- Email: [email protected]
Al2O3 composite coatings
Journal Pre-proof Effect of alumina particle size on characteristics, corrosion, and tribological behavior of Co/Al2O3 composite coatings S. Mahdavi, A. Asghari-Alamdari, M. Zolola Meibodi PII:
S0272-8842(19)33167-0
DOI:
https://doi.org/10.1016/j.ceramint.2019.10.289
Reference:
CERI 23357
To appear in:
Ceramics International
Received Date: 22 August 2019 Revised Date:
23 October 2019
Accepted Date: 31 October 2019
Please cite this article as: S. Mahdavi, A. Asghari-Alamdari, M. Zolola Meibodi, Effect of alumina particle size on characteristics, corrosion, and tribological behavior of Co/Al2O3 composite coatings, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.10.289. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Effect of alumina particle size on characteristics, corrosion, and tribological behavior of Co/Al2O3 composite coatings
S. Mahdavi*, A. Asghari-Alamdari, M. Zolola Meibodi Research Center for Advanced Materials, Faculty of Materials Engineering, Sahand University of Technology, Tabriz, Iran. Postal Code: 5331817634
*
Corresponding author:
Soheil Mahdvai, Ph.D., Assistant Professor, Research Center for Advanced Materials, Faculty of Materials Engineering, Sahand University of Technology, Tabriz, Iran. Email: [email protected], Tel: +98-4133459413, Fax: +98-4133444333 Abstract The electrodeposition technique was used to produce Co, Co/micro-Al2O3, and Co/nano-Al2O3 coatings. The effect of particle size on current efficiency, thickness, composition, morphology, and structure of the coatings was investigated. The potentiodynamic polarization technique and electrochemical impedance spectroscopy was used to study the corrosion behavior of the coatings. Tribological behavior of the micro/nano-composite coatings was also examined, and the results were compared to the pure Co and the steel substrate. The results revealed that the incorporation of alumina nano-particles decreased the current efficiency of Co electrodeposition. Although the weight percent of the co-deposited nano-particles was much less than the micro-particles under the same electrodeposition conditions, their effect on morphology was more pronounced. The Co/micro-Al2O3 coating had a higher hardness and corrosion resistance than the other samples. The incorporation of nano-particles
1
decreased the hardness and polarization resistance of the pure Co by about 15 and 59%, respectively. Unlike the corrosion results, both the composite coatings had better wear resistance than the pure Co film. However, the volume loss of the Co/micro-Al2O3 coating was 1.6 times lower than the Co/nano-Al2O3 electrodeposit. Keywords: Composites (B); Corrosion (C); Wear resistance (C); Al2O3 (D).
1 Introduction Electrodeposition is a well-known technique for the preparation of pure metal, alloy or composite coatings [1-8]. Composite coatings are formed by suspending ceramic particles in an electrodeposition bath and applying an electric field [3]. Co-deposition of ceramic particles within the metal matrix improves properties of pure metals such as hardness, wear and corrosion resistance [4-11]. Therefore, these coatings are extensively used for corrosion and wear-resistant purposes in mechanical, chemical, biomedical and cosmonautic industries [5,12]. Type, amount, size, shape, and dispersion of the reinforcing particles have a key role in the final properties of the composite coatings [8,10]. Different types of ceramic particles such as CeO2 [8], TiN [3,5], TiO2 [7,13], Mn3O4 [14], SiC [15-17], Al2O3 [11], B4C [4], WC [6], ZrC [18], SiO2 [19], CeO2 [8], etc. with various sizes were used in previous research works. Gupta et al. [6] produced Ni matrix composite coatings containing WC particulates with an average size of 10 µm at different electrodeposition conditions. They exhibited that the Ni/WC film had higher microhardness and corrosion resistance than the pure Ni coating. Jian et al. [20] showed that Ni/SiC nano-composite film had better corrosion resistance than pure Ni coating. Corrosion current density of the composite film was about half of the pure Ni one. Wang et al. [9] compared the
2
mechanical properties of Ni and Ni/Al2O3 nano-composite coatings and found that the incorporation of nano-alumina particles increased the hardness and modulus of the Ni matrix. Tolumoye et al. [19] studies revealed that the incorporation rate of the microsized SiO2 particles within the Zn-Ni film was higher than the nano-sized ones. However, they did not compare the properties of these coatings. Effect of ZrC particle size on morphology, hardness and electrochemical behavior of Ni matrix composite coatings have been studied by Zhang et al. [18]. According to their results, the surface morphology of both the micro and nano-composite coatings was less uniform and rougher than the pure Ni film. They also found that the effect of nano-sized ZrC particles on enhancing the hardness and corrosion resistance of Ni films was more significant than micro-sized ones. In the past 30 years, nickel-based micro/nano-composite coatings have been extensively examined [5]. However, in spite of great interest to cobalt matrix composites in a large range of applications, due to their strength, good corrosion and wear resistance, electrocatalytic activity, and magnetic properties [2,12,21-25], studies on these coatings are much more limited than the Ni-based films. Apelt et al. [14] investigated the effect of electrodeposition condition on the incorporation of Mn3O4 powder with an average particle size of 2.5 µm in the cobalt coatings for using on the ferritic interconnect in solid oxide fuel cell (SOFC). They found that the solution pH and the particle loading have the greatest influence on the co-deposition of Mn3O4. Carac et al [8] expressed that microstructure and hardness of Co/micro-CeO2 composite coatings could be influenced by electrolyte composition and process parameters of current density, temperature, and stirring condition. Rudnik et al. [17] investigated the effect of surfactant concentration on the incorporation of SiC micro-particles within the Co matrix. They reported that
3
adding surfactant to the deposition bath increased the co-deposition of particles while decreased the current efficiency. All of the above studies have only focused on the effect of electrodeposition parameters and particles concentration of the bath on characteristics of cobalt composite films. Particle size has also a great influence on the final properties of the coatings [18,19,26]. In this research, micro and nano-sized alumina particles were used for electrodeposition of Co matrix composites. Alumina is an attractive ceramic material because of its relatively low density, notable hardness, high erosion and corrosion resistance, and high melting temperature [27,28], which can improve the corrosion and tribological behavior of the coatings. This study aims to investigate the effect of alumina particle size on characteristics and hardness of the cobalt coatings. Corrosion and tribological behavior of Co/Al2O3 micro/nanocomposites are also examined and the results are compared to the pure Co and the steel substrate. 2 Experimental 2.1 Electrodeposition process Cobalt coatings were electrodeposited from the baths containing 200 g L-1 CoSO4.7H2O, 50 g L-1 H3BO3, 0.4 g L-1 C7H4NNaO3S.2H2O (Sodium saccharin), and 0.1 g L-1 NaC12H25SO4 (SDS) by using the direct current. These values were selected based on our previous research [29]. Current density, temperature, stirring speed, and pH of the bath were 80 mA cm-2, 45˚C, and 3, respectively. The pH was adjusted by using 1M NaOH or 1M H2SO4 solutions. Alumina particles with the average sizes of 2 µm and 50 nm were used for electrodeposition of micro/nano-composite coatings. 15 g L-1 of these particles were added to the distilled water (with the same pH of the main bath) in a separate container.
4
After stirring for 24 h and ultrasonic treatment for 1 h, the suspension was added to the main bath. A double electrode cell was used for electrodeposition of Co and Co/Al2O3 coatings. The anode and cathode were pure cobalt and St 37 steel plates, respectively. Before electrodeposition, the steel substrates were abraded with emery papers (from 120 to 800 grit), and rinsed with distilled water. Degreasing was ultrasonically done in acetone at room temperature for 5 min, and then in an alkaline solution (50 g L-1 Na2CO3, 50 g L-1 Na3PO4.12H2O, 30 g L-1 NaOH) at 70°C for 15 min. Finally, the substrates were pickled in 20% HCl solution. 2.2 Characterization A scanning electron microscope (CAMSCAN MV2300 SEM) and the attached energy dispersive X-ray spectrometer were employed to examine the morphology and chemical composition of the samples. The structure of the coatings was studied by the X-ray diffraction (XRD) technique. The preferred orientation was investigated by using the relative texture coefficient (RTChkl) [13,30]: =∑ In this equation,
/ /
× 100
(1)
represents the diffraction intensities of the (hkl) lines in the XRD
patterns of the samples, and
denotes the diffraction intensities of the randomly
oriented cobalt powder (JCPDS no. 5-0727). Four diffractions lines of (100), (002), (101), and (110) of Co were considered. 2.3 Microhardness measurements Microhardness measurements were carried out by using an applied load of 0.2 N for 15 s. The measurements were conducted on at least five different areas of each sample. 2.4 Corrosion tests
5
Corrosion behavior of the samples was examined by the potentiodynamic polarization and
the
electrochemical
impedance
spectroscopy
(EIS).
A
Zive
SP2
potentiostat/galvanostat was used for this purpose. The tests were performed in a three electrode cell containing 3.5 wt.% NaCl solution at room temperature. The counter electrode was a square platinum plate with the dimensions of 1×1 cm. An Ag/AgCl (3 M KCl) probe was used as the reference electrode. The potential scan rate during the polarization tests was 1 mV/s. The EIS measurements were done at the open circuit potential. The frequency range and the sinusoidal signal amplitude were 100 kHz to 0.1 Hz, and 10 mV, respectively. 2.5 Wear tests Dry sliding pin-on-disc wear tests were done at room temperature. The cylindrical steel pins (1.5Cr, 1C, 0.35Mn, 0.25Si) with the hardness of 64HRC were used as the counterbody. The diameter of the used pins was 5 mm, and they had a curved ends. The normal load, sliding speed, and sliding distance were 4 N, 0.1 ms-1, and 300m, respectively. The friction coefficient was also measured during the tests. 3 Results and discussion 3.1 Current efficiency and thickness of the coatings The effect of adding 15 g L-1 alumina particles with different sizes to the electrodeposition bath on the current efficiency and thickness of the coatings is shown in Fig. 1. Although the electrodeposition current efficiency and thickness of the Co film somewhat increase with co-deposition of the micro-particles, they decrease with the incorporation of the nano-particles. The highest current efficiency of about 90% is obtained in the presence of the micro-alumina particles in the bath. This could be explained by the incorporation of relatively large ceramic particles in the metal matrix,
6
along with decrement of the metallic surface in contact with the solution and increment of the current density, as also reported by Benea et al. [31,32]. While the same concentrations of micro or nano-particles (15 g L-1) was added to the electrodeposition baths, the number and the total surface area of the nano-particles is considerably higher than the micro-sized ones. This can cause an effective increment of the solution resistance [33], as well as a significant rise of the current density during the deposition process, which in turn increases solution pH and the probability of metal hydroxide formation on the cathode surface. The formation of these hydroxides inhibits cobalt ion reduction and reduces the current efficiency. The reasons for the thickness changes are the same. 3.2 Morphology and chemical composition SEM images from the surface of Co, Co/micro-Al2O3, and Co/nano-Al2O3 coatings at two different magnifications have been shown in Fig. 2. The bumpy morphology of Co film becomes coarser and more irregular with the incorporation of the micro-particles, which can be due to the formation of a Co layer on the coarse alumina particles. However, the morphology has changed to the needle-shaped with co-deposition of the alumina nano-particles. The incorporated nano-particles can increase the nucleation sites and change the preferred growth direction, and as a result, the morphology of the coating. SEM images from the polished surfaces of the composite coatings are shown in Fig. 3. The uniform distribution of the particles is noticeable in this figure, indicating suitable preparation of the electrodeposition baths (stirring for 24 h and ultrasonic treatment for 1 h in a separate container containing water). It is also evident that the agglomeration of the particles is negligible, which can be due to the employing of SDS in the
7
electrodeposition bath. The SDS, as an anionic surfactant, forms a negative layer on surface of the particles in the bath, and increases the electrostatic repulsion between them [34]. Therefore, the probability of the agglomeration is reduced. According to the EDS analysis results (Fig. 4), a higher volume fraction of the microalumina particles has been incorporated within the cobalt matrix as compared to the nano-sized ones. The aluminum peaks are clear in the EDS spectrum of Co/micro-Al2O3 composite film. The weight percent of the micro-alumina particles in this composite film is about 15%, which is considerably higher than that for the nano-alumina particles with about 1%. The absorbance and holding time of the particles on the cathode surface determine the amount of their incorporation. It has been said that the fine particles are more prone to agglomeration and detachment from the surface of the substrate, resulting in their lower incorporation [35]. However, assuming that the particles are spherical and by considering the average particle sizes, the mass of a micro-sized alumina particle is about 64000 times higher than the nano-sized one. Therefore, the effect of gravity and turbulence of the electrolyte on the micro-particles, and thus the probability of their detachment, are higher than the nano-particles. Nevertheless, the influence of the codeposition of an alumina micro-particle on the weight percent is equal to the incorporation of about 64000 nano-particles. Thus, although the number of incorporated nano-particles is higher than the number of micro-particles, their weight percent is much lower. 3.3 Structure The X-ray diffraction patterns of the coatings are shown in Fig. 5. All the coatings have a hexagonal close-packed (hcp) structure. The relative texture coefficient of four diffraction lines of the deposits was calculated according to Eq. (1), and have been
8
presented in Table 1. It is seen that the RTC(100) decreases with the incorporation of alumina particles. The RTC(002) and RTC(101) are increased by the co-deposition of micro-particles, but corresponding peaks of (002) and (101) planes are not observed in the XRD pattern of the Co/nano-Al2O3 composite coating. However, the main reason for the morphological changes with the incorporation of the alumina particles can be the variation of RTC(110). The RTC(110) increases from 1.1 to 12.3% with the incorporation of the nano-alumina particles, resulting in the needle-shaped morphology of Co/nanoAl2O3 composite film (Fig. 2c). Matte white appearance of the pure cobalt coating is also changed to the black by the incorporation of the nano-alumina particles.
3.4 Microhardness The microhardness of the pure cobalt and the composite coatings have been compared in Fig. 6. The microhardness of the Co film increases from 318 to 396 Hv with the incorporation of the micro-particles, but it is decreased by about 12% by co-deposition of the nano-sized alumina particles. The hard ceramic particles can restrain the matrix deformation and the motion of the dislocations, resulting in higher hardness values [13,32,35]. The effect of nano-particles on the increment of the hardness should be more pronounced than the micro-particles due to their higher number and the larger total surface area [35]. However, the microhardness is reduced by co-deposition of the nanoparticles, which can be attributed to the structural and morphological changes with the incorporation of alumina nano-particles in this study (Figs. 2 and 5). 3.5 Corrosion behavior
9
The polarization curves of Pure Co and composite coatings are shown in Fig. 7. The polarization curve of the substrate is also presented for comparison. The extracted electrochemical parameters are presented in Table 2. All the coatings have more positive corrosion potential than the substrate, indicating their nobler behavior. The corrosion current densities of the electrodeposits are also lower than the steel substrate, which means that the corrosion rate has been decreased by applying the coatings. Comparing the corrosion current density values, the incorporation of the micro-sized alumina particles improves the corrosion resistance of the Co film. The icorr of this composite is 2.5 times lower than the pure Co coating, which can be due to the reduction of the metallic surface in contact with the corrosive environment with codeposition of the alumina micro-particles. However, the incorporation of nano-particles does not have the desired effect on the corrosion resistance of the Co film. The corrosion resistance has gotten even worse with the incorporation of the nano-particles, while it has been said in the literature that these particles can decrease the icorr through the grain refinement, structural modifications, and embedment in crevices and pores [36,37]. This discrepancy can be a result of the morphological changes with the inclusion of the nano-particles (Fig. 2), which may increase the metallic surface where the corrosion occurs. The Nyquist plots of the samples are shown in Fig. 8. All the plots represent a single semicircle, indicating the presence of one time constant in the equivalent circuit (Fig. 8). For a further investigation of the corrosion behavior at the high frequencies, the Bode plots are shown in Fig. 9. This figure also confirms the correctness of the used equivalent circuit. In this circuit, Rs is the solution resistance, Rp shows the polarization
10
resistance, and CPE is the constant phase element and represents the double layer capacitance [37-39]. The impedance of CPE is calculated by using the following equation [37,40-43]: =
(
(2)
)
where Y0 is the admittance function, j2=-1, ω is the angular frequency, and n is the CPE power (n=απ/2, where α is the phase angle constant). The double layer capacitance can also be calculated by using these parameters and the following equation [41]: =!
"#
×
( "#)/# $
(3)
The calculated electrochemical parameters are presented in Table 3. These results confirm that the incorporation of micro-sized alumina particles within the cobalt matrix has the greatest effect on increasing the corrosion resistance. The polarization resistance of the Co/micro-alumina composite coating is about 13.8, 1.4, and 3.4 times higher than the substrate, the pure Co, and the Co/nano-alumina film, respectively. The double layer capacitance has also been presented in Table 3. The capacity of the substrate is significantly higher than the coatings, which means that the corrosion reactions occur in a larger area of the substrate (abraded to 800 grit) as compared to the electrodeposits. The double layer capacitance of the micro-composite film is smaller than the pure Co and the nano-composite coatings. The smaller capacity of the Co/micro-alumina than the Co film can be due to its coarser morphology (Fig. 2b) and the presence of alumina particles on the surface of this composite coating, which decrease the metallic surface where the electrochemical reactions occur. However, although the nano-particles may decrease the metallic surface, the double layer capacitance of Co/nano-alumina film is higher than the two other coatings. This
11
behavior may be a result of the morphological changes, along with the increment of the porosity and the total surface area (Fig. 2c). 3.6 Tribological behavior The volume loss and the average coefficient of friction of the samples are presented in Table 4. As expected from the hardness results (Fig. 6), the substrate and the Co/microalumina composite coating have the worst and the best wear resistance between the samples, respectively. The wear volume loss of the substrate is 5.4 times higher than the micro-composite film. According to the Archard theory, the wear resistance is directly related to the hardness [44]. As the hardness of the micro-composite coating is higher than the other samples, it also has the minimum volume loss. However, although the Co film is harder than the Co/nano-alumina electrodeposit, its volume loss is 25% higher than the nano-composite coating. The presence of nano-particles can hinder the movement of the grains and the grain boundary migration during the sliding, and thus increase the wear resistance [13,45]. The average coefficient of friction values have also been presented in Table 4. The friction coefficient of Co/micro-alumina coating is lower than the other samples, which is consistent with the volume loss results. However, unlike the volume loss results, the Co/nano-alumina film has a higher friction coefficient than the pure Co coating. The variations of the friction coefficient of the samples with the sliding distance are shown in Fig. 10 for further investigations. The friction coefficient of the substrate steeply increases at the first 50 m of the sliding, and after that remains almost constant. The real contact area between the steel pin and the sample, along with the wear mechanism define the friction coefficient. As the steel pin has a curvature at its end, the real contact area increases with sliding distance and the entrance of the pin to the sample surface.
12
Therefore, the coefficient of friction rises with the sliding distance. The fast increment of the friction coefficient of the substrate at low sliding distances is a result of its low hardness value and rapid penetration of the steel pin into the surface. However, the work hardening of the worn surface or formation of a lubricating tribo-layer, and approaching the real contact area to its maximum value can be the reasons for the almost constant friction coefficient of the substrate after 50 m. The friction coefficient of Co and Co/nano-alumina coatings rises until the end of the wear tests. However, this increment occurs relatively fast at the first 100 m of sliding for the nano-composite coating, while the coefficient of friction of the Co film is uniformly increased over the 300 m. The friction coefficient is increased by the gradual flattening of the surface morphology with sliding. As the nano-composite film is softer than the pure Co coating, its surface morphology is worn out faster. This causes a more rapid increment of the real contact area and the friction coefficient of the nanocomposite coating at the first 100 m of the sliding process. However, the presence of the nano-particles may improve the wear resistance and hinder quick increment of the friction coefficient after the disappearance of the relatively porous and mechanically weak (Fig. 6) needle-shaped morphology (Fig. 2c) at longer sliding distances. In the Co/micro-alumina coating, the coefficient of friction gradually enhances at the first 100 m of the sliding and then remains almost constant. This is due to the flattening of the surface roughness at the early stages, and then a balance between smoothing of the surface and protrusion of the micro-particles at the worn surface. The Protruded particles decrease the metal on metal contact and as a result the friction coefficient and the volume loss.
13
SEM micrographs and EDS analysis from the worn surface of the substrate are shown in Fig. 11. Deep grooves on the worn surface indicate that the abrasion is the main wear mechanism. However, a higher magnification SEM image (Fig. 11b) reveals that a flake shape debris is going to be delaminated from the worn surface. Considering the backscattered electron micrograph and the EDS analysis (Fig. 11c) from the defined point in this figure, the dark regions in the BSE image contains about 31 wt.% (61 at.%) oxygen and 69 wt.% iron, which means that the oxidation also occurs on the worn surface. Some of the separated particles from the surface of the substrate are placed between the steel pin and the substrate. These particles are deformed and oxidized due to the increment of the temperature. Sticking of these particles to the worn surface during the sliding forms the dark regions in the BSE image. These oxidized regions may also act as a lubricating layer and decrease the friction coefficient. Fig. 12 shows the SEM images from the worn surfaces of the coatings. The worn surfaces of these samples are relatively smooth. The shallow grooves specify the mild abrasive wear. Protrusion of the alumina micro-particles on the worn surface of the micro-composite coating improves the wear resistance of this sample, and the worn grooves are hardly observable at a high magnification SEM micrograph (the image in the corner of Fig. 12b). 4 Conclusions In the present study, two different sized alumina particles (micro/nano) were used to produce the Co/Al2O3 composite coatings. The effect of micro/nano-particles incorporation on the current efficiency, thickness, chemical composition, morphology, hardness, corrosion, and Tribological behavior of the coatings was investigated. The outcome of the results can be summarized as follows:
14
1. Adding the micro-particles to the electrodeposition bath somewhat increased the thickness and current efficiency of the Co electrodeposition. However, the current efficiency was decreased by about 10% by adding nano-particles to the bath. 2. Both the micro and nano-particles were uniformly dispersed within the Co matrix. The weight percent of the co-deposited nano-particles (1 wt.%) was much less than the micro-particles (15 wt.%), but this small amount also caused the morphological changes of the Co film from bumpy to needle-shaped. 3. The incorporation of different sized alumina particles did not change the hcp structure of the Co film. However, the nano-composite coating had different relative texture coefficient values as compared to the Co and the micro-composite coatings. 4. According to the corrosion results, the co-deposition of the nano-particles deteriorated the corrosion resistance of the Co film. The corrosion current density was decreased from 2 to 0.8 mA cm-2 by incorporation of the micro-particles, while it was increased to 3 mA cm-2 by co-deposition of the nano-particles. 5. The Co/micro-Al2O3 coating had a higher hardness (396 Hv), as well as a smaller volume loss (0.05 mm3) and friction coefficient (0.54) than the steel substrate, the pure Co, and the Co/nano-Al2O3 coating. While the hardness of the nano-composite coating was about 12% less than the Co film, it had a better wear resistance.
Declarations of interest: none This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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Figure Captions: Fig. 1. Effect of alumina particles on current efficiency and thickness of the coatings. Fig. 2. SEM micrographs from the surface of the as-deposited (a) Co, (b) Co/microAl2O3, and (c) Co/nano-Al2O3 coatings at two different magnifications. Fig. 3. SEM micrographs from (a) cross-section of the Co film, and the polished surfaces of (b) Co/micro-Al2O3, and (c) Co/nano-Al2O3 composite coatings. Fig. 4. EDS analysis from the surface of the as-deposited (a) Co/micro-Al2O3 and (b) Co/nano- Al2O3 composite coatings. Fig. 5. X-ray diffraction patterns of the pure cobalt and composite coatings. Fig. 6. The microhardness of the pure Co and the composite coatings. Fig. 7. The polarization curves of St37, pure Co, and composite coatings. Fig. 8. The Nyquist plots of the substrate and the coatings. The solid lines show the simulated diagrams by using the presented equivalent circuit. Fig. 9. The Bode plots of the substrate and the coatings. The solid lines show the simulated diagrams. Fig. 10. The diagrams of the coefficient of friction versus the sliding distance of different samples. Fig. 11. (a,b) Secondary electron images from the worn surface of the substrate at two different magnifications. A backscattered electron image has also been shown at the corner of figure (b). (c) EDS analysis from the defined point in figure (b). Fig. 12. SEM images from the worn surfaces of (a) Co, (b) Co/micro-Al2O3, and (c) Co/nano-Al2O3 coatings. 23
Table 1. The relative texture coefficient (RTC) values of (100), (002), (101), and (110) diffraction lines of Co and composite coatings Sample
RTC(100)
RTC(002)
RTC(101)
RTC(110)
Co
92.1
2.9
3.9
1.1
Co/micro-Al2O3
87.6
3.9
4.4
4.1
Co/nano-Al2O3
87.7
-
-
12.3
24
Table 2. The electrochemical parameters of the samples obtained from the polarization curves Ecorr (mV)
icorr (µA cm-2)
-620
Co
Sample
βa (mV
βc (mV
-1
-1
Rp (Ω cm2)
decade )
decade )
15
120
190
2131
-430
2
125
155
15043
Co/micro-Al2O3
-435
0.8
140
165
41162
Co/nano-Al2O3
-450
3
140
170
11127
St37 steel substrate
25
Table 3. The equivalent circuit parameters of the samples Sample
Rs (Ω cm2) Rp (Ω cm2)
Y0 (µΩ-1 sn cm-2)
n
C (µF cm-2)
St37 steel substrate
10.2
1784
838.3
0.74
1317.2
Co
8.9
17785
11.9
0.83
3.0
Co/micro-Al2O3
50.3
24652
9.0
0.73
1.6
Co/nano-Al2O3
11.6
7233
25.4
0.80
5.0
26
Table 4. The volume loss and average friction coefficient of the samples Volume loss (mm3)
Friction coefficient
St37 steel substrate
0.27
0.85
Co
0.10
0.70
Co/micro-Al2O3
0.05
0.54
Co/nano-Al2O3
0.08
0.76
Sample
27
Fig. 1
28
Fig. 2
29
Fig. 3
30
Fig. 4
31
Fig. 5
32
Fig. 6
33
Fig. 7
34
Fig. 8
35
Fig. 9
36
Fig. 10
37
Fig. 11
38
Fig. 12
39
40
Declarations of interest: none This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
S0272-8842(19)33167-0
DOI:
https://doi.org/10.1016/j.ceramint.2019.10.289
Reference:
CERI 23357
To appear in:
Ceramics International
Received Date: 22 August 2019 Revised Date:
23 October 2019
Accepted Date: 31 October 2019
Please cite this article as: S. Mahdavi, A. Asghari-Alamdari, M. Zolola Meibodi, Effect of alumina particle size on characteristics, corrosion, and tribological behavior of Co/Al2O3 composite coatings, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.10.289. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Effect of alumina particle size on characteristics, corrosion, and tribological behavior of Co/Al2O3 composite coatings
S. Mahdavi*, A. Asghari-Alamdari, M. Zolola Meibodi Research Center for Advanced Materials, Faculty of Materials Engineering, Sahand University of Technology, Tabriz, Iran. Postal Code: 5331817634
*
Corresponding author:
Soheil Mahdvai, Ph.D., Assistant Professor, Research Center for Advanced Materials, Faculty of Materials Engineering, Sahand University of Technology, Tabriz, Iran. Email: [email protected], Tel: +98-4133459413, Fax: +98-4133444333 Abstract The electrodeposition technique was used to produce Co, Co/micro-Al2O3, and Co/nano-Al2O3 coatings. The effect of particle size on current efficiency, thickness, composition, morphology, and structure of the coatings was investigated. The potentiodynamic polarization technique and electrochemical impedance spectroscopy was used to study the corrosion behavior of the coatings. Tribological behavior of the micro/nano-composite coatings was also examined, and the results were compared to the pure Co and the steel substrate. The results revealed that the incorporation of alumina nano-particles decreased the current efficiency of Co electrodeposition. Although the weight percent of the co-deposited nano-particles was much less than the micro-particles under the same electrodeposition conditions, their effect on morphology was more pronounced. The Co/micro-Al2O3 coating had a higher hardness and corrosion resistance than the other samples. The incorporation of nano-particles
1
decreased the hardness and polarization resistance of the pure Co by about 15 and 59%, respectively. Unlike the corrosion results, both the composite coatings had better wear resistance than the pure Co film. However, the volume loss of the Co/micro-Al2O3 coating was 1.6 times lower than the Co/nano-Al2O3 electrodeposit. Keywords: Composites (B); Corrosion (C); Wear resistance (C); Al2O3 (D).
1 Introduction Electrodeposition is a well-known technique for the preparation of pure metal, alloy or composite coatings [1-8]. Composite coatings are formed by suspending ceramic particles in an electrodeposition bath and applying an electric field [3]. Co-deposition of ceramic particles within the metal matrix improves properties of pure metals such as hardness, wear and corrosion resistance [4-11]. Therefore, these coatings are extensively used for corrosion and wear-resistant purposes in mechanical, chemical, biomedical and cosmonautic industries [5,12]. Type, amount, size, shape, and dispersion of the reinforcing particles have a key role in the final properties of the composite coatings [8,10]. Different types of ceramic particles such as CeO2 [8], TiN [3,5], TiO2 [7,13], Mn3O4 [14], SiC [15-17], Al2O3 [11], B4C [4], WC [6], ZrC [18], SiO2 [19], CeO2 [8], etc. with various sizes were used in previous research works. Gupta et al. [6] produced Ni matrix composite coatings containing WC particulates with an average size of 10 µm at different electrodeposition conditions. They exhibited that the Ni/WC film had higher microhardness and corrosion resistance than the pure Ni coating. Jian et al. [20] showed that Ni/SiC nano-composite film had better corrosion resistance than pure Ni coating. Corrosion current density of the composite film was about half of the pure Ni one. Wang et al. [9] compared the
2
mechanical properties of Ni and Ni/Al2O3 nano-composite coatings and found that the incorporation of nano-alumina particles increased the hardness and modulus of the Ni matrix. Tolumoye et al. [19] studies revealed that the incorporation rate of the microsized SiO2 particles within the Zn-Ni film was higher than the nano-sized ones. However, they did not compare the properties of these coatings. Effect of ZrC particle size on morphology, hardness and electrochemical behavior of Ni matrix composite coatings have been studied by Zhang et al. [18]. According to their results, the surface morphology of both the micro and nano-composite coatings was less uniform and rougher than the pure Ni film. They also found that the effect of nano-sized ZrC particles on enhancing the hardness and corrosion resistance of Ni films was more significant than micro-sized ones. In the past 30 years, nickel-based micro/nano-composite coatings have been extensively examined [5]. However, in spite of great interest to cobalt matrix composites in a large range of applications, due to their strength, good corrosion and wear resistance, electrocatalytic activity, and magnetic properties [2,12,21-25], studies on these coatings are much more limited than the Ni-based films. Apelt et al. [14] investigated the effect of electrodeposition condition on the incorporation of Mn3O4 powder with an average particle size of 2.5 µm in the cobalt coatings for using on the ferritic interconnect in solid oxide fuel cell (SOFC). They found that the solution pH and the particle loading have the greatest influence on the co-deposition of Mn3O4. Carac et al [8] expressed that microstructure and hardness of Co/micro-CeO2 composite coatings could be influenced by electrolyte composition and process parameters of current density, temperature, and stirring condition. Rudnik et al. [17] investigated the effect of surfactant concentration on the incorporation of SiC micro-particles within the Co matrix. They reported that
3
adding surfactant to the deposition bath increased the co-deposition of particles while decreased the current efficiency. All of the above studies have only focused on the effect of electrodeposition parameters and particles concentration of the bath on characteristics of cobalt composite films. Particle size has also a great influence on the final properties of the coatings [18,19,26]. In this research, micro and nano-sized alumina particles were used for electrodeposition of Co matrix composites. Alumina is an attractive ceramic material because of its relatively low density, notable hardness, high erosion and corrosion resistance, and high melting temperature [27,28], which can improve the corrosion and tribological behavior of the coatings. This study aims to investigate the effect of alumina particle size on characteristics and hardness of the cobalt coatings. Corrosion and tribological behavior of Co/Al2O3 micro/nanocomposites are also examined and the results are compared to the pure Co and the steel substrate. 2 Experimental 2.1 Electrodeposition process Cobalt coatings were electrodeposited from the baths containing 200 g L-1 CoSO4.7H2O, 50 g L-1 H3BO3, 0.4 g L-1 C7H4NNaO3S.2H2O (Sodium saccharin), and 0.1 g L-1 NaC12H25SO4 (SDS) by using the direct current. These values were selected based on our previous research [29]. Current density, temperature, stirring speed, and pH of the bath were 80 mA cm-2, 45˚C, and 3, respectively. The pH was adjusted by using 1M NaOH or 1M H2SO4 solutions. Alumina particles with the average sizes of 2 µm and 50 nm were used for electrodeposition of micro/nano-composite coatings. 15 g L-1 of these particles were added to the distilled water (with the same pH of the main bath) in a separate container.
4
After stirring for 24 h and ultrasonic treatment for 1 h, the suspension was added to the main bath. A double electrode cell was used for electrodeposition of Co and Co/Al2O3 coatings. The anode and cathode were pure cobalt and St 37 steel plates, respectively. Before electrodeposition, the steel substrates were abraded with emery papers (from 120 to 800 grit), and rinsed with distilled water. Degreasing was ultrasonically done in acetone at room temperature for 5 min, and then in an alkaline solution (50 g L-1 Na2CO3, 50 g L-1 Na3PO4.12H2O, 30 g L-1 NaOH) at 70°C for 15 min. Finally, the substrates were pickled in 20% HCl solution. 2.2 Characterization A scanning electron microscope (CAMSCAN MV2300 SEM) and the attached energy dispersive X-ray spectrometer were employed to examine the morphology and chemical composition of the samples. The structure of the coatings was studied by the X-ray diffraction (XRD) technique. The preferred orientation was investigated by using the relative texture coefficient (RTChkl) [13,30]: =∑ In this equation,
/ /
× 100
(1)
represents the diffraction intensities of the (hkl) lines in the XRD
patterns of the samples, and
denotes the diffraction intensities of the randomly
oriented cobalt powder (JCPDS no. 5-0727). Four diffractions lines of (100), (002), (101), and (110) of Co were considered. 2.3 Microhardness measurements Microhardness measurements were carried out by using an applied load of 0.2 N for 15 s. The measurements were conducted on at least five different areas of each sample. 2.4 Corrosion tests
5
Corrosion behavior of the samples was examined by the potentiodynamic polarization and
the
electrochemical
impedance
spectroscopy
(EIS).
A
Zive
SP2
potentiostat/galvanostat was used for this purpose. The tests were performed in a three electrode cell containing 3.5 wt.% NaCl solution at room temperature. The counter electrode was a square platinum plate with the dimensions of 1×1 cm. An Ag/AgCl (3 M KCl) probe was used as the reference electrode. The potential scan rate during the polarization tests was 1 mV/s. The EIS measurements were done at the open circuit potential. The frequency range and the sinusoidal signal amplitude were 100 kHz to 0.1 Hz, and 10 mV, respectively. 2.5 Wear tests Dry sliding pin-on-disc wear tests were done at room temperature. The cylindrical steel pins (1.5Cr, 1C, 0.35Mn, 0.25Si) with the hardness of 64HRC were used as the counterbody. The diameter of the used pins was 5 mm, and they had a curved ends. The normal load, sliding speed, and sliding distance were 4 N, 0.1 ms-1, and 300m, respectively. The friction coefficient was also measured during the tests. 3 Results and discussion 3.1 Current efficiency and thickness of the coatings The effect of adding 15 g L-1 alumina particles with different sizes to the electrodeposition bath on the current efficiency and thickness of the coatings is shown in Fig. 1. Although the electrodeposition current efficiency and thickness of the Co film somewhat increase with co-deposition of the micro-particles, they decrease with the incorporation of the nano-particles. The highest current efficiency of about 90% is obtained in the presence of the micro-alumina particles in the bath. This could be explained by the incorporation of relatively large ceramic particles in the metal matrix,
6
along with decrement of the metallic surface in contact with the solution and increment of the current density, as also reported by Benea et al. [31,32]. While the same concentrations of micro or nano-particles (15 g L-1) was added to the electrodeposition baths, the number and the total surface area of the nano-particles is considerably higher than the micro-sized ones. This can cause an effective increment of the solution resistance [33], as well as a significant rise of the current density during the deposition process, which in turn increases solution pH and the probability of metal hydroxide formation on the cathode surface. The formation of these hydroxides inhibits cobalt ion reduction and reduces the current efficiency. The reasons for the thickness changes are the same. 3.2 Morphology and chemical composition SEM images from the surface of Co, Co/micro-Al2O3, and Co/nano-Al2O3 coatings at two different magnifications have been shown in Fig. 2. The bumpy morphology of Co film becomes coarser and more irregular with the incorporation of the micro-particles, which can be due to the formation of a Co layer on the coarse alumina particles. However, the morphology has changed to the needle-shaped with co-deposition of the alumina nano-particles. The incorporated nano-particles can increase the nucleation sites and change the preferred growth direction, and as a result, the morphology of the coating. SEM images from the polished surfaces of the composite coatings are shown in Fig. 3. The uniform distribution of the particles is noticeable in this figure, indicating suitable preparation of the electrodeposition baths (stirring for 24 h and ultrasonic treatment for 1 h in a separate container containing water). It is also evident that the agglomeration of the particles is negligible, which can be due to the employing of SDS in the
7
electrodeposition bath. The SDS, as an anionic surfactant, forms a negative layer on surface of the particles in the bath, and increases the electrostatic repulsion between them [34]. Therefore, the probability of the agglomeration is reduced. According to the EDS analysis results (Fig. 4), a higher volume fraction of the microalumina particles has been incorporated within the cobalt matrix as compared to the nano-sized ones. The aluminum peaks are clear in the EDS spectrum of Co/micro-Al2O3 composite film. The weight percent of the micro-alumina particles in this composite film is about 15%, which is considerably higher than that for the nano-alumina particles with about 1%. The absorbance and holding time of the particles on the cathode surface determine the amount of their incorporation. It has been said that the fine particles are more prone to agglomeration and detachment from the surface of the substrate, resulting in their lower incorporation [35]. However, assuming that the particles are spherical and by considering the average particle sizes, the mass of a micro-sized alumina particle is about 64000 times higher than the nano-sized one. Therefore, the effect of gravity and turbulence of the electrolyte on the micro-particles, and thus the probability of their detachment, are higher than the nano-particles. Nevertheless, the influence of the codeposition of an alumina micro-particle on the weight percent is equal to the incorporation of about 64000 nano-particles. Thus, although the number of incorporated nano-particles is higher than the number of micro-particles, their weight percent is much lower. 3.3 Structure The X-ray diffraction patterns of the coatings are shown in Fig. 5. All the coatings have a hexagonal close-packed (hcp) structure. The relative texture coefficient of four diffraction lines of the deposits was calculated according to Eq. (1), and have been
8
presented in Table 1. It is seen that the RTC(100) decreases with the incorporation of alumina particles. The RTC(002) and RTC(101) are increased by the co-deposition of micro-particles, but corresponding peaks of (002) and (101) planes are not observed in the XRD pattern of the Co/nano-Al2O3 composite coating. However, the main reason for the morphological changes with the incorporation of the alumina particles can be the variation of RTC(110). The RTC(110) increases from 1.1 to 12.3% with the incorporation of the nano-alumina particles, resulting in the needle-shaped morphology of Co/nanoAl2O3 composite film (Fig. 2c). Matte white appearance of the pure cobalt coating is also changed to the black by the incorporation of the nano-alumina particles.
3.4 Microhardness The microhardness of the pure cobalt and the composite coatings have been compared in Fig. 6. The microhardness of the Co film increases from 318 to 396 Hv with the incorporation of the micro-particles, but it is decreased by about 12% by co-deposition of the nano-sized alumina particles. The hard ceramic particles can restrain the matrix deformation and the motion of the dislocations, resulting in higher hardness values [13,32,35]. The effect of nano-particles on the increment of the hardness should be more pronounced than the micro-particles due to their higher number and the larger total surface area [35]. However, the microhardness is reduced by co-deposition of the nanoparticles, which can be attributed to the structural and morphological changes with the incorporation of alumina nano-particles in this study (Figs. 2 and 5). 3.5 Corrosion behavior
9
The polarization curves of Pure Co and composite coatings are shown in Fig. 7. The polarization curve of the substrate is also presented for comparison. The extracted electrochemical parameters are presented in Table 2. All the coatings have more positive corrosion potential than the substrate, indicating their nobler behavior. The corrosion current densities of the electrodeposits are also lower than the steel substrate, which means that the corrosion rate has been decreased by applying the coatings. Comparing the corrosion current density values, the incorporation of the micro-sized alumina particles improves the corrosion resistance of the Co film. The icorr of this composite is 2.5 times lower than the pure Co coating, which can be due to the reduction of the metallic surface in contact with the corrosive environment with codeposition of the alumina micro-particles. However, the incorporation of nano-particles does not have the desired effect on the corrosion resistance of the Co film. The corrosion resistance has gotten even worse with the incorporation of the nano-particles, while it has been said in the literature that these particles can decrease the icorr through the grain refinement, structural modifications, and embedment in crevices and pores [36,37]. This discrepancy can be a result of the morphological changes with the inclusion of the nano-particles (Fig. 2), which may increase the metallic surface where the corrosion occurs. The Nyquist plots of the samples are shown in Fig. 8. All the plots represent a single semicircle, indicating the presence of one time constant in the equivalent circuit (Fig. 8). For a further investigation of the corrosion behavior at the high frequencies, the Bode plots are shown in Fig. 9. This figure also confirms the correctness of the used equivalent circuit. In this circuit, Rs is the solution resistance, Rp shows the polarization
10
resistance, and CPE is the constant phase element and represents the double layer capacitance [37-39]. The impedance of CPE is calculated by using the following equation [37,40-43]: =
(
(2)
)
where Y0 is the admittance function, j2=-1, ω is the angular frequency, and n is the CPE power (n=απ/2, where α is the phase angle constant). The double layer capacitance can also be calculated by using these parameters and the following equation [41]: =!
"#
×
( "#)/# $
(3)
The calculated electrochemical parameters are presented in Table 3. These results confirm that the incorporation of micro-sized alumina particles within the cobalt matrix has the greatest effect on increasing the corrosion resistance. The polarization resistance of the Co/micro-alumina composite coating is about 13.8, 1.4, and 3.4 times higher than the substrate, the pure Co, and the Co/nano-alumina film, respectively. The double layer capacitance has also been presented in Table 3. The capacity of the substrate is significantly higher than the coatings, which means that the corrosion reactions occur in a larger area of the substrate (abraded to 800 grit) as compared to the electrodeposits. The double layer capacitance of the micro-composite film is smaller than the pure Co and the nano-composite coatings. The smaller capacity of the Co/micro-alumina than the Co film can be due to its coarser morphology (Fig. 2b) and the presence of alumina particles on the surface of this composite coating, which decrease the metallic surface where the electrochemical reactions occur. However, although the nano-particles may decrease the metallic surface, the double layer capacitance of Co/nano-alumina film is higher than the two other coatings. This
11
behavior may be a result of the morphological changes, along with the increment of the porosity and the total surface area (Fig. 2c). 3.6 Tribological behavior The volume loss and the average coefficient of friction of the samples are presented in Table 4. As expected from the hardness results (Fig. 6), the substrate and the Co/microalumina composite coating have the worst and the best wear resistance between the samples, respectively. The wear volume loss of the substrate is 5.4 times higher than the micro-composite film. According to the Archard theory, the wear resistance is directly related to the hardness [44]. As the hardness of the micro-composite coating is higher than the other samples, it also has the minimum volume loss. However, although the Co film is harder than the Co/nano-alumina electrodeposit, its volume loss is 25% higher than the nano-composite coating. The presence of nano-particles can hinder the movement of the grains and the grain boundary migration during the sliding, and thus increase the wear resistance [13,45]. The average coefficient of friction values have also been presented in Table 4. The friction coefficient of Co/micro-alumina coating is lower than the other samples, which is consistent with the volume loss results. However, unlike the volume loss results, the Co/nano-alumina film has a higher friction coefficient than the pure Co coating. The variations of the friction coefficient of the samples with the sliding distance are shown in Fig. 10 for further investigations. The friction coefficient of the substrate steeply increases at the first 50 m of the sliding, and after that remains almost constant. The real contact area between the steel pin and the sample, along with the wear mechanism define the friction coefficient. As the steel pin has a curvature at its end, the real contact area increases with sliding distance and the entrance of the pin to the sample surface.
12
Therefore, the coefficient of friction rises with the sliding distance. The fast increment of the friction coefficient of the substrate at low sliding distances is a result of its low hardness value and rapid penetration of the steel pin into the surface. However, the work hardening of the worn surface or formation of a lubricating tribo-layer, and approaching the real contact area to its maximum value can be the reasons for the almost constant friction coefficient of the substrate after 50 m. The friction coefficient of Co and Co/nano-alumina coatings rises until the end of the wear tests. However, this increment occurs relatively fast at the first 100 m of sliding for the nano-composite coating, while the coefficient of friction of the Co film is uniformly increased over the 300 m. The friction coefficient is increased by the gradual flattening of the surface morphology with sliding. As the nano-composite film is softer than the pure Co coating, its surface morphology is worn out faster. This causes a more rapid increment of the real contact area and the friction coefficient of the nanocomposite coating at the first 100 m of the sliding process. However, the presence of the nano-particles may improve the wear resistance and hinder quick increment of the friction coefficient after the disappearance of the relatively porous and mechanically weak (Fig. 6) needle-shaped morphology (Fig. 2c) at longer sliding distances. In the Co/micro-alumina coating, the coefficient of friction gradually enhances at the first 100 m of the sliding and then remains almost constant. This is due to the flattening of the surface roughness at the early stages, and then a balance between smoothing of the surface and protrusion of the micro-particles at the worn surface. The Protruded particles decrease the metal on metal contact and as a result the friction coefficient and the volume loss.
13
SEM micrographs and EDS analysis from the worn surface of the substrate are shown in Fig. 11. Deep grooves on the worn surface indicate that the abrasion is the main wear mechanism. However, a higher magnification SEM image (Fig. 11b) reveals that a flake shape debris is going to be delaminated from the worn surface. Considering the backscattered electron micrograph and the EDS analysis (Fig. 11c) from the defined point in this figure, the dark regions in the BSE image contains about 31 wt.% (61 at.%) oxygen and 69 wt.% iron, which means that the oxidation also occurs on the worn surface. Some of the separated particles from the surface of the substrate are placed between the steel pin and the substrate. These particles are deformed and oxidized due to the increment of the temperature. Sticking of these particles to the worn surface during the sliding forms the dark regions in the BSE image. These oxidized regions may also act as a lubricating layer and decrease the friction coefficient. Fig. 12 shows the SEM images from the worn surfaces of the coatings. The worn surfaces of these samples are relatively smooth. The shallow grooves specify the mild abrasive wear. Protrusion of the alumina micro-particles on the worn surface of the micro-composite coating improves the wear resistance of this sample, and the worn grooves are hardly observable at a high magnification SEM micrograph (the image in the corner of Fig. 12b). 4 Conclusions In the present study, two different sized alumina particles (micro/nano) were used to produce the Co/Al2O3 composite coatings. The effect of micro/nano-particles incorporation on the current efficiency, thickness, chemical composition, morphology, hardness, corrosion, and Tribological behavior of the coatings was investigated. The outcome of the results can be summarized as follows:
14
1. Adding the micro-particles to the electrodeposition bath somewhat increased the thickness and current efficiency of the Co electrodeposition. However, the current efficiency was decreased by about 10% by adding nano-particles to the bath. 2. Both the micro and nano-particles were uniformly dispersed within the Co matrix. The weight percent of the co-deposited nano-particles (1 wt.%) was much less than the micro-particles (15 wt.%), but this small amount also caused the morphological changes of the Co film from bumpy to needle-shaped. 3. The incorporation of different sized alumina particles did not change the hcp structure of the Co film. However, the nano-composite coating had different relative texture coefficient values as compared to the Co and the micro-composite coatings. 4. According to the corrosion results, the co-deposition of the nano-particles deteriorated the corrosion resistance of the Co film. The corrosion current density was decreased from 2 to 0.8 mA cm-2 by incorporation of the micro-particles, while it was increased to 3 mA cm-2 by co-deposition of the nano-particles. 5. The Co/micro-Al2O3 coating had a higher hardness (396 Hv), as well as a smaller volume loss (0.05 mm3) and friction coefficient (0.54) than the steel substrate, the pure Co, and the Co/nano-Al2O3 coating. While the hardness of the nano-composite coating was about 12% less than the Co film, it had a better wear resistance.
Declarations of interest: none This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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Figure Captions: Fig. 1. Effect of alumina particles on current efficiency and thickness of the coatings. Fig. 2. SEM micrographs from the surface of the as-deposited (a) Co, (b) Co/microAl2O3, and (c) Co/nano-Al2O3 coatings at two different magnifications. Fig. 3. SEM micrographs from (a) cross-section of the Co film, and the polished surfaces of (b) Co/micro-Al2O3, and (c) Co/nano-Al2O3 composite coatings. Fig. 4. EDS analysis from the surface of the as-deposited (a) Co/micro-Al2O3 and (b) Co/nano- Al2O3 composite coatings. Fig. 5. X-ray diffraction patterns of the pure cobalt and composite coatings. Fig. 6. The microhardness of the pure Co and the composite coatings. Fig. 7. The polarization curves of St37, pure Co, and composite coatings. Fig. 8. The Nyquist plots of the substrate and the coatings. The solid lines show the simulated diagrams by using the presented equivalent circuit. Fig. 9. The Bode plots of the substrate and the coatings. The solid lines show the simulated diagrams. Fig. 10. The diagrams of the coefficient of friction versus the sliding distance of different samples. Fig. 11. (a,b) Secondary electron images from the worn surface of the substrate at two different magnifications. A backscattered electron image has also been shown at the corner of figure (b). (c) EDS analysis from the defined point in figure (b). Fig. 12. SEM images from the worn surfaces of (a) Co, (b) Co/micro-Al2O3, and (c) Co/nano-Al2O3 coatings. 23
Table 1. The relative texture coefficient (RTC) values of (100), (002), (101), and (110) diffraction lines of Co and composite coatings Sample
RTC(100)
RTC(002)
RTC(101)
RTC(110)
Co
92.1
2.9
3.9
1.1
Co/micro-Al2O3
87.6
3.9
4.4
4.1
Co/nano-Al2O3
87.7
-
-
12.3
24
Table 2. The electrochemical parameters of the samples obtained from the polarization curves Ecorr (mV)
icorr (µA cm-2)
-620
Co
Sample
βa (mV
βc (mV
-1
-1
Rp (Ω cm2)
decade )
decade )
15
120
190
2131
-430
2
125
155
15043
Co/micro-Al2O3
-435
0.8
140
165
41162
Co/nano-Al2O3
-450
3
140
170
11127
St37 steel substrate
25
Table 3. The equivalent circuit parameters of the samples Sample
Rs (Ω cm2) Rp (Ω cm2)
Y0 (µΩ-1 sn cm-2)
n
C (µF cm-2)
St37 steel substrate
10.2
1784
838.3
0.74
1317.2
Co
8.9
17785
11.9
0.83
3.0
Co/micro-Al2O3
50.3
24652
9.0
0.73
1.6
Co/nano-Al2O3
11.6
7233
25.4
0.80
5.0
26
Table 4. The volume loss and average friction coefficient of the samples Volume loss (mm3)
Friction coefficient
St37 steel substrate
0.27
0.85
Co
0.10
0.70
Co/micro-Al2O3
0.05
0.54
Co/nano-Al2O3
0.08
0.76
Sample
27
Fig. 1
28
Fig. 2
29
Fig. 3
30
Fig. 4
31
Fig. 5
32
Fig. 6
33
Fig. 7
34
Fig. 8
35
Fig. 9
36
Fig. 10
37
Fig. 11
38
Fig. 12
39
40
Declarations of interest: none This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.