**2.2. Chemical dispersion**

4 Tungsten Carbide – Processing and Applications

after the reaction.

particles from the gas mixture.

abrasive wear and resistance.

Let us consider the possibilities of these processes.

In order to obtain ultrafine and nanosized products in the processes of solid-flame combustion, one must use the reagents of the same dispersion. In solid-phase systems with the intermediate melted layer the possibility of nano-crystal formation depends on crystallization and recrystallization processes, combustion heat modes and product cooling

In the case of gas-phase SHS (gas combustion followed by a condensed product formation) the product elemental particles consolidate with each other and form nuclei on the surface of which the following reactions occur. If fast artificial cooling is used, it is possible to arrest the particle size growth at a required stage and obtain nanopowders by depositing the

At gas-phase combustion the initial reagents, intermediate and final compounds remain in

The SHS method has provided the possibility of obtaining a great number of compounds in the dispersed state (powder). Among the materials for which the technological backgrounds are well developed the main ones are powders of refractory compounds. They are widely used in industry due to their outstanding properties such as hardness, thermal stability,

There are several directions of the SHS technologies. The widest and well-developed type of SHS reactions is the synthesis reactions of refractory compounds from elements. It is oxygen-free combustion. Both powders and gaseous elements take part in the chemical reactions. Besides, some regulating additions R are introduced into the initial mixture. They

Another direction is combination of SHS with thermal reduction (SHS with a reducing stage) when the compounds of elements (oxides, halogenides, etrc.) and metal-reducers – Mg, Ca, Al, Zn, etc. are used for the synthesis. The advantages of this method are a low price and availability of raw materials. Besides, metallothermal powders are characterized by such valuable properties as high dispersion and homogeneous granulometric composition.

The interaction of the reagents in the combustion wave occurs within two stages. The first one (reduction of the main metal oxide) is a metalthermal reaction. The second stage (SHS itself) is the interaction of the reduced metal with a non-metal followed by a refractory compound formation. There are a lot of secondary reactions which should be suppressed when optimum technological terms of the process are worked out. In the complicated systems of oxide – metal-reducer – carbon (hydrocarbon), carbon-containing components take part in carbide formation and reduction of metal oxides as well. It defines the

As a result of the SHS with a reducing stage a "semiproduct" is obtained which contains the main compound and the secondary products which can often be metal-reducer oxides. In metallothermic powders the secondary product is distributed uniformly in the whole

can be synthesis products (as diluents), various inorganic and organic compounds.

requirements to the choice of the initial components ratio.

the condensed state (either liquid or solid) during the entire reaction [16, 23].

SHS products are cakes or ingots which should be processed for obtaining powders. It can be achieved by either mechanical milling or chemical treatment.

Mechanical milling (conventional milling by balls, friction milling, planetary milling) is the easiest method for obtaining ultrafine and nano-sized powders. It is possible to obtain fine powders (up to 10-20 nm), but the problems of the long duration of the process, powder contamination with the ball and vessel materials, high power consumption require some additional solution.

One of the promising methods of obtaining nano-sized powders is the method microparticle dissolution. Recently, the efficiency of the dissolution processes for converting microparticle size to the nano-level has been confirmed. The method is based on the property of particles to decrease their volume uniformly due to their dissolution in acid and alkali media. But simultaneously the structure and the properties of the central part of the substance or phase remain the same [26].

The main aim of powder application is to obtain a dense product with homogeneous microstructure after compaction. The common reason restricting the refractory material strength is existence of agglomerates in the powder [27]. So in order to make the powder strong, it is necessary to disintegrate or remove large solid agglomerates from the initial powder. In the case of ultrafine powders the agglomerates are disintegrated by dispergating and milling in suitable solutions.

The influence of various solutions on the powder structure, dispersion degree and specific surface area has been already studied for SHS powders of boron nitride and aluminum nitride.

After synthesis, the materials were mechanically disintegrated and subjected to thermochemical treatment in neutral, acid, and alkali media at temperatures ranging from 20 to 100°C [28]. Such treatment is termed "chemical dispersion" of SHS products, as suggested by Merzhanov [29]. Chemical dispersion in a neutral medium resulted in increased total, outer, and inner specific surfaces. Mean grain size decreased. This implies that chemical dispersion provided for disintegration of the materials, as well as leading to formation of new channels and pores and the appearance of new defects, finally resulting in improved specific surface.

In [25] describes thoroughly the application of chemical dispersion for separating ultrafine and nanosized powders of boron nitride obtained by various methods under the SHS mode: from elements, with participation of boron and boron oxide, and from boron oxide with the stage of magnesium reduction.

Self-Propagating High-Temperature Synthesis of Ultrafine Tungsten Carbide Powders 7

**Figure 1.** X-ray pattern of WC∙MgO∙Mg intermediate product.

**Figure 2.** Microstructure of WC∙MgO∙Mg intermediate product.

were dissolved.

The process of chemical dispersion in various solutions is necessary for separation of the target products from the cakes forming during SHS and their further purification from

The milled cake was treated with water solutions of hydrochloric acid (1:1) or sulfuric acid (1:5) (acid enrichment) for tungsten carbide separation from the semiproduct. Unreacted metal magnesium and magnesium oxide which was formed during the synthesis process

At first the powder was treated by chloride solutions since it is known that water solutions of haloid salts destroy metal magnesium. Magnesium, potassium and ammonium salts were

admixtures with simultaneous change in the obtained powder dispersion.

Possible production of tungsten carbide of ultrafine and nanosized structure by the SHS technology with a reducing stage with using chemical dispersion for separation of submicron powders was of great practical interest.

This paper demonstrates the investigation results of the dependence of SHS tungsten carbide powder dispersion on the SHS process parameters and composition of the solutions used for chemical dispersion of the synthesis products and separation of the final product. The aim is producing single phase tungsten carbide with ultrafine and nanosized structure.
