Aluminothermic reduction surface modification for enhanced wettability and interfacial bonding in brazing of SiO_2f/SiO₂ composite to Invar alloy

SiO_2f/SiO₂ composite has attracted considerable attention for advanced structural applications due to its low dielectric constant, excellent wave-transmitting capability, and outstanding thermal shock resistance. However, their poor wettability with AgCuTi filler limits the metallurgical bonding when brazing to Invar alloy. In this work, an aluminothermic reduction of SiO_2f/SiO₂ composite surface was proposed by depositing a 6 μm-thick Al layer and annealing at 700 °C. The aluminothermic reduction generates a Si and Al₂O₃-modified surface layer, which promotes the formation of continuous TiSi-TiO-Al₂Ti₇O₁₅ and Cu₃Ti₃O interfacial reaction layers during brazing, thereby improving wettability. Consequently, the contact angle was reduced from 82° to 45°. An AgCuTi/Nb/AgCu composite interlayer was introduced to regulate the Fe-Ti and Ni-Ti brittle reaction products with the help of Nb barrier layer. Owing to the aluminothermic reduction treatment, the joint achieved a shear strength of 56 MPa at 860 °C for 30 min, about 10 times higher than that without aluminothermic reduction treatment.