**1. Introduction**

Transition metal carbides, particularly tungsten carbide, are rather attractive due to their physical and mechanical properties [**1**]. They are characterized by the high melting point, unusual hardness, low friction coefficient, chemical inertness, oxidation resistance, and excellent electric conductivity. Nowadays, highly dispersed tungsten carbide powders appear to be very important for production of wear-resistant parts, cutters, non-iron alloys, etc.

It is well known, that fine-grained alloys demonstrate better mechanical properties in comparison with coarser alloys of the same composition under the same terms [2-4]. Use of ultrafine or nanosized powders is one of the most efficient ways to produce new materials with required properties.

That is why nowadays the production technologies of nanopowders play the leading role among the widely used directions.

There are several phases of tungsten carbide; the most important ones are WC and W2C [5]. Though W2C is unstable at T=1300°C, in most cases the mixture of WC and W2C is observed in the synthesis products. Precipitation of the single phase of WC is only possible in the narrow area of the technological parameters [6].

There are different ways to obtain tungsten carbide powders, and each process changes the characteristics of the forming product.

Tungsten carbide powders are obtained by direct carbonization of tungsten powder. This process implies production of pure highly dispersed powder of metal tungsten within the first stage. The initial material in this case is very pure WO3, tungsten acid or ammonium tungstate [7-9].

The second stage includes carbonization of tungsten by carbon in the graphite furnace with hydrogen atmosphere. Depending on the type of the furnace, atmosphere, and carbon content the reaction occurs according to the scheme:

Self-Propagating High-Temperature Synthesis of Ultrafine Tungsten Carbide Powders 3

composition and dispersion degrees. Therefore the possibility of application of SHS technology for preparing ultrafine and nanosized tungsten carbide powders represented

The new scientific direction SHS was developed at the interface of three scientific fields: combustion, high-temperature inorganic chemistry and materials science. SHS is an autowave process analogous to propagation of the combustion wave with the chemical reaction being localized in the combustion zone propagating spontaneously along the chemically active medium [15, 16]. The essence of the process is occurrence of exothermic reactions at temperatures developing as a result of self-heating of the substance; the synthesis temperature is up to 4000°C, the temperature growth rate – 103-106 K/s, the

Thorough fundamental investigations of the SHS process have proved that chemical transformation in combustion waves and product structure formation occur simultaneously with high velocity and at significant temperature gradients. These peculiarities of the process provide practically complete chemical transformation of the mixture and a specific structure of the combustion products. Application of SHS allows avoiding the main disadvantages of conventional technological processes – high power consumption,

The extreme terms which are characteristic of SHS of chemical compounds affect chemical and phase composition of the products as well as their morphology and particle size [17, 18]. The experiments in product quenching by special cooling methods immediately after the combustion front propagation have proved that "primary" product particles of 0.1-0.2 μm in

The product structure formation during the chemical reaction was called primary structure formation while the structure formed in this case was called the primary structure of the product. The characteristic time of the chemical reaction is 10-3-10-1 s; the time of the primary structure formation being the same. After the chemical reaction the particle size increases as a result of the secondary structure formation process followed by collecting recrystallization [21]. The duration of the process depends on the sample cooling mode and is usually about

Transformation of initial reagents to final SHS products is a complicated multiparametric process. There are various ways to govern it. The main types of the occurring processes are solid-flame combustion in the solid-solid system (one of the varieties is combustion with the intermediate melted layer), gas-phase SHS (chain flames, combustion of condensed systems with gaseous intermediate zone), combustion of solid-gas systems (filtration combustion,

**2.1. Self-propagating high-temperature synthesis (SHS)** 

practical interest.

**2. Experimental** 

combustion velocity – 0.1-10 cm/s.

some or tens seconds.

complicated equipment, low product output.

size can be formed in the combustion front [19, 20].

combustion of gaseous suspensions) [22].

$$\text{2W} \star \text{C} \to \text{WxC}$$

or

$$\text{W} + \text{C} \to \text{W} \text{C} \dots$$

The obtained tungsten carbide powder has particles of the indefinite melted form, minimum 3 – 5 μm in size and contains 5 % of W2C minimum. The reduction terms greatly influence the characteristics of the metal powder and forming carbide.

Thermochemical synthesis of nano-phased tungsten carbide powders was also studied. It consisted of two stages [10, 11]. At first, nano-phased powders of metal tungsten were synthesized by reduction of various tungsten salts and chemical decomposition of vapor of volatile tungsten compounds. Then nano-phased tungsten carbide with the particle size of ~30 nm was obtained by carbonization at low temperature in the medium of controlled active carbon-containing gas phase.

The method suitable for tungsten carbide synthesis at low temperatures (~800°C) during 2 hours was suggested [12]. It is based on the gas-solid reaction between a tungsten source (ammonium paratungstate or tungsten oxide) and carbon-containing gas phase which includes a mixture of H2 and CH4.

The conventional calcination–reduction–carburization (CRC) process offers the potential to manufacture commercial tungsten carbide powders with median grain sizes below 0.5 μm (ultrafine grades) [13].

In [14] point to that transferred arc thermal plasma method is more economical and less energy intensive than the conventional arc method and results in a fused carbide powder with higher hardness. Coatings of high wear resistance can be produced using fused tungsten carbide powder with WC and W2C phases, which can be economically synthesized by thermal plasma transferred arc method [14].

However, it is not economically efficient to use very pure and fine tungsten powder obtained from tungsten compounds at the stage of its reduction for producing a large quantity of tungsten carbide powder.

The existing economical and technological restrictions make the problem of the development of large-scaled cheap production of ultrafine and nanosized tungsten carbide powders very actual. Nowadays, a promising ecologically safe method, discovered in 1967 by academician A.G Merzhanov and his co-workers I.P. Borovinskaya and V.M. Shkiro – Self-propagating High-temperature Method (SHS) – is used for obtaining refractory compounds of high quality. This method combines a simple technology with low power consumption and allows obtaining products with regulated chemical and phase composition and dispersion degrees. Therefore the possibility of application of SHS technology for preparing ultrafine and nanosized tungsten carbide powders represented practical interest.
