Interfacial energies and mass transport in the Ni(Al)-Al2O3 system: The implication of very low oxygen activities

Adhesion and capillary-driven mass transport at ceramic-metal interfaces play a very important role in the performance and durability of materials for many applications, and the influence of the oxygen activity is a critical issue. This work systematically investigates the variation of interfacial energies and atomic transport mechanisms at metal-oxide interfaces at very low oxygen activities by bonding Ni-Al alloys and pure polycrystalline alumina under controlled conditions in sessile drop experiments. The angles and the evolution of the grain boundary grooves were analyzed by scanning electron microscopy, atomic force microscopy and focused ion beam milling to calculate the interfacial and grain boundary energies and the transport rates at the metal-Al2O3 interface. In parallel, high-resolution structural and chemical analysis of selected grain boundaries was performed using advanced transmission electron microscopy. Our results confirm that all the interfacial energies (metal-Al2O3, Al2O3 surface and grain boundary energy) are smaller at reduced p(O2) than those of stoichiometric interfaces. The atomic transport at the metal-Al2O3 interface was found to decrease initially with decreasing p(O2) but increased significantly with a further decrease in the oxygen activity.