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Microstructure and characterization of a novel cobalt coating prepared by cathode plasma electrolytic deposition
Applied Surface Science 353 (2015) 1320–1325
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Microstructure and characterization of a novel cobalt coating prepared by cathode plasma electrolytic deposition Cheng Quan, Yedong He ∗ Beijing Key Laboratory for Corrosion, Erosion and Surface Technology, University of Science and Technology Beijing, 100083 Beijing, China
a r t i c l e
i n f o
Article history: Received 30 April 2015 Received in revised form 7 July 2015 Accepted 13 July 2015 Available online 15 July 2015 Keywords: Cathode plasma electrolytic deposition Microstructure Nanocrystalline Cobalt coating
a b s t r a c t A novel cobalt coating was prepared by cathode plasma electrolytic deposition (CPED). The kinetics of the electrode process in cathode plasma electrolytic deposition was studied. The composition and microstructure of the deposited coatings were investigated by SEM, EDS, XRD and TEM. The novel cobalt coatings were dense and uniform, showing a typically molten morphology, and were deposited with a rather fast rate. Different from the coatings prepared by conventional electrodeposition or chemical plating, pure cobalt coatings with face center cubic (fcc) structure were obtained by CPED. The deposited coatings were nanocrystalline structure with an average grain size of 40–50 nm, exhibited high hardness, excellent adhesion with the stainless steels, and superior wear resistance. The properties of the novel cobalt coatings prepared by CPED have been improved significantly, as compared with that prepared by conventional methods. It reveals that cathode plasma electrolytic deposition is an effective way to prepare novel cobalt coatings with high quality. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Plasma electrolysis is an advanced surface engineering technique, which is a hybrid of conventional electrolysis and atmospheric plasma processing [1]. It has been a field of research for many years, since the first significant work done by Kellogg in 1950 [2]. There are two kinds of electrode processes in plasma electrolysis: an anodic process such as the plasma electrolytic oxidation (PEO), and a cathodic process such as the plasma electrolytic saturation (PES) [3]. Most studies are concentrated on the anodic process of plasma electrolysis in the past decades, such as the micro-arc oxidation (MAO) [4–9], and very few are on the cathodic process. However, in recent years, more and more researchers are paying attention to the study on the cathodic process of plasma electrolysis, for example, the plasma electrolytic nitriding/carburizing/boriding [10–14], surface cleaning [15–17] and coating [3], and they have made great progress. Cathode plasma electrolytic deposition (CPED) is an important part of plasma electrolysis. It is a cathodic atmospheric plasma process which has been successfully used in preparing metal coatings (such as Ni [18], Cr [19], Al [20], Zn [15]), ceramic coatings (such as Al2 O3 [21], ZrO2
∗ Corresponding author. E-mail address: [email protected] (Y. He). http://dx.doi.org/10.1016/j.apsusc.2015.07.080 0169-4332/© 2015 Elsevier B.V. All rights reserved.
[22]), carbon nanotubes and diamond-like carbon films [23,24]. The coatings prepared by CPED exhibit excellent adhesion to the substrate, high hardness, nanocrystalline structure, and favorable corrosion and wear resistance [15,18–20,25–27]. In CPED process, the presence of the high-temperature and high-enthalpy plasma arcs over the cathode (sample) will lead to the melting of localized micro-zones on the surface. The subsequent quenching by the electrolyte and the mechanical effects of the plasma discharge have shown to form coatings with unique microstructure and characteristics [1,3]. Cobalt and its alloys are important engineering materials in many applications due to their unique properties, for instance, the wear and corrosion resistance, heat conductivity, magnetism and electrocatalysis [28–30]. Jorge Vazquez-Arenas et al. have proposed the transient and steady-state model of cobalt deposition in borate-sulfate solutions [31]. Lupi et al. have studied the composition, morphology, structural aspects of Ni–Co alloy coatings [32]. Babak Bakhit et al. have investigated the corrosion resistance of Ni–Co/SiC nanocomposite coatings electrodeposited by sediment co-deposition [33]. Fei Cai et al. have analyzed the effects of Co contents on the microstructures and properties of electrodeposited Ni–Co–Al composite coatings [34]. Han Xiao et al. have found a novel shell-like cobalt nanostructure by galvanostatic electrochemical deposition which exhibits prominent superhydrophobic property [35]. Particularly, there have been
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Table 1 The composition of electrolyte and the parameters for CPED. Parameters
CPED
CoSO4 (g L−1 ) H2 SO4 (g L−1 ) Depositing voltage (V) Depositing current density (A cm−2 ) Time (min)
25 60 75–85 2–4 3
composition of the electrolyte for CPED was based on the previous study [47]. The detailed parameters for cathode plasma electrolytic depositing Co coatings are shown in Table 1. 2.2. Composition and microstructure analysis
Fig. 1. The electrolytic cell for cathode plasma electrolytic deposition.
increasing interests concentrated on the magnetic applications in the fields such as micro-electromechanical system devices, data storage media, magnetic recording and reading heads, and so on [36–40]. In recent years, the electrodeposition and chemical plating of cobalt and its alloys have attracted a lot of attentions, for the usability of cobalt and cobalt alloy coatings in the protective and functional fields [41–43]. However, there are many factors which will influence the properties of the coatings. Firstly, it is well known that the nanocrystalline coatings show high hardness and considerably good wear resistance with respect to their conventional microcrystalline coatings [44]. Secondly, three different crystalline structures—face center cubic (fcc) structure, hexagonal close packed (hcp) structure, and the mix structure of fcc and hcp—have been found in the cobalt and cobalt alloy coatings with different depositing parameters [45]. The coatings with different phase structures exhibit relatively large differences in the morphology, the grain size, and the properties such as the corrosion resistance [32,46]. In certain cases, there is phase transition of the coatings in the depositing process, which will have an effect on the properties of the coatings. In the present work, a novel cobalt coating was developed by cathode plasma electrolytic deposition. The kinetics of the electrode process for preparing cobalt coatings by CPED was studied. The microstructure and mechanical properties of the deposited coatings were investigated. 2. Experimental details 2.1. Materials and methods Fig. 1 shows the electrolytic cell for cathode plasma electrolytic deposition to (a) measure the depositing current density–voltage (I–U) curve and (b) prepare Co coatings. A platinum plate electrode of 100 mm × 50 mm was used as the anode. 304 stainless steels of 10 mm × 10 mm × 2 mm as the cathode samples were prepared by usual machining. After being polished to 2000-grit and cleaned in the ethanol ultrasonically, the samples were embedded into the polytetrafluoroethylene groove to ensure that only the side of 10 mm × 10 mm was toward the anode. The distance between the anode and the cathode was approximately 20 mm. A DC power supply (500 V/30 A) was used for cathode plasma electrolytic deposition. A computer was connected to the DC power supply to output the values of the instantaneous current and voltage in CPED process. In process a, the voltage was increased steadily with the rate of 0.5 V/s, from 0 to 100 V. In process b, the voltage was increased to the designated value directly and kept at this value for 3 min. The
The scanning electron microscope (SEM, JSM-6480A) was used to investigate the surface and cross-sectional morphologies of the deposited coating. The thickness of the coating was estimated from the cross-sectional morphology, and the depositing rate of cobalt was evaluated qualitatively. The energy dispersive spectrometer (EDS) was used to analyze the composition of the deposited coating. The X-ray diffraction (XRD, PW3710, Phillips) analysis was performed to determine the phase constituent of the deposited coating, at a scanning rate of 6◦ /min in the range of 10–100◦ , with ˚ Cu K␣ radiation ( = 1.5418 A). The high-resolution transmission electron microscope (HRTEM, TECNAI F20) was used to reveal the grain size of the deposited coating. In the TEM analysis, the deposited coating was prepared into powders by mechanical method. The powders were dispersed in ethanol ultrasonically for 20 min until they were mixed uniformly. The mixture was taken out by glass capillary and dropped on the Cu grid (3 mm) as the TEM sample. 2.3. Mechanical properties The Vickers microhardness tester (HXD-1000TMSC/LCD) was used to measure the microhardness values of the deposited coating, using a load of 50 g with the holding time of 15 s. The surface and cross-sectional microhardness of each sample were measured, respectively for more than five times and the average microhardness values were calculated to reduce the errors. Three samples were used for the microhardness measurements to ensure the accuracy of the results. The automatic scratch tester (WS-2005, China) was used to evaluate the adhesion between the deposited coating and the substrate, according to the change of the Acoustic Emission (AE) signal. The scratching procedures were carried out by a diamond indenter with a rounded tip (200 m tip radius) at room temperature. The diamond tip was loaded against the surface of the deposited coating at an initial load of 0.5 N. Then it was sliding on the surface under linearly increasing load at a rate of 100 N/min until the load was reaching the maximum value of 200 N. The scratching speed was 5 mm/min. Three samples were used for the scratch tests and three single scratches of each sample were performed in order to ensure the result repeatability. The high temperature tribometer (HT-600, China) was used to investigate the friction and wear properties of the deposited coating. Before the friction and wear test, the initial weight of each sample was measured and recorded. The friction experiment was conducted using a ball-on-disk tester in air at room temperature. A WC-8Co ball with a diameter of 2 mm was used. The applied load was 200 N and the rotating speed of the motor was 2000 r/min. The friction coefficient was measured continuously during the experiment. After a duration of 20 min, the experiment was stopped and the final weight of the sample was measured and recorded. The
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Fig. 4. EDS analysis of the deposited coating prepared by CPED.
Fig. 2. Depositing current density–voltage (I–U) curve of CPED process.
weight loss of the sample was calculated to evaluate the wear resistance of the deposited coating. One bare sample and three coated samples were used for the friction and wear test and three independent measurements of each sample have been done to reduce the errors. 3. Results and discussion Fig. 2 shows the depositing current density–voltage (I–U) curve in the process of cathode plasma electrolytic depositing Co coatings, in the electrolyte with 25 g L−1 CoSO4 and 60 g L−1 H2 SO4 . At a relatively low voltage, the increase of the voltage will lead to a proportional rise in the current density (Section A). The kinetics of the electrode process conforms to the Faraday’s law and the I–U characteristics almost follow the Ohm’s law. With the subsequent increase of the voltage, the rise of the current density is gradually limited by a partial shielding action of hydrogen evolution reaction over the cathode surface, and the areas where the electrode remains in contact with the electrolyte are reduced greatly (Section B) [3,15]. At the peak of the curve, the cathode is covered with a continuous gaseous envelope of low electrical conductivity. Nearly all the voltage drop across the electrolytic cell is concentrating in this region. The strength of the electric field within this region is so large (about 106 –108 V/m [3]) that the gaseous envelope is broken through with discharge phenomenon (critical condition). Bright micro-arcs can be observed on the surface of the cathode. Owing to the generation and energy release of the plasma arcs, the current is oscillating [1]. With the initial increase of the voltage beyond the critical value, there are more and more uniform and tiny micro-arcs generating and acting on the surface of the sample (Section C). In this condition, the deposits can be modified by the micro-arcs sufficiently.
However, it will do harm to the sample if excessively high voltage is applied since the micro-arcs are too dense and severe. Therefore, the electrolytic parameters of section C in the I–U curve were selected for cathode plasma electrolytic depositing Co coatings. Fig. 3 shows the SEM micrographs of the surface and crosssection of the deposited coating prepared by CPED. From Fig. 3(a) it can be found that the surface morphology of the deposited coating prepared by CPED is different from that prepared by conventional electrodeposition. The deposited coating is dense and uniform, showing molten morphology. This is due to the effect of plasma arcs. In CPED process, the generated plasmas are of high temperature and high enthalpy. The local temperature of the arcing area in the near-electrode region can reach up to 2000 ◦ C [48], and the transient temperature of the plasma arcs is calculated at about 8000 K [49]. Therefore, the deposits can be melted by plasma arcs completely. However, the temperature drops down rapidly with the energy release of the plasmas, causing freezing of the deposits on the substrate [15]. This process is repeated continuously, so the deposits can be modified by plasma arcs sufficiently. As a result, the defects are made up, and the deposited coating becomes uniform and dense with a typically molten morphology. From the cross-sectional micrograph in Fig. 3(b), it can be estimated that the thickness of the deposited coating is approximately 20–25 m, which indicates that the depositing rate of CPED is much faster than that of conventional electrodeposition. Fig. 4 shows the EDS analysis of the deposited coating prepared by CPED. It indicates that the coating is composed of only cobalt without other elements. No other compounds have been deposited as well. This is different from the conventional electrodeposition of Co coatings. In conventional electrodeposition process, some additives are necessary in the electrolyte, for example, H3 BO3 as the buffering agent, Na2 SO4 as the conductive salt and so on. The selection of the anode is mainly concentrated on high purity cobalt, in order to improve the dissolving efficiency of the anode. However,
Fig. 3. SEM micrographs of the deposited coating prepared by CPED: (a) surface and (b) cross-section.
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Fig. 7. TEM micrograph of the cobalt deposits prepared by CPED. Fig. 5. XRD patterns of the cobalt deposits under the electrolytic parameters of section A and B in the I–U curve.
Fig. 6. XRD pattern of the deposited Co coating prepared under the electrolytic parameters of section C in the I–U curve by CPED.
there is no restriction on the choice of anode in CPED process. The inert electrodes such as platinum and graphite are also suitable to be selected as the anode, since they can reduce the change of Co2+ concentration effectively. Pure Co coatings can be obtained directly from the cobalt sulfate or chloride electrolyte with a high depositing rate. The process is simplified. Fig. 5 shows the XRD patterns of the cobalt deposits prepared under the electrolytic parameters of section A and B in the I–U curve. It is found that all of the diffraction peaks in pattern A and B match well with the standard peaks of cobalt (JCPDS No. 05-0727), and the substrate peaks have no effect on these peaks. The diffraction peaks at 2 = 41.7◦ , 44.8◦ , 47.6◦ , 75.9◦ , 92.5◦ correspond to the crystallographic planes (1 0 0), (0 0 2), (1 0 1), (1 1 0) and (1 1 2) of cobalt, and all of them are indexed to the hcp phase. This is basically the same with the Co coatings prepared by conventional electrodeposition or chemical plating, which are also with hcp structure. The intensity of the diffraction peaks are similar under the electrolytic parameters of section A, and the grains mainly grow along the crystallographic planes (0 0 2) and (1 1 0) (Pattern A). However, with the rise of the voltage ahead of the arcing value, the intensity of the characteristic diffraction peak at 2 = 75.9◦ is increased obviously under the electrolytic parameters of section B, which indicates that the growth of grains with (1 1 0) plane is preferred (Pattern B). Fig. 6 shows the XRD pattern of the deposited Co coating prepared under the electrolytic parameters of section C in the I–U curve by CPED. As there are only diffraction peaks matching with the
standard peaks of cobalt (JCPDS No. 15-0806) without any other peaks, the result from the XRD pattern of the deposited Co coating is consistent with that from EDS analysis. However, the diffraction peaks in pattern C are different from that of the cobalt with hcp structure. In Fig. 6, the intensity of the characteristic diffraction peak at 2 = 44.2◦ is increased obviously. There appears a diffraction peak at 2 = 51.5◦ , which is a characteristic diffraction peak of cobalt with fcc structure. The diffraction peaks at 2 = 75.9◦ and 92.2◦ also correspond to the crystallographic planes (2 2 0) and (3 1 1) of cobalt with fcc structure. It reveals that the structure of Co coating is changed from hcp to fcc with the thorough modification of plasma arcs. This is different from the conventional electrodeposition or chemical plating process, because the Co coatings prepared by conventional methods are always with hcp structure. They need to be treated with annealing to be transformed into fcc structure in some cases, in order to improve their dielectric and soft magnetic properties [50,51]. It is complicated and inconvenient. However, Co coatings with fcc structure can be obtained by CPED directly without any heat treatment. As there is no very broad peak in pattern C, the deposited Co coating shows crystalline structure. Fig. 7 shows the TEM micrograph of the cobalt deposits prepared by CPED. It can be estimated that the grain size is approximately 40–50 nm, which is far much smaller than that of conventional electrodeposition. During the CPED process, the high voltage between the electrodes causes the concentration of the positive ions in the electrolyte close to the cathode, especially on the surface of the gas envelope. This results in the high localized electric field strength between the positive charges and the cathode. The power generated from the near-electrode area is estimated to be typically 0.1–1 MW/m2 , which is very significant [3]. The temperature of the plasmas is very high as is mentioned above, and the plasma discharge temperature is much higher, reaching up to 6000–7000 K [52]. The high-temperature plasmas will lead to the localized melting of the deposits. These deposits are surrounded by the relatively cool electrolyte and the substrate, thereby resulting in the freezing of them. It is reported that the duration of the plasma discharge process is expected to be 10−6 s for each individual event and the cooling rate can reach 108 K/s [3]. The freezing that follows the localized melting causes a quenching effect, leading to form an ultra-fine microstructure of the deposited coating. The nanocrystalline grains formed by the CPED process will offer potential to improve the properties of the deposited Co coatings as compared with the large-grained Co coatings. Fig. 8 shows the microhardness of the surface and cross-section of the deposited Co coatings prepared by CPED. The surface and cross-sectional microhardness of the three samples are 709.5 HV and 744.7 HV (sample 1), 753.7 HV and 838.2 HV (sample 2), 691.6 HV and 710.8 HV (sample 3), respectively. No significant
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Fig. 8. The microhardness of the surface and cross-section of the deposited Co coatings prepared by CPED.
Fig. 10. The friction coefficient curves of the substrate and the deposited Co coating prepared by CPED under the load of 200 N. Table 2 The comparison of the weight loss in the friction and wear test.
Substrate With Co coating
Fig. 9. The scratch test of the deposited Co coating prepared by CPED.
change of the microhardness is observed with the surface and the cross-section of the deposited Co coating, even with different samples. As is reported, the microhardness of conventional Co coatings is about 106.4 HV. It reveals that the microhardness of the Co coatings prepared by CPED is increased greatly by comparing with that prepared by conventional electrodeposition. This is mainly attributed to the grain refinement of rapid quenching in CPED process. As a result, the nano-sized grain structure contributes to the increase of the microhardness of the deposited Co coatings prepared by CPED. Fig. 9 shows the scratch test of the deposited Co coating prepared by CPED. It can be found that there is a peak on the Acoustic Emission (AE) signal at the load of 108.9 N, which corresponds to the critical load for the failure of the deposited coating. As the welldefined failure of the deposited coating can represent the coating detachment from the substrate, the critical load can be considered approximately as the qualitative measure of the coating–substrate adhesion [53]. With the linear increase of the load in the x direction, the AE signal changes continuously, which indicates that the deposited Co coating has been damaged. It has been reported that there is electrolyte elemental diffusion into the surface of the electrode during plasma electrolytic heating process [48]. Also, there is diffusion of the elements inward to the substrate and outward to the coating in CPED process, so the deposited coating exhibits excellent adhesion with the substrate [1]. From the SEM micrograph of the cross-section (Fig. 3(b)), it can be found that the deposited Co coating is dense and conformal to the substrate as well. The result of the scratch test is consistent with the observation of the cross-sectional morphology of the deposited coating.
Initial weight (g)
Final weight (g)
Weight loss (mg)
9.56703 9.56719
9.56411 9.56704
2.92 0.15
Fig. 10 shows the friction coefficient curves of the substrate and deposited Co coating prepared by CPED under the load of 200 N. It is clear that the friction coefficient of the substrate is much higher than that of the deposited Co coating prepared by CPED. There are many factors which will influence the friction coefficients of the materials, such as the change of load, the speed of rotation, the temperature and so on. However, the result of the friction test reveals that the tribological property of the deposited Co coating is relatively good. The comparison of the weight loss in the friction and wear test is shown in Table 2. It can be found that the weight loss of the sample coated with Co coating by CPED is far much smaller by comparing with that of the bare substrate. These values demonstrate that the deposited Co coating exhibits superior wear resistance. This is more likely due to the localized melting of the surface region and its subsequent quenching by the hypothermal electrolyte, for which the deposited Co coating is plasma enhanced. The elemental diffusion in the bonding layer and the nanocrystalline structure may also contribute to the superior wear resistance of the deposited coating prepared by CPED [1]. 4. Conclusions In conclusion, a novel cobalt coating was prepared by cathode plasma electrolytic deposition. The best electrolytic parameters for cathode plasma electrolytic depositing cobalt coatings were determined by the depositing current density–voltage (I–U) curve. The novel cobalt coating is dense and uniform with a typically molten morphology. The depositing rate of CPED is much faster than that of conventional methods. EDS analysis indicates that the deposited coating is composed of only cobalt without other elements. X-ray diffraction analysis reveals that the coating is pure cobalt with face center cubic structure, which is different from the coatings prepared by conventional electrodeposition or chemical plating. TEM analysis shows that the deposited cobalt coating is nanocrystalline structure with an average grain size of 40–50 nm. The microhardness of the coating is up to approximately 700 HV without any other processing. The adhesion between the coating and the substrate is about 108.9 N. The friction coefficient and weight loss of the coated sample are much smaller than that of the bare substrate, which indicates that the wear resistance of the coating is excellent. The
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microstructure and properties of the novel cobalt coatings prepared by CPED are very different from that prepared by conventional methods. All of the above are due to the effect of plasmas. With the high-temperature and high-enthalpy plasmas, the deposits are melted, the defects are made up, the grains are refined, and the mechanical properties are improved. It reveals that cathode plasma electrolytic deposition is an effective way to prepare novel cobalt coatings with high quality. Acknowledgement The authors would like to thank the National Natural Science Foundation of China (Grant No. 51171021) for the support of this work. References [1] P. Gupta, G. Tenhundfeld, E. Daigle, D. Ryabkov, Electrolytic plasma technology: science and engineering—an overview, Surf. Coat. Technol. 201 (2007) 8746–8760. [2] H.H. Kellogg, Anode effect in aqueous electrolysis, J. Electrochem. Soc. 97 (1950) 133–142. [3] A. Yerokhin, X. Nie, A. Leyland, A. Matthews, S. Dowey, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (1999) 73–93. [4] X. Nie, A. Leyland, A. Matthews, Deposition of layered bioceramic hydroxyapatite/TiO2 coatings on titanium alloys using a hybrid technique of micro-arc oxidation and electrophoresis, Surf. Coat. Technol. 125 (2000) 407–414. [5] H. Guo, M. An, Growth of ceramic coatings on AZ91D magnesium alloys by micro-arc oxidation in aluminate–fluoride solutions and evaluation of corrosion resistance, Appl. Surf. Sci. 246 (2005) 229–238. [6] L.-H. Li, Y.-M. Kong, H.-W. Kim, Y.-W. Kim, H.-E. Kim, S.-J. Heo, J.-Y. Koak, Improved biological performance of Ti implants due to surface modification by micro-arc oxidation, Biomaterials 25 (2004) 2867–2875. [7] H. Guo, M. An, H. Huo, S. Xu, L. Wu, Microstructure characteristic of ceramic coatings fabricated on magnesium alloys by micro-arc oxidation in alkaline silicate solutions, Appl. Surf. Sci. 252 (2006) 7911–7916. [8] X. Nie, A. Leyland, H. Song, A. Yerokhin, S. Dowey, A. Matthews, Thickness effects on the mechanical properties of micro-arc discharge oxide coatings on aluminium alloys, Surf. Coat. Technol. 116 (1999) 1055–1060. [9] D.-Y. Kim, M. Kim, H.-E. Kim, Y.-H. Koh, H.-W. Kim, J.-H. Jang, Formation of hydroxyapatite within porous TiO2 layer by micro-arc oxidation coupled with electrophoretic deposition, Acta Biomater. 5 (2009) 2196–2205. [10] D.-J. Shen, Y.-L. Wang, P. Nash, G.-Z. Xing, A novel method of surface modification for steel by plasma electrolysis carbonitriding, Mater. Sci. Eng., A 458 (2007) 240–243. [11] M. Béjar, R. Henríquez, Surface hardening of steel by plasma-electrolysis boronizing, Mater. Des. 30 (2009) 1726–1728. [12] J. Guo, H. Wang, J. Zhu, K. Zheng, M. Zhu, H. Yan, M. Yoshimura, Boron nitride synthesized at ambient pressure and room temperature by plasma electrolysis, Electrochem. Commun. 9 (2007) 1824–1827. [13] F. C¸avus¸lu, M. Usta, Kinetics and mechanical study of plasma electrolytic carburizing for pure iron, Appl. Surf. Sci. 257 (2011) 4014–4020. [14] P. Taheri, C. Dehghanian, M. Aliofkhazraei, A.S. Rouhaghdam, Nanocrystalline structure produced by complex surface treatments: plasma electrolytic nitrocarburizing, boronitriding, borocarburizing, and borocarbonitriding, Plasma Process. Polym. 4 (2007) S721–S727. [15] E. Meletis, X. Nie, F. Wang, J. Jiang, Electrolytic plasma processing for cleaning and metal-coating of steel surfaces, Surf. Coat. Technol. 150 (2002) 246–256. [16] B.O. Aronsson, J. Lausmaa, B. Kasemo, Glow discharge plasma treatment for surface cleaning and modification of metallic biomaterials, J. Biomed. Mater. Res. 35 (1997) 49–73. [17] P. Krüger, R. Knes, J. Friedrich, Surface cleaning by plasma-enhanced desorption of contaminants (PEDC), Surf. Coat. Technol. 112 (1999) 240–244. [18] G. Zhao, Y. He, Plasma electroplating Ni coating on pure copper sheet—the effects of H2 SO4 concentration on the microstructure and mechanical properties, Surf. Coat. Technol. 206 (2012) 4411–4416. [19] C. Quan, Y. He, Properties of nanocrystalline Cr coatings prepared by cathode plasma electrolytic deposition from trivalent chromium electrolyte, Surf. Coat. Technol. 269 (2015) 319–323. [20] M. Aliofkhazraei, A. Sabour Roohaghdam, A novel method for preparing aluminum diffusion coating by nanocrystalline plasma electrolysis, Electrochem. Commun. 9 (2007) 2686–2691. [21] E. Bahadori, S. Javadpour, M. Shariat, F. Mahzoon, Preparation and properties of ceramic Al2 O3 coating as TBCs on MCrAly layer applied on Inconel alloy by cathodic plasma electrolytic deposition, Surf. Coat. Technol. 228 (2013) S611–S614. [22] Z. Yao, H. Gao, Z. Jiang, F. Wang, Structure and properties of ZrO2 ceramic coatings on AZ91D Mg alloy by plasma electrolytic oxidation, J. Am. Ceram. Soc. 91 (2008) 555–558.
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Microstructure and characterization of a novel cobalt coating prepared by cathode plasma electrolytic deposition Cheng Quan, Yedong He ∗ Beijing Key Laboratory for Corrosion, Erosion and Surface Technology, University of Science and Technology Beijing, 100083 Beijing, China
a r t i c l e
i n f o
Article history: Received 30 April 2015 Received in revised form 7 July 2015 Accepted 13 July 2015 Available online 15 July 2015 Keywords: Cathode plasma electrolytic deposition Microstructure Nanocrystalline Cobalt coating
a b s t r a c t A novel cobalt coating was prepared by cathode plasma electrolytic deposition (CPED). The kinetics of the electrode process in cathode plasma electrolytic deposition was studied. The composition and microstructure of the deposited coatings were investigated by SEM, EDS, XRD and TEM. The novel cobalt coatings were dense and uniform, showing a typically molten morphology, and were deposited with a rather fast rate. Different from the coatings prepared by conventional electrodeposition or chemical plating, pure cobalt coatings with face center cubic (fcc) structure were obtained by CPED. The deposited coatings were nanocrystalline structure with an average grain size of 40–50 nm, exhibited high hardness, excellent adhesion with the stainless steels, and superior wear resistance. The properties of the novel cobalt coatings prepared by CPED have been improved significantly, as compared with that prepared by conventional methods. It reveals that cathode plasma electrolytic deposition is an effective way to prepare novel cobalt coatings with high quality. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Plasma electrolysis is an advanced surface engineering technique, which is a hybrid of conventional electrolysis and atmospheric plasma processing [1]. It has been a field of research for many years, since the first significant work done by Kellogg in 1950 [2]. There are two kinds of electrode processes in plasma electrolysis: an anodic process such as the plasma electrolytic oxidation (PEO), and a cathodic process such as the plasma electrolytic saturation (PES) [3]. Most studies are concentrated on the anodic process of plasma electrolysis in the past decades, such as the micro-arc oxidation (MAO) [4–9], and very few are on the cathodic process. However, in recent years, more and more researchers are paying attention to the study on the cathodic process of plasma electrolysis, for example, the plasma electrolytic nitriding/carburizing/boriding [10–14], surface cleaning [15–17] and coating [3], and they have made great progress. Cathode plasma electrolytic deposition (CPED) is an important part of plasma electrolysis. It is a cathodic atmospheric plasma process which has been successfully used in preparing metal coatings (such as Ni [18], Cr [19], Al [20], Zn [15]), ceramic coatings (such as Al2 O3 [21], ZrO2
∗ Corresponding author. E-mail address: [email protected] (Y. He). http://dx.doi.org/10.1016/j.apsusc.2015.07.080 0169-4332/© 2015 Elsevier B.V. All rights reserved.
[22]), carbon nanotubes and diamond-like carbon films [23,24]. The coatings prepared by CPED exhibit excellent adhesion to the substrate, high hardness, nanocrystalline structure, and favorable corrosion and wear resistance [15,18–20,25–27]. In CPED process, the presence of the high-temperature and high-enthalpy plasma arcs over the cathode (sample) will lead to the melting of localized micro-zones on the surface. The subsequent quenching by the electrolyte and the mechanical effects of the plasma discharge have shown to form coatings with unique microstructure and characteristics [1,3]. Cobalt and its alloys are important engineering materials in many applications due to their unique properties, for instance, the wear and corrosion resistance, heat conductivity, magnetism and electrocatalysis [28–30]. Jorge Vazquez-Arenas et al. have proposed the transient and steady-state model of cobalt deposition in borate-sulfate solutions [31]. Lupi et al. have studied the composition, morphology, structural aspects of Ni–Co alloy coatings [32]. Babak Bakhit et al. have investigated the corrosion resistance of Ni–Co/SiC nanocomposite coatings electrodeposited by sediment co-deposition [33]. Fei Cai et al. have analyzed the effects of Co contents on the microstructures and properties of electrodeposited Ni–Co–Al composite coatings [34]. Han Xiao et al. have found a novel shell-like cobalt nanostructure by galvanostatic electrochemical deposition which exhibits prominent superhydrophobic property [35]. Particularly, there have been
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Table 1 The composition of electrolyte and the parameters for CPED. Parameters
CPED
CoSO4 (g L−1 ) H2 SO4 (g L−1 ) Depositing voltage (V) Depositing current density (A cm−2 ) Time (min)
25 60 75–85 2–4 3
composition of the electrolyte for CPED was based on the previous study [47]. The detailed parameters for cathode plasma electrolytic depositing Co coatings are shown in Table 1. 2.2. Composition and microstructure analysis
Fig. 1. The electrolytic cell for cathode plasma electrolytic deposition.
increasing interests concentrated on the magnetic applications in the fields such as micro-electromechanical system devices, data storage media, magnetic recording and reading heads, and so on [36–40]. In recent years, the electrodeposition and chemical plating of cobalt and its alloys have attracted a lot of attentions, for the usability of cobalt and cobalt alloy coatings in the protective and functional fields [41–43]. However, there are many factors which will influence the properties of the coatings. Firstly, it is well known that the nanocrystalline coatings show high hardness and considerably good wear resistance with respect to their conventional microcrystalline coatings [44]. Secondly, three different crystalline structures—face center cubic (fcc) structure, hexagonal close packed (hcp) structure, and the mix structure of fcc and hcp—have been found in the cobalt and cobalt alloy coatings with different depositing parameters [45]. The coatings with different phase structures exhibit relatively large differences in the morphology, the grain size, and the properties such as the corrosion resistance [32,46]. In certain cases, there is phase transition of the coatings in the depositing process, which will have an effect on the properties of the coatings. In the present work, a novel cobalt coating was developed by cathode plasma electrolytic deposition. The kinetics of the electrode process for preparing cobalt coatings by CPED was studied. The microstructure and mechanical properties of the deposited coatings were investigated. 2. Experimental details 2.1. Materials and methods Fig. 1 shows the electrolytic cell for cathode plasma electrolytic deposition to (a) measure the depositing current density–voltage (I–U) curve and (b) prepare Co coatings. A platinum plate electrode of 100 mm × 50 mm was used as the anode. 304 stainless steels of 10 mm × 10 mm × 2 mm as the cathode samples were prepared by usual machining. After being polished to 2000-grit and cleaned in the ethanol ultrasonically, the samples were embedded into the polytetrafluoroethylene groove to ensure that only the side of 10 mm × 10 mm was toward the anode. The distance between the anode and the cathode was approximately 20 mm. A DC power supply (500 V/30 A) was used for cathode plasma electrolytic deposition. A computer was connected to the DC power supply to output the values of the instantaneous current and voltage in CPED process. In process a, the voltage was increased steadily with the rate of 0.5 V/s, from 0 to 100 V. In process b, the voltage was increased to the designated value directly and kept at this value for 3 min. The
The scanning electron microscope (SEM, JSM-6480A) was used to investigate the surface and cross-sectional morphologies of the deposited coating. The thickness of the coating was estimated from the cross-sectional morphology, and the depositing rate of cobalt was evaluated qualitatively. The energy dispersive spectrometer (EDS) was used to analyze the composition of the deposited coating. The X-ray diffraction (XRD, PW3710, Phillips) analysis was performed to determine the phase constituent of the deposited coating, at a scanning rate of 6◦ /min in the range of 10–100◦ , with ˚ Cu K␣ radiation ( = 1.5418 A). The high-resolution transmission electron microscope (HRTEM, TECNAI F20) was used to reveal the grain size of the deposited coating. In the TEM analysis, the deposited coating was prepared into powders by mechanical method. The powders were dispersed in ethanol ultrasonically for 20 min until they were mixed uniformly. The mixture was taken out by glass capillary and dropped on the Cu grid (3 mm) as the TEM sample. 2.3. Mechanical properties The Vickers microhardness tester (HXD-1000TMSC/LCD) was used to measure the microhardness values of the deposited coating, using a load of 50 g with the holding time of 15 s. The surface and cross-sectional microhardness of each sample were measured, respectively for more than five times and the average microhardness values were calculated to reduce the errors. Three samples were used for the microhardness measurements to ensure the accuracy of the results. The automatic scratch tester (WS-2005, China) was used to evaluate the adhesion between the deposited coating and the substrate, according to the change of the Acoustic Emission (AE) signal. The scratching procedures were carried out by a diamond indenter with a rounded tip (200 m tip radius) at room temperature. The diamond tip was loaded against the surface of the deposited coating at an initial load of 0.5 N. Then it was sliding on the surface under linearly increasing load at a rate of 100 N/min until the load was reaching the maximum value of 200 N. The scratching speed was 5 mm/min. Three samples were used for the scratch tests and three single scratches of each sample were performed in order to ensure the result repeatability. The high temperature tribometer (HT-600, China) was used to investigate the friction and wear properties of the deposited coating. Before the friction and wear test, the initial weight of each sample was measured and recorded. The friction experiment was conducted using a ball-on-disk tester in air at room temperature. A WC-8Co ball with a diameter of 2 mm was used. The applied load was 200 N and the rotating speed of the motor was 2000 r/min. The friction coefficient was measured continuously during the experiment. After a duration of 20 min, the experiment was stopped and the final weight of the sample was measured and recorded. The
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Fig. 4. EDS analysis of the deposited coating prepared by CPED.
Fig. 2. Depositing current density–voltage (I–U) curve of CPED process.
weight loss of the sample was calculated to evaluate the wear resistance of the deposited coating. One bare sample and three coated samples were used for the friction and wear test and three independent measurements of each sample have been done to reduce the errors. 3. Results and discussion Fig. 2 shows the depositing current density–voltage (I–U) curve in the process of cathode plasma electrolytic depositing Co coatings, in the electrolyte with 25 g L−1 CoSO4 and 60 g L−1 H2 SO4 . At a relatively low voltage, the increase of the voltage will lead to a proportional rise in the current density (Section A). The kinetics of the electrode process conforms to the Faraday’s law and the I–U characteristics almost follow the Ohm’s law. With the subsequent increase of the voltage, the rise of the current density is gradually limited by a partial shielding action of hydrogen evolution reaction over the cathode surface, and the areas where the electrode remains in contact with the electrolyte are reduced greatly (Section B) [3,15]. At the peak of the curve, the cathode is covered with a continuous gaseous envelope of low electrical conductivity. Nearly all the voltage drop across the electrolytic cell is concentrating in this region. The strength of the electric field within this region is so large (about 106 –108 V/m [3]) that the gaseous envelope is broken through with discharge phenomenon (critical condition). Bright micro-arcs can be observed on the surface of the cathode. Owing to the generation and energy release of the plasma arcs, the current is oscillating [1]. With the initial increase of the voltage beyond the critical value, there are more and more uniform and tiny micro-arcs generating and acting on the surface of the sample (Section C). In this condition, the deposits can be modified by the micro-arcs sufficiently.
However, it will do harm to the sample if excessively high voltage is applied since the micro-arcs are too dense and severe. Therefore, the electrolytic parameters of section C in the I–U curve were selected for cathode plasma electrolytic depositing Co coatings. Fig. 3 shows the SEM micrographs of the surface and crosssection of the deposited coating prepared by CPED. From Fig. 3(a) it can be found that the surface morphology of the deposited coating prepared by CPED is different from that prepared by conventional electrodeposition. The deposited coating is dense and uniform, showing molten morphology. This is due to the effect of plasma arcs. In CPED process, the generated plasmas are of high temperature and high enthalpy. The local temperature of the arcing area in the near-electrode region can reach up to 2000 ◦ C [48], and the transient temperature of the plasma arcs is calculated at about 8000 K [49]. Therefore, the deposits can be melted by plasma arcs completely. However, the temperature drops down rapidly with the energy release of the plasmas, causing freezing of the deposits on the substrate [15]. This process is repeated continuously, so the deposits can be modified by plasma arcs sufficiently. As a result, the defects are made up, and the deposited coating becomes uniform and dense with a typically molten morphology. From the cross-sectional micrograph in Fig. 3(b), it can be estimated that the thickness of the deposited coating is approximately 20–25 m, which indicates that the depositing rate of CPED is much faster than that of conventional electrodeposition. Fig. 4 shows the EDS analysis of the deposited coating prepared by CPED. It indicates that the coating is composed of only cobalt without other elements. No other compounds have been deposited as well. This is different from the conventional electrodeposition of Co coatings. In conventional electrodeposition process, some additives are necessary in the electrolyte, for example, H3 BO3 as the buffering agent, Na2 SO4 as the conductive salt and so on. The selection of the anode is mainly concentrated on high purity cobalt, in order to improve the dissolving efficiency of the anode. However,
Fig. 3. SEM micrographs of the deposited coating prepared by CPED: (a) surface and (b) cross-section.
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Fig. 7. TEM micrograph of the cobalt deposits prepared by CPED. Fig. 5. XRD patterns of the cobalt deposits under the electrolytic parameters of section A and B in the I–U curve.
Fig. 6. XRD pattern of the deposited Co coating prepared under the electrolytic parameters of section C in the I–U curve by CPED.
there is no restriction on the choice of anode in CPED process. The inert electrodes such as platinum and graphite are also suitable to be selected as the anode, since they can reduce the change of Co2+ concentration effectively. Pure Co coatings can be obtained directly from the cobalt sulfate or chloride electrolyte with a high depositing rate. The process is simplified. Fig. 5 shows the XRD patterns of the cobalt deposits prepared under the electrolytic parameters of section A and B in the I–U curve. It is found that all of the diffraction peaks in pattern A and B match well with the standard peaks of cobalt (JCPDS No. 05-0727), and the substrate peaks have no effect on these peaks. The diffraction peaks at 2 = 41.7◦ , 44.8◦ , 47.6◦ , 75.9◦ , 92.5◦ correspond to the crystallographic planes (1 0 0), (0 0 2), (1 0 1), (1 1 0) and (1 1 2) of cobalt, and all of them are indexed to the hcp phase. This is basically the same with the Co coatings prepared by conventional electrodeposition or chemical plating, which are also with hcp structure. The intensity of the diffraction peaks are similar under the electrolytic parameters of section A, and the grains mainly grow along the crystallographic planes (0 0 2) and (1 1 0) (Pattern A). However, with the rise of the voltage ahead of the arcing value, the intensity of the characteristic diffraction peak at 2 = 75.9◦ is increased obviously under the electrolytic parameters of section B, which indicates that the growth of grains with (1 1 0) plane is preferred (Pattern B). Fig. 6 shows the XRD pattern of the deposited Co coating prepared under the electrolytic parameters of section C in the I–U curve by CPED. As there are only diffraction peaks matching with the
standard peaks of cobalt (JCPDS No. 15-0806) without any other peaks, the result from the XRD pattern of the deposited Co coating is consistent with that from EDS analysis. However, the diffraction peaks in pattern C are different from that of the cobalt with hcp structure. In Fig. 6, the intensity of the characteristic diffraction peak at 2 = 44.2◦ is increased obviously. There appears a diffraction peak at 2 = 51.5◦ , which is a characteristic diffraction peak of cobalt with fcc structure. The diffraction peaks at 2 = 75.9◦ and 92.2◦ also correspond to the crystallographic planes (2 2 0) and (3 1 1) of cobalt with fcc structure. It reveals that the structure of Co coating is changed from hcp to fcc with the thorough modification of plasma arcs. This is different from the conventional electrodeposition or chemical plating process, because the Co coatings prepared by conventional methods are always with hcp structure. They need to be treated with annealing to be transformed into fcc structure in some cases, in order to improve their dielectric and soft magnetic properties [50,51]. It is complicated and inconvenient. However, Co coatings with fcc structure can be obtained by CPED directly without any heat treatment. As there is no very broad peak in pattern C, the deposited Co coating shows crystalline structure. Fig. 7 shows the TEM micrograph of the cobalt deposits prepared by CPED. It can be estimated that the grain size is approximately 40–50 nm, which is far much smaller than that of conventional electrodeposition. During the CPED process, the high voltage between the electrodes causes the concentration of the positive ions in the electrolyte close to the cathode, especially on the surface of the gas envelope. This results in the high localized electric field strength between the positive charges and the cathode. The power generated from the near-electrode area is estimated to be typically 0.1–1 MW/m2 , which is very significant [3]. The temperature of the plasmas is very high as is mentioned above, and the plasma discharge temperature is much higher, reaching up to 6000–7000 K [52]. The high-temperature plasmas will lead to the localized melting of the deposits. These deposits are surrounded by the relatively cool electrolyte and the substrate, thereby resulting in the freezing of them. It is reported that the duration of the plasma discharge process is expected to be 10−6 s for each individual event and the cooling rate can reach 108 K/s [3]. The freezing that follows the localized melting causes a quenching effect, leading to form an ultra-fine microstructure of the deposited coating. The nanocrystalline grains formed by the CPED process will offer potential to improve the properties of the deposited Co coatings as compared with the large-grained Co coatings. Fig. 8 shows the microhardness of the surface and cross-section of the deposited Co coatings prepared by CPED. The surface and cross-sectional microhardness of the three samples are 709.5 HV and 744.7 HV (sample 1), 753.7 HV and 838.2 HV (sample 2), 691.6 HV and 710.8 HV (sample 3), respectively. No significant
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Fig. 8. The microhardness of the surface and cross-section of the deposited Co coatings prepared by CPED.
Fig. 10. The friction coefficient curves of the substrate and the deposited Co coating prepared by CPED under the load of 200 N. Table 2 The comparison of the weight loss in the friction and wear test.
Substrate With Co coating
Fig. 9. The scratch test of the deposited Co coating prepared by CPED.
change of the microhardness is observed with the surface and the cross-section of the deposited Co coating, even with different samples. As is reported, the microhardness of conventional Co coatings is about 106.4 HV. It reveals that the microhardness of the Co coatings prepared by CPED is increased greatly by comparing with that prepared by conventional electrodeposition. This is mainly attributed to the grain refinement of rapid quenching in CPED process. As a result, the nano-sized grain structure contributes to the increase of the microhardness of the deposited Co coatings prepared by CPED. Fig. 9 shows the scratch test of the deposited Co coating prepared by CPED. It can be found that there is a peak on the Acoustic Emission (AE) signal at the load of 108.9 N, which corresponds to the critical load for the failure of the deposited coating. As the welldefined failure of the deposited coating can represent the coating detachment from the substrate, the critical load can be considered approximately as the qualitative measure of the coating–substrate adhesion [53]. With the linear increase of the load in the x direction, the AE signal changes continuously, which indicates that the deposited Co coating has been damaged. It has been reported that there is electrolyte elemental diffusion into the surface of the electrode during plasma electrolytic heating process [48]. Also, there is diffusion of the elements inward to the substrate and outward to the coating in CPED process, so the deposited coating exhibits excellent adhesion with the substrate [1]. From the SEM micrograph of the cross-section (Fig. 3(b)), it can be found that the deposited Co coating is dense and conformal to the substrate as well. The result of the scratch test is consistent with the observation of the cross-sectional morphology of the deposited coating.
Initial weight (g)
Final weight (g)
Weight loss (mg)
9.56703 9.56719
9.56411 9.56704
2.92 0.15
Fig. 10 shows the friction coefficient curves of the substrate and deposited Co coating prepared by CPED under the load of 200 N. It is clear that the friction coefficient of the substrate is much higher than that of the deposited Co coating prepared by CPED. There are many factors which will influence the friction coefficients of the materials, such as the change of load, the speed of rotation, the temperature and so on. However, the result of the friction test reveals that the tribological property of the deposited Co coating is relatively good. The comparison of the weight loss in the friction and wear test is shown in Table 2. It can be found that the weight loss of the sample coated with Co coating by CPED is far much smaller by comparing with that of the bare substrate. These values demonstrate that the deposited Co coating exhibits superior wear resistance. This is more likely due to the localized melting of the surface region and its subsequent quenching by the hypothermal electrolyte, for which the deposited Co coating is plasma enhanced. The elemental diffusion in the bonding layer and the nanocrystalline structure may also contribute to the superior wear resistance of the deposited coating prepared by CPED [1]. 4. Conclusions In conclusion, a novel cobalt coating was prepared by cathode plasma electrolytic deposition. The best electrolytic parameters for cathode plasma electrolytic depositing cobalt coatings were determined by the depositing current density–voltage (I–U) curve. The novel cobalt coating is dense and uniform with a typically molten morphology. The depositing rate of CPED is much faster than that of conventional methods. EDS analysis indicates that the deposited coating is composed of only cobalt without other elements. X-ray diffraction analysis reveals that the coating is pure cobalt with face center cubic structure, which is different from the coatings prepared by conventional electrodeposition or chemical plating. TEM analysis shows that the deposited cobalt coating is nanocrystalline structure with an average grain size of 40–50 nm. The microhardness of the coating is up to approximately 700 HV without any other processing. The adhesion between the coating and the substrate is about 108.9 N. The friction coefficient and weight loss of the coated sample are much smaller than that of the bare substrate, which indicates that the wear resistance of the coating is excellent. The
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