Synthesis of WC-Co Composite Powder by In-Situ Solid Carbothermic Reduction Method

1. Introduction

Engineering ceramics, due to their strong atomic bonds, exhibit high refractoriness and thermal resistance, including hot strength and creep resistance. For instance, carbides generally do not have a melting point and begin to sublimate at temperatures above 2500°C [1]. Most ceramics are lightweight yet possess high hardness, wear resistance, oxidation resistance, and chemical stability. However, they also have drawbacks such as brittleness, thermal and electrical insulation, and poor thermal shock resistance [2]. To mitigate these drawbacks, they can be combined with metals and their alloys to form cermets [3]. One notable engineering ceramic is tungsten carbide, which is gray, has a Mohs hardness of about 9, and a density roughly twice that of steel [4].

Cermets are predominantly ceramic in their primary phase but feature a continuous metallic phase along the particle boundaries, which enhances ductility and impact resistance while reducing brittleness [5, 6]. For example, a mixture of tungsten carbides with 10% by weight of Co metal produces cutting tools with reduced brittleness, which neither metal nor ceramic alone can achieve. This composite exhibits high toughness and hardness simultaneously, making it highly wear-resistant without being brittle, which is why it is used for cutting tools for hard steels, known as hard metals [7].

Cemented carbides consist of a complex phase, such as WC (tungsten carbide), and a binder phase, like Co (cobalt). The primary constituent of these materials is tungsten carbide, which provides hardness and wear resistance to the composite. The secondary component, cobalt, imparts toughness and helps bind the tungsten carbide particles together. Cobalt is predominantly used in tungsten carbide-based cermets due to its properties, such as the good wettability of tungsten carbide particles and forming strong bonds with them.

In addition to the mentioned properties of Co, recent studies indicate that the presence of a certain amount of pure Co alongside the raw materials (C + WO₃) for producing WC can act as a catalyst and improve the reduction conditions. In this role, cobalt accelerates the reduction of WO₃ during the carburization process. Specifically, the presence of Co reduces the oxygen pressure of the environment required to form the intermediate phase CoWO₄, which lowers the phase transition temperature of WO₃ to WO₂ and subsequently to W to about 750°C [8].

The hardness of tungsten carbide makes it brittle. To increase its flexibility and toughness, cobalt is used as the secondary phase. Therefore, this composite is utilized in the manufacturing of cutting tools and wear-resistant tools that require desirable toughness. The particle size of tungsten carbide in WC-Co cermets ranges from 0.3 to 40 microns, and the cobalt content varies from 3 to 30% by weight [9]. The grain size significantly affects performance; for general industrial cutting applications, tungsten carbide particle sizes are typically 1–2 microns [10]. Recently, ultrafine WC particles less than 0.1 microns have been developed, improving wear behavior [11].

According to the Hall-Petch effect (σ = σ₀ + k.d^−0.5), as grain size decreases, hardness, strength, and toughness increase. Smaller particles also enhance the driving force for sintering, leading to better composite formation. Particle size directly impacts the pressing pressure during powder compaction; smaller particles require higher pressing pressure, resulting in higher densification [12]. Over the past 20 years, significant attention has been given to nanocomposites of WC-Co. Applications include cutting tools, drilling, machining, drilling tools, molds for metal forming, ceramic and composite part molds in powder metallurgy, aerospace industries, turbine blades, milling balls, and sandblasting nozzles [13, 14, 15].

The excellent properties of this composite depend on various factors, but grain size has the most significant impact. As the grain size decreases, the material’s hardness and strength increase. Additionally, finer particles increase the driving force for sintering, resulting in better composite formation conditions. The particle size directly affects the pressing pressure during the powder compaction stage; finer particles require higher pressure, leading to higher density [16, 17, 18].

One of the measures taken to reduce particle size is pressure-assisted sintering, including methods such as SPS (Spark Plasma Sintering), MS (Microwave Sintering), UPRC (Ultra-High Pressure Rapid Consolidation), HFIHS (High-Frequency Induction Heating Sintering), and HIP (Hot Isostatic Pressing). It is worth mentioning that the carbo-thermic reduction method and the in-situ production of the WC-Co composite yield finer grains compared to the method of carburizing tungsten in the initial mixture (W + Co + C) or the powder metallurgy method for the (WC + Co) mixture [19, 20, 21, 22, 23, 24].

Refractory metals are commonly used in cermets because, in addition to providing toughness, they do not significantly reduce refractoriness and mechanical properties at high temperatures. Refractory metals have melting points above 1500°C and generally belong to the transition elements of the periodic table. These metals act as barriers to crack propagation at the ceramic-metal interfaces, requiring cracks to expend more energy cutting through the metal phase. As a result, the crack loses its energy and stops, thereby increasing the toughness of the piece [13].

2. In-situ methods for composite production

In-situ synthesis refers to forming composite phases (matrix or reinforcement) during milling or thermal treatment through chemical reactions. This can occur in liquid, solid, or gas phases or their combinations:

1. Direct nitration

2. Indirect nitration

3. Pyrolysis of organic materials

4. Chemical vapor deposition

5. Plasma arc spark process

6. Sol-gel process

7. Mechanical methods

8. Atomization process

9. Combustion and self-combustion synthesis

10. Carbo-thermic reduction (solid or gaseous or their combination) [25, 26, 27].

The most important parameters influencing this method are:

1. Characteristics of raw materials

2. Milling conditions

3. Effect of heat treatment temperature and holding time in the furnace (heat treatment cycle): The reaction conditions are optimized when the temperature and holding time in the furnace are minimized, which is more economically favorable and cost-effective.

4. Effect of internal furnace pressure and its atmosphere

5. Appropriate composition and purity of hydrogen gas as an auxiliary agent for the reduction process.

3. Solid carbo-thermic reduction method

In this process, composite matrix and reinforcement phases are formed through carbon reduction, which can use solid carbon (active carbon powder) or gaseous carbon (methane, CH₄, which constitutes about 80% of natural gas) or CO [28]. The reducing agent can also combine carbonaceous materials and H₂ gas. Notably, carbon polymers can serve as a carbon source. For instance, in a research study, a combination of ethylene glycol, polyethylene glycol, and polypropylene glycol was used as a carbon source to reduce tungsten oxides for WC powder production (Figure 1) [29].

Advantages of carbo-thermic reduction:

1. The powders produced have high compressibility due to their high porosity.

2. Efficient for producing composites where melting the metallic part on an industrial scale is challenging.

3. Easy control over factors like temperature, atmosphere, and chemical composition.

4. Potential for producing nanocomposite powders.

5. Reduced environmental pollution and hazards.

6. Better and more uniform particle distribution, improving mechanical and physical properties.

7. Clean interface between soft and hard phases, enhancing interface strength.

8. Thermodynamic stability at higher temperatures.

9. Cost-effectiveness [30, 31, 32, 33].

The main drawback of solid carbo-thermic reduction is the lengthy process due to diffusion-controlled reduction reactions and the high activation energy required for initial oxide materials.

4. WC ceramic and WC-Co cermets formed by in-situ and solid carbo-thermic reduction methods

One of the significant differences in the fabrication of WC-Co composites is the use of different raw materials, which can be categorized as follows:

1. Using tungsten and cobalt oxides as the primary materials and performing gaseous and solid reduction with various carbon sources [34, 35].

2. Using aqueous and salt compounds containing W and Co and introducing different C sources for reduction [36, 37, 38, 39].

3. Direct use of W, Co, and C through powder metallurgy, which, although not cost-effective, is very efficient and precise for critical components [40].

In this research, the primary focus is on the first method, which offers higher economic benefits and aligns with the objectives of this study. Other methods are briefly mentioned.

For instance, in one study, (NH₄)₆(H₂W₁₂O₄).4H₂O and (CoNO₃)₂.6H₂O salts were used as the raw materials. These materials were mixed via spraying in an environment at 100–130°C, and then heated to 800°C for 2 hours to remove the molecular water of the salts, resulting in a powder mixture of WO₃ and CoWO₄. This study is mentioned here because the final raw materials are similar to those obtained via solid carbo-thermic reduction. In the next step, the calcined powder was mixed with activated carbon as the reducing agent and then heat-treated at 950°C for 4 hours under a nitrogen gas atmosphere, ultimately producing WC-Co composite powder [41].

Figures 2–4 illustrate SEM images with different magnifications of WC-Co powder. The composite powder size ranges from 100 to 300 nm, larger than the initial oxide powders.

The reactions during the carbo-thermic reduction are as follows:

In a previous study [42], the reduction mechanism of WO₃ during carburization under a methane and hydrogen gas mixture atmosphere was examined. The results indicated that WO₃ is first converted to W₂₀O₅₈, then sequentially to WO₂, W, W₂C, and WC. A similar study showed that in a powder mixture of tungsten oxide, cobalt, and carbon, the CoWO₄ phase forms first, followed by W, W₂C, ƞ, and WC phases. Notably, the ƞ phase appears in two forms, Co₃W₃C and Co₆W₆C, which are intermediate and unstable phases for synthesizing WC-Co cermet [43]. It was also noted that at 900–923 K, solid-state diffusion on the surface of tungsten oxide particles intensifies and gradually penetrates the core.

For the reduction of WO₂ powder to WC using CO gas as the reducing agent, a recent study provided exciting results [44]. The conclusion was that at relatively low temperatures (813–908°C), the final product of the reduction process is WC. In contrast, at higher temperatures (1179°C), the final product is W₂C or W. Thermodynamic studies of this reduction mechanism showed that WO₂ is directly converted to WC at low temperatures (813–908°C) during carburization with CO gas. However, at higher temperatures (1179°C), the reduction mechanism is two-staged, first converting WO₂ to W and then to W₂C and WC. The reason for this is the increased difficulty of carburization at higher temperatures, requiring more CO. In essence, CO loses its effective reducing capability at high temperatures.

A similar study demonstrated the two-stage synthesis of tungsten carbide powder from tungsten oxide using direct reduction and carburization by a mixture of hydrogen and methane gases. Tungsten oxide powder was first converted to tungsten powder under a hydrogen atmosphere at 800–900°C. Subsequently, with the addition of methane to the hydrogen gas at various ratios (100, 50, and 20%), the behavior of tungsten carbide powder synthesis was studied at 950°C for 60–210 minutes.

The results indicated that complete carburization of tungsten carbide was not achieved within 210 minutes, and the methane-to-hydrogen ratio was optimized at 20% methane and 80% hydrogen. Spherical tungsten carbide particles with an average size of about 1.5 microns were obtained after 210 minutes [45].

One current application of nano-structured WC powder is its use as a platinum substitute. In one study, ammonium para tungstate was used as the reducing agent and subjected to a thermal gradient of 1.1°C/min under a 20% methane flow in a hydrogen environment. Samples were withdrawn at specific temperature intervals and analyzed. The results indicated that the tungsten phase formed around 550°C, initially converting to W₂C and then to WC at higher temperatures. The heat treatment was conducted in the 350–950°C range [46, 47]. Recently, in 2018, studies showed that glucose, as a solid carbon source, is more active than carbon black, successfully reducing tungsten and cobalt oxides at 900°C with a holding time of 1 hour [48].

A literature review indicates that many studies have used tungsten and cobalt oxide materials as the initial raw materials. For example, one study used WO_2.9, Co₃O₄, and C as the raw materials. The materials were homogenized in a ball mill with ethanol, a ball-to-powder weight ratio of 3:1, and a rotation speed of 500 rpm for 5 hours. Subsequently, the powder mixture was compacted and sintered, during which the reduction reaction occurred, forming a WC-Co composite [31]. It is worth noting that compaction in the initial material mixture, especially in the solid reduction method, increases the contact surface between particles, improving the reduction reaction kinetics.

The initial reduction temperature of tungsten oxide by carbon or carbon monoxide is lower due to the more negative change in energy, the lower equilibrium temperature of the reduction reaction, the higher electronegativity difference between tungsten and carbon, and the less dense crystalline structure of tungsten oxide (BCC) compared to cobalt oxide’s compact structure (HCP) [49]. DTA-TGA analysis results showed that tungsten oxide reduction reactions occurred within the 850–1000°C range, and then the cobalt oxide reduction and WC-Co cermet formation were completed simultaneously up to 1200°C. If the reduction temperature reaches the sintering temperature of cobalt (the melting point of cobalt is 1495°C), the reduction and sintering processes can be observed in a two-stage process, as reported in the study related to the source [49]. However, most studies have synthesized WC-Co cermet powder and sintered it in two separate stages, initially obtaining WC-Co cermet powder at around 1200°C using solid or gaseous reduction methods, and then performing sintering with methods such as hot pressing, spark plasma, etc. [50].

In 2020, a new two-stage reduction method was employed [51], resulting in a WC-Co composite powder with improved toughness after spark plasma sintering (SPS). As known, WC-Co composites exhibit high hardness but poor toughness. This method used pure tungsten, cobalt, and carbon powders as raw materials. The materials were combined in a ball mill at 350 rpm for 25 hours with a ball-to-powder ratio of 3:1 in an ethanol environment. The resulting mixture underwent partial reduction heat treatment in a vacuum furnace to form amorphous and unstable Co₃W₉C₄ and Co₂W₄C phases. The resulting products were re-milled with the necessary carbon to form a WC-10wt%Co composite for 15 hours. Finally, the combined powder was subjected to SPS, heating to 850°C for 5 minutes, holding for 4 minutes, then rapidly heating to 1050°C in 1 minute, holding for 3 minutes, cooling to 600°C in 3 minutes, and finally reaching room temperature in the furnace. During heating, the SPS pressure increased from 10 to 30 N. Figure 5 schematically illustrates the above process.

This two-stage heat treatment modifies the interface between WC and Co phases from non-coherent to coherent, increasing interfacial strength and resistance to intergranular fracture. The coherent twin boundaries in the cobalt phase allow greater plastic deformation during crack propagation, enhancing fracture toughness.

A 2017 study [52] utilized a mixture of coke and alumina with a 50–50 weight ratio as a protective layer instead of using vacuum or hydrogen gas atmospheres to prevent reoxidation of tungsten carbide and cobalt metal and the formation of complex oxides. The results confirmed that alumina as an inert filler between coke particles prevents oxygen penetration, reduces the activity of coke, and slows down the oxidation or combustion of carbon. The discussion continues on the influential parameters in the synthesis of WC-Co composite powder by in-situ solid carbo-thermic reduction.

4.1 Milling conditions

Microscopic results in Figure 6 indicate that the primary powder size decreases with increased milling time. The results show that the initial powder mixture, milled for 20, 30, and 40 hours, reduced the WO_2.9 and Co₃O₄ grain sizes to around 40–50 nm after 20 hours of milling, while the carbon grain size remained larger, ranging from 200 to 500 nm, indicating carbon’s tendency to re-agglomerate.

By increasing the milling time to 30 hours, the size of the oxide powder particles becomes finer, ranging from 20 to 40 nm, and the size of the carbon grains also becomes slightly finer. Subsequently, by increasing the milling time to 40 hours, the size of the oxide powder particles does not change much, but the distribution and dispersion of the carbon particles increase. Their grain size becomes about 250 nm, which is suitable for the reduction process due to having the highest surface energy and lower activation energy for reduction [25].

In a similar example, XRD results on the products show that the samples milled for 40 hours do not contain the ƞ phase (the ƞ phase includes compounds like Co₃W₃C, Co₆W₆C, and Co₂W₄C, all of which have an FCC structure but differ in the number of atoms in the crystal lattice). This phase persists if powder synthesis is done at 1100°C for 1 hour [53]. Moreover, the main phases are WC and Co. Figure 7 indicates that the ƞ phase is present in the final product for shorter milling times.

Increasing the milling time before reduction not only affects the formed phases but also facilitates the production of nanocomposite particles. Results of three samples with different milling times show that the grain size of the final composite powder after 20, 30, and 40 hours of milling (Figure 8) is 424, 355, and 306 nm, respectively [25].

In a new study conducted in 2019 [54], it was shown that the particle size of the initial WO₃ powder can affect the size and distribution of the resulting WC powder. Four types of WO₃ powder with different grain sizes were used as raw materials, and the results indicated that WO₃ powder with the smallest particles (200 nm) had the least agglomeration. Also, its distribution and uniformity after milling were higher, which reduced the reduction temperature of tungsten oxides and increased the production efficiency of WC powder. Under thermal treatment at 1100°C for 3 hours in a vacuum atmosphere, tungsten oxides were reduced with carbon, and WC powder with a size of 100 nm was obtained. In contrast, when using WO₃ powder with the largest particle size (20–100 microns), the agglomeration and clustering of oxide particles increased, and after milling, good uniformity and homogeneity were not achieved, resulting in WC powder with a particle size of 250 nm. Even the milling temperature can affect the formation conditions of the powder [55]. A study showed that at low milling temperatures (−30°C) compared to ambient temperature milling, the particle shape disorder was less, and the final WC particle size was larger (26 vs. 21 nm). This factor can influence the sintering conditions of the WC-Co cermet powder.

Therefore, according to available sources, the raw materials for producing tungsten carbide ceramics and tungsten carbide-cobalt cermet must be well-milled; otherwise, the powder synthesis will not be successful. One of the main issues in industrial-scale production is that to increase production efficiency, only a mixer is used, and the use or time of ball milling is minimized, which can be a weakness during powder synthesis. (For the synthesis of WC and WC-Co, the raw materials must be milled for at least 10 hours. However, if the aim is to use an industrial mixer since little strain energy is stored in the raw materials, the time and temperature of thermal treatment must be increased).

4.2 Effect of carbon content and material ratios in initial powder mixture

If the carbon content in the initial powder mixture exceeds the permissible limit, free carbon or intermediate ƞ phases can form in the composite powder, which can affect the properties of the WC-Co composite. The ƞ phase includes compounds like Co₃W₃C and Co₆W₆C. Therefore, the amount of carbon added to the initial powder is crucial. The amount of carbon to be added is determined experimentally. Different amounts of carbon are examined under constant conditions for composite fabrication. In a study to determine the carbon content in the initial materials, carbon amounts ranged from 16.8 to 17.1% by weight under 40 hours of milling and 1080°C reduction for 4 hours. XRD results are shown in Figure 9 [25].

According to the results in Table 1, the free carbon in the 16.8% weight sample is less than all other samples. The theoretical carbon amount under these conditions is 5.75% by weight, and since the actual amount is less, there is a carbon deficiency for composite production. For the second sample containing 16.9% carbon by weight, free carbon is still low, but intermediate ƞ phases are present in the composite structure. However, for the third and fourth samples with 17 and 17.1% carbon by weight, only WC and Co phases are observed. Therefore, according to these results, the initial powder should contain 17% carbon by weight to achieve the best and most optimized structure in the composite. An optimal amount of the ƞ phase can be suitable for the WC-Co composite and increase its corrosion resistance, especially in zinc-containing environments. Additionally, recent studies have shown that forming the ƞ phase reduces the energy required for WC phase formation, thus lowering the reduction temperature and significantly impacting the final powder grain size [43].

Sample	Excess carbon in raw materials (wt%)	Total actual carbon (wt%)	Free carbon (wt%)	Theoretical carbon (wt%)
C 16.8	16.8	5.71	0.05	5.66
C 16.9	16.9	5.88	0.06	5.82
C 17.0	17.0	5.97	0.20	5.77
C 17.1	17.1	6.11	0.34	5.77

Table 1.

Free carbon and total carbon amounts in four samples with different theoretical and actual initial carbon contents [25].

4.3 Effect of furnace atmosphere and pressure

The furnace atmosphere plays a crucial role in preparing tungsten carbide and tungsten carbide-cobalt. A vacuum atmosphere can reduce the reduction reaction temperature and improve the final powder phases, eliminating unwanted phases. Figure 10 shows the XRD pattern for powder produced at 1050°C under different vacuum levels. The best powder production conditions are under gas pressure of 4 × 10⁻⁴ Pa, where no additional phases are present and high-quality powder is obtained.

4.4 Effect of thermal treatment temperature

Figure 11 shows the XRD patterns of composites produced in the 850–1050°C [25]. Initially, when the temperature rises to about 850°C, the reduction of tungsten oxide and the release of tungsten become possible, forming the WC phase. At temperatures above 850°C, the formation of ƞ and W phases increases. With further temperature increase to 1050°C, the formation of ƞ phases decreases, and cobalt oxide reduction completes, resulting only in WC and Co phases.

Besides the effects of furnace temperature on formed phases, it can also influence the grain size of the produced composite powder (Figures 12 and 13). Results show that the initial powder grain size was around 115 nm, indicating fine raw materials. With thermal treatment in the temperature range of 900–950°C, grain growth occurs, possibly due to the high activation energy generated during milling, causing particles to reduce their surface area by increasing size significantly.

Therefore, a review of the literature shows that the temperature range for the reduction and formation of tungsten carbide and tungsten carbide-cobalt varies depending on the type of raw materials used, milling conditions, and the synthesis equipment and method. In most references, an approximate temperature range of 1000–1200°C has been chosen to synthesize tungsten carbide and tungsten carbide-cobalt powders.

4.6 Thermodynamic analysis for the formation of WC-Co composite powder (WC powder) from metal oxides using in-situ and carbo-thermic reduction methods

When discussing chemical reactions, thermodynamic laws come into play. Specifically, the first law of thermodynamics deals with the energy of reactions, and the second law addresses the transfer of this energy, guiding the direction of reactions. For instance, if ∆G = 0, the reaction is at equilibrium and can proceed in both directions. If ∆G < 0, the reaction is spontaneous and moves forward, while if ∆G > 0, the reaction is non-spontaneous and requires energy input. ∆G can be calculated using the following formula:

∆G=−∆A1TLnT−12×10−3∆A2T2−12×105∆A3T−1−16×10−6∆A4T3−16×108∆A5T−2+A6T+A7E3

Where ∆Ai and i=1−5 pertain to the thermodynamic data of reactants and products, and A7andA6 are constants related to stability. The reaction’s starting temperature can be found under conditions where ∆G = 0.

When tungsten oxide (WO₃) and cobalt oxide are combined and heated in a carbon environment at specific temperatures, numerous reactions occur, including forming intermediate phases before yielding the final product. Thus, controlling the conditions of these reactions is complex [30].

The following reactions occur during the carbo-thermic reduction of WO₃:

WO3+0.05C=WO2.9+0.05CO2E4

WO3+0.1C=WO2.9+0.1COE5

WO2.9+0.09C=WO2.72+0.09CO2E6

WO2.9+0.18C=WO2.72+0.18COE7

WO2.72+0.36C=WO2+0.36CO2E8

WO2.72+0.72C=WO2+0.72COE9

WO2+C=W+CO2E10

WO2+2C=W+2COE11

The ∆G values for these reactions can be obtained using formula 3 and thermodynamic tables. The free energy changes with temperature for oxides WO₂, WO_2.72, WO_2.9, and WO₃ with gaseous products CO₂ and CO are illustrated in Figure 14.

In Figure 14(a), reactions (4) and (5) are depicted. When the temperature exceeds 390 K and 705 K, respectively, the free energy for both reactions is less than zero, indicating they can occur under these conditions. Below 970 K, the free energy for reaction (4) is lower than for reaction (5), making CO₂ stable at temperatures below 970 K and CO at higher temperatures.

Figure 14(b) concerns reactions (6) and (7), showing that at room temperature, the free energy for both reactions is below zero, indicating WO_2.9 reduces well at lower temperatures. Above room temperature, the free energy for reaction (6) is lower than for reaction (7), thus CO₂ is produced in this case.

Figure 14(c) indicates that reactions (8) and (9) have free energies below zero above 905 K and 940 K, meaning they can occur under these conditions. Below 970 K, the free energy for reaction (8) is lower than for reaction (9), making CO₂ stable, while above 970 K, reaction (9) has a lower free energy, making CO stable.

In Figure 14(d), reactions (10) and (11) have free energies below zero above 1015 K and 1055 K, meaning they can proceed under these conditions. Below 970 K, reaction (10) is more stable, but since ∆G > 0, the reaction does not occur and CO₂ is not produced. Above 970 K, CO is produced.

In summary, the gas produced for the reduction of WO₃ and WO_2.27 can be CO and CO₂ together, but for WO₂ and WO_2.9, only one type of gas, either CO or CO2, exists, with CO being more stable at higher temperatures.

Next, the thermodynamic analysis of the formation of tungsten carbide (WC) is presented. W₂C and WC are two different chemical compounds of tungsten carbide formed during the carburization of tungsten, as shown in reactions (12)–(14):

W+0.5C=0.5W2CE12

W+C=WCE13

0.5W2C+0.5C=WCE14

Figure 15 shows the curves of free energy changes with temperature for these reactions. The ∆G for reaction (13) is significantly lower than for reactions (12) and (14). Below 1500 K, WC is more likely to form and is more stable than W₂C. However, above 1500 K, the ∆G for reaction (12) becomes lower than for reaction (13), making W2C stable, and even increasing carbon content at higher temperatures cannot convert W₂C to WC.

This section continues with the thermodynamic analysis of the carbo-thermic reduction reactions of Co₃O₄ by carbon. The carbo-thermic reduction reactions of Co₃O₄ producing CoO and Co are listed below:

Co3O4+2C=3Co+2CO2E15

Co3O4+4C=3Co+2COE16

Co3O4+0.5C=3CoO+0.5CO2E17

Co3O4+C=3CoO+COE18

Figure 16 shows the curves of free energy changes with temperature for these reactions. All these reactions can occur at low temperatures. The starting temperature for reaction (16) is around 625 K, and for other reactions, it is below 360 K. At this low-temperature range, reactions (15) and (17) start sooner, both producing CO₂ gas. Below 515 K, reaction (17) has a lower ∆G and occurs more quickly, forming CoO. Above 515 K, reaction (18) has a lower ∆G and proceeds more easily, forming Co. At 970 K, reaction (16) has the lowest ∆G and is thermodynamically more stable, with the final product being cobalt and CO gas.

It is worth noting that one of the differences in the synthesis of tungsten carbide and tungsten carbide-cobalt is the presence of cobalt oxide in the initial material composition. This combination leads to the formation of the intermediate compound “CoWO₄” in the structure, which can improve the synthesis process of tungsten carbide–-. Therefore, it has been stated that it can act as a catalyst in synthesizing tungsten carbide alongside cobalt. The thermodynamic analysis of the intermediate compound CoWO₄ is provided below. The reaction between WO₃, Co₃O₄, and carbon at lower temperatures results in forming the intermediate and unstable compound CoWO₄, which can decompose at higher temperatures. The formation and decomposition reactions of CoWO₄ are presented below.

Co3O4+0.5C+3WO3=3CoWO4+0.5CO2E19

Co3O4+C+3WO3=3CoWO4+COE20

CoO+3WO3=CoWO4E21

CoWO4+2.5C=0.5W2C+Co+2CO2E22

CoWO4+4.5C=0.5W2C+Co+4COE23

CoWO4+2C=W+Co+2CO2E24

CoWO4+4C=W+Co+4COE25

CoWO4+3C=WC+Co+2CO2E26

CoWO4+5C=WC+Co+4COE27

Figure 17, parts (a) and (b), corresponding to the curves of Gibbs free energy changes with temperature related to reactions 19–27. Part (a) in Figure 17 depicts the formation reactions of CoWO₄, which occur at low temperatures around 300 Kelvin. Under these conditions, the Gibbs free energy change (ΔG) for reactions 19–21 is negative. Reaction 17 has a more negative ΔG than reactions 20 and 21, indicating the stability of the phases CoWO₄ and CO₂ under these conditions. It is noteworthy that reaction 21 indicates that CoWO₄ can be produced from the combination of CoO and WO₃, and the temperature does not affect the ΔG of the reaction.

In part (b) of Figure 17, the decomposition reactions of CoWO₄ are shown, which occur at relatively high temperatures. The onset temperature for transformations corresponds to reaction 26 (945 Kelvin). Under these conditions, the products of reaction 26, Co, WC, and CO₂, are stable, and the highest formation temperature of the products is 1050 Kelvin, corresponding to reaction 24. Within this temperature range, when the temperature reaches 975 Kelvin, the ΔG of reaction 27 is at its minimum, and WC, CO gas, and Co(s) are formed.

Therefore, when CoWO₄ reacts with carbon, the gaseous reaction products at low temperatures are CO₂ and CO, and the solid components in the final products depend on the reaction temperature and the amount of carbon. At a constant temperature, with an increase in the amount of carbon, the products change from W to W₂C and WC.

5. Conclusion

The literature review indicates the optimal conditions for various factors influencing the carbo-thermic reduction process and the synthesis of tungsten carbide (tungsten carbide-cobalt). For the milling of raw materials in the solid-state method, a duration of 5–20 hours with a ball-to-powder ratio of 3 (up to 10) to 1 and a speed of 300 rpm is suitable. Additionally, the best atmosphere for solid-state reduction is a vacuum, while an oxidizing atmosphere or argon protective gas does not perform well. The optimal time and temperature for the heat treatment process (thermal cycle) for the solid-state reduction method are within the temperature range of 1000–1200°C and a holding time of at least 3 hours. Information on the amount of carbon in the initial material composition is incomplete and requires further investigation.

Conflict of interest

The authors declare no conflict of interest.