Preparation, Sintering Behavior and Microstructure of Al_2O_3-Coated Ni Powders

Alumina precursor-coated nanosized Ni powders were prepared by the heterogeneous precipitation method in the aluminium salt solution. The chemical composition of the as-prepared coated powder was characterized by energy dispersive spectra and Auger electron spectra. It revealed that nanosized Ni particles were homogeneously coated with layers of alumina hydrate. The calcined powders that were performed in argon were hot pressed in a protective atmosphere of argon. The experimental results indicate that the calcined powders can be sintered to nearly fully dense at a lower temperature as compared with that from sol-gel prepared powder mixtures. The microstructures of these sintered specimens are different from that of ball-milled and hotpressed counterparts. Irrespective of the location of Ni particles in alumina matrix, there exist holes at the interfaces between alumina grains and Ni particles, which is attributed to thermal expansion mismatch between alumina and nickel. The fracture mode of these composites is mainly an intergranular-type.