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Studies on enhancement of surface mechanical properties of electrodeposited Ni–Co alloy coatings due to saccharin additive
Surface & Coatings Technology 258 (2014) 225–231
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Studies on enhancement of surface mechanical properties of electrodeposited Ni–Co alloy coatings due to saccharin additive A.C. Lokhande ⁎, J.S. Bagi Department of Production Engineering, KIT's College of Engineering, Kolhapur 416 234, MS, India
a r t i c l e
i n f o
Article history: Received 8 April 2014 Accepted in revised form 10 September 2014 Available online 18 September 2014 Keywords: Electrodeposition Ni–Co alloy Corrosion resistance Contact angle Friction coefficient Wear resistance
a b s t r a c t NiCo alloy coatings were galvanostatically electrodeposited at room temperature from sulfate bath using saccharin (0 to 12 g L−1) as an additive agent. These alloy coatings were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic absorption spectroscopy (AAS), contact angle, microhardness, Electrochemical impedance spectroscopy (EIS), salt immersion test and tribology techniques. The XRD study confirmed the formation of Ni–Co alloy with a face centered cubic (FCC) structure. Depending on saccharin addition, the Co content varied from 27 to 7 at.%. The microhardness and corrosion resistance showed enhancement due to saccharin addition. Ni–Co alloy with a 12 at.% Co content obtained from 10 g L−1 saccharin addition showed reduced grain size (12 nm) and hydrophobic surface (113°) with the highest corrosion and wear resistant values as compared to others. Such coating on medium carbon steel fasteners showed significant enhancement in corrosion protection during salt immersion test. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Electrodeposited cadmium and zinc based alloy coatings have been widely utilized in aerospace, electrical and industrial applications due to their good corrosion resistance. Cadmium coatings are produced from cyanide bath which is very toxic. Similarly, the deposition of zinc coating produces harmful hexavalent compounds. On the other hand, Ni based alloy coatings can serve the best alternative for the toxic cadmium coating [1]. These alloy coatings have been used for the purpose of industrial fasteners and decorative coatings. Particularly, Ni–Co alloy coatings have been utilized in many applications due to their advantages like strength, better wear resistance, corrosion resistance, hardness and their esthetic value [2,3]. Preparation parameters employed during electrodeposition such as the composition of bath, pH, current density and temperature noticeably affect the microstructure and mechanical properties of the Ni–Co alloy [4]. These alloys have been electrodeposited from simple and complex baths. This deposition technique produces a nanosized grain structure and has advantages such as low cost, high production rate, and uniform deposition [5]. Deposition from sulfamate bath produces low stressed deposits [6–9]. The cobalt content in the deposit greatly affects the mechanical properties of the Ni–Co alloy. Co rich alloy possesses high wear and friction resistance due to the hexagonal close packed (HCP) crystal structure. The microhardness also depends on the Co content in the alloy and increasing the Co content in the alloy beyond 50 at.% causes
⁎ Corresponding author. Tel.: +91 9049626517. E-mail address: [email protected] (A.C. Lokhande).
http://dx.doi.org/10.1016/j.surfcoat.2014.09.023 0257-8972/© 2014 Elsevier B.V. All rights reserved.
reduction in the microhardness due to the HCP phase structure of the Co [10,11]. The mechanical properties of the Ni–Co alloy depend upon the grain size. Generally, alloys with a grain size between 20 and 100 nm show enhancement in mechanical properties. Wang et al. [11] reported the effect of grain size on the microhardness of the electrodeposited Ni–Co alloy coating additive agents are used to enhance the mechanical properties, control the grain size and compactness of the film. Therefore, many additives have been added during the deposition. Ni coatings have been obtained using organic additives like saccharin, naphthalene and trisulfonic acid. Saccharin plays a very important role in the deposition of Ni based alloy coatings. It produces smooth, compact, low stressed and refined grain deposits [12]. These effects are produced due to the variation in the composition of the Helmholtz layer and adsorption of the saccharin on the active sites of cathode inducing chemical barrier layer for adions and adatoms on the surface of cathode resulting growth inhabitation of grain deposit [13]. The presence of the impurities such as sulfur and carbon in the saccharin restricts the sliding of grain boundaries and the grain growth. Chang et al. [14] studied the corrosion resistance of Ni–Co alloy at 800 °C in air. Li et al. [5] studied the effects of saccharin and Co concentration on the microhardness of Ni–Co alloy. Scanty data are available on the electrochemical behavior of cobalt rich Ni–Co alloy. So far, studies on the mechanical properties, wettability and corrosion resistance of nickel rich Ni–Co alloy coatings obtained from the bath containing additive agent are not reported. This paper reports the electrodeposition of nickel rich Ni–Co alloy coatings from the bath containing saccharin as an additive agent and its effect on the quality of electrodeposited Ni–Co alloy coatings. The effect of the saccharin content on the surface wettability, corrosion
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resistance and its tribological behavior has been studied. The effect of the saccharin content on microstructure and surface mechanical properties is reported. The industrial fasteners coated with Ni–Co alloy are used for the corrosion protection study. 2. Experimental details Nickel rich Ni–Co alloys were prepared using an electrodeposition technique. In electrodeposition, preparative parameters such as the concentration and composition of solution, complexing agent, pH of bath, temperature, plating current density, deposition time, applied voltage and nature of substrate play important roles in deciding the quality of coating. By making various trial and errors, the following preparative parameters were fixed for the deposition of Ni–Co alloys. Ni–Co alloys were electrodeposited at room temperature (300 K) from an aqueous bath consisting of nickel sulfate (250 g L−1), nickel chloride (40 g L− 1) and cobalt sulfate (20 g L− 1). Boric acid (35 g L−1) was used as a buffer to maintain pH, Triton X-100 (0.08 g L−1) as an antipitting agent and saccharin (6–12 g L−1) as an additive. The pH of the solution was adjusted to 2.5 ± 0.1 using nickel carbonate and sulfuric acid. All solutions were prepared using double distilled water. Fresh solutions were used for all depositions. The electrodeposition was carried out on medium carbon steel (1 cm × 5 cm area) strip substrates. The substrates were polished with a sand paper followed by ultrasonic cleaning in double distilled water. After that, the substrates were activated for 10 s in the 10 vol.% HCl solution followed by final rinsing in double distilled water. A three electrode system consisting of carbon steel substrate as a cathode, graphite as an anode, and saturated calomel electrode (SCE) as a reference electrode, was used. Electrodeposition was carried out in a galvanostatic mode at a current density of 15 mA cm− 2 using WonATech battery cycler potentiostat. The plating parameters like current density, electrolyte composition and temperature were kept fixed and the concentration of the additive agent, saccharin, was varied from 0 to 12 g L − 1 . The Ni–Co alloy coatings were electrodeposited for 15 min which resulted into the thickness between 10 and 12 μm. The atomic percentage of the cobalt content in Ni–Co alloy deposits was determined by atomic absorption spectroscopy (AAS). The structural characterization and the stress induced in the electrodeposited Ni–Co alloy films were carried out by analyzing X-ray diffraction (XRD) patterns obtained with CuKα (α = 1.5418 Å) radiation from a D2 PHASER model (Bruker AXS Analytical Instruments Pvt Ltd) in the span of angle 2θ between 10 and 90°. Surface morphology of the Ni–Co alloy was studied with a scanning electron microscope (SEM) (JEOL, JSM 6360). The elemental composition of the alloy coating was studied with an atomic absorption spectrophotometer (Perkin Elmer Analyst 300). The surface wettability studies of the coating were carried out a using contact angle meter (Rame-hart USA equipment) with a CCD camera. The topographic images were recorded with a multimode atomic force microscope (AFM) (Innova-1B3BE) operated in the contact mode. Corrosion resistance was studied with a electrochemical impedance spectrometer workstation (Won-ATech, zive SP5) and also with a salt immersion test in a 5% NaCl solution. The hardness was measured using a Vicker's microhardness tester with a load of 150 g applied for 10 s. The friction coefficient (μ) and the wear resistance were determined with an applied load of 6 N on a pin on disk tribometer (Ducom instruments, India).
brings about reduction in the deposition of the cobalt content and facilitates that of nickel as seen in Table 1. Also, the Co content in the deposit is higher than that in the electrolyte bath indicating an anomalous codeposition of Ni–Co alloy, where less noble metal Co is preferentially deposited as an effect of pH change of the electrode, which is in agreement with the earlier results [5,14]. The atomic percent of the cobalt in the Ni–Co alloys affects the mechanical properties of the alloy. Co rich Co–Ni alloys (Co N 50 at.%) show improved properties of friction coefficient and wear resistance. The microstructure becomes less compact as the Co content increases beyond 30 at.% [2,5]. Hence in the present work, Ni:Co alloy composition was obtained as 73:27 to 93:7 at.% by varying the saccharin concentration. 3.2. Structure and induced stress in Ni–Co alloys coatings Fig. 1 shows XRD patterns of Ni–Co alloy deposited with saccharin addition between 0 and 12 g L−1. The XRD pattern (a) without saccharin addition shows only (111) peak and inhibits other peaks due to the mixed phase of FCC and HCP structures. With saccharin addition, the intensity of (111) peak increases up to 10 g L−1 and decreases afterwards. The other two peaks corresponding to (200) and (220) planes appear for higher saccharin addition exhibiting more of the Ni (α) phase of FCC. The XRD pattern matches with standard JCPDS card number 01-074-5694. A similar type of XRD results is obtained in electrodeposited Ni–Co alloy [15-17]. The more intense peak long (111) plane indicates that the films are textured along (111) plane, which may due to the combined effect of saccharin and presence of cobalt [5]. The crystallite size, D, of Ni–Co alloy was calculated for the (111) plane at fullwidth half maximum (FWHM) using the Scherrer’s formula as, D¼
0:9λ β cos θ
ð1Þ
where β is the broadening of diffraction line measured at half maximum intensity (radians), λ = 1.5404 Å is the wavelength of the CuKα X-ray, and θ is the Bragg's angle. The deposited alloy consists of nanocrystals of about 12.0 nm. It is well known that residual stresses are induced in the deposit during electrodeposition. Grain boundary relaxation may be a major factor leading to induce residual stress in the deposited films. The stress induced in the electrodeposited Ni–Co alloy coatings and lattice strains are determined from the XRD study for the characteristic intense peak (111) using the ‘H Digital’ software. The microstrain was calculated using the formula. β ¼ λ=D cos ðθÞ − ε tan ðθÞ
ð2Þ
where ε⁎ is the dimensionless strain value and β, λ, θ and D⁎ are parameters from Eq. (1). Table 1 shows the variation of induced stress and lattice strains in the electrodeposited Ni–Co alloy. The variation in the values of stress and strain can be seen as an effect of saccharin. The formation of compact morphology due to the addition of saccharin from 0 to 10 g L−1 reduces induced stress. The nucleation and grain growth process is inhabited by the saccharin which produces smooth, compact and low stress deposits. As the saccharin content increased up to 12 g L−1, the induced stress is increased due to the formation of a non-compact morphology.
3. Results and discussion 3.3. Microstructure of Ni–Co alloy coatings 3.1. Composition of Ni–Co alloy coatings The AAS study was carried out on alloys obtained by varying amount of saccharin as 0, 6, 10 and 12 g L−1. The AAS results confirmed the composition of cobalt as 27, 18, 12 and 7 at.%, respectively in the Ni–Co alloy. The analysis depicts that saccharin addition in the electrolyte bath
Fig. 2 shows the SEM images of Ni–Co alloy coatings with saccharin addition as (a) 0, (b) 6, (c) 10 and (d) 12 g L−1. The morphology with no saccharin addition (Fig. 2(a)) with Co composition as 27 at.% has a non-compact morphology with a grain size between 170 and 200 nm. The induced stress is more with no grain growth inhabitation process
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Table 1 The effect of saccharin content on the properties of NiCo alloy coating. Saccharin content in deposition bath (g L−1)
Ni:Co composition (at.%)
Compressive stress induced (MPA)
Lattice strain
Microhardness value (HV)
Impedance value (Ω cm–2)
0 6 10 12
73:27 82:18 88:12 93:7
−550 −515 −450 −612
2.10 1.32 0.77 1.74
360 340 320 280
290 340 524 360
due to saccharin. Fig. 2(b) shows a smooth, bright and compact microstructure consisting of a cluster of grains. The grain size lies in the range of 100–120 nm. The smooth and compact morphology resulted due to the FCC crystal structure of Ni rich alloy along with 6 g L− 1 saccharin as saccharin restricts the Co deposition and favors Ni deposition. This result is in agreement with others [18,19]. Fig. 2(c) shows a smooth and densely packed morphology with further reduced grain size in the range of 40–50 nm due to an increased saccharin concentration (10 g L−1) inducing grain refinement effect along with restriction to the grain boundary sliding. It is quite clear that the density of grain boundaries is reduced, reflecting in the uniform and compact deposit. Fig. 2(d) shows the formation of a non-compact and cracked morphology. This may be attributed due to the variation in the composition of the Helmholtz layer and inducing carbon and sulfur impurities in the deposits due to the excess addition of saccharin [13].
3.4. Wettability of Ni–Co alloy coatings Controlling the surface wettability of solid surfaces is important in many situations. If the wettability is high, the contact angle (θ b 90°) is small and the surface is hydrophilic and, if the wettability is low, the contact angle (θ N 90°) is large and the surface is hydrophobic. The microstructure, chemical composition, local inhomogeneities and surface cleanliness decide the contact angle [20]. The contact angle of the Ni–Co alloy deposited without saccharin addition was 84°, indicating that the alloy is hydrophilic. With increasing the saccharin content from 0 to 10 g L−1, the contact angle increased up to 113°, indicating the hydrophobic nature of the alloy. This increment in the contact angle of the alloy is due to the smooth and compact morphology obtained with increased saccharin addition. Increasing saccharin addition to 12 g L−1 showed contact angle value as 74° resulting in the hydrophilic nature of the Ni–Co alloy due to the production of a non-compact
Fig. 1. The XRD patterns of electrodeposited Ni–Co alloy coating from saccharin addition as, (a) 0 g L−1, (b) 6 g L−1, (c) 10 g L−1 and (d) 12 g L−1 in the bath.
microstructure. Fig. 3 shows the typical contact angle measurement of the Ni–Co alloy with 10 g L−1 saccharin addition. In order to avoid corrosion, hydrophobic surface is very desirable as it provides very limited scope for the liquid medium to wet the coated surface. This avoids the maximum contact between liquid medium and coated surface making the coated surface as a water repellent surface and reducing the chances of chemical reaction between liquid and coated surface. Hydrophilic surface increases the contact between liquid medium and coated surface. As the wettability or the contact between liquid medium and coated surface increases, liquid spreads over the coated surface as well as enters the pores and cracks of the coated surface increasing the chances of chemical reaction which leads to the formation of undesirable oxide products. In the present work, due to the saccharin addition, coating surface becomes compact and water repellent. Such coatings are desirable in the corrosion protection applications. 3.5. Surface topography and microhardness of Ni–Co alloy deposit The three-dimensional (3D) surface topography of Ni–Co alloy coatings was studied using an AFM technique. The AFM 3D images in Fig. 4(a–c) are of the Ni–Co alloy coatings produced from 0, 6 and 10 g L−1 saccharin containing baths, respectively. Table 2 shows the values of surface roughness, grain size and grain height for various saccharin contents in the bath obtained from AFM studies. A low roughness is obtained as compared to electrodeposited Ni–Co alloy under a magnetic field [21]. The significant reduction in the grain size from 162 to 56 nm and roughness value from 17 to 3.16 nm resulted from increasing saccharin addition from 0 to 10 g L− 1, producing a grain refinement effect with a smooth and compact structure, as evident from SEM images. There is a considerable reduction in the grain height from 79.6 to 12 nm due to the effect of grain growth inhabitation by saccharin inducing chemical barrier to adions and adatoms on the coated surface acting as the cathode [13]. From the AFM study, it is seen that nanograins of uniform size and shapes are formed with low roughness value. Fig. 5 represents the variation of the microhardness with an at.% of Co in the electrodeposited Ni–Co alloy. The microhardness of the alloy increased from 280 to 360 Hv with decreasing saccharin content from 12 to 0 g L−1, producing a Co content from 7 at.%. to 27 at.%. The microstructure of a coating depends on many factors such as the phase structure, crystallite size and formation of solid solution. The microhardness of alloy increases with the formation of a solid solution which may be studied using a solid solution hardening mechanism [22]. In our case, the microhardness of 27 at.%. of Co in Ni–Co alloy results from the formation of the mixed phase of FCC and HCP crystal structures [23].The alloy compositions with 6, 10 and 10 g L−1 added saccharin resulted into a pure FCC structure. This is clear from (111) and (200) reflections of the Ni–Co coating with 7, 12 and 18 at.% of the Co content. However, the reduced intensities of (111) plane and absence of (200) and (220) planes corresponding to Ni–Co alloy prepared without saccharin addition indicates the formation of mixed phase which in turn helps to improve the microhardness. In addition, the hardness decrease with saccharin content can also be explained using an inverse Hall–Petch relation [24]. According to this relation the microhardness is decreased
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Fig. 2. The SEM images of Ni–Co alloy coatings deposited from saccharin addition as, (a) 0 g L−1, (b) 6 g L−1, (c) 10 g L−1 and (d) 12 g L−1 in the bath. Magnification of 2000×.
when the crystallite size is reduced below some critical level as observed in our case.
3.6. Electrochemical impedance spectroscopy (EIS) study of Ni–Co alloy coatings EIS is an important technique to determine the corrosion of coatings. Fig. 6(a–d) shows the Nyquist plots of the Ni–Co alloys in a 5% NaCl solution with an applied bias of 10 mV, where Z′(ω) and Z″(ω) are the real and imaginary parts of the measured impedance respectively and ω is the angular frequency. The higher impedance value and larger diameter of the (incomplete) semicircle in the spectra of the Ni–Co alloy show its higher corrosion resistance, which can be related to the variation in the film (coating) capacitance. The capacitive impedance at high
frequencies is well related to the thickness and the dielectric constant of the coating [25]. Two semicircles appear in the plots indicating two relaxation processes in the lower and higher frequency ranges. The semicircle in the lower frequency range indicates the oxidation resistance of the Ni–Co alloy and the semicircle in the higher frequency range represents the corrosion resistance of the alloy [26]. The higher impedance value in the lower and higher frequency ranges indicates the higher oxidation and corrosion resistance. Fig. 6(a–c) shows the Nyquist plots of Ni–Co alloy with 0, 6 and 10 g L−1 saccharin addition with impedance value as 290, 340 and 524 Ω cm2, respectively. The increased impedance value from 290 to 524 Ω cm2 resulted due compact morphology, due to saccharin addition. Additional saccharin content in bath up to 12 g L− 1 resulted in the decrease in the impedance value to 360 Ω cm2 (Fig. 6(d)), due to the formation of a non-compact and cracked microstructure. The reduced impedance is attributed due to the spreading and entering of an NaCl solution into the pores and cracks of the alloy inducing chemical reaction, leading to the formation of undesirable oxide products in the coating. Table 1 shows the variation in the impedance with saccharin addition in the bath.
3.7. Salt immersion test (5% NaCl solution test)
Fig. 3. Contact angle measurement on Ni–Co alloy coatings with 10 g L−1 saccharin addition.
In this test, the Ni–Co coated medium carbon steel fasteners were kept immersed in a 5% NaCl solution at room temperature and the time taken for the appearance of the red rust on the coated surface was noted. Fig. 7(a) shows the photograph of medium carbon steel fasteners without and with electrodeposited Ni–Co alloys. Fig. 7(b) shows the photograph of the formation of red rust along with the time taken for rusting of the Ni–Co alloy coated fasteners with 0, 6, 10 and 12 g L−1 saccharin addition in the bath. Ni–Co alloy deposited on fasteners with 10 g L−1 saccharin addition in the bath showed a maximum time for the formation of red rust as compared to other fasteners such as 0, 6 and 12 g L−1 saccharin addition. EIS study also supports that Ni–Co alloy prepared from 10 g L−1 saccharin addition in the bath has a maximum corrosion resistance.
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Fig. 4. The 3 dimensional (3-D) AFM images of Ni–Co alloy coatings deposited from bath containing saccharin addition as, (a) 0 g L−1, (b) 6 g L−1 and (c) 10 g L−1.
Fig. 8(a, b) shows the variation of friction coefficient value (μ) with sliding time for Ni–Co alloy coatings without and with saccharin addition. Fig. 8(a) shows the plot without saccharin addition. Initially μ value starts from 0 and reaches a value of about 0.25. After time lapse of 3000 s, the μ value shoots to 0.71 and then attains a stable constant average value of 0.62. Significant fluctuations are observed in the graph due to the wear of the Ni–Co alloy coating surface during sliding and its effect of acting as third body resistance between the pin and coated Ni–Co alloy surfaces. The value of μ as 0.62 is mainly attributed due to the face centered cubic (FCC) phase structure of the Ni rich Ni– Co alloy along with the hexagonal close packed (HCP) phase structure of Co. Fig. 8(b) represents the μ of the Ni–Co alloy with 10 g L−1 saccharin addition. The value of μ increases up to 0.69. This effect of increase in μ value is mainly due to Ni rich Ni–Co alloy with Ni content as 88 at.%. Hence, it is clear that μ increases as the saccharin content increases which results to produce Ni rich Ni–Co alloy with restricted Co content deposits. It is known that μ depends upon the content of Co in Ni–Co alloy. Ni rich alloy exhibits an increased μ value due to its FCC phase structure and reduced HCP structure of Co. Similar results are reported
Table 2 The effect of saccharin content on surface roughness, grain size and grain height of Ni–Co alloy coating. Saccharin content in deposition bath (g L−1)
Surface roughness (nm)
Grain size (nm)
Grain height (nm)
0 6 10
17 9 3.16
162 120 56
79.2 31 12
for Ni–Co alloy with increased Co content up to 80% with reduced friction coefficient value up to 0.25 by Wang et at [11]. Hamid et al. [27] reported a high friction coefficient value of deposited Ni up to 1.52. Fig. 9(a, b) shows the variation of wear with a sliding time of the Ni– Co alloy coating without and with saccharin addition. Fig. 9(a) shows the wear of Ni–Co alloy without saccharin addition. The wear starts from 0 μm and reaches 460 μm within a time span of 1500 s. After 1500 s, the wear then attains a constant stable average value of 300 μm for a time lapse of 4000 s. Fluctuations in the wear are generated due
360
Hardness Value (HV)
3.8. Friction coefficient and wear resistance of Ni–Co alloy coatings
340
320
300
280 5
10
15
20
25
30
at. % Co in Ni-Co Alloy Fig. 5. The variation of the microhardness of electrodeposited Ni–Co alloy coating with different cobalt contents due to saccharin addition.
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Fig. 6. The Nyquist plots of the Ni–Co alloy in a 5% NaCl solution with saccharin addition in the bath as, (a) 0 g L−1, (b) 6 g L−1, (c) 10 g L−1 and (d) 12 g L−1.
to the wear of Ni–Co alloy coating during sliding and its effect as acting third body resistance, as explained earlier. After the period of 4000 s, the wear significantly reduces and attains an average constant value of
Fig. 7. (a) The photograph of electrodeposited Ni–Co alloy coating on medium carbon steel fasteners used for thin sheet metal clamping and (b) photograph of the formation of red rust along with the time for rusting on the Ni–Co alloy coated fasteners with 0, 6, 10 and 12 g L−1 saccharin addition.
Fig. 8. (a) The plot of friction coefficient value (μ) with sliding time of the Ni–Co alloy coating without saccharin addition and (b) The plot of friction coefficient value of the Ni–Co alloy coating with 10 g L−1 saccharin addition.
220 μm for a time span of 10,000 s. This plot shows a smooth curve with less fluctuation due to the total wear out of the Ni–Co alloy coating from the medium carbon steel substrate and the sliding action occurs between the pin and the substrate. Hence it is clear that Ni–Co alloy coating without saccharin addition has a wear of about 300 μm for a time span of 4000 s with an applied load of 6 N. Fig. 9(b) shows the plot of wear resistance of the Ni–Co alloy coating with 10 g L−1 saccharin addition. The wear attains a steady peak value of 140 μm within a time span of 6500 s. After 6500 s, the wear significantly increases up to 520 μm and then attains a stable average 370 micron (μm) for a time span up to 9500 s. After 9500 s the plot shows a smooth curve with very less fluctuations due to the total wear out of the Ni–Co alloy coating from the medium carbon steel substrate and the sliding action occurs between the pin and the substrate as mentioned before. Hence it is seen that Ni–Co alloy coating with 10 g L− 1 saccharin addition lasts the wear test up to 9500 s with a wear of 370 μm with an applied load of 6 N. Thus, it can be concluded that Ni–Co alloy coating with 10 g L− 1 saccharin addition exhibits a good resistance to wear. The main reason for improved wear resistance is due to the more pure FCC phase structure of the Ni in Ni–Co alloy coating along with good
Fig. 9. (a) The plot of wear resistance with sliding time of the Ni–Co alloy coating without saccharin addition and (b) the plot of wear resistance of the Ni–Co alloy coating with 10 g L−1 saccharin addition.
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adhesion on the medium carbon steel substrate due to reduced residual compressive stress.
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Development & Innovation, MIT, Pune, Maharashtra, India for helping in tribometry results. References
4. Conclusions The electrodeposition of nickel rich Ni–Co alloy coatings from sulfate bath at pH 2.5 using saccharin as an additive agent has been carried out. The saccharin content influenced the Ni–Co alloy composition in the coatings. The Co content in the Ni–Co alloy varied from 27 to 7 at.%. Saccharin addition hinders the deposition of Co and favors the deposition of Ni. The electrodeposited Ni–Co alloy from sulfate bath with 10 g L −1 saccharin addition at a 12 at.% Co content has bright and smooth deposits. Ni–Co alloy possesses an FCC phase structure with major orientation along (111) plane. Ni–Co alloy containing 10 g L− 1 saccharin addition produces low stressed deposits. Microhardness increases as the Co content in the Ni–Co alloy increases and this increase in the value of hardness is attributed to the HCP structure of cobalt. Friction coefficient and wear resistance of the Ni–Co alloy depend upon the saccharin content. Saccharin with 10 g L−1 addition in the bath at 12 at.% Co possesses improved wear and corrosion resistance as compared to other compositions due to the compact morphology and hydrophobic nature of surface.
Acknowledgment The authors thank Prof C. D. Lokhande, Head, Department of Physics, Shivaji University, Kolhapur, India for providing characterization facilities and support. Also the authors thank Prof. G. S. Barpande, Mechanical Engineering Department and Prof S. Radhakrishanan, Director, Research
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Studies on enhancement of surface mechanical properties of electrodeposited Ni–Co alloy coatings due to saccharin additive A.C. Lokhande ⁎, J.S. Bagi Department of Production Engineering, KIT's College of Engineering, Kolhapur 416 234, MS, India
a r t i c l e
i n f o
Article history: Received 8 April 2014 Accepted in revised form 10 September 2014 Available online 18 September 2014 Keywords: Electrodeposition Ni–Co alloy Corrosion resistance Contact angle Friction coefficient Wear resistance
a b s t r a c t NiCo alloy coatings were galvanostatically electrodeposited at room temperature from sulfate bath using saccharin (0 to 12 g L−1) as an additive agent. These alloy coatings were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic absorption spectroscopy (AAS), contact angle, microhardness, Electrochemical impedance spectroscopy (EIS), salt immersion test and tribology techniques. The XRD study confirmed the formation of Ni–Co alloy with a face centered cubic (FCC) structure. Depending on saccharin addition, the Co content varied from 27 to 7 at.%. The microhardness and corrosion resistance showed enhancement due to saccharin addition. Ni–Co alloy with a 12 at.% Co content obtained from 10 g L−1 saccharin addition showed reduced grain size (12 nm) and hydrophobic surface (113°) with the highest corrosion and wear resistant values as compared to others. Such coating on medium carbon steel fasteners showed significant enhancement in corrosion protection during salt immersion test. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Electrodeposited cadmium and zinc based alloy coatings have been widely utilized in aerospace, electrical and industrial applications due to their good corrosion resistance. Cadmium coatings are produced from cyanide bath which is very toxic. Similarly, the deposition of zinc coating produces harmful hexavalent compounds. On the other hand, Ni based alloy coatings can serve the best alternative for the toxic cadmium coating [1]. These alloy coatings have been used for the purpose of industrial fasteners and decorative coatings. Particularly, Ni–Co alloy coatings have been utilized in many applications due to their advantages like strength, better wear resistance, corrosion resistance, hardness and their esthetic value [2,3]. Preparation parameters employed during electrodeposition such as the composition of bath, pH, current density and temperature noticeably affect the microstructure and mechanical properties of the Ni–Co alloy [4]. These alloys have been electrodeposited from simple and complex baths. This deposition technique produces a nanosized grain structure and has advantages such as low cost, high production rate, and uniform deposition [5]. Deposition from sulfamate bath produces low stressed deposits [6–9]. The cobalt content in the deposit greatly affects the mechanical properties of the Ni–Co alloy. Co rich alloy possesses high wear and friction resistance due to the hexagonal close packed (HCP) crystal structure. The microhardness also depends on the Co content in the alloy and increasing the Co content in the alloy beyond 50 at.% causes
⁎ Corresponding author. Tel.: +91 9049626517. E-mail address: [email protected] (A.C. Lokhande).
http://dx.doi.org/10.1016/j.surfcoat.2014.09.023 0257-8972/© 2014 Elsevier B.V. All rights reserved.
reduction in the microhardness due to the HCP phase structure of the Co [10,11]. The mechanical properties of the Ni–Co alloy depend upon the grain size. Generally, alloys with a grain size between 20 and 100 nm show enhancement in mechanical properties. Wang et al. [11] reported the effect of grain size on the microhardness of the electrodeposited Ni–Co alloy coating additive agents are used to enhance the mechanical properties, control the grain size and compactness of the film. Therefore, many additives have been added during the deposition. Ni coatings have been obtained using organic additives like saccharin, naphthalene and trisulfonic acid. Saccharin plays a very important role in the deposition of Ni based alloy coatings. It produces smooth, compact, low stressed and refined grain deposits [12]. These effects are produced due to the variation in the composition of the Helmholtz layer and adsorption of the saccharin on the active sites of cathode inducing chemical barrier layer for adions and adatoms on the surface of cathode resulting growth inhabitation of grain deposit [13]. The presence of the impurities such as sulfur and carbon in the saccharin restricts the sliding of grain boundaries and the grain growth. Chang et al. [14] studied the corrosion resistance of Ni–Co alloy at 800 °C in air. Li et al. [5] studied the effects of saccharin and Co concentration on the microhardness of Ni–Co alloy. Scanty data are available on the electrochemical behavior of cobalt rich Ni–Co alloy. So far, studies on the mechanical properties, wettability and corrosion resistance of nickel rich Ni–Co alloy coatings obtained from the bath containing additive agent are not reported. This paper reports the electrodeposition of nickel rich Ni–Co alloy coatings from the bath containing saccharin as an additive agent and its effect on the quality of electrodeposited Ni–Co alloy coatings. The effect of the saccharin content on the surface wettability, corrosion
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resistance and its tribological behavior has been studied. The effect of the saccharin content on microstructure and surface mechanical properties is reported. The industrial fasteners coated with Ni–Co alloy are used for the corrosion protection study. 2. Experimental details Nickel rich Ni–Co alloys were prepared using an electrodeposition technique. In electrodeposition, preparative parameters such as the concentration and composition of solution, complexing agent, pH of bath, temperature, plating current density, deposition time, applied voltage and nature of substrate play important roles in deciding the quality of coating. By making various trial and errors, the following preparative parameters were fixed for the deposition of Ni–Co alloys. Ni–Co alloys were electrodeposited at room temperature (300 K) from an aqueous bath consisting of nickel sulfate (250 g L−1), nickel chloride (40 g L− 1) and cobalt sulfate (20 g L− 1). Boric acid (35 g L−1) was used as a buffer to maintain pH, Triton X-100 (0.08 g L−1) as an antipitting agent and saccharin (6–12 g L−1) as an additive. The pH of the solution was adjusted to 2.5 ± 0.1 using nickel carbonate and sulfuric acid. All solutions were prepared using double distilled water. Fresh solutions were used for all depositions. The electrodeposition was carried out on medium carbon steel (1 cm × 5 cm area) strip substrates. The substrates were polished with a sand paper followed by ultrasonic cleaning in double distilled water. After that, the substrates were activated for 10 s in the 10 vol.% HCl solution followed by final rinsing in double distilled water. A three electrode system consisting of carbon steel substrate as a cathode, graphite as an anode, and saturated calomel electrode (SCE) as a reference electrode, was used. Electrodeposition was carried out in a galvanostatic mode at a current density of 15 mA cm− 2 using WonATech battery cycler potentiostat. The plating parameters like current density, electrolyte composition and temperature were kept fixed and the concentration of the additive agent, saccharin, was varied from 0 to 12 g L − 1 . The Ni–Co alloy coatings were electrodeposited for 15 min which resulted into the thickness between 10 and 12 μm. The atomic percentage of the cobalt content in Ni–Co alloy deposits was determined by atomic absorption spectroscopy (AAS). The structural characterization and the stress induced in the electrodeposited Ni–Co alloy films were carried out by analyzing X-ray diffraction (XRD) patterns obtained with CuKα (α = 1.5418 Å) radiation from a D2 PHASER model (Bruker AXS Analytical Instruments Pvt Ltd) in the span of angle 2θ between 10 and 90°. Surface morphology of the Ni–Co alloy was studied with a scanning electron microscope (SEM) (JEOL, JSM 6360). The elemental composition of the alloy coating was studied with an atomic absorption spectrophotometer (Perkin Elmer Analyst 300). The surface wettability studies of the coating were carried out a using contact angle meter (Rame-hart USA equipment) with a CCD camera. The topographic images were recorded with a multimode atomic force microscope (AFM) (Innova-1B3BE) operated in the contact mode. Corrosion resistance was studied with a electrochemical impedance spectrometer workstation (Won-ATech, zive SP5) and also with a salt immersion test in a 5% NaCl solution. The hardness was measured using a Vicker's microhardness tester with a load of 150 g applied for 10 s. The friction coefficient (μ) and the wear resistance were determined with an applied load of 6 N on a pin on disk tribometer (Ducom instruments, India).
brings about reduction in the deposition of the cobalt content and facilitates that of nickel as seen in Table 1. Also, the Co content in the deposit is higher than that in the electrolyte bath indicating an anomalous codeposition of Ni–Co alloy, where less noble metal Co is preferentially deposited as an effect of pH change of the electrode, which is in agreement with the earlier results [5,14]. The atomic percent of the cobalt in the Ni–Co alloys affects the mechanical properties of the alloy. Co rich Co–Ni alloys (Co N 50 at.%) show improved properties of friction coefficient and wear resistance. The microstructure becomes less compact as the Co content increases beyond 30 at.% [2,5]. Hence in the present work, Ni:Co alloy composition was obtained as 73:27 to 93:7 at.% by varying the saccharin concentration. 3.2. Structure and induced stress in Ni–Co alloys coatings Fig. 1 shows XRD patterns of Ni–Co alloy deposited with saccharin addition between 0 and 12 g L−1. The XRD pattern (a) without saccharin addition shows only (111) peak and inhibits other peaks due to the mixed phase of FCC and HCP structures. With saccharin addition, the intensity of (111) peak increases up to 10 g L−1 and decreases afterwards. The other two peaks corresponding to (200) and (220) planes appear for higher saccharin addition exhibiting more of the Ni (α) phase of FCC. The XRD pattern matches with standard JCPDS card number 01-074-5694. A similar type of XRD results is obtained in electrodeposited Ni–Co alloy [15-17]. The more intense peak long (111) plane indicates that the films are textured along (111) plane, which may due to the combined effect of saccharin and presence of cobalt [5]. The crystallite size, D, of Ni–Co alloy was calculated for the (111) plane at fullwidth half maximum (FWHM) using the Scherrer’s formula as, D¼
0:9λ β cos θ
ð1Þ
where β is the broadening of diffraction line measured at half maximum intensity (radians), λ = 1.5404 Å is the wavelength of the CuKα X-ray, and θ is the Bragg's angle. The deposited alloy consists of nanocrystals of about 12.0 nm. It is well known that residual stresses are induced in the deposit during electrodeposition. Grain boundary relaxation may be a major factor leading to induce residual stress in the deposited films. The stress induced in the electrodeposited Ni–Co alloy coatings and lattice strains are determined from the XRD study for the characteristic intense peak (111) using the ‘H Digital’ software. The microstrain was calculated using the formula. β ¼ λ=D cos ðθÞ − ε tan ðθÞ
ð2Þ
where ε⁎ is the dimensionless strain value and β, λ, θ and D⁎ are parameters from Eq. (1). Table 1 shows the variation of induced stress and lattice strains in the electrodeposited Ni–Co alloy. The variation in the values of stress and strain can be seen as an effect of saccharin. The formation of compact morphology due to the addition of saccharin from 0 to 10 g L−1 reduces induced stress. The nucleation and grain growth process is inhabited by the saccharin which produces smooth, compact and low stress deposits. As the saccharin content increased up to 12 g L−1, the induced stress is increased due to the formation of a non-compact morphology.
3. Results and discussion 3.3. Microstructure of Ni–Co alloy coatings 3.1. Composition of Ni–Co alloy coatings The AAS study was carried out on alloys obtained by varying amount of saccharin as 0, 6, 10 and 12 g L−1. The AAS results confirmed the composition of cobalt as 27, 18, 12 and 7 at.%, respectively in the Ni–Co alloy. The analysis depicts that saccharin addition in the electrolyte bath
Fig. 2 shows the SEM images of Ni–Co alloy coatings with saccharin addition as (a) 0, (b) 6, (c) 10 and (d) 12 g L−1. The morphology with no saccharin addition (Fig. 2(a)) with Co composition as 27 at.% has a non-compact morphology with a grain size between 170 and 200 nm. The induced stress is more with no grain growth inhabitation process
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Table 1 The effect of saccharin content on the properties of NiCo alloy coating. Saccharin content in deposition bath (g L−1)
Ni:Co composition (at.%)
Compressive stress induced (MPA)
Lattice strain
Microhardness value (HV)
Impedance value (Ω cm–2)
0 6 10 12
73:27 82:18 88:12 93:7
−550 −515 −450 −612
2.10 1.32 0.77 1.74
360 340 320 280
290 340 524 360
due to saccharin. Fig. 2(b) shows a smooth, bright and compact microstructure consisting of a cluster of grains. The grain size lies in the range of 100–120 nm. The smooth and compact morphology resulted due to the FCC crystal structure of Ni rich alloy along with 6 g L− 1 saccharin as saccharin restricts the Co deposition and favors Ni deposition. This result is in agreement with others [18,19]. Fig. 2(c) shows a smooth and densely packed morphology with further reduced grain size in the range of 40–50 nm due to an increased saccharin concentration (10 g L−1) inducing grain refinement effect along with restriction to the grain boundary sliding. It is quite clear that the density of grain boundaries is reduced, reflecting in the uniform and compact deposit. Fig. 2(d) shows the formation of a non-compact and cracked morphology. This may be attributed due to the variation in the composition of the Helmholtz layer and inducing carbon and sulfur impurities in the deposits due to the excess addition of saccharin [13].
3.4. Wettability of Ni–Co alloy coatings Controlling the surface wettability of solid surfaces is important in many situations. If the wettability is high, the contact angle (θ b 90°) is small and the surface is hydrophilic and, if the wettability is low, the contact angle (θ N 90°) is large and the surface is hydrophobic. The microstructure, chemical composition, local inhomogeneities and surface cleanliness decide the contact angle [20]. The contact angle of the Ni–Co alloy deposited without saccharin addition was 84°, indicating that the alloy is hydrophilic. With increasing the saccharin content from 0 to 10 g L−1, the contact angle increased up to 113°, indicating the hydrophobic nature of the alloy. This increment in the contact angle of the alloy is due to the smooth and compact morphology obtained with increased saccharin addition. Increasing saccharin addition to 12 g L−1 showed contact angle value as 74° resulting in the hydrophilic nature of the Ni–Co alloy due to the production of a non-compact
Fig. 1. The XRD patterns of electrodeposited Ni–Co alloy coating from saccharin addition as, (a) 0 g L−1, (b) 6 g L−1, (c) 10 g L−1 and (d) 12 g L−1 in the bath.
microstructure. Fig. 3 shows the typical contact angle measurement of the Ni–Co alloy with 10 g L−1 saccharin addition. In order to avoid corrosion, hydrophobic surface is very desirable as it provides very limited scope for the liquid medium to wet the coated surface. This avoids the maximum contact between liquid medium and coated surface making the coated surface as a water repellent surface and reducing the chances of chemical reaction between liquid and coated surface. Hydrophilic surface increases the contact between liquid medium and coated surface. As the wettability or the contact between liquid medium and coated surface increases, liquid spreads over the coated surface as well as enters the pores and cracks of the coated surface increasing the chances of chemical reaction which leads to the formation of undesirable oxide products. In the present work, due to the saccharin addition, coating surface becomes compact and water repellent. Such coatings are desirable in the corrosion protection applications. 3.5. Surface topography and microhardness of Ni–Co alloy deposit The three-dimensional (3D) surface topography of Ni–Co alloy coatings was studied using an AFM technique. The AFM 3D images in Fig. 4(a–c) are of the Ni–Co alloy coatings produced from 0, 6 and 10 g L−1 saccharin containing baths, respectively. Table 2 shows the values of surface roughness, grain size and grain height for various saccharin contents in the bath obtained from AFM studies. A low roughness is obtained as compared to electrodeposited Ni–Co alloy under a magnetic field [21]. The significant reduction in the grain size from 162 to 56 nm and roughness value from 17 to 3.16 nm resulted from increasing saccharin addition from 0 to 10 g L− 1, producing a grain refinement effect with a smooth and compact structure, as evident from SEM images. There is a considerable reduction in the grain height from 79.6 to 12 nm due to the effect of grain growth inhabitation by saccharin inducing chemical barrier to adions and adatoms on the coated surface acting as the cathode [13]. From the AFM study, it is seen that nanograins of uniform size and shapes are formed with low roughness value. Fig. 5 represents the variation of the microhardness with an at.% of Co in the electrodeposited Ni–Co alloy. The microhardness of the alloy increased from 280 to 360 Hv with decreasing saccharin content from 12 to 0 g L−1, producing a Co content from 7 at.%. to 27 at.%. The microstructure of a coating depends on many factors such as the phase structure, crystallite size and formation of solid solution. The microhardness of alloy increases with the formation of a solid solution which may be studied using a solid solution hardening mechanism [22]. In our case, the microhardness of 27 at.%. of Co in Ni–Co alloy results from the formation of the mixed phase of FCC and HCP crystal structures [23].The alloy compositions with 6, 10 and 10 g L−1 added saccharin resulted into a pure FCC structure. This is clear from (111) and (200) reflections of the Ni–Co coating with 7, 12 and 18 at.% of the Co content. However, the reduced intensities of (111) plane and absence of (200) and (220) planes corresponding to Ni–Co alloy prepared without saccharin addition indicates the formation of mixed phase which in turn helps to improve the microhardness. In addition, the hardness decrease with saccharin content can also be explained using an inverse Hall–Petch relation [24]. According to this relation the microhardness is decreased
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Fig. 2. The SEM images of Ni–Co alloy coatings deposited from saccharin addition as, (a) 0 g L−1, (b) 6 g L−1, (c) 10 g L−1 and (d) 12 g L−1 in the bath. Magnification of 2000×.
when the crystallite size is reduced below some critical level as observed in our case.
3.6. Electrochemical impedance spectroscopy (EIS) study of Ni–Co alloy coatings EIS is an important technique to determine the corrosion of coatings. Fig. 6(a–d) shows the Nyquist plots of the Ni–Co alloys in a 5% NaCl solution with an applied bias of 10 mV, where Z′(ω) and Z″(ω) are the real and imaginary parts of the measured impedance respectively and ω is the angular frequency. The higher impedance value and larger diameter of the (incomplete) semicircle in the spectra of the Ni–Co alloy show its higher corrosion resistance, which can be related to the variation in the film (coating) capacitance. The capacitive impedance at high
frequencies is well related to the thickness and the dielectric constant of the coating [25]. Two semicircles appear in the plots indicating two relaxation processes in the lower and higher frequency ranges. The semicircle in the lower frequency range indicates the oxidation resistance of the Ni–Co alloy and the semicircle in the higher frequency range represents the corrosion resistance of the alloy [26]. The higher impedance value in the lower and higher frequency ranges indicates the higher oxidation and corrosion resistance. Fig. 6(a–c) shows the Nyquist plots of Ni–Co alloy with 0, 6 and 10 g L−1 saccharin addition with impedance value as 290, 340 and 524 Ω cm2, respectively. The increased impedance value from 290 to 524 Ω cm2 resulted due compact morphology, due to saccharin addition. Additional saccharin content in bath up to 12 g L− 1 resulted in the decrease in the impedance value to 360 Ω cm2 (Fig. 6(d)), due to the formation of a non-compact and cracked microstructure. The reduced impedance is attributed due to the spreading and entering of an NaCl solution into the pores and cracks of the alloy inducing chemical reaction, leading to the formation of undesirable oxide products in the coating. Table 1 shows the variation in the impedance with saccharin addition in the bath.
3.7. Salt immersion test (5% NaCl solution test)
Fig. 3. Contact angle measurement on Ni–Co alloy coatings with 10 g L−1 saccharin addition.
In this test, the Ni–Co coated medium carbon steel fasteners were kept immersed in a 5% NaCl solution at room temperature and the time taken for the appearance of the red rust on the coated surface was noted. Fig. 7(a) shows the photograph of medium carbon steel fasteners without and with electrodeposited Ni–Co alloys. Fig. 7(b) shows the photograph of the formation of red rust along with the time taken for rusting of the Ni–Co alloy coated fasteners with 0, 6, 10 and 12 g L−1 saccharin addition in the bath. Ni–Co alloy deposited on fasteners with 10 g L−1 saccharin addition in the bath showed a maximum time for the formation of red rust as compared to other fasteners such as 0, 6 and 12 g L−1 saccharin addition. EIS study also supports that Ni–Co alloy prepared from 10 g L−1 saccharin addition in the bath has a maximum corrosion resistance.
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Fig. 4. The 3 dimensional (3-D) AFM images of Ni–Co alloy coatings deposited from bath containing saccharin addition as, (a) 0 g L−1, (b) 6 g L−1 and (c) 10 g L−1.
Fig. 8(a, b) shows the variation of friction coefficient value (μ) with sliding time for Ni–Co alloy coatings without and with saccharin addition. Fig. 8(a) shows the plot without saccharin addition. Initially μ value starts from 0 and reaches a value of about 0.25. After time lapse of 3000 s, the μ value shoots to 0.71 and then attains a stable constant average value of 0.62. Significant fluctuations are observed in the graph due to the wear of the Ni–Co alloy coating surface during sliding and its effect of acting as third body resistance between the pin and coated Ni–Co alloy surfaces. The value of μ as 0.62 is mainly attributed due to the face centered cubic (FCC) phase structure of the Ni rich Ni– Co alloy along with the hexagonal close packed (HCP) phase structure of Co. Fig. 8(b) represents the μ of the Ni–Co alloy with 10 g L−1 saccharin addition. The value of μ increases up to 0.69. This effect of increase in μ value is mainly due to Ni rich Ni–Co alloy with Ni content as 88 at.%. Hence, it is clear that μ increases as the saccharin content increases which results to produce Ni rich Ni–Co alloy with restricted Co content deposits. It is known that μ depends upon the content of Co in Ni–Co alloy. Ni rich alloy exhibits an increased μ value due to its FCC phase structure and reduced HCP structure of Co. Similar results are reported
Table 2 The effect of saccharin content on surface roughness, grain size and grain height of Ni–Co alloy coating. Saccharin content in deposition bath (g L−1)
Surface roughness (nm)
Grain size (nm)
Grain height (nm)
0 6 10
17 9 3.16
162 120 56
79.2 31 12
for Ni–Co alloy with increased Co content up to 80% with reduced friction coefficient value up to 0.25 by Wang et at [11]. Hamid et al. [27] reported a high friction coefficient value of deposited Ni up to 1.52. Fig. 9(a, b) shows the variation of wear with a sliding time of the Ni– Co alloy coating without and with saccharin addition. Fig. 9(a) shows the wear of Ni–Co alloy without saccharin addition. The wear starts from 0 μm and reaches 460 μm within a time span of 1500 s. After 1500 s, the wear then attains a constant stable average value of 300 μm for a time lapse of 4000 s. Fluctuations in the wear are generated due
360
Hardness Value (HV)
3.8. Friction coefficient and wear resistance of Ni–Co alloy coatings
340
320
300
280 5
10
15
20
25
30
at. % Co in Ni-Co Alloy Fig. 5. The variation of the microhardness of electrodeposited Ni–Co alloy coating with different cobalt contents due to saccharin addition.
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Fig. 6. The Nyquist plots of the Ni–Co alloy in a 5% NaCl solution with saccharin addition in the bath as, (a) 0 g L−1, (b) 6 g L−1, (c) 10 g L−1 and (d) 12 g L−1.
to the wear of Ni–Co alloy coating during sliding and its effect as acting third body resistance, as explained earlier. After the period of 4000 s, the wear significantly reduces and attains an average constant value of
Fig. 7. (a) The photograph of electrodeposited Ni–Co alloy coating on medium carbon steel fasteners used for thin sheet metal clamping and (b) photograph of the formation of red rust along with the time for rusting on the Ni–Co alloy coated fasteners with 0, 6, 10 and 12 g L−1 saccharin addition.
Fig. 8. (a) The plot of friction coefficient value (μ) with sliding time of the Ni–Co alloy coating without saccharin addition and (b) The plot of friction coefficient value of the Ni–Co alloy coating with 10 g L−1 saccharin addition.
220 μm for a time span of 10,000 s. This plot shows a smooth curve with less fluctuation due to the total wear out of the Ni–Co alloy coating from the medium carbon steel substrate and the sliding action occurs between the pin and the substrate. Hence it is clear that Ni–Co alloy coating without saccharin addition has a wear of about 300 μm for a time span of 4000 s with an applied load of 6 N. Fig. 9(b) shows the plot of wear resistance of the Ni–Co alloy coating with 10 g L−1 saccharin addition. The wear attains a steady peak value of 140 μm within a time span of 6500 s. After 6500 s, the wear significantly increases up to 520 μm and then attains a stable average 370 micron (μm) for a time span up to 9500 s. After 9500 s the plot shows a smooth curve with very less fluctuations due to the total wear out of the Ni–Co alloy coating from the medium carbon steel substrate and the sliding action occurs between the pin and the substrate as mentioned before. Hence it is seen that Ni–Co alloy coating with 10 g L− 1 saccharin addition lasts the wear test up to 9500 s with a wear of 370 μm with an applied load of 6 N. Thus, it can be concluded that Ni–Co alloy coating with 10 g L− 1 saccharin addition exhibits a good resistance to wear. The main reason for improved wear resistance is due to the more pure FCC phase structure of the Ni in Ni–Co alloy coating along with good
Fig. 9. (a) The plot of wear resistance with sliding time of the Ni–Co alloy coating without saccharin addition and (b) the plot of wear resistance of the Ni–Co alloy coating with 10 g L−1 saccharin addition.
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adhesion on the medium carbon steel substrate due to reduced residual compressive stress.
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Development & Innovation, MIT, Pune, Maharashtra, India for helping in tribometry results. References
4. Conclusions The electrodeposition of nickel rich Ni–Co alloy coatings from sulfate bath at pH 2.5 using saccharin as an additive agent has been carried out. The saccharin content influenced the Ni–Co alloy composition in the coatings. The Co content in the Ni–Co alloy varied from 27 to 7 at.%. Saccharin addition hinders the deposition of Co and favors the deposition of Ni. The electrodeposited Ni–Co alloy from sulfate bath with 10 g L −1 saccharin addition at a 12 at.% Co content has bright and smooth deposits. Ni–Co alloy possesses an FCC phase structure with major orientation along (111) plane. Ni–Co alloy containing 10 g L− 1 saccharin addition produces low stressed deposits. Microhardness increases as the Co content in the Ni–Co alloy increases and this increase in the value of hardness is attributed to the HCP structure of cobalt. Friction coefficient and wear resistance of the Ni–Co alloy depend upon the saccharin content. Saccharin with 10 g L−1 addition in the bath at 12 at.% Co possesses improved wear and corrosion resistance as compared to other compositions due to the compact morphology and hydrophobic nature of surface.
Acknowledgment The authors thank Prof C. D. Lokhande, Head, Department of Physics, Shivaji University, Kolhapur, India for providing characterization facilities and support. Also the authors thank Prof. G. S. Barpande, Mechanical Engineering Department and Prof S. Radhakrishanan, Director, Research
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