Fabrication of nanocrystalline WC and nanocomposite WC–MgO refractory materials at room temperature

Abstract

Reactant material powders of pure WO3, Mg and graphite have been milled at room temperature using a high-energy ball mill. After a few kiloseconds of milling (11 ks), numerous fresh surfaces of the reactant materials are created as a result of the repeated impact and shear forces generated by the balls. After 86 ks of milling, a mechanical solid state reduction is successfully achieved between the fresh Mg and WO3 particles to form a product of nanocrystalline mixture of MgO and W. A typical mechanical solid state reaction takes place between the W particles and graphite powders to obtain fine grains of nanocrystalline WC. Towards the end-stage of ball-milling (173 ks), the nanocrystalline MgO grains (10 nm) are embedded into the fine matrix of WC to form fine nanocomposite powders (1 μm in diameter) of WC–18% MgO material with spherical-like morphology. This composite powder was then consolidated under vacuum at 1963 K, with a pressure ranging from 19.6 to 38.2 MPa for 0.3 ks, using a plasma activated sintering method. In addition, pure nanocrystalline WC powders (7 nm in diameter) obtained by removing the MgO from the milled powders, using a simple leaching technique have been also consolidated by the same consolidation technique. The consolidation step does not lead to a dramatic grain growth and the compacted samples that are fully dense still maintain their unique nanocrystalline characteristics. The elastic properties and the hardness of both consolidated samples have been investigated. A model for fabrication of refractory nanocrystalline WC and nanocomposite WC–18% MgO materials at room temperature is proposed.