Fabricating porous Si₃N₄ ceramics joint by ultrasonic brazing at 450 °C

Joint seam is always the weak region of the welded joint. The soldered/brazed ceramic joint always fails through the joint seam due to lower strength of the solder/braze. In this work, to fabricate ceramic joint stronger than its substrate, ultrasonic vibration was used when brazing porous Si₃N₄ ceramics. Results show that the brazing can be directly realized at 450 °C in air because ultrasonic cavitation can break the oxide layers of braze. Under ultrasonication, high acoustic pressure is obtained in braze, thus it rapidly infiltrates into the porous ceramics. Then, tight bond is realized between the ceramic and braze within several seconds because of the cavitation phenomenon. The transmission electron microscope results show that the main bonding medium at the ceramic/braze interface is an amorphous Al₂O₃ layer. Shear test reveals that long ultrasonic time is beneficial to the shear strength of the joint. Wetting times longer than 10 s produce joint stronger than the ceramic substrate. This work provides a new insight when joining other porous materials.