Brazing of SiO2f/SiO2 composite modified with few-layer graphene and Invar using AgCuTi alloy.

Due to the poor wettability of the AgCuTi alloy on the SiO2f/SiO2 composite, direct brazing of the composite with an Invar alloy could hardly achieve a reliable joint. To overcome that, the SiO2f/SiO2 composite was decorated with few-layer graphene (FLG) by a plasma enhanced chemical vapor deposition (PECVD) method. Sessile drop experiments indicate that the contact angle dropped from 123.8° to 50.7° after FLG was grown on the surface of the SiO2f/SiO2 composite. Afterwards, the effects of brazing temperature and Ti contents on the microstructure evolution and mechanical properties of joints (Invar/SiO2f-SiO2 modified with FLG) were investigated. The typical interface structure of the joint is SiO2f- SiO2/Ti5Si3 + TiO2 + CuxTi6 - xO(x = 2,3)/Ag(s,s) + Cu(s,s) + Cu-Ti blocks/wave-like Fe2Ti + Ni3Ti/Ag(s,s) + Cu(s,s) + Fe2Ti + Ni3Ti blocks/Invar. As the brazing temperature and Ti contents increase, the reaction layer on the SiO2f/SiO2 side becomes thicker and cracks gradually propagate. Meanwhile, a few dispersive Fe2Ti + Ni3Ti phases change into large-area wave-like compounds and more Cu-Ti compounds form with the increase of the Ti content. The microstructure evolution significantly affects the shear strength of the brazed joints. The highest shear strength is 26 MPa brazed at 860 °C for 10 min with 4.5 wt.% Ti content. [ABSTRACT FROM AUTHOR]