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 s pinel compound NiAl2O4, are used to form metal-ceramic microstructures in situ. Two different morphologies of nearly pure Ni particles, equi axed and rod-like, form within a ceramic matrix, which is either alpha -Al2O3 or a metastable 'defect spinel' depending on the choice of proc essing parameters, such as reduction tempera oxygen partial pressure a nd time. Electron microscopic studies were performed for microstructur al characterization, phase identification and chemical analysis. The f racture toughness of the Ni-Al2O3 mixture was significantly improved w ith respect to that of the original spinal phase. The important issues that must be addressed to form useful metal-ceramic microstructures b y partial reduction reactions are discussed. It is shown that cracking at the original spinel grain boundaries, probably due to the large vo lume change associated with the reduction reaction, can be avoided by the addition of small amounts of ZrO2. Initial results are presented i llustrating the effect of ZrO2 on the microstructures.