Microstructure, mechanical properties, and bonding mechanism of ultrasonic-assisted brazed joints of SiC ceramics with ZnAlMg filler metals in air

SiC ceramic samples were brazed to each other in air using ultrasonic-assisted brazing, filling with ZnAlMg alloys at 420 °C. The microstructure, mechanical properties, and interfacial bonding mechanism of the joint were investigated. The ultrasonic vibration duration time (UVDT) significantly affected the strength of the joint. A joint with a high strength of ∼148 MPa was obtained when the UVDT was increased to 8 s. An amorphous SiO2 film with a thickness of ∼6 nm was formed on the surface of SiC during heating in air. When ultrasonically vibrated, the liquid ZnAlMg filler metal eroded the SiO2 film nonuniformly. This erosion behavior was induced by the impact from liquid micro-jets, shock waves, and localized high temperatures, all of these effects being ascribed to the implosion of the cavitation bubble. The atoms generated from the erosion of the SiO2 film diffused away quickly in the liquid alloy, aided by the acoustic streaming effect. This erosion behavior belonged to a special phenomenon of cavitation erosion. Other amorphous phases not eroded by the liquid ZnAlMg alloy were preserved at the final interface. Strong bonding between the SiC and the ZnAlMg filler metal could be ascribed to cavitation erosion-induced mass transfer from SiO2 to ZnAlMg; this mass transfer behavior was enhanced by prolonging the UVDT.