Wetting transition behavior of Ag–Cu alloy braze on BaCo0.7Fe0.2Nb0.1O3-δ ceramic

Abstract To achieve reliable sealing between the BaCo0.7Fe0.2Nb0.1O3-δ (BCFN) ceramic oxygen permeable membrane and its metal support, the wetting transition behavior of Ag–Cu braze with different copper content on BCFN ceramic at 980 °C in air were revealed. As the copper content in Ag–Cu alloy increased, the wetting status showed a complex transition law of non-wetting → wetting → wetting with a ridge. The formation of ridges led to the inconsistency of the macroscopic and the microscopic contacting angles. TEM-EDS characterization confirmed that an exchange reaction (Cu2+↔Co2+) occurred between the Cu–O in the liquid braze and the BCFN substrate during the brazing process. With the increase of copper content, the increasing amount of interfacial products formed at the solid/liquid/vapor triple line enhanced the pinning effect and capillary effect on the spreading of the Ag–Cu braze, resulting in the transition of wetting status. The lower density of BCFN substrate corresponded to the higher critical copper content for wetting transition. The reason is that the Cu–O in the alloy braze reacted with the BCFN substrate rapidly along the pores, while the products at the joining interface were less, which weakened the hindering effect on the spreading of the Ag–Cu braze. Controlling the accumulation of interfacial products at the joining interface to avoid the formation of ridges is the key to improve the sealing quality of membrane components.