Interface Reactions during Metal/Ceramic Diffusion Bonding

Abstract

The strength of annealed ZrO2/Pd diffusion bonds was found to be weakened after annealing in both air and vacuum. Annealing in air reduces the strength much faster and more severely than in vacuum. Fracture surfaces of as‐bonded joints and those annealed in air and vacuum were studied to characterise the different effects of air and vacuum on the bonded interfaces. Various sizes of precipitates and voids were observed and their distribution on the fractured surfaces was examined by light microscopy. Large precipitates and voids were found close to the edges of the specimen. It is believed that the loss of strength after annealing is an effect of these defects at the highly stressed specimen edges. Transmission and analytical electron microscopy of as‐bonded joints show that an interface layer of very fine grains about 1 micron thick was formed during the bonding process. This layer has a different crystalline structure and composition from both Pd and ZrO2. Characterisation of this layer by electron microscopy is presented in this report. The formation of such a thick interface layer is probably not a pure diffusion process, rather a diffusion and melting process. From the Pd‐Zr phase diagram, there is a range of compositions near a eutectic point where a liquid phase is possible at the bonding temperatures used (1100°C). Taking the Pd‐Zr system as a qualitatively comparable system to Pd‐ZrO2, it is deduced that, at the very beginning of the bonding, Zr and Pd diffuse into each other until the melting composition is reached. The formation of the liquid phase will promote the contact and bonding processes dramatically. This explains why strong bonding cannot be achieved at lower bonding temperatures as was reported in an earlier paper. Similar experiments on Ni/ZrO2 diffusion bonds have also been studied to identify the mechanism of bonding and to compare it with Pd/ZrO2. No reaction was observed at the interface in Ni/ZrO2. Thus the wetting mechanism is absent which explains the formation of a large amount of interface voids and the much weaker bonding strength found in Ni/ZrO2 bonds.