Co-electrodeposition of inorganic fullerene (IF-WS2) nano-particles with cobalt from a gluconate bath with anionic and cationic surfactants

Electrochimica Acta 56 (2011) 6837–6846

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Co-electrodeposition of inorganic fullerene (IF-WS2 ) nano-particles with cobalt from a gluconate bath with anionic and cationic surfactants D.P. Weston a,b,∗ , Y.Q. Zhu a,c , D. Zhang a , C. Miller a,d , D.G. Kingerley a , C. Carpenter a , S.J. Harris a , N.J. Weston a a

Department of Mechanics, Materials and Structures, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK Mechanics of Materials Group, Department of Engineering, University of Leicester, LE7 1RH, UK c Functional Materials Group, College of Engineering, Mathematics and Physical Sciences, University of Exeter, EX4 4QF, UK d School of Clinical Dentistry, University of Sheffield, S10 2TA, UK b

a r t i c l e

i n f o

Article history: Received 19 January 2011 Received in revised form 20 April 2011 Accepted 23 May 2011 Available online 2 June 2011 Keywords: Electrodeposition Nanoparticles Particle volume % and distribution Mechanisms of codeposition Inorganic fullerene

a b s t r a c t The codeposition of IF-WS2 particles (130 nm diameter) in a cobalt matrix has been investigated. The influence of cobalt content and anionic and cationic surfactants on composite coatings produced using direct current (DC) and pulse reverse plating (PRP) are described. The coatings were assessed by XRD, SEM and EDX. The cationic surfactant promoted particle encapsulation and high oxygen content in DC coatings, whilst anionic surfactants do not offer controlled particle codeposition. Improved particle content and distributions were obtained by PRP, on the high cobalt bath containing anionic surfactant. The particle content was optimised by fixing the anodic cycle time ta , and varying the cathodic cycle time, tc . Sound coatings with a high percentage (11 vol.%) of particles were produced at tc of 60 s. This was encouraged by electrophoretic attraction of particles in the anodic phase and then encapsulation during the cathodic phase of the cycle. Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved.

1. Introduction Codeposition of micron sized solid lubricant particles into metallic electrodeposits is a technique used for the past 30 years for the production of coatings with enhanced tribological properties [1–8]. Recently the development of this type of composite coating produced by electrodeposition has concentrated on the inclusion of smaller particulate materials in the size range 10–200 nm. The codeposition of these nano-particles can significantly affect the growth of the metallic coating promoting the formation of fine grain sizes [1,9–13]. Such composites have improved properties, e.g., hardness and wear resistance, when compared with metal or alloy coatings [14–19]. The inclusion of nanoparticles into an electrodeposit is influenced by electrolyte composition and particle loading in the bath, current density and controlled agitation [14]. The supply of nanoparticles and the mechanism of their subsequent incorporation into an electrodeposit are less clear. The creation of a suitable dispersion of particles in the electrolyte and the possi-

∗ Corresponding author at: Department of Mechanics, Materials and Structures, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK. Tel.: +44 1162522538. E-mail address: [email protected] (D.P. Weston).

bilities of electrophoretic migration of particles are known to be important. There has been much work done on easily dispersed particles, such as ␥-alumina [14,20–22]. If particles have a low zeta potential they disperse poorly in aqueous solution. Cationic surfactants can facilitate particle dispersion and have the added benefit of attracting the positively charged particles electrophoretically to the cathode during electrodeposition thus promoting particle inclusion. However, the cationic surfactant may interfere with the electrodeposition of the metal which can lead to a brittle coating [19]. Attempts have been made at producing coatings with a dispersion of particles in the deposit by using either direct current (DC), pulse plating (PP) or pulse reverse plating (PRP). The majority of investigative work has been carried out on ␥-alumina nanoparticles which have been introduced into the nickel or copper containing electrolytes. These ␥-alumina particles have not required surfactants to promote their dispersion in the electrolyte. Early work by Podlaha and Landolt [20] used a bath with 12 g l−1 ␥-alumina dispersed in a copper citrate solution maintained at pH 4 and showed that PRP (with a duty cycle, ([Qc ] − [Qa ])/Qc = 0.16, where Qc is the cathodic charge per cycle and Qa is the anodic charge per cycle), could give a five-fold increase in particle content when compared to DC plating. Later work by Vidrine and Podlaha [22] examined the influence of the cathodic and anodic part of the cycle on matrix and particle deposition in nickel chloride–␥-alumina baths. PRP

0013-4686/$ – see front matter. Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.05.093

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Table 1 Plating parameters and compositions common to all baths. H3 BO3 (g l−1 ) NaCl (g l−1 ) Na gluconate (g l−1 ) pH Magnetic stirrer (rpm) US probe power (W) T (◦ C) j (A dm−2 )

40 30 110 6 100 20 80 4

produced an increase in particle content to 4.7 wt.% (11.7 vol.%) ␥alumina compared with 1.68 wt.% (4.6 vol.%) by DC plating. Vidrine and Podlaha discussed these findings in terms of electrolyte type and how this influences anodic dissolution in PRP. Thiemig et al. [23] have demonstrated that PRP can promote nanocrystalline matrix microstructures and at the same time enhanced the inclusion of 13 nm diameter ␥-alumina particles in the deposit. In this case nickel–␥-alumina composite coatings were produced with more than three times ␥-alumina present as coatings produced by DC plating [24]. A potential particle for inclusion in a composite coating is inorganic fullerene tungsten disulphide (IF-WS2 ) which is produced in the form of 130 nm particles by the sulphidation of WO3 . These IF-WS2 particles have been shown to have superior tribological properties and have been used as additives to oils [25]. The inclusion of such particles in a nanocrystalline metallic cobalt deposit has the potential to produce a low friction coating provided that they are present in sufficient quantities. An attempt has been made at producing an electrodeposited cobalt coating incorporating IFWS2 particles. The process made use of a cationic surfactant and a low cobalt bath concentration to produce coatings with poor adhesion to the substrate and also containing high porosity levels [26]. The present work details ways of codepositing IF-WS2 particles in a porosity-free cobalt matrix from plating baths with different cobalt concentrations together with sodium gluconate added as a complexing agent, additions of IF-WS2 particles and either cationic or anionic surfactants. The bath chemistry was developed from a Co–W alloy plating bath [27] which was operated at pH 6. Co has been deposited in the pH range 1.5–6.0. As the pH increases a fine dispersion of hydroxide becomes occluded in the coating [28]. To overcome this, additions of boric acid are made and complexants e.g., citrate and gluconate have been used [29,30]. Weston et al. [27] have demonstrated that if the gluconate to cobalt ratio is sufficiently high, then cobalt–gluconate complexes are the dominant species in the bath at pH 6. Finally a controlled pulse reverse deposition technique has been applied which promotes the possibilities of ensuring adhesion of the coating to the substrate together with a low porosity coating containing a well dispersed IF-WS2 particle content.

an ELGASTAT OPTION 3 water purifier to make up 1 l of solution. All chemicals were ACS grade supplied by Sigma Aldrich and baths were adjusted to pH 6 by adding sodium hydroxide pellets. Baths 1a, 2a and 3a all contained 0.05 M cobalt sulphate, whilst baths 1b, 2b and 3b contained 0.25 M cobalt sulphate. Sodium gluconate (0.5 M) is added as a complexant for the cobalt, in baths 1a, 2a and 3a at ten times the cobalt content and in baths 1b, 2b and 3b it is twice the cobalt content. Sodium chloride was added to enhance conductivity and boric acid was present as a buffer. No surfactants were added to baths 1a and b, but cationic surfactant, cetyl trimethyl ammonium bromide (CTAB) was added to baths 2a and b, whilst an anionic surfactant, sodium dodecyl sulphate (SDS) was added to baths 3a and b. Finally, the IF-WS2 particles (130 nm in size) supplied by Nanoscience Ltd. were introduced at a concentration of 5 g l−1 into the bath and then the ultrasonic probe operating at 20 W was then used to break down the agglomerated particles to form an effective dispersion in solution. 2.2. Electrodeposition of coatings Initially, direct current (DC) coatings were prepared by galvanostatic controlled deposition from the above baths at a current density of 4 A dm−2 and with the temperature controlled at 80 ± 1 ◦ C. The substrates were sheets of mild steel, 5 mm × 10 mm, which were etched in concentrated nitric acid for 5 s and washed in 10% (w/v) hydrochloric acid prior to deposition. Samples were plated for 1 h with bath agitation by an electromagnetic stirrer and the ultrasonic (US) probe continued to operate at 20 W to maintain the dispersion. Pulse reverse (PRP) deposition experiments were conducted upon baths 3a and 3b. The initial current cycle consisted of an anodic pulse of 8 A dm−2 for 10 s followed by a cathodic pulse of 4 A dm−2 for 60 s. A subsequent set of four coatings was produced using the PRP technique to investigate the effect of altering the length of the cathodic pulse on the particle incorporation in the coatings. PRP parameters are listed in Table 5 and the total tc amounted to 3600s in each experiment. 2.3. Characterisation All coatings were examined in cross section in a Philips XL30 environmental scanning electron microscope fitted with a field emission gun (ESEM FEG) and energy dispersive X-ray (EDX) analyser. EDX area analysis was conducted at three representative sites in the central 4 mm2 of each sample and an average taken. Coatings were also subjected to X-ray diffraction studies with a Siemens D500 diffractometer for structural analysis, Cu K␣ radiation was employed with a 2 range of 10–100◦ at a step size of 0.05◦ and a dwell time of 2 s.

2. Experimental details

3. Results

2.1. Bath composition for deposition experiments

3.1. Direct current (DC) deposition

The compositions of baths which have been investigated are given in Tables 1 and 2. All baths were prepared by combining all chemicals in a volumetric flask and adding deionised water from

During deposition from baths 1a and 1b, which did not have an addition of a surfactant, it was observed that the bulk of the particles were taken up in a froth which formed at the top of the plating baths during deposition due to the action of the ultrasonic probe. In baths 2a, 2b, 3a and the presence of either a cationic or anionic surfactants combined with stirring and ultrasonic agitation encouraged the particles to disperse within the plating baths and no foam was observed. SEM pictures of the cross sections of coatings are used to give reliable distribution of particles and porosity throughout the thickness of the coating and are shown in Fig. 1. In all cross sections the

Table 2 Bath composition. Bath

1a

1b

2a

2b

3a

3b

CoSO4 ·7H2 O (g l−1 ) SDS (g l−1 ) CTAB (g l−1 )

16 – –

80 – –

16 – 0.2

80 – 0.2

16 0.2

80 0.2

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Fig. 1. Cross sectional back scattered electron SEM images of coatings produced by direct current (DC) deposition at the same current density for the same time from (i) bath 1a – no surfactant and 0.05 M Co, (ii) bath 1b – no surfactant and 0.25 M Co, (iii) bath 2a – 0.2 g l−1 CTAB and 0.05 M Co, (iv) bath 2b – 0.2 g l−1 CTAB and 0.25 M Co, (v) bath 3a – 0.2 g l−1 SDS and 0.05 M Co, (vi) bath 3b – 0.2 g l−1 SDS and 0.25 M Co, all containing 5 g l−1 IF-WS2 particles. Variations in Co concentration and surfactant produce significant changes in coating thickness, IF-WS2 particle numbers and distribution.

direction of growth is from left to right. In the absence of surfactant and 0.05 M cobalt present, bath 1a produced a coating approximately 15 ␮m thick with only a few clustered particles of IF-WS2 present, see Fig. 1(i). The coating is well bonded to the substrate, whilst the surface had some irregularities which could relate to the etched surface of the substrate and the presence of clustered particles in the cross section of the coating. Bath 1b with 0.25 M cobalt and no surfactant produced a coating that was 33 ␮m thick, containing fewer particle clusters than bath 1a, but with a smoother surface, see Fig. 1(ii). EDX analysis of these coatings in cross section, see Table 3, indicated the presence of tungsten and sulphur in small amounts from the IF-WS2 particles together with an oxygen content of 6 at.% associated with the cobalt matrix. Baths 2a and 2b, both of which had CTAB surfactant present, produced coatings with a metallic bluish lustre on their surfaces. The SEM picture in Fig. 1(iii) shows that bath 2a produced a coating 5 ␮m thick which contains a high proportion of IF-WS2 particles which are dispersed and only contained a few clusters. The coating from bath 2b with the

higher cobalt content is ∼50 ␮m thick and consists of two distinct layers of different atomic contrast with particles well distributed in the cobalt matrix, see Fig. 1(iv). Both coatings are richer in oxygen content than those produced from the baths without surfactant additions. The coating from bath 2a contains 16.4 at.% oxygen whilst the coating from bath 2b demonstrated two oxygen contents i.e., 13.6 at.% in the near substrate region and 9.7 at.% towards the outer surface.

Table 3 EDX results for coatings produced by DC method.

O (at.%) W (at.%) S (at.%) Co (at.%) Efficiency (%)

1a

1b

2a

2b

6.0 0.7 0.3 92.9 26

5.6 0.4 0.1 93.9 61

16.4 3.9 5.2 75.5 9

13.6 1.6 1.3 83.5

9.7 1.2 1.7 87.4 76

3a

3b

5.6 3.8 4.3 86.3 38

6.2 0.6 0.5 92.6 59

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3.2. Effect of pulse-reverse deposition Coatings produced by pulse reverse deposition (anodic pulse 8 A dm−2 for 10 s and cathodic pulse 4 A dm−2 for 60 s; duty cycle of 0.67) in baths 3a and b are shown in cross-section form in Fig. 3(i) and (ii) respectively. Coating 3a1 was black and dull in appearance and the cross section shows a spongy porous coat-

(i)

counts / a.u.

{100}

{110} {112} {200} {201} 1b

{002} {101}

1a 20

40

60

80

100

2θ

(ii)

=WS2 =Co =?

counts / a.u.

Baths 3a and 3b contained the anionic surfactant, SDS, and produced coatings with quite different morphology, see Fig. 1(v) and (vi). Bath 3a produces a black dull coating which is seen to be porous in cross-section as well as containing a large number of particles in the cobalt matrix. The coating consists of an initial layer which is cobalt rich up to ∼5 ␮m thick containing a smaller number of IF-WS2 particles. From this initial layer, irregular nodules of cobalt grow outwards for ∼10 ␮m and as they do they grow wider and this encourages an increasingly high proportion of particles to be adsorbed. Eventually these nodules merge leaving a band of large irregularly shaped pores, approximately 10 ␮m in size. The outer face of the coating is made up of protruding nodules contains a high proportion of particles which gives a very rough surface. The whole of the porous structure is ∼30 ␮m thick from the substrate to outer surface, see Fig. 1(v). Coatings from bath 3b were also approximately ∼30 ␮m thick and were well adhered to the substrate as was the case with coatings from bath 3a. They had a matt pale gray surface which was much smoother than the etched substrate. Some IF-WS2 particles were encapsulated within the first ∼5 ␮m of cobalt near the substrate but as the coating grows beyond 5 ␮m the number of encapsulated particles diminishes rapidly. EDX analysis showed that ∼ 6 at.% oxygen was found in the cobalt matrix which was similar to the amount found in coatings produced from surfactant free baths 1a and b and hence, much lower than in the coating produced from the cationic surfactant containing baths, 2a and b. XRD patterns of the coatings produced under the DC conditions from baths with and without surfactants are shown in Fig. 2. The coating from bath 1a with 0.05 M cobalt and no surfactant was shown to have a low intensity peaks in positions which correspond to those associated with hcp cobalt with no evidence of IF-WS2 being present. Bath 1b with 0.25 M cobalt and no surfactant produced a coating showing some higher intensity peaks, (1 0 0) and (1 1 0), which indicated preferred orientation. There was no evidence of the presence of IF-WS2 particles in these deposits. Evidence of fcc cobalt being produced at low pH has been found [28]. Abd El Rehim et al. [29] working on a gluconate bath at pH 5 demonstrated the absence of the second strongest fcc (2 0 0) peak as was the case with deposits produced here at pH 6. Coatings from baths 2a and b with the cationic surfactant, CTAB, showed XRD patterns with similar cobalt peaks to those found on deposits from baths 1a and b, see Fig. 2. There is a strong non attributable peak at 2 ≈ 49◦ . There was more evidence of the preferred orientation in the cobalt matrix produced from the 0.25 M cobalt bath with the presence of more highly intense peaks. Significant diffraction peaks were detected which were attributable to IF-WS2 particles particularly in the deposits produced from the 0.05 M cobalt bath. XRD of the coatings produced from bath 3a with 0.05 M cobalt and the anionic surfactant, SDS, shows the presence of small peaks associated with IF-WS2 particles, see Fig. 2(iii). Deposits from bath 3b with 0.25 M cobalt again showed evidence of a high intensity cobalt peak, (0 0 2), and thus the presence of crystal growths with a high degree of preferred orientation. All XRD patterns from the low Co baths indicated nanocrystalline hcp coatings. With higher Co content baths the coatings are still hcp but no longer nanocrystalline.

2b

20

40

60

80

2a 100

2θ

(iii) =WS2 =Co

counts / a.u.

6840

3b

20

40

60

80

3a 100

2θ Fig. 2. XRD patterns for coatings from (i) baths 1a and b, (ii) baths 2a and b, (iii) baths 3a and b. The 0.25 M Co solutions all show evidence of strong hcp preferred orientation, and the introduction of surfactants promotes greater IF-WS2 codeposition.

ing with a high IF-WS2 content. It has a thin Co coating adjacent to the substrate about 2 ␮m thick with few particles and then a mass of porous nodules up to a coating thickness of 50 ␮m. The adhesion to the substrate is very good. Coating 3b1, Fig. 3(i), is similar in thickness to coating produced from bath 3b under DC conditions, Fig. 3(ii), but the surface is a little rougher and there is a significant increase in IF-WS2 particle content. EDX analysis of coating 3b1 shows the oxygen content of the coating is 5.1 at.% a value similar to that of 1b and 3b but significantly less than 2a and b and that from 3a1. In addition, the EDX analysis demonstrates an increased tungsten and sulphur content in coating 3b1 produced by PRP when compared with the DC coating 3b, see Tables 3 and 4. In the case of coating 3a1, it has much further enhanced tungsten and sulphur contents compared with coating 3b1. XRD on the pulse reversed coatings 3a1 and 3b1 showed similar patterns to those obtained on the DC coatings 3a and b, see Fig. 4. IFWS2 peaks are clearly shown for the 3a1 coating and strong cobalt

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Fig. 3. Cross sectional back scattered electron SEM images of coatings produced by pulse reverse deposition from (i) bath 3a, 0.2 g l−1 SDS, 0.05 M Co, (ii) bath 3b, 0.2 g l−1 SDS, 0.25 M Co. PRP conditions; ta = 10 s, tc = 60 s, ia = +80 mA, ic = −40 mA, 60 cycles. The 0.25 M Co bath promotes a more even dispersion of IF-WS2 particles in (ii), whilst the low cobalt bath encourages higher IF-WS2 content but with significant porosity in (i). Table 4 EDX results for coatings produced by reverse pulse method. PRP conditions; ta = 10 s, tc = 60 s, ia = +80 mA, ic = −40 mA, 60 cycles.

O (at.%) W (at.%) S (at.%) Co (at.%) Efficiency (%)

3a1

3b1

10.7 4.0 4.0 81.3 30

5.1 1.8 1.7 91.4 74

counts / a.u.

=WS2 =Co

3b1

3a1 20

40

60

80

100

2θ Fig. 4. XRD patterns of coatings produced by pulse reverse deposition from (i) bath 3a, 0.2 g l−1 SDS, 0.05 M Co, (ii) bath 3b, 0.2 g l−1 SDS, 0.25 M Co. The IF-WS2 peaks are much more visible in 3a1.

peaks (1 0 0) and (0 0 2) are found with 3b1. There is no evidence of IF-WS2 in the XRD pattern of 3b1. 3.3. Influence of cathodic time and number of reverse pulse cycles on composite deposition These experiments were carried out on bath 3b with 0.25 M cobalt content and an SDS surfactant addition. The pulse reverse

Table 6 EDX results for coatings produced by reverse pulse method with variable cathodic time period. 120

O (at.%) W (at.%) S (at.%) Co (at.%)

Inner

Outer

6.8 2.0 1.4 89.8

4.8 1.1 0.6 93.5

60

40

6.1 2.4 2.3 89.2

5.2 3.0 3.3 88.5

30 Inner

Outer

8.8 3.8 4.0 83.4

7.4 4.1 5.1 83.3

plating conditions are detailed in Table 5. Each coating was examined in section (see Fig. 5) and the following measurements were made: coating thickness, weight % and volume % IF-WS2 and cathodic efficiency. From the measurements the following were calculated: mass of cobalt deposited, particle density and thickness of cobalt deposited per cycle. These values are all recorded in Table 5 which also contains data for the DC plated sample from this bath. Table 6 gives the EDX analysis results for the PRP samples. For the PRP samples the coatings decreased in thickness as the cathodic time decreased and the number of cycles increased. At the same time the wt.% (vol.%) of IF-WS2 increased whilst the mass of cobalt deposited and hence, the thickness of cobalt deposited per cathodic cycle decreased. These results clearly demonstrate that the % of IF-WS2 rises with a reducing value of the duty cycle i.e., from 3.9 to 15.4 wt.% IF-WS2 (4.7–18.0 vol.%) as the duty cycle reduces from 0.83 to 0.33. EDX and XRD analysis on these coatings has also demonstrated that there is evidence for the highest tungsten and sulphur content being associated with the tc = 30 s, 120 cycle sample 0.33 duty cycle, see Table 6 and Fig. 6. It should also be noted that the IF-WS2 content in deposits produced from bath 3b under DC conditions was less than 1 wt.% (<1 vol.%). The adhesion between coating and substrate is good for coatings shown in Fig. 5(i)–(iii), but less good for Fig. 5(iv) (120 cycles, tc = 30 s) because the first cycle is cathodic and if this cycle is short it is not able to establish an effective bond.

Table 5 Analysis of coatings produced by reverse pulse method with variable cathodic time period. tc (s)

ta (s)

ic (A dm−2 )

ia (A dm−2 )

Duty cycle

Cycles

Thickness (␮m)

IF-WS2 (wt.%)

IF-WS2 (vol.%)

Mass Co deposited (g)

Cathodic efficiency (%)

Thickness of Co per cycle (␮m)

120 60 40 30 3600

10 10 10 10 10

4 4 4 4 4

8 8 8 8 8

0.8 0.67 0.5 0.33 0.99

30 60 90 120 1

37.5 25 18 15 34

3.9 8.8 11.1 15.4 <1

4.7 10.4 13.0 18.0 <1

0.0318 0.0199 0.0140 0.0109 0.0303

72 65 64 80 60

1.19 0.373 0.175 0.1025 –

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Fig. 5. Cross sectional back scattered electron SEM images of coatings produced by pulse reverse deposition from bath 3b, with variable cathodic deposition time (tc ) between anodic pulses; (i) 120 s, (ii) 60 s (iii) 40 s and (iv) 30 s. Variations in coating thickness, IF-WS2 particles numbers and distribution occur with a variation of tc values and a constant ta .

=WS2 =Co

Counts / a.u.

tc 30 s

4.2. DC plating with CTAB surfactant

40 s

The addition of cationic CTAB in bath 2a produces a smooth metallic coating containing a concentrated dispersion of IFWS2 particles. The cracked coating delaminated readily and EDX revealed the coating is richer in oxygen than other coatings at 16.4 at.%. The oxygen content of the coating is believed to be due to the presence of the CTAB in the bath forming a hydrophobic layer on the cathode, see Fig. 7a1 during deposition which hinders the cobalt deposition and leads to the incorporation of hydrated cobalt oxide and/or hydroxide species within the coating which have not been identified by XRD. The incorporation of Co(OH)2 in electrodeposited coatings produced at pH > 5 is well known in the literature [28]. The addition of CTAB to the path gives a greatly increased quantity of codeposited oxygen. A similar argument may be made for coating 2b which has unidentified peaks in its XRD pattern. Both matrix phases shown in the BSE image of 2b (Fig. 1(iv) and Table 3) contained significant oxygen contents, the initial layer contained 13.6 at.% oxygen and the outer layer 9.7 at.% oxygen. These lower oxygen contents when compared with those in coating 2a are due to the increased cobalt content of the bath 2b and a reduced level of complexing by gluconate. It has been noted that many plating baths which use a cationic surfactant to assist in the codeposition of nanoparticles do use very high metal concentrations e.g., in sulphamate nickel solutions. The coatings from bath 2 were the only coatings to crack and delaminate. It is of note that a recent paper [25] which attempts to codeposit IF-WS2 particles and cobalt uses a cationic surfactant in the bath and produces a coating requiring

60 s 120 s 20

40

deposition as most of the IF-WS2 particles seem to have reclustered or separated out of the bath into the foam. The increased cobalt deposition rate combined with the poor dispersion has led to a decreased concentration of particles in 1b compared with 1a.

60

2θ Fig. 6. XRD patterns for PRP samples produced from a 0.25 M Co bath with SDS surfactant present with a constant ta and variable tc values of 30, 40, 60 and 120 s. IFWS2 peaks are more evident at low tc values.

4. Discussion 4.1. DC plating from baths without surfactant The DC plating experiments provided good examples of coatings which are good examples of the problems encountered in the codeposition of IF-WS2 nanoparticles with an electrodeposit. Bath 1a produced a coating with a few clustered particles. Bath 1b produces a thicker coating which is assisted by the presence of more cobalt which is complexed by gluconate to a lesser degree. Few particles codeposit in both baths and those which are present are clustered due to the poor maintenance of particle dispersion during

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heat treatment in a reducing atmosphere to prevent delamination of the coating. Although no oxygen content is reported it is possible that the reason for the delamination is also due to codeposition of hydrated oxides within the coating. A similar effect was observed by Chen et al. [19] where another cationic surfactant, hexadecylpyridinium bromide, caused nickel from a Watts bath to be brittle when it was added in concentrations greater than 0.2 g l−1 . The composite coatings 2a and b produced here do possess some very attractive features even though the metal matrix of a film with them contains possible oxide phases, i.e., they have uniform thickness and contain a high proportion of well dispersed IF-WS2 particles with only a few obvious clusters. This is due to the combined action of the US probe, the stirring and the CTAB to maintain a good dispersion of particles in the plating bath during deposition. It may be inferred that the CTAB enhances electrophoresis of positively charged IF-WS2 particles towards the working electrode during deposition, as shown in Fig. 7a2. The lower cobalt bath 2a contained a higher density of particles as less metal is deposited than from bath 2b whilst a similar number of particles arrive at the surface. 4.3. DC plating with SDS surfactant

Fig. 7. Model for behaviour of particles during plating in the different baths studied. (a) CTAB cationic surfactant and DC plating. (b) SDS anionic surfactant and DC plating. (c) SDS anionic surfactant with PRP.

The use of anionic SDS in baths 3a and b confers a negative charge on the particles which should provide an electrophoretic effect on the particles away from the working electrode during the cathodic phase, see Fig. 7b. The SDS demonstrates that it is able to disperse the particles in the plating bath in a similar way to CTAB but it will not promote their attraction to the cathode during deposition. Comparing the SEM micrographs of coatings 3a and b, see Fig. 1(v) and (vi) it can be shown that the IF-WS2 content of 3a is very high and that of 3b is very low. The formation of the large porous regions in 3a can be explained by two possible mechanisms, i.e., the initial growth pattern of composite coatings on a rough, freshly etched electrode or by the evolution of hydrogen at the electrode and the subsequent retention of molecular hydrogen at the surface as proposed by Stroumbouli et al. [13] in the DC plating of Ni–WC composite coatings on a rotating disc electrode. With a rough electrode surface particles will be entrapped and then encapsulated by the growing cobalt nodules. These nodules then serve to entrap further particles which are then encapsulated by cobalt until the surface is covered with large nodules made up from particle rich cobalt connected to the surface by thin asperities which take up a “tree-like” appearance. Some of the nodules eventually meet and create voids below which are pores visible in the cross section. The rough surface of the coating entraps further IF-WS2 particles mechanically due to the agitation of the solution. Across the surface there are regions of greater asperities which are even richer in IF-WS2 , lending weight to the theory of physical entrapment of particles by the rough surface. If the growth pattern is controlled by hydrogen evolution, then it might be assumed that all baths exhibiting low efficiency should exhibit high levels of porosity. Bath 3a is highly porous but produces deposits at an intermediate porosity (∼38%) as shown in Table 3. No porosity is observed in deposits from bath 1a which has an efficiency of 26%. The porosity difference might be explained by the presence of SDS in the bath 3a however, no evidence of porosity was found in the coating from bath 3b which had a similar SDS content. Comparing coatings produced from baths 3b and 1b demonstrates that there is no significant difference in efficiency arising from the addition of SDS. Coating 3b is significant in that it contains so few particles compared to 3a. Here, the initial etched rough surface of the substrate does trap some particles which are then encapsulated by cobalt and the first 3–5 ␮m of 3a and b are similar. However, the crucial difference here is that the cobalt is deposited at a faster rate from bath 3b with higher cobalt content than in 3a due to a higher cobalt concentration and less complexing of the cobalt by

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gluconate. Thus, the cobalt is capable of growing over the particle and subsuming it with smoother cobalt before another particle can be entrapped. With increased deposition time the coating became smoother than the original substrate so fewer particles are physically entrapped and fewer particles are observed in the coating. The outer surface of the coating is visibly much smoother than the substrate coating interface. The small number of entrapped particles in 3b beyond 5 ␮m growth on the substrate is good evidence for the power of the electrophoretic effect on the adsorption and retention of particles in this bath, compared with the simple entrapment mechanism. Thus the negatively charged particles in 3b are not attracted to the surface as the positively charged ones are in bath 2b and hence, fewer particles are encapsulated. In 3a the microstructure is dominated by the high rate of particle capture compared to Co deposition, whereas in 3b the opposite is true, except for the region close to the substrate where some particle capture is observed. It might be surmised that there is an intermediate cobalt concentration which balances these two effects and allows deposition of a dense coating with high particle content. Such a plating bath would be difficult to operate and likely to function under strict control of composition, agitation and current density which is not always practical in an industrial process. There are, therefore, two modes by which particles may be transported to a surface, adsorbed and encapsulated by cobalt under the conditions applied here. A particle may randomly strike a rough surface and be physically entrapped for sufficient time for it to be encapsulated by cobalt. If the concentration of cobalt in the bath is high and less complexed by gluconate, then the particle is covered rapidly and the surface of the metal gets smoother before another particle can be entrapped, this results in fewer particles as the coating grows further from the rough substrate. If the cobalt concentration is low and highly complexed, then the surface of the coating becomes rougher as it grows as more particles arrive and are entrapped by the slowly growing cobalt. Since this method is difficult to control, it is better to transport particles to the surface by electrophoresis, by imposing a positive charge on the particles using a cationic surfactant. This produces smooth metal coatings which contain an excellent dispersion and loading of IF-WS2 particles, but the metal matrix is compromised by the inclusion of oxygen which has been shown to necessitate a further reduction process before providing a practical coating. The coatings are so brittle that handling is a major problem and where a heat treatment step might be impractical.

The effect of a pulse reverse technique has been examined here on baths 3a and b to produce coatings 3a1 and 3b1 respectively. Coating 3a1 has a very rough spongy mass of particles as the cobalt deposition from the low concentration bath is insufficient to encapsulate all the available particles from physical capture let alone those attracted electrophoretically. Coating 3b1 shows the effect of electrophoretic attraction of the negatively charged particles to the surface by an anodic pulse acting in conjunction with a cathodic pulse which deposits cobalt at a greater rate from the more concentrated solution. The coating is ∼30 ␮m thick and contains a good dispersion of particles similar in distribution to that attained by coating 2b with CTAB. The matrix in 3b1 is of greater integrity when compared to 2b with excellent coating-substrate adhesion which results from the low oxygen content (5 at.%) and implies a lower oxide and hydroxide content. The oxygen content of 3b1 is similar to that of 3b and 1b and is typical of Co coatings produced at similar pH [28]. As described earlier, this relates to the pulse reverse technique employed by Vidrine and Podlaha [22] and Xiong-Skiba et al. [21] to produce ␥-alumina dispersions in a nickel matrix from a Watts baths. During the anodic pulse other possible processes occur at the working electrode besides the attraction and adsorption of particles, these are metal dissolution and/or passivation. Vidrine et al. drew attention to these processes and demonstrated possible differences in metal removal between chloride and citrate containing electrolytes in their ability to influence the overall process. 4.5. Effect of pulse parameters on the mechanism of pulse-reverse deposition The EDX analysis of the PRP coatings showed an increase in the tungsten and sulphur contents with decreasing tc . By assuming that the coatings consist of pure cobalt and IF-WS2 particles. The volume % of IF-WS2 particles in the coating was calculated and plotted versus the thickness of cobalt deposited per cycle, normalised to the particle diameter of 130 nm (Fig. 8). Normalised metal thickness = metal thickness deposited per cycle/particle diameter. The plot shows that a shorter tc (i.e., less cobalt deposited per cycle), gives a higher particle density, which is to be expected since the amount of metal deposited per cycle is less at shorter tc , e.g., at a tc of 120 s IF-WS2 content is 4.7 vol.% and this increases to 18.0 vol.% at tc of 30 s. This level of inclusion is comparable with that obtained by Podlaha and Landolt [20] using PRP with a copper solution con-

4.4. Possible solutions to effective Co-deposition 35 Current work Podlaha [20]

30

particles / vol.%

An answer to this problem has been to employ a cationic surfactant in plating baths which contain a high concentration of electroactive species such as in the nickel sulphamate bath. This promotes metal deposition but also requires correspondingly high particle loadings in the bath to compete with the high metal deposition rate. The DC plating experiments have demonstrated that the surface chemistry of the particles is largely governed by the addition of cationic or anionic surfactants and these have been shown to promote or inhibit codeposition depending upon the plating conditions. Particles may be codeposited in a more controllable manner than is available using DC conditions by employing an anionic surfactant such as SDS and then using pulse reversal during deposition. In this case the particles are negatively charged and the imposition of a pulse of positive charge attracts the particles electrophoretically to the surface where they may become adsorbed. The polarity of the current is then rapidly reversed in order that metal deposition occurs which can entrap the adsorbed particles. Once the particles are covered another pulse attracts more particles to the surface and so on until a coating of desired thickness is obtained.

25 20 15 10 5

0

2

4

6

8

10

Normalised metal thickness per cycle Fig. 8. Plots of the volume fractions of IF-WS2 particles in Co matrix and Al2 O3 particles in Cu matrix (Podlaha and Landolt [20]) in coatings produced by PRP plotted against metal thickness per cycle normalised to particle diameter.

Volume of particles deposited per cycle / m3

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6.0x10

-12

5.0x10

-12

4.0x10

-12

3.0x10

-12

2.0x10

-12

1.0x10

-12

Podlaha [20] Current work

0.0 0

1

2

3

4

5

6

7

8

9

10

Metal thickness per cycle normalised to particle diameter Fig. 9. Plots of (i) the number of WS2 particles deposited per cycle against Co thickness with variable tc and (ii) the number of Al2 O3 particles deposited per cycle against Cu thickness with variable ta (Podlaha and Landolt [20]) against normalised metal deposited per PRP cycle.

taining ␥-alumina particles with tc constant and a variable ta as plotted in Fig. 8. Whilst volume % of particles is a useful parameter to describe a composite coating and its relationship to microstructure, it may not be possible to discern directly how the particles arrive at the working electrode and become encapsulated within the cobalt matrix. In this regard, a more useful piece of information would be the number of particles which are encapsulated per cycle. By using the volume % of particles and the thickness of the coating it is possible to calculate the number of particles in each PRP deposit. Fig. 9 is a plot of particle volume captured per cycle versus the normalised cobalt thickness deposited on the same basis, which shows that the volume of particles captured increases with metal thickness deposited per cycle. The varying slope of the plot demonstrates the changing rate at which particles are encapsulated during the cathodic phase of the cycle. At the shortest tc of 30 s, the calculated cobalt thickness is close to one particle diameter and the volume of particles per cycle is 2.2 × 10−12 m3 . This volume represents all particles encapsulated during the cathodic cycle. By tc = 60 s, three particle diameters of cobalt have been deposited, whilst the particle capture had almost doubled to 4.3 × 10−12 m3 . At tc = 120 s, particle capture per cycle is 5.5 × 10−12 m3 indicating that after tc = 60 s the coating is less effective at adsorbing and capturing particles, even though this longer time deposits a further 6 diameters of cobalt compared to the tc = 60 s sample. This behaviour can be explained by considering the results obtained from the DC plating experiments from the anionic surfactant plating baths. In Fig. 1(vi), very few particles are encapsulated within the bulk of the deposit produced from bath 3b, with the exception of a marked increase in particle density in the coating close to the substrate. The rough surface of the substrate promotes particle capture in the initial few microns of the deposit. Particle capture is less effective with increased tc as more metal is deposited. The decrease in the rate of particle capture with increased tc may be due to a combination of reasons. Firstly, the anodic phase of the cycle creates a reservoir of particles in close association with the working electrode. These particles consist of those released by the working electrode due to dissolution and those presented to the surface by electrophoresis (see Fig. 7c). During the cathodic phase, particles are codeposited from the reservoir until they become depleted and the rate of particle codeposition decreases. Secondly, the encapsulation of particles creates a rough surface. With increased tc , the surface of the surface becomes smoother and fewer particles are entrapped. As more particles for a given Co thickness deposited are encapsulated at

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low tc , the number of particles in the reservoir from dissolution in the anodic phase will be greater than those provided by a coating with long tc . Thus there is a synergistic effect between the use of an anodic surfactant, the anodic dissolution phase parameters, and the cathodic phase parameters which all contribute to produce the composite coating. With tc = 120 s, the particle concentration close to the substratecoating interface is higher than at the surface as more particles are captured due to mechanical entrapment by the rough substrate surface. As the coating grows the surface becomes smoother and less particles are captured. Little information is available in the literature which provides a comparison with the current model, since previous work has not recognised the formation of a particle reservoir in the anodic phase due to dissolution and electrophoresis. Previous work by Podlaha and Landolt [20] investigating ␥-alumina particles in a copper solution without a surfactant present, looked at the influence of PRP with anodic time ta as a variable and fixed tc (or the net cathodic charge). The coating was shown to contain ∼3 wt.% (6.5 vol.%) ␥-alumina when plated under DC conditions, and the particles are assumed to be encapsulated as a result of mechanical capture. This assumption is based on the work of Lee and Wan [31] who showed that the zeta potential of ␥-Al2 O3 particles in copper sulphate solution (<0.1 M) is negative, and although the concentration used by Podlaha et al. was 0.25 M it seems unlikely that this would result in a positive zeta potential for the suspended particles. In the following arguments has been assumed that the zeta potential of the particles in Podlaha’s work was negative. Data from this work has been interpreted and is presented here in Fig. 8 where the behaviour follows a similar pattern to the Co–WS2 system. The effect of a fixed tc is to set a maximum on the metal thickness deposited per cycle. This data is also presented in Fig. 9. The volume of particles encapsulated per cycle is lower than in the current work, but of the same order. It is interesting to observe the effect of variable ta on the number of particles captured per cycle. With decreasing ta , the metal thickness deposited per cycle (normalised to particle diameter) increases. Initially, with long ta , there is much dissolution and few particles are captured per cycle. The volume of particles captured per cycle increases to a maximum and then decreases as ta becomes very short and the reservoir of particles near the working electrode is not so large. Xiong-Skiba et al. [21] have carried out DC and PRP work on nickel sulphamate solution containing a high loading of 50 g l−1 ␥-alumina (50 nm diameter) together with an addition of the anionic surfactant SDS. DC plating produced composite coatings with 1.82 wt.% (4.8 vol.%) ␥-alumina, whilst PRP plating with tc set at 22 s and ta at 7 s deposited coatings with a very high (23.0 wt.%, 44.6 vol.%) ␥-alumina content. Vidrine and Podlaha [22] did not use a surfactant and obtained a particle content of only 4.7 wt.% (11.7 vol.%) despite using a similar ratio of Ni:␥-alumina and working under similar bath chemistry and PRP conditions. Xiong-Skiba plated more than four times as many particles and correctly deduced the contribution of particles to the near working electrode reservoir from dissolution, however no mention is made of the electrophoretic contribution to the reservoir. If the anodic dissolution of cobalt is assumed to be 100% efficient, then the cathodic efficiency may be calculated, see Table 5. PRP is more efficient than DC plating, and this is attributed to the replenishment of cobalt in the near working electrode region during the anodic dissolution phase.

5. Conclusions The codeposition of IF-WS2 particles into a cobalt matrix can produce different microstructures which depend upon the use of two different surfactants (cationic and anionic) and the application of different electrodeposition methodologies, i.e., DC and PRP.

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In an acid cobalt ion solution 5 g l−1 of 130 nm IF-WS2 particles flocculate and therefore need the application of ultrasonic agitation and the introduction of either cationic or anionic surfactant to disperse them. PRP deposition at fixed charges, Qa and Qc , with the low cobalt solution and the anionic surfactant again produced high IF-WS2 particle clusters and regions of porosity. The more concentrated cobalt solution with anionic surfactant and the same fixed charges produced a coating with an intermediate level of well dispersed particles without porosity. The volume % of well dispersed particles produced by PRP from the 0.25 M cobalt solution with anionic surfactant added can be optimised by fixing Qa and varying Qc . It was demonstrated that at a fixed current density and as tc decreased from 120 to 30 s the volume percent of particles increased, but at 30 s the coating contained porosity, which implied that the optimum particle content without porosity was at a tc of 60 s (∼11 vol.%). The observations made during DC and PRP deposition and anionic surfactant present has allowed a clearer understanding of relevant mechanisms by which the PRP of Co–IF-WS2 coatings take place. The major steps involve the electrophoretic attraction of particles with surfactant adsorbed on their surfaces to the working electrode during the anodic cycle and then their mechanical entrapment within the eletrodeposited cobalt coating during the cathodic cycle. During the anodic cycle, previously deposited cobalt is subjected to dissolution and the release of IF-WS2 particles which remain at or near the working electrode. These particles are then augmented by those arriving by electrophoresis. The dissolution process will together with adsorbed particles promote electrode surface roughening. The number or volume of particles in the coating has been shown to increase with cathodic cycle length, tc . Of the number of particles encapsulated at tc = 120 s in ∼9 particle diameters of composite deposition, ∼80% were encapsulated rapidly in the first ∼3 particle diameters, i.e., when tc = 60 s. In the remaining 60 s, the particles in the near electrode region had been depleted and this allowed the surface of the deposit to become smoother as fewer particles were available. Acknowledgements The authors would like to thank partners of the Foremost project and the European Commission for providing funding for this work.

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