Diffusion bonding of a nickel (chromium) alloy to zirconia: Mechanical properties and interface microstructures

Diffusion bonding of a Ni(Cr) alloy with ZrO₂ has been studied. It was found that the processes were controlled by chemical reactions at the metal/ceramic interface forming compounds within the composition range NiO_1–x Cr₂O_3–y ZrO_2–z (0<x,y,z<1). Reaction products were characterized by examination of the fractured metal/ceramic halves and from cross-sections using optical, scanning and transmission electron microscopy and energy dispersive X-ray microanalysis. These metallographic studies indicate that the interface reactions are accompanied by local melting indicating a possible eutectic reaction following interdiffusion. Pre-oxidizing the metal foil enhances these reactions and lowers the reaction temperatures. Differential thermal analysis found endotherms at 1050 and 1110 °C with sintered NiCr/ZrO₂ powders, and at 980 and 1100 °C when pre-oxidized powders were used. These are shown to be associated with local melting at the metal/ceramic interface. This thus explains the existence of a critical bonding temperature for good-quality diffusion bonding, where a minimum temperature has to be satisfied for the eutectic reaction to occur.