Wear 301 (2013) 551–561

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Wear

0043-16

http://d

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E-m

alamarg

Brezard

journal homepage: www.elsevier.com/locate/wear

An investigation of fretting wear behaviour of nickel coatings forelectrical contacts application in dry and lubricated conditions

S. Noel a,n, D. Alamarguy a, A. Brezard-Oudot a, P. Gendre b

a LGEP, UMR Supelec-CNRS, 11 rue Joliot Curie, 91190 Gif sur Yvette, Franceb PEM, Siaugues-Saint-Romain, 43300 Siaugues-Sainte-Marie, France

a r t i c l e i n f o

Article history:

Received 14 September 2012

Received in revised form

11 January 2013

Accepted 22 January 2013Available online 8 February 2013

Keywords:

Fretting

Wear

Electrical contact

Nickel

Lubrication

Grafting

48/$ - see front matter & 2013 Elsevier B.V. A

x.doi.org/10.1016/j.wear.2013.01.076

esponding author. Tel.: þ33 169851643; fax:

ail addresses: sophie.noel@supelec.fr (S. Noel)

uy@lgep.supelec.fr (D. Alamarguy),

-oudot@lgep.supelec.fr (A. Brezard-Oudot), p.

a b s t r a c t

Fretting remains a major cause of connector failure and can impair reliability of complex system. Much

work is devoted to minimise the effects of cyclic displacements in the contact interface. One of the

options is to choose proper materials for the final layer and eventually select an underlayer. Another

option is to apply a lubricant compatible with the electrical function of the connecting device. Its effect

must be the reduction of friction with the preservation of conduction. In this work we have investigated

the properties of nickel electrodeposited on cuprous coupons submitted to an imposed sine displace-

ment fretting test (1 Hz, 25 mm, 2.5 N). The contacts are of the sphere on plane type. The plating

conditions are shown to have a large effect on the microstructure, roughness and composition of the

nickel coatings. Consequently the fretting behaviours of the compared coating are different. Fretting

degradation is evaluated for our application as the number of cycles for which the build-up of oxidised

debris gives a value of electrical resistance superior to a threshold. During the fretting test the rough

nickel with large grains is observed to be prone to very high friction and partial slip. Fretting of the

smoother coating with small grains is characterised by gross slip. Contacts were lubricated with

solutions of perfluorinated polyether (PFPE). It is shown that a strong friction reduction takes place and

that the fretting degradation is postponed. Surface analysis shows that fluorine is grafted in the

interface and protects the interface. The electrodeposition process and the properties of the nickel

coating are shown to have an effect on the lubricant behaviour in the contact interface. These results

show the strong potential of lubricated nickel for applications in the field of electrical devices.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Fretting of electrical contacts has been identified to cause thefailure of devices controlling major functions [1] and is still aproblem. The approach we have been working on for several yearsis to optimise the materials and the coatings used for connectorterminals.

Various types of electrical contacts involve electrodeposited nickelcoatings either as an underlayer or as a finish. Nickel has interestingproperties. Much work has been devoted to nickel electroplatingssince the patent on nickel sulfamate bath [2]. Efforts were devoted todevelop processes and also to study the microstructure of nickelcoatings [3–5]. Reddy [6] has shown that a preferred orientationcould develop in nickel electro-deposits and proposed a mechanismfor the development of texture in Ni coatings [7]. More recently, withthe development of the LIthographie, Galvanoformung, Abformung

ll rights reserved.

þ33 169418318.

,

gendre@pem.fr (P. Gendre).

(LIGA) technique, much work has been undertaken on nickel electro-deposition processes in relation to the properties of the coatings. Kellyand Goods have related the properties and the structure of Nielectrodeposited films [8,9]. Recently nanocrystalline nickel coatingshave shown interesting properties [10,11]. It is well known that Nicoatings show a good behaviour in certain corrosive atmospheres,mainly because of their propensity to form a thin passivating film[12,13], due to its particular surface chemistry. It has been reportedby some authors that the properties of nickel electrodeposits could betailored [3]. The first goal of this study was to correlate the structural,electrical and mechanical properties of the nickel coatings underinvestigation in order to find a layer delaying fretting degradation. Ina second step we wanted to investigate the particular behaviour oflubricated nickel contacts submitted to the same fretting test.Lubrication, particularly with perfluorinated polyethers (PFPE), hasbeen shown to reduce fretting degradation of tinned electricalcontacts [14–16], but little work involves nickel contacts. It has beenreported that some metals such as Al and Fe could, in sometemperature or friction conditions, react with PFPE and decompose.[17–19]. Very little work has been done on the behaviour of PFPE onnickel [20]. In this paper we show the effect of PFPE lubrication on

Nomenclature

R radius of curvature of the sphere (mm)a radius of contact disc (mm)Fn normal load (N)F tangential force (N)Ra arithmetic roughness (nm)Rc contact resistance (O)Rco constriction resistance (O)

Rf surface film resistance (O)d displacement (mm)r ratio of metallic nickel/oxidised nickelCm measured compliance of the system (mm/N)sy yield stress (MPa)r resistivity (O m)E Young modulus (GPa)n Poisson ratioHv Vicker hardness (daN/mm2)

S. Noel et al. / Wear 301 (2013) 551–561552

nickel contacts submitted to fretting and we give evidence that somemolecules are grafted on the surface due to friction. The issueaddressed is the possible decomposition of the free lubricant filmand the possible oxidation of the metal [21]. The roughness of thesurface has been shown to have an effect on lubricated fretting [22]and is also investigated here.

2. Samples and experimental set-up

2.1. Samples

Substrates were brass (CuZn30) coupons; they were coatedwith two types of nickel layers from the sulphate baths describedin Table 1. The mechanical properties of the substrate are given inTable 2. The hardness of the nickel coatings were measured onspecially deposited thicker layers (10 mm) with a 50 gf load; thisgives a good estimation of the values for the two types of samples.Matte Ni coatings and bright Ni coatings are compared in thefollowing paragraphs.

The Ni coatings were deposited at 60 1C with a current density of10 A dm�2. The coatings for the study were 2 mm thick. Contactswere of the sphere on plane type. The planes were 20 mm�20 mmcoupons; a spherical shape (radius of curvature R¼1.3 mm) wasstamped on dedicated 10 mm�50 mm coupons of the same sub-strate. Lubricant was a branched perfluorinated polyether (PFPE)manufactured by Solvay Solexis S.p.A.. It has a kinematic viscosityof about 1854 cSt. PFPE are known for their inertness, stability and

Table 1Nickel baths compositions.

Type of Ni Bath Concentration

Matte nickel Ni sulphate (NiSO4, 6H2O) 400 g L�1

Ni chloride (NiCl2) 30 g L�1

Boric acid 40 g L�1

Wetting agent 2–4 mL L�1

Bright nickel Ni sulphate (NiSO4, 6H2O) 400 g L�1

Ni chloride (NiCl2) 30 g L�1

Boric acid 40 g L�1

Wetting agent 2–4 mL L�1

Brightener 1 5 mL L�1

Brightener 2 2 mL L�1

Table 2Mechanical properties of tested materials. CuZn30 substrate, matte and bright Ni

coatings.

E (Gpa) n HV sy (MPa) r (Om)

CuZn30 114 0.33 190 370 6.3�10–8

Matte Ni – – 200 – –

Bright Ni – – 600 – –

good temperature pressure curve.

Coupons were dipped in solutions (5% vol) of lubricant inGaldens SV90 and dried in air.

2.2. Experimental set-up

The composition of the coatings, their oxidation state, theirtopography and microstructure were analysed with the appro-priate techniques.

2.2.1. Characterisations

3D images were made with a Wyko optical profiler allowing thecalculation of topography parameters. AFM topographic imageswere performed with a Veeco multimode Nanoscope IIIa system.

XPS surface analyses were obtained with a PHI Versaprobemultitechnique XPS microprobe comprising an Al Ka monochro-mated source. The pressure in the analyser chamber was around10�7 Pa; for all the experiments the power was set at 50 W andthe analysed zone was 420 mm�420 mm. Survey spectra (0–1400 eV) were acquired with a pass energy of 187.85 eV, anenergy step of 0.4 eV and a dwell time of 20 ms. High resolutionspectra for chosen energy ranges were taken with energy pass of23.5 eV, an energy step of 0.1 eV and a dwell time of 50 ms. Asystem of charge neutralisation involving a low energy ion sourceand a low energy electron source avoided charging of thesurfaces. The system was calibrated by setting Cu 2p3/2 and CuLMM of clean copper at 932.7 eV and 918.7 eV respectively. Thetake off angle was 451 unless specified. All the spectra were fittedusing the PHI Multipak software; the background was removedby the Shirley method. Gaussian, Lorentzian and asymmetricpeaks could be chosen for the fits. The stoichiometries of thesamples were determined using the appropriate sensitivity fac-tors. (see http://www.lasurface.com/xps/index.php for basics).Linear Least Squares (LLS) fitting was used to obtain maps ofdifferent chemical states for a given element [23].

2.2.2. Fretting test

A dedicated device, already described [24], comprising anelectro-dynamic shaker applied controlled cyclic movements(50 mm peak to peak, 1 Hz) to the contact under a 2.5 N constantnormal load. A feedback loop imposed the sine displacement; themaximum displacement value remained rigorously constant dur-ing the test. The compliance of the fretting device was measuredfrom the slope of friction cycles. This slope was constant for allthe samples investigated and gave a value of Cm¼1.8 mm/N. Thisparameter has been shown to be very important [25,26].

During the test, the voltage drop in the contact Uc was measuredwith an acquisition card (333�103 samples per second) with the

S. Noel et al. / Wear 301 (2013) 551–561 553

current value set at 20 mA and the voltage limit at 250 mV. Duringeach cycle, 500 values of the position d, of the friction force F and ofthe voltage drop Uc were acquired. The mean value of Rc for a cyclewas calculated during the experiment from the 500 measurements ofUc. The mean Rc values were plotted versus the number of frettingcycles every 10 cycles. A characteristic value Ft was also calculatedevery 10 cycles and plotted versus the number of cycles. Ft during acycle was defined as the average between the maximum andminimum values of a cycle Ft¼(9Fmax9þ9Fmin9)/2. We chose to plotthis value because the parallelogram shape of the cycles evolvesduring the fretting test.

Table 4Peak assignment for Ni 2p3/2.

3. Results and discussion

3.1. Characterisations

3.1.1. Composition and identification of the oxide peaks

Survey XPS analyses were performed to determine the composi-tions of the two nickel surfaces. The results are gathered in Table 3.The sulphur and chlorine found on the bright Ni come from thebrighteners specific to the bright nickel bath. Both samples have alarge amount of carbon contamination on the upper surface.

High resolution analyses were then performed at lower passenergy in order to study chosen peaks: C 1s, O 1s, Ni 2p. The Ni 2p3/2

and O 1s peaks for matte Ni are shown in Fig. 1(a) and

Ni 2p3/2matte Ni

A.U

.

Binding Energy (eV)875 870 865 860 855 850 845 840 5

5875 870 865 860 855 850 845 840

Ni 2p3/2bright Ni

A.U

.

Binding Energy (eV)

Fig. 1. Nickel and oxygen core levels for various Ni coatings showing the experimental c

coating; (b) O 1s matte coating; (c) Ni 2p3/2 bright coating and (d) O 1s bright coating

Table 3Composition in at% of the matte and bright nickel coatings.

C O Ni Ca S Cl

Matte Ni 49.3 33.0 17.7 – – –

Bright Ni 56.7 29.4 12.3 – 0.8 0.8

(b) together with the decompositions spectra corresponding tothe different chemical forms. They can be compared to the Ni andO peaks for the bright nickel in Fig. 1(c) and (d).

The results are in good agreement with other authors [13]; theassignments of the different chemical forms of nickel and oxygenare summarised in Tables 4 and 5. In these tables, the componentsare listed in order of increasing binding energy; they appear fromleft to right on Fig. 1(a–d). Metallic nickel [component (1) and(5)], NiO [component (2), (4) and (6)] and Ni(OH)2 [component(3) and (6)] can be identified. Component (6) is unresolved andcorresponds to satellites of NiO and Ni(OH)2. The oxygen peak atabout 529.6 eV corresponds to NiO while the components athigher energy correspond to Ni(OH)2 and various C–O speciesfrom contamination. It has been proposed [27] that the oxidisednickel surface is a bilayer composed of NiO with a top layer ofNi(OH)2. This is confirmed here by the analyses performed atdifferent take off angles from 151 to 751. At low take off angle (151here) the photo-electrons come from the uppermost surface,while for higher take off angles (751 here) the analysis involvesa thicker layer. By varying the take off angle, a non destructivedepth analysis can be performed. Calculating the amount ofmetallic Ni from the value of the peak surface and the amount

A.U

.

Binding Energy (eV)

O 1smatte Ni

A.U

.

Binding Energy (eV)

O 1sbright Ni

40 538 536 534 532 530 528 526

40 538 536 534 532 530 528 526

urve (–�–) and the different components (–) obtained by fitting: (a) Ni 2p3/2 matte

.

No. Component Matte Ni (eV) Bright Ni (eV)

1 Ni metal 852.73 852.73

2 NiO 854.22 854.41

3 Ni(OH)2 855.95 856.05

4 NiO 856.52 857.10

5 Shake up 858.34 858.35

6 Satellite 861.30 861.30

7 – 864.00 863.92

Table 6Ratio r representing the quantity of metallic Ni/

quantity of oxidised Ni at different take off angles

for matte and bright nickel coatings.

151 751

Matte Ni 0.45 0.56

Bright Ni 0.43 1.19

0 10µm Height : 0-608 nm

0 10µm Height : 0-58 nm

Fig. 2. AFM images of the nickel coatings: (a) matte Ni; (b) bright Ni. Scanning

10 mm�10 mm; load: 126 nN.

0 50 100 150 200 250 3000

50

100

150

200

250

300

350

400

Ra

(nm

)

Fn (nN)

Fig. 3. Summary of the various Ra values obtained for matte Ni: ’ spot 1 and

spot 2 (1 mm scan), spot 1 and 2 (10 mm scan), and for bright Ni: spot

1 and spot 2 (1 mm scan), spot 1 and 2 (10 mm scan).

Table 5Peak assignment for O 1s.

No. Matte Ni (eV) Bright Ni (eV)

1 O in NiO 529.64 529.63

2 O in Ni(OH)2 531.65 531.73

3 O – 533.18 533.21

S. Noel et al. / Wear 301 (2013) 551–561554

of oxidised Ni from the corresponding surfaces, a ratio r can beestimated. Table 6 summarises the values of r for the matte andthe bright nickel versus the take off angle. It shows that in theuppermost layer, the quantity of oxidised nickel in the analysedvolume is twice that of metallic nickel for both types of coating.Deeper in the layer, the bright Ni coating is composed of about asmuch metallic Ni as oxidised Ni.

The initial composition of the nickel surface was important toestablish as a reference. Investigating the evolution of the com-position can give a good insight into tribological transformation ofthe surface.

3.1.2. Roughness and microstructure

The average roughness Ra was measured by optical 3Dprofilometer on 200 mm�200 mm areas: the values are rathersimilar for the two coatings. For matte nickel Ra¼199 nm and forbright nickel Ra¼140 nm. A corrugated pattern due to the rollingprocess of the brass substrate is observed and affects the mea-surement for both coatings. Thus the topography was alsomeasured by AFM with different scanning amplitudes varyingfrom 1 mm�1 mm to 10 mm�10 mm. Fig. 2(a) and (b) depictsAFM images (10 mm�10 mm) of the matte and bright Ni coatingsand show very different aspects of the two coatings. Large andmedium size bumps revealing columnar grain terminations areobserved for the matte Ni and very small bumps for bright Ni. Thearithmetic roughness Ra values calculated from these images giveRa¼151.5 nm for matte Ni and Ra¼34.7 nm for bright Ni.

The bright surface is observed to be very smooth; thus the Ra

values measured with the profilometer were taking into account thelarge amplitude grooves of the substrate. In AFM measurements boththe load applied to the cantilever tip and the scanning amplitudemight affect the results. Thus the topography is analysed in variousconditions. The results of the different values of Ra obtained in thesevarious conditions are gathered in Fig. 3. It shows that the Ra valuesare rather constant when increasing the normal load on the cantilevertip. The values for the matte coating are always higher than for thebright ones. Values corresponding to scans of 10 mm amplitude arehigher than the 1 mm ones. Some scattering is observed for differentspots. The micro-hardness measurements performed on specialsamples showed as expected higher values of hardness for the brightnickel samples, as expected.

3.2. Fretting results

3.2.1. Dry nickel

Fretting experiments (normal load 2.5 N, amplitude 25 mm andfrequency 1 Hz) were performed on the matte and bright nickel

samples. Fig. 4(a) and (b) shows the mean Rc values per cycle andthe maximum tangential force Ft per cycle for the two coatings.The behaviours are very different. For matte Ni Rc is almostconstant and only shows a very slow increase; the threshold valueof RcZ10 mO is reached after more than 700 cycles. Simulta-neously the tangential force Ft increases very quickly to a highvalue of 2.5 N. For the bright nickel Rc increases very quickly tovalues around 500 mO (RcZ10 mO after 80 cycles); at the sametime the tangential force Ft reaches a value of 1.5 N and then stayalmost constant at a value of 1.2 N. After about 2000 cycles asudden decrease of Rc is observed as well as a slow increase of Ft.

These results were duplicated several times and all the runswere superimposable; thus, Fig. 4(a) and (b) shows only one runper type of Ni. The initial values of the Rc (summarised in Fig. 14)are 3 mO and 9 mO. The contact resistance value is the sum of theconstriction resistance Rco and the surface film resistances Rf. Thematte coating does not have a continuous contamination film andthe value of Rc is about equal to the value Rco.

The constriction resistance can be written as Rco ¼ r=2a wherea is the radius of the contact disc and r is the resistivity. In arough approximation the contact radius can be obtained from the

Hertz equation a¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið3=4ÞðFnR=En

Þ3

qwhere En

¼ E=2ð1�n2Þ.

Since the constriction is a bulk phenomenon the parametersused should be those of the brass substrate. This gives a value ofa¼33.7 mm and thus a value of Rco of about 1 mO which is the

10-3

10-2

10-1

100

101

Rc

(Ω)

Number of cycles

Matte Ni

Bright Ni

0 1000 2000 3000 4000 5000

0 1000 2000 3000 4000 50000.0

0.5

1.0

1.5

2.0

2.5

3.0

Ft (N

)

Number of cycles

Matte Ni

Bright Ni

Fig. 4. (a) Rc(n) for Ni matte and Ni bright for ball/plane contacts submitted to

fretting (1 Hz, 2.5 N, 25 mm). (b) Ft(n) for Ni matte and Ni bright for ball/plane

contacts submitted to fretting (1 Hz, 2.5 N, 25 mm).

Fig. 5. Optical images of the wear scars after fretting experiments: (a) matte Ni

500 cycles; (b) matte Ni 5000 cycles; (c) bright Ni 500 cycles; (d) bright Ni 5000

cycles; a 100 mm scale is shown and applies for all images.

S. Noel et al. / Wear 301 (2013) 551–561 555

right order of magnitude. The roughness of the surfaces should betaken into account and would give a higher value. Some model-ling was performed in [28] where finite elements were used toevaluate the influence of sy of the substrate on critical force toreach the elasto-plastic contact regime. It was found that forsy¼510 MPa the global behaviour of the contact remained elasticup to a normal load of 3 N. This means that in our case someplasticity occurs which also would give larger values of a. The Rc

value for bright Ni is higher (9 mO) and comprises a filmresistance of several mO which is consistent with the XPSmeasurements.

Some experiments were stopped after 500 cycles in order toexamine the wear scars. Fig. 5. shows the optical images of thematte and bright nickel planes after runs stopped at 500 cyclesand a run stopped at 5000 cycles. Profiles were extracted fromtopographic images recorded by a 3D profiler and shown in Fig. 6.For the matte coating, after 500 fretting cycles (Fig. 5(a)), a largebump is observed on the coupon after separating the twoelements; for the bright coating (Fig. 5(c)), almost no degradationis observed. The bump on the mate coupon is more than 17 mmhigh as shown in Fig. 6(a) while a mark of about 1 mm deep isobserved Fig. 6(b) for the bright coupon.

The observation of these scars, as well as the Rc and Ft valuesof Fig. 4(a) and (b), indicate that the matte and bright nickelfretting regimes are very different. Additional information isobtained from the plot of the friction cycles constantly measuredduring the tests. Fig. 7. shows some of the friction cyclescorresponding to Fig. 4 for matte and bright Ni.

For matte Ni a few cycles of gross slip (characterised by almostconstant Ft for a cycle) are followed by a period of partial slip(characterised by deformed cycles). During this period part of thecontact area is blocked and the contact resistance remains almostat its initial value. This has been observed by several authors[29,30] who define a transition displacement value under whichno degradation should occur. After 700 cycles, gross slip occursand Rc starts to increase. During the test, the sliding conditionshave changed from gross slip to partial slip and then back to grossslip after enough modifications of the interface have occurred.This could be related to the formation of a tribologically trans-formed structure [31] several mm deep. For bright nickel gross slipoccurs during the whole test as shown by the constant shape ofthe friction cycles Fig. 7(b).

After 5000 cycles the scar of the matte nickel coupon, Fig. 5(b),showed very large amounts of wear, material removal and debris.The scar of the bright Ni coatings submitted to the same amountof cycles showed little degradation. Fig. 8. shows scanningelectron images of these scars. Because of the small amount ofdegradation of the bright Ni, these scars could be analysed by XPSin order to gain insight into the fretting mechanisms involved.

Having identified the initial composition of the as-depositedsurface of the coatings, it was possible to acquire maps byelements on 250 mm�250 mm areas. It is possible with thisprocedure to localise the different elements (O, Cu, Ni, F) andtheir chemical forms on the fretting tracks. An element is selected(interval of energy corresponding to the peak under investigation)and a map is built (Fig. 9(a)). It is then possible to select a subareaon the map, extract the corresponding spectra (Fig. 9(d)) and plotmaps for a chemical form (Fig. 9(b) and (c)).

Fig. 9 corresponds to the 500 cycles fretting track of the brightnickel coating. It shows the presence of Ni mainly in the wear track:by applying a LLS procedure available in the Multipack PHI software[23], it was further possible to distinguish maps of different chemicalforms of nickel. It is clearly seen in Fig. 9(d) that the chemical state ofnickel in the fretting tack (spectrum 1) is not the same as its state

Fig. 6. 3D Profilometer images and profiles of the 500 cycles fretting scars: (a) for matte Ni and (b) for bright Ni.

Fig. 7. (a) and (b) Tangential force cycles from the fretting test of Fig. 4 for the matte and bright nickel coatings (2.5 N, 1 Hz, 25 mm). For the matte sample during cycle 80

and cycle 580, Ft increases while the shape of the cycles is strongly modified due to sticking.

S. Noel et al. / Wear 301 (2013) 551–561556

outside the track (spectrum 2). Spectrum 1 corresponding to insidethe wear track is assigned to nickel in NiO. Fig. 10 represents theoxygen map for the same scar. The presence of NiO in the frettingtrack is even clearer. Because of the large difference in binding energyof oxygen in NiO (529.6 eV) and in Ni(OH)2 and/or other hydratedcomponents (531.7 eV) it can be deduced that the fretting track isevenly composed of NiO. NiO is known for being very insulating andhard; this is very well correlated to the very high values of Rc inFig. 4(a) and the stable values of Ft in Fig. 4(b).

Such analysis was not possible for the matte scars because ofthe tormented topography of the scars after fretting. The sametype of analysis was performed on the fretting track of the brightnickel after 5000 cycles. Fig. 11. represents the map of nickel andshows a very different pattern (compared to Fig. 10.): the wearscar is composed of an outer ring of NiO. The centre of the wearscar is composed of copper, nickel, oxygen and carbonaceousspecies. At this stage of the fretting test, the tangential force has

increased and some wear occurs; copper is detected as is shownin the map of Fig. 12.

3.2.2. Lubricated nickel

Samples deposited as previously (matte and bright nickel) werecoated with the PFPE film and submitted to the same fretting tests.The electrical and friction behaviours are shown in Fig. 13(a) and (b).For matte nickel the friction force values are very low (Ft is about0.5 N which gives a friction coefficient m¼0.2); gross slip occursduring the whole test. The contact resistance values increase to10 mO quickly (in 50 cycles) but remain below 20 mO for morethan 1500 cycles. Note that two single higher values of Ft can beobserved which could correspond to asperity shearing. For thebright coating, Rc is very high (around 1 O) during the first cyclesbecause of the additional resistance due to the lubricant film and the

57260

3297120 μm

20 μ

m

SEM 85060

4049820 μm

20 μ

m

SEM

Fig. 8. Scanning electron microscopy (SEM) image of: (a) the 500 cycles and (b) the 5000 cycles fretting scars of bright nickel (3 kV, 10 nA); a 20 mm scale is shown on

the image.

851852853854855856857858859100

120

140

160

180

200

220

240

260

280

300

Binding Energy (eV)

c/s

Fig. 9. XPS Ni map for the 500 cycles bright Ni: (a) global map; (b) inside of the track map; (c) outside the track map; (d) corresponding spectra: binding energy (BE) from

859 to 851 eV, 1 eV graduations.

S. Noel et al. / Wear 301 (2013) 551–561 557

low roughness of the surface; the value decreases to 20 mO afterabout 100 cycles. Simultaneously Ft increases from 0.3 N to 0.5 N.

Though the behaviours of the two coatings are not the same,lubrication has a strong friction reduction effect which lasts overthe 5000 cycles for both layers.

The values of the contact resistance Rc for the two coatingsunder dry and lubricated conditions are summarised in Fig. 14. Noerror bars are displayed because the aim of the study is not todiscuss accurately the various values of Rc but to analyse trends.Several experiments were performed and confirm the trendsgiven here. The aim of the study was to compare the behaviours

of the two types of coatings in view of an electrical application.They can be summarised as follows: for the lubricated matte Nilower values of Rc are obtained after 5000 cycles as compared todry conditions. For the bright Ni, after a short running in periodduring which Rc diminishes, the presence of the lubricant filmavoids the very high values of Rc measured during the firstthousand cycles.

The friction results for both lubricated nickel coatings submittedto fretting tests show a strong protection against degradations andrather stable contact resistance values. Previous work on the lubrica-tion of tin coated contacts has shown that fluorinated lubricants

348.2

0.020 μm

20 μ

m

O1s

5275285295305315325335340

50

100

150O1s (View Only)

Binding Energy (eV)

c/s

Fig. 10. XPS O map for the 500 cycles bright Ni: (a) global map; (b) inside of the track map; (c) outside the track map; (d) corresponding spectra: binding energy (BE) from

534 to 527 eV, 1 eV graduations.

68.4

39.420 μm

20 μ

m

Ni2p3

85085185285385485585685785880

100

120

140

160

180

200

220

240

260Ni2p3 (View Only)

Binding Energy (eV)

c/s

Fig. 11. XPS Ni map for the 5000 cycles bright Ni: (a) global map; (b) inside of the track spectra; (c) outside the track spectra; (d) corresponding spectra: BE from 858 to

850 eV, 1 eV graduations.

S. Noel et al. / Wear 301 (2013) 551–561558

could delay the onset of fretting degradation. The issue investigatedhere is how long does this lubricating last taking into account theparticular chemical properties of nickel.

The action of lubricants on tin contacts was shown to stronglydepend on the viscosity of the fluorinated lubricant [14,15]; tinbeing very soft and plastically deformed upon contact, the effect

763.0

0.020 μm

20 μ

m

Cu2p3

Fig. 12. XPS Cu map for the 5000 cycles bright Ni.

10-3

10-2

10-1

100

101

Lubricated Bright Ni

Rc

(Ω)

Number of cycles n

Lubricated Matte Ni

0 1000 2000 3000 4000 5000

0 1000 2000 3000 4000 5000

0.0

0.5

1.0

1.5

Lubricated Bright Ni

Ft (N

)

Number of cycles n

Lubricated Matte Ni

Fig. 13. (a) Rc(n) for Ni matte and Ni bright for lubricated ball/plane contacts

submitted to fretting (1 Hz, 2.5 N, 25 mm). (b) Ft(n) for Ni matte and Ni bright for

lubricated ball/plane contacts submitted to fretting (1 Hz, 2.5 N, 25 mm).

1

10

100

1000

dry lubricated

Rc

(mΩ

)

Rc(1) Rc(500) Rc(5000)

1

10

100

1000

Rc

(mΩ

)

dry lubricated

Rc(1) Rc(500) Rc(5000)

Fig. 14. Bar graphs summarizing the values of Rc for dry and lubricated matte

(a) and bright (b) Ni contacts: after the first cycle, after 500 and 5000 cycles.

Fig. 15. Optical images of the wear scars after fretting experiments on lubricated

surfaces: (a) matte Ni 500 cycles; (b) matte Ni 5000 cycles; (c) bright Ni 500

cycles; (d) bright Ni 5000 cycles. A 50 mm scale is shown and applies for all

images.

S. Noel et al. / Wear 301 (2013) 551–561 559

of the lubricant follows complex mechanisms. It was observedexperimentally that the kinematic viscosity and the structureof the molecule had a strong effect on the efficiency of thelubrication.

In order to investigate if the particular chemical properties ofnickel had consequences during the fretting test, the scars wereanalysed as previously for the dry contacts. The coupons wererinsed before they were put in the ultra high vacuum for spectro-scopic analysis. Fig. 15 shows the wear scars after rinsing the

samples in the PFPE solvent for PFPE, for both Ni coating after 500and 5000 fretting cycles.

Before acquiring the element maps, survey spectra weremeasured in order to have the composition of the surface. Thesespectra were performed on areas including the wear scar of about300 mm�300 mm. The calculated compositions are listed inTables 7 and 8. For the matte Ni fretting scars of 500 cycles and5000 cycles, large amounts of fluorine are detected. After 5000cycles, there is more oxygen and some copper is measured but theratio of nickel and fluorine stays the same. For the bright Nifretting scars, Table 8, there is also a large amount of fluorine. Itcan be seen though, that after 5000 cycles the fluorine concentra-tion has decreased, the nickel and fluorine ratio is much higher.

XPS maps of the wear tracks after the fretting tests are sum-marised in Fig. 16. The fluorine and copper maps are shown for matteand bright nickel fretting after 500 and 5000 cycles. Fig. 16(a) and(b) show the F 1s map for 500 and 5000 fretting cycles of matte Ni.

S. Noel et al. / Wear 301 (2013) 551–561560

Fluorine is located only inside the wear track: there is no fluorineoutside the wear scar; it has been reported that no chemical reactionoccurs between nickel and/or its oxides and PFPE. This indicates thatthe measured fluorine has not been eliminated by the repeatedrinsing operations because it is grafted on the nickel surface due tofriction. Friction removes some of the carbon–oxygen contaminationsand causes metallic nickel to be exposed and react with the PFPEmolecules. The grafted layer reduces friction, the friction coefficientbeing low in Fig. 13(b). In the long run some copper is exposed butthe map of Fig. 16(c) shows that is evenly distributed in the wear scar.

Fig. 16(d) and (e) show the F 1s maps for 500 and 5000 frettingcycles of bright Ni. Fluorine is also grafted in the fretting scar as forthe matte coating, but less fluorine is detected and after 5000 cyclesit can be observed to be localised at specific spots. Fig. 16(f) of Cu2p3/2 shows small amounts of copper localised mainly on a specificarea. This means that the lubricant efficiency has decreased andcould suggest that failure of the lubricating effect is occurring.This can be analysed in terms of roughness of the lubricated

Table 8Composition (at% element) for the lubricated bright Ni after 500 and 5000 fretting

cycles fretting cycles.

C O Ni S F Cu Zn Ni/F

500 cycles 33 30 25 2 11 – – 2.3

5000 cycles 45 33 18 – 3 2 – 6

Table 7Composition (at% element) for the lubricated matte Ni after 500 and 5000 fretting

cycles.

C O Ni S F Cu Zn Ni/F

500 cycles 28 27 30 o1 15 o1 – 2

5000 cycles 25 39 20 o1 10 3 3 2

20 μm

20μm

20 μm

20 μ

m

20 μm

20 μ

m

F1s

F1s

20 μm

20 μ

m

F

F

Fig. 16. XPS maps for: (a) F 1s map for matte Ni 500 cycles; (b) F 1s map for matte Ni 5

cycles; (e) F 1s map for bright Ni 5000 cycles and (f) Cu 2p3/2 map for bright Ni 5000

surfaces. The rougher surface tends to retain the lubricant longerand the efficiency is extended.

It is interesting to note that the same kinds of analyses havebeen performed with fretting scars from tinned contacts and thatno reaction of metallic tin with the lubricant was evidenced.

4. Conclusion

Nickel layers are largely used in the electrical contact industryeither as a barrier or as a final coating because it has a fairly goodbehaviour under most corrosive atmospheres. Much work hasbeen aimed at developing electroplating baths and showed theinfluence of various process parameters on the characteristics ofthe coatings.

The first results of this study compare the physico-chemical,electrical and mechanical properties of matte and bright nickelcoatings. A fretting test simulating the micro-displacementscaused by vibrations shows that different behaviours areobserved for coatings with different properties such as composi-tion, microstructure and roughness. The following conclusionscan be made:

–

1s

1s

000

cycl

With matte nickel a partial slip regime occurs during whichthe failure of the contact is postponed. This partial slip isfollowed by a gross slip one causing large degradations of thesurface leading to high increase of the resistance.

–

With bright nickel an initial drastic increase of the resistance isobserved. Surface analyses show that with low roughnessnickel coatings such as bright Ni, slow wear during frettingleads to the formation of an insulating NiO layer.

The effect of a fluorinated lubricant on these mechanisms isinvestigated. It is shown that a fluorinated compound is created inthe nickel contact interface during fretting.

Cu2p3

20 μm

20 μ

m

Cu2p3

cycles; (c) Cu 2p map for matte Ni 5000 cycles; (d) F 1s map for bright Ni 500

es.

S. Noel et al. / Wear 301 (2013) 551–561 561

The results also indicate that the presence of the lubricant filmcan avoid the adhesion and sticking phenomena occurring in thecase of matte coatings. This avoids the drastic increase of Rc

occurring after the transition from partial slip to gross slip.Clear evidence is given that a thin PFPE lubricant film brings a

protection from the atmosphere minimising the oxidation of thesurface. In the case of bright nickel the lubricant film can increasethe initial value of Rc and a running period is needed beforemeasuring low Rc.

The lubrication effect during the fretting test is observed tolast longer in the case of rougher nickel surfaces such as the matteone because of a reservoir effect.

These observations and analyses can be helpful during thedesign of connector terminals by engineers who have to assessdifferent solutions with pros and cons.

Acknowledgements

The Phi Versaprobe equipment was acquired thanks to fundingsfrom CNRS, ANR and Conseil Regional Ile de France.

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