Mechanical and tribological properties of NiCr–Al2O3 composites atelevated temperatures

Feng Liu a,b,c, Junhong Jia a,n, Gewen Yi a, Wenzhen Wang a, Yu Shan a

a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR Chinab School of Materials Science and Engineering, Xi’an Shiyou University, Xi’an 710065, PR Chinac University of Chinese Academy of Sciences, Beijing 100039, PR China

a r t i c l e i n f o

Article history:Received 27 March 2014Received in revised form3 November 2014Accepted 26 November 2014Available online 4 December 2014

Keywords:NiCr–Al2O3 compositesMechanical propertiesTribological propertiesElevated temperatures

a b s t r a c t

The effect of Al2O3 content on the mechanical and tribological properties of Ni–Cr alloy was investigatedfrom room temperature to 1000 1C. The results indicated that NiCr–40 wt% Al2O3 composite exhibitedgood wear resistance and its compressive strength remained 540 MPa even at 1000 1C. The valuesobtained for flexural strength and fracture toughness at room temperature were 771 MPa, 15.2 MPa m1/2,respectively. Between 800 1C and 1000 1C, the adhesive and plastic oxide layer on the worn surface of thecomposite was claimed to be responsible for low friction coefficient and wear rate.

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1. Introduction

During the past few years, researchers had developed a lot ofhigh temperature antifriction materials with low friction coeffi-cient and high wear resistance suited to advanced technologicalfields such as high-performance gas turbine engines and aero-space applications [1–6]. Metal–ceramic composites have attractedmuch attention due to their extraordinary mechanical propertiesand wear resistance, along with their thermal and chemicalstability at high temperature, which makes them to be excellentcandidates to fabricate high temperature antifriction materials.Nickel based alloys are potential materials for long term hightemperature applications [7–9]. But the high temperature wear isthe major technical limitation to wide use of nickel based alloys inthe tribology system [10–12]. To improve wear resistance atelevated temperature, alumina is incorporated into nickel basedalloys because alumina is a promising material for tribologicalapplication at elevated temperature. It has been reported that thefriction and wear coefficient of alumina (Coors AD 998) slidingagainst alumina ball (Coors AD 995) reached to 0.4 and a wearvalue less than 10�6 mm3/N m above 800 1C due to the formationof silicon-rich layer on the wear track by diffusion and viscous flowof the grain-boundary phase [13]. The compressive strength ofalumina was reported to be 700 MPa at 1000 1C [14]. Meanwhile,flexural strength and fracture toughness of alumina ceramic will

be improved by incorporating the second ductile phase such asmetallic phase or intermetallic compound (Ni [15–17], Cr [18], Mo[19,20] and Ni3Al [21] etc.)

As high temperature antifriction materials, recent investiga-tions have led to the development of the interpenetrating metal–ceramic composites because the continuous metallic networkprovided effective crack bridging, while the ceramic networkprovided dimensional stability and good wear resistance at hightemperature. Al2O3–35 vol% Ni composite with interpenetratingmicrostructure prepared by reactive hot pressing exhibited animpressively high combination of strength and toughness. Thefour-point bending strength was in excess of 600 MPa with afracture toughness of more than 12 MPa m1/2, which was asso-ciated with the co-continuous microstructure of Al2O3–Ni compo-sites that allowed the Ni ligaments to bridge cracks, and, in turn,increase the toughness of the composite [15]. The oxidationmechanism of Al2O3–Ni composite was that the exposed nickelon the composite surface reacted with oxygen in the air at hightemperature, then the oxidation of its surface was controlled bydiffusional process, nickel ions diffused out and oxygen ionsdiffused in. It was confirmed that the oxidation resistance ofalumina was degraded due to the incorporation of metallic nickel[22]. However, it has also been found that the spinel layers(NiCr2O4) offered more protection against oxidation than Cr2O3

films on Ni–Cr alloy at elevated temperature when the spinelexisted between the alloy and the Cr2O3 [23,24]. So far, the studieson the tribological performance of Ni–Al2O3 composites at hightemperature were not available. But Ni–Al2O3 composite coatingexhibited a good tribological properties at elevated working

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Tribology International

http://dx.doi.org/10.1016/j.triboint.2014.11.0230301-679X/& 2014 Elsevier Ltd. All rights reserved.

n Corresponding author. Tel.: þ86 931 4968611; fax: þ86 931 8277088.E-mail address: jhjia@licp.cas.cn (J. Jia).

Tribology International 84 (2015) 1–8

temperatures [25]. Compared to the tribological properties ofnickel, Ni–20 wt%Cr alloy exhibited distinct reduction in frictionand wear rate at 800 1C due to the formation of a smooth softenedoxide layer (termed a glaze) with low shear strength on thebearing areas during sliding process. It was confirmed that theglaze layer consisted of NiCr2O4 and Cr2O3 on the worn surface ofthe Ni–20 wt%Cr alloy [26]. Stott proposed that the reduction ofwear rate at higher temperature was related to the increased rateof generation of wear debris particles, particularly oxide debris,which enabled the wear-debris layers to be established morerapidly. The compaction and sintering of the oxides particles inthe layers led to the development of the glaze surface at elevatedtemperature [27]. In addition, aluminum and chromium oxideshad the same crystal structures and formed solid solutions overthe entire range of composition, which may help to achieve a goodbonding between Al2O3 and Cr [18,28]. Therefore, it is expectedthat the addition of chromium will improve the interfacial adhe-sion of nickel and alumina and the formation of the glaze layer ofNi–Cr alloy oxides will provide good tribological properties atelevated temperature. In this paper, Ni–Cr alloy powders wereprepared by mechanical alloying in order to obtain fully denseNiCr–Al2O3 composites using powder metallurgy method, and theinfluence of Al2O3 content on the mechanical and tribologicalproperties of Ni–Cr alloy over a temperature range from roomtemperature to 1000 1C was investigated. Meanwhile, the tribo-films formed on the rubbing surface and their effect on thetribological properties of the composites at elevated temperaturewere analyzed and discussed.

2. Experiment

2.1. Materials preparation and selection

NiCr–Al2O3 composites containing alumina mass percentages of20, 30, 40, 50, and 60 were fabricated by powder metallurgicalmethod using vacuum-hot-pressing furnace (ZT-45-20Y, China),which were listed in Table 1. The starting materials were acommercially available Ni (60 μm), Cr (45 μm) and α-Al2O3

(30 nm), all in form of powder. Ni (80 wt%) and Cr (20 wt%)powders were first ball-milled for 20 h in Pulverisette 5 Planetaryhigh-energy-mill (Fritsch, Germany), then Al2O3 powder was addedand continuously ball-milled for 20 h. The ratio of ball to powderswas 10:1 in weight. The milled powders were subsequently coldcompacted in a graphite mold at a pressure of 20 MPa. Then, thepressed specimens were sintered at 1300 1C for 1 h with a heatingrate of 10 1C/min under the pressure of 25 MPa in a hot presssintering furnace at a dynamic vacuum of about 10�2 Pa.

2.2. Mechanical and tribological tests

The density of the sintered composites was determined by using ahelium pycnometry (AccuPyc 1330, Micromeritics Int. Corp, USA) aftercalculating the volume. The hardness measurements were conductedusing MH-5 Vickers microhardness instrument with a load of 300 g

and a dwell time of 10 s, and the average value of ten repeat tests wasgiven in this article. The compressive specimens with the size of4 mm�4mm�10mm were cut from the composites with differentstates by diamond wire machining (Shenyang Kejing Auto-instrumentCo Ltd, China) and all surfaces were mechanically ground with 800-grit SiC abrasive prior to compression test. The compression tests wereconducted in Gleeble-1500D test machine in the air at a constantstrain rate of 1�10�4 s�1 over a temperature range from roomtemperature to 1000 1C. As shown in Fig. 1a, the flexural strength ofthe composites with dimensions of 3 mm�4mm�36mm wasevaluated by three-points bending method using a DY35 universalmechanical testing machine (Adamel Lhomargy, France), with a spanof 30 mm and a crosshead speed of 0.5 mm/min. The fracturetoughness was measured on the DY35 universal mechanical testingmachine by a single-edge notched beam method with a span of30 mm and a crosshead speed of 0.05 mm/min, and the dimensions ofthe sintered composites were shown in Fig. 1b.

The friction and wear tests of the sintered composites were carriedout on a high temperature tribometer with a ball-on-disk configura-tion (CSM Instruments LTD, Switzerland). The disk was the sinteredcomposites with the size ofΦ40mm�8mm and the friction surfacewas polished to a roughness of 0.05 μm, while the counterpart ballwith a diameter of 3 mmwas made of Al2O3 ceramic (Shanghai UniteTechnology Co. Ltd, China). The chemical composition (wt%) of Al2O3

ball was 99.5%Al2O3 and 0.5% sintering aids (SiO2, MgO, Fe2O3 andNa2O). The Al2O3 ball had a surface roughness of 0.032 μm, a hardnessof 16.5 GPa, and density of 3.92 g/cm3. The tests were run at a slidingvelocity of 0.1 m/s, normal load of 10 N, duration of 60 min, wear trackradius of 5 mm and the selected test temperatures were RT, 200 1C,400 1C, 600 1C, 800 1C, 1000 1C. The wear depth profiles of all the weartracks were examined by Nano Map 500LS contact surface mappingprofiler (AEP Technology, USA) to obtain wear rate of sinteredcomposites. All the tribological tests were carried out at least threetimes in the same condition in order to make sure the reproducibilityof the experimental results, and the average results were reported. Thephase compositions of the composites and worn surfaces wereanalyzed by Rigaku D/max-RB X-ray diffractometer (XRD) with40 kV operating voltage and Cu Ka radiation in the 2θ range of20–801. The microstructure and morphologies of worn surfaces werecharacterized by JSM-5600LV Scanning Electron Microscope equippedwith Energy Dispersive Spectroscopy (EDS). The chemical states ofsome typical elements on the worn surface were examined usingThermon Scientific K-Alpha surface Analysis X-ray photoelectron

Table 1Composition, sintering parameters, density and relative density of NiCr–Al2O3

composites.

Composition (wt%) PM sintering parameters (1C, h, MPa) Density (g/cm�3)

NiCr–20Al2O3 1300, 1, 25 6.77NiCr–30Al2O3 1300, 1, 25 6.20NiCr–40Al2O3 1300, 1, 25 5.68NiCr–50Al2O3 1300, 1, 25 5.34NiCr–60Al2O3 1300, 1, 25 4.91 Fig. 1. The schematic diagram of the sintered composites: (a) flexural strength,

(b) fracture toughness.

F. Liu et al. / Tribology International 84 (2015) 1–82

spectroscope equipped with X-ray Monochromatisation and thebinding energy of carbon contaminant (C1s-284.8 eV) as the reference.

3. Results and discussion

3.1. Density and microstructure characterization of sinteredcomposites

The density and relative density of NiCr–Al2O3 composites are listedin Table 1. It can be seen that the density of the composites graduallydecreases with the increase of Al2O3 content, due to the fact that thedensity of Al2O3 is lower than that of the Ni–Cr alloy. The micro-structures of NiCr–Al2O3 composites are shown in Fig. 2. EDS analysisdemonstrates that gray phase is Ni (Cr) solid solution, whereas dark

gray phase is Al2O3 phase. It can be seen that NiCr–Al2O3 compositeswith different contents of alumina exhibit the similar microstructure,but the distribution of Ni (Cr) phase is transformed from continuousdistribution to dispersed distribution with the addition of Al2O3

content. For NiCr–40 wt% Al2O3 composite (Fig. 2c), it can be observedthat Ni (Cr) phase and Al2O3 phase are interpenetrating through themicrostructure, and phases appear to be homogeneously distributed.

3.2. Mechanical properties of sintered composites

The mechanical properties at room temperature of the sinteredcomposites with various Al2O3 contents are presented in Fig. 3. Asshown in Fig. 3a, it can be observed that the microhardnessincreases with the addition of Al2O3 content in the composites,which is mainly attributed to the hardness of the Al2O3 phase in

Al2O3

Ni (Cr)

Al2O3

Ni (Cr)

Al2O3

Ni (Cr)

Al2O3

Ni (Cr)

Al2O3

Ni (Cr)

Fig. 2. The SEM micrographs of NiCr–Al2O3 composites: (a) NiCr–20 wt%Al2O3, (b) NiCr–30 wt%Al2O3, (c) NiCr–40 wt%Al2O3, (d) NiCr–50 wt%Al2O3, (e) NiCr–60 wt%Al2O3.

F. Liu et al. / Tribology International 84 (2015) 1–8 3

the sintered composites. The compressive strength of the compo-sites at room temperature exhibits a similar trend and reaches amaximum value of 1696 MPa. It indicates that the continuousdistribution of the Al2O3 phase in the sintered composite is helpfulto enhance the compressive strength of the composites. Theflexural strength and fracture toughness of NiCr–Al2O3 compositeswith different Al2O3 contents are presented in Fig. 3b. It can beseen that the flexural strength of the sintered composites

decreases from 1101 to 525 MPa with the increase of Al2O3

content. The fracture toughness exhibits a similar trend andreaches to a minimum value of 10.6 MPa m1/2 due to the increaseof brittle Al2O3 phase in the composites. Fig. 4 shows thefractographs of the fracture surface of NiCr–40 wt%Al2O3

Fig. 3. The mechanical of NiCr–Al2O3 composites with different Al2O3 contents atroom temperature: (a) microhardness and compressive strength, (b) flexuralstrength and fracture toughness.

Fig. 4. The fractographs of fracture surface of NiCr–40 wt%Al2O3 composite with different magnifications: (a) low magnification, (b) high magnification.

Fig. 5. The tribological properties of NiCr–Al2O3 composites with different Al2O3

contents as a function of the test temperature: (a) friction coefficient, (b) wear rate.

F. Liu et al. / Tribology International 84 (2015) 1–84

composite with different magnifications, which exhibits the char-acteristic feature of the transgranular fracture. It is confirmed thatthe satisfactory interfacial bond between Ni (Cr) phase and Al2O3

phase. Intergranular fracture of Al2O3 phase is also observed onthe fracture surface (Fig. 4b). The plastic deformation of Ni (Cr)ligaments bridges the opening crack, leading to the increase of thetoughness of the composites. Compared with the pure Al2O3

ceramic [14], the flexural strength and toughness of NiCr–Al2O3

composites are improved, which are associated with the highrelative density of the sintered composites and the good interfacebonding between the Ni(Cr) phase and Al2O3 phase. More impor-tantly, the sintered composites demonstrate high toughness due toductile Ni (Cr) phase acting as bridging ligament sites.

3.3. Tribological properties of sintered composites

The friction coefficient and wear rate of NiCr–Al2O3 compositesover a temperature range from room temperature to 800 1C areshown in Fig. 5a and b. From Fig. 5a, it can be seen that NiCr–Al2O3

composites with different amounts of alumina exhibit high frictioncoefficient from room temperature to 600 1C. However, the frictioncoefficient of the composites decreases around 0.4 at 800 1C(except NiCr–60 wt%Al2O3). The results reveal that it is difficultto establish well-defined relationships between composition, testtemperature, and friction coefficient. As shown in Fig. 5b, itdemonstrates that the wear rates of the composites with differentamounts of Al2O3 reach a minimum value at 800 1C with increas-ing of test temperature, which is attributed to the formation oflubricious oxidized layer in the tribo-contact region. In addition, atdifferent test temperatures, the wear rates of NiCr–Al2O3 compo-sites decrease with the increase of Al2O3 content (except NiCr–30wt%Al2O3 at 200 1C) and reach a minimum value at 40 wt%Al2O3 concentrations, and their wear rates increase with thefurther addition of Al2O3 content (except NiCr–60 wt%Al2O3 atroom temperature) (Fig. 5b). It is confirmed that NiCr–40 wt%Al2O3 composite exhibits satisfactory wear resistance over atemperature range from room temperature to 800 1C. It is reason-able to explain the reason for the variation of wear rate of thecomposites with the increase of Al2O3 content at different tem-peratures: for lower Al2O3 contents (o40 wt%), the wear resis-tance of the composites is very poor owing to destruction of the Ni(Cr) phase followed by pullout of small Al2O3 grains from therubbing surface during the sliding process, while the contents ofAl2O3 are more than 40 wt%, the composites will fail in mainlybrittle fashion, resulting in fracture of large Al2O3 grains and Al2O3

framework after multiple scratching [29,30]. Damage and materialremoval occur as a result of the crack initiation and propagation ofthe composites surface during the sliding process.

3.4. Compressive and tribological properties of NiCr–40 wt%Al2O3

composite at elevated temperature

For high temperature antifriction materials, it is important thatcompressive strength at elevated temperature. The compressivestrength of NiCr–40 wt% Al2O3 composite over a temperature rangefrom room temperature to 1000 1C is shown in Fig. 6. As can be seenin Fig. 6, although the compressive strength of the sintered compo-site decreases significantly with the increase in the test temperature,it remains above 540 MPa even at 1000 1C. The self-supportingcontinuous skeletal structure of alumina endows the composite witha good compressive strength at elevated temperature.

The tribological properties of NiCr–40 wt%Al2O3 composite fromroom temperature to 1000 1C are shown in Fig. 7. From Fig. 7a, it canbe seen that the friction coefficients of the sintered composite remainsaround 0.7 from room temperature to 600 1C and then decreases withthe increase in the test temperature, and reaches to a minimum valueof 0.29 at 1000 1C. While its wear rate remains at level of6�10�5 mm3/Nm from 200 1C to 600 1C and reaches to a minimumvalue of 2.62�10�6 mm3/Nm at 800 1C. But the wear rate of thesintered material increases to 5.79�10�6 mm3/Nm at 1000 1C. Asshown in Fig. 7b, at 800 1C, the friction coefficient of the composite

Fig. 6. The compressive strength of NiCr–40 wt% Al2O3 composite over a tempera-ture range from room temperature to 1000 1C.

Fig. 7. The tribological properties of NiCr–40 wt%Al2O3 composite as a function ofthe temperature: (a) friction coefficient and wear rate, (b) friction coefficient as afunction of time at 800 1C and 1000 1C.

F. Liu et al. / Tribology International 84 (2015) 1–8 5

decreases to a relatively low value after sliding for a long period.However, its friction coefficient reaches to the steady-state in a shortsliding time at 1000 1C.

Fig. 8 shows the morphologies of the worn surfaces of NiCr–40 wt%Al2O3 composite after wear test at different temperatures. As shownin Fig. 6a, at room temperature, the worn surface of the sinteredcomposite is covered with ploughed micro-grooves and plastic sme-aring of materials illustrates that the wear mechanism is micro-cutting and abrasive wear. As temperature increases to 200 1C, the

worn surface is covered with discontinuous tribofilms. These dis-continuous films are deformed, and containing some delaminationpits (Fig. 8b). The worn surfaces of the composite at 400 1C and600 1C exhibit similar morphologies, except long and deep parallelgrooves are observed on the worn surface at 600 1C [31]. It is rationalto explain why its wear rate remained at level of 6�10�5 mm3/N min a temperature range from 200 1C to 600 1C. When the temperatureup to 800 1C, the continuous glaze layer is found on the worn surface,and there is no evidence that the micro-plowing and scratch areobserved (Fig. 8c). So the wear mechanism of the sintered material at800 1C is characterized by plastic deformation. At 1000 1C, the glazelayer of the bearing surface turns smooth, and debris with differentshapes appear on the worn surface together with plastic deformation(Fig.8d). It is confirmed that the formation of the plastic oxide layeron the worn surface results in relatively low friction coefficient in ashort sliding time. The corresponding to the XRD patterns of thesintered material on the rubbing surface is showed in Fig. 9. It revealsthat Cr2O3 and NiCr2O4 phases are formed after wear test above800 1C, which is ascribed to the oxidation of chromium and nickel inNi–Cr matrix and the tribo-chemical reaction between NiO andCr2O3. Fig. 10 shows the variations of chemical states of Cr, Ni andAl on the worn surface of NiCr–40 wt%Al2O3 composite after weartests at 800 1C and 1000 1C. The binding energies occurred at577.1 eV, 576.1 eV are assigned to Cr in Cr2O3 [32] and NiCr2O4

[33], respectively. The Ni3p3/2 peaks at 856.2 eV [34] and 854.5 eV[35] indicate the presence of NiCr2O4 and NiO species on the wornsurface (Fig. 10b). In Fig.10c, according to the handbook of X-rayphotoelectron spectroscopy, the Al2O3 is identified on the rubbingsurface. It indicates that the formation of NiCr2O4 and Cr2O3 in thebearing region might result in the reduction of friction and wear ofthe composites above 800 1C.

Glazer layer

Glazer layer

Debris

Fig. 8. The worn surfaces of NiCr–40 wt%Al2O3 composite tested at different temperatures: (a) room temperature, (b) 200 1C, (c) 800 1C, (d) 1000 1C.

Fig. 9. XRD patterns of the worn surface of NiCr–40 wt%Al2O3 composite testedat 800 1C and 1000 1C.

F. Liu et al. / Tribology International 84 (2015) 1–86

Above 800 1C, the worn surfaces of NiCr–40 wt%Al2O3 compositeare not only subjected to the stresses associated with frictional andcontact forces but are also susceptible to oxidation by reaction with

oxygen during sliding process, which was attributed to the incorpora-tion of Ni–Cr matrix. At 800 1C, the transient oxide layers were formedon the rubbing surface, it continued to grow following diffusion of Niand Cr ions to the scale/gas interface or oxygen to the scale/alloyinterface due to the interfacial reaction between oxygen in the air andthe exposed Ni–Cr matrix on the composite surface. Therefore, NiOand Cr2O3 were formed on the surface of the sinteredmaterials. Due tothe high concentration of chromium in NiCr–40 wt%Al2O3 composite,the amount of Cr-rich oxide was observed to be more than that of NiOat 800 1C [36]. With further oxidation, NiCr2O4 was formed on thesurface, which was attributed to the chemical reaction between NiOand Cr2O3 (Figs. 9 and 10). However, in the subsequent sliding process,the removal of the oxide layer consisting of NiCr2O4 and Cr2O3 and re-oxidation of the fresh surface played an important role in theformation of the glaze layer on the worn surface. The glaze layerwould be formed from the smearing of large compacted oxide debris,which was attributed to the fragmentation and rearrangement andplastic flow of the oxide debris particles under frictional forces. Thesubsequent development of the glaze layer led to a distinct reductionin friction and wear (Fig. 7). It indicated that the softening and plasticoxide layer of NiCr2O4 and Cr2O3 provided the low friction coefficientand wear rate [26]. When the temperature up to 1000 1C, as comparedto the tribological properties of the pure Al2O3 ceramic at 1000 1C [13],the sintered composite exhibited low friction coefficient, which wasattributed to the formation of NiCr2O4 and Cr2O3 oxide layer on thecontact region. However, the temperature of the bearing surface couldbe sufficiently high to greatly soften the oxide layer (Fig. 8d), which ledto plastic flow and then resulted in relatively high wear rate(5.79�10�6 mm3/Nm) than that of the sintered composite at800 1C (Fig. 7a). Its good tribological properties together with decentmechanical properties in the temperature range from 800 1C to1000 1C rendered them promising materials for various high tempera-ture tribological applications.

4. Conclusions

NiCr–Al2O3 composites were prepared by powder metallurgymethod. The effect of Al2O3 content on the mechanical andtribological properties of Ni–Cr alloy was investigated from roomtemperature to 1000 1C. Results from these studies revealed that:

(1) At room temperature, microhardness and compressivestrength of NiCr–Al2O3 composites increased with the additionof Al2O3 phase, which was attributed to the hardness and thecontinuous distribution of the Al2O3 phase. But their flexuralstrength and fracture toughness exhibited an opposed trenddue to the increase of brittle Al2O3 phase in the composites.

(2) NiCr–40 wt% Al2O3 composite exhibited the satisfactory wearresistance over the entire temperature range. From roomtemperature to 1000 1C, the self-supporting continuous skele-tal structure of alumina was responsible for the good com-pressive strength of the sintered composite.

(3) For NiCr–40 wt% Al2O3 composite, the formation of the glazelayer consisting of NiCr2O4 and Cr2O3 led to the distinctdecrease in the friction coefficient and wear rate between800 1C and 1000 1C.

Acknowledgements

The authors acknowledge the financial supports by theNational Natural Science Foundation of China (Grant nos.51175490, 51101166, 51471181).

Fig. 10. XPS spectra of elements on the worn surface of NiCr–40 wt%Al2O3

composite tested at 800 1C and 1000 1C: (a) Cr2p3/2, (b) Ni3p3/2, (c) Al2p.

F. Liu et al. / Tribology International 84 (2015) 1–8 7

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