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Effects of Co contents on the microstructures and properties of the electrodeposited NiCo–Zr composite coatings
Materials Research Bulletin 65 (2015) 195–203
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Effects of Co contents on the microstructures and properties of the electrodeposited NiCo–Zr composite coatings Fei Cai a , Chuanhai Jiang a, * , Yuantao Zhao a , Vincent Ji b a b
School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China ICMMO/LEMHE, UMR 8182, Université Paris-Sud 11, Orsay Cedex, 91405 France
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 August 2014 Received in revised form 16 January 2015 Accepted 19 January 2015 Available online 1 February 2015
In this study, the NiCo–Zr composite coatings were prepared from the electrolytes with different Co2+ concentrations by electrodeposition method. The effects of Co contents on the crystal structure, surface morphology, grain size, microstrain and residual stress were examined by X-ray diffractometer (XRD), field emission scanning electron microscopy (FESEM), Energy dispersive X-ray spectroscopy (EDX) and atomic force microscope (AFM). The corrosion resistance of the composite coatings was also examined by the potentiodynamic polarization and electrochemical impedance (EIS) measurements. The results revealed that the crystal structures of the coatings were dependent on the Co contents and addition of Co content of 58 wt% resulted in the formation of hexagonal (hcp) Co. The increasing Co contents in the NiCo–Zr composite coatings resulted in the smoother and more compact surface, decreased the grain size and increased the microstrain. The micro-hardness and residual stress also increased with increasing Co contents. The addition of Co increased the corrosion resistance of the NiCo–Zr composite coatings compared with the Ni–Zr coating while the corrosion resistance of the NiCo–Zr composite coatings decreased as the Co contents increased. ã 2015 Published by Elsevier Ltd.
Keywords: A. Composites B. Microstructure C. Atomic force microscopy C. X-ray diffraction D. Surface properties
1. Introduction In recent years, the electro-deposited metal matrix composite (MMCs) coatings have got considerable research interests due to their combined properties, such as better corrosion resistance, excellent oxidation resistance, higher hardness and improved wear resistance. The prepared MMCs were comprised of the Ni matrix and the incorporated particles, such as SiC [1], TiO2 [2], Al2O3 [3], Y2O3 [4], Cr [5] particles. All the MMCs have been well investigated with respect to the microstructures and properties. The properties of the pure Ni coating could also be enhanced by alloying with Co element [6–8]. Yang et al. [9] found that addition of about 20 wt% Co in the pure Ni coating decreased the grain size and made the surface smoother which was beneficial to the improvement of the corrosion resistance of the composite coating. Kang et al. [7] pointed out that the presence of Co greatly enhanced the stability of the passive film, which increased the corrosion resistance of the NiCo coatings. Recently, there have been increasing interests in fabricating the particles reinforced NiCo composite coatings [8,10–15] and the incorporated particles could further enhance the properties of the composite coatings compared with the NiCo alloy coatings.
* Corresponding author. Tel.: +86 21 34203096; fax: +86 21 34203096. E-mail address: [email protected] (C. Jiang). http://dx.doi.org/10.1016/j.materresbull.2015.01.046 0025-5408/ ã 2015 Published by Elsevier Ltd.
Shi et al. [16] fabricated the NiCo–SiC composite coatings and found that the SiC nano-particles increased the microhardness, wear and corrosion resistance of the composite coatings. Similar enhancement effects of particles were also found for carbon nanotubes (CNTs) [11] and TiO2 [12]. Tian et al. [17] found that incorporation of Al2O3 particles in the coating enhanced the codeposition of Ni and Co and the increasing Al2O3 particle amount in the composite coating increased the micro-hardness and wear resistance in oil and sand slurry. The size of the incorporated particle also had an effect on the hardness and anti-corrosion properties of the composite coatings. Bakhit et al. [8] pointed out that the SiC nano-particle reinforced NiCo–SiC composite coating showed the higher hardness and better corrosion resistance than the SiC micro-particle reinforced composite coatings. However, the effects of Co contents on the NiCo based composite coatings were few investigated. Srivastava et al. [18] found that the presence of 25 wt% cobalt in the NiCo–CeO2 composite coatings increased the hardness and improved its wear resistance. And their result also showed that addition of 85 wt% cobalt in the composite coating imparted the composite coating better thermal stability. In reference [19], addition of 28 wt% Co content in the NiCo–SiC matrix showed the higher hardness than other coatings. Although the particles reinforced NiCo composite coating were widely studied, the fabrication of Zr particles reinforced NiCo–Zr
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[(Fig._3)TD$IG]
Fig. 3. XRD patterns of NiCo–Zr composite coatings with different Co contents. Fig. 1. Element content in the deposits as a function of CoSO47H2O concentrations in the bath.
composite coatings were few reported and the effects of Co contents on the NiCo based composite coatings need further investigation. In this study, the NiCo–Zr composite coatings with different Co contents were successfully prepared by electrodeposition from the modified Watts baths containing different Co2+ concentrations. Then, the effects of Co contents on the surface morphologies, crystal structure, grain size, micro-strain, hardness and residual stresses were investigated. In addition, the corrosion resistance of the NiCo–Zr composite coatings was discussed in detail.
the electrolytes were ultrasonic dispersion for 30 min followed by magnetically stirring for about 4 h at a stirring rate of 300 rpm. The surfactant C12H25NaSO4 (0.2 g/L) was also added in the bath to disperse the particles. The solution temperature and pH value were maintained at 50 C and 4.2, respectively. The applied current density was maintained at a constant of 4 A/dm2 for 1 h to maintain a thickness of about 30 mm for the deposited coating. Pure nickel plate and stainless steel plate with an area of 1 1 cm2 were used as anode and cathode, respectively. The stainless steel specimens were grounded with grade 600, 800 and 1200 emery papers step by step. Then, the substrates were degreased in acid (10% HCl) and washed with distilled water before deposition.
2. Experimental
2.2. Coatings characterization
2.1. Coating processes
Morphology observation of the composite coatings was carried out using a field emission scanning electron microscope (FESEM, JSM-7600F), and the chemical composition was checked with the energy dispersive X-ray spectroscopy (EDX) attached with the FESEM. In addition, the X-ray photoelectron spectroscopy (XPS) was also used to examine the chemical composition of the
Electrodeposition of the NiCo–Zr composite coatings was carried out in the Watt baths containing NiSO46H2O (240 g/L), NiCl26H2O (40 g/L) and H3BO3 (30 g/L). Co was added as CoSO47H2O and the additions ranged from 5 g/L to 40 g/L of Co contents. The Zr particles of 20 g/L with a mean diameter of 1 mm were added into the solutions. In order to ensure a good dispersion of the Zr particles, all
[(Fig._4)TD$IG]
[(Fig._2)TD$IG]
Co/Ni in the deposite
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16
Co/Ni in the solution Fig. 2. Effect of electrolyte Co/Ni ratio on deposit Co/Ni ratio.
Fig. 4. Typical XRD patterns of Ni–0Co/Zr, Ni–29Co/Zr and Ni–58Co/Zr composite coatings.
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[(Fig._5)TD$IG]
composite coatings. All XPS spectra were collected with a Kratos Axis Ultra DLD spectrometer (Krato Analytica-A Shimadzu Group Company), using a monochromatic Al Ka X-ray source. In order to remove the surface oxides of the coating, Ar+ sputtering of 10 min was carried out with a minibeam IV ion gun under 1.5 108 Torr in the sample analysis chamber. The atomic force microscope (dimension edge) was also used to examine the surface morphology and roughness of the composite coatings. The phase structures of the as-deposited coatings was determined by a Rigaku Ultima IV X-ray diffractometer (Cu Ka radiation, l = 1.54056 Å) in standard 2u–u mode and the voltage and current were 40 kV and 30 mA, respectively. The grain size and the microstrain of the coatings were obtained by using the Modified Williamson-Hall method (MWH) [20]. The lattice parameter a was also obtained using Nelson–Riley method [21]. The instrumental line broadening of the measured profiles was corrected using a Ni powder as the standard sample, which was annealed thoroughly at 450 C for 6 h. The structural broadening only due to crystallite size and lattice strain (B(struct)) was obtained using the following equation qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi BðstructÞ ¼ B2obs B2std , where, Bstd and Bobs are full width at half-
Lattice parameter a, nm
0.3532
0.3528
0.3524
0.3520
0.3516
Ni-0
Co/Z r
Ni-1
8 Co
/Zr
Ni-2
9Co
/Zr
Ni-4
2Co
/Zr
Ni-5
8Co
/Zr
Fig. 5. Effect of Co content on the lattice parameter of the NiCo–Zr composite coatings.
maximum of any particular reflection from the standard sample (Si powder) and the sample, respectively. Residual stress examination of the as-deposited Ni–Zr composite coatings was carried out on a Proto LXRD Residual Stress Analyser using the classical sin2 c method [22]. The voltage and current were 30 kV and 25 mA, respectively. The peaks of (3 3 1)a of Ni were selected to calculate the residual stress. The Vickers microhardness measurement was carried out with a load of 200 g and an indentation time of 15 s. The corresponding final values were determined as the average of 8 measurements. The potentiodynamic polarization measurements were conducted in a standard three-electrode method. A platinum sheet and a Ag/AgCl electrode [+207 mV(SHE)] were used as the auxiliary electrode (AE) and the reference electrode, respectively. The sample was used as working electrode (WE). The measurements were conducted in 3.5 wt% NaCl solution at the temperature of 25 C using an electrochemical apparatus (CHI660E) at a scan rate of 1 mV/s. The polarization potentiodynamic curves were recorded after 40 min of immersion. The corrosion potential Ecorr and corrosion current density Icorr were determined from Tafel curves. The electrochemical impedance (EIS) measurements were conducted at the open circuit potential after immersion of a sample into solution for 30 min. The applied sinuous potential amplitude was 5 mV and the frequency ranged from 0.01 Hz to 100,000 Hz.
The electro-codeposition mechanism of the composite coatings has been widely studied in the past several decades and several models have been developed, such as the Guglielmi’s model [23], Celis’s model [24] and the trajectory model [25]. In this study, the depositing of the NiCo–Zr composite coating could be explained by the Guglielmi’s model containing successive adsorption. In the first step, the Zr particles with adsorbed ions were physically and loosely adsorbed on the cathode surface. In the second step, the metal ions adsorbed on the Zr particles were reduced, making the Zr particles strongly absorb on the growing surface and to be embedded in the coating. Furthermore, addition of Co2+ in the bath resulted in the formation of alloyed NiCo–Zr composite coating. Fig. 2 shows the effect of Co/Ni ratio in electrolytes on the Co/Ni ratio in the deposits. It could be found that the Co contents in the composite coatings were significantly higher than those in the baths, which showed an anomalous co-deposition. This phenomenon has also been reported in references [10,27,28] and several mechanisms have been proposed to explain this phenomenon, such as a local increase in the pH [29], two-step codeposition of cobalt cations and decreasing the nickel deposition in the second step [30], and the preferential deposition of cobalt cations [31]. Previous researches [26,32,33] pointed out that increase of Co contents could enhance the codeposition of the incorporated
3. Results and discussion
[(Fig._6)TD$IG] 100
3.1. Chemical component content analysis
0.7
80
0.6 0.5
60
0.4 0.3
40
0.2 20
0.1
Ni-0
Co/Z
r
Ni-1
8Co
/Zr
Ni-2
9Co
/Zr
Ni-4
2Co
/Zr
Ni-5
8Co
/Zr
Fig. 6. Grain size and microstrain of the composite coatings.
Microstrain, %
Grain Size, nm
Fig. 1 shows the element contents of the NiCo–Zr composite coatings as a function of CoSO47H2O concentrations in the bath. The graph clearly showed that the Co contents increased linearly while Ni contents decreased linearly with increasing the CoSO47H2O concentrations in the bath. The average Co contents obtained from the results of EDX were 18%, 29%, 42% and 58 wt% for the coatings deposited at CoSO47H2O concentrations of 5 g/L, 10 g/ L, 20 g/L and 40 g/L, respectively. The XPS experiments were also done to confirm the element contents of the composite coatings and it could be found that the variation trend of Ni, Co and Zr contents determined from the XPS were in agreement with that obtained from the EDX. However, the Zr particle contents showed no obvious variation with increasing CoSO47H2O concentrations. Therefore, the NiCo–Zr composite coatings could be referred to as Ni–0Co/Zr, Ni–18Co/Zr, Ni–29Co/Zr, Ni–42Co/Zr and Ni–58Co/Zr.
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particles such as Si3N4, SiC and Al2O3, which was due to that the adsorption of Co2+ cations on the particles surface was easier than the Ni2+ cations. However, in reference [34], the Co contents increased while the SiC particle contents decreased with reducing the current densities. Similar results are also observed for the ZrC and Al particles that the ZrC and Al particle contents decreased with increasing Co contents in the composite coatings [35,36]. However, In this study, the Zr particle contents showed no evident variation with increasing Co contents as shown in Fig. 1, which might be due to the different adsorption capacity of Zr microparticles with the Co2+ cations compared with the particles mentioned above.
3.2. Coating microstructures Fig. 3 displays the XRD patterns of the NiCo–Zr composite coatings with different Co contents. As shown in Fig. 3, the Ni or NiCo matrix peaks corresponding to the (111), (2 0 0), (2 2 0), (3 11) and (2 2 2) were detected and all the coatings exhibited the random orientation. With the increasing Co contents, the NiCo matrix peaks decreased in intensities and the diffraction peaks broadened. To further investigate the effects of Co contents on the microstructures of the composite coatings, the XRD patterns of Ni–0Co/ Zr, Ni–29Co/Zr and Ni–58Co/Zr were selected to present in Fig. 4. The shifting of the NiCo matrix peaks to the lower angle was found
[(Fig._7)TD$IG]
Fig. 7. FESEM images of NiCo–Zr composite coatings with different Co contents (a) Ni–0Co/Zr (b) Ni–18Co/Zr (c) Ni–29Co/Zr (d) Ni–42Co/Zr and (e) Ni–58Co/Zr.
[(Fig._8)TD$IG]
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Fig. 8. AFM images of NiCo–Zr composite coatings with different Co contents (a) Ni–0Co/Zr (b) Ni–18Co/Zr (c) Ni–29Co/Zr (d) Ni–42Co/Zr and (e) Ni–58Co/Zr.
for the Ni–29Co/Zr composite coating compared with the Ni–Zr composite coating, which was due to that the smaller Ni atoms were partly substituted by the larger Co atoms. With increasing the Co content to 58 wt%, the peaks of the NiCo further shifted to the lower angle. Fig. 5 shows the lattice parameter a calculated from the Nelson–Riley method and it was clearly shown that the lattice parameter a increased linearly with increasing Co contents duo the solid solution. It could also be found from the XRD patterns that the hcp Co were detected for the Ni–58Co/Zr composite coatings. Previous studies [27,28] pointed out that the crystal structure of the NiCo coatings were closely dependent on the Co contents in the coatings. The crystal structures of the composite coatings changed from the typical FCC structures to mixed structures of FCC + HCP, and then to the HCP structures with increasing Co contents. Fig. 6 demonstrates the grain size and microstrain of the NiCo– Zr composite coatings. As seen in Fig. 6, the grain size of the Ni–Zr composite coating was 92 nm and the addition of Co decreased the grain size of the composite coatings. As the Co contents increased from 18 wt% to 58 wt%, the grain size of the composite coatings decreased from 70 nm to 31 nm. As mentioned above, the Zr particle contents showed no evident changes with increasing Co contents. Thus, the decrease in grain size could be ascribed to the addition of Co, which was also reported in references [34,37,38]. During the electrodeposition, Co+2 and Ni+2 could react with OH to form the metal mono-hydroxides Co(OH)+ and Ni(OH)+, which usually adsorbed on the surface of electrodeposits [39]. The greater adsorption of Co(OH)+ than Ni(OH)+ in Ni–Co/Zr electrolyte could block the growth centers, which resulted in the smaller grain size [40]. It could also be found from Fig. 6 that the microstrain of the composite coatings increased with increasing Co contents and obtained the maximum at Co content of 58 wt%. The increase of the microstrain was related to the increasing Co contents in the coatings. As mentioned above, the Ni atoms were partly
substituted by Co atoms to form the solid solution NiCo phases, which resulted in the distortion of the composite coatings. Thus, the microstrain of the composite coatings increased with increasing Co contents. 3.3. Surface morphology and cross sectional morphology characteristics FE-SEM morphology images of the NiCo–Zr composite coatings with different Co contents are shown in Fig. 7. The Ni–Zr composite coating demonstrated some polygonal structures as shown in Fig. 7a. With increasing the Co contents from 18 wt% to 42 wt%, a change in the structure from polygonal structures to the globular crystal structure occurred and the structure became smaller (Fig. 7b–d). However, some cauliflower structures were observed (Fig 7e) with further increasing the Co content to 58 wt%. Fig. 8 shows the AFM images of the NiCo–Zr composite coatings with different Co contents. It was observed that surfaces of the composite coatings were refined and the average roughness of the composite coatings were 190 nm, 161 nm, 151 nm, 114 nm and 125 nm with increasing contents. The surface morphology observation indicated that addition of Co could make the surface of the Ni–Zr composite coating smoother and more compact, which was also reported in NiCo–Si3N4 composite coating [26]. The cross sectional FESEM images of NiCo–Zr composite coatings with different Co contents are shown in Fig. 9. It could be found that all the coatings showed fairly uniform thickness and the Zr particles were also found to show fairly uniform distribution over the cross sectional surface of the composite coatings. In order to further study the Zr particle distribution uniformity across the composite coating, the Ni, Co and Zr particle dot-maps on the cross sectional FESEM image of the Ni–42Co/Zr composite coating is shown in Fig. 10. The Zr dot-map patterns confirmed that the Zr
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Fig. 9. Cross sectional images of NiCo–Zr composite coatings with different Co contents of (a) 18 wt%, (b) 29 wt% (c) 42 wt% and (d) 58 wt%.
particles showed fairly uniform distribution over the cross sectional surface of the composite coating. 3.4. Hardness measurements Fig. 11 shows the measured microhardness of the NiCo–Zr composite coatings. It was clear that the measured hardness values of the composite coatings increased with increasing Co contents and obtained the maximum at Co content of 42 wt% beyond which the hardness decreased. The microhardness of the composite
coatings was determined by both the matrix material and the reinforced particles. As mentioned above, the incorporated Zr particle contents showed a constant with increasing Co contents. Thus, the hardness enhancement of the composite coating was related to the changes of the matrix material. Firstly, the grain size of the coatings decreased with increasing Co contents (as shown in Fig. 6), which resulted in a higher hardness consistent with the well-known Hall–Petch relationship [41]. Secondly, the solid solution strengthening caused by the substitution of Ni atoms by Co atoms also contributed to the increase of hardness [38], and
[(Fig._10)TD$IG]
Fig. 10. The Ni, Co and Zr particle dot-map on the cross sectional FESEM image of the Ni–42Co/Zr composite coating.
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[(Fig._13)TD$IG]
-2
500
Microhardness, Hv
Log (current density), Log (Acm )
[(Fig._1)TD$IG]
400 300 200 100 0
Ni-0
Co/Z r
Ni-1
8Co
/Zr
Ni-2
9Co
/Zr
Ni-4
Ni-5 2Co 8Co /Zr /Zr
-1
-3 -4 -5 -6 -7 -8 -9 -0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
Potential / V
Fig. 11. Hardness of composite coating with different Co contents.
the more Co contents the composite coatings contained, the higher the hardness were. However, the hardness of the composite coating decreased as the Co content increased to 58 wt%, which was due to the formation of hcp Co [32].
Ni-0Co/Zr Ni-18Co/Zr Ni-42Co/Zr
-2
Fig. 13. Potentiodynamic polarization curves for NiCo–Zr composite coatings in 3.5% NaCl solution.
atoms by Co atoms might also contribute to the increase of the residual stress of the NiCo–Al composite coatings.
3.5. Residual stress analysis 3.6. Potentiodynamic polarization Fig. 12 presents the residual stresses of the NiCo–Zr composite coatings with different Co contents and all the coatings exhibited the tensile residual stress. It could be found from Fig. 10 that the residual stress of 113 MPa for Ni–Zr composite coating was obtained. As the Co contents increased from 18 wt% to 58 wt%, the residual stresses of the NiCo–Zr composite coatings increased from 169 MPa to 438 MPa. The residual stress results showed that addition of Co increased the residual stresses of the NiCo–Zr composite coatings. Coalescence of the islands and decreasing structural mismatch were the main mechanisms for the occurrence of tensile stress [42]. The increase in residual stress with increasing Co contents was due to the decrease in grain size. Nguyen et al. [43] pointed out that the smaller grain possesses higher surface free energy and the crystallite coalescence could lead to higher tensile stresses in the coating. Therefore, the smaller the grain sizes were, the higher the tensile stresses would be. In addition, the increasing distortions caused by substituting of Ni
[(Fig._12)TD$IG] Residual Stress, MPa
500 450 400 350
Fig. 13 presents the potentiodynamic polarization curves of the NiCo–Zr composite coatings and the corrosion parameters, such as corrosion potential (Ecorr) and corrosion current (Icorr) derived from the polarization curves, are concluded in Table 1. It could be found that the corrosion current (Icorr) of the NiCo–Zr composite coatings (except for the Ni–58Co/Zr) were less than that of the Ni–Zr (Ni– 0Co/Zr) coating. In term of the NiCo–Zr composite coatings, the corrosion current (Icorr) increased with increasing Co contents. Two important conclusions could be obtained by comparing the corrosion current (Icorr) usually used to measure the corrosion resistance of the composite coatings. On one hand, the addition of Co enhanced the corrosion resistance of the NiCo–Zr composite coatings, which was ascribed to the smoother and more compact surfaces compared with the Ni–Zr composite coating [6,9]. On the other hand, the corrosion resistance of the NiCo–Zr composite coatings decreased with increasing Co contents. The decrease in the corrosion resistance were related to several factors such as chemical composition, phase structure and grain size. It is acknowledged that alloying can decrease the corrosion resistance by changing the nobility of materials [10,44]. Because Co is more active than Ni [10,45], it is expected that the electrochemical activity of the NiCo alloy is greater than the pure Ni coating. Therefore, the corrosion resistance of the NiCo–Zr alloy coating decreased with increasing Co contents. Another factor to deteriorate the corrosion resistance of the coating was the formation of
300 250
Table 1 Corrosion potential Ecorr and corrosion current Icorr of coatings with various Co content.
200 150 100
Ni-0
Co/Z r
Ni-1
8Co
/Zr
Ni-2
9Co
/Zr
Ni-4
Ni-5 2Co 8Co /Zr /Zr
Fig. 12. Residual stress of composite coatings with different Co contents.
Composite coating with different Co contents
Ecorr (mV)
Icorr (mA/cm2)
Ni–0Co/Zr Ni–18Co/Zr Ni–29Co/Zr Ni–42Co/Zr Ni–58Co/Zr
27.5 113.7 65.3 134.3 96.3
0.552 0.263 0.311 0.448 0.594
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F. Cai et al. / Materials Research Bulletin 65 (2015) 195–203 Table 2 Fitting of charge-transfer resistance from the measured EIS plots on composite coatings. Composite coating with different Co contents
Rct (kV cm2)
Ni–0Co/Zr Ni–18Co/Zr Ni–29Co/Zr Ni–42Co/Zr Ni–58Co/Zr
119.57 236.92 186.33 174.17 98.76
4. Conclusions
Fig. 14. Nyquist impedance diagrams for NiCo–Zr composite coatings in 3.5% NaCl solution.
hcp Co. In general, two phase structures are less corrosion resistant than the single-phase structure because of galvanic cells can be easily formed between phases with different nobilities [10]. The last factor influencing the corrosion resistance of the composite coating was the grain size. Previous studies pointed out that the grain boundaries are prone to corrosion attacks, especial for the multi-phase structure materials [46]. In this case, the grain size for the NiCo–Zr composite coatings decreased with the increasing Co contents, which resulted in the decrease of the corrosion resistance of the composite coatings. 3.7. Electrochemical impedance spectroscopy (EIS) The Nyquist impedance plots obtained for the composite coatings in 3.5 wt% NaCl solution are shown in Fig. 14. All the composite coatings exhibited a single semicircle and the diameters of the semicircle of the NiCo–Zr composite coatings (except for the Ni–58Co/Zr) were larger than that of Ni–Zr composite coating. Moreover, the diameters of the semicircle for the NiCo–Zr composite coatings decreased with increasing Co contents, indicating the highest corrosion resistance for the Ni–18Co/Zr composite coating. To account for the corrosion resistance of the composite coating, the equivalent circuit model (Fig. 15) was used fit the impedance spectra and a good fit with this model was obtained. The Rs represented the solution resistance. The constant phase element, C.P.E, was introduced in the circuit instead of a pure double layer capacitor to give a more accurate fit. The Rp represented the charge transfer resistance whose value could be used to measure the corrosion resistance. Table 2 shows the date extracted from the EIS by simulation. It could be found that the Ni– 18Co/Zr composite coatings (except for the Ni–58Co/Zr) possessed the highest Rp, indicating the highest corrosion resistance.
[(Fig._15)TD$IG]
Fig. 15. Equivalent circuits used for fitting of impedance plots for NiCo–Zr composite coatings in 3.5% NaCl solution.
The NiCo–Zr composite coatings were prepared from the electrolytes with different Co2+ concentrations. The Co contents in the deposited composite coatings increased from 18 wt% to 58 wt% as the Co2+ concentration increased from 5 g/L to 40 g/L and have an effect on the microstructures and properties of the composite coatings. The surface morphology of the composite coating became smoother and more compact with the addition of Co. The crystal structures of the coatings were dependent on the Co content. With increasing the Co contents, the grain size decreased while the microstrain increased. The addition of Co also increased the microhardness of the composite coatings and the microhardness attained the maximum of 498 Hv at the Co content of 42 wt%. The residual stress of the composite coatings increased with increasing the Co contents. The corrosion experiments showed that the addition of Co increased the corrosion resistance of the Ni– Zr composite coating due to the smoother and more compact surface. However, the corrosion resistance of the composite coatings decreased with increasing Co contents. References [1] A. Abdel Aal, S.M. El-Sheikh, Y.M.Z. Ahmed, Mater. Res. Bull. 44 (2009) 151–159. [2] W. Shao, D. Nabb, N. Renevier, I. Sherrington, Y.Q. Fu, J.K. Luo, J. Electrochem. Soc. 159 (11) (2012) D671. [3] R.K. Saha, T.I. Khan, Surf. Coat. Technol. 205 (2010) 890–895. [4] R. Sen, S. Das, K. Das, Mater. Res. Bull. 47 (2012) 478–485. [5] J. Zhao, X. Peng, F. Wang, Mater. Res. Bull. 45 (2010) 420–424. [6] S. Hassani, K. Raeissi, M. Azzi, D. Li, M.A. Golozar, J.A. Szpunar, Corros. Sci. 51 (2009) 2371–2379. [7] J. Kang, Y. Yang, X. Jiang, H. Shao, Corros. Sci. 50 (2008) 3576–3580. [8] B. Bakhit, A. Akbari, Surf. Coat. Technol. 206 (2012) 4964–4975. [9] X. Yang, Q. Li, S. Zhang, H. Gao, F. Luo, Y. Dai, J. Solid State Electrochem. 14 (2010) 1601–1608. [10] B. Bakhit, A. Akbari, F. Nasirpouri, M. Ghasem Hosseini, Appl. Surf. Sci. 307 (2014) 351–359. [11] L. Shi, C.F. Sun, P. Gao, F. Zhou, W.M. Liu, Surf. Coat. Technol. 200 (2006) 4870– 4875. [12] B. Ranjith, G. Paruthimal Kalaignan, Appl. Surf. Sci. 257 (2010) 42–47. [13] L.M. Chang, H.F. Guo, M.Z. An, Mater. Lett. 62 (2008) 3313–3315. [14] Y. Yang, Y.F. Cheng, Surf. Coat. Technol. 216 (2013) 282–288. [15] Y. Yang, Y.F. Cheng, Electrochim. Acta 109 (2013) 638–644. [16] L. Shi, C. Sun, P. Gao, F. Zhou, Appl. Surf. Sci. 252 (2006) 3591–3599. [17] B.R. Tian, Y.F. Cheng, Electrochim. Acta 53 (2007) 511–517. [18] M. Srivastava, V.K. William Grips, K.S. Rajam, Appl. Surf. Sci. 257 (2010) 717– 722. [19] M. Srivastava, V.K. William Grips, K.S. Rajam, Appl. Surf. Sci. 253 (2007) 3814– 3824. [20] T. Unger, I. Dragomir, A. Revesz, A. Borbely, J. Appl. Cryst. 32 (1999) 992–1002. [21] M. Razavi, M.R. Rahimipour, A.H. Rajabi-Zamani, J. Alloys Compd. 436 (2007) 142–145. [22] P.J. Withers, H. Bhadeshia, Mater. Sci. Tech. 17 (2001) 355. [23] N. Guglielmi, J. Electrochem. Soc. 119 (8) (1972) 1009–1012. [24] J.P. Celis, J.R. Roos, J. Electrochem. Soc. 124 (10) (1977) 1508–1511. [25] J. Fransaer, J.P. Celis, J.R. Roos, J. Electrochem. Soc. 139 (1992) 413–425. [26] M. Srivastava, V.K. William Grips, K.S. Rajam, J. Alloys Compd. 469 (2009) 362– 365. [27] C. Lupi, A. DellEra, M. Pasquali, P. Imperatori, Surf. Coat. Technol. 205 (2011) 5394–5399. [28] L. Tian, J. Xu, S. Xiao, Vacuum 86 (2011) 27–33. [29] A. Bai, C.C. Hu, Electrochim. Acta 47 (2002) 3447–3456. [30] E. Gómez, J. Ramirez, E. Valles, J. Appl. Electrochem. 28 (1998) 71–79. [31] G. Qiao, T. Jing, N. Wang, Y. Gao, X. Zhao, J. Zhou, W. Wang, Electrochim. Acta 51 (2005) 85–92. [32] B. Bakhit, A. Akbari, J. Alloys Compd. 560 (2013) 92–104.
F. Cai et al. / Materials Research Bulletin 65 (2015) 195–203 [33] G. Wu, N. Li, D.L. Wang, D.R. Zhou, B.Q. Xu, K. Mitsuo, Mater. Chem. Phys. 87 (2004) 411–419. [34] S.M. Lari Baghal, M. Heydarzadeh Sohi, A. Amadeh, Surf. Coat. Technol. 206 (2012) 4032–4039. [35] Z. Zhang, C. Jiang, N. Ma, J. Mater. Eng. Perform. 23 (11) (2014) 4065–4071. [36] F. Cai, C. Jiang, P. Fu, J. Vincent, Appl. Surf Sci. 324 (2015) 482–489. [37] L. Wang, Y. Gao, Q. Xue, H. Liu, T. Xu, Appl. Surf. Sci. 242 (2005) 326–332. [38] B. Bakhit, A. Akbari, Surf. Coat. Technol. 206 (2012) 4964–4975. [39] A. Bai, C.C. Hu, Electrochim. Acta 47 (2002) 3447. [40] B. Tury, M. Lakatos-Varsányi, S. Roy, Surf. Coat. Technol. 200 (2006) 6713–6717.
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[41] M. Hakamada, Y. Nakamoto, H. Matsumoto, H. Iwasaki, Y.Q. Chen, H. Kusuda, M. Mabuchi, Mater. Sci. Eng. A 457 (2007) 120. [42] C.V. Thompson, Annu. Rev. Mater. Sci. 30 (2000) 159. [43] V.C. Nguyen, C.Y. Lee, L. Chang, F.J. Chen, C.S. Lin, J. Electrochem. Soc. 159 (6) (2012) D393. [44] ASM Handbook, Corrosion: Fundamentals, Testing, and Protection, vol. 13A, ASM, International, Materials Park, OH, 2003. [45] J. Edward, Coating and Surface Treatment Systems for Metals: A ComprehensiveGuide to Selection, ASM International, Michigan, 1997. [46] B. Bakhit, A. Akbari, J. Coat. Technol. Res. 10 (2) (2013) 285–295.
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Effects of Co contents on the microstructures and properties of the electrodeposited NiCo–Zr composite coatings Fei Cai a , Chuanhai Jiang a, * , Yuantao Zhao a , Vincent Ji b a b
School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China ICMMO/LEMHE, UMR 8182, Université Paris-Sud 11, Orsay Cedex, 91405 France
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 August 2014 Received in revised form 16 January 2015 Accepted 19 January 2015 Available online 1 February 2015
In this study, the NiCo–Zr composite coatings were prepared from the electrolytes with different Co2+ concentrations by electrodeposition method. The effects of Co contents on the crystal structure, surface morphology, grain size, microstrain and residual stress were examined by X-ray diffractometer (XRD), field emission scanning electron microscopy (FESEM), Energy dispersive X-ray spectroscopy (EDX) and atomic force microscope (AFM). The corrosion resistance of the composite coatings was also examined by the potentiodynamic polarization and electrochemical impedance (EIS) measurements. The results revealed that the crystal structures of the coatings were dependent on the Co contents and addition of Co content of 58 wt% resulted in the formation of hexagonal (hcp) Co. The increasing Co contents in the NiCo–Zr composite coatings resulted in the smoother and more compact surface, decreased the grain size and increased the microstrain. The micro-hardness and residual stress also increased with increasing Co contents. The addition of Co increased the corrosion resistance of the NiCo–Zr composite coatings compared with the Ni–Zr coating while the corrosion resistance of the NiCo–Zr composite coatings decreased as the Co contents increased. ã 2015 Published by Elsevier Ltd.
Keywords: A. Composites B. Microstructure C. Atomic force microscopy C. X-ray diffraction D. Surface properties
1. Introduction In recent years, the electro-deposited metal matrix composite (MMCs) coatings have got considerable research interests due to their combined properties, such as better corrosion resistance, excellent oxidation resistance, higher hardness and improved wear resistance. The prepared MMCs were comprised of the Ni matrix and the incorporated particles, such as SiC [1], TiO2 [2], Al2O3 [3], Y2O3 [4], Cr [5] particles. All the MMCs have been well investigated with respect to the microstructures and properties. The properties of the pure Ni coating could also be enhanced by alloying with Co element [6–8]. Yang et al. [9] found that addition of about 20 wt% Co in the pure Ni coating decreased the grain size and made the surface smoother which was beneficial to the improvement of the corrosion resistance of the composite coating. Kang et al. [7] pointed out that the presence of Co greatly enhanced the stability of the passive film, which increased the corrosion resistance of the NiCo coatings. Recently, there have been increasing interests in fabricating the particles reinforced NiCo composite coatings [8,10–15] and the incorporated particles could further enhance the properties of the composite coatings compared with the NiCo alloy coatings.
* Corresponding author. Tel.: +86 21 34203096; fax: +86 21 34203096. E-mail address: [email protected] (C. Jiang). http://dx.doi.org/10.1016/j.materresbull.2015.01.046 0025-5408/ ã 2015 Published by Elsevier Ltd.
Shi et al. [16] fabricated the NiCo–SiC composite coatings and found that the SiC nano-particles increased the microhardness, wear and corrosion resistance of the composite coatings. Similar enhancement effects of particles were also found for carbon nanotubes (CNTs) [11] and TiO2 [12]. Tian et al. [17] found that incorporation of Al2O3 particles in the coating enhanced the codeposition of Ni and Co and the increasing Al2O3 particle amount in the composite coating increased the micro-hardness and wear resistance in oil and sand slurry. The size of the incorporated particle also had an effect on the hardness and anti-corrosion properties of the composite coatings. Bakhit et al. [8] pointed out that the SiC nano-particle reinforced NiCo–SiC composite coating showed the higher hardness and better corrosion resistance than the SiC micro-particle reinforced composite coatings. However, the effects of Co contents on the NiCo based composite coatings were few investigated. Srivastava et al. [18] found that the presence of 25 wt% cobalt in the NiCo–CeO2 composite coatings increased the hardness and improved its wear resistance. And their result also showed that addition of 85 wt% cobalt in the composite coating imparted the composite coating better thermal stability. In reference [19], addition of 28 wt% Co content in the NiCo–SiC matrix showed the higher hardness than other coatings. Although the particles reinforced NiCo composite coating were widely studied, the fabrication of Zr particles reinforced NiCo–Zr
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[(Fig._3)TD$IG]
Fig. 3. XRD patterns of NiCo–Zr composite coatings with different Co contents. Fig. 1. Element content in the deposits as a function of CoSO47H2O concentrations in the bath.
composite coatings were few reported and the effects of Co contents on the NiCo based composite coatings need further investigation. In this study, the NiCo–Zr composite coatings with different Co contents were successfully prepared by electrodeposition from the modified Watts baths containing different Co2+ concentrations. Then, the effects of Co contents on the surface morphologies, crystal structure, grain size, micro-strain, hardness and residual stresses were investigated. In addition, the corrosion resistance of the NiCo–Zr composite coatings was discussed in detail.
the electrolytes were ultrasonic dispersion for 30 min followed by magnetically stirring for about 4 h at a stirring rate of 300 rpm. The surfactant C12H25NaSO4 (0.2 g/L) was also added in the bath to disperse the particles. The solution temperature and pH value were maintained at 50 C and 4.2, respectively. The applied current density was maintained at a constant of 4 A/dm2 for 1 h to maintain a thickness of about 30 mm for the deposited coating. Pure nickel plate and stainless steel plate with an area of 1 1 cm2 were used as anode and cathode, respectively. The stainless steel specimens were grounded with grade 600, 800 and 1200 emery papers step by step. Then, the substrates were degreased in acid (10% HCl) and washed with distilled water before deposition.
2. Experimental
2.2. Coatings characterization
2.1. Coating processes
Morphology observation of the composite coatings was carried out using a field emission scanning electron microscope (FESEM, JSM-7600F), and the chemical composition was checked with the energy dispersive X-ray spectroscopy (EDX) attached with the FESEM. In addition, the X-ray photoelectron spectroscopy (XPS) was also used to examine the chemical composition of the
Electrodeposition of the NiCo–Zr composite coatings was carried out in the Watt baths containing NiSO46H2O (240 g/L), NiCl26H2O (40 g/L) and H3BO3 (30 g/L). Co was added as CoSO47H2O and the additions ranged from 5 g/L to 40 g/L of Co contents. The Zr particles of 20 g/L with a mean diameter of 1 mm were added into the solutions. In order to ensure a good dispersion of the Zr particles, all
[(Fig._4)TD$IG]
[(Fig._2)TD$IG]
Co/Ni in the deposite
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16
Co/Ni in the solution Fig. 2. Effect of electrolyte Co/Ni ratio on deposit Co/Ni ratio.
Fig. 4. Typical XRD patterns of Ni–0Co/Zr, Ni–29Co/Zr and Ni–58Co/Zr composite coatings.
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composite coatings. All XPS spectra were collected with a Kratos Axis Ultra DLD spectrometer (Krato Analytica-A Shimadzu Group Company), using a monochromatic Al Ka X-ray source. In order to remove the surface oxides of the coating, Ar+ sputtering of 10 min was carried out with a minibeam IV ion gun under 1.5 108 Torr in the sample analysis chamber. The atomic force microscope (dimension edge) was also used to examine the surface morphology and roughness of the composite coatings. The phase structures of the as-deposited coatings was determined by a Rigaku Ultima IV X-ray diffractometer (Cu Ka radiation, l = 1.54056 Å) in standard 2u–u mode and the voltage and current were 40 kV and 30 mA, respectively. The grain size and the microstrain of the coatings were obtained by using the Modified Williamson-Hall method (MWH) [20]. The lattice parameter a was also obtained using Nelson–Riley method [21]. The instrumental line broadening of the measured profiles was corrected using a Ni powder as the standard sample, which was annealed thoroughly at 450 C for 6 h. The structural broadening only due to crystallite size and lattice strain (B(struct)) was obtained using the following equation qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi BðstructÞ ¼ B2obs B2std , where, Bstd and Bobs are full width at half-
Lattice parameter a, nm
0.3532
0.3528
0.3524
0.3520
0.3516
Ni-0
Co/Z r
Ni-1
8 Co
/Zr
Ni-2
9Co
/Zr
Ni-4
2Co
/Zr
Ni-5
8Co
/Zr
Fig. 5. Effect of Co content on the lattice parameter of the NiCo–Zr composite coatings.
maximum of any particular reflection from the standard sample (Si powder) and the sample, respectively. Residual stress examination of the as-deposited Ni–Zr composite coatings was carried out on a Proto LXRD Residual Stress Analyser using the classical sin2 c method [22]. The voltage and current were 30 kV and 25 mA, respectively. The peaks of (3 3 1)a of Ni were selected to calculate the residual stress. The Vickers microhardness measurement was carried out with a load of 200 g and an indentation time of 15 s. The corresponding final values were determined as the average of 8 measurements. The potentiodynamic polarization measurements were conducted in a standard three-electrode method. A platinum sheet and a Ag/AgCl electrode [+207 mV(SHE)] were used as the auxiliary electrode (AE) and the reference electrode, respectively. The sample was used as working electrode (WE). The measurements were conducted in 3.5 wt% NaCl solution at the temperature of 25 C using an electrochemical apparatus (CHI660E) at a scan rate of 1 mV/s. The polarization potentiodynamic curves were recorded after 40 min of immersion. The corrosion potential Ecorr and corrosion current density Icorr were determined from Tafel curves. The electrochemical impedance (EIS) measurements were conducted at the open circuit potential after immersion of a sample into solution for 30 min. The applied sinuous potential amplitude was 5 mV and the frequency ranged from 0.01 Hz to 100,000 Hz.
The electro-codeposition mechanism of the composite coatings has been widely studied in the past several decades and several models have been developed, such as the Guglielmi’s model [23], Celis’s model [24] and the trajectory model [25]. In this study, the depositing of the NiCo–Zr composite coating could be explained by the Guglielmi’s model containing successive adsorption. In the first step, the Zr particles with adsorbed ions were physically and loosely adsorbed on the cathode surface. In the second step, the metal ions adsorbed on the Zr particles were reduced, making the Zr particles strongly absorb on the growing surface and to be embedded in the coating. Furthermore, addition of Co2+ in the bath resulted in the formation of alloyed NiCo–Zr composite coating. Fig. 2 shows the effect of Co/Ni ratio in electrolytes on the Co/Ni ratio in the deposits. It could be found that the Co contents in the composite coatings were significantly higher than those in the baths, which showed an anomalous co-deposition. This phenomenon has also been reported in references [10,27,28] and several mechanisms have been proposed to explain this phenomenon, such as a local increase in the pH [29], two-step codeposition of cobalt cations and decreasing the nickel deposition in the second step [30], and the preferential deposition of cobalt cations [31]. Previous researches [26,32,33] pointed out that increase of Co contents could enhance the codeposition of the incorporated
3. Results and discussion
[(Fig._6)TD$IG] 100
3.1. Chemical component content analysis
0.7
80
0.6 0.5
60
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40
0.2 20
0.1
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r
Ni-1
8Co
/Zr
Ni-2
9Co
/Zr
Ni-4
2Co
/Zr
Ni-5
8Co
/Zr
Fig. 6. Grain size and microstrain of the composite coatings.
Microstrain, %
Grain Size, nm
Fig. 1 shows the element contents of the NiCo–Zr composite coatings as a function of CoSO47H2O concentrations in the bath. The graph clearly showed that the Co contents increased linearly while Ni contents decreased linearly with increasing the CoSO47H2O concentrations in the bath. The average Co contents obtained from the results of EDX were 18%, 29%, 42% and 58 wt% for the coatings deposited at CoSO47H2O concentrations of 5 g/L, 10 g/ L, 20 g/L and 40 g/L, respectively. The XPS experiments were also done to confirm the element contents of the composite coatings and it could be found that the variation trend of Ni, Co and Zr contents determined from the XPS were in agreement with that obtained from the EDX. However, the Zr particle contents showed no obvious variation with increasing CoSO47H2O concentrations. Therefore, the NiCo–Zr composite coatings could be referred to as Ni–0Co/Zr, Ni–18Co/Zr, Ni–29Co/Zr, Ni–42Co/Zr and Ni–58Co/Zr.
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particles such as Si3N4, SiC and Al2O3, which was due to that the adsorption of Co2+ cations on the particles surface was easier than the Ni2+ cations. However, in reference [34], the Co contents increased while the SiC particle contents decreased with reducing the current densities. Similar results are also observed for the ZrC and Al particles that the ZrC and Al particle contents decreased with increasing Co contents in the composite coatings [35,36]. However, In this study, the Zr particle contents showed no evident variation with increasing Co contents as shown in Fig. 1, which might be due to the different adsorption capacity of Zr microparticles with the Co2+ cations compared with the particles mentioned above.
3.2. Coating microstructures Fig. 3 displays the XRD patterns of the NiCo–Zr composite coatings with different Co contents. As shown in Fig. 3, the Ni or NiCo matrix peaks corresponding to the (111), (2 0 0), (2 2 0), (3 11) and (2 2 2) were detected and all the coatings exhibited the random orientation. With the increasing Co contents, the NiCo matrix peaks decreased in intensities and the diffraction peaks broadened. To further investigate the effects of Co contents on the microstructures of the composite coatings, the XRD patterns of Ni–0Co/ Zr, Ni–29Co/Zr and Ni–58Co/Zr were selected to present in Fig. 4. The shifting of the NiCo matrix peaks to the lower angle was found
[(Fig._7)TD$IG]
Fig. 7. FESEM images of NiCo–Zr composite coatings with different Co contents (a) Ni–0Co/Zr (b) Ni–18Co/Zr (c) Ni–29Co/Zr (d) Ni–42Co/Zr and (e) Ni–58Co/Zr.
[(Fig._8)TD$IG]
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Fig. 8. AFM images of NiCo–Zr composite coatings with different Co contents (a) Ni–0Co/Zr (b) Ni–18Co/Zr (c) Ni–29Co/Zr (d) Ni–42Co/Zr and (e) Ni–58Co/Zr.
for the Ni–29Co/Zr composite coating compared with the Ni–Zr composite coating, which was due to that the smaller Ni atoms were partly substituted by the larger Co atoms. With increasing the Co content to 58 wt%, the peaks of the NiCo further shifted to the lower angle. Fig. 5 shows the lattice parameter a calculated from the Nelson–Riley method and it was clearly shown that the lattice parameter a increased linearly with increasing Co contents duo the solid solution. It could also be found from the XRD patterns that the hcp Co were detected for the Ni–58Co/Zr composite coatings. Previous studies [27,28] pointed out that the crystal structure of the NiCo coatings were closely dependent on the Co contents in the coatings. The crystal structures of the composite coatings changed from the typical FCC structures to mixed structures of FCC + HCP, and then to the HCP structures with increasing Co contents. Fig. 6 demonstrates the grain size and microstrain of the NiCo– Zr composite coatings. As seen in Fig. 6, the grain size of the Ni–Zr composite coating was 92 nm and the addition of Co decreased the grain size of the composite coatings. As the Co contents increased from 18 wt% to 58 wt%, the grain size of the composite coatings decreased from 70 nm to 31 nm. As mentioned above, the Zr particle contents showed no evident changes with increasing Co contents. Thus, the decrease in grain size could be ascribed to the addition of Co, which was also reported in references [34,37,38]. During the electrodeposition, Co+2 and Ni+2 could react with OH to form the metal mono-hydroxides Co(OH)+ and Ni(OH)+, which usually adsorbed on the surface of electrodeposits [39]. The greater adsorption of Co(OH)+ than Ni(OH)+ in Ni–Co/Zr electrolyte could block the growth centers, which resulted in the smaller grain size [40]. It could also be found from Fig. 6 that the microstrain of the composite coatings increased with increasing Co contents and obtained the maximum at Co content of 58 wt%. The increase of the microstrain was related to the increasing Co contents in the coatings. As mentioned above, the Ni atoms were partly
substituted by Co atoms to form the solid solution NiCo phases, which resulted in the distortion of the composite coatings. Thus, the microstrain of the composite coatings increased with increasing Co contents. 3.3. Surface morphology and cross sectional morphology characteristics FE-SEM morphology images of the NiCo–Zr composite coatings with different Co contents are shown in Fig. 7. The Ni–Zr composite coating demonstrated some polygonal structures as shown in Fig. 7a. With increasing the Co contents from 18 wt% to 42 wt%, a change in the structure from polygonal structures to the globular crystal structure occurred and the structure became smaller (Fig. 7b–d). However, some cauliflower structures were observed (Fig 7e) with further increasing the Co content to 58 wt%. Fig. 8 shows the AFM images of the NiCo–Zr composite coatings with different Co contents. It was observed that surfaces of the composite coatings were refined and the average roughness of the composite coatings were 190 nm, 161 nm, 151 nm, 114 nm and 125 nm with increasing contents. The surface morphology observation indicated that addition of Co could make the surface of the Ni–Zr composite coating smoother and more compact, which was also reported in NiCo–Si3N4 composite coating [26]. The cross sectional FESEM images of NiCo–Zr composite coatings with different Co contents are shown in Fig. 9. It could be found that all the coatings showed fairly uniform thickness and the Zr particles were also found to show fairly uniform distribution over the cross sectional surface of the composite coatings. In order to further study the Zr particle distribution uniformity across the composite coating, the Ni, Co and Zr particle dot-maps on the cross sectional FESEM image of the Ni–42Co/Zr composite coating is shown in Fig. 10. The Zr dot-map patterns confirmed that the Zr
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Fig. 9. Cross sectional images of NiCo–Zr composite coatings with different Co contents of (a) 18 wt%, (b) 29 wt% (c) 42 wt% and (d) 58 wt%.
particles showed fairly uniform distribution over the cross sectional surface of the composite coating. 3.4. Hardness measurements Fig. 11 shows the measured microhardness of the NiCo–Zr composite coatings. It was clear that the measured hardness values of the composite coatings increased with increasing Co contents and obtained the maximum at Co content of 42 wt% beyond which the hardness decreased. The microhardness of the composite
coatings was determined by both the matrix material and the reinforced particles. As mentioned above, the incorporated Zr particle contents showed a constant with increasing Co contents. Thus, the hardness enhancement of the composite coating was related to the changes of the matrix material. Firstly, the grain size of the coatings decreased with increasing Co contents (as shown in Fig. 6), which resulted in a higher hardness consistent with the well-known Hall–Petch relationship [41]. Secondly, the solid solution strengthening caused by the substitution of Ni atoms by Co atoms also contributed to the increase of hardness [38], and
[(Fig._10)TD$IG]
Fig. 10. The Ni, Co and Zr particle dot-map on the cross sectional FESEM image of the Ni–42Co/Zr composite coating.
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-2
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Microhardness, Hv
Log (current density), Log (Acm )
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/Zr
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/Zr
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-1
-3 -4 -5 -6 -7 -8 -9 -0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
Potential / V
Fig. 11. Hardness of composite coating with different Co contents.
the more Co contents the composite coatings contained, the higher the hardness were. However, the hardness of the composite coating decreased as the Co content increased to 58 wt%, which was due to the formation of hcp Co [32].
Ni-0Co/Zr Ni-18Co/Zr Ni-42Co/Zr
-2
Fig. 13. Potentiodynamic polarization curves for NiCo–Zr composite coatings in 3.5% NaCl solution.
atoms by Co atoms might also contribute to the increase of the residual stress of the NiCo–Al composite coatings.
3.5. Residual stress analysis 3.6. Potentiodynamic polarization Fig. 12 presents the residual stresses of the NiCo–Zr composite coatings with different Co contents and all the coatings exhibited the tensile residual stress. It could be found from Fig. 10 that the residual stress of 113 MPa for Ni–Zr composite coating was obtained. As the Co contents increased from 18 wt% to 58 wt%, the residual stresses of the NiCo–Zr composite coatings increased from 169 MPa to 438 MPa. The residual stress results showed that addition of Co increased the residual stresses of the NiCo–Zr composite coatings. Coalescence of the islands and decreasing structural mismatch were the main mechanisms for the occurrence of tensile stress [42]. The increase in residual stress with increasing Co contents was due to the decrease in grain size. Nguyen et al. [43] pointed out that the smaller grain possesses higher surface free energy and the crystallite coalescence could lead to higher tensile stresses in the coating. Therefore, the smaller the grain sizes were, the higher the tensile stresses would be. In addition, the increasing distortions caused by substituting of Ni
[(Fig._12)TD$IG] Residual Stress, MPa
500 450 400 350
Fig. 13 presents the potentiodynamic polarization curves of the NiCo–Zr composite coatings and the corrosion parameters, such as corrosion potential (Ecorr) and corrosion current (Icorr) derived from the polarization curves, are concluded in Table 1. It could be found that the corrosion current (Icorr) of the NiCo–Zr composite coatings (except for the Ni–58Co/Zr) were less than that of the Ni–Zr (Ni– 0Co/Zr) coating. In term of the NiCo–Zr composite coatings, the corrosion current (Icorr) increased with increasing Co contents. Two important conclusions could be obtained by comparing the corrosion current (Icorr) usually used to measure the corrosion resistance of the composite coatings. On one hand, the addition of Co enhanced the corrosion resistance of the NiCo–Zr composite coatings, which was ascribed to the smoother and more compact surfaces compared with the Ni–Zr composite coating [6,9]. On the other hand, the corrosion resistance of the NiCo–Zr composite coatings decreased with increasing Co contents. The decrease in the corrosion resistance were related to several factors such as chemical composition, phase structure and grain size. It is acknowledged that alloying can decrease the corrosion resistance by changing the nobility of materials [10,44]. Because Co is more active than Ni [10,45], it is expected that the electrochemical activity of the NiCo alloy is greater than the pure Ni coating. Therefore, the corrosion resistance of the NiCo–Zr alloy coating decreased with increasing Co contents. Another factor to deteriorate the corrosion resistance of the coating was the formation of
300 250
Table 1 Corrosion potential Ecorr and corrosion current Icorr of coatings with various Co content.
200 150 100
Ni-0
Co/Z r
Ni-1
8Co
/Zr
Ni-2
9Co
/Zr
Ni-4
Ni-5 2Co 8Co /Zr /Zr
Fig. 12. Residual stress of composite coatings with different Co contents.
Composite coating with different Co contents
Ecorr (mV)
Icorr (mA/cm2)
Ni–0Co/Zr Ni–18Co/Zr Ni–29Co/Zr Ni–42Co/Zr Ni–58Co/Zr
27.5 113.7 65.3 134.3 96.3
0.552 0.263 0.311 0.448 0.594
202
[(Fig._14)TD$IG]
F. Cai et al. / Materials Research Bulletin 65 (2015) 195–203 Table 2 Fitting of charge-transfer resistance from the measured EIS plots on composite coatings. Composite coating with different Co contents
Rct (kV cm2)
Ni–0Co/Zr Ni–18Co/Zr Ni–29Co/Zr Ni–42Co/Zr Ni–58Co/Zr
119.57 236.92 186.33 174.17 98.76
4. Conclusions
Fig. 14. Nyquist impedance diagrams for NiCo–Zr composite coatings in 3.5% NaCl solution.
hcp Co. In general, two phase structures are less corrosion resistant than the single-phase structure because of galvanic cells can be easily formed between phases with different nobilities [10]. The last factor influencing the corrosion resistance of the composite coating was the grain size. Previous studies pointed out that the grain boundaries are prone to corrosion attacks, especial for the multi-phase structure materials [46]. In this case, the grain size for the NiCo–Zr composite coatings decreased with the increasing Co contents, which resulted in the decrease of the corrosion resistance of the composite coatings. 3.7. Electrochemical impedance spectroscopy (EIS) The Nyquist impedance plots obtained for the composite coatings in 3.5 wt% NaCl solution are shown in Fig. 14. All the composite coatings exhibited a single semicircle and the diameters of the semicircle of the NiCo–Zr composite coatings (except for the Ni–58Co/Zr) were larger than that of Ni–Zr composite coating. Moreover, the diameters of the semicircle for the NiCo–Zr composite coatings decreased with increasing Co contents, indicating the highest corrosion resistance for the Ni–18Co/Zr composite coating. To account for the corrosion resistance of the composite coating, the equivalent circuit model (Fig. 15) was used fit the impedance spectra and a good fit with this model was obtained. The Rs represented the solution resistance. The constant phase element, C.P.E, was introduced in the circuit instead of a pure double layer capacitor to give a more accurate fit. The Rp represented the charge transfer resistance whose value could be used to measure the corrosion resistance. Table 2 shows the date extracted from the EIS by simulation. It could be found that the Ni– 18Co/Zr composite coatings (except for the Ni–58Co/Zr) possessed the highest Rp, indicating the highest corrosion resistance.
[(Fig._15)TD$IG]
Fig. 15. Equivalent circuits used for fitting of impedance plots for NiCo–Zr composite coatings in 3.5% NaCl solution.
The NiCo–Zr composite coatings were prepared from the electrolytes with different Co2+ concentrations. The Co contents in the deposited composite coatings increased from 18 wt% to 58 wt% as the Co2+ concentration increased from 5 g/L to 40 g/L and have an effect on the microstructures and properties of the composite coatings. The surface morphology of the composite coating became smoother and more compact with the addition of Co. The crystal structures of the coatings were dependent on the Co content. With increasing the Co contents, the grain size decreased while the microstrain increased. The addition of Co also increased the microhardness of the composite coatings and the microhardness attained the maximum of 498 Hv at the Co content of 42 wt%. The residual stress of the composite coatings increased with increasing the Co contents. The corrosion experiments showed that the addition of Co increased the corrosion resistance of the Ni– Zr composite coating due to the smoother and more compact surface. However, the corrosion resistance of the composite coatings decreased with increasing Co contents. References [1] A. Abdel Aal, S.M. El-Sheikh, Y.M.Z. Ahmed, Mater. Res. Bull. 44 (2009) 151–159. [2] W. Shao, D. Nabb, N. Renevier, I. Sherrington, Y.Q. Fu, J.K. Luo, J. Electrochem. Soc. 159 (11) (2012) D671. [3] R.K. Saha, T.I. Khan, Surf. Coat. Technol. 205 (2010) 890–895. [4] R. Sen, S. Das, K. Das, Mater. Res. Bull. 47 (2012) 478–485. [5] J. Zhao, X. Peng, F. Wang, Mater. Res. Bull. 45 (2010) 420–424. [6] S. Hassani, K. Raeissi, M. Azzi, D. Li, M.A. Golozar, J.A. Szpunar, Corros. Sci. 51 (2009) 2371–2379. [7] J. Kang, Y. Yang, X. Jiang, H. Shao, Corros. Sci. 50 (2008) 3576–3580. [8] B. Bakhit, A. Akbari, Surf. Coat. Technol. 206 (2012) 4964–4975. [9] X. Yang, Q. Li, S. Zhang, H. Gao, F. Luo, Y. Dai, J. Solid State Electrochem. 14 (2010) 1601–1608. [10] B. Bakhit, A. Akbari, F. Nasirpouri, M. Ghasem Hosseini, Appl. Surf. Sci. 307 (2014) 351–359. [11] L. Shi, C.F. Sun, P. Gao, F. Zhou, W.M. Liu, Surf. Coat. Technol. 200 (2006) 4870– 4875. [12] B. Ranjith, G. Paruthimal Kalaignan, Appl. Surf. Sci. 257 (2010) 42–47. [13] L.M. Chang, H.F. Guo, M.Z. An, Mater. Lett. 62 (2008) 3313–3315. [14] Y. Yang, Y.F. Cheng, Surf. Coat. Technol. 216 (2013) 282–288. [15] Y. Yang, Y.F. Cheng, Electrochim. Acta 109 (2013) 638–644. [16] L. Shi, C. Sun, P. Gao, F. Zhou, Appl. Surf. Sci. 252 (2006) 3591–3599. [17] B.R. Tian, Y.F. Cheng, Electrochim. Acta 53 (2007) 511–517. [18] M. Srivastava, V.K. William Grips, K.S. Rajam, Appl. Surf. Sci. 257 (2010) 717– 722. [19] M. Srivastava, V.K. William Grips, K.S. Rajam, Appl. Surf. Sci. 253 (2007) 3814– 3824. [20] T. Unger, I. Dragomir, A. Revesz, A. Borbely, J. Appl. Cryst. 32 (1999) 992–1002. [21] M. Razavi, M.R. Rahimipour, A.H. Rajabi-Zamani, J. Alloys Compd. 436 (2007) 142–145. [22] P.J. Withers, H. Bhadeshia, Mater. Sci. Tech. 17 (2001) 355. [23] N. Guglielmi, J. Electrochem. Soc. 119 (8) (1972) 1009–1012. [24] J.P. Celis, J.R. Roos, J. Electrochem. Soc. 124 (10) (1977) 1508–1511. [25] J. Fransaer, J.P. Celis, J.R. Roos, J. Electrochem. Soc. 139 (1992) 413–425. [26] M. Srivastava, V.K. William Grips, K.S. Rajam, J. Alloys Compd. 469 (2009) 362– 365. [27] C. Lupi, A. DellEra, M. Pasquali, P. Imperatori, Surf. Coat. Technol. 205 (2011) 5394–5399. [28] L. Tian, J. Xu, S. Xiao, Vacuum 86 (2011) 27–33. [29] A. Bai, C.C. Hu, Electrochim. Acta 47 (2002) 3447–3456. [30] E. Gómez, J. Ramirez, E. Valles, J. Appl. Electrochem. 28 (1998) 71–79. [31] G. Qiao, T. Jing, N. Wang, Y. Gao, X. Zhao, J. Zhou, W. Wang, Electrochim. Acta 51 (2005) 85–92. [32] B. Bakhit, A. Akbari, J. Alloys Compd. 560 (2013) 92–104.
F. Cai et al. / Materials Research Bulletin 65 (2015) 195–203 [33] G. Wu, N. Li, D.L. Wang, D.R. Zhou, B.Q. Xu, K. Mitsuo, Mater. Chem. Phys. 87 (2004) 411–419. [34] S.M. Lari Baghal, M. Heydarzadeh Sohi, A. Amadeh, Surf. Coat. Technol. 206 (2012) 4032–4039. [35] Z. Zhang, C. Jiang, N. Ma, J. Mater. Eng. Perform. 23 (11) (2014) 4065–4071. [36] F. Cai, C. Jiang, P. Fu, J. Vincent, Appl. Surf Sci. 324 (2015) 482–489. [37] L. Wang, Y. Gao, Q. Xue, H. Liu, T. Xu, Appl. Surf. Sci. 242 (2005) 326–332. [38] B. Bakhit, A. Akbari, Surf. Coat. Technol. 206 (2012) 4964–4975. [39] A. Bai, C.C. Hu, Electrochim. Acta 47 (2002) 3447. [40] B. Tury, M. Lakatos-Varsányi, S. Roy, Surf. Coat. Technol. 200 (2006) 6713–6717.
203
[41] M. Hakamada, Y. Nakamoto, H. Matsumoto, H. Iwasaki, Y.Q. Chen, H. Kusuda, M. Mabuchi, Mater. Sci. Eng. A 457 (2007) 120. [42] C.V. Thompson, Annu. Rev. Mater. Sci. 30 (2000) 159. [43] V.C. Nguyen, C.Y. Lee, L. Chang, F.J. Chen, C.S. Lin, J. Electrochem. Soc. 159 (6) (2012) D393. [44] ASM Handbook, Corrosion: Fundamentals, Testing, and Protection, vol. 13A, ASM, International, Materials Park, OH, 2003. [45] J. Edward, Coating and Surface Treatment Systems for Metals: A ComprehensiveGuide to Selection, ASM International, Michigan, 1997. [46] B. Bakhit, A. Akbari, J. Coat. Technol. Res. 10 (2) (2013) 285–295.