Microstructure and mechanical performance of composite joints of sapphire by ultrasonic-assisted brazing

A method of lowering the coefficient of thermal expansion (CTE) of a bond metal of sapphire joints was examined for inhibiting the interfacial cracking of joints. An interlayer with the sandwich structure of Zn-Al/(SiCp/A356)/Zn-Al was fabricated and the surface of sapphire blocks was coated with a Zn-Al alloy by assistance of ultrasound. The assembly of sapphire and interlayer was brazed under the action of ultrasound. A metallurgical bond is performed between the sapphire and interlayer, and the composite material of SiCp/A356 is dissolved by Zn-Al alloys during brazing, resulting in a joint consisting of a new composite material of SiCp/Zn-Al-Si. It was composed of Zn-Al-Si alloy matrix and SiC particles with ∼40 vol.%, and has a CTE of ∼7.4 × 10−6 /°C estimated by using Turner's model, approaching that of sapphire (∼5 × 10−6 /°C). The maximum shear strength of the composite joints reached ∼155 ± 10 MPa, an increase of ∼250% in comparison with that joined using the Zn-Al alloys.