Diffusion bonding of nickel and zirconia: Mechanical properties and interfacial microstructures

Diffusion bonds of Ni/ZrO2 and Ni/NiO/ZrO2 fabricated in vacuum have been investigated using flexural 4-point bending tests and optical and electron microscopy. It is found that subsequent annealing in air after bonding improves bond strength, and annealing in vacuum reduces strength. This is attributed to the formation of a thin oxide layer during annealing in air which enhances adhesion to the ceramic, whereas annealing in vacuum creates debonding voids at the specimen edges. The transformation of NiO in vacuum to Ni explains why the strength of bonds using preoxidized Ni foil does not show any increase, as it is essentially still the diffusion bonding of Ni to ZrO2 in the configuration of Ni/NiO/Ni/ZrO2. The presence of extensive void necklaces on grain boundaries in the metal where they intersect the bonding interface shows the importance of the metal grain boundaries acting as vacancy sinks during diffusion bonding.