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ZrO2-CeO2 composite coating for enhanced hardness and corrosion resistance
Results in Physics 13 (2019) 102375
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Fabrication and optimization of Ni-W/ZrO2-CeO2 composite coating for enhanced hardness and corrosion resistance Baosong Lia, Dandan Lia, Tianyong Meia, Weiwei Zhangb, a b
T
⁎
College of Mechanics and Materials, Hohai University, Nanjing 211100, China College of Mechanical and Electrical Engineering, Hohai University, Changzhou 213022, China
ARTICLE INFO
ABSTRACT
Keywords: Nanocomposite coating Ni-W alloy Zirconia Ceria Pulse electrodeposition
Ni-W/ZrO2-CeO2 composite coating has been fabricated via pulse electrodeposition. The structure, morphology, composition and the operating parameters of the nanocomposite coating were evaluated and optimized for the enhanced hardness and corrosion resistance. It illustrated that the deposited coatings are dense, crack-free with nodular or granular structure. The average current density of 4–6 A dm−2, duty cycle of 70% is beneficial to the finer structure and higher content of nanoparticles in deposits. The average roughness, crystallite size and growth rate of the composite coating was examined. The hardness as a function of duty cycle, current density and deposition time were investigated. The optimal parameters for the improved performance of the composites were given. It was illustrated that the reinforcing particle phase had been uniformly co-deposited in the coating. The embedded ZrO2 and CeO2 particles could enhance the corrosion resistance and hardness of Ni-W metallic matrix. Ni-W/ZrO2-CeO2 nanocomposite coatings possess higher performance in aggressive environment and exhibit great application values.
Introduction In recent years, particle reinforced metal matrix composite (MMC) coating has attracted much attention for their superior performance including high hardness, wear and corrosion resistance [1]. It has been illustrated that incorporation of particles in metallic matrix could enhance its performance or obtain an entirely new property [2,3]. Luo investigated WC [4], and YSZ [5] reinforced Ni-P nanocomposites and illustrated the enhanced corrosion resistance and improved hardness of the nanocomposite coating. Singh [6] prepared Ni-Fe/In2O3-WO3 nanocomposite coatings and investigated its hardness. It demonstrated the strengthening effects of the incorporated particles in metallic matrix. Aliofkhazraei [7] studied pulse reverse electrodeposited Ni-W-Co/ Al2O3 nanocomposite and illustrated that corrosion and wear behavior of the tailored coatings are heavily affected by electrochemical process parameters. Sengupta [8] developed a bilayered Zn-Ni/Ni-Co-SiC nanocomposite coating and illustrated it’s superiority in hardness, wear resistant and anti-corrosive coating. Recently, as an anti-corrosion and wear-resistant alloy, nickeltungsten (Ni-W) coatings have been investigated by many researchers for their good appearance, high microhardness, anti-corrosion and wear-resistant properties [4]. However, in harsh conditions, it still needs to improve its performance to meet the requirements of corrosion ⁎
and wear resistance. Then, nanoparticle reinforced Ni-W composite coating was developed [9,10]. It was reported that Al2O3 [11], BN [12,13], TiN [14] SiC [15,16], ZrO2 [17], Si3N4 [18] and MWCNT [19] have been added to Ni-W matrix to enhance their properties. Wasekar [20] investigated Ni-W/SiC coating by pulse deposition. Keshri [21,22] studies the SiC reinforced Ni-W coatings and illustrated the enhanced hardness and corrosion resistance. Among various particles, ZrO2 owns high hardness, high strength and good fracture toughness [11,23]. Till now, only several literature [24,25] have reported Ni-W/ZrO2 coating, demonstrating the enhanced properties by the addition of ZrO2 in Ni-W matrix. References [26,27] illustrated that the addition of two kinds of particles in a metallic matrix might provide better performance. Singh [28] has fabricated Ni-Fe/Si3N4-TiN nanocomposite coatings from sulphamate-N, N-dimethylformamide bath and illustrated that Si3N4 and TiN ceramics particles have prodigious effects on the structure and properties of the coating, leading to higher hardness and corrosion resistance compared to Ni-Fe, Ni-Fe/Si3N4 and Ni-Fe/TiN coatings. Wang [29] illustrated that the incorporation of CeO2 into metal matrix could improve the corrosion resistance. Aruna [30] illustrated that embedded CeO2 could enhance the hardness and protective properties of Ni coating. Thus, it was feasible by embedding ZrO2 and CeO2 particles into Ni-W matrix to obtain new or excellent properties. However, till
Corresponding author. E-mail address: [email protected] (W. Zhang).
https://doi.org/10.1016/j.rinp.2019.102375 Received 18 April 2019; Received in revised form 16 May 2019; Accepted 21 May 2019 Available online 25 May 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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now, Ni-W/ZrO2-CeO2 composite coating is less studied. It is necessary to investigate the structure and properties of Ni-W/ZrO2-CeO2 composite coatings. In this work, Ni-W/ZrO2-CeO2 coating was fabricated and optimized. This work could help the development of particle phase reinforced Ni-W composite coating. Experimental Fabrication of Ni-W/ZrO2-CeO2 composite coating The composition of the electrolyte for Ni-W/ZrO2-CeO2 electrodeposition was designed based on our previous work [31,32] on Ni-W/ ZrO2 coating and the related literature [33–35] on CeO2 reinforced nickel coating. The coating were deposited from an ammonia-citrate bath containing: 32 g L−1 nickel sulfate hexahydrate, 66 g L−1 sodium tungstate dihydrate, 118 g L−1 sodium citrate dihydrate, 0.002 g L−1 sodium lauryl sulfate, 30 g L−1 ZrO2 and 10 g L−1 CeO2 nanoparticles. The chemicals were analytical grade. The ZrO2 and CeO2 particles were provided by Shanghai Aladdin biochemical Technology Co., Ltd and their size was in the range of 10–45 nm. The electrolyte temperature, pH, stirring speed was kept at 65 ± 2 °C, 8.3 ± 0.1 and 400 ± 50 rpm, respectively. The deposited coating samples were named 1#–1 A dm−2, 2#–4 A dm−2, 3#–30%, 4#–70%, 5#–10 min and 6#–60 min and their corresponding process parameters were presented in Table 1. Fig. 1 shows the applied pulse current in the electrodeposition process comprising a cathodic current and a zero current. The relationships of the parameters in pulse electrodeposition are as follows:
f=
1 Ton + Toff
(1)
r=
Ton × 100% Ton + Toff
(2)
Fig. 1. Schematic of the applied pulse current in the electrodeposition process.
D=
RTC(hkl) =
× 100%; R(hkl) =
Is (hkl) Ip (hkl)
(5)
It was reported [25] that the particle shape greatly affected the amount and dispersion of particles in coating. The particles in spherical shape resulted in better codeposition in metal matrix than those of irregular shape [24]. Therefore, the zirconia and ceria particles were first examined by TEM observation. Fig. 2(a, b) presents that zirconia particles have polygonal shape which is near spherical shape in diameters of 20–38 nm. This particle size of zirconia is in accordance with the size provided by manufacturer (< 50 nm). Fig. 2(c, d) displays that ceria particles have a similar shape with zirconia. However, the facets of ceria particles are sharp and the uniformity of the particle size is less compared to zirconia particles. It can be seen that the ceria particle size was in dimension from 10 nm to more than 40 nm. Due to the high surface energy of the particles, agglomeration was observed. Comparatively, the agglomeration tendency of nano-ceria is greater than that of nano-zirconia, which was confirmed by the TEM observation.
Table 1 Electrodeposited Ni-W/ZrO2-CeO2 coating and the corresponding process parameters.
1 A dm−2, 50%, 100 Hz, 20 min 4 A dm−2, 50%, 100 Hz, 20 min 30%, 4 A dm−2, 100 Hz, 20 min 70%, 4 A dm−2, 100 Hz, 20 min 10 min, 100 Hz, 4 A dm−2, 50%, 60 min, 100 Hz, 4 A dm−2, 50%,
R(hkl)
Characterization of ZrO2 and CeO2 nanoparticles
TEM (Tecnai G2F30 S-Twin) was utilized to observe the surface microstructure of the coatings. The cross-sectional images were examined by SEM (Hitachi S4800). The morphology, composition, structure, topography and surface element states were characterized by SEM, EDS, XRD, AFM and XPS, respectively. The hardness was measured with a DHV-1000 hardness tester under the applied load of 100 gf lasting for 15 s. The average value of five sites of each sample was reported as the final hardness. The surface roughness was calculated using the Nova software attached to NT-MDT AFM instrument. The grain size was calculated using the Debye-Scherrer Eq. (4)
1#–1 A dm−2 2#–4 A dm−2 3#–30% 4#–70% 5#–10 min 6#–60 min
n 1
Results and discussion
Characterization of Ni-W/ZrO2-CeO2 composite coating
Electrodeposition parameters
R(hkl)
where Is(hkl) and Ip(hkl) were the diffraction intensities of the (hkl) plane measured for the composite deposits and the standard Ni powder sample (JCPDS No. 04-0850), respectively. Corrosion resistance was evaluated by EIS methods at the Eocp in the frequency range of 105–10−2 with the input signal 10 mV on an electrochemical workstation (CHI 660E, CH Instruments). A three-electrode cell is used and platinum foil, saturated calomel electrode and deposited coating with 1 cm−2 exposed area are used as counter, reference and working electrode, respectively.
where f is frequency, Ton is current on time (s), Toff is current off time (s), r is duty cycle, Ipe is peak current density and Iav is average current density. Nickel and copper sheet was used as anode and cathode, respectively. The electrode space were kept at 30 mm. Before each test, the cathode was cleaned and activated in 7.2 wt.% HCl solution for 30 s.
Coating sample
(4)
where D was the mean crystalline size (nm), λ was X-ray wavelength (0.15418 nm), β was the full width at half maxima in intensity and θ is Bragg diffraction angle. The relative texture coefficient (RTC) [36,37] was used to characterize the relative degree of preferred crystal orientation among all crystal planes calculated by the following Eq. (5).
(3)
Iav = Ipe × r
0.94 cos
Cross-sectional and surface morphology The cross section of the deposit was observed by SEM images. Fig. 3a shows the cross-sectional images of the deposited Ni-W/ZrO2CeO2 composite coating, which was prepared under the deposition time of 60 min, frequency of 100 Hz, average current density of 4 A dm−2 and duty cycle of 50%. It can be seen the thickness of the deposited composite coating is about 29 μm. The growth rate is about 0.48 μm/ min. It indicated that the interface between the coating and substrate is crack free. The cross section exhibits uniform structure with some 2
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Fig. 2. TEM images of the nanoparticles used (a, b) ZrO2, (c, d) CeO2.
scratches which were produced in the sanding process. Fig. 3b shows the enlarged morphology of the yellow rectangular marked area A in Fig. 3a. It was found that the nanoparticles in the dimension of 20–40 nm were uniformly embedded in the bulk of the cross section of the coating. It was illustrated that the nanoparticles were homogeneously dispersed in Ni-W matrix, which is expected to enhance the properties of the nanocomposite deposits. TEM analysis was performed to illustrate the incorporation of nanoparticles in coating. Fig. 4 shows the TEM images of the pulse electrodeposited Ni-W/ZrO2-CeO2 coating. It was noticed that zirconia and ceria nanoparticles have been co-deposited and were wrapped in Ni-W metallic matrix. The diameters of these particles in the deposited coating were consistent with the size confirmed by our previous studies on zirconia and ceria powder via TEM, which was in the range of 10–40 nm. The composite coating exhibits a dense and homogeneous structure. No defects such as pores or cracks were found on the coating and the combination of the incorporated particles and the metallic matrix is good, indicating a satisfying phase interface. In general, reinforcing nanoparticles are uniform in size and are well embedded in the Ni-W metallic matrix. Fig. 5 presents the surface SEM images of the Ni-W/ZrO2-CeO2 coating prepared at different conditions. It should be noted that besides the given parameter in the figure caption, other deposition parameters were consistent with the value introduced in the experiment section. As seen in Fig. 5, the deposited Ni-W/ZrO2-CeO2 composite coating exhibits nodular or granular morphology with dense and homogeneous structure. It was reported [38] that the Ni-W crystal shows a long needle shape, which can be observed in these figures. Small particles were dispersed in the large granules formed on the coating. As well known, the granules were the aggregates of many small crystals. The
nodular structure that like a dome-like island can be seen in each sample. The difference lies in the size of the dome-like structure and their quantity and distribution. The size of a dome-like granule in Fig. 5c is more than 3 μm. The distributed particles in the coating are faintly visible, which was confirmed by the HRTEM (Fig. 4). Comparing Fig. 5a and b, the granule size obtained at 4 Adm−2 is more uniform in the dimension of less than 1 μm. Initially, increasing current density could improve the transfer process of particles. But the deposition rate of nickel increased more than particles as the current density increased, leading to the decrease of the particle content in deposits. It also can be found that the granule size of the coating prepared at 70% is the finest, compact and uniform among the investigated samples. The samples of 1 A dm−2 and 30% shows the relatively large granule size and rougher surface. Comparing Fig. 5e and f, it was found the granule size is similar for the coating deposited for 10 min and 60 min, indicating the granule could not grow up continuously. Chemical composition and phase structure Fig. 6(a–c) shows the composition of Ni-W/ZrO2-CeO2 coating deposited at different duty cycle. It illustrated that the coating contains 3.9–4.9 wt.% ZrO2 and 0.45–0.68 wt.% CeO2 nanoparticles. Here, the oxygen content was also detected by EDS. It needs to be noted that the weight % of ZrO2 and CeO2 presented in Fig. 6(a–c) were calculated according to content of Zr and Ce and the stoichiometry of ZrO2 and CeO2. Besides, the nanocomposite contains 65.7–71.9 wt.% nickel and 23.7–28.7 wt.% tungsten. With the increase of duty cycle, th embedded ZrO2 particles initially decreased and then increased with maximum content at 4.9 wt.% at r = 70%. The amount of CeO2 in the composite coating is low and increase with the duty cycle. Fig. 6(d–g) displays the 3
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Fig. 3. Cross-sectional images of Ni-W/ZrO2-CeO2 composite coating deposited at t = 60 min. f = 100 Hz, Iav = 4 A dm−2 and r = 50%. (a) low magnification of 2000×, (b) high magnification of 100,000×.
EDS map of Ni-W/ZrO2-CeO2 coating. It was illustrated that the coating comprises Ni, W, Zr, Ce and O elements. It was demonstrated that nickel and tungsten were evenly distributed. Generally, the particles distribution is uniform. The commercial zirconia powder has been characterized by XRD. Fig. 7a confirms that the crystal size was 15–40 nm. It indicated the ZrO2 powder was a mixture of the tetragonal and monoclinic phases. Fig. 7b shows the XRD pattern of CeO2 powder which reveals typical intense lines corresponding to the fcc structure. Fig. 8 shows the XRD pattern of the deposited coating and indicated that the single phase of Ni(W) solid solution have been formed with fcc structure. The primary peaks at 43.9–44.1°, 51.2–51.4° and 75.3–75.6° correspond to nickel (111), (200) and (220) crystal planes. Compared to the standard XRD pattern of nickel (111) plane at 44.51°, (200) plane at 51.84° and (220) plane at 76.37° (JCPDS No. 04-0850), the 2θ of the crystal plane has shifted to lower angles, due to the lattice distortion by addition of W and particles in nickel matrix. ZrO2 and CeO2 were detected with weak peaks at 30.2° and 28.3°, respectively. The peaks of substrate (JCPDS No. 50-1333) emerged because the X-ray could penetrate more than 10 μm which might be larger than the coating thickness. According to Scherrer's Eq. (4), the crystallite sizes based on the Ni (111) plane were 14–18 nm, as shown in Table 2. The grain sizes slightly increased with the increase of current density. The finer structure was obtained under 4 A dm−2. Fig. 8(c, d) exhibits the XRD spectra of the Ni-W/ZrO2-CeO2 nanocomposites prepared at 30% and 70%. The crystalline grain of Ni-W/ZrO2-CeO2 deposited at r = 70% has the minimum size. With the increase of deposition time, the grain
Fig. 4. TEM images of Ni-W/ZrO2-CeO2 composite coating (4 A dm−2, 50%, 100 Hz, 60 min).
size increases (Fig. 7e and f). In the electrodeposition process, as nucleation sites, the presence of nanoparticles could promote the formation of new grains and inhibit the growth of the already formed grains. Then, the grain refinement was achieved. Relative texture coefficient (RTC) was used to reflect the crystalline preferred orientation using Eq. (5). Here, (111), (200) and (220) reflection lines for Ni have been considered. The preferred orientation through an axis [hkl] was selected by values of RTC ≥ 25%. The strength of the preferred orientation becomes the maximum amount when RTC reaches the value of 100% [37]. The calculated RTC (hkl) values of the Ni-W/ZrO2-CeO2 coatings are shown in Table 2. When current density increases from 1 to 4 A dm−2, the RTC (111) value increases and RTC (200) decreases. At the current density of 4 A cm−2, the value of RTC (111) is 62.03%. When the duty cycle changed from 30% to 70%, the RTC (111), (220) increases and RTC (200) decreases. Contrarily, when the deposition time increases from 10 min to 60 min, the RTC (111) value decreases and the RTC (200) and (220) increases. This illustrates the textures of Ni-W/ZrO2-CeO2 coatings varied at different deposition parameters. For Ni-W/ZrO2-CeO2 coatings, the preferred orientation was (111) texture [39]. 4
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Fig. 5. SEM micrograph of Ni-W/ZrO2-CeO2 composite coating deposited under (a) 1 A dm−2, (b) 4 A dm−2, (c) 30%, (d) 70%, (e) 10 min, (f) 60 min. Other parameters see Section “Fabrication of Ni-W/ZrO2-CeO2 composite coating.”
3D topography and roughness analyzed by AFM
might help to improve the adhesion of the subsequent (top) coating.
Fig. 9 shows the topography of the coatings deposited at Iav = 4 A dm−2, r = 50%, f = 100 Hz and t = 20 min. Fig. 9 reveals that the nanocomposite coating exhibited hill-valley like morphology. The composite coating was made up of micron scale granules where the nanoparticles were dispersed throughout the coating. The average surface roughness(Ra) value was 79 ± 2 nm [2]. As can be seen, the granules were in the dimension of 250–550 nm, which was made up of many small crystals and particles. This hill-valley like morphology
Surface properties analyzed by XPS The surface properties of the coating have critical effects on the protective performances. Hence, it is necessary to investigate the surface properties of the coating by XPS. Fig. 10 indicated that Ni, W, Zr and Ce elements were detected by XPS spectra. Fig. 10a shows the Ni 2p3/2 XPS spectra with three decomposed peaks. The corresponding peaks at 851.5 eV, 855.2 eV and 860.4 eV belongs to Ni(0), Ni-O and
Fig. 6. The composition and element distribution map of Ni-W/ZrO2-CeO2 composite coating prepared at duty cycle of (a) 30%, (b) 50% and (c) 70%, (d–g) element distribution map of nickel, tungsten, zirconium and cerium, respectively. 5
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Table 2 Relative texture coefficient (RTC) and crystallite size of Ni-W/ZrO2-CeO2 composite coating electrodeposited at different conditions. Samples
1#–1 A dm−2 2#–4 A dm−2 3#–30% 4#–70% 5#–10 min 6#–60 min
Relative texture co-efficient (RTC), (%) (111)
(200)
(220)
53.24 62.03 49.78 52.57 60.26 41.29
23.71 15.52 18.66 13.95 16.93 19.45
23.05 22.45 31.56 33.48 22.81 39.26
Crystallite size(nm)
14.5 14.2 16.5 14.1 15.7 17.2
satellite nickel. It indicated the formation of oxide layer on the coating. Fig. 10b shows the W4f XPS spectra with four distinct peaks. The peak at 30.4 eV are attributed to W(0), suggesting that the tungstate ion has been reduced to W(0). Fig. 10c exhibits the Zr 3d XPS spectra of the deposits with two fitted peaks. The peaks at 181.6 eV and 184.0 eV are ascribed to Zr 3d5/2 and Zr 3d3/2, respectively [40]. Fig. 10d shows the Ce3d XPS spectra, illustrating the cerium element was mainly in the form of Ce4+ state originated from the added CeO2 particles [40]. Microhardness and corrosion resistance Fig. 11 shows the microhardness of the Ni-W/ZrO2-CeO2 nanocomposite coating deposited under different conditions. As can be seen, as Iav increased from 1 to 4 A dm−2, the hardness increased. It is well known that the composition and structure might be changed under different current density due to the different mass transfer conditions. Comparing the coatings deposited at r = 30% and r = 70%, the hardness reached the maximum value at r = 70%, which is consistent with literature [35]. Comparing the coating deposited at 10 min and 60 min, the hardness increased. Because the crystallite size was similar (14–18 nm), the change of hardness was mainly due to the nanoparticles content in the metallic matrix, which could be affected by current density, duty cycle and deposition time. The roles of the CeO2 on anti-corrosion properties of composite coatings have been reported by some references [41–44]. The Nyquist and Bode plots are shown in Fig. 12. Here, the equivalent electrical circuit (EEC) models consisting of RQ components (Fig. 13) was used to analyze all the EIS plots, which was a single time constant model. In Fig. 13, the Rs is solution resistance, Rct is charge transfer resistance and CPEdl is the electric double layer capacity. Fig. 12a depicts that as current density increases from 1 to 4 A dm−2, the radius of impedance arc increased. Table 3 shows that the Rct values is 30.8, 32.4 kΩ·cm2 at respectively. As Iav increases from 1 to 4 A dm−2, the 1 and 4 A dm−2 , Rct values increases. Fig. 12b shows that the modulus |Z| at 0.01 Hz of 4 A dm−2 is larger than that of 1 A dm−2. It illustrated that the coating prepared at 4 A dm−2 has better corrosion resistance. Fig. 12a also indicated that as the duty cycle increased from 30% to 70%, the corrosion resistance largely increased with the maximum value at 70%. Table 3 presented that when duty cycle increases from 30% to 70%, the Rct values increases from 24.0 kΩ·cm2 up to 48.5 kΩ·cm2, indicating the best corrosion resistance at duty cycle of 70%. The corresponding Bode plots are shown in Fig. 12b and displayed one hump shape, indicating only one time constant in the frequency range investigated. The modulus |Z| at 0.01 Hz reached the peak value at 70%. Fig. 12c depicts that as deposition time increases from 10 min to 60 min, the Rct values slightly decreased from 56.8 kΩ·cm2 at 10 min to 50.2 kΩ·at 60 min. The decreased Rct value indicates the depressed corrosion resistance of the coating obtained at 60 min. According to the Bode diagrams of Fig. 12d, it illustrated that the deposition time longer than 60 min are disadvantageous to corrosion resistance. Fig. 14 shows the corroded surface morphology of the Ni-W/ZrO2CeO2 coating deposited at Iav = 4 A dm−2 and r = 50% after immersed
Fig. 7. XRD patterns of (a) ZrO2, (b) CeO2 nanoparticles.
Fig. 8. XRD patterns of Ni-W/ZrO2-CeO2 nanocomposites deposited at (a) 1 A dm−2, (b) 4 A dm−2, (c) 30%, (d) 70%, (e) 10 min, (d) 60 min.
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Fig. 9. (a–c) 3D topography, (d) granule size distribution of Ni-W/ZrO2-CeO2 nanocomposite coating (4 A dm−2, 50%, 100 Hz, 20 min).
Fig. 10. (a) Ni 2p, (b) W 4f, (c) Zr 3d and (d) Ce 3d XPS spectra of Ni-W/ZrO2-CeO2 composite coating. 7
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Fig. 13. Equivalent electrical circuit (EEC) model for EIS plots analysis. Table 3 Corrosion parameters of Ni-W/ZrO2-CeO2 coatings.
Fig. 11. Microhardness of Ni-W/ZrO2-CeO2 coating.
in 3.5 wt% NaCl solution. Fig. 14a shows that the coating is still dense and compact without visible defects when immersed 48 h. In this stage, the corrosion products formed on the surface have no destructive effect on the properties of the coating. After immersed 168 h, Fig. 14b exhibits that many tiny cracks have emerged on the local surface of the coating. This corroded surface will reduce the corrosion resistance of the composite coating.
Sample coatings No.
Rs (Ω·cm2)
Rct (kΩ·cm2)
CPEdl (μF cm−2)
1#–1 A dm−2 2#–4 A dm−2 3#–30% 4#–70% 5#–10 min 6#–60 min
4.3 3.8 4.8 7.3 7.3 6.8
30.8 32.4 24.0 48.5 56.8 50.2
46.4 45.0 43.6 40.6 43.8 59.7
Fig. 15 presents the surface morphology of the coating deposited at Iav = 4 A dm−2 and r = 70% after immersed 48 h in 3.5 wt% NaCl solution. It illustrated that the coating was compact without corroded defects. Although corrosion products can be observed on the surface, the microstructure and integrality have not changed, indicating the satisfying corrosion resistance of the coating deposited at duty cycle of
Fig. 12. Nyquist and Bode plots of Ni-W/ZrO2-CeO2 composite coatings. 8
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Fig. 14. Surface morphology of the Ni-W/ZrO2-CeO2 coating deposited Iav = 4 A dm−2 and r = 50% immersed in 3.5 wt.% NaCl solution (a, b) initial stage of corrosion after 48 h, (c, d) local cracks emerged after 168 h.
Fig. 15. Surface morphology of the Ni-W/ZrO2-CeO2 coating deposited at Iav = 4 A dm−2 and r = 70% after immersed 48 h in 3.5 wt.% NaCl solution.
70%. The process parameters could regulate the composition and structure of the deposited coating, especially the content of nanoparticles, then affecting the properties of the deposits. Regulating parameters could co-deposit more uniformly distributed ZrO2 and CeO2 nanoparticles in the coating which could improve the properties.
79 ± 2 nm. The growth rate is about 0.48 μm/min. The ZrO2 and CeO2 reinforcing nanoparticles could improve the corrosion resistance and the hardness of the coating. Optimizing parameters, such as current density, duty cycle has great importance on the amounts of embedded ZrO2 and CeO2 nanoparticles in coating and the microstructure of the deposits, which was crucial to the coating properties. The enhanced hardness and corrosion resistance were ultimately achieved by regulating the composition and structure of the composites.
Conclusions Ni-W/ZrO2-CeO2 nanocomposite coating with dense structure have been synthesized through pulse electrodeposition. The coating exhibited nodular or granular morphology. Finer structure and higher embedded nanoparticles could be obtained at duty cycle of 70%. The hardness of the deposited coating is affected by the applied duty cycle and current density. The average roughness(Ra) value of the coating is
Acknowledgements This work is supported by the National Natural Science Foundation of China (51679076), the Fundamental Research Funds for the Central Universities (2019B15914), China. 9
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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.rinp.2019.102375.
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reinforced pulse electrodeposited nickel-tungsten composite coatings. Appl Surf Sci 2016;364:264–72. Asiq ROS, Wasekar NP, Sundararajan G, KumarKeshri A. Experimental investigation of grain boundaries misorientations and nano twinning induced strengthening on addition of silicon carbide in pulse electrodeposited nickel tungsten composite coating. Mater Charact 2016;116:1–7. Shen X, Sheng J, Zhang Q, Xu Q, Cheng D. The corrosion behavior of Zn/graphene oxide composite coatings fabricated by direct current electrodeposition. J Mater Eng Perform 2018;27:3750–61. Beltowska-Lehman E, Indyka P, Bigos A, Szczerba MJ, Kot M. Ni-W/ZrO2 nanocomposites obtained by ultrasonic DC electrodeposition. Mater Des 2015;80:1–11. Beltowska-Lehman E, Indyka P, Bigos A, Szczerba MJ, Kot M. Effect of hydrodynamic conditions of electrodeposition process on microstructure and functional properties of Ni-W/ZrO2 nanocomposites. J Electroanal Chem 2016;775:27–36. Ahmadi M, Guinel MJF. Synthesis, characterization and understanding of the mechanisms of electroplating of nanocrystalline–amorphous nickel–tungsten alloys using in situ electrochemical impedance spectroscopy. J Alloys Compd 2013;574:196–205. Lv J, Wang Z, Liang T, Ken S, Miura H. Effect of tungsten on microstructures of annealed electrodeposited Ni-W alloy and its corrosion resistance. Surf Coat Technol 2018;337:516–24. Tripathi MK, Singh VB. Microstructure and properties of Si3N4 and TiN nano-particles peinforced electrodeposited functional Ni-Fe matrix nanocomposite. J Electrochem Soc 2016;163:D453–61. Niazi H, Yari S, Golestani-Fard F, Shahmiri M, Wang W, Alfantazi A, et al. How deposition parameters affect corrosion behavior of TiO2-Al2O3 nanocomposite coatings. Appl Surf Sci 2015;353:1242–52. Aruna ST, Bindu CN, Ezhil Selvi V, William Grips VK, Rajam KS. Synthesis and properties of electrodeposited Ni/ceria nanocomposite coatings. Surf Coat Technol 2006;200:6871–80. Li B, Zhang W, Li D, Wang J. Electrodeposition of Ni-W/ZrO2 nanocrystalline film reinforced by CeO2 nanoparticles: structure, surface properties and corrosion resistance. Mater Chem Phys 2019;229:495–507. Li B, Li D, Zhang J, Chen W, Zhang W. Preparation of Ni-W nanocrystalline composite films reinforced by embedded zirconia ceramic nanoparticles. Mater Res Bull 2019;114:138–47. Wang L, Zhao Y, Jiang C, Ji V, Chen M, Zhan K, et al. Investigation on microstructure and properties of electrodeposited Ni-Ti-CeO2 composite coating. J Alloys Compd 2018;754:93–104. Li Y, Geng S, Chen G. Electrodeposited Ni/CeO2 mulriple coating on SUS 430 steel interconnect. Int J Hydrogen Energy 2018;43:12811–6. Sen R, Das S, Das K. Influence of duty cycle on the microstructure and microhardness of pulse electrodeposited Ni-CeO2 nanocomposite coating. Mater Res Bull 2012;47:478–85. Berube LP, LEsperance G. A quantitative method of determining the degree of texture of zinc electrodeposits. J Electrochem Soc 1989;136:2314–5. Lajevardi SA, Shahrabi T. Effects of pulse electrodeposition parameters on the properties of Ni–TiO2 nanocomposite coatings. Appl Surf Sci 2010;256:6775–81. Sangeetha S, Kalaignan GP, Anthuvan JT. Pulse electrodeposition of self-lubricating Ni-W/PTFE nanocomposite coatings on mild steel surface. Appl Surf Sci 2015;359:412–9. Zhang Z, Jiang C, Fu P, Cai F, Ma N. Microstructure and texture of electrodeposited Ni–ZrC composite coatings investigated by Rietveld XRD line profile analysis. J Alloys Compd 2015;626:118–23. Anandan C, Bera P. XPS studies on the interaction of CeO2 with silicon in magnetron sputtered CeO2 thin films on Si and Si3N4 substrates. Appl Surf Sci 2013;283:297–303. Xiang T, Zhang M, Li C, Dong C, Yang L, Chan W. CeO2 modified SiO2 acted as additive in electrodeposition of Zn–Ni alloy coating with enhanced corrosion resistance. J Alloys Compd 2018;736:62–70. Yusuf MM, Radwan AB, Shakoor RA, Awais M, Abdullah AM, Montemor MF, et al. Synthesis and characterisation of Ni–B/Ni–P–CeO2 duplex composite coatings. J Appl Electrochem 2018;48:391–404. Jin H, Wang Y, Wang Y, Yang H. Synthesis and properties of electrodeposited Ni–CeO2 nano-composite coatings. Rare Met 2017;37:148–53. Wang JL, Xu RD, Zhang YZ. Influence of SiO2 nano-particles on microstructures and properties of Ni-W-P/CeO2-SiO2 composites prepared by pulse electrodeposition. Trans Nonferrous Met Soc China 2010;20:839–43.
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Fabrication and optimization of Ni-W/ZrO2-CeO2 composite coating for enhanced hardness and corrosion resistance Baosong Lia, Dandan Lia, Tianyong Meia, Weiwei Zhangb, a b
T
⁎
College of Mechanics and Materials, Hohai University, Nanjing 211100, China College of Mechanical and Electrical Engineering, Hohai University, Changzhou 213022, China
ARTICLE INFO
ABSTRACT
Keywords: Nanocomposite coating Ni-W alloy Zirconia Ceria Pulse electrodeposition
Ni-W/ZrO2-CeO2 composite coating has been fabricated via pulse electrodeposition. The structure, morphology, composition and the operating parameters of the nanocomposite coating were evaluated and optimized for the enhanced hardness and corrosion resistance. It illustrated that the deposited coatings are dense, crack-free with nodular or granular structure. The average current density of 4–6 A dm−2, duty cycle of 70% is beneficial to the finer structure and higher content of nanoparticles in deposits. The average roughness, crystallite size and growth rate of the composite coating was examined. The hardness as a function of duty cycle, current density and deposition time were investigated. The optimal parameters for the improved performance of the composites were given. It was illustrated that the reinforcing particle phase had been uniformly co-deposited in the coating. The embedded ZrO2 and CeO2 particles could enhance the corrosion resistance and hardness of Ni-W metallic matrix. Ni-W/ZrO2-CeO2 nanocomposite coatings possess higher performance in aggressive environment and exhibit great application values.
Introduction In recent years, particle reinforced metal matrix composite (MMC) coating has attracted much attention for their superior performance including high hardness, wear and corrosion resistance [1]. It has been illustrated that incorporation of particles in metallic matrix could enhance its performance or obtain an entirely new property [2,3]. Luo investigated WC [4], and YSZ [5] reinforced Ni-P nanocomposites and illustrated the enhanced corrosion resistance and improved hardness of the nanocomposite coating. Singh [6] prepared Ni-Fe/In2O3-WO3 nanocomposite coatings and investigated its hardness. It demonstrated the strengthening effects of the incorporated particles in metallic matrix. Aliofkhazraei [7] studied pulse reverse electrodeposited Ni-W-Co/ Al2O3 nanocomposite and illustrated that corrosion and wear behavior of the tailored coatings are heavily affected by electrochemical process parameters. Sengupta [8] developed a bilayered Zn-Ni/Ni-Co-SiC nanocomposite coating and illustrated it’s superiority in hardness, wear resistant and anti-corrosive coating. Recently, as an anti-corrosion and wear-resistant alloy, nickeltungsten (Ni-W) coatings have been investigated by many researchers for their good appearance, high microhardness, anti-corrosion and wear-resistant properties [4]. However, in harsh conditions, it still needs to improve its performance to meet the requirements of corrosion ⁎
and wear resistance. Then, nanoparticle reinforced Ni-W composite coating was developed [9,10]. It was reported that Al2O3 [11], BN [12,13], TiN [14] SiC [15,16], ZrO2 [17], Si3N4 [18] and MWCNT [19] have been added to Ni-W matrix to enhance their properties. Wasekar [20] investigated Ni-W/SiC coating by pulse deposition. Keshri [21,22] studies the SiC reinforced Ni-W coatings and illustrated the enhanced hardness and corrosion resistance. Among various particles, ZrO2 owns high hardness, high strength and good fracture toughness [11,23]. Till now, only several literature [24,25] have reported Ni-W/ZrO2 coating, demonstrating the enhanced properties by the addition of ZrO2 in Ni-W matrix. References [26,27] illustrated that the addition of two kinds of particles in a metallic matrix might provide better performance. Singh [28] has fabricated Ni-Fe/Si3N4-TiN nanocomposite coatings from sulphamate-N, N-dimethylformamide bath and illustrated that Si3N4 and TiN ceramics particles have prodigious effects on the structure and properties of the coating, leading to higher hardness and corrosion resistance compared to Ni-Fe, Ni-Fe/Si3N4 and Ni-Fe/TiN coatings. Wang [29] illustrated that the incorporation of CeO2 into metal matrix could improve the corrosion resistance. Aruna [30] illustrated that embedded CeO2 could enhance the hardness and protective properties of Ni coating. Thus, it was feasible by embedding ZrO2 and CeO2 particles into Ni-W matrix to obtain new or excellent properties. However, till
Corresponding author. E-mail address: [email protected] (W. Zhang).
https://doi.org/10.1016/j.rinp.2019.102375 Received 18 April 2019; Received in revised form 16 May 2019; Accepted 21 May 2019 Available online 25 May 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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now, Ni-W/ZrO2-CeO2 composite coating is less studied. It is necessary to investigate the structure and properties of Ni-W/ZrO2-CeO2 composite coatings. In this work, Ni-W/ZrO2-CeO2 coating was fabricated and optimized. This work could help the development of particle phase reinforced Ni-W composite coating. Experimental Fabrication of Ni-W/ZrO2-CeO2 composite coating The composition of the electrolyte for Ni-W/ZrO2-CeO2 electrodeposition was designed based on our previous work [31,32] on Ni-W/ ZrO2 coating and the related literature [33–35] on CeO2 reinforced nickel coating. The coating were deposited from an ammonia-citrate bath containing: 32 g L−1 nickel sulfate hexahydrate, 66 g L−1 sodium tungstate dihydrate, 118 g L−1 sodium citrate dihydrate, 0.002 g L−1 sodium lauryl sulfate, 30 g L−1 ZrO2 and 10 g L−1 CeO2 nanoparticles. The chemicals were analytical grade. The ZrO2 and CeO2 particles were provided by Shanghai Aladdin biochemical Technology Co., Ltd and their size was in the range of 10–45 nm. The electrolyte temperature, pH, stirring speed was kept at 65 ± 2 °C, 8.3 ± 0.1 and 400 ± 50 rpm, respectively. The deposited coating samples were named 1#–1 A dm−2, 2#–4 A dm−2, 3#–30%, 4#–70%, 5#–10 min and 6#–60 min and their corresponding process parameters were presented in Table 1. Fig. 1 shows the applied pulse current in the electrodeposition process comprising a cathodic current and a zero current. The relationships of the parameters in pulse electrodeposition are as follows:
f=
1 Ton + Toff
(1)
r=
Ton × 100% Ton + Toff
(2)
Fig. 1. Schematic of the applied pulse current in the electrodeposition process.
D=
RTC(hkl) =
× 100%; R(hkl) =
Is (hkl) Ip (hkl)
(5)
It was reported [25] that the particle shape greatly affected the amount and dispersion of particles in coating. The particles in spherical shape resulted in better codeposition in metal matrix than those of irregular shape [24]. Therefore, the zirconia and ceria particles were first examined by TEM observation. Fig. 2(a, b) presents that zirconia particles have polygonal shape which is near spherical shape in diameters of 20–38 nm. This particle size of zirconia is in accordance with the size provided by manufacturer (< 50 nm). Fig. 2(c, d) displays that ceria particles have a similar shape with zirconia. However, the facets of ceria particles are sharp and the uniformity of the particle size is less compared to zirconia particles. It can be seen that the ceria particle size was in dimension from 10 nm to more than 40 nm. Due to the high surface energy of the particles, agglomeration was observed. Comparatively, the agglomeration tendency of nano-ceria is greater than that of nano-zirconia, which was confirmed by the TEM observation.
Table 1 Electrodeposited Ni-W/ZrO2-CeO2 coating and the corresponding process parameters.
1 A dm−2, 50%, 100 Hz, 20 min 4 A dm−2, 50%, 100 Hz, 20 min 30%, 4 A dm−2, 100 Hz, 20 min 70%, 4 A dm−2, 100 Hz, 20 min 10 min, 100 Hz, 4 A dm−2, 50%, 60 min, 100 Hz, 4 A dm−2, 50%,
R(hkl)
Characterization of ZrO2 and CeO2 nanoparticles
TEM (Tecnai G2F30 S-Twin) was utilized to observe the surface microstructure of the coatings. The cross-sectional images were examined by SEM (Hitachi S4800). The morphology, composition, structure, topography and surface element states were characterized by SEM, EDS, XRD, AFM and XPS, respectively. The hardness was measured with a DHV-1000 hardness tester under the applied load of 100 gf lasting for 15 s. The average value of five sites of each sample was reported as the final hardness. The surface roughness was calculated using the Nova software attached to NT-MDT AFM instrument. The grain size was calculated using the Debye-Scherrer Eq. (4)
1#–1 A dm−2 2#–4 A dm−2 3#–30% 4#–70% 5#–10 min 6#–60 min
n 1
Results and discussion
Characterization of Ni-W/ZrO2-CeO2 composite coating
Electrodeposition parameters
R(hkl)
where Is(hkl) and Ip(hkl) were the diffraction intensities of the (hkl) plane measured for the composite deposits and the standard Ni powder sample (JCPDS No. 04-0850), respectively. Corrosion resistance was evaluated by EIS methods at the Eocp in the frequency range of 105–10−2 with the input signal 10 mV on an electrochemical workstation (CHI 660E, CH Instruments). A three-electrode cell is used and platinum foil, saturated calomel electrode and deposited coating with 1 cm−2 exposed area are used as counter, reference and working electrode, respectively.
where f is frequency, Ton is current on time (s), Toff is current off time (s), r is duty cycle, Ipe is peak current density and Iav is average current density. Nickel and copper sheet was used as anode and cathode, respectively. The electrode space were kept at 30 mm. Before each test, the cathode was cleaned and activated in 7.2 wt.% HCl solution for 30 s.
Coating sample
(4)
where D was the mean crystalline size (nm), λ was X-ray wavelength (0.15418 nm), β was the full width at half maxima in intensity and θ is Bragg diffraction angle. The relative texture coefficient (RTC) [36,37] was used to characterize the relative degree of preferred crystal orientation among all crystal planes calculated by the following Eq. (5).
(3)
Iav = Ipe × r
0.94 cos
Cross-sectional and surface morphology The cross section of the deposit was observed by SEM images. Fig. 3a shows the cross-sectional images of the deposited Ni-W/ZrO2CeO2 composite coating, which was prepared under the deposition time of 60 min, frequency of 100 Hz, average current density of 4 A dm−2 and duty cycle of 50%. It can be seen the thickness of the deposited composite coating is about 29 μm. The growth rate is about 0.48 μm/ min. It indicated that the interface between the coating and substrate is crack free. The cross section exhibits uniform structure with some 2
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Fig. 2. TEM images of the nanoparticles used (a, b) ZrO2, (c, d) CeO2.
scratches which were produced in the sanding process. Fig. 3b shows the enlarged morphology of the yellow rectangular marked area A in Fig. 3a. It was found that the nanoparticles in the dimension of 20–40 nm were uniformly embedded in the bulk of the cross section of the coating. It was illustrated that the nanoparticles were homogeneously dispersed in Ni-W matrix, which is expected to enhance the properties of the nanocomposite deposits. TEM analysis was performed to illustrate the incorporation of nanoparticles in coating. Fig. 4 shows the TEM images of the pulse electrodeposited Ni-W/ZrO2-CeO2 coating. It was noticed that zirconia and ceria nanoparticles have been co-deposited and were wrapped in Ni-W metallic matrix. The diameters of these particles in the deposited coating were consistent with the size confirmed by our previous studies on zirconia and ceria powder via TEM, which was in the range of 10–40 nm. The composite coating exhibits a dense and homogeneous structure. No defects such as pores or cracks were found on the coating and the combination of the incorporated particles and the metallic matrix is good, indicating a satisfying phase interface. In general, reinforcing nanoparticles are uniform in size and are well embedded in the Ni-W metallic matrix. Fig. 5 presents the surface SEM images of the Ni-W/ZrO2-CeO2 coating prepared at different conditions. It should be noted that besides the given parameter in the figure caption, other deposition parameters were consistent with the value introduced in the experiment section. As seen in Fig. 5, the deposited Ni-W/ZrO2-CeO2 composite coating exhibits nodular or granular morphology with dense and homogeneous structure. It was reported [38] that the Ni-W crystal shows a long needle shape, which can be observed in these figures. Small particles were dispersed in the large granules formed on the coating. As well known, the granules were the aggregates of many small crystals. The
nodular structure that like a dome-like island can be seen in each sample. The difference lies in the size of the dome-like structure and their quantity and distribution. The size of a dome-like granule in Fig. 5c is more than 3 μm. The distributed particles in the coating are faintly visible, which was confirmed by the HRTEM (Fig. 4). Comparing Fig. 5a and b, the granule size obtained at 4 Adm−2 is more uniform in the dimension of less than 1 μm. Initially, increasing current density could improve the transfer process of particles. But the deposition rate of nickel increased more than particles as the current density increased, leading to the decrease of the particle content in deposits. It also can be found that the granule size of the coating prepared at 70% is the finest, compact and uniform among the investigated samples. The samples of 1 A dm−2 and 30% shows the relatively large granule size and rougher surface. Comparing Fig. 5e and f, it was found the granule size is similar for the coating deposited for 10 min and 60 min, indicating the granule could not grow up continuously. Chemical composition and phase structure Fig. 6(a–c) shows the composition of Ni-W/ZrO2-CeO2 coating deposited at different duty cycle. It illustrated that the coating contains 3.9–4.9 wt.% ZrO2 and 0.45–0.68 wt.% CeO2 nanoparticles. Here, the oxygen content was also detected by EDS. It needs to be noted that the weight % of ZrO2 and CeO2 presented in Fig. 6(a–c) were calculated according to content of Zr and Ce and the stoichiometry of ZrO2 and CeO2. Besides, the nanocomposite contains 65.7–71.9 wt.% nickel and 23.7–28.7 wt.% tungsten. With the increase of duty cycle, th embedded ZrO2 particles initially decreased and then increased with maximum content at 4.9 wt.% at r = 70%. The amount of CeO2 in the composite coating is low and increase with the duty cycle. Fig. 6(d–g) displays the 3
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Fig. 3. Cross-sectional images of Ni-W/ZrO2-CeO2 composite coating deposited at t = 60 min. f = 100 Hz, Iav = 4 A dm−2 and r = 50%. (a) low magnification of 2000×, (b) high magnification of 100,000×.
EDS map of Ni-W/ZrO2-CeO2 coating. It was illustrated that the coating comprises Ni, W, Zr, Ce and O elements. It was demonstrated that nickel and tungsten were evenly distributed. Generally, the particles distribution is uniform. The commercial zirconia powder has been characterized by XRD. Fig. 7a confirms that the crystal size was 15–40 nm. It indicated the ZrO2 powder was a mixture of the tetragonal and monoclinic phases. Fig. 7b shows the XRD pattern of CeO2 powder which reveals typical intense lines corresponding to the fcc structure. Fig. 8 shows the XRD pattern of the deposited coating and indicated that the single phase of Ni(W) solid solution have been formed with fcc structure. The primary peaks at 43.9–44.1°, 51.2–51.4° and 75.3–75.6° correspond to nickel (111), (200) and (220) crystal planes. Compared to the standard XRD pattern of nickel (111) plane at 44.51°, (200) plane at 51.84° and (220) plane at 76.37° (JCPDS No. 04-0850), the 2θ of the crystal plane has shifted to lower angles, due to the lattice distortion by addition of W and particles in nickel matrix. ZrO2 and CeO2 were detected with weak peaks at 30.2° and 28.3°, respectively. The peaks of substrate (JCPDS No. 50-1333) emerged because the X-ray could penetrate more than 10 μm which might be larger than the coating thickness. According to Scherrer's Eq. (4), the crystallite sizes based on the Ni (111) plane were 14–18 nm, as shown in Table 2. The grain sizes slightly increased with the increase of current density. The finer structure was obtained under 4 A dm−2. Fig. 8(c, d) exhibits the XRD spectra of the Ni-W/ZrO2-CeO2 nanocomposites prepared at 30% and 70%. The crystalline grain of Ni-W/ZrO2-CeO2 deposited at r = 70% has the minimum size. With the increase of deposition time, the grain
Fig. 4. TEM images of Ni-W/ZrO2-CeO2 composite coating (4 A dm−2, 50%, 100 Hz, 60 min).
size increases (Fig. 7e and f). In the electrodeposition process, as nucleation sites, the presence of nanoparticles could promote the formation of new grains and inhibit the growth of the already formed grains. Then, the grain refinement was achieved. Relative texture coefficient (RTC) was used to reflect the crystalline preferred orientation using Eq. (5). Here, (111), (200) and (220) reflection lines for Ni have been considered. The preferred orientation through an axis [hkl] was selected by values of RTC ≥ 25%. The strength of the preferred orientation becomes the maximum amount when RTC reaches the value of 100% [37]. The calculated RTC (hkl) values of the Ni-W/ZrO2-CeO2 coatings are shown in Table 2. When current density increases from 1 to 4 A dm−2, the RTC (111) value increases and RTC (200) decreases. At the current density of 4 A cm−2, the value of RTC (111) is 62.03%. When the duty cycle changed from 30% to 70%, the RTC (111), (220) increases and RTC (200) decreases. Contrarily, when the deposition time increases from 10 min to 60 min, the RTC (111) value decreases and the RTC (200) and (220) increases. This illustrates the textures of Ni-W/ZrO2-CeO2 coatings varied at different deposition parameters. For Ni-W/ZrO2-CeO2 coatings, the preferred orientation was (111) texture [39]. 4
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Fig. 5. SEM micrograph of Ni-W/ZrO2-CeO2 composite coating deposited under (a) 1 A dm−2, (b) 4 A dm−2, (c) 30%, (d) 70%, (e) 10 min, (f) 60 min. Other parameters see Section “Fabrication of Ni-W/ZrO2-CeO2 composite coating.”
3D topography and roughness analyzed by AFM
might help to improve the adhesion of the subsequent (top) coating.
Fig. 9 shows the topography of the coatings deposited at Iav = 4 A dm−2, r = 50%, f = 100 Hz and t = 20 min. Fig. 9 reveals that the nanocomposite coating exhibited hill-valley like morphology. The composite coating was made up of micron scale granules where the nanoparticles were dispersed throughout the coating. The average surface roughness(Ra) value was 79 ± 2 nm [2]. As can be seen, the granules were in the dimension of 250–550 nm, which was made up of many small crystals and particles. This hill-valley like morphology
Surface properties analyzed by XPS The surface properties of the coating have critical effects on the protective performances. Hence, it is necessary to investigate the surface properties of the coating by XPS. Fig. 10 indicated that Ni, W, Zr and Ce elements were detected by XPS spectra. Fig. 10a shows the Ni 2p3/2 XPS spectra with three decomposed peaks. The corresponding peaks at 851.5 eV, 855.2 eV and 860.4 eV belongs to Ni(0), Ni-O and
Fig. 6. The composition and element distribution map of Ni-W/ZrO2-CeO2 composite coating prepared at duty cycle of (a) 30%, (b) 50% and (c) 70%, (d–g) element distribution map of nickel, tungsten, zirconium and cerium, respectively. 5
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Table 2 Relative texture coefficient (RTC) and crystallite size of Ni-W/ZrO2-CeO2 composite coating electrodeposited at different conditions. Samples
1#–1 A dm−2 2#–4 A dm−2 3#–30% 4#–70% 5#–10 min 6#–60 min
Relative texture co-efficient (RTC), (%) (111)
(200)
(220)
53.24 62.03 49.78 52.57 60.26 41.29
23.71 15.52 18.66 13.95 16.93 19.45
23.05 22.45 31.56 33.48 22.81 39.26
Crystallite size(nm)
14.5 14.2 16.5 14.1 15.7 17.2
satellite nickel. It indicated the formation of oxide layer on the coating. Fig. 10b shows the W4f XPS spectra with four distinct peaks. The peak at 30.4 eV are attributed to W(0), suggesting that the tungstate ion has been reduced to W(0). Fig. 10c exhibits the Zr 3d XPS spectra of the deposits with two fitted peaks. The peaks at 181.6 eV and 184.0 eV are ascribed to Zr 3d5/2 and Zr 3d3/2, respectively [40]. Fig. 10d shows the Ce3d XPS spectra, illustrating the cerium element was mainly in the form of Ce4+ state originated from the added CeO2 particles [40]. Microhardness and corrosion resistance Fig. 11 shows the microhardness of the Ni-W/ZrO2-CeO2 nanocomposite coating deposited under different conditions. As can be seen, as Iav increased from 1 to 4 A dm−2, the hardness increased. It is well known that the composition and structure might be changed under different current density due to the different mass transfer conditions. Comparing the coatings deposited at r = 30% and r = 70%, the hardness reached the maximum value at r = 70%, which is consistent with literature [35]. Comparing the coating deposited at 10 min and 60 min, the hardness increased. Because the crystallite size was similar (14–18 nm), the change of hardness was mainly due to the nanoparticles content in the metallic matrix, which could be affected by current density, duty cycle and deposition time. The roles of the CeO2 on anti-corrosion properties of composite coatings have been reported by some references [41–44]. The Nyquist and Bode plots are shown in Fig. 12. Here, the equivalent electrical circuit (EEC) models consisting of RQ components (Fig. 13) was used to analyze all the EIS plots, which was a single time constant model. In Fig. 13, the Rs is solution resistance, Rct is charge transfer resistance and CPEdl is the electric double layer capacity. Fig. 12a depicts that as current density increases from 1 to 4 A dm−2, the radius of impedance arc increased. Table 3 shows that the Rct values is 30.8, 32.4 kΩ·cm2 at respectively. As Iav increases from 1 to 4 A dm−2, the 1 and 4 A dm−2 , Rct values increases. Fig. 12b shows that the modulus |Z| at 0.01 Hz of 4 A dm−2 is larger than that of 1 A dm−2. It illustrated that the coating prepared at 4 A dm−2 has better corrosion resistance. Fig. 12a also indicated that as the duty cycle increased from 30% to 70%, the corrosion resistance largely increased with the maximum value at 70%. Table 3 presented that when duty cycle increases from 30% to 70%, the Rct values increases from 24.0 kΩ·cm2 up to 48.5 kΩ·cm2, indicating the best corrosion resistance at duty cycle of 70%. The corresponding Bode plots are shown in Fig. 12b and displayed one hump shape, indicating only one time constant in the frequency range investigated. The modulus |Z| at 0.01 Hz reached the peak value at 70%. Fig. 12c depicts that as deposition time increases from 10 min to 60 min, the Rct values slightly decreased from 56.8 kΩ·cm2 at 10 min to 50.2 kΩ·at 60 min. The decreased Rct value indicates the depressed corrosion resistance of the coating obtained at 60 min. According to the Bode diagrams of Fig. 12d, it illustrated that the deposition time longer than 60 min are disadvantageous to corrosion resistance. Fig. 14 shows the corroded surface morphology of the Ni-W/ZrO2CeO2 coating deposited at Iav = 4 A dm−2 and r = 50% after immersed
Fig. 7. XRD patterns of (a) ZrO2, (b) CeO2 nanoparticles.
Fig. 8. XRD patterns of Ni-W/ZrO2-CeO2 nanocomposites deposited at (a) 1 A dm−2, (b) 4 A dm−2, (c) 30%, (d) 70%, (e) 10 min, (d) 60 min.
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Fig. 9. (a–c) 3D topography, (d) granule size distribution of Ni-W/ZrO2-CeO2 nanocomposite coating (4 A dm−2, 50%, 100 Hz, 20 min).
Fig. 10. (a) Ni 2p, (b) W 4f, (c) Zr 3d and (d) Ce 3d XPS spectra of Ni-W/ZrO2-CeO2 composite coating. 7
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Fig. 13. Equivalent electrical circuit (EEC) model for EIS plots analysis. Table 3 Corrosion parameters of Ni-W/ZrO2-CeO2 coatings.
Fig. 11. Microhardness of Ni-W/ZrO2-CeO2 coating.
in 3.5 wt% NaCl solution. Fig. 14a shows that the coating is still dense and compact without visible defects when immersed 48 h. In this stage, the corrosion products formed on the surface have no destructive effect on the properties of the coating. After immersed 168 h, Fig. 14b exhibits that many tiny cracks have emerged on the local surface of the coating. This corroded surface will reduce the corrosion resistance of the composite coating.
Sample coatings No.
Rs (Ω·cm2)
Rct (kΩ·cm2)
CPEdl (μF cm−2)
1#–1 A dm−2 2#–4 A dm−2 3#–30% 4#–70% 5#–10 min 6#–60 min
4.3 3.8 4.8 7.3 7.3 6.8
30.8 32.4 24.0 48.5 56.8 50.2
46.4 45.0 43.6 40.6 43.8 59.7
Fig. 15 presents the surface morphology of the coating deposited at Iav = 4 A dm−2 and r = 70% after immersed 48 h in 3.5 wt% NaCl solution. It illustrated that the coating was compact without corroded defects. Although corrosion products can be observed on the surface, the microstructure and integrality have not changed, indicating the satisfying corrosion resistance of the coating deposited at duty cycle of
Fig. 12. Nyquist and Bode plots of Ni-W/ZrO2-CeO2 composite coatings. 8
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Fig. 14. Surface morphology of the Ni-W/ZrO2-CeO2 coating deposited Iav = 4 A dm−2 and r = 50% immersed in 3.5 wt.% NaCl solution (a, b) initial stage of corrosion after 48 h, (c, d) local cracks emerged after 168 h.
Fig. 15. Surface morphology of the Ni-W/ZrO2-CeO2 coating deposited at Iav = 4 A dm−2 and r = 70% after immersed 48 h in 3.5 wt.% NaCl solution.
70%. The process parameters could regulate the composition and structure of the deposited coating, especially the content of nanoparticles, then affecting the properties of the deposits. Regulating parameters could co-deposit more uniformly distributed ZrO2 and CeO2 nanoparticles in the coating which could improve the properties.
79 ± 2 nm. The growth rate is about 0.48 μm/min. The ZrO2 and CeO2 reinforcing nanoparticles could improve the corrosion resistance and the hardness of the coating. Optimizing parameters, such as current density, duty cycle has great importance on the amounts of embedded ZrO2 and CeO2 nanoparticles in coating and the microstructure of the deposits, which was crucial to the coating properties. The enhanced hardness and corrosion resistance were ultimately achieved by regulating the composition and structure of the composites.
Conclusions Ni-W/ZrO2-CeO2 nanocomposite coating with dense structure have been synthesized through pulse electrodeposition. The coating exhibited nodular or granular morphology. Finer structure and higher embedded nanoparticles could be obtained at duty cycle of 70%. The hardness of the deposited coating is affected by the applied duty cycle and current density. The average roughness(Ra) value of the coating is
Acknowledgements This work is supported by the National Natural Science Foundation of China (51679076), the Fundamental Research Funds for the Central Universities (2019B15914), China. 9
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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.rinp.2019.102375.
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