SiC

Article pubs.acs.org/JPCC

Elucidation of the Structural Texture of Electrodeposited Ni/SiC Nanocomposite Coatings Abouzar Sohrabi,†,‡ Abolghasem Dolati,*,‡ Mohammad Ghorbani,†,‡ Mohammad Reza Barati,§ and Pieter Stroeve∥ †

Institute for Nanoscience & Nanotechnology, Sharif University of Technology, P.O. Box 11155-8639, Tehran, Iran Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11155-9466, Tehran, Iran § Department of Materials Engineering, Monash University, Clayton, VIC 3800, Australia ∥ Department of Chemical Engineering and Materials Science, University of CaliforniaDavis, Davis, California 95616, United States ‡

ABSTRACT: Crystallographic texture is one of the most sensitive parameters for controlling the microstructure of electrodeposited layers. In this work, we study the crystallographic texture of electrodeposited nickel−silicon carbide coatings. The nickel coatings containing SiC nanoparticles and microparticles were electrodeposited from an additive-free sulfate bath containing nickel ions and SiC particles. The effect of current density on the codeposited Ni/SiC was studied. The coatings were analyzed with scanning electron microscopy and X-ray diffraction. Pole figure studies were done to characterize the evolved crystallographic texture. The X-ray scans followed by Rietveld analysis using the Rietquan program were used for the determination of texture, lattice parameter, and grain size of the matrix. The Ni/SiC nano-electrocomposites, prepared at 7 A/dm2, exhibited improved properties in comparison to pure nickel electrodeposits. The properties of the composite coatings are associated with structural modifications of the nickel crystallites as well as the morphology of the electrodeposited layers. For the optimum current density, the preferred orientation is ⟨110⟩. Grain refinement is observed for an optimum range of 7−10 A/dm2 associated with the evolution of the ⟨110⟩ texture toward a less marked ⟨210⟩ texture. From pole figure and inverse pole figure studies, a mechanism of crystallographic texture changes as a function of current density and pH values of the electrolyte bath is proposed.

1. INTRODUCTION With the emergence of nanostructured materials over the past decade, investigations have been focused on the need to produce coatings with superior mechanical, corrosion, and tribological properties. As a result of the current interest in nanomaterials, reports have appeared on the deposition of composite coatings, with built-in particles smaller than 100 nm, and the corresponding properties of the coatings.1−4 Nickel coatings have been used in numerous applications for many years and electrodeposition of nickel produces polynanocrystalline structure.5 For single-phase coatings, smaller grain size contributes to improved hardness, strength, surface roughness, and wear properties.5 The incorporation of an inert second phase stabilizes the grain structure of nickel by blocking grain growth.6−8 Ni/SiC composites in particular, due to their high wear resistance, have been investigated and commercialized for the protection of friction parts, combustion engines, and casting molds.9,10 The functionality and reliability of such coatings depend on the microstructure; however, the understanding of the growth characteristics and the evolution of microstructure in finite structures is limited. Crystallographic texture is one of the most sensitive parameters for controlling © 2012 American Chemical Society

the microstructure of electrodeposited layers. The electrolyte composition and the current density applied on the substrate have an essential influence on the resulting microstructure and preferred crystallographic orientation of the deposits.11,12 The presence of a strong crystallographic texture in a material may affect all its properties (e.g., mechanical, chemical, and magnetic) and make them anisotropic. The texture present in coatings is a fibrous texture, because of the planar isotropy of the coating process, but the preferred direction (“fiber”) will vary as a function of coating conditions.12−14 From our previous study,1 a nickel layer deposited in the presence of SiC particles shows a completely different crystal orientation from what has been obtained previously for nickel deposits. Pure Ni deposit is characterized by an intense (200) diffraction line corresponding to a (100) texture, while those of Ni/SiC deposits exhibit a reinforcement (111) line accompanied with an attenuation of the (200) diffraction. Electrocomposites show a preference for (111) grain growth; however, Received: October 4, 2011 Revised: December 29, 2011 Published: January 4, 2012 4105

dx.doi.org/10.1021/jp2095714 | J. Phys. Chem. C 2012, 116, 4105−4118

The Journal of Physical Chemistry C

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the volume density of that growth direction is lower in the Ni/SiC composites compared to pure Ni. The reduction of the amount of (111) grains is compensated by (100) type growth.1 This reinforcement of diffraction lines can be linked with the dispersed ⟨211⟩ orientation.15 Hence, it is now obvious that the embedding of SiC particles in the Ni matrix modifies the soft-mode (100) texture to a mixed preferred orientation of Ni crystallites through the (111) axis. The textural modification is less pronounced in the case of nanocomposite coatings. The codeposition of SiC nanoparticles changes the electrocrystallization of nickel electrodeposited layer and, as a result, changes the crystal orientation and morphology of the matrix surface. In addition, the nanoparticles may act as nucleation sites that may be a detriment to nickel crystal growth.16 Researchers such as Pavlatou et al.,15 Wang et al.,16 Ari-Gur et al.,17 and Erler et al.18 studied texture orientation of electrocomposites and obtained similar results. Rashkov and Pangarov19 and Pangarov et al.20 discussed in detail the influence of deposition parameters on the texture of nickel layers. The authors constructed a sequence of predominant texture types corresponding with increasing overvoltage. Rasmussen and co-workers21 gave a more general systematic overview of the effect of current density and deposition temperature on the microstructure and texture of the electrodeposited nickel. The main focus was placed on nickel and copper layers with incorporated particles of alumina. In this paper, the non-oxide compounds silicon carbide and silicon nitride were also covered. A texture investigation of nickel composite layers with incorporated silicon carbide particles was published by Pavlatou et al.15 Similar investigations on materials with titania particle codeposition were presented by Lampke and collaborators.22 The authors detected a reduction of the crystallite size and a change of the grain shape caused by the nanoparticles. Furthermore, a change of the preferred orientation was also reported. The paper of Thiemig and Bund23 describes a change of texture in nickel layers with codeposited titania nanoparticles. The particle incorporation in this instance reduced the prominence of the texture parallel to ⟨100⟩ direction while the texture component ⟨111⟩ was increased. Erler et al.18 discussed the properties of similar composites from a more electrochemical viewpoint. In the study of Ferkel et al. the mechanical and structural properties of nickel layers with codeposited alumina nanopowder were discussed.7 The authors postulate an increased microhardness of such composites. Socha et al. describe the electrochemical codeposition of nickel layers with SiC and SiO2 particles.24 A short work on the system Ni−ZrO2 was presented by Möller et al.25 and Angerer et al.26 Although a number of authors have studied mechanical and structural properties of composite films of nickel and nanoparticles, the changes in crystallographic texture for electrodeposited Ni/SiC in the presence of SiC nanoparticles is not fully understood. The present paper focuses on the crystallographic texture of Ni/SiC nanocomposites on a polycrystalline Cu-substrate and the effect on coating properties. Texture analysis is performed by means of X-ray diffraction (XRD) with Rietveld analysis using Rietquan program analysis27,28 and pole figures obtained from the XRD technique. The orientation distribution function (ODF) is calculated for quantification of the texture components.

Table 1. Overview of the Composition and Electrodeposition Parameters of the Composite Bath electrodeposition conditions

amount

NiSO4·6H2O NiCl2·6H2O H3BO3 SiC (APS 50 nm) temperature pH current density magnetic stirring speed

300 g/L 35 g/L 40 g/L 20 g/L 50 ± 2 °C 4.5 ± 0.1 1−20 A/dm2 250 rpm

distilled water were used to prepare the bath. All chemicals were purchased from Merck Chemicals Co., and the purity of them was higher than 99.90%. Prior to deposition, the SiC particles (50 nm with purity of 99.00%, synthesized by Nanoshel Co.) with concentration of 20 g L−1 were dispersed in distilled water for 24 h and then SiC slurry was added to electrolyte. Disk cathodes, made of copper and with diameter of 15 mm and thickness of 1 mm, were inserted in a PTFE holder with a 10 mm circular window exposed to electrolyte. The most widely used electrodeposition method, i.e., conventional electrocodeposition (CECD) with solution being stirred at all times by a magnetic stirrer, was used as the cell configuration. The cathodes were positioned in vertical alignment with the anode. The distance between anode and cathode was 3 cm. A reference saturated calomel electrode (SCE) and a 15 mm × 15 mm counter electrode of pure nickel was used. The reference electrode was placed in a glass arm separated from the main cell and the counter electrode was placed as far as possible from the cathode in order not to disturb the uniformity of the current flow to the electrode or the particle distribution at the surface. Before each experiment, the cathode was ultrasonically cleaned in ethanol, acetone, and distilled water, respectively, each for 10 min, and then activated in 1:1 HCl:H2O for 30 s, washed in distilled water, and then immersed immediately in the plating bath to allow the electrodeposition of the target composite coatings. Prior to the electrodeposition process, the bath was ultrasonicated for 30 min with a Misonix Sonicator 4000. An EG&G potentiostat/galvanostat model 273A device was employed for the electrochemical studies. The surface morphology of composite coatings was examined with a field emission scanning electron microscope (FESEM) using a JEOL model JSM-7800F operated at 30 kV. The chemical composition of the deposits was determined using an energy dispersive spectrometer attached to the SEM. All chemical composition values are quoted in weight percent and represent the average of at least five measurements. The volume fraction of SiC particles was obtained with eq 2:

VS(X ) = VF =

M (X ) ρ(X )

atom %(Si) × VS(SiC) atom %(Ni) × VS(Ni) + atom %(Si) × VS(SiC)

(1)

(2)

M(X) and ρ(X) indicate the atomic mass and the density of the species between parentheses, VS is the specific volume of X and VF is the volume fraction of SiC particles in Ni/SiC electrodeposits. As stated above, EDS atom % results were obtained from at least five points. The surface topography and roughness was characterized using Veeco Wyko NT1100 Optical Profilometer. A Philips

2. MATERIALS AND METHODS The composition of solution and operating parameters for electrodeposition are given in Table 1. Analytical reagents and 4106



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Figure 1. Cyclic voltammetry curves for pure Ni and Ni/SiC nanocomposite with a scan rate of 10 mV/s.

(vs SCE), as seen in the logarithmic scale of current density in the inset of Figure 1. This shift at reduction potential is attributed to an increase in the active surface area due to the adsorbed particles on the cathode and to a possible increase in ionic transport by the nanoparticles. It is likely that the negatively charged nanoparticles will adsorb Ni2+ and H3O+ ions after the particles are introduced to the electrolyte. More negatively charged particles will be rejected to the bulk of the solution. A similar displacement of the curve to more positive potential was observed by Watson31 using micrometer-sized SiC particles codeposited with nickel in a sulfate nickel bath. It is worth noting that the fixation of the particles has been often ascribed to the reduction of metallic ions adsorbed at the particle surface.32,33 3.2. Codeposition of SiC Particles with Nickel. Nickel− silicon carbide composite coatings were electrodeposited under the conditions described in Table 1. Galvanostatic control was employed for codeposition of SiC nanoparticles with nickel, as shown in Figure 2, to obtain nanostructured electrocomposite coatings. This technique is based on the concept that the incorporation of nanoparticles occurs simultaneously with the reduction reaction of the ionic metal species to form the metal surface. As seen from the EDS and XRD analyses in Figure 2, the codeposition of SiC nanoparticles increases as current density increases to a value of 10 A/dm2 but then decreases and saturates to the constant value. Two of the main factors governing the particle content of the layers are the current density and the particle concentration in the suspension.34,35 Deposits prepared between 7 and 10 A/dm2 give the highest incorporation of SiC particles. At high current densities, nickel ions are transported faster than SiC particles are transported by mechanical agitation and Coulombic force, which should result in a low and then constant volume percent of the codeposited SiC. The occurrence of hydrogen evolution avoids a reduction in the current efficiency as well by hindering the adsorption of nanoparticles to the metal surface.2 At low current densities, nickel ions move slowly, leading to a low flux to the cathode. Near the cathode the nickel ion concentration is low and therefore fewer ions

Xpert-Pro X-ray diffractometer with a Cu Kα1 radiation (λ = 1.5405 Å) was employed to obtain XRD spectra using standard θ−2θ geometry to determine the phase(s) present in the deposits. A computer-base search and match was used for phase identification. The X-ray scans followed by Rietveld analysis using the Rietquan program27,28 were used for the determination of the texture, lattice parameter, and grain size of the matrix. For a more detailed examination of the texture, the pole figures of the reflections (111), (200), and (220) were determined. The textures of Ni/SiC specimen were measured using a Bruker X8 APEX CCD diffractometer X-ray goniometer and Mo Kα radiation. A pole figure can be interpreted as a twodimensional distribution function of the normal direction of the corresponding lattice plane. Therefore, one pole figure lacks the full information of the orientation distribution function (ODF) which describes the complete orientation of the crystallites by all three Euler angles. The intensities of each diffraction maximum were measured up to a sample tilt angle or pole distance of ψ = 85° in steps of 5°. The precession at constant ψ has been performed in azimuthal increments (in φ) of 5°. The zero position of φ were chosen randomly. The results were corrected for absorption and defocusing using a standard random specimen prepared from Ni powder.

3. RESULTS AND DISCUSSION 3.1. Electrodeposition Behavior of Ni/SiC Coatings. Figure 1 shows the cyclic voltammetry (CV) curves of pure Ni and Ni/SiC electrodeposition. In the range from −700 to −1500 mV, the current density gradually increases with increasing potential. Applying overpotential of more than −700 mV shows that the current density of pure nickel deposition is less than the current density evolved in Ni/SiC electrodeposition. This effect is because of the semiconducting properties of SiC nanoparticles due to their allowance of electric current by the Schottky effect in the SiC nanoparticles barrier layer.29,30 As seen in Figure 1, SiC nanoparticles influence the nucleation and growth process of Ni/SiC composites. Actually, the addition of silicon carbide nanoparticles displaces the nickel reduction curve to more positive potentials from −764 to −740 mV 4107



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Figure 2. Volume percent of embedded SiC particles codeposited with nickel as a function of current density.

adsorb on the SiC nanoparticles. As a result, the Coulombic force between anions adsorbed on particles and the cathode may be weak, causing a lower content of the codeposited SiC. Thus, an optimum current density exists, which gives a maximum volume percent of codeposited SiC. In this research, the optimum current density is seen to be between 7 and 10 A/dm2, as shown in Figure 2. 3.3. X-ray Diffraction Patterns and Crystallite Size Obtained from Diffractograms. The influence of SiC particles on the structure and crystallization of the electrodeposited layers was investigated by the XRD method. Figure 3

shown that the intensity of Ni(111) and Ni(200) is changed using different current densities. For better interpreting of X-ray diffraction data, Rietveld analysis using the Rietquan program27,28 was used to analyze the diffraction patterns. Crystallite size, density, and SiC particle incorporation into the nickel matrix of nano-electrocomposites were obtained from X-ray diffraction data. Figure 4 shows the crystallite size of the nickel matrix of electrocomposites as a function of current density. As seen in Figure 4, Ni crystallites in all reflections show that the crystallite size decreases as the current density increases to 7−10 A/dm2 and then increases after this optimum value. The codeposition of SiC nanoparticles changes the electrocrystallization of the nickel layer and, as a result, changes the crystal orientation and morphology of the matrix surface. SiC nanoparticles inhibit grain growth of crystallites, and this phenomenon is observed in Figure 4. 3.4. Surface Morphology and Topography of Ni/SiC Nano-Electrocomposites. Surface topography and roughness characterizations of Ni/SiC nanocomposites deposited at current densities of 1−20 A/dm2 were carried out by using an optical profilometer. Figure 5a−f shows the topography and roughness of the electrodeposited layers. Also, Figure 6 shows the roughness average (Ra) and the maximum peak-to-valley height (Rt) as a function of current density. As seen in Figures 5a−f and 6, the roughness of deposits increases with increasing current density to 7 A/dm2 and then decreases. This is due to the incorporation of SiC nanoparticles on the surface of the nickel electrodeposits. Maximum incorporation of nanoparticles occurs when the current density reaches 7 A/dm2 and thus maximum roughness is obtained at the same current density value. Surface morphology characterizations of the size and the shape of the grains of Ni/SiC nano-electrocomposites deposited at current density of 7 A/dm2 was carried out by field emission scanning electron microscopy (FE-SEM). Figure 7 shows the micrograph of the electrodeposited nanocomposite. In the case of pure nickel electrodeposition (Figure 7a of ref 1), a bimodal grain structure of truncated pyramidal type can be formed. As seen in Figure 7, the morphology of Ni/SiC changes from

Figure 3. X-ray diffraction patterns for the Ni/SiC nanocomposites obtained with current density of (a) 1 A/dm2, (b) 5 A/dm2, (c) 7 A/dm2, (d) 10 A/dm2, (e) 15 A/dm2, and (f) 20 A/dm2.

shows the diffractograms of Ni/SiC nanoelectrodeposits with (hkl) reflections of different current densities. In XRD diffraction patterns seen in Figure 3 it is obvious that Ni and SiC and also the Cu substrate show different reflections. In Figure 3 it is 4108



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Figure 4. Crystallite size with different reflections of nickel structure as a function of current density.

Figure 5. Surface topography and profile of Ni/SiC deposits at current densities of (a) 1 A/dm2, (b) 5 A/dm2, (c) 7 A/dm2, (d) 10 A/dm2, (e) 15 A/dm2, and (f) 20 A/dm2.

nodular disturbed surface to cauliflower structure. Twodimensional growth mechanism dominates at a current density of 7 A/dm2. As seen in Figures 5c and 6 maximum roughness is obtained at a current density of 7 A/dm2. This is related to the cauliflower structure morphology, as seen if Figure 7, which is

formed and come together to yield two-dimensional growth. The incorporation of nanoparticles occurs in the form of clusters. It is possible that the electrodeposition conditions as well as the particle incorporation perturb the nickel growth and induce an increase of the number of nucleation sites, resulting 4109



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Figure 6. Roughness average (Ra) and maximum peak-to-valley height (Rt) of deposits a function of current density.

Figure 7. SEM micrograph of Ni/SiC nanocomposite obtained at a current density of 7 A/dm2.

in a refined grain structure. The perturbation of the nickel growth results from a change in the adsorption−desorption phenomena at the nickel/electrolyte interface, which may be caused by a local pH increase induced by an adsorption of H+ on the dispersed particles. The codeposition of SiC particles strongly affects the microstructure of the metal matrix. As a result of the SiC addition to the plating electrolyte, the microstructure of the nickel matrix changed to UD (unoriented dispersion).1

The incorporation of nanoparticles has different consequences with respect to the particles material. During their incorporation they act as new nucleation sites; therefore, the mean crystallite size is reduced, as seen in Figure 4, when maximum embedment of SiC nanoparticles occurs at 7 A/dm2. The incorporation of semiconducting SiC particles is initially connected with undisturbed grain growth until the particle is adsorbed on the newly developed surface. Solely on top of the particle, new 4110



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Figure 8. Calculated residual stress and density of dislocations of electrodeposits as a function of current density.

DPFs; thus, the deposits have no ⟨111⟩ texture. These two maxima in (111) DPFs are related to the formation of ⟨110⟩ texture, which features significant scattering of ∼35°−40° in (111) pole figures. In Figure 10a,e,f also two maxima are seen. The formation of the ⟨110⟩ texture is detected, which is characterized by rather strong scattering of ∼45°−50° in (200) DPFs. As seen in Figure 10b−d with weaker scattering of intensities, the intensity region is not continuous; i.e., a crystallite fraction is oriented along the ⟨110⟩ direction. As current density increases from 5 to 10 A/dm2, attenuation of the ⟨110⟩ orientation is again observed with a simultaneous decrease in its scattering. It is possible that a weaker partial ⟨110⟩ orientation arises as well. The (220) DPFs with the ⟨110⟩ direction of the deposits investigated in this work show approximate circular symmetry, as seen in Figure 11a−d, which is a clear indication of a fiber texture. Upon increasing current density from 1 to 10 A/dm2, the ⟨110⟩ texture exists, but it is stronger at current densities of 7 and 10 A/dm2. In Figure 11e,f the maximum intensity is scattered around 70°, which is related to ⟨210⟩ texture. As the current density increases to 20 A/dm2, ⟨110⟩ texture also exists, but it is weaker than that at i = 7, 10 A/dm2; however, its fraction is insignificant. The DPF also contains stronger maxima apparently corresponding to the ⟨210⟩ orientation. This texture in the DPFs is not continuous, which means that the formed texture is incomplete. The formed crystallite orientation is insufficiently exact, and normals to nodal planes of crystallites are arranged within a certain cone. To determine the probability that the emergence of normals to texture planes will coincide with the normal to the sample surface, inverse pole figures (IPFs) for all deposits are calculated and plotted using pole density, i.e., the fraction of crystallites, for which the normal to the (hkl) plane coincides with the normal to the sample surface. Figure 12 shows the IPFs for the normal direction of Ni/SiC nanocomposites deposited at current densities of 1−20 A/dm2. It is found that the maximum pole density of various poles of the standard stereographic triangle vary with the current density. As seen in the figures, all deposits have relatively strong intensity near the (110) lattice plane. Current densities of 7 and 10 A/dm2 are maxima for the (110) pole, which also confirms

crystals similar to the initial layer are formed as a new basis of continued columnar growth.36 3.5. Residual Stress and Density of Dislocations of Nickel-Matrix Electrodeposits. As residual stress measurement and obtaining density of dislocations are important in a precise description of texture formation of nano-electrocomposites, the following discussion is considered. The materials residual stress (σR) can be expressed in the term of eq 337

σR = −

E ⎛ d n − d0 ⎞ ⎜ ⎟ ν ⎝ d0 ⎠

(3)

where ν is Poisson’s ratio, E is Young’s modulus for nickel, dn is the spacing of the planes reflecting at normal incidence under stress, and d0 is the spacing of the same planes in the absence of stress. The calculated residual stress of the nickel matrix of Ni/SiC nanocomposites as a function of current density can be seen in Figure 8. For composite materials, the dislocations density ρ can be represented in terms of grain size (D) and microstrain (⟨ε2⟩1/2) by38−40

ρ=

2 3 ⟨ε2⟩1/2 Db

(4)

where b is Burgers vector and equals to √2a/2 for an fcc nickel. The calculated ρ in Ni/SiC nanocomposites is shown in Figure 8. As seen in Figure 8, the maximum value of both residual stress and density of dislocations is related to a current density of 7 A/dm2. Incorporation of SiC nanoparticles in the Watt’s bath disturbs the growth of electrodeposited nickel, and thus, defects of nickel coating layers are increased and thereby residual stress increases also. 3.6. Analysis of Texture Evolved in Electrodeposited Ni/SiC Nanocomposites. Figures 9, 10, and 11a−f show the (111), (200), and (220) and direct pole figures (DPFs) of electrodeposited Ni/SiC layers at current densities of 1−20 A/dm2, respectively. As seen in Figure 9a−f, two split peaks in all DPFs are observed. There are no intensities in the center of (111) 4111



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Figure 9. (111) Pole figures of Ni/SiC deposits at current densities of (a) 1 A/dm2, (b) 5 A/dm2, (c) 7 A/dm2, (d) 10 A/dm2, (e) 15 A/dm2, and (f) 20 A/dm2.

the existence of the preferred orientation with the ⟨110⟩ texture. This result indicates that a more pronounced texture formed under electrodeposition of Ni/SiC nanocomposites at the optimum current density. As the current density increases, the Ni/SiC nanocomposites deposited at i = 15, 20 A/dm2 show clustering of orientations near the (210) pole in the inverse pole figure, which indicates the formation of the ⟨210⟩ texture. The ⟨110⟩ axial texture characteristics exists in this case but is more pronounced. These results confirm DPFs of these deposits for Figure 11e,f. Therefore, as the current density increases to 10 A/dm2, the ⟨110⟩ texture is the sole preferred orientation, and when the current density passes from that value, the ⟨210⟩ texture is formed in the deposits, but the ⟨110⟩ texture is more pronounced. The pole density is the highest in the Ni/SiC nanocomposite deposited at 7 A/dm2, and the ⟨110⟩ axial texture dominates all preferred orientations. The experimental pole figures were used to calculate the orientation distribution functions (ODF) of the electrodeposited Ni/SiC nanocomposites. The ODF is an isointensity contour plot for various crystal orientations with respect to the same reference frame. According to Bunge’s notation,41 the standard coordinates (φ, φ1, φ2) are applied in φ1 projection. The ODFs

of the Ni/SiC deposits are displayed in Figure 13, indicating that the texture exhibits ⟨110⟩ fiber component. Our results obtained with the sulfate bath are in good agreement with published results that report a strong crystallographic texture along the ⟨110⟩ axis associated with small grains for deposition currents between 7 and 10 A/dm2.26,42−47 3.7. Mechanisms of Texture Formation in Ni/SiC Nano-Electrocomposites. Many theoretical explanations have been put forward to account for the different textures observed experimentally. According to the first hypothesis, anisotropy is the result of competitive nucleation,48−50 while, for the second theoretical treatment, the same anisotropy stems from growth competition.51−53 A diagram of texture stability for Watt’s bath expressed as a function of current density and pH is shown in Figure 14 using data from refs 54−56. In this case, the preferred orientation axes change as a function of current density increase according to the following sequence:

110 → 110 + 211 → 211 → 211 + 100 → 100 → 210 In this research the interface line between ⟨110⟩ and ⟨100⟩ shifted to the dashed line. Growth leading to texture during 4112



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Figure 10. (200) Pole figures of Ni/SiC deposits at current densities of (a) 1 A/dm2, (b) 5 A/dm2, (c) 7 A/dm2, (d) 10 A/dm2, (e) 15 A/dm2, and (f) 20 A/dm2.

and regardless of the texture axis, these electrodeposits have a very low residual stress. In the case of Ni/SiC electrodeposits, as seen in Figure 8, residual stress is not low and one of the reasons for texture formation is residual stress and dislocations pileup. Due to the presence of the SiC nanoparticles in the bath and at the interface of electrodeposited nickel/electrolyte, growth is disturbed. This phenomenon means that silicon carbide particles do not allow nickel to grow in habitual texture direction and thus defects, such as dislocations, pile up. Increasing residual stress and dislocations pileup affect texture formation of nickel electrodeposits, and thereby, the electrodeposited nickel’s preferred orientation is changed. 3.7.2. Differences in Growth Velocity of Crystal Faces. Texture development due to differences in the growth velocity of crystal faces is partly due to surface energy differences and partly due to kinetics of crystal growth. This hypothesis suggests that it is the difference in growth velocity of crystal planes of different indices {hkl} that leads to faceting and thereafter texture of electrodeposited metals. Using a purely geometrical argument, it can be seen that fast growing faces grow out of existence and slow growing crystal faces survive.62 To understand the concept, consider a two-dimensional crystal with the shape of a rhombus, and let the upper line be a crystal face with

electrodeposition has variously been considered to be a result of (i) a cold work mechanism, caused by large intrinsic or extrinsic deposition stresses (the texture formation is believed to result from the same plastic deformation mechanisms and dislocation pileup as that which occurs during cold drawing of wires), (ii) a multiple twinning process also caused by intrinsic stresses, (iii) the differences in the work of formation of different nuclei having crystallographic directions, (iv) an inherent difference in crystal face growth velocities due to differences in atomic density of crystal faces (the density of atoms can also be expressed as relative surface energy) and modification of these differences in growth velocity by hydrogen adsorption, and (v) the presence of blocking or growth retarding agents, which inhibit the growth of certain crystal directions and allow other directions to grow.57 3.7.1. Intrinsic Electrodeposition Stress Mechanism: Residual Stress and Dislocations Pileup. There is in general very little belief in the hypothesis that preferred orientation of electrodeposits should stem from plastic deformation mechanisms or twinning mechanisms. This hypothesis has been introduced by Wyllie,58 Bozorth,59 Wilman,60 and Rashkov et al.,61 and from their work Pd, Zn, Cd, Au, and Ag can be electrodeposited with different preferred orientations of the crystals, 4113



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Figure 11. (220) Direct pole figures of Ni/SiC deposits at current densities of (a) 1 A/dm2, (b) 5 A/dm2, (c) 7 A/dm2, (d) 10 A/dm2, (e) 15 A/dm2, and (f) 20 A/dm2.

3.7.3. Growth Competition under Influence of Crystal Face Specific Inhibitors. Texture is the result of a competition that occurs during the growth process. Even though such a first conclusion resembles the starting point of geometrical selection theories, it is no longer possible to suppose that anisotropy is only due to geometrical factors, since it is known that some of the various nuclei are necessarily nonsingle crystals: the presence inside them of structural defects gives rise to a sharp anisotropy far more pronounced than the anisotropy arising out of geometrical factors only. Moreover, one must wonder how the various chemical species adsorbed on the cathodic surface play a role in the growth competition. Additional considerations of crystal face specific growth inhibition is given by Amblard.63 It can be divided into two major points or assumptions: (i) during electrodeposition, two- or three-dimensional nuclei are deposited, if not with the same probability in all directions, then with a definite probability distribution of the low index directions, and (ii) during electrodeposition, different growthdisturbing species might be present on the crystal facets. These species may be hydrogen, hydrides, hydroxides, organic complexes, or inorganic species, depending on the bath chemistry and plating parameters.

a higher growth velocity than the two lines with an angle to the upper line. As the lines grow outward, the rhombus will get an upper line which becomes narrower, and in the end, the rhomboid shape has transformed into a triangle. In this case, the densest faces will have the slowest growth velocity, v, meaning for the fcc case that v{111} < v{100} < v{110} < v{211}; this is the Bravais law of crystal growth. As an example, a growing fcc crystal with {100} and {111} faces would end up only leaving {111} faces. The Bravais law shows, however, only that crystal faces develop, but not in what way a given texture is produced. For the outward growth, Reddy51 states “As the deposit crystals have an outward mode of growth, the velocity of growth in a direction parallel to the substrate surface is least.” The slowest growing facets should, therefore, be developed normal to the substrate. The axis of texture would then be the lattice row at the common intersection of the selectively developed facets (i.e., the zone axis). The above statements assume that the growing crystal faces do not experience any growth disturbing species such as SiC nanoparticles and organic brightener or inhibitors. However, the electrochemical reactions that lead to the deposition of metal atoms also involve generation of atomic and molecular hydrogen.51,63,64 4114



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Figure 12. Inverse pole figures for normal direction of the electrodeposited Ni/SiC layers at current densities of (a) 1 A/dm2, (b) 5 A/dm2, (c) 7 A/dm2, (d) 10 A/dm2, (e) 15 A/dm2, and (f) 20 A/dm2.

conditions that decide the concentration of the individual adsorbed species and thereby the chosen texture. In the case of electrodeposition of nickel, there is a close correlation between the stability of a given texture and the prevalence of a definite foreign species near the cathode/electrolyte interface:63 (i) ⟨110⟩ appears at low current densities when it is mostly Hads that inhibits the deposition. (ii) ⟨210⟩ has a domain of stability restricted to conditions where gaseous H2 evolution is the strongest. (iii) ⟨211⟩ exists only for high pHsol values and relatively low current densities, in the very domain where we expect a strong perturbation because of Ni(OH)2. Taking into account the effects of Hads inhibition, which shifts both pH and potential scales, it is possible to ascribe ⟨211⟩ texture of Ni deposits to a specific inhibition by Ni(OH)2.

On the basis of electrochemical impedance measurements, Amblard63 showed that there are codeposited species that inhibit the deposition of nickel. A reasonable inhibiting species is adsorbed hydrogen, as it is generated as part of the electrochemical process. Another inhibiting specie is believed to be Ni(OH)2, which is caused by a pH rise in the electrolyte when H+ ions arriving at a nickel cathode are discharged and adsorbed.65,66 Further, inorganic species such as SiC nanoparticles may be adsorbed on the cathode and are added to produce changes in morphology and to alter the texture axes of electrodeposits.63,67 Assuming that there are inhibiting adsorbed species on the cathode and that they are responsible for the texture formation, Amblard concluded that the adsorbing species behave specifically on different crystal faces. When electrodeposits are textured, it is only one crystal direction that is not blocked in its growth, thereby creating a growth texture. It is the electrochemical 4115



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Figure 13. φ1-Sections ODF maps for Ni/SiC electrodeposits obtained at 7 A/dm2.

Figure 14. Dependence of the texture of nickel on current density and pH for Watt’s bath.54−56

impedance diagrams obtained at low current densities and corresponding to ⟨110⟩ texture formation have an inductive loop that can be attributed to hydrogen adsorption. When the intensity increases, this loop and the ⟨110⟩ texture disappear at the same time (Wiart70). The growth competition is governed by the dynamic composition of the cathodic layer. This composition depends both on the local pH value and on the electrochemical balance of the various cathodic reactions occurring at a given imposed potential (which means mainly the different reactions of the competitive discharge of Ni2+ and H+ ions). The chemical species that are then formed are adsorbed, more or less strongly, on the different available sites, in such a way that they may block growth sites, slow down propagation steps, or modify the activity of emerging structural defects, etc. It appears that at low current densities the amount of Hads is significantly high and the SiC nanoparticles play a catalytic role in conversion of H+ to Hads, and thus ⟨110⟩ texture can evolve. At intermediate current

(iv) Conversely, [100] cannot be associated with any definite inhibition. This texture appears under conditions where the cathodic surface becomes relatively free from Hads, Ni(OH)2, or gaseous H2. Thus, we consider it to correspond to the free mode of growth for electrolytic Ni. 3.7.4. SiC Nanoparticle Inhibition to Texture Formation. In the case of Ni/SiC electrodeposition, it is known that Ni deposition begins as soon as the H+ discharge becomes diffusioncontrolled;68 thus, in the low current density range, the metallic surface must be almost entirely covered with Hads, which has been experimentally confirmed.69 This is not the case for higher current densities because of a release of the cathodic surface (thanks to its renewal) without newly arriving H+ becoming discharged and adsorbed. It must be noted that the ⟨110⟩ preferred orientation is stable in the very domain where we may expect the strongest inhibition by Hads. This result has been confirmed by electrochemical impedance measurements carried out by Bressan and Wiart.70 These authors showed that 4116



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densities this hypothesis is right; therefore, evolution of ⟨110⟩ texture is the most probable. At high current densities, Hads converts to H2 gas, but the amount of Hads is very high in this situation, and thereby, ⟨210⟩ texture appears, but the strongest texture is ⟨110⟩.

(21) Rasmussen, A. A.; Møller, P.; Somers, M. A. J. Surf. Coat. Technol. 2006, 200, 6037. (22) Lampke, Th.; Wielage, B.; Dietrich, D.; Leopold, A. Appl. Surf. Sci. 2006, 253, 2399. (23) Thiemig, D.; Bund, A. Surf. Coat. Technol. 2008, 202, 2976. (24) Socha, R. P.; Nowak, P.; Laajalehto, K.; Väyrynen, J. Colloids Surf., A Physicochem. Eng. Asp. 2004, 235, 45. (25) Möller, A.; Hahn, H. Nanostruct. Mater. 1999, 12, 259. (26) Angerer, P.; Simunkova, H.; Schafler, E.; Kerber, M. B.; Wosik, J.; Nauer, G. E. Surf. Coat. Technol. 2009, 203, 1443. (27) Lutterotti, L.; Scardi, P. J. Appl. Crystallogr. 1990, 23, 246. (28) Wenk, H. R.; Lutterotti, L.; Vogel, S. C. Powder Diff. 2010, 25 (3), 283. (29) Benea, L.; Bonora, P. L.; Borello, A.; Martelli, S.; Wenger, F.; Ponthiaux, P.; Galland, J. Solid State Ionics 2002, 151, 89. (30) Benea, L.; Bonora, P. L.; Borello, A.; Martelli, S.; Wenger, F.; Ponthiaux, P.; Galland, J. J. Electrochem. Soc. 2001, 148 (7), C461. (31) Watson, S. W. J. Electrochem. Soc. 1993, 140, 2235. (32) Guglielmi, N. J. Electrochem. Soc. 1972, 119, 1009. (33) Maurin, G.; Lavanant, A. J. Appl. Electrochem. 1995, 25, 1113. (34) Stojak, J. L.; Fransaer, J.; Talbot, J. B. in Advances in Electrochemical Science and Engineering; Alkire, R. C., Kolb, D. M., Eds.; Wiley−VCH: Weinheim, 2002; Vol 7. (35) Musil, J. Surf. Coat. Technol. 2000, 125, 322. (36) Lampke, Th.; Steinhauser, S.; Richter, D.; Wielage, B. Mat.-Wiss. u. Werkstofftech 2007, 38, 23. (37) Cullity, B. D. J. Appl. Phys. 1964, 35, 1915. (38) Zhao, H.; Liao, X. Z.; Jin, Z.; Valiev, R. Z.; Zhu, Y. T. Acta Mater. 2004, 52, 4589. (39) Williamson, G. K.; Smallman, R. E. Philos. Mag. 1956, 1, 34. (40) Smallman, R. E.; Westmacott, K. H. Philos. Mag. 1957, 2, 669. (41) Bunge, H. J. Texture Analysis in Materials Science: Butterworth: London, 1982. (42) Wielage, B.; Lampke, Th.; Zacher, M.; Dietrich, D. Key Eng. Mater. 2008, 384, 283. (43) Kim, I.; Lee, S. G. Texture Microstruct. 2000, 34, 159. (44) Wielage, B.; Steinhauser, S.; Lampke, Th.; Hofmann, U.; Jakob, C. Metalloberflache 2003, 57, 25. (45) Ari-Gur, P.; Alogab, K.; Alamr, A.; Alkhasawneh, H.; Mirmiran, S. J. Metastable Nanocryst. Mater. 2005, 24−25, 619. (46) Pelletier, H.; Krier, J.; Mille, P. Mech. Mater. 2006, 38, 1182. (47) Wielage, B.; Podlesak, H.; Steinhauser, S.; Nickelmann, D. Messund Pruftechnik 1998, 52, 386. (48) Pangarov, N. A. Electrochim. Acta 1962, 7, 139. (49) Pangarov, N. A. Electrochim. Acta 1964, 9, 721. (50) Pangarov, N. A. Electroanalyt. Chem. 1965, 9, 70. (51) Reddy, A. K. N. Electroanalyt. Chem. 1963, 6, 141. (52) Reddy, A. K. N.; Rajagopalan, S. R. Electroanal. Chem. 1963, 6, 153. (53) Reddy, A. K. N.; Rajagopalan, S. R. Electroanal. Chem. 1963, 6, 159. (54) Amblard, J.; Froment, M.; Spyrelis, N. Surf. Coat. Technol. 1977, 5, 205. (55) Froment, M.; Maurin, G. J. Microscopie 1968, 7, 37. (56) Czerwinski, F.; Szpunar, J. Corros. Sci. 1999, 41, 729. (57) Nielsen, C. B.; Horsewell, A.; Østerhåerd, M. J. L. J. Appl. Electrochem. 1997, 27, 839. (58) Wyllie, M. J. Chem. Phys. 1948, 16, 52. (59) Bozorth, R. Phys. Rev. 1925, 26, 390. (60) Wilman, H. Trans. Inst. Metal. Finish. 1955, 32, 281. (61) Rashkov, S.; Stoichev, D.; Tomov, I. Electrochim. Acta 1972, 17, 1955. (62) Buckley, H. Crystal Growth; John Wiley & Sons: New York, 1951. (63) Amblard, J.; Epelboin, I.; Froment, M.; Maurin, G. J. Appl. Electrochem. 1979, 9, 233. (64) Snavely, C. Trans. Electrochem. Soc 1948, 92, 537. (65) Ives, A.; Edington, J.; Rothwell, G. Electrochim. Acta 1970, 15, 1797.

4. CONCLUSIONS In this work, we elucidate the changes in the texture of electrodeposited Ni/SiC on a copper substrate, in the presence of SiC nanoparticles, and as a function of the current density. An optimum current density of 7 A/dm2 is observed where the codeposited SiC nanoparticles have the highest content in the Ni/SiC coating. From XRD measurements a minimum crystal size of about 75 nm is obtained for the electrodeposited coating at the optimum current density. Due to the minimum value of the crystal size, the residual stress and the highest dislocation density are observed at the optimum current density. From pole figure and inverse pole figure studies, a mechanism of texture changes with current density and pH values of the electrolyte bath is proposed. At an intermediate current density of about 7 A/dm2 evolution of ⟨110⟩ texture is the most probable. At higher current densities, ⟨210⟩ texture appears, but the strongest texture is still ⟨110⟩.

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REFERENCES

(1) Sohrabi, A.; Dolati, A.; Ghorbani, M.; Monfared, A.; Stroeve, P. Mater. Chem. Phys. 2010, 497−505, 121. (2) Low, C. T. J.; Wills, R. G. A.; Walsh, F. C. Surf. Coat. Technol. 2006, 201, 371. (3) Zimmerman, A. F.; Palumbo, G.; Aust, K. T.; Erb, U. Mater. Sci. Eng., A 2002, 328, 137. (4) Garcia, I.; Fransaer, J.; Celis, J.-P. Surf. Coat. Technol. 2001, 148, 171. (5) Ebrahimi, F.; Bourne, G. R.; Kelly, M. S.; Matthews, T. E. Nanostruct. Mater. 1999, 343, 11. (6) Muller, B.; Ferkel, H. Z. Metallkd. 1999, 868, 90. (7) Muller, B.; Ferkel, H.; Riehemann, W. Mater. Sci. Eng., A 1997, 474, A234−236. (8) Lin, C. S.; Hsu, P. C.; Chang, L.; Chen, C. H. J. Appl. Electrochem. 2001, 925, 31. (9) Hu, F.; Chan, K. C. Appl. Surf. Sci. 2005, 243, 251. (10) Lampke, Th.; Leopold, A.; Dietrich, D.; Alisch, G.; Wielage, B. Surf. Coat. Technol. 2006, 201, 3510. (11) Kollia, C.; Spyrellis, N.; Amblard, J.; Froment, M.; Maurin, G. J. Appl. Electrochem. 1990, 20, 1025. (12) Gyftou, P.; Stroumbouli, M.; Pavlatou, E. A.; Asimidis, P.; Spyrellis, N. Electrochim. Acta 2005, 50, 4544. (13) Ari-Gur, P.; Alogab, K.; Alamr, A.; Alkhasawneh, H.; Mirmiran, S. J. Metastable. Nanocrystal. Mater. 2005, 619−622, 24−25. (14) Pantleon, K.; Somers, M. A. J. Mater. Sci. Forum 2005, 495−497, 1455. (15) Pavlatou, E. A.; Stroumbouli, M.; Gyftou, P.; Spyrellis, N. J. Appl. Electrochem. 2006, 36, 385. (16) Wang, L.; Gao, Y.; Xue, Q.; Liu, H.; Xu, T. Mater. Sci. Eng., A 2005, 390, 313. (17) Ari-Gur, P.; Sariel, J.; Vemuganti, S. J. Alloys Compd. 2007, 434− 435, 704. (18) Erler, F.; Jakob, C.; Romanus, H.; Spiess, L.; Wielage, B.; Lampke, T. Electrochim. Acta 2003, 48, 3063. (19) Rashkov, S.; Pangarov, N. Electrochim. Acta 1969, 14, 17. (20) Pangarov, N. A.; Uvarov, L. A.; Vagramyan, A. T. Electrochim. Acta 1968, 13, 1905. 4117



Article

(66) Berezina, S. I.; Burnasheva, L. V.; Gilmanov, A. N.; Sageeva, R. M. Elektrokhimiya 1974, 10, 948. (67) Fischer, H. Electrodeposition Surf. Treat. 1972/73, 1, 319. (68) Dorsch, R. K. J. Electroanal. Chem. 1969, 21, 495. (69) Epelboin, I.; Morel, P.; Takenouti, H. J. Electrochem. Soc. 1971, 118, 1282. (70) Wiart, R. Oberflache-Surface 1968, 9, 213.

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