Effect of Oxygen Content in Copper on the Tensile Strength of Diffusion Bonding Joints between Copper and Several Metals

Abstract

Diffusion bonding is a useful method to join different metals, though it is indicated that the tensile strength of the joint decreases due to the formation of oxide at the bonding interface.
In this study, diffusion bonding process was employed to make the joint of copper (tough pitch copper described as TPC, and oxygen free high conductivity copper expressed as OFHC) and several metals (molybdenum, nickel and platinum). The effect of oxygen content in the copper and kind of coupling metal on the tensile strength of the joint was investigated using XRD and SEM.
When TPC was joined to molybdenum or nickel, molybdenum oxide or nickel oxide formed at the bonding interface and the tensile strength of the joint decreased extremely. While when platinum was used as the coupling metal, the oxide did not form there.
In the case of OFHC, the oxide did not form at the bonding interface between OFHC and any metal.
These results could be explained thermodynamically. If TPC was joined to a metal, whose standard free energy of formation of oxide was lower than that of Cu₂O, Cu₂O dispersed in TPC was reduced and a metal oxide formed at the bonding interface. If TPC was joined to a metal, which had a higher standard free energy of formation of oxide than that of Cu₂O, Cu₂O remained stable and the oxide did not form at the interface.
In OFHC, oxygen is dissolved in copper, therefore the activity of oxygen must be considered for the calculation of free energy change. The calculated free energy change indicated that the oxide did not form at the bonding interface between OFHC and any metal.