Silver-copper oxide based reactive air braze for joining yttria-stabilized zirconia

We investigated a new method of ceramic-to-metal joining, referred to as reactive air brazing, as a potential method of sealing ceramic components in high-temperature electrochemical devices. Sessile drop wetting experiments and joint strength testing were conducted using yttria stabilized zirconia (YSZ) substrates and CuO–Ag-based air brazes. Results from our studies indicate that the wettability of the braze improves substantially with increasing CuO content, over a compositional range of 1–8 mol% CuO, which is accompanied by an increase in the bend strength of the corresponding brazed YSZ joint. The addition of a small amount of TiO₂(0.5 mol%) to the CuO–Ag braze further improves wettability due to the formation of a titanium zirconate reaction product along the braze/substrate interface. However, with one notable exception, the bend strength of these ternary braze joints remained nearly identical to those measured in comparable binary braze joints. Scanning electron microscopy analysis conducted on the corresponding fracture surfaces indicated that in the binary braze joints, failure occurs primarily at the braze/YSZ interface. Similarly in the case of the ternary, TiO₂-doped brazes joint failure occurs predominantly along the interface between the braze filler metal and the underlying titanium zirconate reaction layer.