Strengthening mechanism of Al/Sn interfaces: Study from experiments and first-principles calculation

The joining of aluminum using a metal of pure Sn as an interlayer or solder was investigated by reactive air soldering and vacuum diffusion bonding. The interfacial structure and mechanical property of the joints were examined. A nanometer-scale interlayer of amorphous Al2O3 dotted with nanocrystalline γ-Al2O3 was observed and identified at the Al/Sn interface by reactive air soldering. The shear strength of the joints (∼40 MPa) by reactive air soldering was higher than that (∼20 MPa) by vacuum diffusion bonding. The reaction and diffusion behaviors of oxygen dopants at the Al/Sn interface were studied through geometry optimization and a transition state search, respectively. The work of separation values of these two interfaces between Al/O-terminated or Sn/O-terminated was 6.46 and 2.83 J/m2, respectively, which were both larger than that of a Al/Sn interface (1.06 J/m2). The results indicated that oxygen atoms could strengthen the interfacial adhesion of Al/Sn. In addition, the interfacial reaction of Al and O and the formation process of Al2O3 were discussed. It has the potential to be used to realize a high-strength joint in other weak bonding systems such as solder/ceramics, through O doping in air.