Tribology International 58 (2013) 7–11

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Tribology International

0301-67

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journal homepage: www.elsevier.com/locate/triboint

Tribological performance of CuS–ZnO nanocomposite film: The effectof CuS doping

Yidong Zhang a,n, Baojun Huang a, Pinjiang Li a, Xinmin Wang b, Yange Zhang a

a Key Laboratory of Micro–Nano Materials for Energy Storage and Conversion of Henan Province, Institute of Surface Micro and Nano Materials, Xuchang University,

no. 88, Bayi Road, Xuchang 461000, PR Chinab Zhengzhou College of Animal Husbandry Engineering, Zhengzhou 450011, PR China

a r t i c l e i n f o

Article history:

Received 1 August 2012

Received in revised form

4 September 2012

Accepted 6 September 2012Available online 26 September 2012

Keywords:

Sol–gel

CuS–ZnO

Nanocomposite film

Tribological performance

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

x.doi.org/10.1016/j.triboint.2012.09.004

esponding author. Tel.: þ86 374 2968783.

ail addresses: zyd630@126.com, zyd630@xcu

a b s t r a c t

CuS–ZnO nanocomposite films were prepared by a simple sol–gel process. The microstructure,

morphology, and tribological properties of the prepared films were investigated via X-ray diffracto-

metry (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning

electron microscopy (SEM), and by an UMT-2 tribometer. Results show that the films are uniform

and compact with hexagonal ZnO phase. The films display excellent wear resistance and friction-

reduction performance. Particularly, the molar ratio of 5% CuS of sample ‘‘c’’ obtains the best result.

Frictional trace observation suggests that the wear mechanism of the film has slight scuffing, adhesion,

and abrasion.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

There is a great scientific and technological interest in sol–gelceramic films because of their excellent tribological and mechan-ical properties [1,2]. Zinc oxide (ZnO), as a well-known functionalfilm, has been widely used in transparent electronics [3], randomlasers [4], UV light emitting devices [5], thin film transistors [6],dye-sensitized solar cells [7], photo-detectors [8], and surfaceacoustic wave devices [9]. Recently, tribological investigation ofZnO has attracted much attention [10,11]. Usually, nano-ZnO wasused as a filler in the polymer to improve the tribological propertyof the polymer composite coating [12]. For example, Wang et al.[13] reported that ZnO particles and ZnO whiskers can improvethe mechanical and tribological properties of nylon compositeswithout affecting the crystallinity of the nylon matrix. Hardness,tensile strength and scratch coefficients of composites areincreased by the addition of ZnO particles and ZnO whiskers.Bahadur et al. [14] also reported that the tribological behavior ofpolyphenylene sulfide (PPS) could be improved by a certainamount of ZnO dopant. However, few reports have mentionedthe tribological property of ZnO sol–gel film.

CuS, with a hexagonal crystal and approximate laminar struc-ture, will slide between the interlayer easily, leading to excellenttribological performance. As a result, it becomes an important

ll rights reserved.

.edu.cn (Y. Zhang).

lubricant additive widely used in many tribological fields [15,16].For example, the tribological performance of some polymers suchas PTFE and nylon can be improved by a certain amount of CuSadditive [17]. Further, the wear rate of the different CuS contentsdepends upon their ability to form transfer films on the steelcounterface [18]. Unfortunately, few reports have studied theinfluence of CuS additive on the tribological performance of sol–gel ceramic films. For the first time, CuS additive was added to theZnO sol–gel film to improve the tribological behavior of the ZnOfilm. In this article, ZnO sol–gel film was fabricated by a glucose-assisted sol–gel method using Zn(Ac)2 �2H2O as a precursor. Thena certain amount of CuS additive was added to the ZnO sol to formCuS–ZnO composite film by a spin coating method on a quartzsubstrate.

2. Experimental procedure

All the reagents used in the experiments were in analyticgrade (purchased from Shanghai Sinopharm Chemical ReagentCo., Ltd.) and used without further purification.

2.1. The preparation of CuS–ZnO thin films

The quartz substrates had been ultrasonically cleaned byethanol, acetone, and distilled water for 30 min each. ZnO solwas fabricated by a sol–gel method using zinc acetate, anhydrousethanol and diethanolamine (DMA) as the solute, solvent, and sol

Fig. 1. XRD patterns of the obtained films with different CuS contents at 500 1C for

1 h: (a) pure ZnO film; (b) 1% CuS; (c) 5% CuS and (d) 10% CuS.

Y. Zhang et al. / Tribology International 58 (2013) 7–118

stabilizer, respectively, which marked solution ‘‘A’’, which hasbeen reported in our previous research [19]. The Cu(Ac)2, �HAc,and Cu(Ac)2 were dissolved in absolute ethanol, then slowlyadded drop by drop to CS(NH)2 ethanol solution, which markedsolution ‘‘B’’. Then solution ‘‘B’’ was added to solution ‘‘A’’ understirring for 3 h at room temperature, followed by aging for 24 h,so the transparent composite sol was obtained. CuS–ZnO thinfilms were prepared by a spin coating method on quartz sub-strates for 30 s with a spinning speed of 2500 rpm. The resultingthin films with molar ratio of CuS of 0%, 1%, 5%, and 10% wereannealed at 500 1C in a vacuum muffle oven for 1 h, which weremarked ‘‘a’’, ‘‘b’’, ‘‘c’’, and ‘‘d’’, respectively.

2.2. The characterization of CuS–ZnO thin films

The crystal structure of the prepared samples was determinedby XRD (Bruker D8 advance) with Cu Ka radiation (l¼1.5418 A)and a scan rate of 0.01 2y s�1. The element states of the samplesurfaces were obtained by XPS (Kratos AXIS-ULTRA) equippedwith a standard and monochromatic source (Al Ka). The bindingenergy (BE) scale was calibrated against the BE of C1s at 284.8 eV.The surface morphologies of CuS–ZnO thin films were obtained byAFM (Veeco Digital Instruments, Nanoscope 3d) in contact modeat a scan rate of 1.0 Hz with a commercial silicon nitride micro-cantilever probe under ambient conditions (2472 1C). The tipradius and spring constant of the probe are less than 10 nm and0.12 N m�1, respectively.

2.3. Tribological properties test

The tribological properties of the prepared films against astationary GCr15 steel ball (diameter 4 mm) were evaluated on aUMT-2 reciprocating friction and wear tester (made in USA,providing a reciprocal sliding configuration) at ambient condi-tions (RH:�50%). The sliding velocity and stroke length werefixed to 1000 mm min�1 and 6 mm, respectively. The normal loadwas 1.5 N. The physical and mechanical properties of the counter-part GCr1.5 steel ball are shown in Table 1. The friction coefficientfinally increased sharply to a higher stable value after sliding for acertain number of cycles. It was recognized that the film failed atthis point. The corresponding number of sliding cycles wasrecorded as the wear life. Then replicate tests were carried outfor each specimen. The average coefficient of friction and wear lifeof the three replicate tests were cited in this article. The relativeerror for the replicate tests was no more than 5%. Prior to thefriction and wear test, all the samples were cleaned in anultrasonic bath with ethanol and acetone for 10 min and thendried in a vacuum oven at 60 1C for 10 min.

3. Results and discussion

3.1. The characterization of CuS–ZnO thin films

Fig. 1 shows the XRD patterns of pure ZnO and CuS doped ZnOfilms annealed at 500 1C for 1 h on quartz substrates. For allsamples, the (100) and (002) peaks can be observed, indicatingthat the samples were polycrystalline, hexagonal wurtzite

Table 1Physical and mechanical properties of the GCr15 steel ball.

Relative density

(%)

Hardness

(HRC)

Fracture toughness

(Mpa)

Fracture strength

(Mpa)

98 485 5–8 400–600

structures (a¼3.250 A, b¼5.207 A, JCPDS 36-1451). Obviously,with increasing concentration of CuS, the diffraction intensity ofthe ZnO film decreased. The (002) diffraction peak is dominant,indicating that the samples have a preferred orientation along thec-axis direction, which is consistent with the previous report [20].The size of the nanocrystals (D) was calculated on the basis of theScherrer formula, D¼kl/bcosy, where k (0.9) is the shape factor, lis the X-ray wavelength of Cu Ka radiation (1.5418 A), y is theBragg diffraction angle, and b (0.461) is the full width at halfmaximum (FWHM) of the (002) peak. Accordingly, the averagenanocrystallite size of ZnO, 1CuS–ZnO, 5 CuS–ZnO, and 10CuS–ZnO are estimated to be 34, 30, 21, and 45 nm, respectively. Theresults indicate that the relatively small proportion of CuS ishighly dispersed through the ZnO network homogeneously. CuSdopant can restrain the crystallization of ZnO.

The ZnO film composition with 5% CuS of sample ‘‘c’’ wasevaluated by XPS analysis. The spectra of Zn2p, O1s, Cu2p, andS2p are shown in Fig. 2(a–d), respectively. The binding energies ofZn2p1/2 and Zn2p3/2 are observed at 1042.5 eV and 1019.4 eV,respectively. The peak of 529.2 eV is ascribed to O1s. The bindingenergies of Cu2p1/2 and Cu2p3/2 are observed at 952.5 eV and932.5 eV, respectively. The peaks at 169.2 eV and 161.1 eV areascribed to S2p1/2 and S2p3/2, respectively. From the aboveanalysis, the CuS had been stably doped into the ZnO matrix.

Fig. 3(a–d) shows three-dimensional AFM surface heightmorphologies of the CuS–ZnO composite films on quartz sub-strates. The scanning area is 1�1 mm2.The ZnO thin films havesmooth surfaces and compact grains. At the right side of the eachimage, an intensity strip is shown, indicating the depth and heightalong the z-axis. The root-mean-square (RMS) roughness calcu-lated by the equipment’s software of the ZnO thin films samples‘‘a’’, ‘‘b’’, ‘‘c’’, and ‘‘d’’ are 15.6, 12.4, 9.2, and 20.5 nm, respectively.The average particle sizes of the samples ‘‘a’’, ‘‘b’’, ‘‘c’’, and ‘‘d’’ are�38, 32, 24, and 47 nm, respectively, which are consistent withthe crystal size by the XRD analysis. The whole measurement wasrepeated six times at locations randomly selected with the sameAFM tip and the resulting data were averaged. The relative errorfor six tests is no more than 5%. Obviously, the RMS roughness ofsample ‘‘c’’ is the smallest one. This is probably due to the factthat the suitable CuS dopant can inhibit the growth of the ZnOfilm to improve the surface quality. With increase of theCuS dopant content, the grain size decreased and surface mor-phology became smoother. This revealed that the relatively small

Fig. 2. XPS spectra of (a) Zn2p; (b) O1s; (c) Cu2p and (d) S2p of the prepared film.

Fig. 3. Three-dimensional AFM images of obtained films with different CuS contents at 500 1C for 1 h: (a) pure ZnO film; (b) 1% CuS; (c) 5% CuS and (d) 10% CuS.

Y. Zhang et al. / Tribology International 58 (2013) 7–11 9

Fig. 4. Friction coefficients and wear cycles of the obtained films as functions of

CuS molar ratio in air at 20 1C under a load of 1.5 N.

Fig. 5. Friction coefficients of the obtained films as functions of wear cycles in air at

20 1C under a load of 1.5 N: (a) pure ZnO film; (b) 1% CuS (c) 5% CuS; and (d) 10% CuS.

Fig. 6. SEM micrographs of the worn surfaces at 1000 mm min�1 sliding speed

Y. Zhang et al. / Tribology International 58 (2013) 7–1110

proportion of CuS is highly dispersed in the ZnO network homo-geneously. However, in Fig. 4(d), excessive CuS cannot disperseuniformly due to the phase separation caused by the aggregationof superfluous CuS, as a result, the RMS roughness of sample ‘‘d’’ isincreased. The thickness obtained by ellipsometer (type: SMART-SE) of samples ‘‘a’’, ‘‘b’’, ‘‘c’’, and ‘‘d’’ are 75, 86, 102, and 121 nm,respectively, i.e., with increase in the amount of CuS dopant, thethickness of the samples increased due to increased viscosity ofthe ZnO sol.

3.2. Tribological properties of CuS–ZnO thin films

Fig. 4 shows the tribological behavior of CuS–ZnO compositefilms with different CuS molar ratio contents under load of 1.5 Nand sliding speed of 1000 mm min�1. When the molar ratio was1% and 5%, the wear life and friction coefficient (COF) wasimproved compared with the pure ZnO film, especially in the 5%molar ratio of sample ‘‘c’’. However, when the molar ratio was10%, the tribological behavior was seriously destroyed. Fig. 5shows the resulting COF of CuS doped and pure ZnO films under aload of 1.5 N. For the pure ZnO film, the COF sharply rises to veryhigh value only after 2700 cycles, indicating the poor wearresistance of pure ZnO film. However, after doping a certainamount of CuS, the wear life was increase. In particular, sample‘‘c’’ with 5% CuS obtained the longest wear life of 6180 cycles.However, the wear life of sample ‘‘d’’ with excessive CuS dopant(10%) was decreased to 1700 cycles. The embedded CuS mayimprove the adhesion and mechanical stability of thin film on thequartz substrate, which are all possibly responsible for the bettertribological performances of the 5CuS–ZnO film. Differently,sample ‘‘d’’ of 10CuS–ZnO shows shorter wear life than that ofpure ZnO film because the addition of excessive CuS into ZnO filmseriously deteriorates wear resistance due to the phase separationbetween CuS and ZnO. The tribological behavior of CuS doped ZnOfilm was possible supposed to be greatly influenced by inhibitoryeffect of additives. The CuS modifier can greatly influence thechemical composition and structure of the film strongly as well asthe tribological properties.

To explore the friction and wear mechanism further, the wornsurfaces of the obtained thin films after failure sliding against a

and 1.5 N load: (a) pure. ZnO film; (b) 1% CuS; (c) 5% CuS and (d) 10% CuS.

Y. Zhang et al. / Tribology International 58 (2013) 7–11 11

GCr15 steel ball have been observed by SEM, as shown in Fig. 6. Itis noteworthy that the frictional track of the pure ZnO film after2700 sliding cycles is rough. Some grooves, plastic deformationand microfracture features can be found on the worn surface ofpure ZnO film, as shown in Fig. 6a. However, the 5CuS–ZnO filmafter 6180 sliding cycles is smooth without obvious grooves,deformation and microfracture features, as shown in Fig. 6c,indicating that extremely light wear happened under this slidingcondition. Therefore, sample ‘‘c’’ obtained the best friction reduc-tion and wear resistance behavior among the tested films.Differently, signs of severe brittle fracture and abrasive particlesare visible on the worn surface of the 10CuS–ZnO film after only1700 sliding cycles under a load of 1.5 N, as shown in Fig. 6d. Onthe basis of analysis of the AFM results, the weaker interfacialinteraction between the two phases due to phase separation arereally present, which possibly caused more surface defects. Thismay be the reason for the difference in failure modes and thereduction in wear protection of the 10CuS–ZnO film.

4. Conclusions

The effect of CuS addition into ZnO thin film on tribologicalproperties was investigated. The prepared thin films are poly-crystalline compact hexagonal wurtzite structures. XRD and AFMinvestigations showed that the suitable addition of CuS into ZnOfilm could effectively prevent the growth of ZnO grains andensure the durability of ZnO due to the crystal seed role of CuS.The finer grain size would greatly improve the resistance of theZnO film to microfracture. All these effects contributed to theexcellent antiwear and friction-reduction performance of themutually soluble 5CuS–ZnO film over the pure ZnO film in slidingagainst a GCr15 steel ball under low load. However, it is notice-able that excessive addition of CuS will deteriorate the tribologi-cal performance of ZnO film probably due to phase separation.SEM observations revealed that the wear mechanism of compo-site thin film sliding against a GCr15 steel ball was light scuffingand abrasion, whereas that of pure ZnO film was severe plasticdeformation and abrasive wear.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (Grant nos. 50842027 and61106125), National Science Foundation of Henan Province (nos.102300410165 and 2010B150028), Henan Province of Interna-tional Science and Technology Cooperation (no. 124300510054),

and the Program for Science and Technology of Henan Province(nos. 122102210419, 122300410005, and 122300410267).

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