Electroless Plating of Ni Nanoparticles on WC to Assist Its Pressureless Sintering of WC-Ni Cemented Carbide with Enhanced Mechanical and Corrosion-resistant Performance

MIN Fanlu, YANG Hao, YAO Zhanhu, LI Xinggao, ZHANG Jianfeng, LIU Hai

(1. Key Laboratory of Geomechanics and Embankment Engineering, Hohai University, Ministry of Education, Nanjing 210098, China; 2. CCCC Tunnel Engineering Company Limited, Beijing 100102, China; 3. Key Laboratory of Urban Underground Engineering of Ministry of Education, Beijing Jiaotong University, Beijing 100044, China; 4. College of Mechanics and Materials, Hohai University, Nanjing 211198, China)

Abstract: Ni nanoparticles were coated uniformly on the surface of WC powder via a facile electroless plating method (abbreviated as WCN-EP), and then consolidated for mechanical and corrosion resistance performance characterization, in comparison with hand mixed WC-Ni (WCN-H). Under the optimized electroless plating parameters, Ni particles, less than 1 μm in average diameter, were found to be uniformly and densely wrapped on the surface of the tungsten carbide matrix of WCN-EP. In comparison, in WCN-H, the Ni particles about 1.8 μm in average diameter, were randomly distributed together with irregular WC particles. The uniform coating of Ni was found to assist the densification process of WCN-EP effectively, with higher densities and less pores than those of WCN-H at the Ni content of 10.6wt%, 25.5wt%, and 30.3 wt%. However, at the Ni content of 18.8wt%, the relative densities of WCN-EP and WCN-H both increased to the maximum value of 98%. The maximum hardness of the consolidated WCN-EP was 82.6 HRA, about 1.2 HRA higher than that of WCN-H. In addition, the consolidated WCN-EP also exhibits a superior corrosion resistance by the polarization curve analysis at an electrochemical workstation..

Key words: electroless plating; tungsten carbide (WC); WC-Ni; preparation process; corrosion-resistant performance

1 Introduction

Cemented carbide is a type of alloy materials sintered with hard phases (such as WC and TiC) and bonded phases (such as Co, Ni, and Fe) for cutting and many other functional applications[1,2]. The presence of the hard phase directly determines the hardness, wear resistance, whereas the metal phase exists to bond the hard phase tightly by a plastic flow existing at a high temperature[3,4]. Among the bonding metal candidates, Co is most widely used for its excellent wettability to WC to get good mechanical properties. However, the oxidation resistance and the corrosion resistance of the WC-Co cemented carbide are not so satisfactory. In addition, as a national strategic resource, Co is also expensive due to its limited global reserve. Ni is a periodic element close to Co, and thus also considered possible to bond WC due to its relatively abundant resource and low price. Simultaneously, the incorporation of Ni is promising to enhance the corrosion resistance of WC-based cemented carbides than Co. Therefore, some literatures have appeared dealing with the preparation and characterization of Ni-bonded WC cemented carbides. Wentzelet al[5]and Martinset al[6]studied the influence of Ni content on the polarization behavior of WC-Co-Ni tungsten carbide. Changet al[7,8]explored the optimal sintering process, sintering behavior and mechanical properties (bending strength, fracture toughness, hardness) of WC-15 (Fe, Ni) and WC-13.5 (Co, Ni, Fe) carbides and compared them with conventional WC-Co tungsten carbide. Chen[9]studied the feasibility of applying WC-Co/Ni tungsten carbide to roll ring materials.

The uniform mixing of powders is a prerequisite for the preparation of high-performance cemented carbides. At present, the common methods include mechanical ball grinding, spray drying and electroless plating; Hu[10]studied the influence of TaC, Cr3C2and Mo added to WC-Ni cemented carbide on the structure and mechanical properties. When additives were not added, the hardness of 10%Ni of WC-Ni was 85HRA. Ahmad et al.[11]prepared WC-10% Ni coating by spray drying and tested the corrosion resistance in NaCl solution. The highest polarization resistance (Rp) value was 19576.436 Ω; Chenet al[12]studied the effect of different Ni content on the properties of WC-Ni by electroless plating. When Ni content was 9%, the alloy hardness was only 950HV. Among them, electroless plating is a relatively new one involving a catalytic redox reaction to produce the target metal coating on the WC substrate surface. Compared with the traditional ball milling, the electroless-plating-induced metal coating is more compact and uniform. By controlling the content of components in the chemical bath and the process conditions, the content and morphology of the bonded phase to meet different needs can be easily tailored[13]. Juet al[14]prepared WC-based cemented carbide cutting tool materials using a vacuum sintering method and studied the effects of different modification methods on their microstructure and properties. Electroless plating of Co nanoparticles on WC effectively improved the relative density of the cemented carbide cutting tool materials. Phuonget al[15]prepared WC-8%Ni powder by ball milling and then consolidated it by vacuum sintering technology. The effect of the change of sintering temperature from 1 375 ℃ to 1 500 ℃ on the structure and mechanical properties of WC-8Ni cemented carbide was studied. However, the literatures dealing with the electroless plating of Ni on WC and the consolidation behavior can still be hardly reviewed.

In this study, electroless plating was also tried to mix WC with Ni (abbreviated as WCN-EP), for which the component concentration, bath temperature and pH value were optimized for uniform and dense coating of Ni nanoparticles on WC. After that, WCN-EP powders were consolidated by pressureless sintering at a high temperature for densification behavior analysis. The relative density, hardness and corrosion resistance of the consolidated WCN-EP were characterized in comparison with those of hand mixed WC with Ni (abbreviated as WCN-H). The effects of powder mixing strategies on the densification mechanism were discussed.

2 Experimental

2.1 Materials

The WC matrix material used in the experiment was purchased from Zigong Cemented Carbide Co. Ltd., China, with an average particle size of 12 μm and a purity of 99.99%. The other materials and chemicals used in the experiment are listed in Table 1 for brevity.

Table 1 List of experimental materials

2.2 Powder mixing and consolidation

Electroless plating: The detailed description for electroless plating process can be found in the Ref.[16].As Ni instead of Co was electroless plated on WC in this paper, the experimental parameters vary only slightly. In brief, WC powder was roughened by a mixed solution of HF and HNO3. The concentrations of HF and HNO3were 0.67 mol/L and 0.46 mol/L respectively, and the activation solution was a mixed aqueous solution of NiSO4·6H2O and NaH2PO2·H2O. Then the NiSO4·6H2O was coated on the surface of WC, and reduced by Na2HPO2·H2O to form Ni fine particles. Thus WCN-EP powders were obtained with different Ni contents at 10.6wt%, 18.8wt%, 25.5wt%, and 30.3wt%, respectively.

Hand mixing: WC and Ni powders were also hand mixed (WCN-H) and consolidated for comparison with WCN-EP. In brief, WC and Ni powders were placed into an agate mortar with the Ni weight content of 10.6wt%, 18.8wt%, 25.5wt%, and 30.3wt%, respectively. After anhydrous ethanol was added as the solution, they were ground with a pestle for 2h until the anhydrous ethanol volatilized, which process was repeated 5 times. Finally, the as-obtained WCN-EP powders were dried in an oven and sieved ready for use.

Consolidation: The as-obtained WCN-EP or WCN-H powders with different Ni content (0, 10.6wt%, 18.8wt%, 25.5wt%, and 30.3wt%) were respectively added with an appropriate amount of the SD-E molding agent for doping and granulation, and then uniaxially pressed at 20 MPa for 30 min. The sintering process was carried out in a vacuum furnace at 1 450 ℃ for 60 min[17]. The cooling stage adopted cooling with the furnace to reduce the temperature in the furnace to room temperature.

2.3 Microstructure and properties

The microstructural morphology and the elemental distribution was observed by scanning electron microscopy (SEM, Hitachi SU8020, Japan) equipped with energy dispersive spectroscopy (EDS). The particle size of Ni particles falling on the drawn straight line in the SEM photograph was measured using a line intercept method, and measure the particle size of a total of 50 Ni particles falling on the drawn line in the SEM photograph, then count the proportion of the total number of Ni particles in different particle size ranges. Specially, the metallargical microscope observation (Metallographic observation, HX-MD50, China) was also conducted for the corroded bulk samples.

The density of the sintered sample was measured by the Archimedes drainage method. The hardness was tested by Rockwell hardness tester (Hardness test, 200HR-150, China). Each reported value was averaged from 5 tests.

A three-electrode test system was used to determine the polarization curve of the cemented carbides at an electrochemical workstation (Corrosion test, IM6e, China), and a NaCl solution with a concentration of 1 mol/L was used as the etching solution. The platinum electrode was used as the auxiliary electrode, and the cemented carbide sample as the working electrode. Then the sample’s self-corrosion current density (Jcorr) and corrosion potential (Ecorr) were analyzed[18].

3 Results and discussion

3.1 Powder mixing

Fig.1 shows the SEM images of WC powders in the intermediate steps before the final electroless plating treatment.

Fig.1 SEM images of WC powders in the intermediate steps before the final electroless plating: (a1, a2) Raw powder, (b1, b2) Coarsening, and (c1, c2) Activation

The raw WC particles exhibited a smooth surface contoured with clear oval grains. After being treated in a roughening solution, the WC surface appeared to have pits and stepped depressions because of the erosion of the strong acid solution, which would like to increase the specific surface area of the tungsten carbide particles, ready for catalytic substances and precipitated Ni metal particles to adhere in the following electroless plating process. After being immersed in the activation solution and heat-treated in a box furnace, the WC surface was covered by obvious fine particulates as the center for the catalytic reaction of the electroless plating process[19].

Overall, the four component parameters all have significant effects on the powder weight increment and the reaction plating rate. For example, the weight gain and reaction rate increased with increasing the concentration of NiSO4·6H2O until 45 g/L. However, the concentration of NiSO4·6H2O should not be too high, otherwise it would form nickel hydroxide precipitation with the OH−in the solution and affect the quality of the coating. At the same time, too high a concentration of Ni2+would make the concentration of the reducing agent sodium hypophosphite relatively low, resulting in a reduction in reducing ability, leading to a decrease in the reaction rate of electroless plating.

Fig.2 Effect of component concentration on weight gain and reaction rate of electroless plating reaction, including (a) Na3C6H5O7·2H2O, (b) Na2HPO2·H2O, (c) H3BO3, and (d) NiSO4·6H2O

Fig.3 shows the effects of (a) bath temperature and (b) bath pH on weight gain and reaction rate of WCN-EP powders.

Fig.3 Effects of (a) bath temperature and (b) bath pH on weight gain and reaction rate of WCN-EP powders

When the temperature of the plating solution was low during the electroless Ni plating reaction, the ionic activity in the solution was low and the reaction plating speed was very low[20]. As the temperature increased to 70 ℃, the precipitation mechanism of Ni is shown in Fig. 4, the ionic activity in the solution and the total redox potential of the reaction increased due tothe following reactions[21]:

Fig.4 Precipitation mechanism of Ni during electroless plating

A further increase in temperature to 70 ℃ caused a reduction in the weight gain of the plating solution, which might be attributed to the decomposition of the plating solution[22]. On the one hand, as the temperature of the plating solution increased, the activity of the reactants increased. Then, some Ni2+did not need to trigger the Ni active sites on the WC surface but reacted directly and quickly with NaH2PO2·H2O in the plating solution, which was not conducive to the co-wrapping of the WC surface. On the other hand, NaH2PO2·H2O was consumed by this side reaction[23-25]:

However, when the temperature of the plating solution was too high at 90 ℃, the stability of the plating solution will be decreased, causing the precipitation of Ni into the solution and the decrease of weight gain of WCN-EP powders.

On the other hand, the electroless plating process involves a redox reaction in nature. Therefore, the bath pH value is also one of the key factors affecting the powder increase and reaction rate[26]. It is known that the pH value of the solution not only affects the complexation of Ni2+and (C6H8O7)−but also plays an important role in the electroless plating process[27-29]. That is, a low concentration of the OH−solution will lead to a lower degree of complexation, and therefore, only a small amount of Ni distributed on the WC surface will be reduced. When the pH value is 9, an appropriate concentration of the OH−solution will make Ni(C6H5O7)−stable and result in a well-polymerized complex reaction, which is conducive to the subsequent complete redox reaction, leading toa uniform Ni coating on the WC layer[30]. However, when the pH is too high, most Ni forms Ni(OH)2precipitates, and the free Ni in the solution is unevenly deposited on the WC surface, both forming cluster-like co-plating layers on the surface of WC.

The use of the standard electrode potential to calculate the potential difference is an important way to judge whether the redox reaction can proceed. The redox reaction can proceed only when the potential difference is greater than 0, and the larger the potential difference is, the easier it is for the reaction to proceed. In an acidic environment, the standard electrode potentials of metallic Ni and the reducing agent sodium hypophosphite are -0.25 V and -0.50 V, respectively. At this time, the standard electrode potential difference of the redox reaction is 0.25 V[16]. In an alkaline environment, the standard electrode potential of the reducing agent sodium hypophosphite is −1.57 V, and the standard electrode potential difference at this time is 1.32 V. Therefore, the electroless Ni plating reaction is more easily performed in an alkaline environment. In an alkaline environment, the following reactions occur during electroless plating:

Fig.5 shows SEM images and Ni particle size distribution of WCN-EP powders at different bath temperatures.

When the temperature was 60 ℃, there were fewer particles on the surface of the tungsten carbide powder and the coating was discontinuous and not dense. When the temperature was 70 ℃, the surface particles of the tungsten carbide powder were significantly increased. The coating was uniform, and the density was increased. When the temperature was 90 ℃, the surface particles on WC powder were significantly reduced and the coating density and was reduced, which might be attributed to the increased ionic activity and the electroless plating reaction in the solution stimulated by the increase of temperature.

Fig. 6 shows SEM images and the Ni particle size of WCN-EP powders at different pH values of the bath.

Fig.6 SEM images and Ni particle size distribution of WCN-EP powders at different pH values of the bath: (a1-a3) pH = 8, (b1-b3) pH = 9, and (c1-c3) pH = 11

When pH = 8, there were fewer particles on the surface of the powder and the particle size was approximately 0.7 μm. When pH = 9, the number of granular materials on the surface of the powder increased, their distribution was uniform, and the particle size increased to 1 μm. When pH=11, both the number of particles on the surface of the powder and the particle size decreased significantly, and the particle size range was approximately 0.3 μm. The decrease of the particles size could be attributed to the decrease of the phosphorus content and the change of Ni-phosphorus cemented carbide from amorphous to crystalline. The above analysis results showed that the pH value would not only affect the coating effect of the powder but also have a significant impact on the particle size of the coating particles.

Fig.7 SEM images of WC powders of WCN-H, (a1, a2) WCN-H(10.6wt%), (b1, b2) WCNH(18.8wt%), (c1, c2) WCN-H(25.5wt%), and (d1, d2) WCN-H(30.3wt%). When the Ni content was 18.8wt%, the Ni particles hardly adhered to the WC. As the Ni content increased, the agglomeration of Ni powders were observed apparently besides the irregular WC particles.

Fig.7 SEM images of WC powders of WCN-H: (a1, a2) WCN-H (10.6wt%), (b1, b2) WCN-H(18.8wt%), (c1, c2) WCN-H(25.5wt%), and (d1, d2) WCN-H(30.3wt%)

3.2 Pressureless sintering

Fig.8 shows the corresponding photographic images of consolidated WCN-H and WCN-EP.

Fig.8 Metallographic images of consolidated WCN-H and WCN-EP: (a1) WCN-H(10.6wt%), (a2) WCN-H(18.8wt%), (a3) WCNH(25.5wt%), and (a4) WCN-H(30.3wt%); (b1) WCN-EP(10.6wt%), (b2) WCN-EP(18.8wt%), (b3) WCN-EP(25.5wt%), and (b4) WCN-EP(30.3wt%)

When the Ni content was 10.6wt%, the porosity of WCN-H was less than that of WCN-EP. When the Ni content was 18.8wt%, the pores were significantly reduced for both types of samples. When the Ni content was 25.5wt%, the cemented carbide grains of WCN-H had obvious notches and cracks. When the Ni content was 30.3wt%, the phenomenon of grain gaps in WCN-H is becoming increasingly serious, but at the same content, the WCN-EP did not exhibit the same phenomenon, indicating that the electroless plating process improved the cemented carbide performance.

Fig.9 shows the effects of Ni content on (a) relative density and (b) Rockwell hardness of consolidated WCN-EP and WCN-H samples, respectively.

Fig.9 Influences of Ni content on (a) relative density and (b) Rockwell hardness of consolidated WCN-EP and WCN-H samples, respectively

Without Ni addition, the relative density of the WC was only 72.5%. With Ni addition, the relative densities of the WCN-EP and WCN-H were both enhance apparently. But the maximum value both reached 98% when the Ni content was 18.8wt%. As the Ni content increased further, the relative densities both decreased, although WCN-EP were slightly higher than that of WCN-H. When the Ni content was 30.3wt%, the relative density of WCN-EP dropped to 96%, about 2% higher than that of WCN-H. On the other hand, the Rockwell hardness of WC was also enhanced with the addition of Ni. WC without Ni addition had the lowest hardness of 54 HRA due to the high porosity[31-32]. When the Ni content was 10.6wt%, the hardness of WCN-H reached the maximum value of 81.4 HRA, almost the same as that of WCN-EP. When further increasing the Ni content to 18.8wt%, the hardness of the electroless plating process reached 82.6HRA, and then decreased with the Ni content increased. But the hardness value of WCN-H started to decreased after the Ni content of 10.6wt%[33]. The above-mentioned phenomenon clearly indicates the beneficial effect of electroless plating mixing for WC and Ni, which was more obvious at a high Ni content.

Fig.10 shows the polarization curves of WCN-H and WCN-EP in a 1mol/L NaCl solution with the Ni contents of 10.6wt% and 18.8wt%, respectively, and Table 2 shows the corresponding electrochemical corrosion data.

Fig.10 Polarization curves of cemented carbide with different Ni contents: (a) 10.6wt%Ni and (b) 18.8wt%Ni

Table 2 Electrochemical corrosion data of cemented carbide with different Ni contents prepared by two processes

The lower the self-corrosion current density of the cemented carbide was and the more positive the self-corrosion potential was, the greater was the polarization resistance and the better was the corrosion resistance of the cemented carbide[34]. The self-corrosion current density of electroless plating 18.8wt% Ni was the smallest, the potential was the largest, and the resistance was the largest, while the WCN-EP for 10.6wt% Ni exhibited the opposite results, indicating that the anti-oxidation ability of electroless plating 18.8wt% Ni was the best and that of WCN-H for 10.6wt% Ni was the worst.

Hochstrasser (-Kurz)et al[35]used inductively coupled plasma mass spectroscopy (ICP-MS) to determine the amount of WC phase dissolved in cemented carbide in solutions of different pH values, and found the amount of dissolved WC phase was very small even in neutral solutions, which could be ascribed to the corrosion of the binder phase. The electrochemical corrosion reaction process of metal materials mainly includes the oxidation dissolution reaction of metal anode and the reduction of anions. When the cemented carbide was placed in a solution containing NaCl, due to the potential difference between the hard phase and the binder phase, a large number of corroded micro-batteries formed on the non-uniform surface of the cemented carbide, resulting in an effective electrochemical reaction of WC-Ni at room temperature The related electrochemical reactions are listed as follows[36]：

Since the NaCl solution is neutral, it cannot directly react with the anode material, so the anode reaction is mainly controlled by Eq.(4). Near the cathode, as the voltage increases, the dissolved oxygen undergoes a reduction reaction (Eq.(5)). The final products can also be expressed as follows[37]：

After the metal anode lost electrons, water-insoluble Ni(OH)2will form to be attached on the substrate surface, reducing the area of the metal substrate for active dissolution or hindering the transmission of aggressive particles. This will prevent further dissolution of the anode metal effectively. When the surface was more uniformly coated by Ni, the formation of corroded microbatteries will be more difficult, thus showing a better corrosion resistance. Compared with the corrosion performance and Rockwell hardness of WC-9%Ni tested at 40 ℃ in the Ref.[38], the alloy properties of different Ni contents prepared by WC-EP in this study were all lower than those in the literature, and the hardness only decreased by 2.4HRA compared with that in this study, showing no significant difference, self-corrosion potential (Ecorr) was -0.297 V, with a large gap, the specific reasons for which need to be further explored. In addition, Compared with the corrosion resistance of WC-12wt%Co in NaCl solution in literature[39], The highest value of polarization resistance (Rp) in the literature was 13.42 Ω and the data of chemical plated WC@18.8wt% Ni was 2.9 times higher. Therefore, the addition of Ni by electroless plating was found to significantly improve the corrosion resistance of the cemented carbide, which is a very promising strategy for fabrication of high corrosion-resistant cemented carbides[40].

4 Conclusions

In comparison with hand mixed WC and Ni, this study explored the effects of electroless plating mixing on the sintering behavior, mechanical and corrosion resistant properties of WC-Ni cemented carbide. The main conclusions are as follows.

a) By the experimental optimization, the WCNi composite powders were successfully prepared at a chemical plating temperature of 70 ℃, the pH value: 9-10, NiSO4·6H2O: 35 g/L-45 g/L, Na2HPO2·H2O: 30 g/L-40 g/L, Na3C6H5O7·2H2O: 50 g/L-55 g/L, and the concentration of H3BO3was 35 g/L. The powder distribution after electroless plating was uniform. There was no agglomeration and the Ni particles were uniformly and densely wrapped on WC surface.

b) When the Ni content was 18.8wt%, the pores were significantly reduced, And the grain distribution was relatively uniform for WCN-EP. While the Ni content was increased to 25.5wt% and 30.3wt%, the WCN-H showed obvious cracks and gaps, while those of WCN-EP did not have, indicating that the electroless plating mixing of WC and Ni was capable for the sintered dense cemented carbides at a high Ni content.

c) Compared with WCN-H, the WCN-EP can improve the overall performance by improving the distribution uniformity of the bonding phase. The hardness of the cemented carbide prepared by the two processes reached the maximum when the Ni content was 10.6wt% and 18.8wt%, which were 82.6HRA and 81.4HRA, respectively. Besides, the cemented carbide prepared by the electroless plating process had superior corrosion resistance in NaCl solution.