The electrification of high-temperature processes, such as steel casting technology, can remarkably increase energy efficiency and reduce costs. Lining parts like slide plates and spider bricks (see figure 1a) made from conventional refractory ceramics, e.g., Al₂O₃, frequently suffer from failure caused by thermomechanical stresses. Internal resistive preheating of these parts would significantly reduce the thermal gradients but requires electrical conductivity of the part. Bulk refractory composites based on Al₂O₃ and refractory metals such as Nb and Ta meet this need and promise an outstanding combination of high thermal shock resistance, fracture toughness, and erosion resistance.

Technical grade raw materials contain impurity elements (e.g., C, Si, O) that cause the formation of secondary phases such as carbides and complex oxides during the synthesis of sintered or cold-isostatically pressed composites. These phases will further develop during application when exposed to typical temperatures of 1500 to 1800°C and O- and C-containing atmosphere and introduce new interfaces that appear to act as crack initiation sites. To understand the principles of formation, we prepare model materials by sputter-coating of α-Al₂O₃ with Nb. The heat-treated composites (1600°C, Ar, low O partial pressure) were investigated with atom probe tomography (APT) and transmission electron microscopy (TEM). When O diffuses along Nb grain boundaries to the Al₂O₃-Nb interface, it induces a complex reaction to form a ternary oxide, while the interface remains unchanged if a grain boundary is absent. The results are contrasted with composites containing an interlayer of C or Si to selectively study the influence of impurity elements on the interface chemistry.