Establishing relationships between bath chemistry, electrodeposition and microstructure of Co–W alloy coatings produced from a gluconate bath

Electrochimica Acta 55 (2010) 5695–5708

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Establishing relationships between bath chemistry, electrodeposition and microstructure of Co–W alloy coatings produced from a gluconate bath D.P. Weston, S.J. Harris ∗ , P.H. Shipway, N.J. Weston, G.N. Yap Division of Materials, Mechanics and Structures, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK

a r t i c l e

i n f o

Article history: Received 30 November 2009 Received in revised form 23 April 2010 Accepted 1 May 2010 Available online 11 May 2010 Keywords: Electrodeposition Cyclic voltammetry UV spectroscopy Amorphous Crystalline coatings

a b s t r a c t Electrodeposited Fe group: W and Mo alloys have the potential to replace hard Cr coatings for use in engineering applications where wear and corrosion resistance are needed. Electrochemical studies have concentrated in the past on Ni–W alloy deposition, but now interest in Co–W alloys has developed as they possess lower coefficients of friction when in contact with another metal. The most attractive coating composition is in the range 14–20 at.% W, if controlled deposition promotes crystalline alloys of high hardness, rather than softer amorphous alloys containing >20 at.% W. This paper employs ammonia free baths with low concentrations of cobalt and sodium tungstate and varying additions of sodium gluconate to produce alloys at close to 50% efficiency. Voltammetry, UV and visible spectrometry, and potentiostatic deposition have been performed on such baths, whilst XRD, SEM and TEM observations have been made on the deposits. This aims to optimise the process and to understanding the relationships between bath contents, electrochemical kinetics and alloy composition. Efficient deposition of coatings with hardness values up to 1000 kgf mm−2 occurred from a bath containing a high concentration of gluconate. Such deposits arise from concentrations of Co–W–gluconate complexes which promote the formation of nanoscale alloy grains. Current densities up to 2.75 A dm−2 in the agitated bath promoted deposition kinetics to form these highly orientated structures. These kinetics produced nano-segregation of W which may be assisted by the migration of Co–W clusters to boundary sites during the growth of the deposit. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Chromium electrodeposits have served industry well for many years in the production of coatings with good aesthetic qualities as well as for their functional properties such as high hardness together with minimal coefficient of friction and excellent wear properties when lubricated. As these properties can be combined with a potential to resist corrosion, it is clear why electrodeposited hard chromium has been used so extensively in engineering applications. In recent years concern has grown over the use of hexavalent chromium in the production of electrodeposits on environmental grounds, and this is the major driver behind the development of wear resistant coatings to replace hard chromium [1–3]. Continuing attempts to develop a wear and corrosion resistant electrodeposit as a replacement for hard chromium have focussed on the co-deposition of tungsten or molybdenum with one or more of the iron group metals Fe, Co or Ni. Tungsten will not deposit by itself from aqueous solution but will co-deposit as an alloy with iron

∗ Corresponding author. Tel.: +44 115 9513739; fax: +44 115 951 5989. E-mail address: [email protected] (S.J. Harris). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.05.005

group metals. Brenner [4], who introduced the concept of induced deposition, showed that aqueous solutions of tungstate plus an iron group metal were generally unstable and precipitated out unless a suitable salt of a hydroxyl organic acid was also present to reduce the quantity of free iron group metal ions in solution. Baths containing these constituents were able to produce alloys which displayed hardness values of 450–650 kgf mm−2 in the asdeposited condition [5,6] and on heat treatment the hardness value was raised to that of hard chromium (>900 kgf mm−2 ) [7]. As deposited, these alloys contained >20 at.% W and were in an amorphous form. On heat treatment the alloys crystallised to form high volume fractions of Co3 W which is an intermetallic compound containing 25 at.% W. The deposition of both tungsten and molybdenum with individual iron group alloys has received significant attention in the past decade to provide an understanding of the electrochemical factors which influence the process. Studies by Podlaha et al. [8], and Podlaha and Landolt [9–11] using a rotating cylinder electrode with a series of solutions containing variable nickel sulphate and sodium molybdate contents together with citrate and ammonia additions demonstrated that mass transport was a major influence in controlling the composition of the Ni–Mo alloy deposit. This was particularly the case when the molybdate to nickel ratio

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in solution was low and the concentration of the citrate complexant was similar to the combined metallic ions. High current densities and electrode rotation rates raised the Mo content of the alloy but decreased its efficiency of deposition. Models based on these experimental observations have demonstrated that the partial current densities of nickel and molybdenum reduction specify the alloy composition and the current efficiency of the process. The molybdenum partial current density is influenced by the concentration of nickel in the solution, i.e. raising the nickel concentration in the bath at a fixed molybdenum concentration could increase the Mo content of the alloy over a limited range. Zech et al. [12–14] as a result of these observations postulated that two absorbed species are present at the cathode surface, one contained molybdenum and the other containing nickel which influence the deposition process. Gileadi et al. [17,18] have more recently concentrated upon the influences of complexes in the plating bath on the deposition in the Ni–W system. In solutions containing citrate they assumed that separate complexes involving nickel and tungstate form, but with appropriate concentration ratios of nickel and tungstate present mixed complexes such as [(Ni)(WO4 )(Cit)(H)]2− can form in the bath and it is this species which promotes W deposition in the alloy. Nickel deposition can take place from either the mixed species or from a less complexed form of nickel. The majority of this investigative work has taken place in solutions which have additions of a ammonia and thus the pH in most cases lies in the range 8–11. Several workers [16,17,20] have claimed that additions of ammonia also produces complexes and influences the composition of the deposited alloy and improved cathodic efficiency, others [18,21] have demonstrated that deposits of Ni–W alloys can be produced from baths maintained at pH <7 and without ammonia present. If nickel, tungsten and citrate concentrations are raised and deposition conditions are controlled, i.e. reducing current densities (0.5–1.5 A dm−2 ), Sridhar et al. [19] have shown that a wider range of coating compositions with W contents up to 75 at.% can be produced. Whilst the major effort on the development of electrodeposited Ni–W, Ni–Mo and Co–W alloys has concentrated on solution chemistry, deposition efficiency and alloy composition have been shown to be important. Structural matters relating to the deposits have been gaining attention with the realisation that alloy composition and microstructure are together responsible for gaining optimum tribological and corrosion properties. In the period up to the turn of the century most efforts were focussed on obtaining amorphous deposits, as in this condition many coatings could produce superior properties. Donten et al. [22,23] reviewed this approach and indicated that several XRD studies had demonstrated that these coatings possessed broad peaks of low intensity. Some researchers claimed that such peaks were not necessarily proof of perfect amorphicity. Such coatings particularly those containing >20 at.% W or Mo had problems with residual stresses and adhesion to certain substrates. It was also claimed these deposits could contain up to 1.5 at.% boron from the presence of boric acid in many plating solutions and this encouraged the amorphous condition and enhanced mechanical properties. More recent structural studies [24] initially by XRD have demonstrated the existence of crystallinity with sharp

peaks replacing broad peaks in alloys containing <20 at.% W. Schuh et al. [24–29] using more advanced techniques such as a three dimensional atom probe (3DAP) has begun to gain important new evidence of nanoscale microstructures in electrodeposited Ni–W alloys in the composition range 10–20 at.% W. With more evidence of this kind as presented in this paper it is becoming possible to link solution chemistry, electrochemical kinetics and fine microstructure observations together and thus optimise alloy compositions and microstructures to produce coatings with outstanding properties. This paper describes efforts to link the above factors together in a more direct and positive way and then use them to demonstrate how a more wear and corrosion resistant coating can be produced without the need for post-deposition heat treatment. This effort is being concentrated on the development of Co–W bath using gluconate rather than citrate as a complexant. Co has been chosen as the iron group metal because it can have superior tribological properties when it possesses a close packed hexagonal crystal structure and a nanoscale microstructure [30–33]. Gluconate was selected instead of citrate as earlier unpublished work by the authors had demonstrated that it stabilised the solution over a wider range of pH values. The influence of bath chemistry and operating conditions on the electrochemical behaviour of the cobalt sulphate–sodium tungstate–sodium gluconate bath operating at pH 6 will be studied without the use of ammonia, which is not well accepted in the plating industry. This will involve cyclic voltammetry, potentiostatic and galvanostatic deposition techniques and the use of UV–visible spectrometry to assess complexes which exist in the plating bath. The results obtained from these techniques are then correlated with the microstructures of the deposits and their properties. 2. Experimental 2.1. Bath compositions for cyclic voltammetry, potentiostatic and galvanostatic deposition The choice of composition for the Co–W baths was based in part on the experimental results of Gileadi et al. [16–18] relating to nickel to tungstate ratios in Ni–W baths, together with the limited amount of work carried out by others [34,35] on the use of sodium gluconate as a complexant in cobalt alloy baths as well as the need to exclude ammonia. Gluconate electrolytes are non-toxic and it is claimed that they cause no adverse effects on the environment. The aim of the exercise was to produce deposits with <25 at.% W from baths containing low concentrations of Co and tungstate at deposition efficiencies of ∼50%. The main compositional variable in the bath was to be the sodium gluconate concentration, i.e. at a level equal to the combined concentrations of Ni and tungstate and at a level five times this concentration. Sodium chloride was added to enhance the conductivity of the bath and boric acid was present as a buffer. Baths 1–7, given in Table 1, were studied in duplicate tests by cyclic voltammetry. All baths were prepared by combining all chemicals in a one litre volumetric flask and adding deionised water from an Elgastat Option 3 water purifier to one litre. All chemicals

Table 1 Co–W bath compositions examined by cyclic voltammetry and UV/visible spectroscopy. Constituents

CoSO4 (M) Na2 WO4 (M) H3 BO3 (M) NaCl (M) Na gluconate (M)

Bath number 1

2

3

4

5

6

7

8

9

– – 0.65 0.51 0.55

0.053 – 0.65 0.51 –

0.053 – 0.65 0.51 0.11

0.053 – 0.65 0.51 0.55

– 0.050 0.65 0.51 0.55

0.053 0.050 0.65 0.51 0.11

0.053 0.050 0.65 0.51 0.55

– 0.050 0.65 0.51 0.11

– 0.050 0.65 0.51 –

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were ACS grade supplied by Sigma–Aldrich. All baths were adjusted to pH 6 using NaOH pellets. Bath temperature was maintained at 80 ± 2 ◦ C. 2.2. Cyclic voltammetry All experiments were performed in a water jacketed reaction vessel holding 50 cm3 of solution. The working electrode was a mild steel disc contained in a plastic holder with a surface area of 3.15 mm2 exposed and a Pt wire counter electrode. The steel electrode was ground and polished to a 1 ␮m, degreased and rinsed in deionised water. The reference electrode was Ag/AgCl in a concentrated KCl solution and the reaction vessel was kept at a controlled temperature of 80 ± 2 ◦ C. All voltammetry plots were recorded using a Voltalab10 Modular Potentiostat at a sweep rate of 20 mV s−1 , thus providing evidence of the cathodic processes taking place on the mild steel electrode in a group of bath compositions. 2.3. UV/visible spectroscopy UV/visible spectra were recorded for each of the solutions for Baths 2–9 given in Table 1 using a Perkin Elmer UV/visible spectrometer lambda 16 in the wavelength range 900–190 nm. 2.4. Potentiostatic electrodeposition These experiments were performed on Bath 7 using the EDT ECP1354 Modular Potentiostat with an Ag/AgCl reference electrode. The counter electrode was a Pt flag and the working electrode was a piece of mild steel of 1 cm2 surface area, coated with a 1–2 ␮m thick Co strike layer produced from Bath 3. Bath electrodes were held in a vertical plane. Deposition of the alloy was performed under quiescent conditions at −800, −850 and −900 mV and at −800, −850, −900, −950 and −1000 mV with vigorous agitation by air. Deposition was allowed to proceed for ∼2 h, at a bath temperature of 80 ± 2 ◦ C, whilst the current was measured continuously. 2.5. Galvanostatic electrodeposition Previous work [7] on Co–W deposition had used galvanostatic techniques. Galvanic deposition was performed using Baths 6 and 7 on mild steel ground sheet (BS080A15) over an area of 4 cm2 . The plates were subjected to a cleaning process before pickling in 10% H2 SO4 to destroy any residual base. A thin Co strike layer was then applied to this cleaned substrate from Bath 3, before deposition of the alloy. The alloys were then galvanostatically plated at 2.5, 3.125, 3.75 and 5.0 A dm−2 for 2 h. The current densities were chosen to relate to those observed in the potentiostatic deposition experiments. All deposits were produced galvanostatically using an ACM Instruments DSP 300W model potentiostat/galvanostat. A commercial inert iridium oxide coated platinised titanium mesh (70 mm × 100 mm) was used as the anode instead of platinum sheet as the bath was about to be considered for industrial use. The temperature of all baths was maintained at 80 ± 2 ◦ C by immersion in a water bath. Throughout the deposition process the solutions were constantly agitated using an air sparge. The efficiency of deposition was also measured under galvanic condition on Bath 7. Three sequentially plated samples were produced at a CD of 2.7 A dm−2 , each for a period of 2 h. The samples were weighed before and after deposition and the deposit was analysed by SEM/EDX. 2.6. Characterisation All coatings were examined in cross-section in a Philips XL30 environmental scanning electron microscope fitted with a field

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emission gun (ESEM FEG) and an energy dispersive X-ray (EDX) analyser. Several position were analysed in each cross-section. Coatings were also submitted to X-ray diffraction studies on a Siemens D500 diffractometer for structural analysis using Cu K␣ radiation scanning a 2Â range of 10–100◦ at a step size of 0.05◦ and a dwell time of 2 s. Selected coatings were examined in cross-section by transmission electron microscopy (TEM) using a JEOL 2000FX microscope. Samples for TEM were mounted in cross-section and thinned using an ion beam miller. Hardness measurements were made on coating cross-sections using a LECO-400 Vickers microhardness indenter under a load of 25 gf for 15 s. Six measurements were made on each cross-section. 3. Results 3.1. Cyclic voltammetry Fig. 1(a)–(c) shows cyclic voltammograms recorded on a mild steel electrode for Baths 1–7 (Table 1) at 80 ◦ C and pH 6. Current density values are based on the surface area of the ground and polished cathode which was replaced for each test. The positive limit of the steel electrode is −550 mV vs Ag/AgCl, a potential where the current was zero and no alloy was deposited. Corrosion of the iron commences at potentials positive of −550 mV. Voltammograms obtained on Baths 1, 2 and 5 are shown in Fig. 1(a). Bath 1 with the base solution of boric acid, sodium chloride and 0.55 M sodium gluconate showed evidence of a small current density flowing up to −850 mV before increasing as the potential became more negative and hydrogen evolution increased. Bath 2 contained 0.053 M CoSO4 but no gluconate showed little evidence of a change in current density up to −670 mV, at which stage it changed rapidly to give a peak value of 1.5 A dm−2 at −700 mV. This was interpreted as being due to Co2+ reduction. Bath 5 was similar to Bath 1 except that 0.05 M sodium tungstate was added and the voltammogram took on a similar form to Bath 1 indicating that the tungstate had little effect on this particular bath. Comparisons were made between Baths 3 and 4 which have different amounts of the complexant gluconate present. Fig. 1(b) demonstrates the effect of gluconate on the reduction of Co2+ which was observed in gluconate free Bath 2. The addition of 0.11 M gluconate shifted the rapid rise in current density to a slightly more negative potential and a marginally smaller peak at −720 mV. Raising the gluconate to 0.55 M in Bath 4 slightly increased the current density at potentials less than −670 mV but then prevented the formation of a distinct Co2+ reduction peak although the current densities did increase significantly beyond −750 mV. This has been brought about by the excess gluconate present in the solution effectively complexing the cobalt. The influence of cobalt and tungstate together were demonstrated in the voltammogarms for Baths 6 and 7, which were again studied at two different gluconate concentrations. Bath 6 with 0.11 M gluconate followed the pattern of Bath 3 except the rapid rise in current density now took place at −760 mV. On this occasion the peak height had increased to over 2.1 A dm−2 which is greater than the 1.2 A dm−2 measured on Bath 3. This would suggest that cobalt and tungstate had both been reduced to form an alloy under these conditions. In Bath 7 with 0.55 M gluconate present the plot follows the form of that found with Bath 4 which did not have a definitive peak. At −800 mV the current density had increased to 1.2 A dm−2 and to 2.0 A dm−2 at −850 mV. Both of these current densities were greater than those measured at similar potentials with Bath 4. Again this indicted that additional deposition had taken place when tungsten was present with cobalt in the bath.

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Fig. 1. Cyclic voltammeter plots on a series of plating baths of differing compositions, as specified in Table 1, held at pH 6 and 80 ± 2 ◦ C for the following baths: (a) Baths 1, 2 and 5, (b) Baths 3 and 4 and (c) Baths 6 and 7.

In an additional experiment with Bath 2, i.e. without gluconate, the addition of the tungstate ion resulted in the immediate formation of copious, pale-lilac coloured precipitate leaving a faintly pink coloured solution as the precipitate settled. This indicated the ability of the gluconate complexant in Bath 7 to maintain Co2+ and tungstate in solution at pH 6. 3.2. Ultra violet/visible light spectroscopy The significant UV/visible spectral peaks recorded for baths/solutions 1–9 are presented in Fig. 2(a)–(d) and in Table 2. Co2+ transitions were assigned to peaks in the range 450–650 nm, whilst the absorptions due to tungstate, boric acid and gluconate exist in the range 190–350 nm but have not been fully assigned and referred to as ‘others’. The spectra have been recorded to determine the effects of the increasing additions of gluconate to electrolytes containing Co2+ as in Baths 2–4 (see Fig. 2(a)). Addition of gluconate appeared to produce an increase in the intensity of the absorption due to the dd

transition ␭1 = 4 T1g (P) ← 4 T1g (F), and a shift in the wavelength of the associated peak from 511 to 535 nm. The low intensity, broad 2 electron ␭2 = 4 A2g ← 4 T1g (F) dd transition in the range 675–750 nm was similarly affected. Absorption peaks were assigned by reference to the appropriate Orgel diagram. With an addition of tungstate to Co–gluconate baths, as in the case of Co–0.55 M gluconate (Baths 4 and 7), there was a slight decrease in the intensity of the ␭1 transition and a slight shift to lower wavelength (Fig. 2(d)). The ␭2 transition was shifted similarly. Addition of tungstate to Co–0.11 M gluconate (Baths 3 and 6) gave an increase in the intensity of the ␭1 transition but no real change in position (Fig. 2(c)). In Bath 3 with 0.11 M gluconate, the absorption in the UV was more intense and covered a significantly wider range of wavelengths than the corresponding absorption for the 0.55 M gluconate Bath 4 (Fig. 2(a)). The effect of a gluconate addition to baths containing tungstate but no cobalt (Baths 9, 8 and 5) was to decrease the width and intensity of the absorption in the UV region of the spectrum (Fig. 2(b)).

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Fig. 2. (a–d) Ultraviolet/visible spectra obtained on baths numbered 2–9 as specified in Table 1. Peaks recorded for the Co2+ transitions are in the range 420–650 nm; those for tungstate, gluconate and boric acid are in the range 190–350 nm. Table 2 Wavelengths of absorption peaks obtained by UV/visible spectroscopy on Co–W baths. Bath number

Wavelength of 4 A2g ← 4 T1g (F) transition (nm) Wavelength of 4 T1g (P) ← 4 T1g (F) transition (nm) Wavelength of other transitions (nm)

1

2

3

4

5

6

7

8

9

– – 190–250

650 511 190–220

660 517 190–250

724 535 190–260

– – 190–310

660 517 190–400

718 532 190–300

– – 190–340

– – 190–370

3.3. Potentiostatic controlled electrodeposition Plating experiments were performed on Bath 7 at a series of controlled cathode potentials. Fig. 3 shows a consistent rise in current

Fig. 3. Variation in mean current density plotted against controlled cathodic potential for potentiostatic deposition in Bath 7 under agitated and quiescent conditions.

density with increasingly negative cathodic potentials for the baths operated with agitation and in quiescent conditions. The quiescent bath had a current density of approximately a half of that of the agitated bath in the potential range −800 to −900 mV. At more negative potentials, the current density in the quiescent bath tended towards those observed in the agitated bath, which was probably due to greater hydrogen evolution promoting local agitation. Fig. 4 shows a plot of W content in the coating against cathodic potential for both quiescent and agitated baths. At less negative potentials the W content of the deposit from the quiescent bath are much greater than those from the agitated bath, i.e. at −800 mV the difference was 6 at.% W and at −850 mV it was 9.5 at.% W. This difference is believed to be promoted by the kinetics of deposition as indicated by the current density, i.e. 1.2 A dm−2 (quiescent) and 2.1 A dm−2 (agitated) at −800 mV and 1.9 A dm−2 (quiescent) and 3.6 A dm−2 (agitated) at −850 mV. The rise in W content (to 25.5 at.%) continues in the quiescent case to −900 mV where the current density had increased to 2.7 A dm−2 . Thereafter the W content begins to fall to 21.5 at.% at −950 mV (current density 4.1 A dm−2 ) and is then maintained at −1000 mV (current density 6.1 A dm−2 ). In the agitated case the W content began to rise at a more negative potential than −850 mV, i.e. to 20.5 at.% at −900 mV,

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Fig. 4. Variations in the W content of coatings vs cathode potential produced under potentiostatic conditions from Bath 7 with 0.55 M gluconate in quiescent and agitated condition. The mean current densities measured at each potential are given on the plot.

22.5 at.% at −950 mV and 25 at.% at −1000 mV with increasing current densities of 4.8, 5.7 and 6.7 A dm−2 , respectively. A selection of XRD patterns obtained from the potentiostatically deposited coatings are shown in Fig. 5(a) and (b) and these demonstrate changes in the structure of the deposit from the quiescent and agitated baths as the potential changed between −800 and −900 mV, i.e. from crystalline to amorphous. The onset of amorphous deposits occurred at less negative potentials in the quiescent case just above −800 mV, whilst in the agitated case it started occurring close to −850 mV. In both cases the W content increases with change in potential as the amorphous deposit starts to form. Beyond −900 mV where the W content of the deposit from the quiescent bath started to fall to 21.5 at.%, it maintains its amorphous

state up to −1000 mV. This behaviour is probably influenced by the supply of W in the quiescent condition by the higher current densities. With the agitated bath the W content continues to rise at more negative potentials than −900 mV to reach 25 at.% W at −1000 mV as the agitation helps supply W to the cathode even at a current density of 6.7 A dm−2 . XRD provides evidence of a strong texture in crystal structures which are based on the cph Co lattice which was formed under quiescent and agitated conditions. The positioning of all the peaks has been modified by the presence of W in solid solution in the Co. There is no evidence of diffraction lines, other than those from the Co cph lattice, relating to other phases or compounds. The deposits tended to grow with a strong preferred orientation which was found to vary with the deposition conditions. For example, from the quiescent bath, the deposit favoured a {1 0 0} texture at −800 mV (Fig. 5(a)) but from the agitated bath the deposit had both {1 0 0} and {1 0 1} textures present with the former showing greater intensity at −800 and −850 mV (Fig. 5(b)). Fig. 6 shows the SEM images of the surface of the Co–W coatings produced potentiostatically under agitated conditions at different cathodic potentials. At −800 mV the coating had large growths, 20–30 ␮m in size, with internal facets with an apparent crystal size of 2–3 ␮m. On moving to the more cathodic potential of −850 mV the growths appear to be smaller, i.e. 10–15 ␮m in size with submicron facets. Beyond and at −900 mV the coatings continued to display 10–15 ␮m growths but now with smooth non-faceted topography which is associated with the amorphous state. A plot of microhardness against potential (see Fig. 7) demonstrates a similar trend for deposits produced from both quiescent and agitated baths, with the high hardness values associated with deposition at less negative deposition potentials and the presence of the crystalline state. In both cases, increasingly negative potentials are associated with a steady decrease in hardness, even though the W content had increased up to 25 at.%. The major factor for the hardness reduction appears to be the increasing presence of the amorphous phase. In Fig. 7 it is clear that hard crystalline coatings are produced over a wider potential range in the agitated bath than in quiescent conditions, which could be an important factor in the operation of commercial baths. 3.4. Galvanostatic electrodeposition

Fig. 5. XRD plot obtained on coating produced under potentiostatic control with (a) quiescent and (b) agitated conditions from Bath 7 (0.55 M gluconate) at 80 ◦ C. Crystalline coatings were produced at potentials of −800 mV when quiescent and −800 and −850 mV when the bath was agitated.

The current density range chosen, 2.5–5 A dm−2 , related to the potentiostatic deposition experiments carried out in the range −800 to −900 mV with the same degree of agitation being imposed on the baths. Plots showing variations of W content of coatings produced from Baths 6 (0.11 M gluconate) and 7 (0.55 M gluconate) with variations in current density are shown in Fig. 8. In both cases the W content remains low at current densities up to 3 A dm−2 but then increases at 3.75 A dm−2 and remained high as current density increased further. The variation of W content with current density is similar to that produced potentiostatically with agitation. However, there is a difference in W content at each current density between coatings produced from Baths 6 and 7. The coatings from the low gluconate bath had the lower W contents, e.g. 13 at.% at 2.5 A dm−2 compared with 18 at.% W from the coating produced from the bath with the higher gluconate content. However, coatings produced at 3.75 A dm−2 from the high gluconate bath had W contents increased from 16 to 24 at.%, whilst coatings from the 0.11 M gluconate bath only showed increases from 11 to 16 at.% W. The stepped increase at 3.75 A dm−2 for the low gluconate bath keeps the W content of deposits below 20 at.%. XRD patterns of coatings produced from Bath 7 are shown in Fig. 9(a). At low current density, 2.5 and 3.125 A dm−2 , the coatings were crystalline with W contents between 16 and 18 at.%, but at 3.75 and 5 A dm−2 the coatings contained >24 at.% W and were

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Fig. 7. Microhardness plotted against cathode potential for coatings produced under potentiostatic control with both agitated and quiescent conditions from Bath 7 at 80 ◦ C. A dividing region of mixed structure is shown which separates the crystalline and amorphous coatings as determined by XRD (see Fig. 5(a) and (b)).

Fig. 8. Variation in W content with current density for coatings produced under galvanostatic control from Baths 6 and 7 under agitated conditions at 80 ◦ C showing a step in W content at and above 3 A dm−2 .

Fig. 6. Changes in topography as shown by SEM photographs on the surfaces of potentiostatic controlled coatings produced from Bath 7 (0.55 M gluconate) with increasingly negative potentials at 80 ◦ C when agitated.

amorphous. The XRD patterns from the coatings produced from the 0.11 M gluconate Bath 6 showed that all were crystalline and based on the cph Co lattice (Fig. 9(b)). It was noted that none of these coatings had a W content greater than 16 at.% W. Changes in the major diffraction line intensity between samples can be attributed to the different orientations of the preferred crystal growth from {0 0 2} at 2.5 A dm−2 to {1 0 1} at 3.125 and 3.75 A dm−2 to {1 0 0} at 5 A dm−2 with the coatings from the low gluconate Bath 6 (Fig. 9(b)). SEM images of the surfaces of coatings produced at current densities up to 3.125 A dm2 from a bath containing 0.55 M gluconate showed the presence of crystallite facets (see Fig. 10(a) and (b)). Similar facets were found on deposits produced over a wider current range (2.5–5 A dm−2 ) from the 0.11 M gluconate bath (see Fig. 10(e)–(h)). This confirms the XRD evidence of the incidence of crystallinity in each of these coatings. At and above 3.75 A dm−2 the

Fig. 9. XRD plots of obtained on coatings produced under galvanostatic control from (a) Bath 7 (0.55 M gluconate) and (b) Bath 6 (0.11 M gluconate) at 80 ◦ C.

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Fig. 10. Surface topography of coatings produced under galvanical control at the current densities specified. Deposits from the higher gluconate Bath 7 show fine crystal facets in the low current density range (a) and (b): then change to amorphous growths above 3.75 A dm−2 (c) and (d). Crystalline facets are to be found over the complete current density range (e–h) for the low gluconate Bath 6.

Fig. 11. SEM images of coating cross-sections produced from (a) Bath 7 (0.55 M gluconate) at 2.7 A dm−2 and (b) Bath 6 (0.11 M gluconate) at 5 A dm−2 under galvanostatic control at 80 ◦ C after 1 h plating time. Note the even uniform coating produced from the higher gluconate containing bath.

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3.5. TEM analysis of selected coatings

Fig. 12. Variation of microhardness with current density for coatings produced under galvanostatic control from Bath 6 (0.11 M gluconate) and Bath 7 (0.55 M gluconate) at 80 ◦ C.

topography of the coatings from the high gluconate baths changes to an amorphous growth form. Cross-sections of the coatings with 16 at.% W produced from both baths showed up differences in growth behaviour (see Fig. 11). The high gluconate bath produced dense deposits with smooth flat surface finishes whilst the low gluconate bath encouraged irregular growth with undulating surfaces. Efficiency measurements were made on a series of coatings produced from the same high gluconate bath at a current density of 2.7 A dm−2 . An efficiency value of 48.8 ± 3.9% was obtained on the coatings which were crystalline and contained 17.2 ± 1.6 at.% W. The microhardness of the coatings produced from the two baths differs as shown in Fig. 12. Coatings from the 0.55 M gluconate bath followed the microhardness pattern observed with the agitated potentiostatic controlled coatings (see Fig. 7), i.e. hardness remains high (>800 kgf mm−2 ) at low current densities but then falls as the current density exceeds 3.25 A dm−2 due to change from crystalline to amorphous growth. Whilst the coatings from the 0.11 M gluconate bath demonstrated an increase in hardness over the whole current density range, despite all of these coatings being crystalline their hardness did not exceed 750 kgf mm−2 . It should be noted that the tungsten content of the coatings from the high gluconate bath produced just below 3.25 A dm−2 and from the low gluconate bath at 5 A dm−2 and above contained similar tungsten contents.

Samples were prepared by ion milling three coatings, two of these coatings were deposited at a current density of 5 A dm−2 from Baths 6 and 7, and a third coating was obtained from Bath 7 operating at a current density of 2.7 A dm−2 , all were produced under agitated galvanostatic conditions. Fig. 13 shows the crosssectional TEM image and the related electron diffraction pattern obtained from a coating produced from Bath 7 at 5 A dm−2 . Here the electron diffraction pattern shows diffuse rings characteristic of an amorphous phase. This evidence together with the broad XRD peaks and non-faceted growths found in the coatings produced at more negative potentials (high current density) in Fig. 6 gives support to the transition from crystalline to amorphous deposits which occurs at high current densities with the high gluconate Bath 7. The cross-sectional TEM image and electron diffraction pattern from the coating produced at 5 A dm−2 from Bath 6 is shown in Fig. 14. The images showed a variety of nano-crystallites sizes, some were equiaxed and ∼5 nm in size, but others were elongated crystallites, 5 nm width but up to 50 nm long. Fig. 15 has a cross-sectional image of a coating produced from Bath 7 at 2.7 A dm−2 . The coating consisted entirely of highly oriented rods or sheets which are ∼5 nm in thickness. This evidence of rod/sheet alignment can be combined with that given by the XRD pattern in Fig. 9(a), and a TEM diffraction pattern produced by centring the electron beam on one rod whilst showing diffraction from three adjacent rods/sheets. Hence, the higher hardness values obtained on these samples, when compared with the high current density deposits produced from the low gluconate bath, can be accounted for by the highly oriented nanostructure even though they have similar tungsten contents. 4. Discussion Initially the discussion will deal with the voltammetry and UV spectroscopy results relating to the role of the gluconate complexant in Co containing solutions and how the addition of tungstate to the solution influences that behaviour. Then information gained from the potentiostatic controlled deposition from a 0.55 M gluconate solution under quiescent and agitated condition will be analysed in terms of the solution chemistry and the resultant structure and composition of the deposits. Structure–property relationships will be discussed in terms of the recent findings about nanostructures in associated Ni–W electrodeposits and finally how the growth of nanostructures and the onset of amorphous deposits

Fig. 13. Bright field TEM micrograph of a thinned cross-section of an amorphous Co–W alloy deposit produced under galvanostatic control from Bath 7 at 80 ◦ C using a current density of 5 A dm−2 . Note the diffuse rings in the diffraction pattern.

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Fig. 14. Cross-sectional TEM image and electron diffraction pattern from the coating produced at 5 A dm−2 from Bath 6 showing numerous nano-crystallites of varying size and shape.

may be influenced by the presence of various complexes in solution and the rate of electrodeposition.

4.1. Influence of gluconate on the deposition of Co The voltammetry plots (Fig. 1(a) and (b)) demonstrate how Co2+ ions behave in solution in the absence of gluconate ion (Bath 2). The onset of Co deposition occurs at the less negative potential of −670 mV than the hydrogen evolution reaction observed in the cobalt free bath (Bath 1). Deposition of cobalt with a peak current density of 1.5 A dm−2 at −700 mV. With 0.11 M gluconate present (Bath 3), the onset of Co deposition is shifted to a slightly more negative potential than in Bath 2 and with a reduced peak current density. This is attributed to the formation of complexes of Co2+ and gluconate, which is consistent with the calculations on speciation made using stability constants [36–38] as plotted in Fig. 16(a). It shows that free Co2+ is removed as pH of the bath rises above 4 and is replaced by a Co–gluconate complex which reaches a peak at pH 6. Bath 4 is used to demonstrate the effect of a further addition of gluconate up to 0.55 M on the deposition of Co. Here with the removal of the peak associated with Co2+ reduction the current density remains below that achieved in Bath 3 until the potential reaches −800 mV where a

crossover takes place (see Fig. 1(b)). This is due to the presence of more highly complexed cobalt species as the ratio of gluconate to Co2+ reaches 10:1. The calculated stability constant data for Bath 4 is shown in Fig. 16(b) which predicts that all the Co2+ is complexed at pH 6, where [Co(C6 H11 O7 )2 (C6 H10 O7 )]2− accounts for 99% of the Co2+ and [Co(C6 H11 O7 )2 (C6 H9 O7 )]3− for the remaining 1%. The effect of gluconate additions to a solution of Co2+ , NaCl and H3 BO3 (Baths 2–4) at pH 6 is shown in the UV/visible spectra in Fig. 2(a). The Co2+ absorption at 510 nm in Bath 2 increases in intensity and wavelength with an increased gluconate addition due to the presence of increasingly complex Co–gluc species which supports the evidence given by the stability constant calculations.

4.2. The introduction of tungstate in the Co–gluconate baths It should be noted that the chemical behaviour of the tungstate ion in solutions in the pH range 5–7.8 is complex, where equilibria can involve WO4 2− , W6 O20 (OH)2 6− , W7 O24 6− , HW7 O24 5− and H2 W12 O42 10− and several cobalt polytungstates have been reported in the literature, e.g. [Co2 W11 O40 H2 ]8− . UV/visible spectroscopy on Bath 9 which contained no gluconate gave a strong absorption in the UV spectrum between 190 and ∼360 nm. This is

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Fig. 15. TEM of thinned cross-section of a coating produce from Bath 7 under galvanostatic control at a current density of 2.7 A dm−2 at 80 ◦ C. Bright field images together with an electron diffraction pattern, providing evidence of fine 5–6 nm thick crystals which have grown with a strong preferred orientation.

believed to be due to absorption from a complex set of equilibria involving many of the species mentioned above. However, absorption is greatly reduced in band width when 0.11 M gluconate is added (Bath 8) and also at 0.55 M (Bath 5). This observation implies that tungstate bonding could be much simpler in the presence of gluconate, e.g. it may reduce the tendency to form para- and metatungstates. The onset of deposition in Bath 6 with 0.11 M gluconate and tungstate occurs at −760 mV compared to −680 mV in Bath 3 without tungstate. This implies that that the addition of tungstate has made the deposition more difficult and that the reduction of Co2+ ion is now being impeded further. Here the tungstate may be acting as a ligand for Co2+ ion and the resulting complex with gluconate required a more negative potential for deposition. As no cobalt tungstate precipitation is observed in Bath 6, it is reasonable to assume that when tungstate acts as a ligand for Co2+ ion in a complex it must also contain gluconate, i.e. [Co–gluc–WO4 2− ]. The formation of analogous species in Ni–W–citrate baths has been suggested by Gileadi et al. [14–19]. According to the stability constant data cobalt is present as 46% Co2+ “free” ion, 40% [Co(C6 H11 O7 )2 (C6 H10 O7 )]2− and 14% [Co(C6 H10 O7 )] in the tungstate free Bath 3. Besides the shift of potential to more negative values the voltammogram for Bath 6 shows a much higher current peak of 2.1 A dm−2 . Thus the addition of tungstate results in the formation of significant quantities of [Co–gluc–WO4 2− ], thus

reducing the quantity of free Co2+ and higher gluconate complexes and promoting the formation of the alloy deposit. In the bath containing 0.55 M gluconate (Bath 7) the onset of deposition occurs at potentials less than −670 mV but with a slightly enhanced current density similar to the pattern which similar to the pattern observed in the tungstate free bath (Bath 4). As the potential becomes more negative no major current density peak was observed which was certainly the case with the lower gluconate containing Bath 6. Instead the current density remains below that obtained with Bath 6 until at a potential close to −900 mV it crosses over the plot for the lower gluconate bath (see Fig. 1(c)). This follows the same pattern of behaviour as found with Baths 3 and 4, except that the crossover of the plots (Fig. 1(b)) occurs at more negative potentials in Baths 6 and 7. The differences in the voltammetry plots obtained on Baths 6 and 7 can be accounted for by changes in the complexes which were present. Stability constant data (see Fig. 16(a) and (b)) calculated for Baths 3 and 4 demonstrate the big differences in species which are present in the case of Bath 7 the majority of cobalt as [Co(C6 H11 O7 )2 (C6 H10 O7 )]2− with virtually no free Co2+ . So that the addition of tungstate in Bath 7 finds a reduced range of species to react with when compared with the addition of tungstate in Bath 6. As a consequence the current density measured in the voltammogram for Bath 7 is reduced below that measured on Bath 6 at potentials immediately more negative than −800 mV. This could arise because of less ternary

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0.05 M tungstate. A more likely explanation for the shift in the Co2+ ␭1 transition is the formation of a Co2+ complex which involves tungstate as a ligand on the Co, replacing some gluconate, e.g. [Co–gluc–WO4 2− ]. This ternary complex may prove to be different to that formed in Bath 6. 4.3. Potentiostatic electrodeposition in the 0.55 M gluconate bath

Fig. 16. Stability data showing the variation of calculated % ionic species with pH for two aqueous solutions containing (a) cobalt sulphate and 0.11 M sodium gluconate and (b) cobalt sulphate and 0.55 M sodium gluconate. Note the differences between plots (a) and (b) in the pH range 5–7.

[Co–gluc–WO4 2− ] complexes or the formation of a different form of ternary complex which may be more difficult to reduce. The fact that the current density in the voltammogram for Bath 7 rises again and crosses that obtained on Bath 6 at −900 mV might suggest that a different ternary complex had formed which is more difficult to reduce. Evidence for the involvement of ternary complexes containing tungstate comes from the comparison of the voltammograms obtained from Baths 3 and 6 and also from Baths 4 and 7 which show that current densities rise when tungstate is present. The evidence of a rise is much stronger with Bath 6. The addition of tungstate to Co–gluconate baths may now be addressed. When 0.05 M tungstate is added to Bath 3 to form Bath 6, the intensity of the Co2+ ␭1 transition increases. The absorption in the UV is still relatively broad, indicating that the effects of any polyand meta-tungstates formation merges with the Co2+ ␭1 transition (see Fig. 2(c)). The re-establishing of the poly- and meta-tungstates equilibrium is understandable in Bath 6 when it is considered that stability constant calculations show that in Bath 3 more than 75% of the gluconate present is acting as ligands for cobalt, hence there is insufficient gluconate available to affect significantly the formation of poly- and meta-tungstates. When the absorption due to complex tungstate equilibrium in the Co free Bath 5 is compared to Bath 6 the broader absorption in Bath 6 may be due to the formation of some hetero-polytungstate species containing cobalt. The effect of the addition of tungstate to the 0.55 M gluconate Bath 4 to form Bath 7 is more straightforward to explain. Here, the strong absorption in the UV is not broadened as much due to complex equilibrium formation because of the excess quantity of gluconate in the bath. The Co2+ ␭1 transition in Bath 7 occurs at 531 nm which is slightly shifted from 535 nm in Bath 4. Accompanying this shift, there is also a slight decrease in the intensity of this absorption. The large excess of gluconate in the bath means that the Co speciation is unlikely to be altered by the addition of

Potentiostatic plating experiments on the high gluconate Bath 7 under agitated conditions show almost a linear increase in current with increasingly negative potential (see Fig. 3). Under quiescent conditions the rate of current increases from −900 mV so that at −1000 mV the current density under quiescent and agitated conditions is almost identical, as hydrogen bubbling may have provided local agitation in the quiescent case. In the agitated Bath 7, high hardness coatings are produced at less negative potential, −800 to −850 mV, which contain 14–15 at.% W. At −900 mV there is a sudden increase in W content to >20 at.% and a marked decrease in hardness by ∼200 kgf mm−2 . The decrease in hardness is thought to be largely due to the transition from a crystalline to an amorphous structure. The crystalline lower W coatings give an XRD pattern which may be interpreted as a solid solution of W in hcp Co. The shift to an amorphous structure occurs as the W content of the coating tends towards 25 at.%. This is due to the baths inability to deposit the crystalline equilibrium phase, Co3 W, due to the larger unit cell of this intermetallic. The change in W content in the coatings could be indicative of the presence of both [Co–gluc] and [Co–gluc–WO4 2− ] species in solution decomposing at different rates dependent on the deposition potential. The voltammogram on Bath 7, Fig. 1(c), showed a more rapid rise in current density in the neighbourhood of −900 mV, which could be associated with the reduction of a second form of ternary complex ions. Under quiescent conditions, the only crystalline coating is produced at −800 mV. It is oriented about the {1 0 0} plane and contains a surprisingly high 21.4 at.% W, though still less than the 25 at.% associated with the intermetallic compound Co3 W. The sample is also the hardest produced under potentiostatic conditions, i.e. 925 kgf mm−2 . At −850 and −900 mV, there is an increase in W content, a change to an amorphous deposit and a corresponding decrease in hardness by ∼170 and 220 kgf mm−2 respectively. The supply of Co and Co–W complexes to the cathode in these quiescent conditions is controlled by diffusion of [Co–gluc] and [Co–gluc–WO4 2− ] species. At a more negative potentials beyond −900 mV the quiescent conditions are not able to maintain the concentration of [Co–gluc–WO4 2− ] and consequently the W content and hardness of the coatings decreases. This may lead to the more complexed Co–gluc species decomposing on a larger scale and this increases the relative rate of Co deposition. Under agitation, less W is deposited at lower potentials, which may be due to the higher current densities which have been recorded together with the more plentiful supply of [Co–gluc] and [Co–gluc–WO4 2− ] to the cathode and the preference for the decomposition of [Co–gluc] over [Co–gluc–WO4 2− ]. At more negative potentials the rate of reduction of [Co–gluc–WO4 2− ] increases more rapidly and more W is deposited provided there is sufficient agitation to encourage the necessary mass transport of this mixed complex to the cathode, as specified for the Ni–W system by Gileadi et al. [18,19]. 4.4. Galvanostatic electrodeposition in the 0.11 and 0.55 M gluconate baths Coatings produced galvanostatically from the high gluconate Bath 7 under agitated conditions, over an increasing range of cur-

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rent densities show increases in W content and a change to the amorphous state with a corresponding decrease in hardness. The pattern of behaviour is very similar to that produced by potentiostatic deposition with agitation. The slight mismatch in the shift from crystalline to amorphous coatings between galvanostatically and potentiostatically deposited coatings is attributed to changes in bath and working electrode geometry and especially differences in agitation. The SEM images of the surface of the galvanostatically produced coatings are also similar to those produced potentiostatically. Galvanostatic deposition was also obtained from an agitated low gluconate Bath 6 to give crystalline coatings over a wider range of current densities, i.e. up to 5 A dm−2 . The coatings were lower in W content (<16 at.%) and possess large crystalline facets as shown in Fig. 10(f) at intermediate current densities. This is thought to be due to the presence of a variety of reducible species (uncomplexed Co2+ and ternary Co–gluc–WO4 2− ) in the bath which permits larger crystalline growths than was the case with the high gluconate Bath 7. Crystalline deposits with large facets were also produced from Bath 6 at higher current densities containing 16 at.% W as shown in Fig. 10(g) and (h).

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the composition range 10–20 at.% W. An increase in hardness (H) is forecast by the Hall–Petch type relationship (H = H0 + ky d−1/2 ) with decreasing grain size and this suggests that it is the major contribution to the hardening of the deposit. Schuh et al. [25,27,28] has also gone on to consider the possibility of short range ordering of W as an alternative process within the grains. However, the controlled refinement of the Ni or Co grains as W supersaturation increases, which has been observed, would appear to demonstrate that boundary segregation of W is the more dominant process. Hardness measurements (giving values >850 kgf mm−2 ) have been used to show how these fine nanostructures can be produced from the highly complexed Bath 7 operating at measured efficiency levels close to 50%. The deposits which are dense and between 5 and 100 ␮m thick could complete with hard chromium coatings. Further work published elsewhere by the authors has demonstrated how other important engineering properties can be enhanced by these nanostructures, e.g. exceptional tribological properties [41] can be developed such as very low coefficients of friction (<0.15) and also highly corrosion resistant behaviour in aggressive conditions [42]. 4.6. Interactions between electrodeposition and microstructure

4.5. Microstructure–property relationships The electro-deposited Co–W alloy coatings with high hardness values need to be related to their microstructures, particularly in the composition range 10–25 at.% W. In these as-deposited coatings there is no evidence of the intermetallic compound, Co3 W, which is found in the Co–W thermal equilibrium diagram determined on cast and wrought alloys. Claims for the existence of this intermetallic in electrodeposits containing 20.3 at.% W have been made by Tsyntsaru et al. [39] but XRD work by Itoh et al. [40] supports our findings on Co–W electrodeposits. They have demonstrated a change from crystalline to amorphous deposits above 21.4 at.% W with no evidence of Co3 W formation. The major contribution to the hardness (450–650 kgf mm−2 ) of the 20–25 at.% W alloys comes from the metastable amorphous phase in which the tungsten atoms are distributed randomly. There is also evidence from SEM micrographs and XRD that mixtures of amorphous and crystalline phases exist in alloys with 20 at.% W. This could imply that some variations in W content exist between these phases, i.e. low W in the crystalline phase and high W in the amorphous regions. The presence of the crystalline phase in such a mixture has been shown by Itoh et al. [40] to encourage further growth at the expense of the amorphous phase during heat treatment in the temperature range 200–500 ◦ C. Transformation from amorphous to the intermetallic, Co3 W, only takes place a temperatures >600 ◦ C [7,40]. The production of crystalline deposits from the more highly complexed Bath 7 in the composition range 10–20 at.% W with even harder properties requires an explanation in terms the contribution of both grain size (Hall–Petch equation relating hardness to (grain size)−1/2 ) and solid solution hardening of W in highly orientated Co crystals. Observations by TEM confirm the presence of fine 5 nm wide grains which have formed in a highly oriented microstructure with no evidence of fine precipitates of a second phase which could have contributed to the hardness of the alloy. The recent work by Schuh et al. [24–29] made use of 3DAP analysis of Ni–W deposits that helps to understand the formation of these alloy microstructures. Besides the discovery of fine nano-grains, Schuh et al. also provides evidence of the nano-segregation of W at Ni grain boundaries. It has been theoretically shown this form of segregation can be more thermodynamically stable than a supersaturated solution of W in Ni. This brings into play a relationship between this nanosegregation of W and the size of the grains as the supersaturation of W increases. A decreasing grain size (d) results as the W supersaturation supplies excess W atoms for grain boundary segregation in

The recent discovery of nanostructures and segregation in Ni–W and Co–W coatings does raise the question of how the deposition process influences the microstructure. The presence of one or more complexed species together in the high gluconate electrolyte could lead to their combined reduction at or close to the cathode surface. This could help create a process of segregation as the final deposit forms, for example by the reduction of different complexed species in the 0.55 M gluconate bath delivering different adatom clusters at controlled rates on to the substrate surface. Time for migration of these adatom species on the substrate could then become a key issue in producing either nano-crystalline or amorphous deposits. At the slower deposition rates (lower current densities) such migration could occur to form the more stable nano-crystalline grains with W segregated at the boundaries. Whilst at high current densities where ternary complexes are reduced, migration does not take place in the shorter time interval available and thus the W containing adatoms remain in their as-deposited positions and produce a random distribution of excess W in the amorphous deposits. The growth plane of the deposit may also have a significant effect on microstructural development. The predominant plane of growth for the hard Co–W coatings is the {1 0 1} hcp plane, with the {1 0 1} plane most often secondary in intensity. The TEM observations made on deposits from the lower gluconate containing Bath 6 are also relevant to the discussion. In this bath with less complexing taking place, Co2+ ions are present and this dilutes the Co–tungstate–gluconate complexes available for reduction. The availability of these Co2+ ions permits larger growths of crystalline facets with lower tungsten contents to occur at intermediate current densities; then at higher current densities, i.e. 5 A dm−2 the coatings did contain more tungsten (16 at.%) and are harder (∼700 kgf mm−2 ). TEM images obtained on these harder deposits demonstrate the presence of nano-crystals many of which are equiaxed whilst others are rod or plate shaped. With the 0.11 M gluconate bath it would appear that a higher current density is required to deliver and reduce a sufficient number of Co–tungstate–gluconate complexes to raise the W content of the deposit and promote boundary segregation of W. This will then promote nano-crystal formation and hardening of the deposit. Hardness values of these coatings are still below those obtained from the 0.55 M gluconate Bath 7 which contain similar W contents but highly oriented and more uniform nano-crystals, encouraged by the kinetics of reduction of the complexed species which are available.

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5. Conclusions (1) By careful control of bath chemistry and plating conditions it is possible to obtain thick, defect-free Co–W coatings with high as-deposited hardness values approaching that of hard electrodeposited chromium. (2) The high hardness Co–W coatings are characterised as having highly oriented nano-crystalline deposits consisting of a solid solution of W in hcp Co together with a form of nanosegregation of W at the grain boundaries. (3) Analysis of voltammetry results obtained on low concentration Co and tungstate plating baths has demonstrated that it is important to provide a significant oversupply of the sodium gluconate complexant, i.e. five times the combined Co + tungstate concentration in order to gain high W containing deposits in the range 14–20 at.% at efficiencies close to 50%. (4) UV/visible absorption spectroscopy has provided evidence of a series of equilibria in the Co and W containing baths. There is evidence for the presence of tungstate–gluconate, cobalt–gluconate, cobalt–tungstate gluconate and cobalt–paratungstate–gluconate species, which depends upon bath chemistry. The presence of the mixed cobalt–paratungstate–gluconate is believed to be important for gaining high W alloy deposits at high deposition efficiencies. (5) The appropriate balance of complexed species in the bath appears to influence the formation of ultra-fine grain structures by promoting the nano-segregation of W at crystal boundaries. The segregation it is suggested arises from the optimised adsorption and redistribution of a variety of adatom clusters on the substrate. At low current densities the bath containing high gluconate allows appropriate clusters to migrate to boundaries to build up segregation which then promotes highly oriented fine 5 nm grains and coatings with high hardness values. High current densities do not allow sufficient migration during deposition from high gluconate bath and this leads to random distribution of W and softer amorphous deposits. Acknowledgements The authors would like to thank the partners of the N-COAT 70 project and the TSB for providing the funding for this work. In particular the contributions by Terry Hirst and colleagues at Goodrich Corporation and John Yellup at M4 Technologies Ltd. to the work described in this paper are gratefully acknowledged.

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