Synthesis and Processing of Nanostructured W-Base Materials

Abstract

Chemical processing is a more direct route for making materials than traditional metallurgical processing. It offers the potential for lower cost production of novel materials with homogeneous ultrafine structures (nanostructures) and improved properties.

High surface area nanocrystalline W and nanostructured WC-Co composite powders have been synthesized by thermochemical conversion of homogeneous precursor compounds. Reductive decomposition of WO3 at T > 625°C yields nanocrystalline α-W (bcc structure), whereas the same treatment at ∼ 500°C gives nanocrystalline β-W (A15 structure). The elemental form of β-W has an A15 defect structure, from which all or nearly all of the oxygen atoms have been removed. Diffusional disordering (amorphization) of high surface area elemental β -W in ammonia occurs at temperatures 350° C. The amorphous state of W(N) decomposes at higher temperatures in the range 650 – 850° C to form nanocrystalline W₂N.

Reductive decomposition of an homogeneous spray dried powder of ammonium metatungstate and cobalt nitrate yields a high surface area reactive intermediate of W-Co, which can be gas phase carburized to nanostructured WC-Co. The carburization treatment may be performed in CO/CO₂ gas mixtures of controlled carbon activity in the range 0.35 – 0.95. Rapid carbon uptake gives rise initially to W₂C-Co, which gradually transforms to the thermodynamically stable WC-Co composition. However, the fastest carburization kinetics at the lowest permissible temperatures, with control of uncombined carbon formation, occurs with CO/H₂ gas mixtures.