Nanoscale Oxide Formation at α-Al2O3-Nb Interfaces

Parts for metallurgical applications made from refractory metal-ceramic composites offer improved thermal shock resistance due to their capability for resistive heating compared to ones made solely from ceramics such as Al2O3. The combination of Al2O3 and Nb is intriguing as both show similar thermal expansion behavior over a wide temperature range. The high affinity of Nb for O to form nonprotective oxides, however, hampers its use in oxidative environments. Formation of such phases at the ceramic-metal interface can have detrimental effects on the cohesion of the composites. For this work, nanocrystalline Nb films are deposited on sapphire substrates by magnetron sputtering to study diffusion of O and high-temperature phase formation at a refractory metal-ceramic interface during heat treatment under Ar at 1600 degrees C. A combined approach of atom probe tomography and transmission electron microscopy for compositional and crystallographic analyses reveals that at triple junctions of the sapphire-Nb interface with Nb grain boundaries, heterogeneous nucleation of nanoscale NbO2 occurs, which further reacts with Al2O3 to form AlNbO4, while the Nb film itself remains metallic. Fast O transport through grain boundaries leads to internal oxidation at the interface, whereas regions further away from Nb grain boundaries remain unchanged.