1. Introduction
High-entropy alloys are usually a new type of alloy formed from five or more metals in equal or relative proportions. The name is “High-entropy alloys” because the entropy increase is substantially higher when many elements are mixed in the mixture, and the ratios are closer to equal[1]. “High entropy” means that the entropy increase of mixing is significantly higher when there are a larger number of elements in the mix and their proportions are more nearly equal. Because high-entropy alloys may have some desirable properties, they have received considerable attention in materials science and engineering[2]. Metal alloys usually have just one or two metal components. For example, iron usually adds carbon, manganese, and other elements to enhance its characteristics, but the resulting alloy is still dominated by iron, and the proportion of other elements is quite low[2]. From past process experience, the more metal is added to the alloy, the more likely the material will be embrittled. However, as a new type of material, high-entropy alloys can effectively avoid embrittlement due to the addition of too many metal components[2,3]. Studies have found that many high-entropy alloys have much better specific strength than traditional alloys, and their fracture resistance, tensile strength, corrosion resistance, and oxidation resistance have been better than traditional alloys. High-entropy alloys have been introduced before 2004, but there have been numerous related studies in the 2010s[2,4-7].
Titanium carbide ceramics are an excellent ceramic reinforcement due to their high hardness, excellent wear resistance, low density, and extraordinary elastic modulus[8]. TiC often be chosen as the strengthening phase in metallic materials[9,10] to improve metal materials' friction and wear performance[11].
Different from conventional high-entropy alloys, the high-entropy alloy used in this research is Co33Cr30Ni33.5 Al1Ti2.5, which is the most corrosion-resistant of the currently available formulation. TiC is selected as the reinforcement material. Phases of TiC are like that of CoCrNiAlTi, and this combination can be well joined. CoCrNiAlTi-TiC composite coatings are prepared by plasma spraying[12-16]. In addition, the composite coatings have both the high hardness and elastic modulus of TiC and the corrosion resistance of CoCrNiAlTi [17]. Wherefore, this coating can be used in a variety of environments. TiC improves the hardness and elastic modulus of CoCrNiAlTi, which enhances the metal's wear resistance and deformation resistance. A good combination of TiC and CoCrNiAlTi may also reduce abrasive wear during friction[18].
In the previous research on metal enhancement, some schemes can only enhance one attribute and often have some side effects. The metal reinforcement scheme in this study not only enhances the metal's hardness but also improves the strength and other properties of the alloy. The enhanced coating has enhanced properties such as hardness, elastic modulus, friction coefficient, and wear resistance. This is a fully enhanced metal reinforcement solution.
2. Sample preparation
2-1. Materials
Prepare pure metal powders (1-5 μm) of Co, Cr, Ni, Al, and Ti according to the metal material compositions in Table 1 and use vacuum sintering technology [19] to fuse and sinter various metal powders into alloy particles with a size of 15-53 μm. The preparation process is conducted in a vacuum state to ensure that the metal does not undergo oxidation reactions.
Table 1. Composition of CoCrNiAlTi
2-2. Coating Preparation
The base material is cylindrical Q235 steel with a diameter of 30 mm and a thickness of 10 mm, and the base material is polished to remove surface pollutants and oxides. Plasma spraying equipment is filled with inert gas during plasma spraying to prevent chemical reactions of metals. The specific spraying conditions are shown in Table 2.
Table 2. Plasma Spray Parameters for Coatings
CoCrNiAlTi high-entropy alloy particles are sprayed on Q235 steel substrate to form HEA coating. In addition, TiC (30 wt%) and CoCrNiAlTi (70 wt%) high-entropy alloy particles are mixed with a particle size of 15-53 μm, and then sprayed on the Q235 steel substrate to form a composite coating. The thickness of both coatings is 1.3 mm (the final thickness is 1 mm, and the excess part will be lost during the polishing process), and finally, the coating surface is polished to Ra<0.1 μm, as shown in Table 3. The plasma sprayed samples are shown in Figure 1.
Table 3. Surface roughness of samples after polishing
Fig. 1. Picture of sample (a) CoCrNiAlTi coating, (b) CoCrNiAlTi-TiC coating.
3. Experiment
3-1. XRD Test And Microhardness Test
The CoCrNiAlTi and CoCrNiAlTi-TiC coatings are evaluated using the Multifunctional X-ray Diffractometer rotating at a scan rate of 5o/min in the range of 10° to 90°. By comparing the XRD test results with the XRD database of Jade software, We can understand the phase-type and distribution of the coating. The results of the XRD test are analyzed in the XRD database to obtain the phase types of the two coatings. The results of the XRD test are compared and analyzed in the XRD database of the Jade software to obtain the phase types of the two coatings. For the reliability of the experiment, five experiments were carried out and analyzed for each coating. Finally, it is deduced whether the coating produced by plasma spraying technology meets the standard by analyzing the results.
A microhardness tester evaluated the microhardness and elastic modulus of CoCrNiAlTi and CoCrNiAlTi-TiC coatings. Cut the coating sample into thin slices of 10 mm × 5 mm × 2 mm, and test according to the conditions in Table 4. Three points are evaluated simultaneously to take the average. The material of the ball used in the wear test is selected according to the microhardness test results.
Table 4. Microhardness test conditions
3-2. Friction and Wear Test
The operating environment of large drill bits is under non-lubricated conditions, using the wear test system to do dry friction and wear tests of the coating samples. As shown in Table 5. As shown in Table 5, the friction method is ball to disk, the radius of the wear test is set to 11.5 mm, the frequency is set to 50 rpm, the load is set to 25 N, and continuous friction is performed for 1800 seconds. It continuously records the data of the friction coefficient changing with time during the wear process. The coated samples need to be cleaned in an ultrasonic cleaner for 30 minutes before and after the wear test to prevent the presence of surface attachments from affecting the test results. Use an electronic balance to record the weight of the sample before and after the wear test and then calculate the weight loss.
Table 5. Test conditions of wear test
3-3. Surface Analysis
Use a scanning electron microscope to continue high-magnification scanning (magnification 5000 times, 30,000 times) on the surface of CoCrNiAlTi coating and CoCrNiAlTi-TiC coating. First, check the coating surface for abnormal crystallization, small cracks, bubbles, and other surface defects. Next, and after the wear test, scan the worn part to check the surface condition of the worn part (magnification 50 times, 500 times), including grooves, peeling, particle embedding, etc., and infer wear type based on this information.
Use a surface roughness tester to evaluate the points shown in Figure 2. First, record the profile of the wear scar and evaluate the depth Ry(min) of the wear scar and the deformation height |Ry(min)| on both sides of the wear scar. The measuring range of the surface roughness test is 4 mm. For a more rigorous measurement, the part of the line segment on the friction surface that rotates around the axis of the cylindrical sample and intersects the wear scar is selected for measurement. The angle between adjacent line segments is 90o, and the measurement results are averaged. According to the data of the measurement points, the line graph of the wear scar contour can be output, the crosssectional area of the worn part can be calculated, and then calculate the wear volume.
Fig. 2. The surface roughness test location of the sample.
\(\begin{aligned}V=2 \pi R \cdot \sum_{n=0}^{n=4}\left(\left|y_{n}\right| \cdot \Delta x\right)\end{aligned}\)
(yn is the depth evaluation value, and ∆x is the evaluation interval.)
the wear rate W can be calculated by the following equation:
W = V/(S × L)
where S is the sliding distance and L is the normal load applied.
4. Results and Discussion
Use Jade software to compare XRD data and database information. The following results are obtained:
As shown in Figure 3 (a), the CoCrNiAlTi coating only has a face-centered cubic, indicating that no by-products of other phases are produced (FCC) single-phase solid solution crystal structure. So, it is during the plasma spraying process. Figure 3 (b) shows the XRD pattern of the CoCrNiAlTi-TiC coating. Unlike Figure 3 (a), the TiC phase is detected in the XRD pattern. In addition, other complex, brittle phases are not formed after adding TiC particles, showing that the addition of TiC did not change the solid solution phase in the high-entropy alloy coating.
Fig. 3. XRD patterns and analysis of coating (a) CoCrNiAlTi coating, (b) CoCrNiAlTi-TiC coating.
The data at the beginning and end of the micro-hardness test are volatile, and only the relatively stable part of the data in the middle period is used in data sorting and calculation. As shown in Figure 4 and Table 6, the microhardness of the CoCrNiAlTi-TiC composite coating is 18.86 GPa, and the elastic modulus is 363.76 GPa, both of which are higher than the CoCrNiAlTi high-entropy alloy coating. Therefore, we can conclude that the hardness of the coating increases due to the presence of the more brutal TiC phase in the CoCrNiAlTi-TiC coating. Furthermore, because the TiC and FCC phases have similar fusion characteristics, the added TiC composite coatings have a higher elastic modulus
Table 6. Microhardness test results of CoCrNiAlTi coatings and CoCrNiAlTi-TiC coatings
Fig. 4. Microhardness test results over a range. (a) CoCrNiAlTi coatings, (b) CoCrNiAlTi-TiC coatings.
The hardness of CoCrNiAlTi and CoCrNiAlTi-TiC coatings is very high, which is much higher than that of ordinary steel balls, tungsten carbide balls with high hardness are selected as wear test balls. In addition, tungsten carbide is a difficult-to-cut material, which can easily cause tool wear, making the wear test results more obvious. The wear of the coating by the steel ball will be a very smooth profile, which does not conform to the actual use of the drill. The wear profile of the WC ball on the coating is irregular and rough, which is in line with the wear situation of the drill bit in actual use.
Plasma spraying is superimposed layer by layer, and the coating is prone to small fragments being peeled off during the friction and wear test. This results in a fluctuating coefficient of friction and a slight increase overall. As shown in Figure 5, To reduce the influence of fluctuations on judgment, in the timefriction coefficient line graph, a curve after dropping fluctuations has been added. The friction coefficients of CoCrNiAlTi and CoCrNiAlTi-TiC coatings gradually increased in the wear test. It shows that the friction coefficient changed during the wear test. This result may be due to abrasive wear or deformation. By comparing the friction coefficient changes of CoCrNiAlTi and CoCrNiAlTi-TiC coatings in wear tests, it can be possible to conclude that the variation of friction coefficient of the CoCrNiAlTi-TiC coating is slightly smaller than that of CoCrNiAlTi coating. This result may be due to the less abrasive wear or deformation of the CoCrNiAlTi-TiC coating.
Fig. 5. The friction coefficient of CoCrNiAlTi and CoCrNiAlTi-TiC coatings.
Tests of friction coefficient are performed on three samples of each type of coating. The friction coefficient result curve is smoothed and averaged to compare the experimental results numerically, and the results shown in Table 7 are obtained. The average friction coefficient of CoCrNiAlTi-TiC coatings is slightly smaller than that of CoCrNiAlTi coatings. This result may be due to the increased wear and deformation of the CoCrNiAlTi coating over time. Since the plasma spraying method is stacked layer by layer, the coating may cause tiny pieces to peel off when worn away. The flakes of microscopic fragments lead to a change in the roughness of the rubbed part, which is responsible for the change in the coefficient of friction evaluated during the tribological and wear test.
Table 7. The results of friction coefficient
As shown in Figure 6. It can be known from the SEM photos of CoCrNiAlTi and CoCrNiAlTi-TiC coating samples. There are no abnormal crystals, fine cracks, and bubbles on the surface of CoCrNiAlTi and CoCrNiAlTi-TiC coatings by plasma spraying, and the metals are well bonded. The SEM photo of the worn part of the CoCrNiAlTi coating is shown in Figure 7 (a), and the CoCrNiAlTi-TiC coating is shown in Figure 7 (b). The small black dots in the SEM photo represent the part where the falling particles are embedded after wear, the white outline represents the edge left after the metal is peeled off, and the strip outline is the worn groove. There are more black spots in the worn part of CoCrNiAlTi coating, which indicates that more falling particles are embedded in the worn part of CoCrNiAlTi coating. The wear parts of CoCrNiAlTi coating and CoCrNiAlTi-TiC coating are densely covered with white contours and many strip contours, indicating that the wear parts of CoCrNiAlTi and CoCrNiAlTi-TiC coatings are mainly worn by peeling and accompanied by many wear grooves.
Fig. 6. Surface SEM photographs of coating samples. (a) 5000 times (left) and 30000 times (right) of CoCrNiAlTi coating, (b) 5000 times (left) and 30000 times (right) of CoCrNiAlTi-TiC coating.
Fig. 7. 3D micrograph of the wear scar. (a) CoCrNiAlTi coating, (b) CoCrNiAlTi-TiC coating.
Observe the notched region of the wear scar profile in Figure 8(a) and (b), and calculate the wear volume by the following formula.
Fig. 8. Wear profile of (a) CoCrNiAlTi coating, (b) CoCrNiAlTi-TiC coating.
\(\begin{aligned}V=2 \pi R \cdot \sum_{n=0}^{n=4}\left(\left|y_{n}\right| \cdot \Delta x\right)\end{aligned}\)
(yn is the depth evaluation value, and ∆x is the evaluation interval.)
It can be concluded that the wear volume of the CoCrNiAlTi coating is significantly larger than that of the CoCrNiAlTi-TiC coating. CoCrNiAlTi-TiC coatings are proven to have higher wear resistance. CoCrNiAlTi-TiC coating has smaller protrusions on both sides of the wear scar. It shows that the shape change of CoCrNiAlTi-TiC coating wear part is smaller.
The surface roughness test results of each part in Figure 2 are averaged, and the wear scar's depth and the protrusions' height on both sides are shown in Table 8. The average wear depth of CoCrNiAlTi coating (|Ry(min)| = 28.61 μm) is much larger than that of CoCrNiAlTi-TiC coating (|Ry(min)| = 12.39 μm). Likewise, the average protrusion height (Ry(max) = 3.36 μm) of CoCrNiAlTi coating is much larger than that of CoCrNiAlTi-TiC coating (Ry(max) = 1.98 μm).
Table 8. Summary of surface roughness test results
The weighing results of the electronic balance before and after the friction and wear experiment are shown in Table 9. After calculation, the average loss mass of the CoCrNiAlTi coating sample is 0.0041 g, much greater than the average loss mass of the CoCrNiAlTi-TiC composite coating sample 0.0018 g. the average wear rate of the CoCrNiAlTi coating sample is 6.8545 × 10-7 mm2/N, much greater than the average wear rate of the CoCrNiAlTi-TiC composite coating sample 2.7953 × 10-7 mm2/N.
Table 9. Wear date of samples before and after wear tests
5. Conclusions
CoCrNiAlTi coatings and CoCrNiAlTi-TiC composite coatings are prepared on Q235 steel discs by plasma spraying. CoCrNiAlTi coating and CoCrNiAlTi-TiC composite coating consist of FCC and FCC + TiC, respectively. Both coatings have no cracks or pores. And has excellent metallurgical bonding with the substrate.
The CoCrNiAlTi-TiC composite coating has higher hardness and elastic modulus than the CoCrNiAlTi coating. The hardness of the CoCrNiAlTi-TiC composite coating reaches 18.86 GPa, which is about 23.43% more than that of the CoCrNiAlTi coating. And the elastic modulus reaches 363.86 GPa, which is about 19.51% more than the CoCrNiAlTi coating.
The average deformation height of CoCrNiAlTi coating at the wear position is 3.36 μm, which is about 1.7 times that of CoCrNiAlTi-TiC composite coating. The average wear depth of CoCrNiAlTi coating at the wear position is 28.61 μm, which is about 2.3 times that of CoCrNiAlTi-TiC composite coating. The wear mass loss of CoCrNiAlTi coating is 0.0041 g, about 2.3 times that of CoCrNiAlTi-TiC composite coating. The wear rate of CoCrNiAlTi coating is 6.8545 × 10-7 mm2/N, about 2.45 times that of CoCrNiAlTi-TiC composite coating
Acknowledgments
This research is supported by a grant(2022-MOIS35-005) of Policy-linked Technology Development Program on Natural Disaster Prevention and Mitigation funded by Ministry of Interior and Safety (MOIS, Korea).
References
- Tsai, M. H., & Yeh, J. W. "High-entropy alloys: a critical review." Materials Research Letters, Vol 2, No.3, pp.107-123, 2014. https://doi.org/10.1080/21663831.2014.912690
- Lavine, M S. "A metal alloy that is stronger when cold." Science, Vol 345, No.6201, pp.1131-1131, 2014. https://doi.org/10.1126/science.345.6201.1131-b
- Wang, S. "Atomic structure modeling of multi-principal-element alloys by the principle of maximum entropy." Entropy, Vol 15, No.12, pp.5536-5548, 2013. https://doi.org/10.3390/e15125536
- Ye, Y. F., Wang, Q., Lu, J., Liu, C. T., & Yang, Y. "High-entropy alloy: challenges and prospects." Materials Today, Vol 19, No.6. pp.349-362, 2016. https://doi.org/10.1016/j.mattod.2015.11.026
- Lavine, M. S. "A metal alloy that is stronger when cold." Science, Vol.345, No.6201, pp.1131-1131, 2014. https://doi.org/10.1126/science.345.6201.1131-b
- Youssef, K. M., Zaddach, A. J., Niu, C., Irving, D. L., & Koch, C. C. "A novel low-density, high-hardness, high-entropy alloy with close-packed single-phase nanocrystalline structures." Materials Research Letters, Vol.3, No.2, pp.95-99, 2015. https://doi.org/10.1080/21663831.2014.985855
- Gludovatz, B., Hohenwarter, A., Catoor, D., Chang, E. H., George, E. P., & Ritchie, R. O. "A fracture-resistant high-entropy alloy for cryogenic applications." Science, Vol.345, No.6201, pp.1153-1158, 2014. https://doi.org/10.1126/science.1254581
- Das, K., Bandyopadhyay, T. K., & Das, S. "A review on the various synthesis routes of TiC reinforced ferrous based composites." Journal of Materials Science, Vol.37, No.18, pp.3881-3892, 2002. https://doi.org/10.1023/A:1019699205003
- Li, T., Liu, B., Liu, Y., Guo, W., Fu, A., Li, L., ... & Fang, Q. "Microstructure and mechanical properties of particulate reinforced NbMoCrTiAl High Entropy based composite." Entropy, Vol.20, No.7, pp.517, 2018.
- Yu, Y., Wang, J., Li, J., Kou, H., & Liu, W. "Characterization of BCC phases in AlCoCrFeNiTix high entropy alloys." Materials Letters, Vol.138, pp.78-80, 2015. https://doi.org/10.1016/j.matlet.2014.09.100
- Ha, M. W., Lee, K. H., Lee, C. H., Choi, J. M., & An, J. W. "Study on the Friction and Wear Characteristics of Tungsten Carbide and Zirconium with Phosphor-Containing Liquid." Journal of Tribology, Vol.139, No.3, 2017, https://doi.org/10.1115/1.4034022
- Park, D. S., Park, J. M., Kim, J. S., & Lee, Y. Z. "EThe Effect of TiN and CrN Coatings on the Fretting Wear of Tubes against Supports in a Nuclear Steam Generators." KSTLE International Journal, Vol.10, No.1-2, pp.33-36, 2009.
- Goo, B. C., & Kim, M. S. "Tribological Properties of Ceramic Composite Friction Materials Reinforced by Carbon Fibers." Tribology and Lubricants, Vol.33, No.1, pp.15-22, 2017, https://doi.org/10.9725/kstle.2017.33.1.15
- Lee, Soo W. "Wear Mechanism of Plasma-Sprayed Coating in Mo-and Co-Based Alloy." Tribology and Lubricants, Vol.11, No.5, pp.108-113, 1995, https://doi.org/10.9725/kstle.1995.11.5.108
- Umarov, R., Pyun, Y. S., & Amanov, A. "Tribological Performance of Ni-Cr Composite Coating Sprayed onto AISI 4340 (SNCM439) Steel by High Velocity Oxygen Fuel." Tribology and Lubricants, Vol.34, No.6, pp.217-225, 2018, https://doi.org/10.9725/kts.2018.34.6.217
- Le Thuong, H., Chae, Y. H., Kim, S. S., Kim, B., & Yoon, S. B. "Sliding Wear Behavior of Plasma Sprayed Zirconia Coatingagainst Silicon Carbide Ceramic Ball." Proc. Fall Conf. Korean Soc. Tribol. Lubr. Engr., Daegu, Korea, November 2004.
- Huynh, Ngoc-Phat, Nga Linh Vu, and Koo-Hyun Chung. "Quantitative Assessment of Wear Characteristics of Cr-based Coating Reinforced with Diamond." Tribology and Lubricants, Vol.38, No.1, pp.15- 21, 2022, https://doi.org/10.9725/kts.2022.38.1.15
- Guo, L., Zhang, X., Lee, C. H., Marinescu, I. D., Zhang, Y., & Hui, J. "An experimental study on the abrasive machining process of electronic substrate material with a novel ultraviolet-curable resin bond diamond lapping plate." IEEE Access, Vol.7, pp.64375-64385, 2019. https://doi.org/10.1109/access.2019.2917304
- Choi, S. I. M., Karimbaev, R., Pyun, Y. S., Amanov, a., "Improvement in Tribological Properties of Carbon Steel Sintered by Powder Metallurgy." Tribology and Lubricants, Vol.36, No.4 pp.244-252, 2020, https://doi.org/10.9725/kts.2020.36.4.244