Self-propagating high-temperature synthesis of ultrafine and nanosized WC and TiC powders

Abstract

The possible application of self-propagating high-temperature synthesis (SHS) for preparing nanosized powders of refractory compounds is examined. The structurization of tungsten carbide and titanium carbide powders produced by SHS with a reduction stage is studied. The synthesis is based on exothermic reactions between tungsten oxide or titanium oxide, carbon, and magnesium metal. The influence of starting-mixture composition, ratio of components, and nature of adjusting additives on the particle size, morphology, and phase composition of WC and TiC powders is analyzed. Procedures are described for recovering tungsten carbide or titanium carbide from intermediate products using chemical dispersion, i.e., chemicothermal treatment of the ground cake in various solutions. As a result, impurities are removed and defect-rich intercrystalline layers are dissolved. Thus the sinter cake breaks into homogenous hexagonal WC or tetragonal TiC particles. The powder is additionally refined when the final product is treated in various solutions: the uniform shrinkage of the particles is observed because of their partial dissolution in acid and alkaline media, while the structure and properties in the central part of the substance or phase remain unchanged. The influence of the dispersion agent on the morphology and particle size of WC and TiC powders is examined. Conditions are determined for producing TiC and WC powders containing up to 80% of particles smaller than 30 nm using SHS with subsequent chemical dispersion. Based on the research, an SHS process is developed for producing ultrafine and nanosized TiC and WC powders on a commercial scale.