Nickel–alumina interfacial fracture toughness using the thick foil technique

The interfacial fracture toughness of the nickel–alumina bond and the influence of interfacial chemistry are studied experimentally. Nickel and alumina form a relatively strong bond, thus many of the well-known methods for experimental determination of toughness fail due to the crack deflecting into the ceramic rather than cracking along the interface. Thus a thick foil technique that forces the crack to run along the interface without kinking into the ceramic was developed. This new technique eliminates any chemical masking agents on the interface, greatly reducing surface contamination and increasing bond strength. By varying the oxygen partial pressure during hot press bonding we show that higher oxygen partial pressure, as long as it remains below the threshold for formation of nickel aluminate spinel, increases the toughness by forming a NiO layer. We also show that even when spinel is formed a strong interface can still be obtained as long as the spinel layer is thin, below approximately 3 μm.