Behavior of superficial oxide film at solid-state diffusion-bonded interface of tin

The microstructure of the solid-state diffusion-bonded interface of tin has been investigated by TEM observations in order to reveal the evolution of the superficial oxide film of tin with the progress in bonding. The diffusion bonding was carried out in a vacuum chamber at bonding temperatures T-j of 373-493 K and at bonding pressures P-j of 90-170 MPa (bonding time = 1.8 ks). The tensile strength of the joint was increased with bonding temperature and pressure; joints having tensile strength comparable with the base metal were obtained at T-j = 493 K at P-j = 130 MPa and at T-j = 443 K at P-j = 130 MPa. When the joint strength was much lower than the base-metal strength, an interfacial region similar to500 nm in width was observed where a number of very fine inclusions that could be regarded as tin oxide were distributed. As the joint strength increased with bonding temperature, these inclusions were coarsened, and their number density was decreased. The increase in the bonding pressure enhanced these changes in the inclusion. The rise in the joint strength with bonding temperature corresponded well with the observed change in the size and density of the inclusion.