Densification of nanocrystalline NiO ceramics by spark plasma sintering

NiO nanopowders with 20 nm particle size were spark plasma sintered (SPS) for 5 min at 100 MPa between 600 °C and 900 °C, and for different durations at 800 °C. Fully dense nanocrystalline compacts were obtained after 20 min at 800 °C. The nano-size grains were dislocation free where the larger grains composed of sub-grains separated by low angle grain boundary dislocation networks. The average grain size and the accurate lattice parameter increased, where the residual stress decreased with the increase in the SPS temperature. The high densification rates ∼10−2 s−1 during the SPS were in agreement with plastic yield and strain recovery, followed by diffusional Nabarro-Herring creep at the final stage of sintering. Grain growth occurs by grain sliding and rotation.