SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 2002; 34: 790–793Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/sia.1412

Work of adhesion and fretting: influence of materialsurface properties and medium

Mathieu Lambert, Sebastien Duluc, Jean-Yves Paris,∗ Yves Baziard and Jean Denape

Laboratoire Genie de Production, Ecole Nationale d’Ingenieurs, BP 1629, 65016 Tarbes Cedex, France

Received 16 July 2001; Revised 3 December 2001; Accepted 29 December 2001

This work attempts to link tribological and adhesion phenomena in fretting conditions. Frettingexperiments were carried out in three environmental conditions (air atmosphere, demineralized waterand kerosene) and with various couples of industrial materials. Contacting materials and liquids werechosen mainly for their surface properties, in order to have a wide range of surface free energies. Thefretting device included a pin-on-plane configuration. The pin (or slider) was fixed and the plane was themoving substrate. The total distance (dT) covered by the moving substrate was chosen as the tribologicalparameter representative of the history of an experiment. The distance dT was then compared to surfaceparameters such as Dupre’s work of adhesion between the two substrates in contact (W12) and with threebodies (W132), the index 3 being the liquid medium. In these conditions, dT = f .W12/ and dT = f .W132/

variations were found to be linear, thus showing a clear link between friction and adhesion. Copyright 2002 John Wiley & Sons, Ltd.

KEYWORDS: fretting; work of adhesion; liquid medium; polyamide–imide coating; steels; sintered ceramics

INTRODUCTION

Fretting is often encountered in industrial systems whenvibrations of very weak amplitude take place between twosolids in contact, leading to surface and bulk degradations ofmaterials. Fretting behaviour of materials has been studiedwidely for many years. The partial understanding of somemechanisms was achieved by analysing both mechanical andmaterial processes involved during fretting.1 – 5 In particular,adhesion phenomena can occur from the initial moment offretting and can lead to the seizure of bodies in contact.Many models allow adhesive friction phenomena on theatomic scale to be described for tip–sample interactions(atomic force and friction force microscopy techniques)6 – 8

and, according to the authors, special attention should bepaid to the relation between the work of adhesion andthe corresponding friction force. Very few studies dealwith the influence of adhesion phenomena on friction onthe macroscopic scale and merely tried to link the frictioncoefficient and the work of adhesion between polymericmaterials.9,10

The aim of this work was to show the influence, in frettingconditions, of the physic-chemistry of the substrate surfacesand liquid medium in which the contact occurs. In this way,the medium and the surfaces were characterized by contactangle measurements that allowed the liquid surface tensionand the work of adhesion between the materials in interactionto be calculated. These thermodynamic parameters were

ŁCorrespondence to: Jean-Yves Paris, Laboratoire Genie deProduction, Ecole Nationale d’Ingenieurs, BP 1629, 65016 TarbesCedex, France. E-mail: paris@enit.fr

compared with mechanical parameters of fretting and onlyone of them (the total distance dT) provided conclusive resultsin the conditions of our experiments.

EXPERIMENTAL

The fretting deviceThis device simulated the phenomena occurring insidethe contact area between two substrates, in pin-on-planeconfiguration, subjected to an oscillatory movement. A forcesensor and a displacement sensor give the friction forcetransmitted by the moving substrate (the plane) and thereal displacement amplitude. The latter was lower than thedisplacement amplitude set-point because the movement ofthe plane was drived in open-loop control. As a consequence,the real displacement was directly linked to the forcesgenerated by the friction of substrates. In this work, all theexperiments were performed for a short duration (600 s), ata low frequency (10 Hz) and for relatively weak appliedloadings, in order to minimize substrate damage. Theamplitude of the displacement set-point was 100 µm.

Materials and liquidsThe materials under investigation were two sintered ceram-ics, a zirconia 3Y-TZP (from Societe des Ceramiques Tech-niques, SCT), a silicon carbide (SiC100 from Ceramiques etComposites, C&C), two steels (Z15 CN17-03 and 100Cr6) anda polyamide–imide-coated AU4G (Fluorimid-9G from Flu-orotechnique). This coating consisted of a polyamide–imideresin filled with PTFE fibres (¾20 wt.%) and MoS2 powder (5wt.%). Its thickness was ¾40 µm.

Copyright 2002 John Wiley & Sons, Ltd.

Work of adhesion and fretting 791

Table 1. Surface free energies of substrates andenvironmental liquids

Materialsand liquidsa

�D

(mJ m�2)�AB

(mJ m�2)�

(mJ m�2)

tac 3Y-TZP 40.6 š 1.2 31.3 š 1.5 71.9 š 2.7ntac 3Y-TZP 32.2 š 0.4 5.0 š 0.5 37.2 š 0.9SiC100 43.0 š 1.0 1.3 š 0.6 44.3 š 1.6Z15 CN17-03 35.2 š 0.5 0.4 š 0.2 35.6 š 0.7100Cr6 37.6 š 0.4 1.0 š 0.3 38.6 š 0.7Fluorimid-9G 31.1 š 0.3 0.8 š 0.3 31.9 š 0.6Deionized water 21.8 š 0.5 51.0 š 0.5 72.8 š 1.0Kerosene 25.2 š 0.4 0.0 25.2 š 0.4

a tac D thermally air cleaned; ntac D not thermally air cleaned.

All the substrates used for fretting experiments andcontact angle measurements (see later) were previouslydegreased in an alkaline solution (pH 10) for 10 minat ambient temperature under ultrasonic motion and airdried (in an oven) for 20 min at 60 °C. In the case ofzirconia substrate, a thermal air cleaning (tac) at 625 °Cisothermally for 1 h in air was applied before analysis.This treatment significantly increase the 3Y-TZP surface freeenergy compared with the uncleaned substrate (see Table 1).

The substrates were associated in pin-on-plane couples.The pins had a hemispherical shape (curvature radius: 25 mmfor steel pins and 21.5 mm for zirconia pins) and the planeswere 5 mm thick disks or squares. The pins were polishedfor zirconia and rectified for steels. The planes were polishedfor zirconia, used ‘as received’ from sintering (SiC100) andwere spray-coated for polymeric Fluorimid-9G substrate.The average (Ra) and total (Rt) surface roughness coefficientswere then, respectively, equal to: 0.04 and 1.04 µm for 3Y-TZPpins and planes; 0.43 and 9.3 µm for SiC100 planes; 1.34 and13.74 µm for Fluorimid-9G planes; Ra D 0.3 µm for Z15 CN17-03 and 100Cr6 pins. The couples were: tac 3Y-TZP pin/tac3Y-TZP plane, ntac 3Y-TZP pin/SiC100 plane, Z15 CN17-03 pin/Fluorimid-9G plane, 100Cr6 pin/Fluorimid-9G planeand 100Cr6 pin/tac 3Y-TZP plane.

Concerning the environmental conditions of fretting, theexperiments were performed at ambient temperature (22 °C),either in air (¾60% relative humidity) or deionized water orkerosene. The latter is an organic solution constituted by amixture of C11 and C12 alkanes.

Contact angle measurementsAn important part of this work was to characterize thematerials and the liquids by wettability. Measurements weremade using the Digidrop goniometer from GBX Instruments.Thus, the surface tension of liquids (�L) were determined bythe pendant-drop technique and the surface free energy ofsubstrates (�S) by the sessile-drop technique. With the latter,contact angles were determined using the one-liquid methodwith the Fowkes’ procedure.11 The surface free energies (�)of materials and liquids are given in Table 1. Accordingto Fowkes,11 � D �D C �AB, with �D for the long distance(>0.4 nm) dispersive (non-polar) Lifschitz–Van der Waalsinteractions and �AB for the short distance (<0.4 nm) Lewisacid–base (polar) interactions.

RESULTS AND ANALYSIS

Figure 1 shows variations of the real displacement amplitude(da) and of the tangential force (FT) as a function of time for atac 3Y-TZP pin/tac 3Y-TZP plane couple. The da D f �t� andFT D f �t� variations are typical of many fretting experimentsperformed during this work: the variations take place byplateaus. Here, da decreases quickly from 80 µm (initial value)to a first short plateau (¾70 µm) and goes down to a secondplateau value (¾40 µm) at 100 s. After this and between 100and 200 s, da decays to ¾10 µm and remains constant untilthe end of the experiment: this corresponds to a seizure of thepin–plane couple. The tangential force and the displacementamplitude vary simultaneously and inversely, except after450 s, from which FT increases up to the constant valueof 6.5 N. The friction coefficient f �f D FT/FN; FN-appliednormal load), not shown in Fig. 1, varies with FT because FN

remains constant.Additional information is provided by da D f �t� curves

of Fig. 2, because the da variations can be compared for thedifferent applied loads FN. From 0.5 to 7.5 N, the first and thesecond plateau can be observed, but for 0.5 and 2.5 N the sec-ond plateau goes on until the end of the experiment because

0

20

40

60

80

100

0 100 200 300 400 500 6000

2

4

6

8

Time (s)

da (µm) FT (N)

Tangential force (FT)

Displacement amplitude (da)

Figure 1. Fretting experiment for a tac 3Y-TZP pin/tac 3Y-TZPplane couple. Experimental conditions: ambient air (60%relative humidity); applied normal force FN D 7.5 N;displacement amplitude set-point D 100 µm; t D 600 s.

0

20

40

60

80

100

0 100 200 300 400 500 600

2.5 N

7.5 N

12.5 N

17.5 N

Time (s)

da (µm)

0.5 N

Figure 2. Real displacement amplitude (da) versus time for atac 3Y-TZP pin/tac 3Y-TZP plane couple and for differentvalues of the applied normal force (FN). Experimentalconditions: ambient air (60% relative humidity); displacementamplitude set-point D 100 µm.

Copyright 2002 John Wiley & Sons, Ltd. Surf. Interface Anal. 2002; 34: 790–793

792 M. Lambert et al.

there is no pin–plane seizure. The latter occurs from 7.5 Nand for shorter times when FN increases. The two plateausare shorter as the applied load increases and they van-ish totally for 17.5 N. Comparative experiments (not shownhere) with a zirconia couple not thermally cleaned haveshown longer plateaus and seizures at longer times, there-fore it can be hypothesized that the first plateau correspondsto the pin–plane couple accommodation through the hydro-carbonated contamination layer and the second plateau toan accommodation through a third-body layer consistingof wear debris (observed by SEM and optical microscopy)in addition to the contamination layer. All the phenomenadescribed here for the tac 3Y-TZP pin/tac 3Y-TZP plane cou-ple in ambient air were also observed in deionized water,but with slightly lower displacement amplitudes for 0.5 and2.5 N and earlier pin–plane seizures from 7.5 N. This meansthat the friction phenomenon was slightly increased in water.On the other hand, it must be noted also that there was nopin–plane seizure in kerosene in our experimental conditionsfor the tac 3Y-TZP pin/tac 3Y-TZP plane couple. The dis-placement amplitude da was identical at the beginning andend of the experiment—da D f �t� curves (not shown here)are totally flat—and no wear mark was observed inside thecontact area, either on the pin or plane surfaces.

The curve profiles of Fig. 2, corresponding to differentexperimental events, show the importance of the history ofan experiment. In order to describe the fretting phenomenaby a single tribological parameter, and with the aim ofcomparing it with surface free energy parameters, we chosethe total distance (dT) covered by the moving substrate(the plane) during an experiment. On the other hand, aninteresting surface parameter is Dupre’s work of adhesionbetween two bodies (W12): W12 D �1 C �2 � �12, where �1, �2

and �12 are, respectively, the surface free energies and theinterfacial surface free energy between the two materials ininteraction. This work depends also on additive dispersiveVan der Waals and Lewis acid–base interactions: W12 DW12

D C W12AB, with W12 D 2��1�2�1/2, W12

D D 2��1D�2

D�1/2

and W12AB D 2��1

AB�2AB�1/2.11 Thus, dT D f �W12� variations

show remarkable results (Fig. 3). Indeed, if there is nopin–plane seizure (which will reduce dT drastically), thetotal distance is linearly dependent on the work of adhesion,whatever the environmental medium. In ambient air and indeionized water, dT is inversely proportional to W12, withinexperimental variation (Figs 3(a) and 3(b)). It can be notedthat pin–plane couple fretting behaviours were very closefor experiments carried out in ambient air and in deionizedwater. Knowing the possible influence of water on fretting,1,4

the hypothesis here is to consider that fretting in ambientair occurred on wet surfaces (relative humidity was ¾60%during our experiments).

On the other hand, in kerosene (Fig. 3(c)), the totaldistance dT is independent of the work of adhesion W12

within experimental variation. A priori, dT D f �W12� plotsprovide information concerning substrate surface propertiesonly, but if results in water and in kerosene are compared(Figs 3(b) and 3(c)), the influence of liquid medium on frettingappears clearly. In deionized water (or in ambient air), asexpected, friction was high (short distance dT) for a high work

a)

b)

0

0.25

0.5

0.75

1

1.25

1.5

0 25 50 75 100 125 150 175 200

0.5 N

7.5 N

2.5 N

17.5 N

12.5 N

W12 (mJ/m2)

dT (m)

Sliding area

Seizure area

0

0.25

0.5

0.75

1

1.25

1.5

0 25 50 75 100 125 150 175 200

W12 (mJ/m2)

dT (m)

Sliding area

Seizure area

c)

0

0.25

0.5

0.75

1

1.25

1.5

0 25 50 75 100 125 150 175 200

W12 (mJ/m2)

dT (m)

Sliding area

Seizure area

0.5 N

2.5 N

7.5 N

2.5 N

7.5 N

12.5 N

Figure 3. The dT D f�W12� variations for the differentpin–plane couples in three different environmental conditions:(a) ambient air; (b) deionized water; (c) kerosene.

of adhesion. This was not the case for fretting experimentsperformed in kerosene. It can be hypothesized then thatkerosene was present inside the contact and screenedthe specific acid–base interactions between substrates. Thepresence of kerosene inside the contact can be explained byits low surface tension (�L D 25.2 mJ m�2), which allows itto fulfil the wetting criterion (�L � �S) and therefore to wettotally the substrate surfaces (�S D 31.9 mJ m�2 for the lowestvalue; see Table 1). On the contrary, the wetting criterionis not fulfilled with the polar water (�L D 72.8 mJ m�2

by comparison with the highest �S value of 71.9 mJ m�2;Table 1). This means that water partly wet the substratesurfaces and would be present partly inside the contact.

At this stage, dT D f �W12� plots cannot provide moreinformation and it would be interesting to examine anotherenergy parameter: Dupre’s work of adhesion with three

Copyright 2002 John Wiley & Sons, Ltd. Surf. Interface Anal. 2002; 34: 790–793

Work of adhesion and fretting 793

bodies W132 D �13 C �23 � �12. All calculations made in thiswork can be explained as W132 D 2�3 � W13 � W23 C W12.This parameter takes into account not only the interactionsbetween substrates in contact (W12) but also the interactionsbetween each substrate and the liquid medium (W13 andW23), along with the surface tension of this liquid (�3).Thus, the dT D W132 plots (not shown here) are flats forthe different pin–plane couples in kerosene. This confirmsthat friction is independent of adhesion in kerosene. Indeionized water, however, dT increases linearly as W132,contrary to what was observed for the previous dT D f �W12�.This would mean that adhesion is not favoured in waterand the partial presence of water inside the contact seemsto be implicated here. At this stage, however, with onlyone polar liquid tried in our experiments it becomesdifficult to give more explanation because of the lack ofinformation.

CONCLUSION

The main goal of this study was to emphasize the role ofadhesion in tribological phenomena. Fretting experimentsof short duration, weak applied loads and with an open-loop control of the displacement amplitude were carriedout with various pin–plane couples and in three differentmedia (ambient air, deionized water and kerosene). In theseexperimental conditions, the total distance (dT) coveredby the moving substrate during an experiment was theonly pertinent parameter that could be compared withthe work of adhesion of two solid bodies (W12) and twobodies plus the liquid medium (W132). Thus, dT varied

linearly with W12 and W132, showing a direct link betweenfriction and adhesion. The experimental conditions forwhich a pin–plane seizure occurred and the role of theliquid medium inside the contact between substrates werespecified. However, if kerosene is present inside the contactand screens the interactions between substrates, the roleof water is still not clearly elucidated. This study allowedus to set in place the basis of an original experimentalapproach. The important results obtained need, however, tobe confirmed and encourages us to plan new experimentswith other dispersive and polar substrates and liquidmedia.

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Copyright 2002 John Wiley & Sons, Ltd. Surf. Interface Anal. 2002; 34: 790–793