Composites Science and Technology 79 (2013) 49–57

Contents lists available at SciVerse ScienceDirect

Composites Science and Technology

journal homepage: www.elsevier .com/ locate/compsci tech

Micro/nano-wear studies on epoxy/silica nanocomposites

Zheng-zhi Wang a,b, Ping Gu a,⇑, Xiao-ping Wu a, Hui Zhang c, Zhong Zhang c, Martin Y.M. Chiang b,⇑a Department of Modern Mechanics, University of Science and Technology of China, Hefei, Anhui 230027, Chinab Biosystems and Biomaterials Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USAc National Center for NanoScience and Technology, Beijing 100080, China

a r t i c l e i n f o

Article history:Received 13 September 2012Received in revised form 5 February 2013Accepted 11 February 2013Available online 20 February 2013

Keywords:A. Nano compositesB. WearB. Mechanical propertiesScanning probe microscopy (SPM)

0266-3538/$ - see front matter Published by Elsevierhttp://dx.doi.org/10.1016/j.compscitech.2013.02.010

⇑ Corresponding authors. Current address: DepartmeUniversity of California, Berkeley, CA 94720, USA (P. G

E-mail addresses: guping@ustc.edu.cn (P. Gu)(M.Y.M. Chiang).

a b s t r a c t

We proposed a new method for quantifying the micro/nano-scale wear volume (i.e., volume of wear loss)to characterize the wear-resistance of nanocomposites. Effects of wear load, pass (a pattern of scancycles), and nanoparticle content on the wear behavior of silica nanoparticle-reinforced composites(EP/SiO2) were studied. The multiple nano-scratch patterning technique was carried out for the wear test.Images of sample surface, before/after the test, obtained using in situ scanning probe microscopy (SPM)were used to calculate the wear volume. Our results indicate that the wear mechanism transits from aplastic-deformation dominated mode to a fatigue-wear dominated mode with increased wear load andpass. The threshold at which transition occurs increases with the nanoparticle content, and correlateswith improvement in wear resistance of nanocomposites. This transition threshold can be a differentmetric, rather than using the conventional mechanical properties (e.g., surface hardness and stiffness),to characterize the wear resistance of materials such that wear load and pass is taken into account.

Published by Elsevier Ltd.

1. Introduction

Nanoparticle-reinforced polymeric composites have attractedsignificant attention due to their improvements over conventionalcomposites in various aspects including optical transparency [1],electrical insulation [2], thermal stability [3], mechanical [4,5]and tribological performances [6,7]. Among those composites,silica nanoparticles were often used in composites to tailor themechanical strength [8], stiffness [9], fracture toughness [10],impact resistance [11] as well as the tribological scratch andabrasion-resistance [12] of epoxy resins (referred to EP/SiO2 com-posites hereinafter). Most existing studies on EP/SiO2 compositeswere aimed at macro-scale properties [8–11]. Information onmicro/nano scale investigations, especially the wear propertieswere scarce. This is mainly due to difficulties in implementationof test and quantification of result at the small scale. The objectiveof this study is to introduce a new method for quantifying themicro/nano-scale wear volume (i.e. the volume loss) for determin-ing the wear-resistance of materials. Also, effects of nanoparticlecontent and wear condition (wear load and pass) on the wearresistance of EP/SiO2 are studied.

A number of current works have pointed out the importance ofestablishing relationship between macro-, micro- and nano-wear

Ltd.

nt of Mechanical Engineering,u)., martin.chiang@nist.gov

mechanisms as well as understanding atomic origins of wear[13–16]. With the developments of scanning force microscopy(SFM) and advances in computational methods, much notable pro-gress in micro/nano-tribology has been accomplished in the pastdecade [16–21]. Many of these studies have been reported on themicro/nano-wear behavior of polymeric and inorganic neat mate-rials [17–20]. Few papers on polymeric nanocomposites have beenreported [22,23]. One of challenges in the micro/nano-wear studiesis the precise quantification of wear volume/rate since traditionalmacroscopic methodologies such as wear mass measurement andstereometry are impracticable at the small scales. Previous studieshave described wear as in terms of surface roughness, wear depth,or critical wear force/cycle [23–28]. Some studies estimated thewear volume or wear rate according to the two-dimensional (2D)profiles of the worn regions obtained from the scanning topo-graphic images [29–32]. These methods may result in non-negligi-ble errors due to the non-uniform distributions of surface wear[13,33].

In this study, we propose a new approach to calculate themicro/nano-wear volume from images of the composite beforeand after wear. The images are obtained based on in situ scanningprobe microscopic (SPM) through single-asperity nano-scratchpatterning technique. Accordingly, the wear behavior of EP/SiO2

with varying nanoparticle content was explored for different com-binations of wear load and pass (a pattern of scan cycles). Also, thesurface hardness and elastic modulus of the EP/SiO2 compositeswere obtained by separate nanoindentation tests, and the resultsare used to correlate with the micro/nano-wear volume for

Fig. 1. Illustrations of the wear volume calculation. Typical sample pre-wear (a) and post-wear (b) surface morphologies of a sample. The corresponding sectional profiles ofpre-wear (c) and post-wear (d) morphologies, along the lines marked in (a) and (b), respectively. The area within the red frame denotes the registration region based on thepre- and post-wear images (e), which is also shown in (b). Result of edge detection (f), where the white region denotes the exact worn region. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version of this article.)

50 Z.-z. Wang et al. / Composites Science and Technology 79 (2013) 49–57

characterizing wear behavior. We show that surface mechanicalproperties and composite wear-resistance increases dramaticallywith increasing nanoparticle content, particularly under higherwear loads and multiple passes. Also, the wear-resistance isassociated with the transition of wear mechanism during the wearprocess, going from a plastic deformation dominated regime to afatigue wear dominated regime. The improvement of wear resis-tance with nanoparticle content is correlated to an increase intransition threshold. This threshold manifests as a change in therate of wear loss and should be a practical metric for the wearresistance due to the consideration of wear condition (wear loadand wear pass).

2. Experiments1

2.1. Materials and methods

The epoxy resin (diglycidyl ether of bisphenol F, DGEBF)containing about 40 wt.% of silica nanoparticles with averagediameter of 25 nm (Nanopox F520) was used as a masterbatchfor nanocomposites studied. This masterbatch was hardened byan anhydride curing agent (Albidur HE600) and supplied by

1 Certain commercial materials and equipment are identified in this manuscript inorder to specify adequately the experimental and analysis procedures. In no case doessuch identification imply recommendation or endorsement by the National Instituteof Standards and Technology (NlST) nor does it imply that they are necessarily thebest available for the purpose.

Nanoresins AG, Germany. The epoxy-based nanocomposites wereobtained by mixing the masterbatch with different amounts ofactivated DGEBF resin to produce nanocomposites with variousnanoparticle contents (0, 1, 3, 6, 7, 10, 13, and 15 vol.%). In thismanuscript the samples corresponding to these different nanopar-ticle contents are symbolized as F0, F1, F3, F6, F7, F10, F13, and F15,respectively. More detailed descriptions about the fabrication,structure, mechanical and thermal characterizations of EP/SiO2

nanocomposites can be found in the literature [11]. TransmissionElectron Microscopy (TEM) (JEM-2100, Japan) with the assistanceof cryoultramicrotome (Leica EM FC7 UC7, Germany) was used toexamine the distribution of nanoparticles in the EP/SiO2.

The samples were polished to get highly smooth surfaces, andthe polishing protocol includes a preliminary manual step (usinga series of abrasive papers from coarse to fine, to ultrafine)followed with an instrumental polishing process. The surfaceroughness was measured for a 15 lm � 15 lm surface area usingthe SPM of TriboIndenter system (standard diamond Berkovichtip, triangular pyramid shape and curvature radius of 50 nm,Hysitron Inc., USA). Also, the nanoindentation test was carriedout using the same system for surface properties (hardness andstiffness). The applied load in the test was controlled accordingto a pre-determined loading function, which consists of a 5 s linearloading to 8000 lN, followed with a 5 s dwelling, and a 5 s linearunloading afterwards. The displacement of indenter was continu-ously monitored during the loading and unloading process. Foreach sample, at least 10 indentation tests were performed at spots50 lm apart to avoid interference.

Fig. 2. Transmission electron microscopy (TEM) images of F13 sample at lower (a) and higher magnification (b). TEM images of F15 sample at lower (c) and higher (d)magnification. Surface roughness of F0 sample, before the wear test, scanned by scanning probe microscopy (SPM) (e).

Z.-z. Wang et al. / Composites Science and Technology 79 (2013) 49–57 51

For the wear test, instead of using a triangular pyramid indentertip, a conical diamond tip with 1 lm radius of curvature and 90�cone angle was used. The wear volume was calculated based onthe pre- and post-wear topographic images of SPM. Major stepsof the wear test are summarized as follows:

(a) A pre-scan was performed on a15 lm � 15 lm square areaunder a load of 1 lN to get the initial topography of sample.

(b) A 2 lm � 2 lm area within the scanning region in step (a)was selected to carry out the wear test, as shown in Fig. S2in the Supplementary Data (SD). The indenter tip was drivenin the longitudinal direction for a round trip under a certainwear load at the rate of 6 lm/s, then the tip was moved by�7 nm (step size in Fig. S2b) laterally for next longitudinalscratch. This longitudinal scratch and lateral movement wererepeated until the scratches covered the whole test region(i.e. 2 lm � 2 lm), which is counted as one pass. Single passand multiple pass (including 5, 9, 13 and 17 passes) weartests were performed under different constant loads rangingfrom 10 to 80 lN. For each wear condition (combination ofwear load and pass), the test was repeated at least 3 times.

(c) The post-scan was performed on the worn sample surfacethrough the image processing described in step (a).

2.2. Calculation of wear volume

The wear volume (i.e. the volume based on the difference be-tween the pre- and post-wear morphologies) is determined fromthe height difference of each pair of corresponding pixel pointsin the wear region. Fig. 1a and b are selected pre- and post-wearimages; Fig. 1c and d are the corresponding line profiles alongthe line marked in Fig. 1a and b, respectively. Before determiningthe height differences between the pre- and post-wear images,there are three necessary procedures: the first one is to have an im-age registration, which means to find the translational relationshipbetween the pre- and post-wear images. This translation is causedby the non-linear piezoelectric effect of the crystal in the scanhead. The registration is implemented by matching the height-pro-file of a region (away from the worn region) in the post-wear imagewith that in the pre-wear image (see Fig. 1b and e). The second stepis the edge detection for a worn region (shown in Fig. 1b and f),such that the calculation of wear volume will not take into accountthe accumulated plastic deformation and/or detached wear debrisaround the worn region due to ploughing in the wear test. The laststep is to define a reference region; accordingly, the average initialheight of pre- and post-wear images in the region can be adjustedas having the same height for future wear volume calculation. This

Fig. 3. Topographic results of wear test for F0 sample under 15 lN and single pass. Pre-wear image (a) and post-wear image (b) of sample. The corresponding sectionalprofiles of pre-wear (c) and post-wear (d) morphologies, along the lines marked in (a) and (b). The profile difference (e). 3D morphologies of the pre-wear (f) and post-wear(g) surface.

Table 1Measured and calculated properties of the neat epoxy resin and EP/SiO2: roughness average (Ra) before test; hardness (H); Elastic modulus (E); the ratios of H/E and H3/E2;plasticity index.

Sample code Ra with in 15 � 15 lm2 (nm) H (GPa) E (GPa) H/E H3/E2 (GPa) Plasticity index

F0 3.206 ± 0.261 0.289 ± 0.009 3.295 ± 0.075 0.0878 0.0022 0.584 ± 0.016F1 2.968 ± 0.196 0.315 ± 0.008 3.460 ± 0.063 0.0911 0.0026 0.581 ± 0.012F3 3.054 ± 0.091 0.332 ± 0.006 3.662 ± 0.043 0.0906 0.0027 0.554 ± 0.020F6 3.495 ± 0.286 0.368 ± 0.005 3.912 ± 0.046 0.0941 0.0033 0.534 ± 0.015F7 3.591 ± 0.351 0.379 ± 0.006 4.040 ± 0.053 0.0936 0.0033 0.501 ± 0.010F10 2.847 ± 0.483 0.412 ± 0.011 4.362 ± 0.089 0.0944 0.0037 0.485 ± 0.008F13 3.159 ± 0.364 0.436 ± 0.004 4.494 ± 0.058 0.0970 0.0041 0.498 ± 0.015F15 3.486 ± 0.591 0.438 ± 0.010 4.585 ± 0.014 0.0956 0.0040 0.510 ± 0.013

52 Z.-z. Wang et al. / Composites Science and Technology 79 (2013) 49–57

reference region should be determined at a location distant fromthe worn region.

Finally the wear volume, V, is calculated using the equation as:

V ¼X

area

ðhpre � hpostÞ �AN

ð1Þ

where hpre and hpost represent the corresponding pixel depths in thepre- and post-wear images, respectively; A is the area of whole imageand N is the total number of pixel points in the image. The final sum-mation is within the range of the worn area, i.e. the result of the edgedetection. Our proposed measurement procedure is different fromthat suggested by Hysitron Inc. (see SD for details and error analysis

about the suggested procedure). Also, our procedure provides aquantitative 3D wear volume to characterize the wear behavior ofmaterials. This is more precise and realistic than procedures men-tioned in the literature [23–32], where 2D wear profile/depth orqualitative critical force/cycle is used for the characterization.

3. Results

3.1. TEM and SPM for morphological characterization

For nanocomposites, nanofiller distribution was crucial indetermining their final properties [34]. The nanoparticles can

Fig. 4. Dependence of wear volume of F0 sample on wear passes for different wear loads (a–e). Dependence of wear volume of F0 sample on wear load for single wear pass (f)and multiple passes (g–j).

Z.-z. Wang et al. / Composites Science and Technology 79 (2013) 49–57 53

effectively bear, transfer and distribute the stress in the matrix ifthey are homogeneously dispersed with good interfacial bondingto the resin [35]. On the contrary, aggregated nano-fillers causedsevere stress concentration and interfacial damage, thus negativelyinfluencing composite properties. Fig. 2a–d shows the TEM imagesof F13 and F15 EP/SiO2 samples at different magnification. Thespherical silica nanoparticles were well dispersed in the matrixwithout apparent agglomerates even at high nanoparticle content(15 vol.%). Fig. 2e gives the surface morphology of sample F0scanned using SPM, from which the roughness Ra (arithmetic aver-age roughness) and Rq (root-mean-square roughness) are about3.1 nm and 3.9 nm, respectively. The surface roughness of all othersamples is close to that of F0, as listed in Table 1.

3.2. Nanoindentation for mechanical properties

Table 1 lists the hardness (H) and elastic modulus (E), which areobtained by using the Oliver & Pharr method for indentation tests[36]. Also displayed are two ratios, H/E and H3/E2, which were re-ported to be closely correlated to the wear resistance and plasticdeformation of materials [37,38]. The value of four parameters H,

E, H/E and H3/E2 increases almost monotonously with the nanopar-ticle content. Also, H3/E2 shows a closely linear relationship with Hof nanocomposites (see Fig. S3 in SD for detailed results of inden-tation), which has good agreement with the result reported in theliterature [38,39]. Table 1 also lists the plasticity index (the ratio ofthe plastic work to the total work during the indentation process,see Fig. S3d in SD), which characterizes the ratio of plastic/elasticdeformation for the material during indentation [12]. The greaterthe plasticity index, the more the proportion of plastic deformationduring the indentation. Our result indicates the index decreases asthe nanoparticle content increases and reaches a minimum valueat 10% of the content, then increases with the content (Fig. S3din SD). The increased hardness (associated with a decreasing plas-tic index) of EP/SiO2 with the nanoparticle content can also be ver-ified by the three-dimensional (3D) surface morphologies of theindentations after complete elastic recovery (Fig. S4 in SD).

3.3. Micro/nano-wear results

In a wear test, if the contact stress at the probe/sample interfaceis very small, the elastic strain of the sample dominates the defor-

Fig. 5. Post-wear surface morphologies of neat resin sample (F0) under various wear loads and passes. The morphologies enclosed in the red frame represent the wearprocess dominated by the fatigue wear. The unit for the numbers (scale bar) shown next to the image is nm, and all the images denote actual area of 15 lm � 15 lm. (Forinterpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

54 Z.-z. Wang et al. / Composites Science and Technology 79 (2013) 49–57

mation due to contact and makes wear detection difficult. There-fore, there always exists a critical load, beyond which the plasticstrain is accessible and the wear volume can then be determined.Fig. 3a and b present pre- and post-wear topographic images of aF0 sample under the wear condition of test with 15 lN load andsingle pass. Fig. 3c and d display the corresponding line profilesalong the line marked in the two images (i.e. Fig. 3a and b).Fig. 3e shows the difference between the two profiles where thewear pit can be clearly observed. This tiny wear pit was also de-tected by comparing 3D surface morphologies of pre- and post-wear sample (Fig. 3f and g). In contrast, F0 samples under a weartest with single pass but 10 lN load did not show any apparentwear pits (Fig. S5 in SD). Accordingly, 15 lN was taken as a mini-mum load in the following studies.

The calculated wear volumes of the neat resin and EP/SiO2 com-posites under various wear conditions are listed in Table S1 in SD.Fig. 4 displays effects of the wear condition on the wear volume ofneat resin. The results shown in Fig. 4a–e indicate the wear vol-umes increase linearly with pass if the wear load is lower than40 lN; this rate becomes exponential with pass if it is over40 lN. For the single pass wear, the wear volume increases linearly(Fig. 4f); for the multi-pass wear, the volume increases exponen-tially with all the wear loads studied (Fig. 4g–j). Fig. 5 showspost-wear surface morphologies of neat resin (F0) samples undervarious wear conditions. The dependence of wear behavior of EP/SiO2 composites on the wear load and pass is similar to that ofF0 (not shown here).

In this study, we determined the effects of nanoparticle contenton the wear volume of nanocomposites. Fig. 6 displays the varia-tions of the wear volume with nanoparticle content under four se-lected wear conditions (i.e. 20 lN 5 passes, 40 lN 9 passes, 60 lN13 passes, and 80 lN 17 passes), such that both the wear load andpass are increased for every condition tested. Comparing with theneat epoxy resin, all the nanocomposites studied show a decreasein the wear volume. This decrease becomes more significant at thehigher wear loads and passes than the lower ones. For example,under 80 lN and 17 passes, the average wear volume of F15 sam-ples is about (1.4 ± 0.1) � 10�19 m3, which is about 6 times lowerthan that of neat resin. However, at lower wear load and pass(for example, 20 lN and 5 passes), it is only about 2.5 times.

4. Discussion

A number of previous studies showed wear resistance of mate-rials were closely related to their mechanical properties, such ashardness (H), elasticity (E) and ductility [39,40]. Matthews andLeyland first pointed out the ratio of H/E could be a useful metricfor the wear resistance [37,41]. Later, Musil and co-workers de-fined a different wear resistance metric, H3/E2 [38,42]. These met-rics were reasonably correlated to the material wear resistance inmany subsequent experimental studies as long as there was nooccurrence of significant interfacial adhesion at the contact surfaceduring the wear tests [39,40,43,44]. In addition, H/E was found to

Fig. 6. Dependence of wear volume on nanoparticle content under four selected wear conditions (a–d).

Z.-z. Wang et al. / Composites Science and Technology 79 (2013) 49–57 55

be inversely related to the plasticity index of material – a reducedplasticity index is correlated to an increased wear resistance[45,46]. From the result shown in Table 1, one can see two ratios(H/E and H3/E2) increase and the plasticity index decreases withthe addition of silica nanoparticles. This trend in the mechanicalproperties (Figs. S3b and S3d in SD) is in line with our wear test re-sults (Fig. 6a), where the wear volume of EP/SiO2 composites wasindeed reduced and the wear-resistance enhanced with theincreasing nanoparticle volume. However, under the wear condi-tions of higher load and pass, the correlation between wear volume(Fig. 6b–d) and mechanical properties (plasticity index or E/H,Fig. S3d in SD) is less accurate. This implies that the wear conditionshould be considered in the characterization of wear behavior,which will be discussed in the following.

For single asperity nanoabrasion experiments, Rabinowicz [47]and Archad [48] proposed empirical equations predicting that thewear volume is linearly proportional to the wear load and abra-sion cycle as long as wear mechanism stays invariant duringthe wear process [30]. Our results of neat resin (F0) samples,shown in Fig. 4a, b, and f, present a linear relationship betweenthe wear volume and wear condition when low wear load (e.g.,15 lN or 20 lN) or single pass is subjected. At this kind of wearcondition, by observing wear morphologies outside the framemarked in Fig. 5, one can notice that the shape and size of wearregion remain mostly unchanged before and after the test, thematerial piles up around the worn region due to ploughing, andno apparent bulk material or wear debris detached from the sam-ple surface. This indicates that the plastic deformation regime atcontact dominates the wear process [16,23,39,40], and the wearmechanism remains invariant for the wear conditions at low loador single pass.

However, when the wear load or pass has a higher value, ourresults of F0 samples shown in Fig. 4c–e and g–j indicate that thewear volume deviates from the aforementioned linear relationship.The corresponding worn surface morphologies are presentedwithin the frame marked in Fig. 5, in which one can see the shape

of worn region became irregular and the periphery of region wereaccumulated with wear debris. Based on related studies on poly-mer nanowear [40,49,50], we infer that plastic deformation domi-nates during the probe ploughing and nucleation of micro-crackoccurs in the sample subsurface under the higher wear loads.The crack can propagate to the surface layer and result in the for-mation of tiny abrasive chips accumulating onto the sides of theploughing furrows. With the increase of wear pass, more micro-cracks and abrasive debris generate, and the dominating wear pro-cess transforms from ploughing to both ploughing and materialdetachment [32,50]. Finally the cumulative damage mode, i.e. fati-gue wear, is formed. These phenomena were not observed in thecase of either low load or single pass wear condition. Therefore,the deviation of the linear relationship between wear volumeand wear load/pass is due to the change in wear mechanism – froma complete material plastic deformation to a combination of plasticdeformation and fatigue wear.

Now we switch our focus from neat resin (F0) to nanocompos-ites (EP/SiO2). For the nanocomposites, one can note from thepost-wear surface morphologies (Figs. S6 to S12 in SD) that thewear regime also transits from a plastic deformation dominatedmode to a combination of plastic deformation and fatigue wearmode (i.e., fatigue wear dominated regime) with increasing wearloads and passes. The load and pass at which the transition occursare dependent on the nanoparticle content. Fig. 7 gives the mappingof the transition threshold at different load-pass combinations fordifferent nanoparticle contents. The light grey area in the figureindicates the wear behavior follows the plastic deformationdominated regime; the dark grey area signifies the fatigue weardominated regime while the white areas are cases out of ourstudies. It is observed that the size of the dark grey area decreasesgradually as the nanoparticle content increases and diminishes atthe maximum content studied (F15) – the transition threshold israised. Combining the results of Fig. 7 and mechanical parametersH3/E2 or H/E listed in Table 1, the increase of parameter valueimplies a higher threshold for the wear mechanism transition.

Fig. 7. Maps of the transition threshold from a plastic deformation to a fatigue weardominated wear process. Neat resin (a), 1% nanoparticle content in volume (b), 3%(c), 6% or 7% (d), 10% (e), 13% (f), and 15% (g).

56 Z.-z. Wang et al. / Composites Science and Technology 79 (2013) 49–57

For the composites with high nanoparticle contents, besides theaforementioned transition, another possible mechanism for the in-crease of wear-resistance may come from the rolling effect ofnanoparticle [51–53]. In other words, detached nanoparticles fromthe matrix can act as rolling balls between the probe and matrix,such that the original two-body abrasion (probe against compos-ite) becomes an effective three-body wear (probe against matrixand nanoparticles). This small-scale rolling effect may causeremarkable reduction of the frictional coefficient and suppressthe grooving/ploughing wear due to non-direct contact betweenthe probe and sample surface. It was found that the abrasion froma three-body wear process is sometimes 10 times lower than thatfrom a two-body process [54]. Although this rolling effect seemedlogical to interpret the enhanced wear-resistance of nanocompos-ites, it lacks direct evidence in our study.

5. Conclusion

This study presents a new approach, for the evaluation of wearvolume at the micro/nano scale, that shows potential in precisequantification of surface wear. Accordingly, the wear behavior ofepoxy resin and its composites filled with silica nanoparticles werestudied using the multiple nano-scratch patterning technique.Also, results of mechanical properties from nanoindentation testwere examined for comparison.

Our results show that the dependence of wear volume on thewear load and pass gradually changes from a linear to exponential

relationship as the load and pass increase. We have demonstratedthis change corresponds to a wear mechanism transition – from aplastic-deformation dominated mode to a fatigue-wear dominatedmode. The transition threshold is increased with the nanoparticlecontent, and the increase of threshold contributes to the improve-ment of wear resistance. Conventionally, mechanical parameterssuch as surface hardness (H) together with elastic modulus (E)have been used to characterize the wear resistance. However, thisstudy indicates the transition threshold, which reflects the changein the wear rate, could be more reliable than those parameterssince the wear condition has been taken into account.

Acknowledgments

The research was partly supported by the FundamentalResearch Funds for the Central Universities. The authors gratefulacknowledge the provision of experimental samples from NCNST(Beijing, China).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.compscitech.2013.02.010.

References

[1] Zhang H, Zhang H, Tang LC, Zhou LY, Eger C, Zhang Z. Compos Sci Technol2011;71:471.

[2] Zhi CY, Bando Y, Terao T, Tang CC, Kuwahara H, Golberg D. Adv Funct Mater2009;19:1857.

[3] Bitinis N, Hernandez M, Verdejo R, Kenny JM, Lopez-Manchado MA. Adv Mater2011;23:5229.

[4] Wang ZZ, Gu P, Zhang Z. Wear 2010;269:21.[5] Wichmann MHG, Schulte K, Wagner HD. Compos Sci Technol 2008;68:329.[6] Satterthwaite JD, Vogel K, Watts DC. Dent Mater 2009;25:1612.[7] Lin-Gibson S, Sung LP, Forster AM, Hu HQ, Cheng YJ, Lin NJ. Acta Biomater

2009;5:2084.[8] Mahrholz T, Stangle J, Sinapius M. Composites: Part A 2009;40:235.[9] Zhang H, Zhang Z, Friedrich K, Eger C. Acta Mater 2006;54:1833.

[10] Adachi T, Osaki M, Araki W, Kwon SC. Acta Mater 2008;56:2101.[11] Zhang H, Tang LC, Zhang Z, Friedrich K, Sprenger S. Polymer 2008;49:3816.[12] Wang ZZ, Gu P, Zhang Z, Gu L, Xu YZ. Tribol Lett 2011;42:185.[13] Bhushan B, Israelachvili JN, Landman U. Nature 1995;374:607.[14] Perry SS, Tysoe WT. Tribo Lett 2005;19:151.[15] Kim SH, Asay DB, Dugger MT. Nano Today 2007;2:22.[16] Szlufarska I, Chandross M, Carpick RW. J Phys D Appl Phys 2008;41:123001.[17] Berman AD, Ducker WA, Israelachvili JN. Langmuir 1996;12:4559.[18] Dinelli F, Leggett GJ, Shipway PH. Nanotechnology 2005;16:675.[19] Gotsmann B, Durig U. Langmuir 2004;20:1495.[20] Gotsmann B, Duerig UT, Sills S, Frommer J, Hawker CJ. Nano Lett 2006;6:296.[21] Mo YF, Turner KT, Szlufarska I. Nature 2009;457:1116.[22] Wang H, Feng J, Hu X, Ng KM. Polym Eng Sci 2008;48:1467.[23] Pihan SA, Emmerling SGJ, Butt HJ, Gutmann JS, Berger R. Wear 2011;271:2852.[24] Berger R, Cheng Y, Forch R, Gotsmann B, Gutmann JS, Pakula T, et al. Langmuir

2007;23:3150.[25] Lee KM, Yeo CD, Polycarpou AA. Exp Mech 2007;47:107.[26] Dinelli F, Leggett GJ, Alexander MR. J Appl Phys 2002;91:3841.[27] Vyas MK, Nandan B, Schneider K, Stamm M. Eur Polym J 2009;45:1367.[28] Vyas MK, Nandan B, Schneider K, Stamm M. J Colloid Interf Sci 2008;328:58.[29] Balani K, Harimkar SP, Keshri A, Chen Y, Dahotre NB, Agarwal A. Acta Mater

2008;56:5984.[30] Degiampietro K, Colaco R. Wear 2007;263:1579.[31] Peng L, Lee H, Teizer W, Liang H. Wear 2009;267:1177.[32] Wang DF, Kato K. J Tribol T ASME 2003;125:430.[33] Dasari A, Yu ZZ, Mai YW. Mater Sci Eng R 2009;63:31.[34] Schmidt CW, Knieke C, Maier V, Hoppel HW, Peukert W, Goken M. Scripta

Mater 2011;64:245.[35] Chawla N, Sidhu RS, Ganesh VV. Acta Mater 2006;54:1541.[36] Oliver WC, Pharr GM. J Mater Res 1992;7:1564.[37] Leyland A, Matthews A. Wear 2000;246:1.[38] Musil J, Kunc F, Zeman H, Polakova H. Surf Coat Technol 2002;154:304.[39] Beake BD, Vishnyakov VM, Valizadeh R, Colligon JS. J Phys D Appl Phys

2006;39:1392.[40] Beake BD, Shipway PH, Leggett GJ. Wear 2004;256:118.[41] Leyland A, Matthews A. Surf Coat Technol 2004;177–178:317.[42] Mayrhofer PH, Mitterer C, Musil J. Surf Coat Technol 2004;177–178:725.[43] Beake BD, Lau SP. Diamond Relat Mater 2005;14:1535.

Z.-z. Wang et al. / Composites Science and Technology 79 (2013) 49–57 57

[44] Shi BG, Sullivan JL, Beake BD. J Phys D Appl Phys 2008;41. 045303-1.[45] Briscoe BJ, Fiori L, Pelillo E. J Phys D Appl Phys 1998;31:2395.[46] Cheng YT, Cheng CM. Mater Sci Eng R 2004;44:91.[47] Rabinowicz E. Friction and wear of materials. 2nd ed. New York: John Wiley

and Sons; 1965.[48] Hutchings IM. Tribology: friction and wear of engineering

materials. Arnold: CRC press; 1995.[49] Beake BD, Leggett GJ, Shipway PH. Polymer 2001;42:7025.

[50] Wang DF, Kato K. J Tribol T ASME 2003;125:437.[51] Rapoport L, Bilik Y, Feldman Y, Homyonfer M, Cohen SR, Tenne R. Nature

1997;387:791.[52] Rapoport L, Leshchinsky V, Lapsker I, Volovik Y, Nepomnyashcy O, Lvovsky M,

et al. Wear 2003;255:785.[53] Chang L, Zhang Z. Wear 2006;260:869.[54] Stachowiak GW, Batchelor AW. Engineering tribology. 2nd

ed. Woburn: Butterworths/Heinemann; 2001.