Carbon nanotubes-Fe-Alumina nanocomposites. Part II: microstructure and mechanical properties of the hot-Pressed composites

Carbon nanotubes±Fe±Al2O3 massive composites have been prepared by hot-pressing the corresponding composite powders, in which the carbon nanotubes are arranged in bundles smaller than 100 nm in diameter and several tens of micrometers long, forming a web-like network around the Fe±Al2O3 grains. In the powders, the quantity and the quality of the carbon nanotubes both depend on the Fe content (2, 5, 10, 15 and 20wt%) and on the reduction temperature (900 or 1000 C) used for the preparation. Bundles of carbon nanotubes are present in the hot-pressed materials but with a decrease in quantity in comparison to the powders. This phenomenon appears to be less pronounced for the powders containing higher-quality carbon, i.e. a higher proportion of nanotubes with respect to the total carbon content. The presence of carbon as nanotubes and other species (Fe carbides, thick and short tubes, graphene layers) in the powders modi®es the microstructure of the hot-pressed specimens in comparison to that of similar carbon-free nanocomposites: the relative densities are lower, the matrix grains and the intergranular metal particles are smaller. The fracture strength of most carbon nanotubes±Fe±Al2O3 composites is only marginally higher than that of Al2O3 and are generally markedly lower than those of the carbon-free Fe±Al2O3 composites. The fracture toughness values are lower than or similar to that of Al2O3. However, SEM observations of composite fractures indicate that the nanotubes bundles, which are very ̄exible, could dissipate some fracture energy.