Wear 255 (2003) 917–923

Communication

Sliding wear behaviour of electrodeposited cobalt–tungstenand cobalt–tungsten–iron alloys

H. Capel, P.H. Shipway∗, S.J. HarrisSchool of Mechanical, Materials, Manufacturing Engineering and Management, Advanced Materials Group,

University of Nottingham, University Park, Nottingham NG7 2RD, UK

Abstract

Hard-chrome plate is traditionally employed as a wear resistant surface. However, concerns that the processing of such materials isnot environmentally friendly have lead to a search for potential replacements. This work examines the behaviour of two alloy systems,namely Co–W and Co–W–Fe. Alloys from these systems were electrodeposited onto steel substrates. The Co–W alloys were further heattreated to increase their hardness, although this was deemed unnecessary for the Co–W–Fe alloys which had high as-deposited hardnesses.The sliding wear behaviour of these materials was examined, along with the behaviour of a hard chrome under similar conditions. It wasfound that under some conditions, the Co–W alloys exhibited a wear resistance and a coefficient of friction similar to that of hard chrome.Moreover, the corrosion resistance of the Co–W alloys was higher than that of the hard-chrome plate. The Co–W–Fe alloys exhibited ahigh as-deposited hardness. At a critical iron level of 30 wt.%, the wear rate of these coatings decreased substantially. All the Co–W–Fecoatings were more corrosion resistant than the hard chrome; however, even the most wear resistant exhibited a high coefficient of frictionand more wear than the hard chrome.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Electroplating; Chrome replacement; Amorphous; Nanocrystalline

1. Introduction

Chromium electrodeposition is a well-established tech-nique for the production of decorative and functionalcoatings which possess attractive properties such as highhardness, excellent wear resistance and low coefficient offriction. Chromium coatings can also provide excellentprotection against corrosion. As a consequence, chromiumelectrodeposits have found extensive use, particularly fortheir resistance to wear, in a wide range of engineeringindustries. However, replacement of hard chromium elec-trodeposits is being actively pursued on environmentalgrounds. The primary concern with the processing of suchmaterials is that normally a hexavalent chromium solutionis employed in the electrodeposition bath, which produceslarge volumes of chrome contaminated toxic waste re-quiring special disposal methods[1]. Expensive breathingapparatus and exhaust systems must also be employed todeal with emissions during plating. Hexavalent chromiumin solution is also a recognised carcinogen and causes otherhealth problems such as skin and lung irritation[2]. A

∗ Corresponding author. Tel.:+44-115-951-3760;fax: +44-115-951-3764.E-mail address: philip.shipway@nottingham.ac.uk (P.H. Shipway).

further technical concern is associated with the fact thatchromium-plating baths are generally only∼15% efficientand that operators often have difficulty maintaining unifor-mity of coating thickness[3]. As a result, the plating indus-try has been forced to consider alternative materials and/orprocesses that do not involve hexavalent chromium[4–6].

The co-deposition of alloys of tungsten with one or moreof the iron group metals provides a number of electrode-posit systems that may be considered as possible replace-ments for conventional hard chromium electrodeposits. It iswell known that alloys such as Co–W/B and Ni–W/B arecharacterised by high surface hardness; Vickers hardnessvalues between 450 and 650 kgf mm−2 have been reportedfor these coatings in the as-deposited condition[6–8]. Itis also known that heat-treatment improves their hardnessfurther, to values exceeding that of hard-chrome electrode-posits. Tungsten-containing alloy coatings can be producedwith widely different compositions and microstructuresdepending on the composition of the plating bath and thedeposition parameters. Amorphous, crystalline and nanocrys-talline forms of Co–W and Ni–W electrodeposits have allbeen reported in the literature[9–11]. The development ofCo–W and Ni–W alloys with amorphous or nanocrystallinestructures is expected to be of particular future interestsince materials with such microstructures often possess

0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0043-1648(03)00241-2

918 H. Capel et al. / Wear 255 (2003) 917–923

better corrosion resistance and tribological properties whencompared to their crystalline counterparts.

The majority of the work reported in the literature forW-containing electrodeposits concerns the optimisation ofthe electroplating process, i.e. electroplating bath chemistryand operating controls, in order to produce high qualitycoatings. A number of studies have been conducted on theeffect of heat-treatment on the microstructure of Co–W andNi–W electrodeposits[6,12,13]and the change in hardnessof these coatings, with respect to ageing conditions, hasbeen well documented. However, whilst heat treatment ofother amorphous electro- and electroless plated materials toimprove wear resistance has been reported[14–19], there isa relative paucity of literature concerning the wear behaviourof Co–W and Ni–W based alloys[6]. If such alloys are to beconsidered as replacements for hard chrome in engineeringapplications, a detailed understanding of their tribologicalperformance and behaviour in corrosive environments mustbe gained. Whilst it is known that heat treatment can beused to improve the hardness of amorphous electrodepositedalloys, it is also known to embrittle them. The aim of thepresent investigation was therefore to examine the wearand corrosion resistance of amorphous/nanocrystalline elec-trodeposits in the Co–W system in both the as-depositedand heat-treated conditions. Alloys in the Co–W–Fe systemwere also examined which, with higher as-deposited hard-nesses, were hoped could be competitive materials withoutheat treatment. The protective properties of hard-chromeelectrodeposited onto mild steel were also measured forcomparison.

2. Experimental details

2.1. Electrodeposition

Polished mild steel sheet (BS080A15 with a hardnessof 225 kgf mm−2 when measured with a 200 gf indentationforce) was used as the substrate material. The plating areaof each specimen was approximately 30 cm2. All specimenswere subjected to a series of cleaning stages and finallyrinsed in dilute 10% H2SO4 to remove any residual alkaliand to activate the surface for electrodeposition. Electrode-position of Co–W was carried out in two ways: (a) directlyonto the mild steel substrate and (b) on top of a thin cobaltstrike applied to the substrate prior to alloy deposition. Alldeposits were produced using an ACM Instruments AutoACDSP 300W model potentiostat/galvanostat, under galvanos-tatic conditions. An iridium oxide coated platinised titaniummesh (70 mm× 100 mm) formed the anode. The platingbath used for the deposition of the cobalt strike was simply25 g l−1 cobalt sulphate in distilled water. The temperaturewas maintained at 80◦C and the plating time was 2 minutesThe Co–W alloy plating bath composition was as follows:15.0 g l−1 cobalt sulphate heptahydrate, 16.5 g l−1 sodiumtungstate dihydrate, 40.0 g l−1 boric acid, 110.0 g l−1 sodium

gluconate, 30.0 g l−1 sodium chloride. The bath was main-tained at pH 6.0, controlled by the addition of either NaOHor H2SO4, and the volume of the bath was adjusted, duringapproximately 6 h of plating, by additions of distilled water.Boric acid was added to the bath since it has been shownthat small amounts of boron co-deposited with the Co–Walloy promotes an amorphous structure, and hence the ex-cellent properties that are characteristic of such materials,such as high hardness[20]. Co–W–Fe electrodeposits ofthree different alloy compositions were also produced fromthe same bath with additions of ferrous sulphate heptahy-drate. The addition of 3.48, 6.95 and 13.9 g l−1 of ferroussulphate produced coatings with iron contents of 15, 23 and30 wt.%, respectively. The cobalt content of the coatingsdecreased as the iron content increased whilst the tungstencontent remained approximately constant. Electrodepositionof the Co–W and Co–W–Fe alloys was carried out at an ap-plied current density of 5 A dm−2 at a constant temperatureof 80◦C. The standard plating time was 1 h to produce adeposit which was approximately 20�m thick.

Hard-chrome plated onto mild steel specimens preparedin the same way as described above was performed commer-cially, yielding a hard-chrome layer of approximately 20�min thickness.

2.2. Heat treatment

Co–W alloy coatings were heat treated in an argon in-ert atmosphere in a tube furnace. Samples were placed inthe furnace at room temperature, which was then heated tothe annealing temperature at a constant rate of 20◦C min−1.Once the annealing temperature of 600◦C was reached thesamples were aged for 2 h. Work conducted by the authorshad previously shown that these conditions produced coat-ings of optimum hardness[21]. A titanium powder getterwas placed in the furnace alongside each of the samples toremove any residual oxygen remaining in the furnace andthereby prevent oxidation of the Co–W alloy.

2.3. Wear testing

The wear performance of the electrodeposited coat-ings was examined using a BICERI universal wear tester.Ball-on-plate geometry was employed with a reciprocatingsliding action in simple harmonic motion driven by a crankand wheel arrangement (seeFig. 1). The counterface was amartensitic stainless steel (AISI 440C) ball with a diameterof 25.4 mm (Dejay Distribution Ltd., Wokingham, UK) anda hardness of 739 kgf mm−2 (20 kg hardness test load). Allof the wear experiments were conducted under dry slidingconditions at ambient temperature (21–23◦C). The ampli-tude of the motion was 11 mm giving a sliding distance of44 mm per cycle. The frequency was 158 cycles per minuteand the duration of testing was 4000 cycles. Any point onthe coating experienced the passage of the slider twice percycle with a velocity which depended upon the position

H. Capel et al. / Wear 255 (2003) 917–923 919

Fig. 1. Schematic diagram of wear testing apparatus.

on the wear track. The normal load on the coated samples,applied by adding weights to the loading arm on the equip-ment, was approximately 61 N in most tests, although asmall number of tests were carried out with the higher loadof 162 N.

Following the wear tests, the depth of the wear tracksproduced was measured using a Surfcom profilometer (Ad-vanced Metrology Systems Ltd., Leicester, UK). The aver-age steady state coefficient of friction of each sample wasalso calculated from data collected during the wear tests.

2.4. Corrosion testing

The breakdown in corrosion resistance of the plated sam-ples was measured as the time until the first spots of redrust appeared on the surface of samples placed in a neutralsalt spray cabinet working to ASTM B117. In the case ofthe iron-containing deposits, some corrosion of the depositsthemselves occurred producing a corrosion product whichlooked similar to that associated with substrate corrosion.These were distinguished by cross-sectional microscopy fol-lowing testing. Also, it was noted that whilst corrosion prod-uct from the electrodeposits occurred in the early stages ofthe salt-spray tests, it did not tend to increase with furthertime in the cabinet. Once substrate corrosion occurred, ittended to proceed rapidly.

2.5. Characterization

Coatings were examined in a Philips XL30 environmen-tal scanning electron microscope fitted with a field emissiongun (ESEM FEG) and energy dispersive X-ray (EDX) anal-yser. Coatings were examined before and after wear testing.Cross-sections were taken through the wear tracks, parallelto the direction of sliding. The top surfaces of some of theworn coatings were also examined. EDX analysis was con-ducted on the wear debris and the transfer layer to identifyits composition. X-ray diffraction (XRD) was conducted ona Siemens D500 Diffractometer for structural and composi-tional analysis using Cu K� radiation, scanning a 2θ range of10–100◦ at a step size of 0.05◦ and a dwell time of 2 s. Hard-

ness measurements were made on coating cross-sections us-ing a Leco-400 Vickers microhardness indenter, under a loadof 25 g for 15 s. A small load had to be used when makingthese measurements in order to prevent damage to the thincoatings. Average hardness values were estimated by mak-ing 20 indents on each sample.

3. Results and discussion

3.1. Coating characteristics

The Co–W and Co–W–Fe deposits were in all casessemi-bright and light grey in colour. All the deposits had anaverage thickness of 20�m. For the Co–W alloys, EDX andXRF measurements revealed an average alloy compositionof Co–53 wt.% W (Co–27 at.% W), which was constantthroughout the thickness of all deposits, both with andwithout the intermediate cobalt layer. A very small amountof boron was detected in the deposits (0.05 wt.%). For theCo–W–Fe alloys, the tungsten content remained approxi-mately constant at 53 wt.%. As the concentration of ferroussulphate in the bath increased, then the iron concentration inthe coatings increased; coatings with iron contents of 15, 23and 30 wt.% were produced, with the balance being cobalt.A cross-sectional micrograph of a typical as-deposited coat-ing is shown inFig. 2. XRD showed that all coatings werein the amorphous/nanocrystalline state on deposition.

The hardnesses of the iron-containing alloys were higherof those of the plain Co–W deposits. The increase is small forthe two lower levels of iron; however, there is a significant in-crease from 605 kgf mm−2 for 23 wt.% Fe to 838 kgf mm−2

for 30 wt.% Fe (Table 3).Heat treatment of Co–W coated mild steel, both with and

without the cobalt strike layer, was carried out in an attemptto improve the mechanical properties of the alloy deposit. Ahardness increase from 540 to almost 1200 kgf mm−2 wasobserved at the optimum heat treatment conditions of 600◦Cfor 2 h in argon (Table 3). This increase in hardness was dueto partial crystallisation of the material[21]. This increasein hardness is higher than those observed in much other

920 H. Capel et al. / Wear 255 (2003) 917–923

Fig. 2. As-deposited coating of Co–53 wt.% W–30 wt.% Fe alloy.

work. For instance, Eskin et al.[6] reported an increasein hardness for a Co–30 wt.% W deposit from around 450to around 700 kgf mm−2 following heat treatment for 1 hat 600◦C in air. The differences may be explained by themuch lower tungsten content in their work and the fact thatheat-treatment in air results in significant oxidation of thedeposit.

Whilst heat treatment improves the hardness of the al-loy, the integrity of the coating is considerably reduced inthat significant through-thickness cracking is observed. Thecracking does not penetrate the cobalt strike when this ispresent[22] and the strike remains in good contact with thesteel substrate following heat treatment.

3.2. Corrosion resistance

The limit of corrosion resistance was measured as the timetaken until the first spots of red rust associated with substratecorrosion appeared on the sample surface in the salt-spraytest. Care was taken to distinguish between deposit corrosionand substrate corrosion as previously described. The resultsof these tests are given inTable 1.

As-deposited Co–W on mild steel has shown very promis-ing corrosion protection. Substrate corrosion was not ob-served for the Co–W alloy without a strike layer until over2560 h in the neutral salt-spray test. As-deposited Co–Wwith an underlying Co strike has shown even greater cor-

Table 1Corrosion and wear properties (under 61 N load) of electrodeposited coatings

Deposit Strike layerthickness (�m)

Onset of substratecorrosion (h)

Wear trackdepth (�m)

Coefficientof friction

Co–53 wt.% W as-deposited 0 2563 20.9 0.41Co–53 wt.% W as-deposited 2 3334 17.5 0.40Co–53 wt.% W heat treated 0 21 1.4 0.47Co–53 wt.% W heat treated 2 100 2.1 0.50Co–53 wt.% W–15 wt.% Fe as-deposited 2 289 23.4 0.91Co–53 wt.% W–23 wt.% Fe as-deposited 2 120 23.9 0.95Co–53 wt.% W–30 wt.% Fe as-deposited 2 35 4.5 0.75Hard-chrome as-deposited 0 8 1.2 0.4

rosion protection with no sign of substrate corrosion afterover 3300 h of testing. A yellow/gold surface layer formedon top of all the as-deposited Co–W samples after approx-imately 20 h, but this layer was too thin to be identified byXRD. Some as-deposited samples, both with and without thestrike layer, have also displayed evidence of surface pittingafter 100 h. These small pits were filled with a black corro-sion product again in quantities too small to be analysed byXRD or EDX.

The benefits of using a Co strike were more evident whenthe corrosion resistance of heat-treated Co–W alloy was in-vestigated. Samples heat treated at 600◦C for 2 h with nounderlying Co strike provided substrate protection for onlyaround 20 h. After this time the whole sample was com-pletely covered with red rust. Heat-treated Co–W with anunderlying 2�m thick Co strike showed no sign of substratecorrosion for 100 h. No surface pitting was observed on anyof the heat-treated samples as seen with the as-depositedsamples, but the as yet unidentified gold/yellow surface layerwas observed on some samples, typically after 100 h oftesting.

The substrate corrosion protection offered by theCo–W–Fe electrodeposits with varying Fe contents wasalso examined. It can be seen that as the iron content in-creases the protection against substrate corrosion markedlydecreases. Deposits containing 30 wt.% Fe prevented sub-strate attack for only 35 h. However, considerable corrosiveattack to the coating itself occurred almost instantaneouslywith exposure to the salt spray environment. The surface ofthese coatings became completely covered in a red corro-sion product after only 2 h. XRD confirmed that this productwas iron oxide. Substrate corrosion for samples coated withCo–W–Fe alloy, with an Fe content of 23 wt.%, was ob-served after 120 h. Corrosion of the coating itself initiatedafter 4 h exposure to the salt spray but the corrosion productcovered only approximately 20% of the coated surface. Thecoating only reached a similar level of attack as observedfor the 30 wt.% Fe deposits in 2 h after 55 h in the salt spray.Deposits containing only 15 wt.% Fe showed even greaterresistance to both attack of the alloy deposit itself and sub-strate corrosion. No change to the surface of the coatingswas observed in this case for over 35 h of exposure. After80 h of exposure, the thin yellow/gold layer, also seen in

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Table 2Wear properties of selected electrodeposited coatings following slidingunder 162 N load

Deposit Strike layerthickness (�m)

Wear trackdepth (�m)

Co–53 wt.% W heat treated 0 4.2Hard-chrome as-deposited 0 4.1

the case of the Co–W deposits, was observed. Attack of theCo–W–Fe coating occurred after 120 h in the salt spray andsubstrate corrosion was prevented for almost 300 h.

SEM observation of some of the corroded Co–W–Fe sam-ples revealed that as deposit attack took place a thick oxidelayer formed across the top surface of the coatings, coveringany pits and cracks that were present. This layer aided pre-vention of corrosion of the substrate at weak points in thecoatings, by blocking the pathway where corrosive mediacould travel directly to the substrate, which would otherwisehave caused immediate substrate attack. This layer howeverdid not prevent further attack to the coating itself and hencethe relatively poor protection properties of the Co–W–Fedeposits in comparison to the Co–W coatings.

Electrodeposited chromium, 20�m thick, consistentlyfailed to provide substrate protection after less than 10 h inthe salt-spray test. This is even less than the time for corro-sion seen for the heat-treated Co–W without the presenceof a cobalt strike and was a result of the cracked nature ofthe deposits.

3.3. Wear resistance

The resistance to reciprocating wear of the as-depositedand heat-treated Co–W alloy, as well as the chromium elec-trodeposit, was measured by the depth of the wear track aftera given number of testing cycles, under the specified load.The results of these tests are shown inTable 1. It can be seenthat after 4000 cycles, substantial wear of the as-depositedCo–W with no underlying cobalt strike layer has occurred,the track depth equal to the thickness of the deposit itself in-

Fig. 3. (a) Cross-section and (b) plan view of wear scar in a heat-treated Co–53 wt.% W coating (with cobalt strike) following 4000 cycles of wear undera 61 N load.

dicating complete coating removal. A slight improvement inthe wear resistance was seen for this material when a strikelayer was present between the substrate material and the al-loy. However, the wear resistance of the as-deposited Co–Walloy still does not approach that of the chromium electrode-posit, which produced a wear scar depth of only 1.2�m after4000 cycles. It should be noted that when these deposits be-gin to break down during sliding, debris is produced whichis hard and abrasive. As such, once such debris has beenproduced, further wear is rapid and often catastrophic. Anexample of the result of catastrophic wear can be observedin Fig. 6bwhere finely comminuted debris has been foldedinto the sample surface during the wear process. Accord-ingly, this wear test tends to measure primarily the ability ofthe coating to survive severe breakdown under the conditionsapplied. If the coating survives such catastrophic failure, dis-crimination can be made on the basis of depth of wear.

A significant improvement in the wear resistance of theCo–W alloy was observed when samples were heat treatedprior to wear testing. For heat-treated Co–W depositeddirectly onto the steel substrate the wear scar after 4000cycles was now consistently only 1.4�m, a value that ap-proaches that of the chromium sample. The resistance towear of Co–W electrodeposits with an underlying cobaltstrike layer was also greatly improved when compared totheir as-deposited counterparts. However, a difference inthe wear depth of the samples with and without the cobaltstrike was observed in both the as-received and heat-treatedconditions. This is attributed to differences in the stress stateand film morphology, both of which will be affected by thecobalt strike layer. However, the details of this interactionare not, as yet, well understood. Also, the coefficient offriction increased from around 0.4 prior to heat-treatment toaround 0.48 after heat-treatment. Amorphous alloys are wellknown to exhibit low coefficients of friction and the increasefollowing heat-treatment is due to partial crystallisation.

Fig. 3shows a cross-section and plan view of a wear scarin the heat-treated deposit. Comparing this withFig. 2, somecracking can be seen to result from the wear testing. Theplan view image (Fig. 3b) shows that the cracking gener-

922 H. Capel et al. / Wear 255 (2003) 917–923

Fig. 4. (a) Cross-section and (b) plan view of wear scar in as-deposited hard chrome following 4000 cycles of wear under a 61 N load.

ally occurred within nodules and was stopped at the nod-ule boundaries, thus preventing catastrophic failure. Muchof the worn surface was covered with a film (the light greyareas towards the edges of the image inFig. 3b) which isthought to be oxide. The increase in wear resistance was dueto the significant increase in hardness upon heat treatmentwhich made it harder than the counterface ball. EDX pro-vided some evidence of iron oxide present on the surface ofthe deposits which had resulted from wear of the ball.

Fig. 4 shows the wear behaviour of the chromium de-posits under the same conditions. Again, the deposit wasseen to be severely cracked and more significant crackingof the deposit was observed on the plan view (Fig. 4b) thanin the heat-treated Co–W coating. In the chromium, thecracking was observed to be more extensive and not lim-ited by any nodular microstructural features. Cracking wasprimarily perpendicular to the sliding direction. Some evi-dence of delamination was also seen. However, it is notablethat although the damage appears worse in the chromium(Fig. 4) than in the heat-treated Co–W (Fig. 3), the wear ofthe chromium was slightly less (Table 1). The friction coef-ficient in the heat-treated Co–W (around 0.48) was higherthan that in the chromium (0.4) and this may have resultedin the higher rates of wear in the former materials.

Fig. 5. Cross-section of wear scar in a heat-treated Co–53 wt.% W coating(without cobalt strike) following 4000 cycles of wear under a 162 N.

Table 2illustrates that the heat-treated Co–W alloy hasa high load bearing capacity. Both this deposit and thechromium were able to withstand wear under a significantlyhigher load of 162 N. Neither coating failed catastrophically,although the depth of wear was now much higher than un-der the lower load, indicating that the deposits were wear-ing attritively. The Co–W and the hard chromium depositswore to a very similar depth in this test.Fig. 5 shows across-section through the heat-treated Co–W coating testedunder these conditions. The deposit exhibits more crackingthan under the lower load (Fig. 3). The nodular structure canbe discerned with the major cracks running between nod-ules. Where material has been removed from the surface,pores have been filled with an oxide which (as indicated byEDX analysis) had iron as the primary metal.

The Co–W–Fe deposits were examined in the as-depositedstate.Table 3shows that the hardness of the two depositswith the lower iron contents were higher than that of theplain Co–W alloy by a small amount.Table 1indicates thatthey showed no advantage in wear over the plain Co–Walloys, exhibiting catastrophic failure of the coating.Fig. 6bshows a cross-section through such a catastrophic failure.Increasing the iron content in the deposit to 30 wt.% resultedin an as-deposited hardness of 838 kgf mm−2, very close tothat of chromium. However,Table 1indicates that the wearresistance of this material was significantly worse than thatof chromium, even though it wore attritively and not catas-trophically.Fig. 6ashows that the degree of cracking in the

Table 3Hardness of electrodeposited coatings and wear counterface

Deposit Hardness (kgf mm−2)

Co–53 wt.% W as-deposited 538Co–53 wt.% W heat treated 1185Co–53 wt.% W-15wt.% Fe as-deposited 551Co–53 wt.% W–23wt.% Fe as-deposited 605Co–53 wt.% W–30wt.% Fe as-deposited 838Hard-chrome as-deposited 848Wear counterface (440C ball) 739

Coatings tested with a 25 gf load; counterface tested with 20 kgf load.

H. Capel et al. / Wear 255 (2003) 917–923 923

Fig. 6. Cross-section through wear scar in as-deposited iron-containing deposits following 4000 cycles of wear under a 61 N load: (a) Co–53 wt.%W–30 wt.% Fe alloy; (b) Co–53 wt.% W–23 wt.% Fe alloy.

iron-containing deposit was far more extensive than thatobserved in the chromium or the plain Co–W deposit fol-lowing heat-treatment. This high level of cracking may haveresulted from the comparatively high coefficients of friction(>0.75) observed for the iron-containing alloys comparedto those of the non-iron-containing deposits (<0.4). Cobaltoxides are known to result in low friction coefficients, andthe higher friction coefficients of the iron-containing alloysmay be associated with differences in the oxide structureon the surface.

4. Conclusions

It has been shown that electrodeposits based on the Co–Wsystem with high tungsten levels can be successfully elec-trodeposited from a near-neutral bath. The deposit showedexceptional corrosion resistance but poor wear resistance.The as-deposited hardness of the Co–W deposit can be sig-nificantly increased by heat treatment which leads to wearresistance close to that of electroplated chromium, but a sig-nificant reduction in corrosion resistance. The corrosion re-sistance is slightly better than that of chromium, but thiscan be significantly improved by the use of a cobalt strikelayer. Thus, plain Co–W electrodeposits can be made com-petitive with electroplated chromium in terms of both wearresistance and corrosion resistance whilst being able to beprocessed under less stringent environmental controls.

Deposits containing Co–W–Fe were made by the addi-tion of ferrous sulphate to the bath. The iron content ofthe coating could be controlled simply by altering the ironcontent in the bath. The as-deposited hardness of the alloycontaining 30 wt.% Fe was close to that of electrodeposited

chromium and its corrosion resistance much better. How-ever, its wear resistance was lower than chromium (althoughno catastrophic wear was observed).

References

[1] J.D. Schell, M. Rechtsteiner, Plating Surf. Finishing 87 (2000) 17–23.[2] P.C. Wynn, C.V. Bishop, Trans. IMF 79 (2001) B27–B30.[3] M. El-Sharif, Trans. IMF 75 (1997) B143–B146.[4] S.K. Ibrahim, D.T. Gawne, A. Watson, Trans. IMF 76 (1998) 161–

165.[5] P.L. Ko, M.F. Robertson, Wear 252 (2002) 880–893.[6] S. Eskin, O. Berkh, G. Rogalsky, J. Zahavi, Plating Surf. Finishing

85 (1998) 79–84.[7] B.N. Maruthi, L. Ramesh, S.M. Mayanna, D. Landolt, Plating Surf.

Finishing 86 (1999) 85–89.[8] C. Gangmin, Y. Fangzu, H. Ling, N. Zhenjiang, X. Shukai, Z.

Shaomin, Trans. IMF 79 (2) (2001) 81–84.[9] T. Wantanabe, New Mater. New Proces. 3 (1985) 307–312.

[10] U. Admon, M.P. Dariel, J. Appl. Phys. 59 (6) (1986) 2002–2009.[11] P. Schloßmacher, T. Yamsaki, Mikrochim. Acta 132 (2000) 309–313.[12] V.M. Fedosyuk, J.M. Riveiro, J. Non-Cryst. Solids 143 (1992) 153–

156.[13] T. Yamasaki, Scripta Mater. 44 (2001) 1497–1502.[14] Y.C. Wu, G.H. Li, L. Zhang, Surf. Eng. 16 (2000) 506–510.[15] C. Yong, J. Mater. Sci. Lett. 15 (1996) 629–631.[16] Y.Z. Zhang, K. Zhang, J.Z. Fan, S.M. Zhang, M. Yao, Trans.

Non-Ferrous Met. Soc. China 8 (1998) 642–647.[17] Y. Choi, J. Mater. Sci. 32 (1997) 1581–1586.[18] M.H. Staia, E.J. Castillo, E.S. Puchi, B. Lewis, H.E. Hintermann,

Surf. Coatings Tech. 86–87 (1996) 598–602.[19] M.H. Staia, C. Enriquez, E.S. Puchi, Surf. Coatings Tech. 94–95

(1997) 543–548.[20] W. Wikiel, J. Osteryoung, J. Appl. Electrochem. 22 (1992) 506–511.[21] H. Capel, P.H. Shipway, S.J. Harris, in press.[22] H Capel, P.H. Shipway, S.J. Harris, in: Proceedings of the AESF

SUR/FIN 2002, American Electroplaters and Surface FinishersSociety, Orlando, FL, 2002, pp. 123–132.