Wetting Of Zirconia By Ag-Cu Fillers And Its Joining With 304 Stainless Steel Under DC Currents

In the process of joining metal to ceramic, due to the great differences of physical and chemical properties in metal and ceramic, wettability and residual stress may be two main problems we must face in the joining process. In this paper, we made full use of the ionic conductivity of ZrO₂-3 mol.% Y2O3 ceramic at high temperature. By applying a DC current on the Ag-Cu/ZrO₂ system, we can improve the wettability of molten Ag-Cu solders on zirconia substrates, which solved the wettability problem. In order to reduce the residual stress, we chose Ag-Cu filler metals which had perfect plasticity. Similarly, by applying a DC current, we brazed the zirconia with 304 stainless steel（record as 304 S-S） successfully using Ag-Cu filler metals and obtained strong joints. This paper aims to propose a new method of joining zirconia to 304 S-S. The major results of this paper are as follows:（1） By applying a current flowing from zirconia to Ag-Cu solders, the wettability of molten Ag-Cu on zirconia substrates can be improved significantly, which is due to the transformation of stoichiometric ZrO₂ to non-stoichiometric ZrO₂-δ at the interface. Current gives a more remarkable influence on improving the wettability of Ag-Cu/ZrO₂ system than temperature. With current increasing, the final contact angle（the contact angle under current for 10 min） decreases. Current has a more apparent promoting effect on 72Ag-28Cu/ZrO₂ system. The poor wettability of Ag-Cu/ZrO₂ system in Ar atmosphere is related to the formation of zirconium oxide at the melt surface.（2） When there was no current, Ag-Cu filler metals can’t join 304 S-S to ZrO₂; when a DC current was applied from zirconia to 304 S-S, Ag-Cu filler metals can join 304 S-S to ZrO₂ strongly. For 304 S-S/72Ag-28Cu/ZrO₂ joints, the reason of the improvement of joints’ strength was the generation of m（AgCu4Zr） phase at interface which enhanced the shear strength. For 304 S-S/60Cu-40Ag/ZrO₂ joints, the key factor of determining shear strength of the joints was the bonding strength of the 60Cu-40Ag/ZrO₂ interface.（3） For 304 S-S/72Ag-28Cu/ZrO₂ joints, current, temperature, dwell time and coolingrate had little effect on shear strength of the joints, and their shear strength changed between200 to 230 MPa. The shear fracture of the joints indicates that cracks propagated in Ag-rich layer. For 304 S-S/60Cu-40Ag/ZrO₂ joints, when the current was too low or cooling rate was too speed, the bonding strength of the 60Cu-40Ag/ZrO₂ interface was weak, and cracks occur at interface preferentially; on the contrary, interface bonding strength can be enhanced,cracks occur in filler metals close to 304 S-S.