Brazing Of Zro₂ Ceramics With Metal Fillers Using Electrical Current

Abstract

Zirconium dioxide is characterized by significant mobility of anions at high temperatures and is thus capable of passing current in these conditions, which can affect the processes of its interaction with metals. The effect of current passing through the interface between ZrO₂ ceramics with metal melts (copper, nickel, Cu–17.5 Ga, Ni–20 Cr) was studied. The current leads to the complete spreading of the melts on the ZrO₂ ceramic surface. Nickel and nickel–chromium melts spread faster, probably because of higher melting points of these metals and, accordingly, experimental temperatures. These results are explained by the fact that the ceramics is depleted of oxygen near the contact with the metal melt because anions move to the electrode after connection of a positive current collector (cathode) to ZrO₂. 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 ZrO₂ ceramic cathode in the melt and formation of a thick, developed transition layer at the ZrO₂ anode. Since wetting was achieved, current was passed for brazing the ZrO₂ ceramics to molybdenum. Dry joints were observed with copper used as a filler metal because the metal melt was squeezed out of the brazing gap when wetting still was not achieved. In some cases, the ceramics cracked due to local overheating. Nickel and Ni–20 Cr fillers produced strong joints without dry joints areas. When Cu–45 Ni was used as a filler, the brazing gap was not filled in all samples, the ceramic cracked due to local overheating in some samples, and other samples were quite strong. Zirconia ceramics and heat-resistant steel were brazed with Ni–20 Cr. Therefore, current can be used to braze ceramic materials.