Structural evolution in copper layers during sliding under different lubricant conditions

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Acta Materialia 58 (2010) 4685–4692 www.elsevier.com/locate/actamat

Structural evolution in copper layers during sliding under different lubricant conditions Alexey Moshkovich a, Vladislav Perfilyev a, Tatyana Bendikov b, Igor Lapsker a, Hagai Cohen b, Lev Rapoport a,* b

a Holon Institute of Technology, Holon 58102, Israel Chemical Research Support, The Weizmann Institute of Science, Rehovot 76100, Israel

Received 29 October 2009; received in revised form 25 April 2010; accepted 2 May 2010 Available online 2 June 2010

Abstract Friction and wear of copper rubbed in a wide range of loads and sliding velocities were studied. The results of friction and wear experiments in PAO-4 lubricant are presented as the Stribeck curve where the boundary, mixed and elasto-hydrodynamic lubrication regimes are considered. The structural state of surface layers in different lubricant regimes is studied by optical and scanning electron microscopy and X-ray photoelectron spectroscopy analyses. The dominant friction and wear mechanisms in different lubrication regimes are discussed. Severe plastic deformation of subsurface layers under friction is correlated with the nanocrystalline structure obtained by different methods of grain refinement. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Friction; Wear; Lubrication, copper; Severe plastic deformation; Microstructure

1. Introduction Copper (Cu) and its alloys are widely used as engineering materials for various moving machine parts. Strong plastic deformation (SPD), leading to the formation of a cellular/vein or fragmented structures, occurs during the running-in process under dry friction [1–5]. The cellular vein or fragmented structures have been found to be similar to the structure of materials prior to failure due to fatigue and cyclic creep [3,6,7]. It is known that the structure of single metallic materials in fatigue is characterized by the formation of persistent slip bands and by attaining of saturated stress (e.g. [8,9]). Dislocation density approaches the saturation value of 1015 m2 [10]. It has been well documented that SPD of subsurface layers causes the formation of nanocrystalline tribolayers [11,12]. Grain refinement leading to the formation of a nanocrystalline structure is of considerable scientific interest *

Corresponding author. Tel./fax: +972 3 5026616. E-mail address: [email protected] (L. Rapoport).

due to the superior mechanical properties of these structures. A number of methods, e.g. equal channel angular pressing, high-pressure torsion and surface mechanical attrition treatment, have been used in order to obtain an ultrafine grain structure in copper (e.g. [13–15]). Indeed, the nanocrystalline structure of Cu gives rise to low friction and wear in comparison to samples without any preliminary deformation (e.g. [13]). In order to distinguish between the various friction and wear behaviors of metals under different contact conditions, Lim and Ashby developed wear maps [16,17]. Following Lim and Ashby’s maps, friction and wear maps for different materials and different contact conditions have been obtained. In contrast, very few studies have quantified the effect of lubrication on the structural evolution of Cu surfaces under varying friction conditions. Similarly to Lim and Ashby’s wear maps, the Stribeck curve [18] is widely used in order to identify different lubrication regimes. The Stribeck curve distinguishes between boundary lubrication (BL), mixed lubrication (ML) and hydrodynamic lubrication (see e.g. [19,20]). Elasto-hydro-

1359-6454/$36.00 Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2010.05.001

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dynamic lubrication (EHL) is the part of the ML regime where low friction and wear are usually observed. The transition from one lubrication regime to the other is determined by the Sommerfeld number, as used by Hersey [19], or other, similar, parameters, such as the lubrication number of Gelinck and Schipper [21]. Despite years of extensive studies on the lubrication of metals and alloys, little is still known about the evolution of structure in different lubrication regimes. Understanding of the mechanism of structural evolution under different contact conditions is crucial for the development of wear maps of materials rubbing under lubricated and unlubricated conditions. Accordingly, the present investigation was initiated with three objectives in mind: first, to study the friction and wear behavior of Cu surfaces under different lubricant conditions; secondly, to examine the structural evolution of Cu subsurface layers under friction in different lubricant conditions; and thirdly, to compare the structure of subsurface layers obtained by friction with a nanocrystalline structure after grain refinement. 2. Experimental procedure The friction experiments were performed using a blockon-ring rig (Fig. 1a). The rings made from steel (AISI 51100) hardened up to HRc = 60 slide against a block of pure (99.98 wt.%) Cu. The constant area of the block ensures a constant pressure over the duration of the test. This is important for the analysis of friction and structural parameters under different loads and sliding velocities. Furthermore, careful polishing and etching of wear surfaces provide a unique opportunity to characterize the ploughing tracks and grain structure around the grooves at different depths (Fig. 1b). The roughness of the rings and blocks, Ra, was about 0.12 and 0.2 lm, respectively. The average grain size of the Cu samples was 35 ± 15 lm. Cu samples were annealed at 250 °C for 2 h in order to minimize the effect of mechanical history on the structure of surface layers. The Vickers hardness of virgin Cu samples was 800 ± 50 MPa. The effects of sliding velocity and load on the friction coefficient and wear rate of Cu block-hardened steel ring pairs were studied. The sliding velocity was varied in the range of 0.1–1 m s1 and the load was varied between 90 and 420 N (contact pressure was within 5– 22 MPa). Six drops of the synthetic oil, PAO-4, with a viscosity of 18 MPa s at 40 °C, were added to the contact area each minute. The friction force, block displacement (wear loss) and a temperature near the contact were measured. A displacement sensor was used in order to measure in situ the change of the clearance between the contact pair and the sensor (wear loss). Friction results were presented as the Stribeck curve. gu The lubrication number of Schipper, Z ¼ pR [21], was used a as the horizontal axis of the Stribeck curve. Here, g is the viscosity of the lubricant, u is the sliding velocity, p is the

Fig. 1. (a) Scheme of a ring-on-block pair. (b) Cross-sectional imaging of sub-layers around the plough grooves. The sliding direction is shown by the arrow.

pressure and Ra is the arithmetic mean of the departures of the profile from the mean line for the harder sample (i.e. the roughness of the hard ring surface). According to Archard [22], the wear volume w is proportional to the load F and sliding distance L. Therefore w = KFL, where K is the wear constant, usually presented in the units mm3 Nm–1. The Vickers microhardness tests were used extensively for characterization of the surface layers before and after friction test. In order to measure the hardness of a thin surface layer of about 1 lm, a nanohardness test at a load of 20 mN was performed. The hardness variation with depth was measured using cross-sectional Cu samples. The load in hardness measurement was 0.1 N. In order to analyze the grain structure around the plough tracks, worn surfaces were carefully polished down to different depths (see Fig. 1b). After the polishing, the samples were etched, rinsed and dried. This procedure enabled analysis of the grain structure and plastic deformation around the plough tracks on the wear surface. Wear particles were collected after friction testing and then carefully rinsed in an ultrasonic bath prior to the analysis in a scanning electron microscope. The microstructural evolution of Cu before and after the friction test under different lubricant conditions was characterized using optical microscopy, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques. All SEM images were observed

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using a Stereoscan–430i scanning electron microscope (20 kV acceleration voltage and standard Everhart–Thornley detector). XPS measurements were performed using a Kratos AXIS-Ultra spectrometer, using monochromatic Al Ka radiation and detection pass energy ranging between 10 and 80 eV. An XPS imaging mode was used to compare selected regions across the rough surfaces. These showed no significant differences in chemical composition between the heights and valleys of the friction surfaces. Argon sputtering was applied at a pressure of 2  107 torr under a 4 kV beam accelerating voltage. 3. Results 3.1. Friction and wear experiments

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A correlation between the friction coefficient and wear rate during the friction of Cu was observed: the smaller the values of the friction coefficient, the lower are the values of the wear rate. It may be seen that, although the values of the friction coefficient and wear rate are remarkably different in the EHL and BL regimes, their values remain practically constant (steady friction states). Plough grooves are observed on the surface of Cu samples in the BL regime, while a relatively smooth surface with thin fine grooves developed in the EHL regime. Ploughing of soft Cu surface layers in the BL regime led to the formation of relatively large wear particles (Fig. 3a). The size of the wear debris varied from 10 to 50 lm and the thickness was close to 0.2 lm. In contrast, in the EHL regime, which is characterized by low friction

Fig. 2 summarizes the dependence of the friction coefficient and the wear rate on the Z parameter for the copper– steel pairs rubbed under different loads and sliding velocities. Three regimes were observed under lubricated friction of copper–steel pairs with different loads and sliding velocities.  The EHL regime with low values of the friction coefficient (l = 0.01–0.03) and the wear rate close to 108 mm3 Nm–1 (Z = 6–10  103).  The ML regime where the friction coefficient and wear rate increased remarkably with time up to the values observed in the BL regime (Z = 3–6  103).  The BL regime with high values of the friction coefficient (l = 0.23–0.25) and wear rate close to 5  106 mm3 Nm–1 (Z = 1–3  103).

Fig. 2. Dependence of the friction coefficient and wear rate on the Z parameter for copper–steel pairs rubbed under different loads and sliding velocities. The circles indicate the effect of sliding velocity on the friction coefficient. The triangles indicate the effect of load on the friction coefficient. The crosses indicate the effect of sliding velocity and load on the wear rate. The black marks characterize the steady friction state; the blank marks, the unsteady range.

Fig. 3. SEM image of the wear particles collected after friction in the BL (a) and EHL (b) regimes.

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(l = 0.01–0.03) and wear rates (K  108 mm3 Nm–1), the wear particles were much smaller in the size (0.1–0.2 lm) (Fig. 3b). 3.2. Structural state of the surface layers under lubricated conditions Since ploughing plays an important part in the damage of surface layers in the BL regime, the formation and destruction of plough grooves are considered in greater detail. Fig. 4 shows the grain structure around the plough grooves. The sliding direction is shown by the arrow. Two regions of subsurface deformation around the plough grooves are observed: a region of strong plastic deformation with layered lamellar structures of about 10 lm and a deformed structure region of about 100 lm with slowly increasing grain size up to the values of the virgin state. The formation of plough grooves on a worn surface is accompanied both by scratching of soft surface layers of the Cu by hard asperities of a counter-body and by free debris particles that moved into the interface of the rubbing surfaces. The movement of free wear particles in the interface leads to the formation of complex shaped grooves on the wear surface. Thus both straight and round grooves can be seen on the wear surface (Fig. 5a). The magnified micrograph shown in Fig. 5b displays strong plastic deformation around the plough grooves, leading to the formation of a nanocrystalline structure with grain sizes of 100–200 nm.

A cross-section of subsurface layers of a worn Cu sample rubbed in the BL region is shown in Fig. 6. To summarize this section, we have presented experimental evidence of a cross-sectional region in which the deformation hardening due to friction in the BL regime is more than 100 lm (Fig. 6a). Cross-sectional observations reveal the formation of layered lamellar structure in the subsurface layers around the plough grooves (Fig. 6b). The structural state of surface layers under friction in the BL regime can be observed by optical and scanning electron microscopy, but these methods were not effective in the analysis of the subsurface structure due to friction in the EHL regime. Severe plastic deformation was localized in thin layers of about 10 lm under friction in the EHL regime. The hardness variations in the subsurface layers after friction in the BL and EHL regions are shown in Fig. 7. The nanohardness to a depth of about 1 lm was 1700 ± 250 and 1600 ± 140 MPa for the Cu samples rubbed under BL and EHL lubricant conditions, respectively. These values were used as the hardness of the surface in Fig. 7. The hardness of subsurface layers was evaluated with cross-sectional samples starting from a depth of about 15 lm. High values of indentation hardness of thin surface layers indicate that strong plastic deformation occur in both the EHL and BL regimes. The average values of hardness on the wear surfaces under friction in the EHL and BL regimes are similar (taking into account a spread of the results). Two ranges of strong and weak deformation hard-

Fig. 4. (a) SEM micrograph of the plough groove on a worn surface of Cu after friction in the BL regime. (b) The region of severe plastic deformation with lamellar layered structure and the transition region, where the size of grains is increasing slowly. (c) Subsurface layers at a depth of about 300 lm. Sliding direction is shown by arrow.

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ening are revealed under friction in both the EHL and BL regimes: it can be seen that deformation hardening was stretched up to a depth of about 300 lm under friction in the BL regime, whereas it decreased to the value of virgin microhardness at a depth of about 100 lm under friction in the EHL regime.

Fig. 6. Cross-section of the subsurface layers of a worn Cu sample after friction in the BL regime. Plough grooves are noted by arrows (a). Laminated layered structure is formed around the plough grooves (b).

2000

Microhardness, MPa

Fig. 5. SEM image of worn Cu surface after friction in the BL regime. Plough grooves associated with asperity contact and a rolling of free wear particles in the interface are observed on the worn surface (a). Magnification of the region between grooves reveals the nanocrystalline structure with grain size of about 100–200 nm (b). The sliding direction is shown by the arrow.

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1500 1 1000 2 500

3.3. XPS results XPS analysis was conducted to study the chemical composition of the surface films for both the BL and EHL regimes. Fig. 8 shows the binding energies of Cu2p (a), O1s (b), S2p (c), C1s (d) Cu LMM (e) and the valence bands (f), respectively, on the worn surface of Cu after friction in the EHL regime (l = 0.03; red dots) and in the BL regime (l = 0.25; black solid line). Several distinct differences are observed, e.g. in the Cu LMM line (Fig. 8e) and in the top region of the valence bands (Fig. 8f) near the zero of the energy scale. These differences are associated mainly with the appearance of a metallic signal at the high-friction (BL) regime, which is absent from the low-friction (EHL) regime. The XPS-derived atomic concentrations are summarized ˚ for in Table 1, suggesting an average thickness of 22 A

0

100

200

300 Depth, µm

400

500

Fig. 7. The variation of the microhardness as a function of depth after friction in the BL (1) and EHL (2) regimes.

the carbonic layer on the low-friction surface and only ˚ for the high-friction surface. This layer also includes 16 A oxygen, the signal of which appears at the high binding energy side of the line in Fig. 8b. The major component of the oxygen signal is attributed to the formation of copper oxide. Interestingly, the surface passivation of the low-friction sample also includes sulfur. We reach this conclusion from the following considerations. First, the Cu signal is definitely that of Cu(I) oxide and not Cu(II) oxide, as is evident from both the Auger parameter (see the line positions in Table 2, Ref. [23]) and the absence of characteristic shake-up signals next to

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Cu 2p

400

Intensity, cps

Intensity, cps

a

2000

b

O 1s

300

200

1000 100

960

950

940

930

540

535

Binding Energy, eV

c

530

525

Binding Energy, eV

S 2p

400

d

C 1s

Intensity, cps

Intensity, cps

120

100

300

200

100

80

172

168

164

160

156

295

290

Binding Energy, eV

e

280

Valence

0.8

Intensity, counts

Intensity, cps

f

Cu LLM

600

285

Binding Energy, eV

400

0.4

200

0.0 580

570

560

550

540

Binding Energy, eV

9

6

3

0

-3

Binding Energy, eV

Fig. 8. A comparison between spectra of low-friction (solid line) and high-friction (dotted line) samples. Valence band spectra are normalized. Note the absence of the metallic conduction band in the valence spectrum (f) of the low-friction sample (as indicated by the arrow just above the zero binding energy).

Table 1 XPS-derived atomic concentrations of the various samples. Sample

Cu3p

O1s

C1s

S2p

Low friction, before sputtering Low friction, after 20 s of sputtering Low friction, after 80 s of sputtering High friction

28.94 66.91 73.38 38.92

19.86 19.00 17.57 22.34

49.21 7.20 3.33 39.8

2.0 6.88 5.72 –

the Cu2p line in Fig. 8a. Secondly, direct bonding of S to Cu, but not to oxygen, is indicated by the S2p binding energy. Based on the above Cu chemical bonding information and on the magnitude of the Cu metal signals, the possibility of highly heterogeneous overlayers, which include both thick and very thin regions, can be excluded. Hence, the

A. Moshkovich et al. / Acta Materialia 58 (2010) 4685–4692 Table 2 Energy positions (in eV) of the characteristic XPS lines, experimental and literature values.

Metal – experiment Metal – literature Passivation – experiment Passivation literature Cu2S Cu2O

Cu2p3/2

Cu LMM (kinetic energy)

932.48 932.6 932.55

918.64 918.6 916.5

932.5 932.5

917.4 916.6

S2p3/2

O1s

162.1

530.35

162.4

530.3

All lines are given as binding energy positions except for the Cu LMM, which is given in kinetic energy. The literature data are taken from the PHI Handbook [23].

metal surfaces are believed to be covered by relatively thin layers (oxide plus organic molecules), on the scale of a few nanometers. This layer is significantly thicker in the lowfriction samples. Furthermore, a couple of very short sputtering steps confirm the above conclusions. As shown in Table 1, a very short sputtering step was sufficient to remove the organic layer (i.e. the carbon signal) and leave the layer of copper passivation nearly untouched. At that stage the O:S ratio is slightly less than three. A second sputtering step already shows an early decrease in the passivated layer signal, in full agreement with the above estimation of the layer thickness, P10 nm (the sputtering rate here is 4 nm min–1). In conclusion, the wear process under the BL regime is found to expose fresh metallic regions repeatedly, while a reduced load (friction under the EHL regime) allows the formation of a thicker and more uniform overlayer, such that the metal signal is not observed by this surface sensitive technique. 4. Discussion Friction and wear behavior of copper–steel pairs rubbed under different lubricant conditions was studied. Friction in the EHL region is accompanied by low values of the friction coefficients (l = 0.01–0.03) and wear rate (K = 108 mm3 Nm–1). The very small size of the wear particles (100– 200 nm) indicates a strong deformation localized at thin surface layers. Since the friction coefficient and wear loss are small in the EHL region, the hydrodynamic pressure of a lubricant film is the dominant parameter in this contact region. As the friction coefficient and wear rate rise, the role of direct asperity contact increases. High values of the friction coefficient (l = 0.23–0.25) and wear rate (K = 105 mm3 Nm–1) indicate that the dominant mechanism in the BL regime is severe asperity contact, leading to ploughing of the soft surface layers of Cu and delamination of the wear particles. It is expected that during repeated sliding in the BL regime subsurface cracks are developed in the depth close to 0.2 lm (thickness of wear particles), leading ultimately to delamination of the wear particles, as described in Refs. [24,25].

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As can be seen, the steady friction state is preserved for both the EHL and BL regimes, although the dominant friction and wear mechanisms are different. It has been shown that the steady friction state is characterized by the balance between geometrical and structural parameters of the contact [26]. This balance implies equilibrium between deformation, relaxation and fracture in the deformed layers on the one hand and a preservation of the sizes of the asperity contacts and the wear particles on the other hand. It is expected that the balance of geometrical and structural parameters of contact in the EHL and BL regimes are preserved but that each process occurs at different scale levels. Indeed, the damage under friction in the EHL regime is localized in thin surface layers in comparison to thick hardened layers under the friction in the BL regime. This is confirmed by the strong ploughing of surface layers and by the formation of large wear particles observed under friction in the BL regime in comparison to the EHL region. Similar to other processes leading to SPD, two-dimensional planar microstructures, e.g. layered lamellar structures, are developed in the direction of friction. The hardness of the thin surface layers saturates with repeated sliding of Cu. These high values for the hardness are close to the hardness of nanocrystalline Cu produced by different SPD processes [14,27]. The saturated state as characterized by the formation of a nanocrystalline structure in the surface layers under friction is similar to that obtained for Cu subjected to strong plastic deformation during different grain refinement processes. In this case, it was interesting to compare structural evolution under unlubricated and lubricated friction of Cu. Rigney evaluated the dislocation density under friction and predicted that the saturation would be 3.7(5)  1015 m2 and the strain of saturation would be above 5 [28]. Similar results were obtained in the model of Kuhlmann-Wilsdorf [3], q = 2.4  1015 m2. Accepting that, it may be concluded that the saturated hardness of thin surface layers under unlubricated and lubricated friction conditions are similar. It thus seems that answer to the question “Should limiting concentration of dislocations for different substructure produced by different modes of deformation?” should be “Yes” [10]. This important conclusion is based on the same values of hardness in the thin surface layers under steady friction state in the BL and EHL regimes: the same SPD is obtained in thin surface layers under friction both in the BL and EHL regimes. The main difference between these two friction conditions is, apparently, the number of spots subjected to direct asperity contact. Some of the spots are covered by thin lubricant film and thus protect the rubbing surfaces and decrease the friction coefficient (l = 0.23–0.25 in the BL regime) in comparison to unlubricated friction (l = 0.6–0.8). It is thus suggested that the different gradients of strain, hardness and temperature will also determine the variations in friction and wear behavior between the lubricated and unlubricated conditions. This aspect of the problem will be considered in the future.

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The XPS surface analysis confirmed the dominant role of a tribofilm created due to friction under the EHL regime. A passivated Cu2S/Cu2O layer is formed, protecting the Cu surface from damage during direct contact. The tribofilm can physically separate two rubbed surfaces, avoiding direct asperity contact and thus providing both a low friction coefficient and a low wear rate. On the other hand, under high loads the wear experiment repeatedly exposes fresh metallic regions. In fact, the atomic concentration of Cu in this case is close to the concentration obtained for untreated samples. This indicates that the tribofilm cannot remain on the rubbed surface and direct asperity contact is the dominant mechanism under friction in the BL regime. 5. Conclusions 1. Friction and wear behavior of copper–steel pairs rubbed under different lubricant conditions were studied. Friction in the EHL regime is accompanied by low values of the friction coefficient (l = 0.01–0.03) and the wear rate (K = 108 mm3 Nm–1), while in the BL regime the friction coefficient is l = 0.23–0.25 and the wear rate is K = 105 mm3 Nm–1. 2. The dominant friction mechanism of Cu in the EHL regime is hydrodynamic pressure through the lubricant film, whereas direct asperity contact with formation of the plough grooves is responsible for the high values of the friction coefficient and wear rate. 3. Friction of Cu in lubricated conditions is characterized by the formation of layered lamellar structures in the direction of friction. The thickness of deformed layers was about 300 and 100 lm under friction in the BL and EHL regimes, respectively. The hardness of thin surface layers saturates during repeated sliding of Cu to a value close to the hardness of nanocrystalline Cu produced by different SPD processes. 4. It has been shown that the saturated hardness of thin surface layers under both unlubricated and lubricated friction conditions is similar. The main difference between these friction conditions is, apparently, the different gradients of strain, hardness and temperature. 5. The XPS surface analysis confirmed the dominant role of the tribofilm created under friction in the EHL regime. The wear process under the BL regime is found

to expose fresh metallic regions repeatedly, while friction under the EHL regime leads to the formation of thicker, more uniform overlayers which are responsible for the low friction and wear.

References [1] Rigney DA. In: Suh NP, Saka N, editors. Fundamental of tribology. Cambridge, MA: The MIT Press; 1978. p. 119. [2] Heilmann P, Rigney DS. In: Dowson D, Taylor CM, Godet M, Berthie D, editors. The running-in process in tribology. Proceedings of the 8th Leeds–Lyon symposium on tribology; 1981, p. 25. [3] Kulhmann-Wilsdorf D. In: Rigney DA, editors. Fundamentals of friction and wear of materials. In: Papers presented at the 1980 ASM Materials Science Seminar. Metals Park, OH: American Society of Metals; 1980. p. 119. [4] Garbar II. Wear 1995;181–183:50. [5] Garbar II. Wear 1986;7:1043. [6] Hirth JP, Rigney DA. Wear 1976;39:133. [7] Hirth JP, Rigney DA. In: Nabarro FRN, editor. Dislocations in solids, vol. 6. North-Holland; 1983. p. 1. [8] Cheng AS, Laird C. Mater Sci Eng 1981;4:331. [9] Buque C. Int J Fatigue 2001;23:671. [10] Rigney DA. Mat Res Innovat 1998;1:231. [11] Emge A, Karthikeyan S, Rigney DA. Wear 2009;267:562. [12] Emge A, Karthikeyan S, Kim HJ, Rigney DA. Wear 2007;263:614. [13] Zhang YS, Han Z, Wang K, Lu K. Wear 2006;260:942. [14] Mishra A, Kad BK, Gregory F, Meyers MA. Acta Mater 2007;55:13. [15] Gubicza J, Chinh NQ, Csanadi T, Lagdon TG, Ungar T. Mater Sci Eng A 2007;462:86. [16] Lim SC, Ashby MF. Acta Metall 1987;35:1. [17] Lim SC, Ashby MF, Brunton JH. Acta Metall 1987;35:1343. [18] Stribeck R. Zeitschrift des Vereines Deutscher Ingenieure (Zeit Ver Deut Ing) 1902;46:1341, 1432, 1463. [19] Hersey MD. Theory and research in lubrication. New York: Wiley; 1966. [20] Hamrock B. Fundamentals of fluid film lubrications. New York: McGraw-Hill International; 1994. [21] Gelinck ERM, Schipper DJ. Tribol Int 2000;33:175. [22] Archard JF. J Appl Phys 1953;24:981. [23] Moulder JF, Stickle WE, Sobol PE, Bomben KD. Handbook of Xray photoelectron spectroscopy. A reference book of standard spectra for identification and interpretation of XPS data. Perkin–Elmer; 1992. [24] Suh NP. In: Suh NP, Saka N, editors. Fundamental of tribology. Cambridge, MA: The MIT Press; 1978. p. 443. [25] Kulhmann-Wilsdorf D. In: Nabarro FRN, editor. Dislocations in solids, vol. 6. North-Holland; 1983. p. 119. [26] Rapoport L. Wear 2009;267:1305. [27] Balogh L, Ungar T, Zhao Y, Zhu YT, Horita Z, Xu C, et al. Acta Mater 2008;56:809. [28] Rigney DA. Scripta Met 1979;13:353.