Building a nano-crystalline α-alumina layer at a liquid metal/sapphire interface by ultrasound

• Sapphire blocks were ultrasonically soldered with Sn–Zn–Al alloy at 230 °C.
• Oxidation of aluminum at the liquid/solid interface was promoted by ultrasound.
• The aluminum oxide layers deposited on sapphire were made of nano-crystalline Al2O3.
• Shear strength of the joints was 43–48 MPa, highest in those soldered by Sn alloys.
• The mechanism for ultrasonic soldering of oxide materials was revealed.

Transitional layers at the metal/ceramic interface play an very important role in ceramic joining. In this study, sapphire blocks were ultrasonically dipped in liquid Sn–Zn–Al alloy. It is found that the ultrasound promoted rapid oxidation reaction of aluminum at the Sn–Zn–Al/sapphire interface at 230 °C in the ambient atmosphere, resulting in the formation of a nano-crystalline α-Al2O3 layer (NCAL). In a ∼2 nm boundary layer of the NCAL, the lattice matches the sapphire substrate well. Thus, a smooth transition of the lattice from sapphire to metal was formed through the NCAL. Ultrasonically soldered sapphire joints were made with Sn–Zn–Al as the filler alloy. Compressive shear strength of the joints reached 43–48 MPa, which is relatively high comparing to other Al2O3 joints made of Sn alloys doped with Ti or Rear Earth elements. Thus, a new mechanism of ultrasonic soldering, i.e. building an oxide transitional layer on the surface of the solid, was revealed. We expect this sonochemical process to be applicable to other metal/oxide systems.