Influence of oxygen induced during high-energy ball milling process on the mechanical properties of sintered nickel by SPS

Starting from micrometric pure nickel powder, high-energy ball milling process allows to produce nanostructured agglomerate powders associated with both a decrease of crystallite size and an increase of structural defects. With the help of a fast sintering process, such as the Spark Plasma Sintering technique, dense nanostructured materials can be elaborated. However, a drawback of this approach concerns the powder oxidation preventing the development of a perfectly densified metallic material. A study was conducted on the oxidation characteristics of nickel powders revealing a significant oxidation during milling process not only on the surface but, also, inside the agglomerated particle powder. The origin of oxygen contamination has been identified by thermogravimetric and X-ray photoelectron spectrometer analyses. It was found that achieving the milling under argon confinement makes it possible to limit the surface oxidation of particles. A best compromise between mechanical activation (i.e. reactivity) and powder contamination was obtained under argon atmosphere with slow rotation speeds of the plate and bowls composing the planetary mill. Such condition provides a nanostructured dense material with a significant improvement of the true ductility (ϵ increase from 28 to 38%) without a limited reduction of the mechanical tensile strength (Rm decrease from 740 to 720 MPa).