Dry sliding wear behaviour of ZE41A magnesium alloy

Dry sliding wear behaviour of ZE41A magnesium alloy

Wear 271 (2011) 2836–2844 Contents lists available at ScienceDirect Wear journal homepage: www.elsevier.com/locate/wear Dry sliding wear behaviour ...

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Wear 271 (2011) 2836–2844

Contents lists available at ScienceDirect

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

Dry sliding wear behaviour of ZE41A magnesium alloy A.J. López, P. Rodrigo, B. Torres, J. Rams ∗ Dpto. Ciencia e Ingeniería de Materiales, ESCET, Universidad Rey Juan Carlos, C/Tulipán s/n, Móstoles 28933 Madrid, Spain

a r t i c l e

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Article history: Received 7 October 2010 Received in revised form 13 May 2011 Accepted 27 May 2011 Available online 6 June 2011 Keywords: Sliding wear Non-ferrous metals Electron microscopy Other manufacturing processes

a b s t r a c t Wear resistance of the ZE41A magnesium alloy was tested using pin-on-disc technique and steel as counterbody on dry sliding conditions. Wear rates and friction coefficients were measured in a sliding velocity range of 0.1–1 m s−1 and in a normal forces range of 5–40 N. Worn tracks and wear debris were studied by scanning electron microscope (SEM) and energy dispersive X-Ray spectrometer (EDX) to define the main wear mechanism for each testing condition. Wear mechanism map of the studied alloy was proposed. Low sliding velocities led to oxidative wear mechanism regardless of the load used, with a small participation of abrasion and delamination mechanisms. Intermediate speeds led to a predominant abrasion mechanism with participation of oxidation. At high speeds the main mechanism changed from abrasion at low loads to delamination at intermediate loads and to plastic deformation at high loads. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Magnesium alloys are attracting high attention due to their reduced density, which provides them high specific values of stiffness and strength. These properties, combined with a great facility of processing and reasonable cost, makes them suitable for application in many sectors and especially in the transportation one, including aircraft structures, aerospace components, as well as structural parts of mobile phones and portable computers [1]. Nevertheless, the use of magnesium alloys is limited because of their low surface properties such as corrosion and wear resistance, particularly in as-cast magnesium alloys [2]. During casting of magnesium alloys, it is not unusual the formation of porosity so that other treatments are frequently applied to this materials. In particular, the use of extruded magnesium alloys has attracted the interest as it improves the mechanical properties of the alloys, especially after heat treatments because it implies the lowest processing work and usually the highest mechanical properties [3]. The addition of zinc to magnesium significantly improves the strength of both cast and wrought alloys. In particular, the alloys containing Zn and rare earths (RE), such as the ZE41A used in this work, have moderate strength and creep resistance combined with good castability. Although the ZE41A alloy exhibits poor corrosion resistance, it is preferred for certain applications because of its moderate cost [4] and because it keeps good mechanical resistance up to 93 ◦ C, although it exhibits poor surface properties [5].

∗ Corresponding author. Tel.: +34 91 6647460; fax: +34 91 488 8150. E-mail address: [email protected] (J. Rams). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.05.043

This alloy, as well as many other magnesium ones, is not intended for use in unlubricated wear conditions, but as structural material it may be submitted to wear conditions. There are some works dealing on the wear behaviour of magnesium alloys: thixoformed and cast AZ91D magnesium alloys [6,7], of the AZ61 during dry sliding [8] and with other cast alloys such as Mg97Zn1Y2 [9]. Recently, some works have dealt with the dry sliding behaviour of ZE41A alloy in the as-cast and hot extruded conditions [10–12], and also it has been studied the improvement of the wear resistance of the ZE41 using aluminium matrix composite coatings reinforced with SiC particles deposited by thermal spraying [13]. In the different cases, the wear tests were made using the pin-on-disc configuration against steel or alumina couterfaces. Severe and mild wear regimes were detected and mechanism such as abrasion, oxidation, delamination, thermal softening and melting were observed for different wear conditions. All this regimes are common in many materials; in the case of magnesium and its alloys, the low surface hardness and the high reactivity in oxidative media causes that phenomenon such as oxidative wear, abrasion and delamination may play an important role even in mild wear conditions. In the present work, dry sliding wear behaviour of extruded ZE41A magnesium alloy in the T5 heat treated condition has been investigated using a pin-on-disc type wear apparatus against F112 steel. The effect of load and sliding velocity on the wear phenomena has been studied and different regimes were observed in the tested material. The main mechanisms observed were abrasion, delamination, plastic deformation and oxidation, and this last one has been evaluated as the main wear mechanism of the ZE41A magnesium alloy.

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Fig. 1. Backscattered electron micrograph of ZE41A alloy: (a) plain view and (b) cross-section. EDX spectra of the different phases: (c) bright precipitates, (d) dark gray zones and (e) light gray ones.

2. Experimental The ZE41A substrates used were supplied by Magnesium Elektron. Its main alloying element was Zn (4.1 wt.%) and its composition was (wt.%): 4.09 Zn; 1.68 rare earths (Pr + Nd + La + Ce); 0.68 Zr; 0.6 O; 0.03 Mn and balance Mg. The material was supplied in the form of extruded bar with a diameter of 60 mm in T5 condition. The hardness of the samples measured using an Instron Testor 2100 machine with a load of 100 g (HV0.1 ) was 81.7 ± 1.2 HV0.1 , which corresponds with that expected for the T5 condition. This hardness is significantly higher than that measured in other studies for the same alloy in the hot extruded and in the as-cast conditions that was 74.5 and 65.1 HV, respectively [12]. The microstructure of as-received ZE41A alloy, the wear tracks, the worn surface in the cross-section and the debris formed during

the wear tests were examined by Scanning Electron Microscope (SEM), equipped with an Energy Dispersive X-ray Spectrometer (EDX, XFlash 5010, Bruker). Metallographic samples were cut using a diamond disc cutter, ground on emery paper up to 1200 grade and polished with diamond paste of 3 ␮m particle size. ZE41A substrate was etched in glycol reagent to reveal its microstructure [14]. Wear tests were carried out under dry sliding condition on a pin-on-disc tribometre (Wazau) using the ZE41A as pin with sizes in mm of 18 × 5.9 × 3.6. The counterbody was a F112 steel disc and both materials were ground with different emery papers up to 1200 grit. Specimen and counterbody surfaces were cleaned with acetone to avoid the presence of humidity and non-desirable deposits. Wear tests were carried out onto surfaces that were perpendicular to the extrusion direction. At least three samples were tested for each wear condition.

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The wear test was carried out with loads of 5 N, 10 N, 20 N and 40 N; and sliding velocities of 0.1, 0.3, 0.5 and 1 m s−1 . The test was maintained until wear depth of 400 ␮m. The wear testing machine recorded continuously the friction coefficient and the wear depth. The samples were weighted before and after the wear test in order to determine the mass lost during the test. Volume loss during the wear test was determined from the mass lost using the alloy density, allowing determining the wear rate. To evaluate the wear mechanisms of the material under different conditions, the Archard’s law [15] was applied: V W =K = kW L H

(1)

In this equation V is the wear volume, L is the sliding distance, being the coefficient V/L the wear rate, W is the applied load, H is the hardness of the sample, K is the Archard’s constant and k is the specific wear rate. 3. Results 3.1. Substrate characterization Fig. 1 shows the microstructure and composition of the different phases of the ZE41A magnesium alloy used. The surface of the alloy, as observed in SEM in backscattered mode (Fig. 1a), revealed the presence of three main phases. There was a precipitated phase that appeared bright indicating its higher mass number and two other phases in different gray levels. The transversal section (Fig. 1b) evidences that the bright precipitates were nearly equiaxial and that the other phases were strongly elongated in the extrusion direction. EDX analyses showed that the bright precipitates contained, apart from Mg, Zn and Ce in a proportion that was about eight times that of the alloy (Fig. 1c). The darkest zones corresponded to a phase that was mainly constituted by Mg with lower content in rare earths and in Zn and Zr than the rest of the alloy (Fig. 1d). The light gray zones in these micrographs corresponded to phases with lower content of magnesium and higher contents in Zr and Zn than the dark gray ones (Fig. 1e). The precipitates rich in Ce and Zn were clearly placed in the dark gray zones, indicating that this may the cause of the depletion of alloying elements in these zones. Most of the La and part of the Ce were also found homogenously dispersed in all the phases. The microstructure of this tested alloy strongly differed from that tested in other works with similar alloys [10–12] in which equiaxial grains appeared and in which the precipitated alloying elements accumulated at the grain boundaries. 3.2. Wear testing Fig. 2 presents the variation of mass loss per sliding distance for the different tested conditions. Fig. 2a shows the mass lost per sliding distance as a function of velocity for the different loads tested. For all loads, the maximum mass loss rate was obtained for the lowest sliding speed, i.e. 0.1 m s−1 . Mass loss reduced by increasing the sliding velocity. For the highest load tested, it increased again when sliding at 1 m s−1 . For the lowest load the condition of 5 N with 1 m s−1 could not be made because the system was unstable, but wear rate increased at 0.5 m s−1 . Fig. 2b shows the mass lost per sliding distance as a function of applied load for the different sliding velocities. The mass lost increased with the applied force in all tested speeds, although it did not increased linearly. To evidence this, Fig. 3a shows the specific wear rate k (mm3 /N m) as defined by Eq. (1). It can be observed that the specific wear rate decreased in all cases with the applied load and, in many cases, with the sliding speed. This suggests that

Fig. 2. Mass loss/sliding of the different specimens tested in terms of: (a) sliding velocity of the tests and (b) normal force applied.

different wear mechanisms are taking place for the different tested conditions. The friction coefficient changed much from one test condition to another (Fig. 3b), especially for the lowest loads applied. In this case, the highest value measured was close to 0.7 and corresponded to intermediate speed of 0.5 m s−1 . As load increased, the friction coefficients of the different tests tended to 0.25 but for the lowest tested speed, for which remained above 0.3. These values indicate that different wear mechanisms are taking place at different test conditions, especially at low sliding velocities. To better understand the wear rates measured, contour maps of the results were constructed (Fig. 4). In Fig. 4a it can be observed that the highest mass loss per sliding distance corresponded to the highest load and lower velocity. There was a second maximum at the highest tested load and an intermediate zone in which minimum wear rate values were recorded. When the same figure is made in terms of the friction coefficients (Fig. 4b) it can be observed that for the highest speed, the behaviour did not change much with speed, showing a homogeneous behaviour in all these zones. In the lowest load range strong changes take place with the tested speed.

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Fig. 3. Variation in terms of normal force applied of: (a) specific wear rate and (b) friction coefficient. Fig. 4. Contour maps of the wear results: (a) mass loss/sliding distance and (b) friction coefficient.

3.3. Wear mechanisms In order to identify the different mechanisms that take place during wear of the ZE41A alloy, the worn pins and the wear debris were observed by SEM. Wear mechanisms such as abrasion, delamination, oxidation and, for the most exigent conditions, plastic deformation were observed for the different test conditions. 3.3.1. Abrasion Fig. 5a shows the surface of a pin tested at 0.1 m s−1 sliding velocity and 20 N of applied load and Fig. 5b shows the surface of a pin tested at 0.5 m s−1 sliding velocity and 20 N. In both cases it can be observed in the wear surface the presence of many fine grooves aligned, which are the proof of abrasion. These lines are usually caused by the presence of hard particles that plough into the pin. The movement of these particles over the surface cause the removal of material along its path on the surface of the Mg alloy. This wear mechanism predominates in the intermediate regimes of load and speed, but it is also present in nearly all the conditions tested mixed with other wear mechanisms. The effects of abrasion were also observed by SEM in the debris collected (Fig. 5c) for the test conditions 0.5 m s−1 and 20 N. There were short lathy strips indicating the presence of abrasion mechanism of ploughing and adhered to them there were some oxidized particles (as confirmed by EDX analyses) that confirmed the pres-

ence of oxidative wear mechanism in a low extent, as will be seen later.

3.3.2. Delamination Delamination of the surface of the worn ZE41A alloy was observed in some conditions as a result of the lost of material in the surface (Fig. 6). During the wear process subsurface cracks grow and in combination with cracks perpendicular to the sliding direction cause the detachment of sheet-like fragments of the worn material. The size of the voids was 30–60 ␮m in length, and 10–20 ␮m in depth and they allowed observing the undeformed material beneath. This mechanism was only observed for the lowest (0.1 m s−1 , Fig. 6a) and for the highest sliding velocity (1 m s−1 , Fig. 6b) at medium and high loads. In the case of the intermediate velocity of 0.3 m s−1 this mechanism was only observed at the highest load used and it did not appear for the sliding velocity of 0.5 m s−1 . The debris of these samples (Fig. 6c for the sample tested at 1 m s−1 and 20 N) shown the presence of platelets of 30–60 ␮m of the ZE41A alloy. These sizes are equivalent to those of the voids observed in the pin, confirming that delamination was the predominant wear mechanism.

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Fig. 5. SEM micrographs of the worn surfaces of the ZE41A pins tested at 20 N and: (a) 0.1 m s−1 ; (b) 0.5 m s−1 and (c) wear debris for 0.5 m s−1 .

3.3.3. Plastic deformation The surface of the samples at the highest sliding velocity and load (1 m s−1 and 40 N) was the only one that presented clear signs of massive plastic deformation of the surface (Fig. 7a and b). In these cases, it can be observed that the grooves caused by abrasion cannot be observed and that the surface of the samples was clearly

deformed. Platelets of about 500 ␮m detached from the magnesium alloy could be observed in the wear debris (Fig. 7c). They were plastically deformed and showed the progression of fracture lines in its interior. At lower speeds or loads (Fig. 7d) this mechanism was not dominant and can even be considered only as marginal.

Fig. 6. SEM micrographs of the worn surfaces of the ZE41A pins tested at: (a) 0.1 m s−1 20 N; (b) 1 m s−1 20 N and (c) image of the debris from sample shown in (b).

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Fig. 7. SEM micrographs of the worn surfaces of the ZE41A tested at: (a) 1 m s−1 40 N, detail in (b) and its corresponding debris in (c) and (d) 1 m s−1 and 20 N.

3.3.4. Oxidative wear Magnesium alloys strongly tend to oxidation even in the absence of aggressive conditions. Therefore, it is feasible that oxidative wear may play an important role in their behaviour. In all the samples there was evidence of oxidation, as the EDX peak of oxygen appeared for every sample. The maximum oxygen peaks were obtained for the samples worn at the smallest sliding speeds and loads. Fig. 8a shows a SEM image of the surface of a pin tested at 0.1 m s−1 sliding speed and 5 N load and Fig. 8b shows the surface of a pin tested at 0.3 m s−1 and 10 N. In both cases, but especially in the first one, the surface was covered by a thin layer of fine particles. The EDX of both images (Fig. 8c and d, respectively) revealed a strong oxygen peak in addition to the magnesium one. The debris observed for these conditions (Fig. 8e) and its corresponding EDX (Fig. 8f) revealed that particles were strongly oxidized. The origin of oxidative wear lies in the own oxidation sensitivity of Mg alloys that is enhanced by means of frictional heating caused by sliding. The presence of this oxidation layer prevented from metallic contact and resulted in small wear rates, which were in most conditions below 0.3. Some authors have suggested that oxidation layers are stable at sliding speeds below 1 m s−1 [10–12,16]. However, we have observed that at sliding speeds of 0.1 m s−1 , and in some cases at 0.3 m s−1 , the oxidation layer is not compact enough and that it tends to being removed form the surface of the sample, giving rise to a behaviour in which metallic contact between the surfaces causes the increase of the friction coefficient and of the wear rate. These results are similar to those observed in MgAl alloys and composites [17]. To evaluate the extent of oxidation of the different testing conditions, the amount of oxygen in the surface of the samples was evaluate by means of EDX testing. This semi-quantitative technique does not provide precision on the amount of oxygen present in the surface of the samples, but if the EDX analysis is made in the same conditions for all the different samples, it is possible to make a proper comparison between the different samples. Fig. 9a shows that the amount of oxidation in the surface of the samples for all testing speeds was highest for the lowest loads and

that it decreased when increasing the applied load. Similarly, the amount of oxygen in the sample surface was highest for the lowest testing speed and reduced as the speed increased. This indicates that oxidative wear dominates for low loads and sliding velocities, and that other mechanism such as delamination dominates for higher loads and velocities. This behaviour can be clearly seen in a surface plot (Fig. 9b). It can be appreciated that this contour plot is somehow perpendicular to that shown in Fig. 4a. In order to determine the role played by oxidation on the wear of ZE41A, a correlation between the amount of oxygen in the surface and the value of the specific wear rate (k) is plot in Fig. 10a. It can be clearly observed that the highest values of the specific wear rate corresponded with the highest values of oxidation, i.e. oxidative wear is the main mechanism that controls wear of the ZE41A magnesium alloy. Finally, the combined analysis of the contour map of specific wear rate (Fig. 10b) with the friction map (Fig. 4b) and with the oxidation rate of the samples (Fig. 9b) shows that they are strongly correlated. 3.4. Wear mechanism map From all the data and contour plots shown before, it can be extracted the wear mechanism map of the studied alloy (Fig. 11). This map has been derived from the mechanisms observed in the samples after the wear test. The map defines regions where the main wear mechanisms predominate, although the wear rates vary in each zone depending on the load and on the sliding velocity. These zones are separated by transition lines that depend on these parameters. It is important to consider that magnesium alloys have a strong tendency to oxidation at all temperatures that may be favoured by friction during the wear test. At low sliding velocity the formation of the oxide layer is promoted and the oxidative mechanism is the predominant one, with participation of abrasion and delamination. Increasing the sliding velocity promotes the abrasion mechanism, although the oxidative one is also present but in a lower extent, probably because during the test the oxide layer formed in the early stages is detached.

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Fig. 8. SEM micrographs of the worn surfaces of the ZE41A pins tested at: (a) 0.1 m s−1 5 N; (b) 0.3 m s−1 10 N; (c) EDX spectrum of the surface of substrate shown in (a); (d) EDX of the surface of substrate shown in (b); (e) debris from sample shown in (a); (f) EDX spectrum of the particle shown in (e).

At higher speed and moderate forces the delamination mechanism becomes the predominant one. And at with high applied forces the main wear mechanism is the plastic deformation of the magnesium alloy.

4. Discussion At low sliding velocity the magnesium is not removed from the pin and an oxide layer is formed in its surface. The magnesium oxides formed modify the contact between the sample and the disc and causes a decrease in the friction coefficient. However, magnesium oxides are porous and brittle, so when the thickness of the oxidized layer reaches a critical value it is easily removed from the surface. This mechanism is favoured with the increase in the applied load, causing maximum wear rates at high loads and low sliding velocity. In this zone of the wear map, oxidation starts the wear process and afterwards abrasion and delamination cause the detachment of the material. Increasing the sliding velocity promotes the abrasion mechanism. A wide range of the wear map developed corresponds to this zone. The increase in the velocity causes that, although oxida-

tion of the samples takes place, the oxide formed does not rule the behaviour of the contact between the magnesium alloy and steel. Abrasion takes place because hard particles plough into the pin and the movement of these particles over the surface cause the removal of material along its path on the magnesium alloy. The debris is constituted by short lathy strips that are partially oxidized. Increasing the load applied favours the oxidation of the alloy and reduces the friction coefficient. In this zone, the wear rate is smaller than in the previously analyzed region, it is stable and homogenous, with little sensitivity to load or sliding velocity. For practical applications, this regime can be regarded as “safe” operation regime since the wear rates are typically low and wear proceeds under steady-state condition. At higher speed and medium and high loads the delamination mechanism of the Mg alloy becomes the predominant one. At a difference from what was observed at low sliding velocity, the delamination observed corresponded to the Mg alloy and not to the oxidized material. In this case, the progression of subsurface cracks caused by the limited plastic deformation of the Mg alloys causes the detachment of big platelets from the alloy, although they appear in small numbers. Wear rates slightly increase from previous region and friction coefficient is quite stable.

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Fig. 9. Rate of oxidation of the surface of the samples tested: (a) at different sliding velocities in terms of normal force applied and (b) contour plot in terms of loads and sliding velocities.

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Fig. 10. (a) Correlation between the specific wear rate and the oxidation of the magnesium alloy substrate for the different sliding speeds and (b) contour plot of the specific wear rate in terms of load and sliding velocity of the wear test.

Fig. 11. Wear mechanism map of the studied alloy with a representative image of the main wear mechanism of the different zones.

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Finally, at high loads and high sliding velocity, the main wear mechanism is plastic deformation of the magnesium alloy. For these wear conditions, frictional heat cause an increase in the contact temperature, which causes a reduction in the yield strength. As a result of this mechanism, this zone becomes prone to plastic deformation and spreads out of the contact surface in the sliding direction in very big platelets. 5. Conclusions For the test conditions of this study, i.e. load in the range 5–40 N and sliding velocity in the range of 0.1–1 m s−1 , the wear rate increased with an increase of the applied load and, for most conditions, reduced with an increase of sliding velocity. The dominant wear mechanisms of the ZE41A in the low sliding velocities have been identified and summarized in the wear mechanism map. At small sliding velocity and loads, the predominant wear mechanism is oxidation, although some other mechanisms, such as abrasion and delamination, are also observed. The contribution of this mechanism is present in all conditions, but its relevance reduces as sliding velocity increases. As the sliding velocity increases, and for the biggest part of the wear map developed, abrasion becomes the main wear mechanism. This regime is the most adequate for operation because wear rates are low and the wear process proceeds under steady-state condition. As load and sliding speeds increase, delaminating of the Mg alloy becomes the most relevant wear mechanism and at the highest

tested conditions of load and sliding speed plastic deformation is the most relevant one. Acknowledgements Authors wish to thank Ministerio de Ciencia e Innovación (project MAT2009-09845-C02-02) and to the Comunidad de Madrid (project S-0505/MAT/0077) for the funding. References [1] B.L. Mordike, T. Ebert, Mater. Sci. Eng. A 302 (2001) 37–45. [2] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88–113. [3] A.A. Luo, W. Wu, R.J. Mishra, L. Jin, A.K. Sachdev, W. Dinge, Metall. Mater. Trans. A 41 (2010) 2662–2674. [4] E. Aghion, B. Bronfin, F. von Buch, S. Schuman, H. Friedrich, JOM 55 (11) (2003) 30–33. [5] Magnesium and Magnesium alloys. ASM Specialty Handbook , Michael M. Avedesian, 1999. [6] T.J. Chen, Y. Ma, B. Li, Y.D. Li, Y. Hao, Mater. Mater. Des. 30 (2009) 235–244. [7] T.J. Chen, Y. Ma, B. Li, Y.D. Li, Y. Hao, Mater. Mater. Sci. Technol. 23 (2007) 937–944. [8] A. El-Morsy, A. Ismail, M. Waly, Mater. Sci. Eng. A 486 (2008) 528–533. [9] J. An, R.G. Li, Y. Lu, C.M. Chen, Y. Xu, X. Chen, L.M. Wang, Wear 265 (2008) 97–104. [10] S. Anbu selvan, S. Ramanathan, Mater. Des. 31 (2010) 1930–1936. [11] S. Anbu selvan, S. Ramanathan, Mater. Sci. Eng. A 527 (2010) 1815–1820. [12] S. Anbu selvan, S. Ramanathan, J. Alloys Compd. 502 (2010) 459–502. [13] P. Rodrigo, M. Campo, B. Torres, M.D. Escalera, E. Otero, J. Rams, Appl. Surf. Sci. 255 (2009) 9174–9181. [14] Z. Yang, J.P. Li, J.X. Zhang, G.W. Lorimer, J. Robson, Acta Metall. Sin. (Engl. Lett.) 21 (2008) 313–328. [15] J.F. Archard, J. Appl. Phys. 24 (1953) 981–988. [16] S.C. Lim, Tribol. Int. 35 (2002) 717–723. [17] C.Y.H. Lim, S.C. Lim, M. Gupta, Wear 255 (2003) 629–637.