**3. Results and discussion**

The study on SHS stages and chemical dispersion has proved that the final dispersion of the target tungsten carbide product depends on various factors. It was established that the initial mixture composition and density, reactant ratio, their aggregative state in the combustion area, gas pressure, and the nature of regulating additives influenced the size of powder particles.

When calcium chloride or hydride as well as ammonium chloride are used as regulating additives, the final product contains two phases WC and W2C. When the mixture of ammonium chloride and high-molecular polyethylene or that of metal magnesium and WC∙MgO∙Mg semiproduct are used, the single-phase target product is obtained.

**Figure 4.** Microstructure of oxidized tungsten carbide powder

solution Dispersion time, h Weight percent

H2SO4 (1:4) 3,0 7,97 2,05 0,063 0,16

H2SO4(conc) 3,0 6,12 0,03 0,24

KOH 0,5 6,13 0,03 0,005 0,03 0,005

Elemental analysis of WC∙MgO∙Mg semiproduct: Wtotal = 44.1 %; Ctotal = 4.1 %; Oxygen = 9.3 % Mgacid.sol. = 37.7 %; Mgmetal

The study on SHS stages and chemical dispersion has proved that the final dispersion of the target tungsten carbide product depends on various factors. It was established that the initial mixture composition and density, reactant ratio, their aggregative state in the combustion area, gas pressure, and the nature of regulating additives influenced the size of

When calcium chloride or hydride as well as ammonium chloride are used as regulating additives, the final product contains two phases WC and W2C. When the mixture of ammonium chloride and high-molecular polyethylene or that of metal magnesium and WC∙MgO∙Mg semiproduct are used, the single-phase target product is

**Table 2.** Effect of chemical dispersion on the elemental composition of tungsten carbide powder

С total Сfree Mg O Cr

Dispersion

5% K2Cr2O7 in

Aqueous 1%

~ 15.7 %; Mgwater sol. = 0.8 %

powder particles.

obtained.

**3. Results and discussion** 

**Figure 5.** X-ray pattern (a) and microstructure (b) of purified tungsten carbide powder.

The carbon content influenced the phase composition of the product (W2C content). The single phase product WC is formed in the case of the following ratio of the initial components in the green mixture:

$$2\text{ }\text{\textasciicircum}\text{\textasciicircum}\text{\textasciicircum}\text{\textasciicircum}\text{\textasciicircum}\text{\textasciicircum}\text{\textasciicircum}\text{\textasciicircum}\text{\textasciicircum}\text{\textasciicircum}\text{\textasciicircum}\text{\textasciicircum}\text{\textasciicircum}\text{\textasciicircum}\text{\textasciicircum}\text{\textasciicircum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquotesicum}\text{\textquth}\text{\textquth}\text{\textquth}\text{\textquth}\text{\textquth}\text{\textquth$$

The content of magnesium in the starting mixture has a substantial effect on the size of carbide particles: the stoichiometric amount of magnesium results in coarse powders, while it excess leads to a fine product (**Figure 6**).

Self-Propagating High-Temperature Synthesis of Ultrafine Tungsten Carbide Powders 13

**≤ 300 nm, % ≤ 500 nm, %** 

In studying chemical dispersion, the above results were used to analyze how the composition of the solutions, used to recover tungsten carbide from synthesized products, influenced the structure and particle size of the final tungsten carbide powders. The

It was established, that the tungsten carbide particle size depends on the composition of solutions used at the first chemical dispersion stage: recovery of carbide from intermediate

**Acid enrichment conditions Volume fraction, %** 

This result can be explained by the following way. Tungsten carbide is thermodynamically unstable and can be oxidized in the medium of water or oxygen at the room temperature [30]. X-ray phase analyses of tungsten carbide powder state in the humid medium show, that the surface of tungsten carbide particles is the first to be oxidized. The thickness of the

In water the oxide film is entirely removed due to its dissolution and formation of tungstate-

 WO3+H2O→WO42-+2H+ (8) When the milled semiproduct is dispersed by ammonium chloride or potassium chloride solutions, the pH of solution changes from low acid values to high alkali ones. The forming medium provides acceleration of oxide film dissolution by Reaction 8 and deeper tungsten carbide particle hydrolysis leading to a decrease in the particle size due to dissolution from the surface. So, chloride application at the stage of acid enrichment allows obtaining tungsten carbide powder with the number of particles of less than 300 nm in size being 80 % of the total number **(Figure 7)**. Using suitable emulsifiers can disintegrate the agglomerates and separate tungsten carbide particles of less than 100 nm

Application of ultrasound in the process of chemical dispersion decreases the time of the process and affects the dispersion degree of the product. In the case of mechanical mixing refining of tungsten carbide powders with chromium mixture takes several hours. The

**Table 3.** Fraction volumes of refined tungsten carbide powders with minimum particle sizes.

HCl (1:1) 61,3 87,5 H2SO4 (1:5) 66,7 86,3 30 %NH4Cl + HCl 81,6 96,5

following systems were used: diluted sulfuric acid (1 : 5), diluted hydrochloric acid (1 : 1),

product (**Table 3**).

ions by the reaction:

from ultrafine ones.

 ammonium chloride and hydrochloric acid solutions, potassium chloride and hydrochloric acid solutions.

oxide film increases with an increase in humidity [31].

**Figure 6.** Particle size distributions in tungsten carbide powders: **(a)** stoichiometric amount of magnesium in the starting mixture, **(b)** excess of magnesium in the starting mixture.

The excess of magnesium in the mixture seems to inhibit the growth of tungsten carbide crystals and to form a liquid phase when carbides are crystallized; the liquid phase and adjusting additives prevent intensive crystal growth. Introduction of WC∙MgO∙Mg into the green mixture also decreases the dispersion degree of the final product. Probably, the introduced additives as well as metal magnesium form a liquid phase under the terms of crystallization. Tungsten carbide ultrafine crystals contained in the introduced semiproduct can accelerate tungsten carbide crystallization and appear to be crystallization centers but a rather viscous medium prevents intensive crystal growth. Coating of tungsten carbide particles with liquid melt results in better stability of tungsten carbide to hydrolysis and oxidation after the synthesis process.

In studying chemical dispersion, the above results were used to analyze how the composition of the solutions, used to recover tungsten carbide from synthesized products, influenced the structure and particle size of the final tungsten carbide powders. The following systems were used:

diluted sulfuric acid (1 : 5),

12 Tungsten Carbide – Processing and Applications

**Figure 6.** Particle size distributions in tungsten carbide powders: **(a)** stoichiometric amount of

The excess of magnesium in the mixture seems to inhibit the growth of tungsten carbide crystals and to form a liquid phase when carbides are crystallized; the liquid phase and adjusting additives prevent intensive crystal growth. Introduction of WC∙MgO∙Mg into the green mixture also decreases the dispersion degree of the final product. Probably, the introduced additives as well as metal magnesium form a liquid phase under the terms of crystallization. Tungsten carbide ultrafine crystals contained in the introduced semiproduct can accelerate tungsten carbide crystallization and appear to be crystallization centers but a rather viscous medium prevents intensive crystal growth. Coating of tungsten carbide particles with liquid melt results in better stability of tungsten carbide to hydrolysis and

(b)

(a)

magnesium in the starting mixture, **(b)** excess of magnesium in the starting mixture.

oxidation after the synthesis process.


It was established, that the tungsten carbide particle size depends on the composition of solutions used at the first chemical dispersion stage: recovery of carbide from intermediate product (**Table 3**).



This result can be explained by the following way. Tungsten carbide is thermodynamically unstable and can be oxidized in the medium of water or oxygen at the room temperature [30]. X-ray phase analyses of tungsten carbide powder state in the humid medium show, that the surface of tungsten carbide particles is the first to be oxidized. The thickness of the oxide film increases with an increase in humidity [31].

In water the oxide film is entirely removed due to its dissolution and formation of tungstateions by the reaction:

$$\mathrm{WO} \succ \mathrm{HxO} \to \mathrm{WO} \succ \mathrm{2H}^{+} \tag{8}$$

When the milled semiproduct is dispersed by ammonium chloride or potassium chloride solutions, the pH of solution changes from low acid values to high alkali ones. The forming medium provides acceleration of oxide film dissolution by Reaction 8 and deeper tungsten carbide particle hydrolysis leading to a decrease in the particle size due to dissolution from the surface. So, chloride application at the stage of acid enrichment allows obtaining tungsten carbide powder with the number of particles of less than 300 nm in size being 80 % of the total number **(Figure 7)**. Using suitable emulsifiers can disintegrate the agglomerates and separate tungsten carbide particles of less than 100 nm from ultrafine ones.

Application of ultrasound in the process of chemical dispersion decreases the time of the process and affects the dispersion degree of the product. In the case of mechanical mixing refining of tungsten carbide powders with chromium mixture takes several hours. The

ultrasound effect decreases the time to 30 – 40 minutes. It can be explained by disintegration of tungsten carbide agglomerates and carbon coarse particles and acceleration of the reduction-oxidation reaction of chromium mixture with free carbon.

Self-Propagating High-Temperature Synthesis of Ultrafine Tungsten Carbide Powders 15

(a)

(b)

**a** - HCl (1:1); **b** – NH4Cl (30 % solution) + HCl (1:1); **c** - KCl (30 % solution) + HCl (1:1)

**Figure 7.** Tungsten carbide powder microstructure depending on the terms of acid enrichment

(c)

The ultrasound effect on tungsten carbide composition and dispersion has been studied (**Table 4**).


**Table 4.** Ultrasound effect on tungsten carbide powder composition at final product refinement

After refining with chromium mixture, the carbon content decreases to ~0.1 % but oxygen content increases greatly (in comparison with mechanical mixing) due to oxidation of tungsten carbide particle surface. The lower the refinement temperature and the higher time of ultrasound action are used, the higher dispersion of tungsten carbide powder is achieved (**Figure 8**). Under these terms the process of tungsten carbide particle surface oxidation is more active; therefore the particle size is actively decreased (powder A). An increase in the refinement temperature results in obtaining less dispersed powder B due to dissolution of fine particles under the strict terms of the process.

The powder **(a)** consists of agglomerates of fine and coarse particles. It is possible to separate ultrafine and nanosized tungsten carbide particles using proper technological terms. In the powder **(b)** fine tungsten carbide particles are situated on the surface of coarser particles and it makes their further separation much more difficult. Therefore, the ultrasound application results in additional milling of tungsten carbide powders and more complete purification from admixtures.

The results of the work on SHS of tungsten carbide powder with the reduction stage led to the development of the industrial technology of ultrafine and nanosized tungsten carbide powders synthesis. **Figure 9** demonstrates the curve of the particle size distribution of tungsten carbine powder synthesized in the industrial reactor. Obviously, the product is a mixture of particles of different sizes. The prevailing particles are ultrafine and nanosized ones.

Tungsten carbide powders synthesized by the developed technology were tested in making alloys and items thereof.

We studied sinterability of fine-particle of SHS tungsten carbide powders. **Table 5** compares the physicochemical properties and structure of WC-Co alloy prepared with the use of SHS tungsten carbide and the commercial alloy VK6-OM (containing tungsten carbide produced by a furnace process).

Self-Propagating High-Temperature Synthesis of Ultrafine Tungsten Carbide Powders 15

14 Tungsten Carbide – Processing and Applications

(**Table 4**).

Refinement time

ultrasound effect decreases the time to 30 – 40 minutes. It can be explained by disintegration of tungsten carbide agglomerates and carbon coarse particles and acceleration of the

The ultrasound effect on tungsten carbide composition and dispersion has been studied

30 min 145°C 5,72 0,015 1,40 0,14 45 min 85°C 5,18 0,013 2,35 0,07 **Table 4.** Ultrasound effect on tungsten carbide powder composition at final product refinement

After refining with chromium mixture, the carbon content decreases to ~0.1 % but oxygen content increases greatly (in comparison with mechanical mixing) due to oxidation of tungsten carbide particle surface. The lower the refinement temperature and the higher time of ultrasound action are used, the higher dispersion of tungsten carbide powder is achieved (**Figure 8**). Under these terms the process of tungsten carbide particle surface oxidation is more active; therefore the particle size is actively decreased (powder A). An increase in the refinement temperature results in obtaining less dispersed powder B due to dissolution of

The powder **(a)** consists of agglomerates of fine and coarse particles. It is possible to separate ultrafine and nanosized tungsten carbide particles using proper technological terms. In the powder **(b)** fine tungsten carbide particles are situated on the surface of coarser particles and it makes their further separation much more difficult. Therefore, the ultrasound application results in additional milling of tungsten carbide powders and more complete

The results of the work on SHS of tungsten carbide powder with the reduction stage led to the development of the industrial technology of ultrafine and nanosized tungsten carbide powders synthesis. **Figure 9** demonstrates the curve of the particle size distribution of tungsten carbine powder synthesized in the industrial reactor. Obviously, the product is a mixture of particles of different sizes. The prevailing particles are ultrafine and nanosized

Tungsten carbide powders synthesized by the developed technology were tested in making

We studied sinterability of fine-particle of SHS tungsten carbide powders. **Table 5** compares the physicochemical properties and structure of WC-Co alloy prepared with the use of SHS tungsten carbide and the commercial alloy VK6-OM (containing tungsten carbide produced

Cfree, mass %

Oxygen, mass % (non-purified product)

Oxygen, mass % (purified product)

reduction-oxidation reaction of chromium mixture with free carbon.

Ctotal, mass %

Refinement temperature

fine particles under the strict terms of the process.

purification from admixtures.

alloys and items thereof.

by a furnace process).

ones.

(a)

(b)

(c)

**a** - HCl (1:1); **b** – NH4Cl (30 % solution) + HCl (1:1); **c** - KCl (30 % solution) + HCl (1:1)

**Figure 7.** Tungsten carbide powder microstructure depending on the terms of acid enrichment

Self-Propagating High-Temperature Synthesis of Ultrafine Tungsten Carbide Powders 17

The bending strength, durability coefficient, and dispersion degree of the alloy produced

As a result of the realized research, the technology of Self-propagating High-temperature Synthesis has been developed and is being introduced for production of ultrafine and nanosized tungsten carbide powder with the use of chemical dispersion for separation,

Organization of industrial SHS production of submicron tungsten carbide powders

development of hydrometallurgical stage of submicron tungsten carbide powder

organization of design work in modernization of non-standard equipment and in

preparation of the workshop for tungsten carbide semiproduct treatment (leaching,

The processes of Self-propagating High-temperature Synthesis were studied for obtaining nanosized powders of refractory compounds, particularly, tungsten carbide. The SHS terms influence crystallization of the obtained powders. Varying the SHS parameters (reactant ratio, regulating additives, inert gas pressure, combustion and cooling velocities) allows

SHS tungsten carbide powder differs from its furnace and plasmochemical analogs in structure and purity. The grain size can be governed during the SHS processes. Powders of less than 100 nm in particle size can be obtained at complete suppression of recrystallization in combustion products. Separation of the powders from the milled cakes by chemical dispersion with various solutions and choice of chemical dispersion terms (the solution composition, the process time and temperature) allow obtaining SHS materials with the nanostructure characterized by high specific surface area and particle size less than 100 nm

As a result of the realized research, the technology of Self-propagating High-temperature Synthesis has been developed for production of ultrafine and nanosized tungsten carbide powder with the use of chemical dispersion for separation, purification and additional milling of the target product. The sinterability of the synthesized tungsten carbide powder was studied. The bending strength, durability coefficient, and dispersion degree of WC-Co

The proposed technology of ultrafine and nanosized tungsten carbide powder synthesis has

with simultaneous preserving the phase and chemical composition of the product.

alloy produced from SHS tungsten carbide exceed those of the commercial alloy.

some advantages in comparison with the available technologies:

development of the production line with complete or partial automation;

The annual production output is 150 tons. The profitableness is up to 80 %.

changing tungsten carbide powder morphology and particle size.

from SHS tungsten carbide exceed those of the commercial alloy.

purification and additional milling of the target product.

selection of standard additional devices;

utilization and regeneration of wastes).

includes:

separation;

**4. Conclusion** 

**Figure 8.** Dependence of refined tungsten carbide powder microstructure on the terms of ultrasound treatment: A – T=85ºC; B – T=145ºC.

**Figure 9.** Particle size distribution of tungsten carbide powder synthesized in industrial reactor.


**Table 5.** Physicochemical properties of WC-Co alloys prepared by using WC-SHS and WC-furnace process.

The bending strength, durability coefficient, and dispersion degree of the alloy produced from SHS tungsten carbide exceed those of the commercial alloy.

As a result of the realized research, the technology of Self-propagating High-temperature Synthesis has been developed and is being introduced for production of ultrafine and nanosized tungsten carbide powder with the use of chemical dispersion for separation, purification and additional milling of the target product.

Organization of industrial SHS production of submicron tungsten carbide powders includes:


The annual production output is 150 tons. The profitableness is up to 80 %.
