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Article

Study on the Friction Performance of Cerium Oxide on Supersonic Flame-Sprayed WC-10Co-4Cr Coating

by
Wenxiao Ma
,
Yun Ge
*,
Lixin Zhang
,
Fei Chen
,
Yijiang Zheng
and
Zhuhui Qi
College of Mechanical and Electrical Engineering, Shihezi University, Shihezi 832000, China
*
Author to whom correspondence should be addressed.
Coatings 2021, 11(1), 24; https://doi.org/10.3390/coatings11010024
Submission received: 11 November 2020 / Revised: 17 December 2020 / Accepted: 21 December 2020 / Published: 28 December 2020
Browse Figures
Versions Notes

Abstract

:
In order to improve the surface wear resistance of the key parts of agricultural machinery, this article uses 65Mn steel, which is commonly used in agricultural machinery, as the matrix and tungsten carbide-based coating as the basis. In total, two coating materials are selected as WC-10Co-4Cr with and without CeO2 added. The coating was prepared by the supersonic flame spraying process (HVOF), and the tribological characteristics and coating of the three samples of 65Mn, WC-10Co-4Cr, and WC-10Co-4Cr+CeO2 coatings were investigated by the comprehensive material surface performance tester. Layer porosity, microhardness, and X-ray powder diffraction (XRD) phase analysis reveal the modification mechanism of WC-10Co-4Cr coating by rare earth CeO2. The experimental comparison results show that WC-10Co-4Cr is prepared on the 65Mn steel substrate, and the WC-10Co-4Cr+CeO2 coating significantly improves the surface hardness, the coating structure is uniform and dense, and the pores are less. The coating hardness value is relatively high, reaching 1200–1500 HV(N/mm2), and the porosity is less than 1%. When the rare earth cerium oxide is added to WC-10Co-4Cr, the porosity of the coating decreases and the hardness increases. It is proved that rare earth has the modification effect of refining the coating structure. Therefore, adding CeO2 to the WC-10Co-4Cr coating and using the supersonic flame spraying process to prepare the coating can obtain more excellent wear resistance.

1. Introduction

The working environment of agricultural machinery is harsh, and it is often necessary to withstand high-torque alternating loads. Many key components will quickly fail due to wear and cause huge economic losses. Therefore, how to improve the wear resistance of agricultural machinery surfaces under the existing matrix materials is currently one of the research hotspots [1].
Surface treatment of metal materials and preparation of wear-resistant coatings are effective ways to improve the wear resistance of metal materials [2,3,4,5]. At present, the research on wear-resistant metal coatings mostly focuses on the wear performance of tungsten carbide-based coatings [6,7,8,9,10,11,12,13,14]. For example, Karoonboonyanan et al. compared the wear rate of two different coating materials (WC/Co and Al2O3–TiO2/NiAl), which showed that the coating can greatly improve the wear protection of carbon steel rotary tiller blades [15]. Amardeep compared three different spray coatings (WC-Co–Cr, Cr3C2NiCr and Stellite-21) on high-strength steel rotor blades. Experiments show that the wear resistance of the blade is significantly better than that of the uncoated blade. Tungsten carbide–cobalt–chromium coated blades have the strongest wear resistance and can extend the service life of rotor blades [16]. Liu et al. [17] found that the addition of rare earth cerium oxide (CeO2) during thermal spray feeding can promote the “decarburization” of WC and generate a new intermediate compound hard phase, which has a significant effect on enhancing the microhardness of the structure. Rastkar et al. [18] conducted a mutual grinding test on the WC ceramic protective coating without adding rare earth cerium oxide, and the results showed that only slight scratches appeared on the wear surface of the coating modified by rare earth cerium oxide. It has good anti-friction and wear properties.
To sum up, adding an appropriate amount of rare earth cerium oxide to the tungsten carbide-based coating has a good effect on increasing the hardness of the coating, the density of the structure, and improving the wear performance of the coating [19,20,21,22,23,24]. In this paper, 65Mn steel, which is commonly used in agricultural machinery, is used as the substrate, and WC-10Co-4Cr with and without CeO2 is used as the coating material. In total, two different coatings are prepared by supersonic flame spraying. The porosity and microhardness of the coating, XRD phase analysis, friction, and wear performance analysis and three-dimensional morphology analysis after wear reveal the modification mechanism of WC-10Co-4Cr coating by rare earth CeO2. It can provide a theoretical reference for the significant improvement of the wear resistance of key parts of agricultural machinery.

2. Experimental Method

2.1. Test Materials

The three samples used in this study were heat-treated 65Mn substrate (Sample A), 65Mn substrate with WC-10Co-4Cr coating (Sample B) and 65Mn substrate with WC-10Co-4Cr+CeO2 coating (Sample C). The particle size of the WC-10Co-4Cr powder was 15–45 μm. Agglomerated powder is quasi-spherical particles. Agglomerated powder is observed to the naked eye as dark gray powder. The particle size of the rare earth CeO2 powder was 5–8 μm. Observed by naked eyes, it is a light-yellow powder. Figure 1a,b are the low- and high-magnification scanning electron microscopy (SEM) images of the WC-10Co-4Cr powder, respectively, and Figure 1c,d are the low- and high-magnification SEM images of the rare earth CeO2 powder.

2.2. Coating Preparation

The surface of the substrate was degreased and cleaned to enhance the bonding force between the coating and the substrate and eliminate the stress effect. Then, the 65Mn steel matrix was pretreated by sandblasting at 0.5 MPa using a 24# brown corundum. After sandblasting, the surface of the substrate was blown with compressed air, and the supersonic spraying experiment was carried out within 2 h after sandblasting. The spraying equipment used is the HV-8000 fuel oil HVOF system of Zhengzhou Lijia Thermal Spraying Machinery Co., Ltd. (Zhengzhou, China). The flow of oxygen and gas was determined according to the requirements of the equipment to ensure that the spray gun’s flame flow reached the designed power level. The oxygen–fuel ratio is determined to be 1.7–2. The spraying distance was 320–380 mm and the particle velocity was maintained above 880 m/s to avoid the adverse effects of high-temperature flame flow on the heat transfer of the substrate and obtain a high-performance coating. The two different powder coatings were sprayed on the surface of the treated 65Mn steel. The spraying process parameters are shown in Table 1.

2.3. Friction and Wear Test

Reciprocating friction and wear tests were carried out on the CFT-I material surface performance comprehensive tester (Figure 2) to study the influence of the sample on the severity of the friction and wear of Samples A, B and C. The upper sample material was 9Cr18Mo pellets with a diameter of 4 mm and a mass of 0.8595 g. The testing machine was controlled by a computer to maintain a speed of 100 r/min, a load of 20 N, and a time of 10 min and a load of 100 N and a time of 30 min. Friction coefficient was measured during the entire test using a pressure sensor installed in the ball and disc tester. The size of the wear scar on the ball specimen was measured after the test. An electronic balance with an accuracy of 0.1 mg was used to determine the mass before and after the wear.
Samples A, B, and C were made according to the size requirements of the testing machine. Before the friction and wear test, the samples were ground and polished to control their surface roughness to about 0.1. Then, the samples were cleaned with absolute ethanol and dried. The friction and wear properties of the three samples were studied by comparing their friction coefficient, the size of the wear scar, and the amount of wear.

2.4. Microstructure Characterization

An upright metallographic microscope (Axio Lab.Al, Carl Zeiss, Oberkochen, Germany) was used for metallographic analysis and coating porosity measurement. Scanning electron microscopy (SU8010, Hitachi, Tokyo, Japan) and energy-dispersive spectrometry (EDS) were used to observe and analyze the microstructure morphology of the coating. A digital microhardness tester (THV-1MDT, LABTT, Jiaxing, China) and its own microhardness measurement system (THVS-MA, LABTT) were used to measure the microhardness of the coatings. A D8 X-ray diffractometer (BRUKER, Bremen, Germany) was used for the phase analysis of the prepared coating. The three-dimensional profiler (ST400, Nanovea, Irvine, CA, USA) was used to observe the surface morphology of the sample after wear.

2.5. Phase Composition

The phase compositions of the WC-10Co-4Cr and WC-10Co-4Cr+CeO2 coatings were determined using a D8 X-ray diffractometer in Figure 3. Cu target X-ray tube voltage was less than or equal to 40 KV, current was less than or equal to 40 MA. Using continuous scanning, from 10° to 90° degrees, the scanning speed was 4°/min. The main phases of the two coatings are the WC phase. In addition, a small amount of Co phase could be seen in the XRD pattern. The presence of a small amount of W2C phase indicates that WC was decarburized and decomposed during the HVOF spraying process. However, the W2C peak intensity was relatively small; thus, the degree of decarburization and decomposition of WC was relatively light [25]. Further, the peak shape was sharp, the diffraction intensity was high, and it can be inferred that the sample product had a good crystal shape. The XRD pattern had a narrower half-width at the corresponding position, so it could be judged that the average grain size of the prepared sample was smaller. The XRD spectrum of Sample C showed the rare earth CeO2, but its peak value was not particularly obvious because of the less rare earth metal content, and no diffraction peaks of other substances were found, indicating that the prepared product was pure and no other impurity products were generated. In addition, the addition of rare earth elements can further refine the structure, stabilize the grain boundaries and slow down internal diffusion, and enhance the high temperature oxidation resistance of the coating [26]. A fatigue testing machine (8801, INSTRON, Norwood, MA, USA) was used to measure the elastic modulus of the three samples. Among them, the elastic modulus of Sample A was 12,679.33 MPa, the elastic modulus of Sample B was 13,000.03 MPa, and the elastic modulus of Sample C was 13,103.93 MPa. The results show that the WC-10Co-4Cr coating sample with CeO2 added was not easy to deform, and had a stronger rigidity and hardness.

3. Results and Discussion

3.1. Coating Characteristics

3.1.1. Metallographic Analysis

The result of metallographic structure analysis is shown in Figure 4. Choose 10 locations for thickness measurement. The thicknesses of the WC-10Co-4Cr and WC-10Co-4Cr+CeO2 were 183 ± 21 μm and 171 ± 28 μm, respectively. Figure 4a,b are the low- and high-magnification images of WC-10Co-4Cr coating under a metallographic microscope, respectively. Figure 4c,d are the low- and high- magnification photos of WC-10Co-4Cr+CeO2 coating. Both coatings had good structural compactness and few cracks and pores. Figure 4d shows that the WC-10Co-4Cr+CeO2 coating is denser than WC-10Co-4Cr, because the fine CeO2 powder and WC-10Co-4Cr powder have better meltability at high temperature after being fully mixed. This result proves that the rare earth CeO2 has a modification effect on the coating structure. After spraying, the temperature of each layer of coating became higher, cooling was slow, and most of the coating remained in a molten state. Therefore, the layers can be well integrated, the gas between the layers can escape in time. The bonding part of the substrate and the coating had no obvious defect.

3.1.2. SEM Topography and EDS Analysis

During the thermal spraying process, working distance is 18,500 µm and emission current is 6600 nA. The particle temperature gradually decreased as the spray distance increased. The particles within 150–280 mm maintained a temperature of about 1775 °C. The sprayed powder rushed to the surface of the 65Mn steel substrate at a high speed because of the gas effect in the spraying process. The WC-10Co-4Cr powder melted and adhered together under high temperature and high-speed heat source. A small amount of powder particles were not completely melted because of the small heating effect. These particles relatively maintained their initial surface morphology. WC-10Co-4Cr was observed under the scanning electron microscope and showed small lumpy partitions (Figure 5c). The surface of the prepared WC-10Co-4Cr+CeO2 coating had no obvious lumps as shown in Figure 5d. The high-magnification morphology comparison of the two coatings under by SEM is shown in Figure 5a,b. Compared with Sample B, the porosity of Sample C is decreased, the sharp edges and corners of WC particles are dulled, the shape tends to be smooth, and the size of WC particles is fine. The figures show that the WC-10Co-4Cr+CeO2 coating had fewer blocky partitions and a smoother surface. This shows that the addition of rare earths can inhibit the growth of WC particles, and the segregation of rare earths on the grain boundaries will also hinder the coarsening of WC grains [27,28]. At the same time, the rare earth elements gathered at the interface can also lower the melting point of the alloy system. The sharp corners of the WC particles have higher surface energy, and the sharp corners are easily melted during heating and become smooth [29]. In addition, rare earth elements have strong chemical activity and can combine with impurities in the coating to form refractory compounds, reduce grain boundary impurities, and improve wettability between particles.
We analyzed the chemical composition of the elements using the scanning electron microscope’s own energy spectrometer. As shown in Figure 6, EDS analysis was performed at point A on the prepared WC-10Co-4Cr coating and point B on the WC-10Co-4Cr+CeO2 coating. The two coatings contained W elements, but the latter has a small amount of Ce and O elements. This result was consistent with the XRD phase analysis results and verified the consistency of the sprayed materials.

3.1.3. Microhardness and Porosity Analyses

The two coatings had no cracks and delamination. The hardness of the coating along the thickness direction is shown in Figure 7. The hardness distribution is relatively uniform. Compared with the WC-10Co-4Cr coating, the WC10-Co4-Cr+CeO2 coating had greater hardness. The average microhardness values of the WC-10Co-4Cr and WC-10Co-4Cr+CeO2 coatings were 1217 and 1316 HV, respectively, whereas the hardness value of the 65Mn steel substrate was about 500 HV. The hardness value of the 65Mn steel matrix at a distance of −30 μm from the joint was slightly higher than that at −60 μm. This result indicates that the shot peening effect of flying particles increases the microhardness value of the matrix material at the joint. Compared with the WC-10Co-4Cr coating, the WC-10Co-4Cr+CeO2 coating has higher hardness. This is because the addition of CeO2 makes the coating structure refined, the porosity decreases, and the coating density is increased. Therefore, the hardness of the coating is increased, which can more effectively protect the substrate from wear and reduce its wear rate.
The porosity of the two coatings was tested by the grayscale method. It can be seen from Table 2 that the porosity of the coating with rare earth added is low, mainly because the addition of rare earth oxide can lower the melting point of the ceramic material, so that the WC-10Co-4Cr powder can be more fully melted, which is more conducive to spreading on irregular surfaces to form a dense and uniform coating. The fluidity of the coating was low, and each layer of the coating maintained more particles in the molten state; thus, the coating had fewer pores, smaller gaps, a relatively flat surface, and smaller porosity.

3.2. Tribological Properties of the Coating

3.2.1. Friction Coefficient

Figure 8a,b show the friction coefficients of three samples that increase with time under two test conditions. The friction coefficient of the 65 Mn steel substrate was considerably higher than those of the two coatings. The WC-10Co-4Cr and WC-10Co-4Cr+CeO2 coatings showed similar behavior in the time– friction coefficient, but the friction coefficient of WC-10Co-4Cr+CeO2 was lower than that of WC-10Co-4Cr. The friction coefficients of the three samples increased remarkably in the first minute and then slowly increased. The friction coefficients of the WC-10Co-4Cr and WC-10Co-4Cr+CeO2 coatings increased slightly. From the overall trend, the friction coefficients of the three samples increased with time. The friction coefficient of the 65Mn steel substrate fluctuated greatly with a fluctuation range of about 0.6–0.8. The friction coefficient of the WC-10Co-4Cr coating was stable at about 0.45, whereas that of the WC-10Co-4Cr+CeO2 coating was stable at about 0.4. Sample A wears more severely, produces too much wear debris, and the friction coefficient fluctuates greatly. Samples B and C wear less, and the friction coefficient fluctuates little.
Figure 9 a,b show the wear of three samples under two test conditions. It can be seen from the figure that the wear of the 65Mn steel substrate is the largest, while the wear of the WC-10Co-4Cr+CeO2 coating is the smallest. This indicates that the 65Mn steel with the largest friction coefficient has the most serious wear, and the WC-10Co-4Cr coating with CeO2 added shows good wear resistance compared with the WC-10Co-4Cr coating without CeO2.

3.2.2. Three-Dimensional Shape Observation

Figure 10 shows the three-dimensional morphology of the sample surface wear under different loads and times. Comparing the three-dimensional morphology diagram of wear, it can be seen that when the load is 20 N and the wear time is 10 min, there are obvious groove marks in Sample A, and only slight wear marks appear in Samples B and C as shown in Figure 10a–c as shown. When the load is 100 N and the wear time is 30 min, the three kinds of samples have obvious wear groove marks. Among them, the wear debris of Sample A appeared accumulation phenomenon and molten particles, and peeling pits, as shown in Figure 10d Show. It presents the abrasive wear morphology, so the main wear forms of the sample surface wear marks are abrasive wear, adhesive wear and spalling wear. From Figure 10e,f, it can be seen that the main wear form of Samples B and C is abrasive wear. Comparing the three samples, Sample C has the lightest wear, and the law of wear traces is consistent with the amount of wear.

4. Conclusions

(1) The addition of WC-10Co-4Cr and WC-10Co-4Cr+CeO2 coatings on the 65Mn steel substrate substantially increased the surface hardness of the material, strengthened the wear resistance of the prepared coating, and effectively avoided substrate wear.
(2) The coating structure was uniform, dense, and had few pores. The coating hardness value was relatively high at 1200–1500 HV, and the porosity was less than 1%. The porosity of the WC-10Co-4Cr coating decreased and its hardness increased when CeO2 was added.
(3) This paper studied the friction and wear characteristics of three samples (friction coefficient, wear amount, wear scar morphology), and the results show that WC-10Co-4Cr+CeO2 coatings had the lowest coefficient of friction, the smallest amount of wear, had only slight traces of wear in the three-dimensional profiler, and had good wear resistance. There are differences in the wear loss effect of the three samples under different loading conditions, but in general, the wear failure mechanisms mainly include abrasive wear, adhesive wear, and surface spalling caused by fatigue wear.

Author Contributions

Conceptualization, W.M.; methodology, W.M.; software, F.C.; validation, W.M.; formal analysis, Z.Q.; investigation, Y.Z.; resources, Y.G.; data curation, Y.G.; writing—original draft preparation, W.M.; writing—review and editing, W.M.; visualization, Y.G.; supervision, L.Z.; project administration, L.Z.; funding acquisition, Y.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Key Research and Development Program of China (2019YFC1710905), the National Natural Science Foundation of China (No. 52065057) and the Construction Project of Demonstration Platform for National New Materials Production & Application-Demonstration Platform for Production & Application of Materials on The Agricultural Machinery Equipments (TC200H01X-5).

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. SEM image of WC-10Co-4Cr and CeO2 powder: (a,c) low magnification; (b,d) high magnification.
Figure 1. SEM image of WC-10Co-4Cr and CeO2 powder: (a,c) low magnification; (b,d) high magnification.
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Figure 2. CFT-I type material surface performance comprehensive tester. (a) overall picture; (b) partial enlarged view.
Figure 2. CFT-I type material surface performance comprehensive tester. (a) overall picture; (b) partial enlarged view.
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Figure 3. XRD Phase Analysis of Sample A, B, and C.
Figure 3. XRD Phase Analysis of Sample A, B, and C.
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Figure 4. Metallographic micrograph of two coating cross section: low magnification of (a) WC-10Co-4Cr coating and (c) WC-10Co-4Cr+CeO2 coating; high magnification of (b) WC-10Co-4Cr coating and (d) WC-10Co-4Cr+CeO2 coating.
Figure 4. Metallographic micrograph of two coating cross section: low magnification of (a) WC-10Co-4Cr coating and (c) WC-10Co-4Cr+CeO2 coating; high magnification of (b) WC-10Co-4Cr coating and (d) WC-10Co-4Cr+CeO2 coating.
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Figure 5. Surface morphology of the two coatings: high magnification of WC-10Co-4Cr coating (a) and WC-10Co-4Cr+CeO2 coating (b); low magnification of WC-10Co-4Cr coating (c) and WC-10Co-4Cr+CeO2 coating (d).
Figure 5. Surface morphology of the two coatings: high magnification of WC-10Co-4Cr coating (a) and WC-10Co-4Cr+CeO2 coating (b); low magnification of WC-10Co-4Cr coating (c) and WC-10Co-4Cr+CeO2 coating (d).
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Figure 6. Energy spectrum analysis of point A and B. (a) point A of WC-10Co-4Cr coating; (b) energy spectrum analysis of point A; (c) point B of WC-10Co-4Cr+CeO2 coating; (d) energy spectrum analysis of point B.
Figure 6. Energy spectrum analysis of point A and B. (a) point A of WC-10Co-4Cr coating; (b) energy spectrum analysis of point A; (c) point B of WC-10Co-4Cr+CeO2 coating; (d) energy spectrum analysis of point B.
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Figure 7. Microhardness of WC-10Co-4Cr coating and WC-10Co-4Cr+CeO2 coating.
Figure 7. Microhardness of WC-10Co-4Cr coating and WC-10Co-4Cr+CeO2 coating.
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Figure 8. Coefficient of friction of different test conditions. (a) a load of 20 N, and a time of 10 min; (b) a load of 100 N and a time of 30 min.
Figure 8. Coefficient of friction of different test conditions. (a) a load of 20 N, and a time of 10 min; (b) a load of 100 N and a time of 30 min.
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Figure 9. Abrasion of different test conditions. (a) A load of 20 N, and a time of 10 min; (b) A load of 100 N and a time of 30 min.
Figure 9. Abrasion of different test conditions. (a) A load of 20 N, and a time of 10 min; (b) A load of 100 N and a time of 30 min.
Coatings 11 00024 g009
Coatings 11 00024 g010 550
Figure 10. Three-dimensional surface wear morphology of specimens under different loading conditions. (a) a load of 20 N, and a time of 10 min: Sample A; (b) a load of 20 N, and a time of 10 min: Sample B; (c) a load of 20 N, and a time of 10 min: Sample C; (d) a load of 100 N and a time of 30 min: Sample A; (e) a load of 100 N and a time of 30 min: Sample B; (f) a load of 100 N and a time of 30 min: Sample C.
Figure 10. Three-dimensional surface wear morphology of specimens under different loading conditions. (a) a load of 20 N, and a time of 10 min: Sample A; (b) a load of 20 N, and a time of 10 min: Sample B; (c) a load of 20 N, and a time of 10 min: Sample C; (d) a load of 100 N and a time of 30 min: Sample A; (e) a load of 100 N and a time of 30 min: Sample B; (f) a load of 100 N and a time of 30 min: Sample C.
Coatings 11 00024 g010
Table
Table 1. Parameters of the supersonic flame spraying process (HVOF).
Table 1. Parameters of the supersonic flame spraying process (HVOF).
ParametersValue
Spraying distance/(cm)37
Oxygen/(m3·h−1)46
Kerosene/(L·h−1)23
Powder feeding rate/(g·min−1)85
Table
Table 2. Porosity of WC-10Co-4Cr and WC-10Co-4Cr+CeO2 coating.
Table 2. Porosity of WC-10Co-4Cr and WC-10Co-4Cr+CeO2 coating.
Coating PorosityField of View 1Field of View 2Field of View 3Field of View 4Field of View 5
WC-10Co-4Cr0.421%0.041%0.052%0.022%0.039%
WC-10Co-4Cr+CeO20.328%0.036%0.006%0.003%0.002%
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Ma, W.; Ge, Y.; Zhang, L.; Chen, F.; Zheng, Y.; Qi, Z. Study on the Friction Performance of Cerium Oxide on Supersonic Flame-Sprayed WC-10Co-4Cr Coating. Coatings 2021, 11, 24. https://doi.org/10.3390/coatings11010024

AMA Style

Ma W, Ge Y, Zhang L, Chen F, Zheng Y, Qi Z. Study on the Friction Performance of Cerium Oxide on Supersonic Flame-Sprayed WC-10Co-4Cr Coating. Coatings. 2021; 11(1):24. https://doi.org/10.3390/coatings11010024

Chicago/Turabian Style

Ma, Wenxiao, Yun Ge, Lixin Zhang, Fei Chen, Yijiang Zheng, and Zhuhui Qi. 2021. "Study on the Friction Performance of Cerium Oxide on Supersonic Flame-Sprayed WC-10Co-4Cr Coating" Coatings 11, no. 1: 24. https://doi.org/10.3390/coatings11010024

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