Growth Of Few-Layer Graphenes On Invar Alloy And The Process And Mechanism Of Brazing With SiO₂-BN Ceramic

Si O2-BN is the main material of missile radome, and Invar alloy always makes up for missile radome transition rings due to its easy molding processing. Therefore, their own advantages are also combined for further use. Currently, we offen use active fillers to braze the two materials. However, Fe, N i elements will be dissolved and generate a large area of brittle compounds with Ti element. These compounds significantly reduce joints’ plastic deformation, leading to large residual stress even cracks. As a new two-dimensional material, graphene is very thin and vertical graphene has a higher chemical activity and a good affin ity with Ti element which is vertical to the growth substrate. Grow vertical fewer- layer graphenes on the surface of Invar, barriering Fe and Ni elements dissolve over to liquid brazing alloy on the one hand, and avoiding large areas of Fe- Ti and N i- Ti brittle compounds appear in interface on the other hand. So increase joints’ plastic deformation, relieve joint residual stress and improve the mechanical properties.Vertical fewer- layer graphenes grow on the Invar alloy surface according to the method of plasma-enhanced chemical vapor deposition(PECVD). Explore the optimal parameters: growth temperature is 800 ℃, the gas flow ratio of CH4: Ar is 20 : 80, a mixed gas total pressure is 900 Pa, radio frequency power is 200 W, and growth time is 60 min. VFG is characterized by Raman spectroscopy. It shows that there are a few layers and defects by the ratio of ID/IG and I2D/IG analysis. TEM shows that fewer- layer graphenes are transparent and nanoscale dimension. Selected area electron diffraction results show few- layer graphenes have same characteristics of polycrystalline graphite. As is shown in the HRTEM results, fewer layers of graphenes are within ten layers, and atomic level spacing is 0.34 nm which matches polycrystalline graphite atomic spacing.Wet vertical growth of graphene Invar alloy(referred to as Invar-G) with Ag C u Ti filler and study the interface structure of the evolution resulted in holding temperature. Braze Invar-G and Si O2-BN composite ceramic with Ag C u Ti, analyze the typical interface organization, and explore the best welding parameters. Optimum welding parameters are: holding temperature is 860 ℃and holding time is 10 min, and shear strength is 25 MPa. With the changes of brazing temperature and holding time, the shear strength both show a trend that it increases firstly and then decreases. Compared to the interface microstructure without graphene, a large area of brittle compounds ―disappear‖ and Fe2 Ti and N i3 Ti are small block phases dispersively distributed in the interface. And the shear strength rises from 21 MPa to 25 MPa. Fracture locations both occur on Si O2-BN base material, and there is no big difference between them.When holding temperature is 860 ℃ and the holding time is 10 min, Fe2 Ti and Ni3 Ti compounds are significantly decrease after adding the graphenes. The shear strength rises from 163 MPa to 205 MPa. Through Invar alloy its own brazing, it can be concluded that VFG indeed inhibit the growth of Fe- Ti and N i- Ti and there are no large numbers of compounds. This change optimizes the interface structure, and improves the mechanical properties of the joint. Calculate the reaction Gibbs free energy of C, Fe, N i and Ti, analyze the interaction of graphene and Ti by high resolution and TEM. Graphene can generate a small amount of Ti C with Ti element, and inhibit other reactions. A small amount of non-continuous distribution of Fe2 Ti and N i3 Ti also helps to strengthen the joint mechanical properties.