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nano-SiC coatings
Surface & Coatings Technology 213 (2012) 33–40
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Electrodeposition and characterization of Ni–Zn–P and Ni–Zn–P/nano-SiC coatings S. Pouladi ⁎, M.H. Shariat, M.E. Bahrololoom Department of Materials Science and Engineering, School of Engineering, Shiraz University, Shiraz, Iran
a r t i c l e
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Article history: Received 2 June 2012 Accepted in revised form 4 October 2012 Available online 12 October 2012 Keywords: Electrodeposition Alloy coating Nanocomposite coating Wear resistance NI–Zn–P Ni–Zn–P/nano-SiC
a b s t r a c t Alloy coatings and metal based composites have been given special attention for their unique properties. In the present study, a novel electroplating bath was introduced. Ni–Zn, Ni–Zn–P nickel rich alloy coatings and Ni–Zn–P/nano-SiC nickel rich composite coating were successfully deposited on low carbon steel substrates. The optimum bath composition was found by comparing the amount of coating cracks checked by a scanning electron microscope (SEM). The presence of nearly homogeneous SiC nanoparticles in the coating was confirmed by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX) and MAP analysis. The effect of operating conditions on the mechanical properties of nanocomposite coatings was investigated. The wear resistance of the coatings was examined by pin on disk wear tests. The nanocomposite coatings had a lower weight loss compared to the Ni–Zn–P coatings in the wear test. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Metallic alloys and composite deposits have been widely used in industry. By electrodeposition we can produce these deposits and control some specific properties of them. Nickel and zinc are often used as base-materials in composite coatings, as they protect steel substrates against corrosion [1]. Although nickel is more noble than zinc, electrodeposition of nickel–zinc alloys produces zinc-rich coatings. The rate of dissolution for these alloy coatings is high in corrosive conditions compared to Ni-based alloy coatings [2]. The mechanism of this preferred deposition has been thoroughly discussed by A. Brenner [3]. Alfantazi et al. [4] achieved a nickel rich Ni–Zn alloy deposit by increasing the temperature of the chloride-based bath. These deposits, produced at 80 °C, contained about 66 wt.% Ni and showed a large number of microcracks [4]. Addition of phosphorous to crystalline Ni–Zn alloy deposit leads to the production of an amorphous structure. The phosphorous content in the deposit must be more than 10% as quoted by Shidharan et al. [5]. Even low phosphorous content (1%) has good effects, such as refining the microstructure, reducing residual tension [6] and increasing coating adhesion [7]. A wide variation in the electrochemical properties of this alloy has been achieved by co-deposition of phosphorous with different phases of Ni–Zn alloy [8]. It was observed that phosphorous refines the microstructure and increases corrosion resistance of the deposit [7]. There are a few papers on the ternary Ni–Zn–P alloy coatings
⁎ Corresponding author at: Material Science and Engineering Dep. Shiraz University, No. 143, Alley 5, Mahallati St., West Ghoudousi Blvd, Shiraz, Iran. Tel.: +98 9173219824; fax: +98 7118434942. E-mail addresses: [email protected] (S. Pouladi), [email protected] (M.H. Shariat), [email protected] (M.E. Bahrololoom). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.10.011
and most of these alloys were produced by electroless technique [2,9–11]. One of the most recent research studies on the electroless plating of Ni–Zn–P and Ni–Zn–P/nano-TiO2 was conducted by Ranganatha et al. [12]. Electroless deposition, an autocatalytic reduction of metals and alloys, offers an alternate and yet attractive method of producing a coating with higher Ni content. This has been known to form a thin and uniform deposit on the substrate compared to electroplating. However, the production of electroless Ni–Zn–P alloy coating needs an expensive reducing agent and also more control is required to maintain Ni composition of the plating bath because the nickel is depleted from the solution, not from the anodes. In addition, this method is more time consuming than electroplating. So, this study used the electroplating technique to produce Ni–Zn–P alloy coatings. Ni–Zn–P alloy was electrodeposited on carbon steel while its phosphorous content was derived from sodium hypophosphite as the phosphorous source by Swathirajan and Mikhail [8]. In the present study, Ni–Zn–P alloys were deposited from different bath compositions and the source of phosphorous in these coatings was phosphoric acid which is cheaper than sodium hypophosphite. One of the primary reasons of this study was investigation on the possibility of using phosphoric acid as the source of phosphorous in this process. Ma et al. [13] have also used phosphoric acid and phosphorous acid as sources of phosphorous to synthesize aluminophosphate. Electrodeposited composites are obtained by adding insoluble solid particles to an electrolytic bath [14–16]. These insoluble particles can be oxides (Al2O3, TiO2, and SiO2) or carbide particles (SiC and WC), or solid lubricants (Polytetrafluoroethylene (PTFE), graphite or MoS2) or even liquid-containing microcapsules [17] to enhance wear resistance and/or to reduce friction. Electroplated composite coatings with microparticles are used as wear-resistant coatings [18–20], e.g., Ni–SiC in car engines [14,21–23].
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S. Pouladi et al. / Surface & Coatings Technology 213 (2012) 33–40
Table 1 Chemical composition and conditions of the electrodeposition bath. Composition of solution
−1
ZnCl2 (g.l ) NiCl2·6H2O (g.l−1) H3BO3 (g.l−1) H3PO4 (g.l−1) Sodium dodecyle sulfate (SDS) (g.l−1) Temperature °C Current density (A.dm−2)
Type of coating Ni–Zn
Ni–Zn–P (1)
Ni–Zn–P (2)
Ni–Zn–P (3)
200 200 40 0 0.1
200 200 40 5 0.1
200 200 40 15 0.1
200 200 40 25 0.1
83 ± 1 3
83 ± 1 3
83 ± 1 3
83 ± 1 3
There is an increasing interest in codeposition of nanoparticles with deposited metals due to the increasing availability of nanoparticles [24]. The main concern in using nanoparticles is the presence of an adequate amount of particles in the deposit and also avoiding the agglomeration of nanoparticles in the solution. In the present work, an electroplating bath for electrodeposition of Ni–Zn–P and Ni–Zn–P/nano-SiC with high nickel content on low carbon steel substrates was introduced. In addition, the effect of current density, the amount of SiC particles in the bath and the stirring speed on microhardness, wear resistance and coefficient of friction of these coatings were investigated. 2. Experimental The Ni–Zn nickel rich alloy was deposited from a chloride-based electrolyte in a 250 ml bath with the chemical composition shown in Table (1) using the DC plating technique at current density of 3 A.dm −2, temperature 83 ± 1 °C and pH = 4. By adding phosphoric acid (85 wt.%) to the electrolyte in the amount of 5, 15 and 25 g.l −1, Ni–Zn–P ternary alloys have been deposited. In order to achieve Ni–Zn–P/nano-SiC composite deposits,
SiC nanoparticles, with an average size of 50 nm, were added in the amount of 5, 10, 15 and 20 g.l −1 of the electrolyte. In order to keep the SiC particles suspended and prevent their agglomeration, cetyltrimethyl ammonium bromide (CTAB) surfactant was added [25] and an ultrasonic device and a magnetic stirrer were used. The SiC powder was added to distilled water and was premixed by ultrasonic agitation (power of 350 W and frequency of 40 kHz) for 30 min and subsequently CTAB and SDS (as an anti-pitting agent) with 0.04 and 0.1 g.l −1 concentrations were added respectively. Again the electrolyte was stirred by ultrasonic agitation for 15 min and then nickel chloride, zinc chloride and boric acid were added. Finally, the electrolyte solution was stirred for 24 h before electrodeposition. A magnetic stirrer was used during the electrodeposition period. These electrolytes were prepared using Merck pro-analysis grade chemicals and double-distilled water. Low carbon steel (st 37) plates were used as substrates. Before electroplating, each substrate was polished with emery papers (with the following grades: 400, 600, 800, 1000, 2000, and 3000) and then electropolished by dipping into a solution containing 94 vol.% acetic acid (90 wt.%) and 6 vol.% perchloric acid (60 wt.%). After electropolishing, the substrates were rinsed with distilled water and then immersed immediately in the electroplating bath. A nickel sheet (The total exposed surface area was 45 cm 2) was used as an anode. The anode surface area was greater than the cathode surface area to ensure that there was no anodic polarization. To ensure the presence of phosphorous in the deposits, the X-ray fluorescence (XRF, XMF-104 High Speed Micro ED X-Ray Spectrometer) analysis was performed. In order to find the optimum electrolyte composition, the number of cracks in the deposits was studied by a scanning electron microscope (SEM, Cambridge S360). In addition, the presence of SiC in the coatings was also detected using energy dispersive X-ray spectroscopy (EDX). A field emission scanning electron microscope (FESEM, Hitachi S-4160 electron microscope) was used to see how well the SiC particles were distributed in the cross section of the coatings.
Fig. 1. SEM micrographs of the coatings: (a) The Ni–Zn coating, (b) Ni–Zn–P (5 g.l−1 H3 PO4), (c) Ni–Zn–P (15 g.l−1 H3 PO4), and (d) Ni–Zn–P (25 g.l−1 H3 PO4).
S. Pouladi et al. / Surface & Coatings Technology 213 (2012) 33–40 Table 2 Chemical composition of Ni–Zn–P alloy deposits. Phosphoric acid (85 wt.%) (g.l−1)
Ni wt.%
Zn wt.%
P wt.%
5 15 25
82.81 86.36 88.47
14.79 10.44 7.43
2.4 3.2 4.1
To evaluate the tribological properties of the coatings, a homemade pin on disk tribometer, with the sample placed horizontally on a turntable, was used. The testing temperature and relative humidity were 24 ± 1 °C and 45%, respectively. The air relative humidity was reported by Shiraz Meteorological Institute. The tests were performed using hardened high-carbon steel pin and at a linear speed of 0.03 m.s −1 for a total sliding distance of 400 m. A normal load of 18 N was used during the wear tests. In wear resistance experiments, specimens were weighed before the test and after rotation of 100, 200, 300 and 400 m. Coating friction factor was measured using
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the same apparatus, resulting in a graph of friction factor as a function of distance. Hardness measurement of the coatings, which is reported as an average of five readings on each specimen, was carried out using a Leitz L137 microhardness testing instrument equipped with a Vickers indenter under a load of 200 g and load exertion time of 15 s. The microhardness of the coatings as a function of SiC content in the bath, current density and stirring rate was studied. The average surface roughness values of the coatings were determined using a Mitutoyo Surftest 201 roughness tester. 3. Results and discussion 3.1. Coating morphology and composition The surface morphology of a Ni–Zn deposit plated on a steel substrate is shown in Fig. 1a. It can be seen that the Ni–Zn coating shows a large number of microcracks, 0.05 μm.μm−2 crack density. A similar
Fig. 2. SEM micrographs of coatings: (a) 3 A.dm−2, (b) 2 A.dm−2, (c) 1 A.dm−2 (2 g.l−1 H3PO4) (d) 3 A.dm−2, (e) 2 A.dm−2, and (f) 1 A.dm−2 (5 g.l−1 H3PO4).
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3.2. The effect of current density on coating cracks The effect of current density on the number of cracks in the deposits for a constant concentration of phosphoric acid in the electrolyte is shown in Fig. 2. As seen, the number of cracks in the deposits decreased with decreasing current density. It has been reported by Narayan and Mungole [26] that increasing the current density leads to a decrease in phosphorous content of the deposit. The results of present investigation showed that increasing the amount of phosphorous improves the quality of the coatings. Considering the results reported by Narayan and Mungole [26] and also the results of this investigation, it may be concluded that decreasing the current density can improve the quality of deposits, as shown in Fig. 2. 3.3. Electrodeposition of nanocomposites
Fig. 3. EDX analysis of a nanocomposite coating on two points.
result was reported by Alfantazi et al. from a chloride-based electrolyte using the square-wave pulse-plating technique [4]. To eliminate these cracks in the deposit and to produce a ternary Ni–Zn–P alloy, addition of phosphorous to the Ni–Zn alloy deposits was proposed. X-ray fluorescence analysis showed that the deposit obtained from an electrolyte containing 5 g.l −1 phosphoric acid was a ternary alloy with the chemical composition of Ni82.81–Zn14.79–P2.4. By increasing the amount of phosphoric acid in the electrolyte in the range of 0–25 g.l −1, the amount of phosphorous in the deposit increased (Table 2). The increase in the amount of phosphorous in the deposit decreased the number of cracks from 0.004 μm.μm −2 (Fig. 1b) to 0.003 μm.μm −2 (Fig. 1c). Ultimately at 25 g.l −1 phosphoric acid (Fig. 1d) no cracks were observed. Thus, the optimum electrolyte composition for the deposition of Ni–Zn–P alloy should contain at least 25 g.l −1 phosphoric acid (85wt.%). Similar results were also reported for Ni–Zn–P alloy electrolessly deposited by Bouanani et al. [11]. They have attributed the presence of cracks in some deposits to the internal stress caused by nickel lattice distortion and increase in the internal stress with decreasing phosphorus in the deposit [11].
Table 3 Chemical composition of Ni–Zn–P/nano-SiC (wt.%).
Average weight percent wt.% Errors wt.%
C
Si
P
Fe
Ni
Zn
3.265
2.290
2.475
0.325
83.505
8.125
±0.135
±0.030
±0.015
±0.035
±0.045
±0.115
As mentioned, for deposition of nanocomposites, SiC nanoparticles were added to the Ni–Zn–P alloy bath. A large number of samples (five samples were prepared for each experimental condition and these five samples were analyzed to investigate their characteristics) were used to be coated by Ni–Zn–P/nano-SiC. To ensure the existence of SiC particles in the coatings and production of nanocomposites, EDX analysis was performed on two different points on one of the nanocomposite coatings which was obtained from an electrolyte containing 10 g.l−1 SiC and at current density of i = 3 A.dm −2 (Fig. 3a, b). Energy dispersive X-ray spectroscopy is an elementary technique and the peaks related to Si and C are the results of the presence of SiC in the coating. Nearly equal intensity of the peaks in both graphs shows approximately equal chemical compositions in different areas. In the EDX graphs, only the elements Ni, Zn, P, Fe, Si and C can be seen. The Fe peak is probably related to the background interference. Table 3 shows the weight percentage and the errors of determination of elements in the composite. The cross section micrograph of the nanocomposite coating which was obtained from an electrolyte containing 10 g.l−1 SiC and at current density of i = 5 A.dm −2, shown in Fig. 4a, indicates a uniform coating even on the cross section corners of the steel plate. The average thickness of the coating and the electrodeposition rate were 25 μm and 50 μm.h−1, respectively. Fig. 4b and c indicates the presence of SiC nanoparticles within the entire coating layer. Although the individual nanoparticles are not well distributed, the distribution of approximately nano-sized clusters of nanoparticles appears uniform. Meanwhile, the element distribution map of Si (white points) in the Ni–Zn–P/nano-SiC composite coating (Fig. 4(d)) reveals the uniform distribution of Si in the composite. 3.4. Microhardness 3.4.1. Effect of SiC content in the bath on microhardness of the coatings Fig. 5 indicates that the microhardness of all Ni–Zn–P/nano-SiC composite coatings is considerably higher than the Ni–Zn–P alloy coatings. Hardness of composite coatings containing SiC has been attributed to the hindrance of dislocation movement by SiC particles [27,28]. High hardness of the Ni–Zn–P/nano-SiC composite coating will provide wear resistance [29]. Fig. 5 shows that at a constant stirring rate of 180 rpm and current density of 5 A.dm −2, the microhardness of the deposit increases with increasing SiC nanoparticle content up to10 g.l −1 in the electrolyte. Microhardness of the coating decreases with SiC content higher than 10 g.l −1. This might be attributed to agglomeration of the SiC nanoparticles in the electrolyte due to their higher concentration and poor wettability. The results shown in Fig. 5 can be explained by the Guglielmi two-step adsorption model [30,31] where a higher particle concentration in the electrolyte increases the adsorption, thus resulting in higher weight percent of SiC nanoparticles in the composite coatings and consequently higher microhardness.
S. Pouladi et al. / Surface & Coatings Technology 213 (2012) 33–40
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Fig. 4. The optical image of the cross section of the nanocomposite coating (a), FESEM micrograph of cross section illustrating the embedding of SiC nanoparticles in the Ni–Zn–P matrix of a Ni–Zn–P/nano-SiC coating (a, b), element distributing map of Si (white points) on the nanocomposite coating (d).
3.4.2. Effect of current density on microhardness There are two different explanations for the effect of current density on the microhardness of these coatings. A change in the current density results in weight percentage change of nickel in the coating which affects the hardness. On the other hand, this changes the amount of SiC in the coating which also influences the hardness [32]. Variation of the microhardness versus current density of the deposits obtained from electrolytes containing 5, 10 and 15 g.l −1 SiC particles is presented in Fig. 6 for a constant stirring rate of 180 rpm. Microhardness was increased with change of the current density from 1 to 7 A.dm −2 and then it was decreased for current densities higher than 7 A.dm −2. Variation of the current density can
Fig. 5. The change in the microhardness with addition of SiC nanoparticles to the bath at 5 A.dm−2.
affect the amount of SiC nanoparticles in the deposits. In a specific current density, the amount of SiC nanoparticles in the deposit is maximum and consequently the microhardness can be maximum [33]. Before this maximum, increasing the content of SiC nanoparticles could be due to the increasing tendency for particles to be adsorbed on the cathode surface, as explained by the Guglielmi model [34]. Adsorption of the particles is the controlling factor and the dominant
Fig. 6. The variation in microhardness of the coatings with current density.
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S. Pouladi et al. / Surface & Coatings Technology 213 (2012) 33–40
Fig. 7. Effect of stirring rate on hardness of composite deposit.
process is the particle deposition. When current density is greater than a specific value, an increase in the current density leads to more metal deposition so fewer particles are deposited and the metal deposition dominates the process. The effect of stirring rate of electrolyte solution on the microhardness of deposits, with a constant current density and constant SiC particle content, 5 A.dm−2 and 5 g.l−1 respectively, is presented in Fig. 7. As seen, variation of stirring rate has a negligible effect on microhardness of the coatings.
3.5. The influence of current density and SiC nanoparticle content on surface roughness Fig. 8 indicates that the average surface roughness of all nanocomposite coatings, obtained from electrolytes containing 5, 10 and 15 g.l−1 SiC, is considerably higher than that of the alloy coatings. Such results were also reported for nickel composite coatings (Ni– Al2O3, Ni–SiC, and Ni–ZrO2) by Borkar et al. [35]. The surface roughness of the coating increases with increasing SiC nanoparticle content up to 10 g.l−1 in the electrolyte. As the SiC content in the electrolyte surpasses 10 g.l−1 the average surface roughness of the coating decreases. The average surface roughness versus current density for the deposits obtained from electrolytes containing 5 and 10 g.l−1 SiC particles has been shown in Fig. 9. As seen, increasing the current density from 1 to 7 A.dm −2 causes an increase in the average surface roughness of the deposits. The surface roughness of deposits decreases for current densities higher than 7 A.dm −2.
Fig. 8. The change in the average surface roughness of coatings with addition of SiC nano-particles to the bath at 5 A.dm−2.
Fig. 9. Average surface roughness of coatings versus current density.
3.6. Coating wear and friction behavior The wear resistance and friction behavior of the coatings were investigated and Fig. 10 illustrates the variations of weight loss as a function of distance for an alloy coating and Ni–Zn–P/nano-SiC composite coatings (deposited with varying content of SiC nanoparticles in the electrolyte). These results are in good agreement with the general understanding that the harder coatings are more wear resistant. The Ni–Zn–P alloy coatings showed severe weight loss compared to all the Ni–Zn–P/nano-SiC composite coatings. The ductile nature of this alloy coating has led to more weight loss than nanocomposite coatings which has been hardened by incorporation of hard reinforcing nano-SiC in the ductile matrix. The better wear resistance of nickel composites compared to nickel alloys is in accordance with Archard's law which states that the wear rate is proportional to the inverse of microhardness of the material [36]. Higher wear resistance for nickel composites than pure nickel and its alloys were also observed in previous studies [29,35,37]. As seen in Fig. 10, wear resistance of the composite coating showed little improvement by increasing the SiC nanoparticles content in the electrolyte. The difference in the wear behavior of the Ni–Zn–P film and the Ni–Zn–P/nano-SiC composite was further verified by the wear tracks
Fig. 10. Weight loss as a function of distance for alloy and nanocomposite coatings, obtained from pin-on-disk tests.
S. Pouladi et al. / Surface & Coatings Technology 213 (2012) 33–40
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Fig. 11. SEM micrographs of worn surfaces of alloy coating (a) and composite coatings, (b) 5 g.l−1, (c) 10 g.l−1, (d) 15 g.l−1 SiC in bath. (e) Magnified worn surface of alloy and (f) composite.
Fig. 12. Average coefficient of friction of alloy coatings and nanocomposite coatings.
as shown in Fig. (11). The wear tracks became progressively narrow by increasing SiC content in the coatings which indicates better wear resistance of these coatings. The magnified worn surface of the alloy compared to that of the nanocomposite coating (Fig. 11e, f) shows severe adhesion and plastic deformation. Additional, striations and scratches are observed on the wear track of the alloy coating. These pieces of evidence also verify the fact that the nanocomposite coatings, produced here, were more wear resistant than the alloy coatings. Fig. 12 shows the average coefficient of friction for Ni–Zn–P alloy and Ni–Zn–P/nano-SiC composite coatings. The average coefficient of friction for all composite coatings was nearly 0.7, a little higher than the average coefficient of friction for the alloy coating with the amount of 0.53. The same result for Ni–P–PTFE–SiC has been reported [38].
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4. Conclusions Effects of the addition of phosphoric acid in the experimental range of 0–25 g.l −1 to the Ni–Zn electroplating bath on the deposit were investigated. In the presence of phosphoric acid in the bath, Ni–Zn–P ternary alloys were deposited. A gradual reducing trend for cracks in the deposits was observed by incorporating and increasing the P content in the deposit. Finally, a crack-free Ni–Zn–P deposit with 4.1 wt.% P was obtained at 25 g.l −1 phosphoric acid. Furthermore, decreasing the current density decreased the number of cracks in the deposit. SiC nanoparticles were successfully embedded in the Ni–Zn–P alloy matrix by electrodeposition. The presence of the SiC nanoparticles in the alloy deposit increased the microhardness, wear resistance, coefficient of friction and surface roughness of the deposit. It seems that microhardness and surface roughness reach their maximum values by the addition of SiC to the bath and also by increasing the electrodeposition current density. But variation in stirring rate of the bath does not have a considerable effect on microhardness.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Acknowledgments
[25] [26] [27] [28]
The authors acknowledge Shiraz University Research Council for the financial support of this project through Grants awarded to professor Bahrololoom and professor Shariat.
[29] [30] [31] [32]
References
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Electrodeposition and characterization of Ni–Zn–P and Ni–Zn–P/nano-SiC coatings S. Pouladi ⁎, M.H. Shariat, M.E. Bahrololoom Department of Materials Science and Engineering, School of Engineering, Shiraz University, Shiraz, Iran
a r t i c l e
i n f o
Article history: Received 2 June 2012 Accepted in revised form 4 October 2012 Available online 12 October 2012 Keywords: Electrodeposition Alloy coating Nanocomposite coating Wear resistance NI–Zn–P Ni–Zn–P/nano-SiC
a b s t r a c t Alloy coatings and metal based composites have been given special attention for their unique properties. In the present study, a novel electroplating bath was introduced. Ni–Zn, Ni–Zn–P nickel rich alloy coatings and Ni–Zn–P/nano-SiC nickel rich composite coating were successfully deposited on low carbon steel substrates. The optimum bath composition was found by comparing the amount of coating cracks checked by a scanning electron microscope (SEM). The presence of nearly homogeneous SiC nanoparticles in the coating was confirmed by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX) and MAP analysis. The effect of operating conditions on the mechanical properties of nanocomposite coatings was investigated. The wear resistance of the coatings was examined by pin on disk wear tests. The nanocomposite coatings had a lower weight loss compared to the Ni–Zn–P coatings in the wear test. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Metallic alloys and composite deposits have been widely used in industry. By electrodeposition we can produce these deposits and control some specific properties of them. Nickel and zinc are often used as base-materials in composite coatings, as they protect steel substrates against corrosion [1]. Although nickel is more noble than zinc, electrodeposition of nickel–zinc alloys produces zinc-rich coatings. The rate of dissolution for these alloy coatings is high in corrosive conditions compared to Ni-based alloy coatings [2]. The mechanism of this preferred deposition has been thoroughly discussed by A. Brenner [3]. Alfantazi et al. [4] achieved a nickel rich Ni–Zn alloy deposit by increasing the temperature of the chloride-based bath. These deposits, produced at 80 °C, contained about 66 wt.% Ni and showed a large number of microcracks [4]. Addition of phosphorous to crystalline Ni–Zn alloy deposit leads to the production of an amorphous structure. The phosphorous content in the deposit must be more than 10% as quoted by Shidharan et al. [5]. Even low phosphorous content (1%) has good effects, such as refining the microstructure, reducing residual tension [6] and increasing coating adhesion [7]. A wide variation in the electrochemical properties of this alloy has been achieved by co-deposition of phosphorous with different phases of Ni–Zn alloy [8]. It was observed that phosphorous refines the microstructure and increases corrosion resistance of the deposit [7]. There are a few papers on the ternary Ni–Zn–P alloy coatings
⁎ Corresponding author at: Material Science and Engineering Dep. Shiraz University, No. 143, Alley 5, Mahallati St., West Ghoudousi Blvd, Shiraz, Iran. Tel.: +98 9173219824; fax: +98 7118434942. E-mail addresses: [email protected] (S. Pouladi), [email protected] (M.H. Shariat), [email protected] (M.E. Bahrololoom). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.10.011
and most of these alloys were produced by electroless technique [2,9–11]. One of the most recent research studies on the electroless plating of Ni–Zn–P and Ni–Zn–P/nano-TiO2 was conducted by Ranganatha et al. [12]. Electroless deposition, an autocatalytic reduction of metals and alloys, offers an alternate and yet attractive method of producing a coating with higher Ni content. This has been known to form a thin and uniform deposit on the substrate compared to electroplating. However, the production of electroless Ni–Zn–P alloy coating needs an expensive reducing agent and also more control is required to maintain Ni composition of the plating bath because the nickel is depleted from the solution, not from the anodes. In addition, this method is more time consuming than electroplating. So, this study used the electroplating technique to produce Ni–Zn–P alloy coatings. Ni–Zn–P alloy was electrodeposited on carbon steel while its phosphorous content was derived from sodium hypophosphite as the phosphorous source by Swathirajan and Mikhail [8]. In the present study, Ni–Zn–P alloys were deposited from different bath compositions and the source of phosphorous in these coatings was phosphoric acid which is cheaper than sodium hypophosphite. One of the primary reasons of this study was investigation on the possibility of using phosphoric acid as the source of phosphorous in this process. Ma et al. [13] have also used phosphoric acid and phosphorous acid as sources of phosphorous to synthesize aluminophosphate. Electrodeposited composites are obtained by adding insoluble solid particles to an electrolytic bath [14–16]. These insoluble particles can be oxides (Al2O3, TiO2, and SiO2) or carbide particles (SiC and WC), or solid lubricants (Polytetrafluoroethylene (PTFE), graphite or MoS2) or even liquid-containing microcapsules [17] to enhance wear resistance and/or to reduce friction. Electroplated composite coatings with microparticles are used as wear-resistant coatings [18–20], e.g., Ni–SiC in car engines [14,21–23].
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Table 1 Chemical composition and conditions of the electrodeposition bath. Composition of solution
−1
ZnCl2 (g.l ) NiCl2·6H2O (g.l−1) H3BO3 (g.l−1) H3PO4 (g.l−1) Sodium dodecyle sulfate (SDS) (g.l−1) Temperature °C Current density (A.dm−2)
Type of coating Ni–Zn
Ni–Zn–P (1)
Ni–Zn–P (2)
Ni–Zn–P (3)
200 200 40 0 0.1
200 200 40 5 0.1
200 200 40 15 0.1
200 200 40 25 0.1
83 ± 1 3
83 ± 1 3
83 ± 1 3
83 ± 1 3
There is an increasing interest in codeposition of nanoparticles with deposited metals due to the increasing availability of nanoparticles [24]. The main concern in using nanoparticles is the presence of an adequate amount of particles in the deposit and also avoiding the agglomeration of nanoparticles in the solution. In the present work, an electroplating bath for electrodeposition of Ni–Zn–P and Ni–Zn–P/nano-SiC with high nickel content on low carbon steel substrates was introduced. In addition, the effect of current density, the amount of SiC particles in the bath and the stirring speed on microhardness, wear resistance and coefficient of friction of these coatings were investigated. 2. Experimental The Ni–Zn nickel rich alloy was deposited from a chloride-based electrolyte in a 250 ml bath with the chemical composition shown in Table (1) using the DC plating technique at current density of 3 A.dm −2, temperature 83 ± 1 °C and pH = 4. By adding phosphoric acid (85 wt.%) to the electrolyte in the amount of 5, 15 and 25 g.l −1, Ni–Zn–P ternary alloys have been deposited. In order to achieve Ni–Zn–P/nano-SiC composite deposits,
SiC nanoparticles, with an average size of 50 nm, were added in the amount of 5, 10, 15 and 20 g.l −1 of the electrolyte. In order to keep the SiC particles suspended and prevent their agglomeration, cetyltrimethyl ammonium bromide (CTAB) surfactant was added [25] and an ultrasonic device and a magnetic stirrer were used. The SiC powder was added to distilled water and was premixed by ultrasonic agitation (power of 350 W and frequency of 40 kHz) for 30 min and subsequently CTAB and SDS (as an anti-pitting agent) with 0.04 and 0.1 g.l −1 concentrations were added respectively. Again the electrolyte was stirred by ultrasonic agitation for 15 min and then nickel chloride, zinc chloride and boric acid were added. Finally, the electrolyte solution was stirred for 24 h before electrodeposition. A magnetic stirrer was used during the electrodeposition period. These electrolytes were prepared using Merck pro-analysis grade chemicals and double-distilled water. Low carbon steel (st 37) plates were used as substrates. Before electroplating, each substrate was polished with emery papers (with the following grades: 400, 600, 800, 1000, 2000, and 3000) and then electropolished by dipping into a solution containing 94 vol.% acetic acid (90 wt.%) and 6 vol.% perchloric acid (60 wt.%). After electropolishing, the substrates were rinsed with distilled water and then immersed immediately in the electroplating bath. A nickel sheet (The total exposed surface area was 45 cm 2) was used as an anode. The anode surface area was greater than the cathode surface area to ensure that there was no anodic polarization. To ensure the presence of phosphorous in the deposits, the X-ray fluorescence (XRF, XMF-104 High Speed Micro ED X-Ray Spectrometer) analysis was performed. In order to find the optimum electrolyte composition, the number of cracks in the deposits was studied by a scanning electron microscope (SEM, Cambridge S360). In addition, the presence of SiC in the coatings was also detected using energy dispersive X-ray spectroscopy (EDX). A field emission scanning electron microscope (FESEM, Hitachi S-4160 electron microscope) was used to see how well the SiC particles were distributed in the cross section of the coatings.
Fig. 1. SEM micrographs of the coatings: (a) The Ni–Zn coating, (b) Ni–Zn–P (5 g.l−1 H3 PO4), (c) Ni–Zn–P (15 g.l−1 H3 PO4), and (d) Ni–Zn–P (25 g.l−1 H3 PO4).
S. Pouladi et al. / Surface & Coatings Technology 213 (2012) 33–40 Table 2 Chemical composition of Ni–Zn–P alloy deposits. Phosphoric acid (85 wt.%) (g.l−1)
Ni wt.%
Zn wt.%
P wt.%
5 15 25
82.81 86.36 88.47
14.79 10.44 7.43
2.4 3.2 4.1
To evaluate the tribological properties of the coatings, a homemade pin on disk tribometer, with the sample placed horizontally on a turntable, was used. The testing temperature and relative humidity were 24 ± 1 °C and 45%, respectively. The air relative humidity was reported by Shiraz Meteorological Institute. The tests were performed using hardened high-carbon steel pin and at a linear speed of 0.03 m.s −1 for a total sliding distance of 400 m. A normal load of 18 N was used during the wear tests. In wear resistance experiments, specimens were weighed before the test and after rotation of 100, 200, 300 and 400 m. Coating friction factor was measured using
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the same apparatus, resulting in a graph of friction factor as a function of distance. Hardness measurement of the coatings, which is reported as an average of five readings on each specimen, was carried out using a Leitz L137 microhardness testing instrument equipped with a Vickers indenter under a load of 200 g and load exertion time of 15 s. The microhardness of the coatings as a function of SiC content in the bath, current density and stirring rate was studied. The average surface roughness values of the coatings were determined using a Mitutoyo Surftest 201 roughness tester. 3. Results and discussion 3.1. Coating morphology and composition The surface morphology of a Ni–Zn deposit plated on a steel substrate is shown in Fig. 1a. It can be seen that the Ni–Zn coating shows a large number of microcracks, 0.05 μm.μm−2 crack density. A similar
Fig. 2. SEM micrographs of coatings: (a) 3 A.dm−2, (b) 2 A.dm−2, (c) 1 A.dm−2 (2 g.l−1 H3PO4) (d) 3 A.dm−2, (e) 2 A.dm−2, and (f) 1 A.dm−2 (5 g.l−1 H3PO4).
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3.2. The effect of current density on coating cracks The effect of current density on the number of cracks in the deposits for a constant concentration of phosphoric acid in the electrolyte is shown in Fig. 2. As seen, the number of cracks in the deposits decreased with decreasing current density. It has been reported by Narayan and Mungole [26] that increasing the current density leads to a decrease in phosphorous content of the deposit. The results of present investigation showed that increasing the amount of phosphorous improves the quality of the coatings. Considering the results reported by Narayan and Mungole [26] and also the results of this investigation, it may be concluded that decreasing the current density can improve the quality of deposits, as shown in Fig. 2. 3.3. Electrodeposition of nanocomposites
Fig. 3. EDX analysis of a nanocomposite coating on two points.
result was reported by Alfantazi et al. from a chloride-based electrolyte using the square-wave pulse-plating technique [4]. To eliminate these cracks in the deposit and to produce a ternary Ni–Zn–P alloy, addition of phosphorous to the Ni–Zn alloy deposits was proposed. X-ray fluorescence analysis showed that the deposit obtained from an electrolyte containing 5 g.l −1 phosphoric acid was a ternary alloy with the chemical composition of Ni82.81–Zn14.79–P2.4. By increasing the amount of phosphoric acid in the electrolyte in the range of 0–25 g.l −1, the amount of phosphorous in the deposit increased (Table 2). The increase in the amount of phosphorous in the deposit decreased the number of cracks from 0.004 μm.μm −2 (Fig. 1b) to 0.003 μm.μm −2 (Fig. 1c). Ultimately at 25 g.l −1 phosphoric acid (Fig. 1d) no cracks were observed. Thus, the optimum electrolyte composition for the deposition of Ni–Zn–P alloy should contain at least 25 g.l −1 phosphoric acid (85wt.%). Similar results were also reported for Ni–Zn–P alloy electrolessly deposited by Bouanani et al. [11]. They have attributed the presence of cracks in some deposits to the internal stress caused by nickel lattice distortion and increase in the internal stress with decreasing phosphorus in the deposit [11].
Table 3 Chemical composition of Ni–Zn–P/nano-SiC (wt.%).
Average weight percent wt.% Errors wt.%
C
Si
P
Fe
Ni
Zn
3.265
2.290
2.475
0.325
83.505
8.125
±0.135
±0.030
±0.015
±0.035
±0.045
±0.115
As mentioned, for deposition of nanocomposites, SiC nanoparticles were added to the Ni–Zn–P alloy bath. A large number of samples (five samples were prepared for each experimental condition and these five samples were analyzed to investigate their characteristics) were used to be coated by Ni–Zn–P/nano-SiC. To ensure the existence of SiC particles in the coatings and production of nanocomposites, EDX analysis was performed on two different points on one of the nanocomposite coatings which was obtained from an electrolyte containing 10 g.l−1 SiC and at current density of i = 3 A.dm −2 (Fig. 3a, b). Energy dispersive X-ray spectroscopy is an elementary technique and the peaks related to Si and C are the results of the presence of SiC in the coating. Nearly equal intensity of the peaks in both graphs shows approximately equal chemical compositions in different areas. In the EDX graphs, only the elements Ni, Zn, P, Fe, Si and C can be seen. The Fe peak is probably related to the background interference. Table 3 shows the weight percentage and the errors of determination of elements in the composite. The cross section micrograph of the nanocomposite coating which was obtained from an electrolyte containing 10 g.l−1 SiC and at current density of i = 5 A.dm −2, shown in Fig. 4a, indicates a uniform coating even on the cross section corners of the steel plate. The average thickness of the coating and the electrodeposition rate were 25 μm and 50 μm.h−1, respectively. Fig. 4b and c indicates the presence of SiC nanoparticles within the entire coating layer. Although the individual nanoparticles are not well distributed, the distribution of approximately nano-sized clusters of nanoparticles appears uniform. Meanwhile, the element distribution map of Si (white points) in the Ni–Zn–P/nano-SiC composite coating (Fig. 4(d)) reveals the uniform distribution of Si in the composite. 3.4. Microhardness 3.4.1. Effect of SiC content in the bath on microhardness of the coatings Fig. 5 indicates that the microhardness of all Ni–Zn–P/nano-SiC composite coatings is considerably higher than the Ni–Zn–P alloy coatings. Hardness of composite coatings containing SiC has been attributed to the hindrance of dislocation movement by SiC particles [27,28]. High hardness of the Ni–Zn–P/nano-SiC composite coating will provide wear resistance [29]. Fig. 5 shows that at a constant stirring rate of 180 rpm and current density of 5 A.dm −2, the microhardness of the deposit increases with increasing SiC nanoparticle content up to10 g.l −1 in the electrolyte. Microhardness of the coating decreases with SiC content higher than 10 g.l −1. This might be attributed to agglomeration of the SiC nanoparticles in the electrolyte due to their higher concentration and poor wettability. The results shown in Fig. 5 can be explained by the Guglielmi two-step adsorption model [30,31] where a higher particle concentration in the electrolyte increases the adsorption, thus resulting in higher weight percent of SiC nanoparticles in the composite coatings and consequently higher microhardness.
S. Pouladi et al. / Surface & Coatings Technology 213 (2012) 33–40
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Fig. 4. The optical image of the cross section of the nanocomposite coating (a), FESEM micrograph of cross section illustrating the embedding of SiC nanoparticles in the Ni–Zn–P matrix of a Ni–Zn–P/nano-SiC coating (a, b), element distributing map of Si (white points) on the nanocomposite coating (d).
3.4.2. Effect of current density on microhardness There are two different explanations for the effect of current density on the microhardness of these coatings. A change in the current density results in weight percentage change of nickel in the coating which affects the hardness. On the other hand, this changes the amount of SiC in the coating which also influences the hardness [32]. Variation of the microhardness versus current density of the deposits obtained from electrolytes containing 5, 10 and 15 g.l −1 SiC particles is presented in Fig. 6 for a constant stirring rate of 180 rpm. Microhardness was increased with change of the current density from 1 to 7 A.dm −2 and then it was decreased for current densities higher than 7 A.dm −2. Variation of the current density can
Fig. 5. The change in the microhardness with addition of SiC nanoparticles to the bath at 5 A.dm−2.
affect the amount of SiC nanoparticles in the deposits. In a specific current density, the amount of SiC nanoparticles in the deposit is maximum and consequently the microhardness can be maximum [33]. Before this maximum, increasing the content of SiC nanoparticles could be due to the increasing tendency for particles to be adsorbed on the cathode surface, as explained by the Guglielmi model [34]. Adsorption of the particles is the controlling factor and the dominant
Fig. 6. The variation in microhardness of the coatings with current density.
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Fig. 7. Effect of stirring rate on hardness of composite deposit.
process is the particle deposition. When current density is greater than a specific value, an increase in the current density leads to more metal deposition so fewer particles are deposited and the metal deposition dominates the process. The effect of stirring rate of electrolyte solution on the microhardness of deposits, with a constant current density and constant SiC particle content, 5 A.dm−2 and 5 g.l−1 respectively, is presented in Fig. 7. As seen, variation of stirring rate has a negligible effect on microhardness of the coatings.
3.5. The influence of current density and SiC nanoparticle content on surface roughness Fig. 8 indicates that the average surface roughness of all nanocomposite coatings, obtained from electrolytes containing 5, 10 and 15 g.l−1 SiC, is considerably higher than that of the alloy coatings. Such results were also reported for nickel composite coatings (Ni– Al2O3, Ni–SiC, and Ni–ZrO2) by Borkar et al. [35]. The surface roughness of the coating increases with increasing SiC nanoparticle content up to 10 g.l−1 in the electrolyte. As the SiC content in the electrolyte surpasses 10 g.l−1 the average surface roughness of the coating decreases. The average surface roughness versus current density for the deposits obtained from electrolytes containing 5 and 10 g.l−1 SiC particles has been shown in Fig. 9. As seen, increasing the current density from 1 to 7 A.dm −2 causes an increase in the average surface roughness of the deposits. The surface roughness of deposits decreases for current densities higher than 7 A.dm −2.
Fig. 8. The change in the average surface roughness of coatings with addition of SiC nano-particles to the bath at 5 A.dm−2.
Fig. 9. Average surface roughness of coatings versus current density.
3.6. Coating wear and friction behavior The wear resistance and friction behavior of the coatings were investigated and Fig. 10 illustrates the variations of weight loss as a function of distance for an alloy coating and Ni–Zn–P/nano-SiC composite coatings (deposited with varying content of SiC nanoparticles in the electrolyte). These results are in good agreement with the general understanding that the harder coatings are more wear resistant. The Ni–Zn–P alloy coatings showed severe weight loss compared to all the Ni–Zn–P/nano-SiC composite coatings. The ductile nature of this alloy coating has led to more weight loss than nanocomposite coatings which has been hardened by incorporation of hard reinforcing nano-SiC in the ductile matrix. The better wear resistance of nickel composites compared to nickel alloys is in accordance with Archard's law which states that the wear rate is proportional to the inverse of microhardness of the material [36]. Higher wear resistance for nickel composites than pure nickel and its alloys were also observed in previous studies [29,35,37]. As seen in Fig. 10, wear resistance of the composite coating showed little improvement by increasing the SiC nanoparticles content in the electrolyte. The difference in the wear behavior of the Ni–Zn–P film and the Ni–Zn–P/nano-SiC composite was further verified by the wear tracks
Fig. 10. Weight loss as a function of distance for alloy and nanocomposite coatings, obtained from pin-on-disk tests.
S. Pouladi et al. / Surface & Coatings Technology 213 (2012) 33–40
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Fig. 11. SEM micrographs of worn surfaces of alloy coating (a) and composite coatings, (b) 5 g.l−1, (c) 10 g.l−1, (d) 15 g.l−1 SiC in bath. (e) Magnified worn surface of alloy and (f) composite.
Fig. 12. Average coefficient of friction of alloy coatings and nanocomposite coatings.
as shown in Fig. (11). The wear tracks became progressively narrow by increasing SiC content in the coatings which indicates better wear resistance of these coatings. The magnified worn surface of the alloy compared to that of the nanocomposite coating (Fig. 11e, f) shows severe adhesion and plastic deformation. Additional, striations and scratches are observed on the wear track of the alloy coating. These pieces of evidence also verify the fact that the nanocomposite coatings, produced here, were more wear resistant than the alloy coatings. Fig. 12 shows the average coefficient of friction for Ni–Zn–P alloy and Ni–Zn–P/nano-SiC composite coatings. The average coefficient of friction for all composite coatings was nearly 0.7, a little higher than the average coefficient of friction for the alloy coating with the amount of 0.53. The same result for Ni–P–PTFE–SiC has been reported [38].
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4. Conclusions Effects of the addition of phosphoric acid in the experimental range of 0–25 g.l −1 to the Ni–Zn electroplating bath on the deposit were investigated. In the presence of phosphoric acid in the bath, Ni–Zn–P ternary alloys were deposited. A gradual reducing trend for cracks in the deposits was observed by incorporating and increasing the P content in the deposit. Finally, a crack-free Ni–Zn–P deposit with 4.1 wt.% P was obtained at 25 g.l −1 phosphoric acid. Furthermore, decreasing the current density decreased the number of cracks in the deposit. SiC nanoparticles were successfully embedded in the Ni–Zn–P alloy matrix by electrodeposition. The presence of the SiC nanoparticles in the alloy deposit increased the microhardness, wear resistance, coefficient of friction and surface roughness of the deposit. It seems that microhardness and surface roughness reach their maximum values by the addition of SiC to the bath and also by increasing the electrodeposition current density. But variation in stirring rate of the bath does not have a considerable effect on microhardness.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Acknowledgments
[25] [26] [27] [28]
The authors acknowledge Shiraz University Research Council for the financial support of this project through Grants awarded to professor Bahrololoom and professor Shariat.
[29] [30] [31] [32]
References
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