Research on Preparation Process of Ultrafine Spherical WC-10Co-4Cr Spraying Powder Based on Spray Granulation

Abstract

HVAF WC-10Co-4Cr coating has been applied to the extreme wear protection of lightweight titanium alloy workpieces. However, the new generation of lightweight titanium alloy inner bore wear-resistant workpieces is faced with strong wear and instantaneous high-temperature airflow erosion during service, which requires a WC-10Co-4Cr wear-resistant coating with low surface roughness, high thickness and high toughness. In addition, its small diameter inner hole also requires the rapid heating, melting and acceleration performance of sprayed powder during spraying. At present, the finest spraying powder used in this system is generally in the range of 5–15 μm, which faces difficulties in meeting the above requirements. In order to solve this problem, the preparation of 2–10 μm spherical spray powder was studied though a spray granulation experiment, and the change law of powder morphology with the solid content of pre-spray slurry was explored. The suitable binder was selected through a slurry sedimentation test and viscosity test, so that the gunable solid content of the pre-sprayed slurry was reduced from 60 wt.% to 12.5% by weight, which significantly reduces the particle size of the powder obtained by spray granulation. When the solid content of pre-sprayed slurry is 12.5 wt.%, sodium carboxymethyl cellulose (CMC-Na) is selected as the binder, and the binder content is 2 wt.%, the particle size range of powder obtained by spray granulation process reaches 2–10 μm, and the median particle size reaches 5 μm. After heat treatment, the powder is spherical and dense inside. The research results provide technical support for preparing high-performance ultrafine WC-10Cr-4Co spherical powder with wear-resistant coating for light titanium alloy.

1. Introduction

WC-Co cermet materials have been widely used in aerospace [1,2], petrochemical [3], machinery [4,5], special titanium alloy equipment [6,7,8,9,10] and other industries because of their excellent wear resistance and thermal spraying performance. As a surface treatment process, the wear-resistant coating of WC-10Co-4Cr by supersonic air spraying (HVAF) has attracted much attention because of its excellent extreme wear resistance and instantaneous high temperature stability [11,12,13]. At present, the WC-10Co-4Cr wear-resistant coating prepared by HVAF technology has been applied in many titanium alloy parts [14], especially the inner hole parts with extreme wear resistance. The inner hole workpiece with extreme wear resistance needs to bear high-strength wear and instantaneous erosion of high-temperature airflow when it is in service. This special service condition matches the excellent performance of WC-10Co-4Cr wear-resistant coating. However, with the popularization and application of WC-10Co-4Cr coating in wear-resistant workpieces with inner holes, the workpieces with smaller diameter inner holes put forward higher requirements for the coating, including thin thickness, high toughness, high performance, low surface roughness and the rapid melting and extremely rapid acceleration of powder during spraying. These strict performance requirements put forward new requirements for the pre-sprayed powder used in WC-10Co-4Cr coating spraying: ultrafine powder particle size. At present, the particle size of WC-10Co-4Cr spherical powder widely used can reach 5–15 μm at the minimum, but the related report [15] shows that the performance of the coating obtained by spraying powder with this particle size range is not satisfactory. In addition, related research reports [16,17] also show that the long-term wear resistance of 30 μm thin coating sprayed with powder with a particle size of 5–15 μm is worse than that of thicker coating, such as that with a particle size of 150 μm, in extreme friction and wear environment. In short, the long-term wear resistance of ultra-thin WC-10CO-4Cr coating will decrease sharply with the decrease in thickness, and the fundamental reason is that the powder used for spraying is not fine and dense [18]. Therefore, in order to prepare coatings with higher requirements by spraying, it is necessary to use finer ultrafine spherical powder of 2–10 μm. However, at present, the method of preparing 2–10 μm ultrafine spherical spray powder of WC-10Co-4Cr coating by spray granulation is lacking, which is a big problem that needs to be solved urgently.

Spray granulation is the main method to prepare WC-10Co-4Cr spherical powder, and its process includes ball milling and mixing, slurry conveying, centrifugal or pressure spraying, sintering heat treatment, screening and so on. According to related research reports [19,20], in the process of preparing WC-10Co-4Cr powder by spray granulation, in addition to the parameters such as initial powder particle size, spray granulation temperature and heat treatment sintering temperature, the solid content of pre-spray slurry and the content of binder have a direct impact on the morphology and particle size of the product powder. Although the research report did not mention how to prepare ultrafine powder by changing the parameters, it has been verified that each of the above-mentioned influencing elements will cause various kinds of product powder. In order to realize the stable batch production of WC-10Co-4Cr ultrafine spherical powder with particle size ranging from 2 to 10 μm, it is necessary to further explore and study the specific effects of solid content of pre-sprayed slurry and binder content on powder preparation during spray granulation. At the same time, the type of binder is also regarded as another key factor affecting the properties of powder obtained by spray granulation. At present, commonly used industrial binders such as polyvinyl alcohol, polyethylene glycol and sodium carboxymethyl cellulose have their uses in related reports [21,22,23]. In this paper, using the industrial production of nano-WC, Co, Cr powder as raw material, by adjusting the solid content of spray slurry and the types and contents of binders, combined with experimental measurement and analysis results, a reliable method for batch preparation of WC-10Co-4Cr wear-resistant coating system 2–10 μm ultrafine spherical powder by spray drying was obtained.

2. Materials and Methods

In this study, WC-10Co-4Cr powder was prepared using the spray granulation method. The viscosity and stability of the pre-sprayed slurry in the preparation process were tested to judge the slurry state. The properties of the powders prepared by different parameters were characterized and correlated, and the expected ultrafine spherical powder was obtained. The particle size distribution, powder phase composition and powder cross section element distribution of ultrafine spherical powder were tested. The specific method is as follows.

2.1. Preparation of Raw Materials and Powder

In this study, 500 nm of WC powder, 800 nm of Co powder and 1 μm of Cr powder were used as raw materials. The binder is prepared from polyethylene glycol (PEG), polyvinyl alcohol (PVA), sodium carboxymethyl cellulose (CMC-Na) and deionized water in a certain proportion. Deionized water is used as the solvent of the pre-sprayed slurry. The specific process of spray granulation is as follows: firstly, WC, Co and Cr powders are weighed according to the mass ratio of 86:10:4, and then the raw powder, binder, deionized water and zirconia grinding balls are added into the ball milling tank of planetary ball mill (QM-3SP4, Nanda instrument, Nanjing, China) for wet grinding and mixing at the rotating speed of 250 r/min for 2 h, so as to prepare pre-spray slurry. In this process, the ratio of the total mass of raw material powder to the total mass of pre-sprayed slurry is the solid content of pre-sprayed slurry, while the ratio of the mass of binder to the total mass of raw material powder in slurry is the binder content of pre-sprayed slurry. Next, the pre-sprayed slurry is sprayed with a centrifugal spray drier (LPG-5, Xianfeng Drying, Changzhou, China). In the spray drying stage, the feed rate of the slurry is controlled to be 40 r/min by peristaltic pump, the diameter of the atomizing disk is 50 mm, the rotating speed of the atomizing disk is 15,000 r/min, the gas flow rate in the spray dryer is 400 m/h, the inlet air temperature of the spray dryer is 100 °C, the outlet air temperature is 200 °C, and the overall electric heating power is 9 kW. After spray granulation, the obtained powder was placed in a tube furnace (BTF-1600C-11, Anhui BEQ Equipment, Hefei, China) and sintered in argon atmosphere. The sintering temperature was 1250 °C, and the heat preservation time was 2 h. After sintering, the WC-10Co-4Cr ultrafine spherical powder with the particle size range of 2–10 μm was obtained by sieving.

2.2. Viscosity and Stability of Pre-Sprayed Slurry

The viscosity of pre-sprayed slurry was tested by industrial viscometer (NK-2, Zhejiang Ximei, Jinhua, China), and each sample was measured three times, and then the average value was taken as the viscosity data of the sample. The steps of slurry sedimentation experiment are as follows: Take the slurry samples in designated areas after the pre-sprayed slurry has been left standing for different periods of time, and test the solid content of these samples. By analyzing the change in solid content with standing time, the suspension dispersion uniformity and stability of raw material powder in slurry can be judged. Similarly, the solid content test of each sample was also measured three times, and the average value was taken as the final result.

2.3. Performance and Characterization of Powder

A scanning electron microscope (ZEISS GeminiSEM 300, ZEISS, Jena, Germany) was used to obtain information about powder morphology, particle size and sphericity. The actual particle size distribution of the powder was measured by a laser particle size distribution instrument (Bettersize3000, Dandong Bettersize Instrument, Dandong, China). The tap density of powder was measured by a tap density tester (BT-311, Dandong Bettersize Instrument, Dandong, China). When testing the actual particle size distribution and tap density of the powder, each sample is measured three times, and the average value is taken as the final result. X-ray diffraction (Rigaku SmartLab SE, Rigaku, Tokyo, Japan) was used to analyze the phase composition of ultrafine spherical powder finally prepared, and scanning electron microscopy–energy-dispersive X-ray analysis (ZEISS GeminiSEM 300) was used to analyze the element distribution in the cross section of ultrafine spherical powder.

3. Results

3.1. Effect of Solid Content of Pre-Sprayed Slurry on Powder

The preparation of pre-spray slurry is one of the key steps in the process of spray granulation, and the solid content is the primary factor affecting the preparation of pre-spray slurry, which has a significant impact on the properties of powder obtained by spray granulation. In Figure 1, the black line is the change curve of the median particle size of each group of powders obtained by spray granulation when PVA is used as a binder to gradually reduce the solid content from 55 wt.% to 20 wt.%, and the red line in Figure 1 is the change curve of tap density of each group of powders under the same variable. It can be seen from Figure 1 that when the solid content of the pre-sprayed slurry decreases, the median particle size of the powder obtained by spray granulation decreases accordingly. When the solid content is in the range of 30 wt.%–55 wt.%, the tap density decreases with the decrease in the solid content, but when the solid content decreases to 25 wt.%, the researchers notice that the tap density no longer decreases, but begins to increase significantly, and at the same time, the median particle size of the powder also decreases rapidly. According to the relationship between the tap density of powder particles and the size of powder particles, under normal circumstances, when the size of powder decreases, the tap density of powder should show an upward trend [24], but in the experimental results shown in this paper, it shows an opposite trend to the theory, which is inconsistent with the theoretical expectation. In order to explore the reason, the researchers observed the morphology of each group of powder particles obtained from the experiment by SEM, as shown in Figure 2.

The morphology and particle size of each group of powder particles can be clearly observed in Figure 2. Figure 2g,h show that when the solid content is reduced to below 25 wt.%, the sphericity of the powder is obviously reduced, and it even begins to be scattered and not spherical. This is the direct reason for the rapid decline in the median particle size of the powder in Figure 1, and the fundamental reason is that when the solid content is too low, the effect of the binder becomes worse, and the slurry cannot form spherical droplets during spraying, resulting in the slurry being irregularly broken and dried during spraying. At the same time, it can also be observed from Figure 2a–f that the overall particle size of the powder decreased significantly during the process of gradually decreasing the solid content from 55 wt.% to 30 wt.%, but in this experiment, the powder used for the experiment at this stage was obtained by spray granulation and 200-mesh coarse screening. When the solid content was high, there were both large particles and small particles in the powder, and the small particles filled the gaps between the large particles when the tap density was tested. However, in the process of decreasing the solid content from 30 wt.% to 20 wt.%, the granulated powder began to appear scattered and broken, and the dense stacking of broken powder led to a significant increase in tap density. This explains the changing trend of tap density in Figure 1, which decreases first and then increases, and is also confirmed by related research reports [25].

To sum up, the experiment at this stage can draw the following conclusions: on the premise that the spray granulation powder can be well formed, the reduction in the solid content of the pre-spray slurry can significantly reduce the overall particle size of the spray granulation powder. However, when the solid content of the pre-spray slurry is further reduced, it is necessary to use a binder that conforms to the properties of the slurry with low solid content to ensure that the powder can be well formed during spray drying. This conclusion provides a clear idea for the following experiments: by adopting the strategy of “selecting suitable binder” and “continuously reducing the solid content of pre-sprayed slurry” at the same time, spray granulated powder with larger proportion of small-sized particles and good morphology can be obtained.

3.2. Properties of Pre-Sprayed Slurry with Different Binders

In order to further reduce the solid content and still obtain well-formed small-sized powder particles, the researchers used the above three kinds of binders (PEG, PVA and CMC-Na) to prepare pre-sprayed slurry, and made a comparative analysis of the viscosity test and slurry sedimentation experiment of these three groups of slurries, in order to screen out a binder that is more suitable for low-solid-content slurry. Figure 3 shows the viscosity of pre-sprayed slurry obtained by using three different binders under the same slurry parameters (solid content is 40% and binder content is 2 wt.%). It can be seen from Figure 3 that the viscosity of pre-sprayed slurry is significantly increased when CMC-Na is used as a binder. It is expected that this characteristic has a significant advantage in maintaining the uniformity and stability of powder dispersion in slurry when the solid content is greatly reduced. Figure 4 shows the results of the slurry sedimentation experiment. It can be observed from Figure 4 that the average solid content of the pre-sprayed slurry using CMC-Na as a binder remains the most stable after standing for a corresponding time. Based on the comparative analysis of the slurry viscosity and sedimentation data in Figure 3 and Figure 4, it can be concluded that when CMC-Na is selected as the binder, the long-term suspension and dispersion effect of raw material powder in the slurry is the best, which is expected to make the pre-sprayed slurry with low-solid-content spray granulation more effectively, and then obtain powder particles with better morphology.

In order to verify the above conclusions and determine whether CMC-Na is suitable for pre-sprayed slurry with low solid content, the researchers used CMC-Na as a binder to prepare four groups of slurry with solid contents of 40 wt.%, 30 wt.%, 20 wt.% and 10 wt.% (binder content is 2 wt.%). Then, the settlement test results of these four groups of slurry were compared with those of the PVA group, and the results were analyzed. From the comparison of data curves in Figure 5, it can be seen that the stability of the slurry with CMC-Na as a binder at 20 wt.% is still better than that with PVA as a binder and 40 wt.% solid content. When the solid content of the slurry using CMC-Na decreased from 20 wt.% to 10 wt.%, the stability of the pre-sprayed slurry showed a rapid declining trend, which laid the foundation and indicated the direction for the next stage of the experiment.

3.3. Ultrafine Powder

It is necessary to explore the usage content of the newly selected binder CMC-Na before the experiment of further reducing the solid content is carried out. Figure 6 shows the change in powder morphology obtained by spray granulation with CMC-Na as a binder when the binder content changes from 1 wt.% to 2.5 wt.%. It can be observed from Figure 6a–d that when the binder content is between 1 wt.% and 2 wt.%, with the increase in the binder content, the morphology of the powder obtained by spray granulation gradually changes from dispersed non-spherical to complete spherical. When the binder content is 2 wt.%, the powder has the best pelletizing effect, and the proportion of small particles in the powder is relatively large. However, when the binder content reaches 2.5 wt.%, the overall size of powder particles begins to increase. Therefore, in order to obtain a higher proportion of small particles in the subsequent experiments, the content of CMC-Na binder should be 2 wt.% for the next study.

The content of the CMC-Na binder was 2 wt.%, and the solid content of pre-sprayed slurry was gradually reduced from 30 wt.% to 10 wt.%. Several groups of powders were prepared by the spray granulation process, and the tap density and median particle size of each group of powders were determined after 400-mesh coarse screening. The results are shown in Figure 7. At the same time, the SEM morphology of each group of powders was observed, and the results are shown in Figure 8. It can be seen from Figure 8 that in the process of reducing the solid content of pre-spray slurry from 30 wt.% to 12.5 wt.%, the proportion of small particles in the powder obtained by spray granulation increased significantly with the decrease in solid content, and when the solid content of pre-spray slurry was 12.5 wt.%, the proportion of small particles reached the maximum, and the morphology of the powder was the most ideal. However, after the solid content of the pre-sprayed slurry was reduced to 10 wt.%, the powder obtained by granulation began to show a state of dispersion without balls. This phenomenon is highly consistent with the curve of “median particle size/tap density-solid content” in Figure 7 and also matches the analysis result of “solid content-standing time” data in Figure 5 in Section 3.2.

Based on the morphology and particle size distribution of the powder, the spray-granulated powder sample prepared when the solid content of the pre-sprayed slurry is 12.5 wt.% is selected for the next screening treatment (the particle size range is −1800 mesh + 6000 mesh) to obtain the pre-sintered powder, as shown in Figure 9. Subsequently, the pre-sintered powder was sintered by heat treatment at 1250 °C for 2 h, and the micro-morphology of the powder obtained after screening is shown in Figure 10. From the observation in Figure 10a,b, it can be seen that the powder is spherical as a whole, with good morphology and density inside.

The laser particle size distribution of the sintered and screened powder is tested, and the results are shown in Figure 11. According to the laser particle size distribution diagram in Figure 11, it can be seen that the particle size distribution of the powder meets the requirement of 2–10 μm, and it is confirmed that this is the expected ultrafine spherical powder.

In order to determine the internal composition distribution of the prepared ultrafine powder, the cross section of the powder was analyzed by energy spectrum, and the element distribution of the cross section of the powder was obtained, as shown in Figure 12. After ignoring the interference of element C in the resin matrix used for embedding powder, it can be seen from the energy spectrum image that the elements W, Co and Cr in the cross section of the powder in the figure are evenly distributed, and only a few areas are enriched with Co or Cr. Secondly, the energy spectrum did not detect other impurity elements except W, C, Co and Cr in the cross section of the powder, so it can also be judged that the binder CMC-Na, as a high molecular polymer, is decomposed into carbon dioxide and water and leaves the powder at a high temperature during the sintering process of the powder. In addition, the expected introduction of impurity sodium due to sodium groups in the polymer has not been detected, which shows that the prepared powder is pure. Then, in order to further determine the phase composition of the ultrafine powder after heat treatment, the powder was analyzed using XRD, and the diffraction peak diagram of the powder is shown in Figure 13. The results showed that WC, Co and Cr did not undergo phase transformation during the preparation process, and they always existed in the expected form, and no impurity phase appeared in the whole powder.

4. Conclusions

It was found that in the process of preparing WC-10Co-4Cr powder by spray granulation, with the decrease in solid content of pre-spray slurry, the particle size of the prepared powder decreased, until the powder could not be granulated into a spherical shape. The choice of binder has a decisive influence on the suspension and dispersion stability of raw material powder in low solid pre-spray slurry and then affects the morphology of spray granulation particles. When “the solid content of pre-sprayed slurry is 12.5 wt.%, the binder type is CMC-Na, and the binder content is 2 wt.%, and it is screened after heat treatment at 1250 °C for 2 h in argon atmosphere” as the process parameters, a compact ultrafine spherical powder of WC-10Co-4Cr with good morphology can be obtained. It is expected that this ultrafine powder can be used for spraying WC-10Co-4Cr coating with low surface roughness, thin thickness and high toughness. The research results provide technical support for preparing high-performance ultrafine WC-10Cr-4Co spherical powder with wear-resistant coating for light titanium alloy.