Formation of Refractory WC Compound by Mechanical Solid State Reduction.

Abstract

A new process for preparation of nanocrystalline WC powder at room temperature is reported. In our process, a mixture of WO3, Mg and graphite powders are mechanically reacted under an argon gas atmosphere using a high energy ball mill. After few kiloseconds of milling (11 ks), numerous fresh surfaces of the powder are created by the repeated impact and shear forces of the balls. After 86 ks, the fresh Mg particles that have high reducing potential are oxidized, whereas the WO3 is reduced completely to a metallic W powder. In parallel, a solid state reaction between W and the unreacted graphite powders takes place to yield a fine powder of WC. At the final stage of milling (173 ks), composite WC/MgO powder particles is obtained. The MgO is removed from the matrix of WC using a leaching technique. The residual WC powder was then consolidated in vacuum at 1963 K with a pressure of 19.6 - 38.2 MPa for 0.3 ks, using a plasma activated sintering method. No binding material was used during the consolidation procedure. Under these consolidation conditions, the obtained sample is fully dense (>99.5 % theoretical density) with nanoscale grain size of about 25 nm in diameter. The elastic properties and the hardness of the consolidated sample have been determined. A model for the formation of WC at room temperature is proposed.