In situ metal-ceramic microstructures by partial reduction reactions in the NiAlO system and the role of ZrO2

Partial reduction reactions in the NiAlO system, starting with the spinel compound NiAl2O4, are used to form metal-ceramic microstructures in situ. Two different morphologies of nearly pure Ni particles, equiaxed and rod-like, form within a ceramic matrix, which is either α-Al2O3 or a metastable ‘defect spinel’ depending on the choice of processing parameters, such as reduction temperature, oxygen partial pressure and time. Electron microscopic studies were performed for microstructural characterization, phase identification and chemical analysis. The fracture toughness of the NiAl2O3 mixture was significantly improved with respect to that of the original spinal phase. The important issues that must be addressed to form useful metal-ceramic microstructures by partial reduction reactions are discussed. It is shown that cracking at the original spinel grain boundaries, probably due to the large volume change associated with the reduction reaction, can be avoided by the addition of small amounts of ZrO2. Initial results are presented illustrating the effect of ZrO2 on the microstructures.