Fabrication of bulk nanocrystalline Ni by spark plasma sintering

Ni nano-particles were prepared by flow-levitation method. And then, bulk nanocrystalline Ni samples, 10 mm in diameter and 2 mm in height with a relative density of 96.8 % and a microhardness of 4.17 GPa, were fabricated by spark plasma sintering using the nano-particles. The phase composition, grain size, microstructure and microhardness of the bulk samples were characterized by X-ray diffractometer, scanning electron microscope, and Vickers microhardness tester, respectively. The results indicate that the relative density and grain size increase with increasing sintering temperature. The optimal relative density has been obtained at the sintering temperature of 650 ℃, at which the average grain size is 44.8 nm. The microhardness first increases and then decreases with increasing sinter