Interfacial noble-metal corrosion in metal to ceramic reaction welding

WELDING refractory ceramic oxides to a range of transition metals in oxidising atmospheres may involve two apparently distinct bonding mechanisms1,2. In type 1 bonds, which involve the noble metals, a sharp phase discontinuity at the metal–ceramic interface is maintained, even after prolonged annealing in oxygen. No intermediate oxide phase, other than the original ceramic, is observed. The substantial bond strengths3 reach their maximum value after a few hours and do not deteriorate with time. Residual thermal stresses are small as shown by high resolution hot-stage electron microscopy4. In contrast, in type 2 bonds, in which the more reactive transition metals are used, an intermediate (quaternary) oxide layer is clearly visible, its thickness growing with time. The bond strength depends on the corrosion behaviour of the metal and the mechanical strength of the intermediate oxide layer, as metal cations diffuse by bulk and grain boundaries into the ceramic. It is influenced by the duration of the anneal, as the oxidation process continues. Here we report the experimental observation that the couples Pd/MgO and Pd/Al2O3 form type 2 bonds below 800 °C and strong type 1 bonds above that temperature, when annealed in air.