Elimination of oxide films during Spark Plasma Sintering of metallic powders: A case study using partially oxidized nickel

We present a case study addressing the issue of surface cleaning of the particles of metallic powders from oxide films during Spark Plasma Sintering (SPS). Based on the results obtained using an intentionally oxidized powder of carbonyl nickel (O/Ni(at.) = 0.2), we show that when the powder is heated in the SPS chamber but is not in direct contact with graphite foil commonly used in the SPS practice, the reduction of the oxide is limited to the surface layers of the powder bed. Comparative sintering experiments with graphite and copper foils indicate that a direct contact with a source of carbon is critical for the elimination of the oxide from the sintered compact. Compacts/regions of the compacts, in which nickel oxide NiO was reduced, showed more pronounced inter-particle necking than the regions maintaining the initial oxide concentration.