THE THERMODYNAMICS OF SPINEL INTERPHASE FORMATION AT DIFFUSION-BONDED NI/AL2O3 INTERFACES

Interface microstructural development during solid state diffusion bonding of Ni to single-crystal alpha-Al2O3 has been studied by electron microscopy. Nickel aluminate spinel (NiAl2O4) interphase layers approximately 1-mu-m thick formed under high vacuum bonding conditions. Very high vacuum (VHV) annealing caused the spinel to disappear, indicating that its stability depends critically upon the oxygen activity. High vacuum diffusion bonding utilizing initially oxygen-free Ni and oxygen-containing Ni established that spinel formation requires a threshold oxygen activity, and furthermore, that the source of the required oxygen can be oxygen initially dissolved in the Ni. Thermodynamic calculations confirm that the threshold oxygen level necessary to stabilize the spinel increases from 0.006 at.% (60 at.ppm) at 1273 K to 0.025 at.% (250 at. ppm) at 1663 K. Further analysis indicates the spinel exhibits a maximum thickness determined by the difference between the initial and threshold oxygen concentrations and the Ni thickness. Considering the solubility limit of oxygen in solid Ni, the spinel thickness is limited to approximately 0.005 times the Ni thickness. The reaction is explored further in the context of diffusion path concepts with a calculated Ni-Al-O phase diagram.