Nanostructered Co-W alloys produced by electrodeposition as replacement for hard chromium on aerospace components S.J. Harris, D.P. Weston, P.H. Shipway.

Nanostructered Co-W alloys produced by electrodeposition as replacement for hard

chromium on aerospace components

S.J. Harris, D.P. Weston, P.H. Shipway and J.M. Yellup.

Faculty of Engineering, Department of Mechanical, Materials and Manufacturing Engineering, University of

Nottingham. UK.

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Introduction

• There is a need to find a replacement for electroplated hard Cr coatings as used in many engineering applications.

• These Cr coatings are deposited from a plating bath containing hexavalent Cr ions which are toxic in the bath and can give problems to the environment during waste disposal.

• The deposition process for Cr is ~15% efficient.

• Replacement coatings need to possess good tribological properties and corrosion resistance.

• Opportunities exist for the development of replacement Co coatings alloyed with W

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Electrodeposition of Co-W alloys.

• Tungsten is difficult to plate without the presence of an iron group metal(Co, Fe, Ni)

• Early work has been concentrated on Ni-W, usually from an alkaline bath, which produced an amorphous deposit.

• Similar Co-W deposits with ~50 wt%Co were also produced with a hardness in the range 450-650 kgf mm-2; heat treatment could raise the hardness to >1000 kgf mm-2

• The deposition process was >50% efficient.

• Recent work has concentrated on as-deposited nano-crystalline Co-W (Grain size 5-10nm) with hardness >750 kgf mm-2 produced from acid solution(pH 6.0).

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Operation of the Co-W Plating Bath

• The bath contains cobalt sulphate, sodium tungstate, a complexing agent (sodium gluconate), boric acid and sodium chloride; pH maintained at 5.9-6.1.

• Prior to deposition the steel substrates were cleaned by conventional methods and rinsed.

• The Co-W bath was operated at 80ºC using a current density in the range 1-5 Adm-2.

• Some deposits were plated on to either a Co or Ni strike if applied to cast iron or stainless steels.

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Characterisation of Co-W deposits.

• XRD analysis of deposits.

• SEM observations on the topography of the deposits and SEM/EDX analysis.

• Microhardness.

• Salt spray corrosion tests to ASTM B117.

• Potentiodynamic corrosion behaviour.

• Sliding wear tests under dry and lubricated sliding conditions with a martensitic stainless steel ball counterface to give measurements of the friction coefficient, wear volume and wear rates.

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Structural Characterisation.

Changes in composition, crystal structure and hardness with the current density.

Current densityAdm-2

Co contentat%

W contentat%

Hardnesskgf mm-2 Structure

2.50 81.8 18.2 843 Crystalline

3.12 83.8 16.2 908 Crystalline

3.75 75.6 24.4 744 Crystalline/amorphous

5.00 74.0 26.0 550 amorphous

The change in XRD pattern from amorphous to highly oriented CPH crystalline Co with W in solution.

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Coating Topography and Microstructure.

SEM pictures show the difference in topography with nodular amorphous and fine faceted crystalline deposits.

Cross-sections of crystalline deposits are even in section up to 150µm thick with a smoother finish. Voidage and valleys exist between amorphous growths.

P1S402 x 1000 20m

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Salt Spray Corrosion Tests

• Test specimens of amorphous and crystalline deposits exposed at 35°C formed golden brown surface films after 20h.

• Protection of the steel substrate is provided by 15µm thick coatings of both types for at least 2,500h exposure.

• An underlying Co strike (2µm thick) can extend the period of protection for the amorphous coatings. Crystalline coatings without a Co strike can provide protection for the steel for up to 11,000h. Hard Cr coatings are extensively microcracked and need an undercoat of Ni or Cu to prevent the onset of red rust after <50h.

• Failure of the coatings took place by a pitting mode; evidence of iron was found in the pits.

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0 h

168 h

1896 h

Pitting of the crystalline Co-W coating after 11,000 h

Results of Salt Spray Corrosion Tests

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Potentiodynamic Corrosion Behaviour.

• Immersed in 0.5 M NaCl solution at 30°C, produced an increase of ~250mV in Ecorr (to a more anodic value) when replacing an amorphous coating by a crystalline type.

• Both coatings showed some evidence of passivation in the anodic sweep.

1E-5 1E-4 1E-3 0.01 0.1 1 10 100-1000

-800

-600

-400

-200

0

200

400

600

800

1000

1200

Co-W alloys

Co-W(Amorphous)

Co-W(Nano-crystalline)

Po

ten

tia

l (m

V w

rt S

CE

)

Log current density, i

(mA cm-2)

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Wear Tests – Coefficient of Friction.

• Dry and lubricated sliding wear tests using a hardened steel ball on to a flat coated (Co, Cr, crystalline Co-W) samples were carried out under loads of 15, 30 and 61 N for a sliding distance of 500m (only 61N over 1,000m for the lubricated case).

• The COF was measured during each test.

• Crystalline Co-W coatings maintained lower COF values at all 3 loads under dry sliding conditions. At 15N it maintained a value of <0.05 for 150m sliding distance, an exceptional result for a metallic material.

• Tests using white spirit as a lubricant on CoW gave COF values less than 0.04 over a sliding distance of 1,000 m

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Coefficient of Friction Results for Unlubricated Samples

15 N

30 N

61 N

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Coefficient of Friction Results for lubricated samples

Variation of coefficient of friction with distance in lubricated wear tests on Co-W and Cr coatings at 61N load.

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Wear Rates of Unlubricated Samples

• Rates were determined from measured wear volumes on the balls and coated flats, loads and sliding distances.

• At all 3 loads the Co-W coatings gave the lowest rate; at 15 N no wear was detected.

• Rates on the steel ball were the lowest with the Co-W coatings.

Coating

Counterface

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Wear Rates of Lubricated Samples

• The wear rate of the Co-W coating was an order of magnitude less than that of the Cr coating under the same conditions.

• The wear rate of the Co-W coating was 2 orders of magnitude less than that of the unlubricated case at the same load.

• Wear on the counterface for the Co-W was similar to that for the dry tests at the same load whereas that for the Cr coating was 2 orders of magnitude less.

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Blue bars - wear rates for coated discsRed bars - wear rates for counterfaces

Wear Scars for unlubricated tests.

Plan view SEM pictures demonstrate the different scars produced at each load on each coating (left) and the corresponding ball (right).

Cr61N

Co30N

Co-W61N

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Wear Scars for unlubricated tests.

Plan views (left) and sections (right) through the scar on the Co-W coating (tested at 61N) demonstrating the integrity of the coating and the presence of some oxide.

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Applications of Co-W Coatings.

• The combination of favourable wear and corrosion resistance of crystalline versions of this coating makes it an attractive possibility for replacing hard Cr in many engineering applications.

• Such coatings can be used in lubricated and non-lubricated conditions.

• A typical example where lubricants are present are in hydraulic equipment.

• Whilst there are many applications in aero- and automotive engines where they may be applied in conditions when lubrication is absent or difficult to maintain.

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Conclusions

1. Co-W alloys can be produced in the as deposited condition into two forms, amorphous and nanocrystalline from a similar plating bath composition operated at high and low current densities.

2. Nanocrystalline coatings (15-20 at%W) can be deposited with hardness values in the range 850-1000 kgf mm-2 comparable with hard chromium deposits.

3. Both as-deposited coatings have a good corrosion resistance and are capable of protecting a steel substrate when exposed in a neutral salt spray cabinet for >2,500 h.

4. The cobalt-tungsten alloy electrodeposit has demonstrated wear rates similar to those observed for the electrodeposited chromium at an intermediate load under dry sliding conditions, but at least an order of magnitude lower when sliding at higher loads under both dry and lubricated conditions.

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Conclusions

5. Dry sliding of the cobalt-tungsten alloy coating against the hard steel counterbody results in the counterbody wear rates being three orders of magnitude less than that observed when sliding against the chromium coating. Under lubricated conditions the wear rates on the counterbody are similar

6. The low wear rates of the as-deposited cobalt-tungsten alloy electrodeposit are attributed to its relatively high hardness, its hcp structure and a capability to withstand high loads. The low friction coefficient and the lack of copious abrasive wear debris in the unlubricated case results in an extremely low rate of wear of the steel counterbody.

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