Formation of Nanocrystalline Phases in Sol-gel Masses in the Fe-W-C(O) System and Densification up to 1100°C

Nanoparticles (50-200 nm) containing W, C and Fe (with W/Fe weight ratios between 1.5 and 9) were produced from alkoxide-mixtures subjected to partial oxidation and reduction in 10%H2-Ar up to 1000°C. During heating in a dilatometer under Ar or 10%H2-Ar flow, pellets made of nanoparticles with a W/Fe=1.5 ratio exhibited an expansion at low temperatures (400-600°C) and then rapid densifications at higher temperatures. For such a material DTA traces under Ar flow showed endothermic/exothermic peaks at about 400°C and 700°C. The formation of pure Fe and FeWO4 at ca. 300°C was detected by XRD at high temperatures (and under vacuum). A clear densification between 650°C and 900°C was detected in the pellet treated in the dilatometer under a flow of 10%H2-Ar (up to about 1030°C), associated to solid state sintering stage of nanoparticles of Fe and FeWCxO1-x (Fe6W6C, Fe3W3C, W2 C, WC, FeWO4) compositions. After a subsequent similar run under 10%H2-Ar the specimen only showed expansion between 500 and 900°C and continued densifying for temperatures higher than 900°C. At the end of the run the sample was completely reduced to a mixture of Fe & Fe2W (Laves phase). The consolidation increased the particle size considerably.