On sinterability of Cu-coated W nanocomposite powder prepared by a hydrogen reduction of a high-energy ball-milled WO3-CuO mixture

Cu-coated W nanocomposite powder was prepared by a combination of high-energy ball-milling of a WO3 and CuO mixture in a bead mill and its two-stage reduction in a H2 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 H2 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.