The transient phase eutectic process for ceramic-metal bonding

A new method of ceramic-metal bonding using a transient gas-metal eutectic liquid is proposed, confirmed, and investigated using nickel/copper-oxygen/alumina as a model system. A low temperature gas-metal eutectic melt may be made transient (by solidification) through interaction with a more refractory metal component providing a ceramic-metal bond with good wetting, high strength, a broad process window (relative to conventional gas-metal eutectic bonds), high thermal stability, and controlled thermoelastic stress; transport of a more active species to the ceramic interface may further improve adherence.; A eutectic between the low-melting component (copper) and a gas (oxygen) forms at the interface between the refractory metal (nickel) and ceramic (alumina). This interfacial liquid wets the surfaces and promotes bonding. Because the entire copper interlayer is melted, the processing window is wider than conventional gas-metal eutectic in terms of temperature, atmosphere, and time. The liquid (Cu-O) dissolves the active, refractory component (Ni) providing transport to the interface where a refractory bond phase (NiAl₂O₄) forms. Interactions at temperature consume the liquid phase causing isothermal solidification. Diffusional homogenization further increases the solidus temperature of the joint.; Multilayer bond structures were produced using both foils and plating. Oxygen additions were investigated using pre-oxidation of each metal and/or oxidation in-situ. The best bonds resulted from foils combining nickel pre-oxidation with a eutectic atmosphere. The oxide layer slows the oxidation kinetics of the nickel which allows eutectic liquid to form providing wetting, reaction, and adherence to the ceramic. The interfacial bond structure consists of a uniform, thin (sub-micron) reaction layer of nickel-aluminate (NiAl₂ O₄) spinel. Adhesion is comparable to current technologies and can exceed the ceramic strength. Typical peel failure occurs at the metal-spinel interface. Residual thermo-elastic stress is reduced relative to conventional direct bond copper. A high-temperature peel test was developed to evaluate thermal stability. It showed that strength was maintained to 800°C, the apparatus limit. Long term exposure at 1000°C did not deteriorate bond strength when interfacial oxidation was limited.