Brazing of ZrO₂ Ceramics with Metallic Fillers Using Electrical Current

Zirconium dioxide is characterized by highly mobile anions at high temperatures and is thus capable of transmitting electrical current in these conditions, which can influence its interaction with metals. The effect of current passing through the interface between the zirconia ceramics and metal melts (copper, nickel, Cu–17.5 Ga, Ni–20 Cr) was studied. Current leads to the complete spreading of melts over the zirconia ceramic surface. Nickel and nickel–chromium melts spread faster, probably because these metals have higher melting points and, accordingly, experimental temperatures are greater. These results are explained by oxygen depletion of the ceramics near the contact with the metal melt as anions move to the electrode after connection of the positive current lead (cathode) to ZrO₂. The oxygen deficiency can be considered to result from excess zirconium that dissolves in the melt, improving wetting. This was confirmed by microstructural studies, demonstrating significant dissolution of the zirconia ceramic cathode in the melt and formation of a thick transition zirconia layer on the anode. Since wetting was achieved, current was induced for brazing the ZrO₂ ceramics to molybdenum. Dry joints were observed with copper used as a filler metal because the metal melt squeezed out of the brazing gap before wetting was achieved. In some cases, local overheating caused the ceramics to crack. Nickel and Ni–20 Cr fillers produced strong joints without dry areas. When Cu–45 Ni was used as a filler, the brazing gap was not filled in all samples, the ceramic cracked in local overheating in some samples, and other samples were quite strong. Zirconia ceramics and heat-resistant steel were brazed together with the Ni–20 Cr alloy. Current can thus be used in brazing ceramic materials.