Tribology International 60 (2013) 147–155

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Tribology International

0301-67

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Tribological properties of graphite-like carbon coatings couplingwith different metals in ambient air and water

Yongxin Wang a,n, Liping Wang b, Jinlong Li a, Jianmin Chen a,b, Qunji Xue a,b

a Ningbo Key Laboratory of Marine Protection Materials, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, PR Chinab State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, PR China

a r t i c l e i n f o

Article history:

Received 8 September 2012

Received in revised form

8 November 2012

Accepted 8 November 2012Available online 23 November 2012

Keywords:

Coating

Sliding

Dry

Aqueous

9X/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.triboint.2012.11.014

esponding author. Tel.: þ86 574 86685175;

ail address: yxwang@nimte.ac.cn (Y. Wang).

a b s t r a c t

Tribological properties of graphite-like carbon (GLC) coatings coupling with different metals of brass,

Al, GCr15 and Ti in ambient air and water were investigated. The order of friction coefficients (FCs) was

GLC–brass4GLC–Al4GLC–GCr154GLC–Ti in both of the two environments, but the FC in water was

lower than that in ambient air for each tribo-pair. Wear rates (WRs) decreases of the GLC coating in

water were observed when sliding against Al, GCr15 and Ti. Though the WR of GLC coating sliding

against brass increased in water, it was still comparatively low.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Presently, the green machining has become focus of attention inecological and environmental protection [1,2]. As a result, the dry orwater-lubricated machining was proposed [3–5]. However, the ineffi-cient lubrication would certainly result in the high friction and wearbetween the tool surface and metals. In order to solve this problem,advanced coatings were emphasized. Among various coating materi-als, the graphite-like carbon (GLC) coating, referring to the amorphouscarbon with significant sp2-hybidized sites, revealed great potentialfor the application of high performance tool surface under dry condi-tion or in water flood due to its self-adapted friction–reduction andanti-wear performances in either ambient air or water [6–9]. Stallardet al. [10] demonstrated that the GLC coating could exhibit lowfriction and wear under three environmental conditions including air,water and oil when sliding against WC. Fujisawa et al. [11] arguedthat the carbon coating possessing a greater sp2 or graphite-likecontent provided more effective solid lubrication in a wet environ-ment after a comparative study of a range of carbon coatings usingone-side-coated Ti–6Al–4 V alloy couples. Wang et al. [12] reportedthe low friction and high load bearing of GLC coating when slidingagainst GCr15 steel in air with the humidity of 70%. And our previousstudy revealed that the GLC coating could perform well as thelubricating and protective surface to various substrates when slidingagainst Si3N4 under both dry and water-lubricated conditions [13].

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fax: þ86 574 86685159.

However, most of the researches only focused on the tribolo-gical performances of GLC coatings with individual counterparts.Comparative studies on the different tribological performances ofGLC coatings sliding against different materials, especially forcommon metals, were needed. In this paper, four common metalsof brass, Al, GCr15 and Ti were chosen as the sliding counterparts.Different tribological performances of the GLC coatings to thesedifferent metals in both ambient air and water were investigatedand discussed systematically.

2. Experimental

2.1. Coating preparation

The GLC coatings were deposited on WC discs by unbalancedmagnetron sputtering deposition in Ar atmosphere. The WCsubstrates with dimensions 30 mm�30 mm�5 mm were pre-pared by Beijing Taimi Machinery Co., Ltd. And the workingsurfaces were diamond polished to a final surface roughness ofRa¼10–20 nm. Ti interlayers was used to improve the adhesionbetween the GLC coatings and WC substrates. Details of thedeposition were published elsewhere [9].

2.2. Coating characterization

The microstructures were characterized by a scanning electronmicroscope (SEM, Hitachi S-4800). The hardness and elasticmodulus were measured by a nano-indenter (Nano Indenter II,MTS Ltd., US).

Table 1Mechanical properties of different metallic counterparts and the maximum Hertz

contact pressures.

Counterpart Brass Al GCr15 Ti

Hardness (HV) 130 73 891 227

Elastic modulus (GPa) 105 72 208 106

Poisson ratio (m) 0.35 0.33 0.30 0.30

Maximum contact pressure (GPa) 1.30 1.10 1.62 1.28

Y. Wang et al. / Tribology International 60 (2013) 147–155148

2.3. Tribological performances

A reciprocating-type ball-on-disc tribo-meter (CSM, Tribo-S-D-0000) was used to investigate the tribological performances ofGLC coating to brass, Al, GCr15 and Ti in both dry and waterenvironments at room temperature. The four metallic counter-parts had the same diameter of 3 mm, whose mechanical proper-ties were revealed in Table 1. The maximum contact pressureswere calculated from the Hertz model for a ball on a flat surface[14,15], which were given in Table 1 as well. All the tests wereperformed under a load of 5 N. The reciprocating amplitudewas 5 mm, and the maximum sliding speed was 7.86 cm/s. Eachtribo-test was performed 20,000 cycles. The friction coefficients(FCs) were recorded automatically. The wear rates (WRs) of GLCcoating against different counterparts were calculated using theequation: K¼V/SF [16], where V is the wear volume of GLCcoating, S is the total sliding distance and F is the normal load.The volume losses of GLC coatings were determined from thewear track profiles obtained by a non-contact 3D (3-dimensional)surface profiler (MicroMAXTM, ADE Phase Shift, AZ US). And thewear surfaces were also analyzed by JSM-5600 SEM, Ramanspectroscopy and electron probe micro-analysis (EPMA).

Fig. 1. SEM images of the GLC coating: (a) surface and (b) cross-sectional

morphologies.

Fig. 2. FCs of GLC coatings against different counterparts.

3. Results

3.1. Characteristics of the GLC coating

The graphite-like microstructure of the as-fabricated GLCcoating has been demonstrated in our previous study [4].Thenano-hardness obtained by nano-indenter was 17 GPa, while theelastic modulus was 170 GPa. Fig. 1 shows the surface and cross-sectional morphologies of the GLC coating on WC substrate withinterlayer of Ti. Seen from Fig. 1(a), the coating surface wasdispersed with fine nanometer-sized particles that were agglom-erated into packs in micro-scale. The cross-sectional SEM pictureshown in Fig. 1(b) reveals that the dense GLC coating bonded wellwith the WC substrate by means of the interlayer of Ti.

3.2. Friction and wear performances

Fig. 2 reveals FCs of GLC coatings sliding against with brass, Al,GCr15 and Ti. For the dry-sliding in ambient air, the difference ofFCs was GLC–brass4GLC–Al4GLC–GCr154GLC–Ti. The highestFC among these tribo-pairs was approximately 0.46 for GLCsliding against brass (GLC–brass), while the lowest FC was�0.09 for GLC–Ti. The FCs of GLC–Al and -GCr15 tribo-pairs were�0.23 and �0.13, respectively. In water, FC for each tribo-pairwas reduced, and the order of FCs was GLC–brass4GLC–Al4GLC–GCr154GLC–Ti as well. The GLC–brass tribo-pairshowed the highest FC of �0.14, and the GLC–Ti tribo-pairrevealed the lowest FC of �0.09. The FC for GLC–Al was �0.11,which for GLC–GCr15 was �0.10. It could be found that waterreduced FCs with different reductions comparing with those inambient air. The FCs for GLC coating sliding against brass, Al andGCr15 were reduced by approximately 70 and 52, and 23%,

respectively. As for the GLC–Ti tribo-pair, FC in water was similarto that in ambient, but limited reduction could still be observed asshown in Fig. 2.

According to the volume losses, WRs of GLC coatings againstthese metals in the two environments were calculated, which areshown in Fig. 3. In ambient air, GLC coating revealed the highest

Fig. 3. WRs of GLC coatings against different counterparts.

Y. Wang et al. / Tribology International 60 (2013) 147–155 149

WR when sliding against Al (�33.53�10�17 m3 N�1 m�1) andthe lowest WR when coupling with brass (�2.53�10�17

m3 N�1 m�1). The WR of GLC coating against GCr15 was �9.94�10�17 m3 N�1 m�1, which against Ti was �17.63�10�17

m3 N�1 m�1. In water, GLC coating showed the highest WR of�25.79�10�17 m3 N�1 m�1 against Al and the lowest WR of�1.28�10�17 m3 N�1 m�1 against Ti. The WR of GLC coatingagainst GCr15 in water was �2.45�10�17 m3 N�1 m�1, and theWR of GLC coating against brass was �3.14�10�17 m3 N�1 m�1.It seemed that water played different roles to the wear of GLCcoatings sliding against these metals. WRs of GLC coatings slidingagainst Al, GCr15 and Ti were reduced by approximately 23, 75 and93% in water comparing with those in ambient air, respectively.While the WR of GLC coating sliding against brass showeda surprising increase (�24%) under the same condition. Butthe WR of GLC coating sliding against brass in water was stillcomparatively low as shown in Fig. 3.

Fig. 4 gives the 3D morphologies of wear tracks of GLC coatingssliding against different metal counterparts. In ambient air, thewear track of GLC against brass was wide but shallow, while thewear track against Al was not only wide but also deep. Weartracks of GLC coatings against Ti and GCr15 were intervenient.In water, more and deeper wear grooves were observed on wearsurface of GLC coating against brass than that in ambient air,while, the wear tracks of GLC coating sliding against Al, GCr15and Ti were comparatively narrower and shallower (Fig. 4 (b),(c) and (d)).

Wear scars of the counterparts in different environments areshown in Fig. 5. It clearly reveals that some tribo-layers wereformed on the wear scars. But the tribo-layers covered on thewear scars in water were less than those in ambient air. Thoughmore or less tribo-layers were formed on the wear scars, wear ofeach counterpart was absolutely unavoidable. It suggested thatthe formation of tribo-layer might be a dynamic process accom-panied with the wear of the two contact faces. Since the metalliccounterparts had the same diameter, the big/small wear scarinformed the severe/mild wear of the counterpart. The biggestwear scar of Al ball in either ambient air or water suggested theseverest wear of Al ball. The wear scars of brass balls wererelatively big, indicating the wear of brass against GLC coatingwere comparatively greater. The wear scars of GCr15 and Ti weresmall, which meant that the corresponding wear were milderthan the other metals. Besides, it could be found that the wear ofAl, GCr15 and Ti coincided with wear of the correspondingcontact surfaces of GLC coatings, except for brass. Though the

wear of GLC coating against brass was mild in either ambient airor water, the wear of brass was severe under each of thecorresponding environmental condition.

4. Discussions

It was found that the GLC coatings showed different frictionand wear properties when coupling with different commonmetals in ambient air and water, which must be closely relatedto the natures of the coupled metals. In ambient air, the GLCcoating was demonstrated low friction when sliding againstvarious counterparts according to references, however, the FCbetween the GLC and brass was still high. Comparatively high FCin ambient air was also observed for GLC–Al tribo-pair, while, FCsfor GLC coating sliding against GCr15 and Ti were low. Thefriction differences under this dry-sliding condition in ambientair could be attributed to the different adhesions of the coupledmetals. The main composition of brass is Cu which has the typicalface-centered cubic lattice (fcc) lattice. Since the fcc-lattice gen-erally has the most slip systems among three common crystallattice structures (fcc, bcc (body-centered cubic) and hcp(hexagonal close-packed)), the fcc metals usually show goodplasticity, low shear resistance as well as the high adhesion. Seenfrom Fig. 6(a), the high adhesion of the Cu even resulted in theformation of Cu-rich transferred bands on the wear surface ofthe GLC in ambient air. Though the GLC coating would havebeen shown good solid-lubricating effect due to its graphite-likemicrostructure and carbonaceous transferred-layer formation[9,16–19], it was strongly inhibited by the adhered Cu-rich bands.Al is another typical fcc-lattice metal, the adhesion is compara-tively high. However, Al also shows high oxidability (the Gibbsfree energy of Al2O3 formation is �1582.3 kJ/mol at 300 K) [20].The EPMA mapping of oxygen on fresh wear surfaces of brass andAl in ambient air are shown in Fig. 7. Due to the unavoidableadsorption in ambient air, signals of oxygen could be probed oneach surface of the two metals. The O signals on fresh wearsurface of brass were less and uniform, informing that theremight be no significant oxides formed during the friction action.However, the fresh wear surface of Al revealed increased andenhanced O signals, suggesting that the considerable oxidationmust occur during the friction action. The oxidation of the freshwear surface of Al resulted in the difficulty for the formation ofthe adhered metallic bands on wear surface of the GLC coating(as shown in Fig. 6 (a)). The solid-lubricating effect of the GLCcoating could not be inhibited strongly. The friction resistancebetween GLC coating and Al was thus lower than that betweenGLC coating and brass. Different to the fcc metal, the bcc latticehas comparatively less slip directions and lower atomic densityon the close-packed planes though the slip systems are not less,and the hcp lattice has the least slip systems among these threecommon lattice structures. Then the adhesions of the bcc and hcpmetals are normally low. The GCr15 could be considered as one ofthe bcc metals since the main composite is the a-Fe with bcclattice, and Ti is the typical hcp metal. Therefore, the adhesions ofGCr15 and Ti were low. The graphite-like microstructure insidethe surface atomic layers lead to the low friction resistancebetween GLC coatings and GCr15 and Ti. In another view point,tribo-layer also played an important role to the friction behaviorof GLC coating. Due to the EPMA analysis as shown in Fig. 8, thetribo-layers on wear surfaces of brass and Al balls only included afew of carbonaceous materials while the tribo-layers on wearsurfaces of GCr15 and Ti were composed of more carbonaceousmaterials. It might be related to the dynamic process of thetribo-layer formation. If the metallic counterpart wore easily dueto its low shear resistance, the stable carbonaceous transferred

Fig. 4. 3D morphologies of wear tracks of GLC coatings sliding against (a) brass, (b) Al, (c) GCr15 and (d) Ti.

Y. Wang et al. / Tribology International 60 (2013) 147–155150

Fig. 5. Wear scars of (a) brass, (b) Al, (c) GCr15 and (d) Ti balls.

Y. Wang et al. / Tribology International 60 (2013) 147–155 151

materials were difficult to form, and vice versa. According to theRaman spectra shown in Fig. 9, the carbonaceous materials insidethe tribo-layers on GCr15 and Ti in ambient air might be deeplygraphitized and crystallized [21–23]. The graphitized and crystal-lized carbonaceous provided solid-lubricating effect to the frictioncontact surfaces, which might be another reason to the lowfriction coefficients for GLC coatings sliding against GCr15 andTi in ambient air. It can be proposed that the differences offriction performances of GLC coatings in ambient air were mainly

due to the different adhesions and wear behaviors of the coupledmetals. In water, the adhesions between GLC coatings and metalscould be alleviated obviously. Comparing wear surfaces of GLCcoatings against brass in the two environments as shown in Fig. 6,the adhered Cu-rich bands on wear surface of GLC coatingdisappeared in water. Then the friction coefficient for GLC coatingsliding against brass dramatically decreased in water. Besides, allthe friction coefficients between the GLC coatings and metalliccounterparts decreased in water with different reductions. Actually,

Fig. 6. EPMA images of wear scars sliding in (a) ambient air and (b) water.

Fig. 7. EPMA mapping of oxygen on fresh wear surfaces of (a) brass and (b) Al in ambient air.

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the friction coefficients of the GLC coating sliding against differentmetals were similar low in water, informing that the differentadhesions of metals would affect the friction behavior of GLCcoating slightly under this environmental condition. Meanwhile,water could provide lubricating effect in some extent thoughthe effective hydrodynamic lubrication was difficult to form dueto its low viscosity [9]. In combination with the graphitized and

crystallized carbonaceous transferred materials (as shown in Figs.8 and 9) on wear surfaces of GCr15 and Ti, the good solid–liquidlubrication might work.

Besides of the friction behaviors, the wear behaviors of GLCcoatings also depended on the adhesion and wear behaviors ofthe coupled metals. Comparing with Table 1 and Fig. 3, the GLCcoating showed lowest WR coupled with brass in ambient air.

Fig. 8. EPMA images of wear tracks sliding in (a) ambient air and (b) water.

Fig. 9. Raman spectra of transferred materials on GCr15 and Ti counterparts.

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It can be ascribed in two reasons. One was the low hardnessand contact pressure, and the other was the protection of theadhered Cu-rich bands arose from the high adhesion of the brass.

However, the softest metal of Al led to the highest WR of the GLCcoating under the same condition. This phenomenon might beclosely related to the high oxidability of Al as well. The highoxidability of Al not only led to the oxidation of the fresh wearsurface, but also generated abundant hard oxidized wear debris.The hard oxidized wear debris would work as abrasive grains,resulting in the severe three-body abrasion between the frictioncontact surfaces of GLC coating and Al. When sliding againstGCr15 and Ti, the GLC coatings underwent higher contact pres-sure than those sliding against brass and Al but milder abrasivewear than that sliding against Al. Consequently, the WRs of GLCcoatings sliding against GCr15 and Ti were higher than thatsliding against brass but lower than that sliding against Al. Eitherthe adhesion or the abrasion between the friction contact surfacescould be alleviated by water. Since the adhered Cr-rich bandsdisappeared, the protection of the Cu-rich bands disappeared.Then the higher WR of GLC coating sliding against brass in waterwas reasonable. However, the WR of GLC coating sliding againstbrass was still low. If the wear behavior was in the controlof abrasion mechanism, the continuous or discontinuous waterfilm between the contact surfaces must play separating and lubri-cating roles, therefore, the abrasion was alleviated. The decreasedWRs of GLC coatings sliding against Al, GCr15 and Ti in waterwere thus understood.

Fig. 10. Models for GLC coatings sliding against metallic counterparts in ambient air and water.

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According to discussions above, the different friction and wearresponses of the GLC coatings to common metals were highlydependent on adhesions and wear behaviors of the coupledmetals, which can be modeled by Fig. 10. In ambient air, therewould be three conditions. If the metal possesses low shearresistance and high plasticity but inert in ambient air (picture Ain Fig. 10), it not only exhibits high adhesion but also easilytransfers to the wear surface of GLC coating. The high adhesionwould low the friction–reduction effect of GLC coating, resultingin the high FC between the two counterparts. Meanwhile, thesevere wear of the metal is unavoidable due to its low shearresistance, thus the carbonaceous transferred materials are diffi-cult to adhere on the metallic counterpart to form the effectivesolid-lubricating layers. It is another factor to the comparativelyhigher FC for this condition. But the transferred metallic tribo-layer would protect the GLC coating in some extent. However,some of the soft metals might have high oxidizability (picture B inFig. 10). The high oxidizability would result in the oxidation of thefresh wear surface in ambient air quickly, which can reduce theadhesion and inhibit the formation of metallic transferred tribo-layer on the wear surface of GLC coating. Accompanied with thedistribution of hard oxide particles on wear surface, severe three-body abrasive occurs. As for the metals with high hardness andlow plasticity, adhesion in ambient air is low (picture C in Fig. 10).Wear debris between the friction contact faces might includehard particles arose from work hardening or oxidization. Theabrasive wear would also in control under this condition. How-ever, carbonaceous transferred materials would be formed on thewear surfaces of the corresponding metallic counterparts. Thenthe comparatively lower FC is obtained. In water, three kinds offriction contact conditions would change to the similar model asshown in Fig. 10. The friction contact surfaces can be lubricatedby the continuous of discontinuous water film, as a result, lowerFC would be observed. Meanwhile, water would alleviate theadhesion or abrasion between the contact faces. The eliminationof protecting metallic tribo-layers on wear surface of GLC coatingresults in a little of increase of the WR, while the alleviatedabrasive wear to the GLC coating leads to distinct decreaseof the WR. Nevertheless, the GLC coating exhibits low WR when

coupling with metals in water. According to the high FC andadhesion of GLC coating to brass and its low FC and WR–GCr15and Ti in ambient air, the GLC coating is not very suitable tomachine Cu-rich metals but would perform well when machiningFe- and Ti-based metals under dry condition in ambient air. And itcan be proposed that the GLC coating is more suitable to machinecommon metal under water-lubricated condition due to its lowFC and WR in water, especially for Cu-rich metals.

5. Conclusions

The GLC coating showed different tribological properties whensliding against different metals in ambient air and water, whichwere highly dependent on the adhesion and wear behaviors of thecoupled metals. In ambient air, the metallic counterparts withhigh adhesion of brass resulted in the high FC but low WR of theGLC coating, while the metallic counterparts with high adhesionas well as high oxidizability of Al lead to the comparatively low FCand high WR of the GLC coating. The GLC coatings exhibited lowFC and WR to metals of GCr15 and Ti due to the low adhesionsand the carbonaceous lubricating transferred materials formation.In water, either adhesion or abrasion between GLC coating andmetallic counterpart was alleviated. As a result, low FC and WR ofthe GLC coating in water were observed. Accordingly, the GLCcoating is not suitable to machine Cu-rich metals but wouldperform well when machining Fe- and Ti-based metals underdry condition. Comparatively, the GLC coating is more suitableto machine common metals under water-lubricated condition,especially for Cu-rich metals.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (Grant no. 51202261) and the NingboMunicipal Nature Science Foundation (Grant no. 2012A610105).The authors acknowledge Zhi Lin of University of Science &

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Technology Beijing for the measurement of the nano-indentiondiscussed in this work.

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