Ia

Za

Ub

a

ARRAA

KPPIS

1

tcbpscntso

ucdHibaps

0d

Wear 269 (2010) 21–25

Contents lists available at ScienceDirect

Wear

journa l homepage: www.e lsev ier .com/ locate /wear

ndentation and scratch behavior of nano-SiO2/polycarbonate composite coatingt the micro/nano-scale

.Z. Wanga, P. Gua,∗, Z. Zhangb

Department of Modern Mechanics, Key Laboratory of Material Mechanical Behavior and Design of Chinese Academy of Science,niversity of Science and Technology of China, No. 96, Jinzhai Road, 230027 Hefei, Anhui, ChinaNational Center for NanoScience and Technology, No. 2, 1st North Street Zhongguancun, 100080 Beijing, China

r t i c l e i n f o

rticle history:eceived 2 December 2009eceived in revised form 23 February 2010

a b s t r a c t

The mechanical and tribological performances of polycarbonate film and nano-SiO2/polycarbonatecomposite coating are studied with micro/nano-scale indentation and scratch tests using Hysitron Tri-boIndenter. The experimental results show that the hardness and stiffness are increased apparently after

ccepted 1 March 2010vailable online 6 March 2010

eywords:olymersolymer–matrix composite

the addition of nano-SiO2. The scratch tests indicate that the nano-SiO2/polycarbonate coating exhibitssmaller scratch depth and lower frictional coefficient. Combined with the examination of infrared spec-trum, the mechanisms of the improvements in mechanical and tribological properties of the coatings areanalyzed.

© 2010 Elsevier B.V. All rights reserved.

ndentationcratch testing

. Introduction

Polycarbonate (PC) is a kind of extensively applied engineeringhermoplastic which exhibits good mechanical properties, electri-al insulation, thermal stability, transparency and the ability toe processed on conventional machinery [1]. Despite its excellenthysical and optical properties, poor tribological properties haveeriously limited its application and working life [2]. So modifi-ation is necessary for this kind of material. Presently, polymericanoparticle-reinforced coatings have drawn considerable atten-ion, due to the improvements in various properties includingcratch resistance, abrasion resistance, heat stability as well asther mechanical properties [3,4].

In recent years, one of the widely used techniques for the eval-ation of the mechanical and tribological properties of metals,eramics, polymers and films at ultra-microscopic level is nanoin-entation and nanoscratch tests using depth-sensing method [5,6].owever, up to now, few papers concerning the tribological behav-

ors at the micro/nano-scale of the PC-based nanocomposites have

een presented. The present paper characterizes the mechanicalnd tribological behaviors of the PC films and the PC/SiO2 nanocom-osite coatings by using nano-mechanics testing system. It aims attudying and analyzing the modification mechanism of the addi-

∗ Corresponding author. Tel.: +86 551 3603694; fax: +86 551 3606459.E-mail address: guping@ustc.edu.cn (P. Gu).

043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2010.03.003

tion of nanoparticles, which provides experimental basis for theapplication of nano-SiO2/PC composites.

2. Materials and methods

2.1. Materials

Samples used in this study included pure PC films and PC/SiO2nanocomposite coatings based on PC films. The thickness of the filmand the coating were 0.3 mm and 2 �m respectively. The size ofthe nano-SiO2 particles incorporated into the PC matrix, was about20 nm on average and the filling amount was 20 wt% (11.4 in vol.%).With special sol–gel technique, the spherical silica nanoparticleswere distributed homogeneously in the matrix. The compositecoatings were coated onto the PC film by the traditional spin coat-ing technique. For each specimen, the average surface roughnessmeasured by the scanning probe microscopy (SPM) part of the Tri-boIndenter on the scan area of 2 �m × 2 �m was about 3 nm.

2.2. Experimental methods

In this research, micro/nano-scale indentation and scratch tests

were performed to study the mechanical and tribological propertiesof the samples. All works were carried out using the commerciallyavailable TriboIndenter system (Hysitron Inc., USA) with a conicaldiamond indenter tip. The curvature radius of the indenter tip was1 �m and the cone angle was 90◦. This type of instrument combines

2 ear 2

imsitSt

2

wlTademsiud

2 Z.Z. Wang et al. / W

ndentation test to determine the local mechanical properties at theicro/nano-scale with high resolution in situ SPM imaging of the

ample. The load resolution is 1 nN and displacement resolutions 0.0002 nm. Infrared tests were taken on the samples by usinghe Fourier transform infrared spectrometer (Nicolet 8700, Thermocientific Instrument Co., USA) in the transmittance mode to studyhe crystalline properties.

.2.1. Indentation testsThe indentation tests were operated in force-control mode

here the load applied is controlled according to a programmedoading function and the displacement continuously monitored.he loading function of the indentation in this work consisted of5-s linear loading and 5-s unloading segment together with a

welling of 5 s at the peak load, which was used to reduce the influ-nce of creeping effect [7]. The maximum load was decided by the

aximum indentation depth of the composite coating, which was

et as 1/10 of the coating thickness (about 200 nm), to reduce thenfluence of the substrate. The final values of hardness and mod-lus were taken as the average of six indentations carried out inifferent spots on the same material.

Fig. 1. (a) Load–displacement curves of PC film and PC/SiO2 coat

69 (2010) 21–25

2.2.2. Scratch testsThe scratch test included three main procedures. Firstly, a pre-

scan under a very small load (1 �N) was carried out. Then theindenter scraped the sample under a certain force and scratchwould be generated. The loads used in this test contained: 500,1000, 2000, and 3000 �N. The length of the scratches was 10 �m.Finally, a post-scan under the same load as the pre-scan was con-ducted to get an image of sample after scratch. An estimate of themagnitude of the residual scratch ditch and the extent of imme-diate recovery can be obtained by comparing the pre-scan imagewith the post-scan image.

3. Results and discussion

3.1. Indentation tests

The typical load–displacement curves for the two samples areshown in Fig. 1(a) and the three-dimensional images of the indenta-tions under a same load after elastic recovery are shown in Fig. 1(b).The indentation depth of PC film is much larger than that of PC/SiO2nanocomposite at the same load. At the peak load, from Fig. 1(a),

ing and (b) three-dimensional images of the indentations.

Z.Z. Wang et al. / Wear 269 (2010) 21–25 23

F t inde

ttPrwnmuTbsovrr

oamelithpp

3

cr

ig. 2. (a) Hardness, (b) modulus, (c) plasticity index of the two samples at differen

he maximum indentation depth of PC film is about 50 nm morehan the composite coating. However, the unloading segment ofC/SiO2 nanocomposite is steeper than PC film. From Fig. 1(b), theesidual pit of PC is larger and much deeper than that of PC/SiO2,hich implies that the latter is harder than the former. The hard-ess and modulus can be obtained using the Olive and Pharr’sethod [8]. Fig. 2(a) and (b) shows the results of hardness and mod-

lus of the two samples at different indentation depth respectively.he plasticity index, which characterizes the relative plastic/elasticehavior of the material when it undergoes external stresses andtrains, is displayed in Fig. 2(c). This index, , is defined in termsf energy: = Wir/(Wir + Wr), where Wir and Wr represent the irre-ersible work done during the indentation and the reversible workecovered by viscoelastic processes during the unloading stageespectively (see Fig. 2(d)) [9].

It can be seen that both the hardness and the elastic modulusf the PC/SiO2 composites are higher than those of the PC filmst each indentation depth. Analysis suggests that it is due to theixing effect of the PC matrix and the nano-SiO2 particles, which

xhibit much higher hardness and elastic modulus. The enhancedoad-carrying capacity based on the strong particle-matrix bondings also a contributory factor to the improved mechanical proper-ies. Results also show that the plasticity index of PC films is a littleigher than that of PC/SiO2 coatings, which indicates that the com-osite coating exhibits better elasticity recovery capacity than theure film during the indentation process.

.2. Scratch tests

With higher hardness and better elastic recovery capability, itould be easily presumed that the composite coating was moreesistant to scratching. The surface morphologies of the scratches of

ntation depth, and (d) schematic diagram of the calculation of plasticity index.

the two samples under 1000 �N and the corresponding initial andresidual scratch profiles are displayed in Fig. 3(a). The arrows in thefigure represent the sliding direction of the indenter tip. From themorphologies, it could be seen that the residual scratch of PC filmis much deeper and wider than that of PC/SiO2 composite. Com-pared the profiles, the recovered scratch depth of PC film is closeto 50 nm, but just about 30 nm for PC/SiO2 coating. The variationsof the average on-load scratch depths with the load are shown inFig. 3(b). It is found that the relationship between the depth andthe load is closely linear, which implies that both the bulks of PCfilms and PC/SiO2 coatings are quite homogeneous. However, underthe same load, the on-load scratch depths of PC films are all largerthan that of PC/SiO2 coatings, and the differences increase withthe increasing scratch load. Under 3000 �N, the depth of the for-mer is about twice of the latter. Based on the comparisons of theon-load and the recovered scratch depths of the two samples, it isobviously that PC/SiO2 composite coatings exhibit better scratchresistance.

The frictional coefficient is defined as the ratio of the tangentialforce to the normal force. The typical curve of frictional coefficientvalues versus time is shown in Fig. 4(a). The coefficient of frictionchanges from 0 to 0.5 continuously at the beginning of scratching,which is corresponding to the loading stage. Then it keeps invariantat 0.5 for about 25 s during the steady scratching process. An abruptchange takes on subsequently after the steady state because of thedetachment of the indenter tip during the unloading stage whenthe scratching completed. The average values of the steady stage

are taken as the effective frictional coefficient. These coefficients ofPC films and PC/SiO2 coatings under different loads are shown inFig. 4(b). It can be seen that the frictional coefficients of the coatingsare all lower than that of the PC films and, the higher the load,the greater the difference between the two samples. That means

24 Z.Z. Wang et al. / Wear 269 (2010) 21–25

F dingo

tl

e

Fc

ig. 3. (a) Surface morphologies of the scratches under 1000 �N and the corresponn-load scratch depth versus scratch load.

he friction reducing effect is more significant under higher scratchoad.

In Chang and Zhang’s study on the tribological properties ofpoxy nanocomposites [10], they proposed a positive rolling effect

ig. 4. (a) Variation of the frictional coefficient during the scratch process (PC film undeoating.

initial and residual scratch profiles of PC film and PC/SiO2 coating and (b) average

of the nanoparticles to interpret the remarkable reduction of thefrictional coefficient after the addition of nano-TiO2. It is also appro-priate for this study, the spherical-like nano-SiO2 particles tend todetach from the matrix during the scratching process when the

r 2000 �N) and (b) frictional coefficient versus scratch load of PC film and PC/SiO2

Z.Z. Wang et al. / Wear 2

lrbcmdu

3

Fat1bowa[o

mhTtttsmccnar

4

bfiwT

[

[

[

[

[

[

[

[

Fig. 5. Infrared spectrogram of PC film and PC/SiO2 coating.

oad exceeds a critical value. And then the separate particles act asolling balls, which can change sliding friction into rolling friction,etween the indenter and the sample. That is why the frictionaloefficients of the composite coatings are lower than that of the PCatrix. Moreover, the higher the load, the more the nanoparticles

etach from the matrix. So the antifriction effect is more apparentnder higher scratch load.

.3. Infrared tests

The infrared spectrums of the two samples are shown inig. 5. From this spectrogram, the carbonyl C O characteristicbsorption peaks at the wavenumber of 1720–1750 cm−1, theriple stretch vibration absorption of esteryl C–O at 1159 cm−1,190 cm−1 and 1229 cm−1 and the C C absorption peaks onenzene ring at 1460 cm−1 are found. Besides, the transmittancef the PC/SiO2 composites is lower than that of PC matrix at thehole frequency range. That means the composite coating exhibitshigher crystallinity than the pure film. As researches indicate

11,12], the increased crystallinity can result in the improvementf mechanical properties.

For polymer-based composites, the particles added into theatrix have strong effects on the crystallization process. Some

inder crystallization, whereas others have positive effects [13].he nucleating agents, namely these impurities that can acceleratehe crystallization, play roles as crystal nuclei during crystalliza-ion, and transform the nucleation mechanism from homogeneouso heterogeneous. Incorporated with nuclei, compact and finepherulite particles can be formed in the polymer matrix, and theolecules present as microcrystalline structure. Consequently, the

rystallization rate of the molecular chains is improved and therystallinity of the polymer is increased. In this study, the silicaanoparticles added into polycarbonate, are used for nucleatinggents. Thus they promote heterogeneous nucleation and grainefinement, and make the crystallinity increase [14].

. Conclusions

In the present paper, micro/nano-scale indentation and scratchehaviors of polycarbonate films and PC-based composite coatingslled with homogeneously distributed nano-SiO2 particles (20 nm)ere systematically studied using Hysitron TriboIndenter system.

he following conclusions can be drawn:

(a) The addition of nano-SiO2 particles can apparently increase the

hardness and the modulus of the PC film. The elastic recov-ery capability of the nanocomposites is also improved. Theimprovements of the mechanical properties are suggested toascribe to the excellent performances of nanoparticles and theincreased crystallinity of nanocomposites [15–17].

[

[

69 (2010) 21–25 25

(b) Scratch resistance is certainly improved by incorporating nano-SiO2 into PC matrix; both the on-load and the recovered scratchdepths of the PC/SiO2 nanocomposite coatings are lower thanthat of PC films. Meanwhile, the frictional coefficients of thecomposite coatings are all lower than that of the pure PCfilms under each scratch load, which imply that the antifrictioneffect of nanoparticles is activated in the tests. The enhancedscratch resistance of the composite coatings is attributed tothe improved mechanical properties [18,19] and the decres-cent frictional coefficient of the composites is due to the rollingeffect of the nanoparticles.

Acknowledgements

The research was supported by the grant of National NaturalScience Foundation of China (10302026) and Science and Tech-nology Key Project of Anhui province: research and applicationof energy conservation and environmental protection techniquein automobile. The authors grateful acknowledge the provisionof experimental samples from NCNST (Beijing, China) and experi-mental instruments from Laboratory of Engineering and MaterialScience of USTC (Hefei, China).

References

[1] F.J. Carrion, A. Arribas, M.D. Bermudez, A. Guillamon, Physical and tribologi-cal properties of a new polycarbonate–organoclay nanocomposite, EuropeanPolymer Journal 44 (2008) 968–977.

[2] L.Y.L. Wu, E. Chwa, Z. Chen, X.T. Zeng, A study towards improving mechanicalproperties of sol–gel coatings for polycarbonate, Thin Solid Films 516 (2008)1056–1062.

[3] E. Amerio, P. Fabbri, G. Malucelli, M. Messori, Scratch resistance of nano-silicareinforced acrylic coatings, Progress in Organic Coatings 62 (2008) 129–133.

[4] X.H. Hou, C.X. Shan, K.L. Choy, Microstructures and tribological propertiesof PEEK-based nanocomposite coatings incorporating inorganic fullerene-likenanoparticles, Surface & Coatings Technology 202 (2008) 2287–2291.

[5] W.C. Oliver, G.M. Pharr, Measurement of hardness and elastic modulus byinstrumented indentation: advances in understanding and refinements tomethodology, Journal of Materials Research 19 (2004) 3–20.

[6] B.J. Cayer, D. Mazuyer, A. Tonck, P. Kapsa, Nanoscratch and friction: an innova-tive approach to understand the tribological behaviour of poly (amide) fibres,Tribology International 39 (2006) 62–69.

[7] B.J. Briscoe, L. Fiori, E. Pelillo, Nano-indentation of polymeric surfaces, Journalof Physics D: Applied Physics 31 (1998) 2395–2405.

[8] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness andelastic-modulus using load and displacement sensing indentation experiments,Journal of Materials Research 7 (1992) 1564–1583.

[9] R.C. Powles, D.R. Mckenzie, S.J. Meure, M.V. Swain, Nanoindentation response ofPEEK modified by mesh-assisted plasma immersion ion implantation, Surface& Coatings Technology 201 (2007) 7961–7969.

10] L. Chang, Z. Zhang, Tribological properties of epoxy nanocomposites. Part 2.A combinative effect of short carbon fibre with nano-TiO2, Wear 260 (2006)869–878.

11] K.S.K. Karuppiah, A.L. Bruck, S. Sundararajan, J. Wang, X.D. Li, Friction and wearbehavior of ultra-high molecular weight polyethylene as a function of polymercrystallinity, Acta Biomaterialia 4 (2008) 1401–1410.

12] P. Bhimaraj, D. Burris, W. Sawyer, C. Toney, R. Siegel, L. Schadler, Tribolog-ical investigation of the effects of particle size, loading and crystallinity onpoly(ethylene) terephthalate nanocomposites, Wear 264 (2008) 632–637.

13] J.R. Zheng, R.W. Siegel, C.G. Toney, Polymer crystalline structure and morphol-ogy changes in nylon-6/ZnO nanocomposites, Journal of Polymer Science PartB: Polymer Physics 41 (2003) 1033–1050.

14] K.P. Pramoda, A. Mohamed, I.Y. Phang, T.X. Liu, Crystal transformation and ther-momechanical properties of poly(vinylidene fluoride)/clay nanocomposites,Polymer International 54 (2005) 226–232.

15] A. Omrani, L.C. Simon, A.A. Rostami, The effect of alumina nanoparticle onthe properties of an exopy resin system, Materials Chemistry and Physics 114(2009) 145–150.

16] L.Q. Liu, A.H. Barber, S. Nuriel, H.D. Wagner, Mechanical properties of function-alized single-walled carbon-nanotube/poly(vinyl alcohol) nanocomposites,Advanced Functional Materials 15 (2005) 975–980.

17] S.C. Tjong, Structural and mechanical properties of polymer nanocomposites,Materials Science & Engineering R: Reports 53 (2006) 73–197.

18] B.D. Beake, V.M. Vishnyakov, R. Valizadeh, J.S. Colligon, Influence of mechanicalproperties on the nanoscratch behaviour of hard nanocomposites TiN/Si3N4

coatings on Si, Journal of Physics D: Applied Physics 39 (2006) 1392–1397.19] A. Chatterjee, N. Kumar, J.R. Abelson, P. Bellon, A.A. Polycarpou, Nanoscratch

and nanofriction behavior of hafnium diboride thin films, Wear 265 (2008)921–929.