thus obtained. The reliable SiO_2f-SiO₂/Invar joint was finally achieved. Based on that, to further relieve the residual stress, the addition of the Nb particles and the Nb interlayers with low CTE were designed to refine the interfacial microstructure and improve the joint property. PECVD method was adopted to grow VFG on the surface of the Cu powder. The optimal parameters are summarized as follows: 700℃ for the holding temperature, 1 h for the holding time, 200 W for the radio frequencey（RF） power, 90sccm:10sccm for the gas flow ratio of the Ar and the CH4, 900 Pa for the gas pressure. From the Raman spectra, HRTEM and SAED results, it shows that the layer of the VFG is less than 10 and is uniformly distributed. The wetting experiments with VFG/CuTi and CuTi brazing filler show that the composite brazing filler with low Ti content could hardly wet and spread on the surface of the ceramic. The brazing filler with the highest Ti content shows a great brittleness. As a result, the final selected brazing filler was VFG/Cu23wt%Ti alloy. The SiO_2f/SiO₂ composites and the Invar alloy were brazed with the VFG/CuTi and CuTi brazing filler. It shows that many Cu-Ti compounds formed in the brazing seam of the joint brazed with the CuTi brazing filler. In contrast, Cu（s,s） mainly occupies the brazing seam of the joints brazed with the VFG/Cu Ti brazing filler.The nanoindentation test results show that the brazing seam mainly occupied by Cu（s,s） processes better ductility than that mainly composed of the Cu-Ti intermetallics. As a result,The shear strength of brazed joints increase from 5MPa to 15 MPa with 200% improvement. Under 930℃/10 min, the typical interfacial microstructure of the joint brazed with the VFG/CuTi composite brazing filler is: SiO_2f/SiO₂/Ti5Si3+Ti-O+Cu-Ti-O/Cu（s,s）+CuTi2+Cu4Ti+Ti2Cu3/Cu（s,s） /Ti2Ni/Fe2Ti/Invar. The microstructure and mechanical property evolution of the SiO_2f-SiO₂/Invar joint were discussed. The results show that the optimal mechanical property of the joint was achieved at 930℃/10 min with the average shear strength of 15 MPa.The SiO_2f-SiO₂/Invar joint was brazed with VFG/CuTi+Nb_p composite brazing filler and the VFG-CuTi/Nb/VFG-CuTi composite interlayer. The wettability of the VFG/CuTi+Nb_p composite brazing filler with different Nb content on the SiO_2f/SiO₂ composite was investigated. It shows that the wettability became better with the increase of the Nb content. While it became worse when the Nb content was too high since it would deteriorate the fludility of the liquid composite filler. When brazing the SiO_2f-SiO₂/Invar joint with the VFG/CuTi+Nb_p composite brazing filler, the shear strength of brazed joints first increase and then decrease with the increase of the Nb content. When the content of Nb reaches 15 wt.%, the shear strength of joints reaches the highest value of 19 MPa, which is 27% higher than the case brazed with the CuTi brazing filler. When brazing the SiO_2f-SiO₂/Invar joint with the VFG-CuTi/Nb/VFGCuTi composite interlayer, the shear strength of the joint increases with the increase of the Nb foil thickness. When the thickness of the Nb interlayer increases to 150μm, the increase of the joint property is no more evident. When the thickness of the Nb foil is 200μm, the shear strength of the brazed joint reaches the highest value of 34 MPa which is 127% higher than the case brazed with single CuTi interlayer.The interaction energies of the Ti-Cu and the Ti-Nb systems were calculated. The results show that the addition of the Nb particle to the brazing filler can improve the activity of the Ti. The finite element method（FEM） was adopted to simulate the residual stress in the joints brazed with single CuTi interlayer, CuTi+Nb_p composite brazing filler and the VFG-CuTi/Nb/VFG-CuTi composite interlayer. It shows that the residual stress joint brazed with CuTi+Nb_p composite brazing filler decreases to a small extent which drops from 340 MPa to 330 MPa. While the VFG-CuTi/Nb/VFG-Cu Ti composite interlayer could narrow the high stress area of the joint and transfers the highest stress location from the inside of the ceramic to the Nb interlayer. The maximum stress drops from 340 MPa to 240 MPa with 30% reduction.