In the preparation and application of cemented carbide materials, coating the powder (such as coating tungsten carbide with nickel) is an important technological means. Its core purpose is to improve the quality and application range of the final cemented carbide products by enhancing the surface characteristics, interface bonding, and overall performance of the powder.

This paper intends to discuss how to prepare nickel-coated WC composite powder (WC-Ni) by combining the chemical coprecipitation process with the high-temperature hydrogen reduction process, using 텅스텐 카바이드 (WC) powder and NiCl₂·6H₂O as raw materials.

 

연구 배경

Cemented carbide is a material made by combining hard phases (such as WC, TiC) and binder phases (such as cobalt, nickel, iron) in a designed form, proportion, and distribution according to performance requirements through powder metallurgy methods. Cemented carbide not only has high hardness and strength but also possesses properties such as wear resistance, heat resistance, and corrosion resistance.

At present, the mainstream industrial process for preparing cemented carbide composite powder is the ball milling method. In this process, several grinding balls of different sizes, tungsten carbide or other hard phase powders, binder phase metal powders, alcohol, and other materials are added to the ball mill.

The grinding balls in the ball mill continuously collide and roll the powders, mixing the powders in the ball mill and finally achieving the preparation of alloy composite powder [8]. However, to achieve a uniform mixing effect for the alloy composite powder prepared by this process, a relatively long ball milling time is required. Moreover, the particle size of the prepared powder is difficult to control, and a large number of binder phase metal agglomerations are prone to occur, which will have an adverse impact on the alloy performance.

In order to find a better new method for preparing cemented carbide, researchers have made many explorations on the preparation methods of cemented carbide composite powder, mainly including hydrothermal hydrogen reduction method, spray conversion method, electroless plating method, and chemical coprecipitation method, etc.

The chemical coprecipitation method has the advantages of simple preparation process, low cost, easy control of preparation conditions, uniform coating, short synthesis cycle, and no introduction of impurity elements during the preparation process.

At present, there are many studies on cobalt coating WC in the literature, but the research on nickel coating WC is insufficient. In addition, the influence law and mechanism of chemical coprecipitation process conditions on the coating effect still need to be further studied.

The effects of different process conditions such as feed liquid concentration, reaction temperature, and precipitant addition flow rate on the nickel precipitation rate and coating morphology were analyzed, and the nucleation and coating mechanism of nickel particles were discussed.

 

Experimental Materials and Characterization Methods

Experimental Materials

The raw materials used in the experiment mainly include: WC powder with a Fisher particle size of 4.8 – 5.2 μm (purity ≥ 99.96%), hydrofluoric acid (purity ≥ 40.0%), nitric acid (purity ≥ 65.0%), NiCl₂·6H₂O (purity ≥ 98.0%), and (NH₄)₂C₂O₄·H₂O (purity ≥ 99.5%). The experimental steps are as follows.

Roughening of raw WC powder

Weigh a certain amount of hydrofluoric acid (HF) and nitric acid (HNO₃), mix them uniformly with deionized water to obtain a roughening solution. Add the weighed tungsten carbide powder into the roughening solution and stir thoroughly for 30 minutes. Then, wash the powder with deionized water 3 times and dry it to obtain the roughened powder.

 

 

Coated mixture precursor (WC – NiC₂O₄·2H₂O)

Prepare NiCl₂·6H₂O and (NH₄)₂C₂O₄·H₂O into NiCl₂ solution and (NH₄)₂C₂O₄ solution with the same volume but different concentrations respectively. Pour the roughened tungsten carbide powder into the previously prepared NiCl₂ solution, and then add a certain amount of hydrochloric acid solution to the mixed solution of NiCl₂ and tungsten carbide to adjust the pH value of the mixed solution. Then, add the (NH₄)₂C₂O₄ aqueous solution as a precipitant into the mixed solution of NiCl₂ and tungsten carbide.

Due to the high density of WC, in order to prevent the settlement of WC from affecting the uniformity, an electric stirrer is used during the coating process, which can not only prevent the settlement of WC but also make the solution mixture mixed uniformly. The chemical coprecipitation reaction process is as follows:

 

Preparation of WC – Ni alloy powder by high – temperature hydrogen reduction method

Filter the aged mixed solution by suction, and then dry it in a vacuum drying oven. Calcinate the mixture precursor powder at a constant high temperature in a hydrogen protective atmosphere to obtain WC – Ni composite powder. The high – temperature hydrogen reduction reaction process is as follows:

 

Experimental Characterization Methods

The precipitation rate of the coprecipitation reaction reflects the efficiency, effectiveness of the chemical coprecipitation reaction and the operability of the process in actual production. In this experiment, the precipitation rate is calculated by the following formula:

In the formula: M_after is the mass of the powder obtained by drying after the coprecipitation reaction; M_theoretical is the theoretical mass of the powder obtained after the coprecipitation reaction.

Phase composition analysis and microstructure analysis were carried out on the raw tungsten carbide powder, the roughened tungsten carbide powder, the mixed precursor powder obtained after the chemical coprecipitation coating reaction, and the composite powder obtained after high – temperature hydrogen reduction. The testing instruments were Bruker D8 Advance X – ray powder diffraction (XRD) instrument, optical microscope and Zeiss EVO18 scanning electron microscope (SEM) respectively. The working parameters of the X – ray diffractometer during the test are as follows: the X – ray tube voltage is 40 kV, the current is 40 mA, the test accuracy is ≤ 0.02°, the test rate is 10 (°)·min⁻¹, and the test angle is 2° – 90°.

 

Results and Analysis

Influence of Process Parameters on Precipitation Rate of Coprecipitation Reaction

It can be seen from Figure 1a) that the precipitation rate increases with the increase of feed liquid concentration. After the feed liquid concentration rises to 0.4 mol·L⁻¹, the increase range of the precipitation rate begins to decrease significantly, and finally tends to a fixed value. For this experiment, when the feed liquid concentrations are 0.4 mol·L⁻¹ for NiCl₂ and 0.2 mol·L⁻¹ for (NH₄)₂C₂O₄, the optimal coating effect and a relatively high precipitation rate can be obtained. It can be seen from Figure 1b) that the precipitation rate first increases gradually with the increase of reaction temperature. When the reaction temperature rises to 65 ℃, the precipitation rate reaches the peak, and then starts to decrease.

For this experiment, when the reaction temperature is 50 ℃, the optimal coating effect and a relatively high precipitation rate can be achieved. As shown in Figure 1c), the precipitation rate of the coprecipitation reaction first increases significantly with the acceleration of the volume flow rate of the precipitant addition, reaches the peak at the addition volume flow rate of 0.17 mL·s⁻¹, and then the precipitation rate begins to decrease gradually and finally tends to be stable. For this experiment, when the volume flow rate of the precipitant addition is 0.17 mL·s⁻¹, the optimal coating effect and the highest precipitation rate can be obtained.

 

In the coprecipitation reaction, the growth rate of crystal nuclei is closely related to the instantaneous concentration of solute in the solution. Figure 2 shows the SEM morphologies of the mixture precursor powders under different volume flow rates of precipitant addition (NiCl₂ concentration is 0.4 mol·L⁻¹; (NH₄)₂·C₂O₄ concentration is 0.2 mol·L⁻¹, reaction temperature is 50 ℃, reaction time is 1.5 h; pH value is 6.2).

With the decrease of the volume flow rate of the precipitant addition, both the dispersion degree of particles in the solution and the tightness and uniformity of the coating of NiC₂O₄·2H₂O particles on the surface of WC particles are improved. After the volume flow rate of the precipitant addition decreases to 0.17 mL·s⁻¹, with the further decrease of the addition volume flow rate, both the tightness and uniformity of the coating of NiC₂O₄·2H₂O particles on the surface of WC particles start to decrease.

Phase Composition of Powders and Analysis of Coating Effects at Various Stages

Figure 3a) shows the XRD pattern of the raw WC powder used in the experiment. It can be seen that there are no obvious impurity elements in the raw WC powder.

Figure 3b) presents the XRD pattern of the precursor powder used in the experiment. It is evident that the WC-nickel salt mixture precursor obtained from the chemical coprecipitation reaction is WC-NiC₂O₄·2H₂O, with no other impurity elements introduced, thus obtaining pure precursor powder.

Figure 3c) displays the XRD pattern of the alloy powder obtained after the reduction of the mixture precursor powder. It can be observed that the WC-Ni alloy powder after reduction consists of pure WC phase and nickel phase, with no other impurity phases present. The XRD patterns can verify the feasibility and effectiveness of the chemical coprecipitation-high-temperature hydrogen reduction method for coating powders.

The micro-morphologies of powders at various stages in the process of preparing coated powders by chemical coprecipitation-high-temperature reduction are shown in Figure 4. It can be seen from Figures 4a) and 4b) that the particle size of the original tungsten carbide powder is approximately 5 μm, with a smooth surface and a clearly contoured elliptical shape.

Figures 4c) and 4d) show the powders after roughening treatment with the roughening solution. Compared with the original tungsten carbide powder, the surface of the roughened powder has relatively clear grooves and step-like depressions due to erosion by the strong acid solution. The occurrence of these defects increases the specific surface area of the tungsten carbide particles, making it easier for the NiC₂O₄·2H₂O particles obtained from the chemical coprecipitation reaction to adhere to the WC surface.

Figures 4e) and 4f) show the WC-NiC₂O₄·2H₂O composite powder obtained through the chemical coprecipitation coating reaction. It can be seen that the NiC₂O₄·2H₂O particles are coated very uniformly and relatively tightly on the surface of the WC particles.

Figures 4g) and 4h) show the WC-Ni composite powder obtained after the reaction of the WC-NiC₂O₄·2H₂O mixed powder under high-temperature conditions in a hydrogen atmosphere. It can be observed that the WC particles maintain the near-spherical shape of the raw material powder, with relatively intact particle morphology. No obvious fragmentation or deformation is found, and the nickel phase is coated relatively uniformly and densely on the surface of the WC particles.

Mechanism Analysis

As shown in Figure 5, the chemical coprecipitation method involves adding a precipitant to a mixed metal salt solution. This causes two or more types of cations in the solution to precipitate together, forming a precipitation mixture or a solid solution precursor. After filtration and washing, composite oxides are obtained.

The addition of the precipitant may lead to a locally high concentration of the precipitant in the metal salt solution, resulting in agglomeration or insufficiently uniform composition. The main reason for coprecipitation is surface adsorption. The ionic charges on the precipitate surface are not balanced, and their residual charges attract ions with opposite charges in the solution. This adsorption is closely related to parameters such as feed liquid concentration, reaction temperature, and the volume flow rate of precipitant addition.

First, the growth rate of crystal nuclei and the size of precipitated particles are closely related to the concentration of solute in the solution. According to Von Weiman’s empirical formula, the higher the feed liquid concentration, the easier the mutual aggregation of product particles. The rate of precipitation formation is proportional to the relative supersaturation of the solution.

When the feed liquid concentration increases, the relative supersaturation of the solution increases. The number of nucleated particles within the same time increases, leading to a higher probability of collisions between crystal nuclei and gradually intensified agglomeration among precursor particles. In addition, an excessively fast precipitation rate will cause impurity ions adsorbed on the surface to be covered by subsequently deposited ions before they can be replaced by lattice ions of the main precipitate. As a result, impurity ions may get trapped inside the precipitate, causing occlusion.

Second, in the chemical coprecipitation reaction, the precipitation rate first increases and then decreases with the rise of reaction temperature. This is because when the temperature is very low, the energy of solute molecules is very low, resulting in a low formation rate of NiC₂O₄·2H₂O crystals and thus a low precipitation rate.

As the temperature rises, on the one hand, the energy of solute molecules gradually increases, leading to a gradual increase in the formation rate of crystals. On the other hand, the increase in temperature causes a decrease in supersaturation, which slows down the precipitation nucleation rate and is beneficial to the growth of crystal nuclei, resulting in relatively dense precipitates.

Continuing to increase the temperature will cause an increase in the solubility of the generated NiC₂O₄·2H₂O crystals in the solution. At the same time, it will also cause the molecular kinetic energy in the solution to increase too quickly, which is not conducive to the formation of stable crystals. Therefore, the formation rate of crystals tends to decrease again.

Moreover, an excessively high temperature, on the one hand, will increase the solubility of the precipitate. On the other hand, since adsorption is an exothermic process, an excessively high solution temperature will reduce the adsorption effect on the surface of WC particles, thereby reducing the precipitation rate and affecting the coating effect.

Finally, in the chemical coprecipitation reaction, the higher the volume flow rate of the precipitant addition, the easier the mutual aggregation of product particles. This is because a higher volume flow rate of precipitant addition leads to a greater number of instantly nucleated NiC₂O₄·2H₂O particles with smaller particle sizes, resulting in a higher probability of collisions between particles and easier mutual adhesion.

 

Conclusions

(1) Under an appropriate feed liquid concentration, the mutual aggregation of product particles is easier. The number of nucleated particles within the same time increases, leading to a higher probability of collisions between crystal nuclei and improving the coating effect. When the feed liquid concentrations are 0.4 mol·L⁻¹ for NiCl₂ and 0.2 mol·L⁻¹ for (NH₄)₂C₂O₄, the surface coating effect and dispersibility of the precursor particles generated by the reaction are optimal.

 

(2) As the temperature rises, on the one hand, the energy of solute molecules gradually increases, leading to a gradual increase in the formation rate of crystals. On the other hand, the increase in temperature causes a decrease in supersaturation, which slows down the precipitation nucleation rate and is beneficial to the growth of crystal nuclei, resulting in relatively dense precipitates. When the reaction temperature is 50°C, the chemical coprecipitation reaction can achieve a good coating effect, the largest particle size, and a relatively high precipitation rate.

 

(3) The higher the volume flow rate of the precipitant addition, the greater the number of instantly nucleated NiC₂O₄·2H₂O particles with smaller particle sizes, resulting in a higher probability of collisions between particles and easier mutual adhesion.

If the precipitant is added in a dispersed manner with a low volume flow rate and accompanied by stirring, it can avoid local over-concentration of the solution, which would otherwise form a large number of crystal nuclei. This is beneficial to the preparation of crystalline precipitates with high purity. When the volume flow rate of the precipitant addition is 0.17 mL·s⁻¹, the surface coating effect and dispersibility of the precursor particles generated by the reaction are optimal, and the highest precipitation rate can be obtained.

 

(4) An excessively high volume flow rate of the precipitant addition may cause local over-concentration of the solution, resulting in agglomeration or insufficiently uniform composition. In addition, an excessively fast precipitation rate will cause impurity ions adsorbed on the surface to be covered by subsequently deposited ions before they can be replaced by lattice ions of the main precipitate. As a result, impurity ions may get trapped inside the precipitate, causing occlusion.