On sinterability of Cu-coated W nanocomposite powder prepared by a hydrogen reduction of a high-energy ball-milled W[O.sub.3]-CuO mixture

Cu-coated W nanocomposite powder was prepared by a combination of high-energy ball-milling of a W[O.sub.3] and CuO mixture in a bead mill and its two-stage reduction in a [H.sub.2] atmosphere with a slow heating rate of 2°C/min. STEM-EDS and HR-TEM analyses revealed that the microstructure of the reduced W-Cu nanocomposite powder was characterized by ~ 50-nm W particles surrounded by a Cu nanolayer. Unlike conventional W-Cu powder, this powder has excellent sinterability. Its solid-phase sintering temperature was significantly enhanced, and this led to a reduction in the sintering temperature by 100°C from the 1,200°C required for conventional nanocomposite powder. In order to clarify this enhanced sintering behavior of Cu-coated W-Cu nanocomposite powder, the sintering behavior during the heating stage was analyzed by dilatometry. The maximum peak in the shrinkage rate was attained at 1,073°C, indicating that the solid-phase sintering was the dominant sintering mechanism. FE-SEM and TEM characterizations were also made for the W-Cu specimen after isothermal sintering in a [H.sub.2] atmosphere. On the basis of the dilatometric analysis and microstructural observation, the possible mechanism for the enhanced sintering of Cu-coated W composite powder in the solid phase was attributed to the coupling effect of solid-state sintering of nanosized W particle packing and Cu spreading showing liquid-like behavior. Homogeneous and fully densified W-20 wt% Cu alloy with ~ 180 nm W grain size and a high hardness of 498 Hv was obtained after sintering at 1,100°C.