Reduction and sintering of alumina/tungsten nanocomposites - Powder processing, reduction behavior and microstructural characterization

Reduction behavior of tungsten oxide mixed with alumina powder and its sintering processes were investigated to fabricate Al2O3/W nanocomposites, Submicron-sized WO3 powder could be obtained by a conventional ball-milling technique. Hygrometry analysis of Al2O3/WO3 mixture revealed that WO3 powder was reduced in two steps at 873 and 973 K, X-ray diffraction and transmission electron microscopy (TEM) analysis confirmed that Magneli phases, such as WO2 and WOx (n=2.89 to 2.92), were obtained by the reduction of WO3 at 873 K. However, WO3 was completely reduced to metallic W when the reduction temperature was higher than 1173 K, A thermodynamic analysis qualitatively agreed with the experimental result that WO2 and WO, coexisted at intermediate temperatures. Reduction followed by hot-press sintering of Al2O3/WO3 mixtures was carried out to obtain Al2O3/W nanocomposites. Agglomeration and/or incomplete reduction was found at high W content; however, dense Al2O3/W composites with >98% of their theoretical density were obtained at low W content (i.e., < 10 vol%). A microstructural investigation of the dense composite revealed that it consisted of both micro- and nano-sized W dispersions. Furthermore, a large number of W particles of around 40 nm were found to be homogeneously dispersed in the Al2O3 matrix. These particles were much finer than the WO3 particles obtained by ball-milling.