Pressure Sintering of Iron, Cobalt, Nickel and Copper Ultrafine Powders and the Crystal Grain Size and Hardness of the Compacts

Abstract

The lowest sintering temperatures (Tsc) for complete densification of Fe, Co, Ni and Cu ultrafine metal powders (average particle size, 0.02–0.05 μm; oxygen content, 15–6 mass%) were investigated under pressure up to 400 or 500 MPa (sintering time, 3.6 ks) after reduction treatment at 453–546 K in H2 gas (oxygen contents were decreased to 0.43–0.03 mass%), and the crystal grain size and hardness of the dense sintered compacts were measured.Under 400 MPa, Tsc for Fe, Co, Ni and Cu powders were about 590, 640, 450 and 450 K respectively, which were 380–620 K lower than those under 0 MPa. The minimum average crystal grain sizes of Fe, Co, Ni and Cu dense sintered compacts were as small as about 0.08, 0.21, 0.12 and 0.40 μm respectively. The highest values of hardness (Hv) of these sintered compacts were about 600, 550, 550 and 130 respectively, which were extremely high compared with usual coarse-grained metals. The Hall-Petch linear relationship between Hv and the inverse of the square root of grain size held for each element.