Wetting and interfacial phenomena in relation to joining of alumina via Co/Nb/Co interlayers

When designing multilayer metallic interlayers for transient-liquid-phase (TLP) bonding of ceramics, the phenomena that occur at the interface between the liquid metal alloy formed at the joining temperature and the solid ceramic to be joined have a major effect on the success or failure of the joining process. To assess the behavior of liquids that could develop when using Co/Nb/Co trilayers to bond Al2O3 ceramics, the wettability and interfacial behavior of pure Co and Co–Nb alloys on sapphire and polycrystalline Al2O3 were studied. Contact angles were measured at 1500 °C using the sessile-drop technique, and the microstructures of the resulting metal/ceramic interfaces were characterized by scanning electron microscopy and energy-dispersive spectroscopy. Observations were assessed and evaluated in the context of predictions of thermodynamic properties. Additions of Nb to Co reduced the contact angle and thus improved the adhesion between the ceramic and metal. Nb additions also enhanced dissolution of Al2O3.