Songbo Xu

Aydar Akchurin

Department of Mechanical Engineering,

North Dakota State University,

Fargo, ND 58108

Tian Liu

Weston Wood

School of Mechanical and Materials Engineering,

Washington State University,

Pullman, WA 99164

X. W. Tangpong1

Department of Mechanical Engineering,

North Dakota State University,

Fargo, ND 58108

e-mail: Annie.Tangpong@ndsu.edu

Iskander S. AkhatovDepartment of Mechanical Engineering,

North Dakota State University,

Fargo, ND 58108;

Center for Micro and Nanoscale

Dynamics of Dispersed Systems,

Bashkir State University,

Ufa 450076, Russia

Wei-Hong ZhongSchool of Mechanical and Materials Engineering,

Washington State University,

Pullman, WA 99164

Wear of Carbon NanofiberReinforced HDPENanocomposites Under DrySliding ConditionHigh density polyethylene (HDPE) is widely used as a bearing material in industrialapplication because of its low friction and high wear resistance properties. Carbon nano-fiber (CNF) reinforced HDPE nanocomposites are promising materials for biomedicalapplications as well, such as being the bearing materials in total joint replacements. Themain objective of the present study is to investigate how the wear of HDPE can be alteredby the addition of either pristine or silane treated CNFs at different loading levels(0.5 wt. % and 3 wt. %). Two types of silane coating thicknesses, 2.8 nm and 46 nm, wereapplied on the surfaces of oxidized CNFs to improve the interfacial bonding strengthbetween the CNFs and the matrix. The CNF/HDPE nanocomposites were preparedthrough melt mixing and hot-pressing. The coefficients of friction (COFs) and wear ratesof the neat HDPE and CNF/HDPE nanocomposites were determined using a pin-on-disctribometer under dry sliding conditions. The microstructures of the worn surfaces of thenanocomposites were characterized using both scanning electron microscope (SEM) andoptical microscope to analyze their wear mechanisms. Compared with the neat HDPE,the COF of the nanocomposites were reduced. The nanocomposite reinforced with CNFscoated with the thicker silane coating (46 nm) at 0.5 wt. % loading level was found toyield the highest wear resistance with a wear rate reduction of nearly 68% compared tothe neat HDPE. [DOI: 10.1115/1.4023244]

Keywords: carbon nanofibers, silane coating, wear, friction

1 Introduction

With the development of polymer industry, polymer, and poly-meric composites are increasingly used as bearing materials invarious applications due to their easy manufacturability, excellentprocessability, and great vibration resistance compared to metals[1]. Polyethylene (PE), one of the engineering thermoplasticswhose products, HDPE and ultra high molecular weight polyeth-ylene (UHMWPE), have been used as bearing materials becauseof their low cost, low friction, good resistance to wear and tochemical attacks [2,3]. The demand for PE in industry is increas-ing due to the vast tribological applications in low speed bearings[3], automotive [4], petroleum and gas industries [5], and in bot-tling sectors [6]. In the early 1960 s, John Charnley [7] introducedmetal-on-plastic joints with the aim of not only minimizing fric-tion and wear rate but also the torque produced by the frictionalforce [8]. PE has been applied as materials for acetabular cups inartificial joints due to their excellent biocompatibility and tribo-logical properties [9]. So far, UHMWPE is almost a universal can-didate that meets the requirements for modern total jointreplacements [10]. The basic requirements for a bearing compo-nent in artificial joint are high mechanical toughness and impactstrength, creep resistance, good biological stability, biocompati-bility, low friction, and low wear [11]. In the market, the most

common type of prosthetic hip replacement is made out of metalfor the ball and stem, and UHMWPE for the cup. Although someceramic-on-ceramic and metal-on-metal designs exist, they onlyaccount for a very small proportion of the market [12]. It is esti-mated that the annual number of total joint replacements in U.Swill escalate to more than 1.8 million by 2015 [13], as comparedwith 773,000 in 2009 [14]. However, most of these total jointreplacements will not survive beyond 25 years [15]. With peopleliving longer and leading more active lives, these prostheses mightactually have to be replaced. Even with the outstanding tribologi-cal properties of UHMWPE, the generation of wear debris result-ing in biological responses that contribute to osteolysis andloosening of the implant is currently considered the major causeof failure and revision of the artificial joints [16,17]. Therefore,there is a critical need of a new implant material which inducesless wear debris and lower biological responses.

Various methods have been carried out to improve the wear re-sistance of UHMWPE, including cross-linking [18–20] and theincorporation of various fillers into the UHMWPE matrix[21–23]. However, it was reported that the cross-linking could notonly decrease the ductility, yield strength, and elongation of fail-ure but also reduce the toughness [24]. Although cross-linking hasbeen shown to significantly enhance the wear resistance, thosedetrimental effects increase the risk of catastrophic failure of theartificial joint. The incorporation of macroscopic-filler intoUHMWPE also was found to be unsuccessful due to the genera-tion of high stress concentrations at the interfaces [25]. One of thefamous examples is Poly II

TM

(short carbon fiber reinforced

1Corresponding author.Manuscript received May 7, 2012; final manuscript received December 11, 2012;

published online March 26, 2013. Assoc. Editor: Mu Chiao.

Journal of Nanotechnology in Engineering and Medicine NOVEMBER 2012, Vol. 3 / 041003-1Copyright VC 2013 by ASME

Downloaded From: http://nanoengineeringmedical.asmedigitalcollection.asme.org/ on 08/16/2013 Terms of Use: http://asme.org/terms

UHMWPE) which reduced wear at the cost of ductility, crack re-sistance and fatigue resistance of UHMWPE [26]. There has beenincreasing interest in the incorporation of nanoscale fillers intoUHMWPE to improve the nanocomposite’s wear resistance with-out compromising other mechanical properties.

Nanocomposites with carbon-based nanofillers, including car-bon nanotubes (CNTs), CNFs, and graphite nanoparticles (GNPs),have gained intense interest due to their high mechanical proper-ties, thermal conductivity, good biocompatibility, as well as solidlubrication role [27]. Johnson et al. [24] added CNTs into HDPEand found that the wear rate and friction coefficient of the nano-composite were decreased by up to 50% and 12%, respectively,by the addition of 5 wt. % of CNTs without negative effects onother material properties. Fouad and Elleithy [9] investigated thethermal, rheological, cytotoxicity, mechanical, and abrasive wearbehavior of HDPE with GNPs at different concentrations, andconcluded that the HDPE/GNPs nanocomposites possessed com-parable tensile strength and Young’s modulus to UHMWPE, aswell as improved abrasion resistance compared to the neat HDPE.Compared with other carbon-based nanofillers, the high aspect ra-tio, multifunctionality, and commercial productivity of CNFs ledthem to be ideal reinforcements for nanocomposites [28]. In addi-tion, no cytotoxic response of biological cells to CNFs has beenreported so far. Elias et al. [29] investigated the cytotoxicity ofCNFs by examining the proliferation and function of osteoblastcells seeded onto compacted CNFs, and concluded that CNFs didnot show any cytotoxic response to the proliferation of those cells.The adhesion properties of osteoblasts, chondrocytes, fibroblasts,and smooth muscle cells with polycarbonate urethane (PU)/CNFnanocomposites were investigated in the follow-up studies[30,31]. The addition of CNFs into PU did not induce any cyto-toxic effect and the CNF-based nanocomposites were demon-strated to have the potential for biomedical applications [30–33].In our previous work, Wood et al. [34,35] successfully producedsilane-treated oxidized CNFs with different silane coating thick-nesses as nanofillers for HDPE, and found that the thick-silanecoated CNFs (thickness of 46 nm) had improved compatibilitywith HDPE. However, it seems that the quantity and content ofthe published works were not sufficient in addressing the tribolog-ical properties of silane-treated CNF/HDPE nanocomposites andtheir wear mechanisms.

Moreover, a guideline for estimating properties of UHMWPE isstill needed [36]. The impossibility of capturing all probable pa-rameters such as the interfacial bonding and nonhomogeneous dis-persion in polymer composites, effects of multidirectional orbidirectional sliding in bioapplications, and lubricant effects,accelerates the need for a proper model which can identify the tri-bological damage, estimate the lifetime of interfacing materials,optimize the tribological systems, and facilitates simultaneousaccess to several tribological data [37]. Huq and Celis [38]showed that good linear correlations between the volumetric wearand dissipated energy always existed for unidirectional or bidirec-tional sliding of TiN and (Ti,Al)N coating against alumina underdifferent humidity conditions. This linear relation was suggestedto be independent of the type of materials in the friction pair, thecontact geometry, type of movement (unidirectional or multidirec-tional), and the lubricating conditions (dry or lubricated) [39–41].Therefore, it was believed that the energy approach to define weardamage might be more effective for material optimization in tri-bological systems [36].

In this paper, HDPE was chosen as the polymer matrix to studythe effect of silane-coated CNFs on the tribological properties ofthe nanocomposites, due to its essentially the same chemicalstructure as UHMWPE and lower viscosity for easier processing.HDPE nanocomposites with both pristine and silane-treated CNFsunder various weight concentrations were fabricated. The frictionand wear experiments of these nanocomposites carried out in thisstudy were performed using a pin-on-disc apparatus under drysliding condition. The wear damage in each material was consid-ered to have resulted from the energy dissipation caused by

friction forces. The wear behavior was quantified by two types ofspecific wear rates, the traditional distance-based wear rate and anenergy-based wear rate. The indentation energy was also deter-mined to assess their influence on the materials’ wear properties.The obtained results will add insight to the development of HDPEnanocomposites as bearing materials for industrial applications.Even though the material’s tribological property in dry slidingcondition does not directly apply to the study of artificial joints,such preliminary study of HDPE nanocomposites may havepotential implications to the study of wear properties ofUHMWPE nanocomposites in lubricated conditions in the nearfuture.

2 Experiments

2.1 Materials Preparation. HDPE (HP54-60 Flake) suppliedby Bamberger polymers Inc., was used as a matrix. The pristineCNFs (PR-24-HTT) and pretreated oxidized CNFs (PR-24-HHT-OX) were purchased from Applied Sciences Inc., with diametersof approximately 60–150 nm and lengths of 30–100 lm.Octadecyltrimethoxy-silane (ODMS) (90% technical grade) wasmanufactured by Sigma-Aldrich. The pretreated oxidized CNFswere modified with ODMS and ethanol solution, and a silanecoating was formed to cover the fiber surface. The thickness ofthe silane coating was controlled by the ratio of ODMS to ox-CNF added as well as the percentage of ethanol and water in thisreaction. Two silane coating thicknesses, 46 nm and 2.8 nm, wereobtained and considered in this study. Two levels of CNF concen-trations of 0.5 wt. % and 3 wt. % for both pristine and silane-treated CNFs were achieved. The detailed descriptions of the sil-ane coating synthesis and nanocomposite manufacturing proce-dures were given in Refs. [34,35]. A neat HDPE reference wasprepared under the same conditions. The descriptions of the CNF/HDPE nanocomposite samples were summarized in Table 1.

2.2 Wear Testing. The wear and friction testing were per-formed using a pin-on-disc tribometer (UMT-2). Although a mul-tidirectional wear simulator under lubricated conditions, such asin bovine serum, would provide a more realistic pattern of slidingfor the application of artificial joints, dry sliding wear test using apin-on-disc tribometer has been widely performed in preliminarystudies of material’s wear properties for both industrial and bio-medical applications [42–44]. The stainless steel balls (diameter4.762 mm) used were from Salem Specialty Ball Inc., (SS440Grade 25). The testing parameters were set as 3 N (normal load),1 rpm (sliding speed), 8 h (test duration), and 10 mm (wear trackdiameter). The tangential friction force (Fx) and normal load (Fz)were continuously monitored by the tribometer directly, and thecoefficient of friction was then calculated as a ratio of Fx/Fz. Themass loss of each specimen was measured using a digital scalewith an accuracy of 0.1 mg. Each material sample was tested forat least four times.

2.3 Indentation Test. Indentation test was performed todetermine the plastic deformation energy for the seven materialslisted in Table 1 using the same tribometer. The samples were

Table 1 Description of the HDPE/CNF nanocomposite samples

Sample Matrix ReinforcementsNanofiberconcentration

Neat HDPE — —P-05 Pristine CNFs 0.5 wt. %P-3 3 wt. %T1-05 Thick-silane (46 nm) treated CNFs 0.5 wt. %T1-3 3 wt. %T2-05 Thin-silane (2.8 nm) treated CNFs 0.5 wt. %T2-3 3 wt. %

041003-2 / Vol. 3, NOVEMBER 2012 Transactions of the ASME

Downloaded From: http://nanoengineeringmedical.asmedigitalcollection.asme.org/ on 08/16/2013 Terms of Use: http://asme.org/terms

indented with the same type of stainless steel ball as used in thewear testing. During the indentation, the normal load was linearlyincreased to 3 N, which was also the normal load used in the weartesting, and then slowly decreased down to zero. The indentationdepth on the sample surface, or deflection of the material, wasrecorded by the tribometer automatically during the loading–unloading process. The area enclosed in the loading and unloadingcurves represents the plastic deformation energy of the sampleand can be obtained through numerical integration.

3 Results and Discussion

3.1 Friction Force and Dissipated Energy. Graphite pos-sesses self-lubricating and dry lubricating properties due to theloose interlamellar coupling between the planes of polycyclic car-bon atoms in the structure [45]. For that reason, we speculatedthat the addition of CNFs might decrease the COF of HDPE/CNFs nanocomposites due to their layered graphitic structures. InFig. 1, the COFs of all samples changed drastically during the ear-lier stage of sliding up to 1000 s, and then slowly reached a rela-tively steady state after about 12,500 s. With the addition ofCNFs, both pristine and silane-treated, the COFs of the nanocom-posites were all lower than that of the neat HDPE. It can also beobserved that the COFs of the nanocomposites with silane-treatedCNFs were higher than those with pristine CNFs at the same con-centration level. Such a relatively higher COF of the silane-treated sample could have been resulted from the three-dimensional cross-linked structure of the silane coating [34]. The3D network of the cross-linked silane coating may have inhibitedthe lubricating effect of the graphite structure of CNFs. The thick(46 nm) silane coated CNFs seemed to be more effective indecreasing the COF of the nanocomposites than the thinner one(2.8 nm), possibly due to the stronger interfacial bonding betweenthe thick silane coated CNFs and the HDPE matrix.

The average COFs of the samples at steady state (after12,500 s) are summarized in Fig. 2. Only half of the error bars ofthe results were shown in the figure for a clearer presentation. Foreach type of CNFs (pristine, thick, or thin silane coated), the nano-composites with higher concentrations of CNFs yielded lowerCOFs. Such phenomenon could be a result of the lubricatingeffect of the CNFs on the dry sliding contact between the sampleand the steel ball. The COF between any two materials dependson many system parameters such as the temperature, geometric,and material properties of the contact surfaces. Any subtle varia-tion in those parameters might lead to unstable measurements ofthe COF, especially during a long sliding duration. Moreover, theCOF measurement of plastic material can be even more compli-cated because the large plastic deformation might occur at thetips of the asperities and the geometric conditions around thetips could easily change [46]. Therefore, some relatively large

standard deviations were observed in the COF measurements ofsamples such as the neat, P-05 and P-3. However, even with theconsideration of these relatively large standard deviations, theaddition of CNF can be regarded as being effective in decreasingthe COFs of the HDPE nanocomposites.

The tangential friction force is an important factor in evaluatingthe wear behavior of a material, not only because it directly con-tributes to the COF but it was also believed to be one of the origi-nal main sources of energy dissipation on the contacting surfaces[47]. There was a fundamental assumption that the wear damageof a material during sliding against another material was resultedfrom the dissipated energy due to friction between them [38].Energy dissipation during sliding under different tribological con-ditions was discussed in various studies [37,41,48,49]. In thiswork, the dissipated energy was determined from the followingequation that was proposed recently in Refs. [37,47]:

Ed ¼ðL

0

Fxdx (1)

where Ed is the total dissipated energy during the entire slidingdistance, dx is the incremental sliding distance, L is the total slid-ing distance, and Fx is the tangential friction force which wasmonitored by the tribometer directly. An example of determiningthe dissipated energy is given in Fig. 3. The area under the curveof Fx represents the dissipated energy of the material and can be

Fig. 1 Variations of the coefficients of friction over time

Fig. 2 Comparison of the coefficients of friction of the sevenmaterials

Fig. 3 An example (T1-05) of the dissipated energy due tofriction

Journal of Nanotechnology in Engineering and Medicine NOVEMBER 2012, Vol. 3 / 041003-3

Downloaded From: http://nanoengineeringmedical.asmedigitalcollection.asme.org/ on 08/16/2013 Terms of Use: http://asme.org/terms

calculated via a numerical integration method. Figure 4 presentsthe amount of dissipated energy during the total dry slidingbetween each material and the steel ball. The dissipated energy ofthe materials had a consistent trend with their COFs and willbe applied into the determination of specific wear volumes inSec. 3.2.

3.2 Wear Resistance. Generally, the wear resistance of amaterial is characterized by the specific wear rate (SWR), whichis defined as the wear volume per unit distance and unit load [50].The lower the SWR, the better the wear resistance. SWR wasdetermined using the following equation [35,51]:

WSWR ¼DV

FzL¼ Dm

qFzL(2)

where WSWR is the SWR, DV is the wear volume (mm3), Fz is thenormal force (N), and L is the sliding distance (m). The wear vol-ume DV can further be expressed as Dm/q, where Dm is the massloss, and q is the density of the material.

In a recent study [47], specific wear volume (SWV) was pro-posed to relate the wear volume and dissipated energy as an effi-cient substitute for the traditional SWR. SWV, as a new parameterfor the characterization of wear resistance, was defined as the vol-umetric material loss per unit of energy dissipation [37]

WSWV ¼DV

Ed¼ Dm

qEd(3)

where WSWV is the SWV and Ed is the dissipated energy duringthe sliding distance determined by Eq. (1).

The SWRs of the seven materials are illustrated in Fig. 5(a).The T1-05 nanocomposite, which was enhanced with 0.5 wt. % ofCNFs treated with 46 nm silane coating, yielded the lowest wearrate, and compared to the neat HDPE, its wear resistance wasenhanced by almost 68%. Moreover, compared to the neat HDPEand other nanocomposites with the same CNF concentration of3 wt. %, the T1-3 nanocomposite had the lowest wear rate. There-fore, the thick silane-treated CNFs seemed to be more effective inimproving the wear resistance of the nanocomposites comparedwith pristine CNFs and those treated with thin silane coatings. Itwas believed that a thicker silane coating might allow the polymerchains to entangle with the coating structure, and such strongphysical entanglements improved the adhesion and interactionbetween the CNFs and the matrix [34]. The thicker silane coatingthus functioned as a bridge, or transition zone, to transfer load andimproved the reinforcement of CNFs. However, the interactionsbetween the nonpolar thinner coating and the nonpolar polymermatrix were limited to weak Van der Waals forces, which mightnot be strong enough to induce an adequate load transfer.

The effect of CNFs concentration on wear rates of the nano-composites can also be discussed through Fig. 5(a). The nanocom-posites with the three types of CNFs at a low concentration of0.5 wt. % all showed decreased wear rates compared to that of theneat HDPE; however, with the concentrations of CNFs increasingto 3 wt. %, the wear rate also increased. From the COF results dis-cussed in Sec. 3.1, it would be expected that the nanocompositesat 3 wt. % CNFs loading, which have lower COFs, would yieldlower wear rates, since in general the lower the COFs are, the

Fig. 4 Comparison of the total dissipated energy of the sevenmaterials

Fig. 5 Comparison of the wear rates of the seven materials by (a) specific wear rate and(b) specific wear volume

041003-4 / Vol. 3, NOVEMBER 2012 Transactions of the ASME

Downloaded From: http://nanoengineeringmedical.asmedigitalcollection.asme.org/ on 08/16/2013 Terms of Use: http://asme.org/terms

lower the shear stresses would be on the sample surfaces. Toexplain this phenomenon, the microstructures of the worn surfacesof T1-05 and P-3 were observed and are shown in Fig. 6. Noobvious agglomerates of CNFs were found in both materials. TheCNFs on the surface of T1-05 were uniformly distributed in theHDPE matrix in Fig. 6(a). However, the CNFs in P-3 were moreinclined to aggregate, shown in Fig. 6(b), although no big agglom-erates were observed. Those small aggregates can be easilyformed during the fabrication due to Van der Waals forces and thehigh aspect ratios of CNFs [52]. It is speculated that the increasedwear rates of the nanocomposites with higher concentrations ofCNFs were due to such small aggregates of the nanofibers whichaffected the dispersion of CNFs and decreased their reinforcementeffect on the matrix. Furthermore, wear debris that containedthese aggregates of CNFs could work as hard abrasive grains inthird-body abrasive wear, thus removing more material from theworn surface [51].

Figure 5(b) illustrates the SWV of the seven materials. The twomeasures of the wear rate, SWR and SWV, showed a consistenttrend. Both measures of the wear rate can be expressed in thesame unit of mm3/Nm. The difference between them comes fromthe force term involved. As Eq. (2) indicates, the normal forcewas referred in SWR, because it was implicitly assumed that thewear volume per unit sliding distance was proportional to the load[53]. However, this force is normal to the sliding direction andhence makes no contribution to the frictional work. Therefore,although with apparently the same unit, the N�1�m�1 in the for-mulation of SWR represents per unit distance and unit normalload, and does not coincide with any known scientific concept[37]. In the formulation of SWV, however, the N�1�m�1 was con-sidered as the frictional dissipated energy in the unit of Joule. Asshown in Fig. 5, the SWV values are about one magnitude largerthan the corresponding SWR values, and therefore could amplifythe differences in wear resistance between the materials. Such dif-ference in magnitude also arises from the different forces (normalforce and friction force, respectively) related in the formulation ofSWR and SWV. In the calculation of SWR, only normal load andsliding distance were considered. However, in the determinationof SWV, the effect of many other conditions, such as friction,lubrication, contact condition, sliding velocity etc. on the wearrate, have been included through the dissipated energy [37]. Forexample, the SWR of P-3 was lower than that of the neat HDPE,but the SWV of P-3 was higher than that of the neat HDPE due tothe effect of the COF of P-3 (Fig. 2). The lower COF, the smaller

friction force, and thus the lower dissipated energy. A lower dissi-pated energy would result in a higher SWV as Eq. (3) indicates.Therefore, compared to the traditional SWR, SWV may haveincorporated more information of the contact materials and condi-tions through the dissipated energy, and thus it could be more reli-able in evaluating the wear resistance of plastic materials.

3.3 Indentation Test. To determine the surface deformationof the nanocomposites and study the correlation between the wearresistance and surface deformation, indentation test was per-formed. Indention test was introduced to determine the relation-ship between the wear resistance of polymer and indentiondeformation of polymer material as it has been validated onUHMWPE and its composite [54]. Budinski [55] performed simi-lar tests on a wide range of plastics by indenting the test specimenwith a 6 mm-diameter spherical ball to a fixed maximum depth of1.25 mm linearly instead of to a fixed maximum load consideredin Ref. [54]. The load-deflection curve was integrated to predictthe ability of a material to deform plastically. It was believed thatthe contact mechanics resulting in wear was load-controlled ratherthan deformation depth-controlled, and generally the comparativewear performance under the same environmental and load condi-tions was of interest [54]. Therefore, a load controlled methodwas adopted in the present work. During the indentation, the nor-mal load was linearly increased to 3 N, which was also the normalload used in the wear testing, and then slowly decreased down tozero. The same stainless steel balls used in the dry sliding testwere used as the indenter balls for the indentation tests.

A typical load-deflection curve of the material (T1-05) is plot-ted in Fig. 7. It includes the loading curve of AB and the

Fig. 6 SEM micrographs of the worn surfaces of (a) T1-05 and(b) P-3

Fig. 7 Load-deflection curve of the nanocomposite T1-05

Fig. 8 Comparison of the plastic deformation energy of theseven materials

Journal of Nanotechnology in Engineering and Medicine NOVEMBER 2012, Vol. 3 / 041003-5

Downloaded From: http://nanoengineeringmedical.asmedigitalcollection.asme.org/ on 08/16/2013 Terms of Use: http://asme.org/terms

unloading curve of BC. The area under the curve AB was the totaldeformation energy, while the area under the curve BC was therecovered energy. The area enclosed between AB and BC,expressed as the difference between the areas under AB and BC

and calculated via a numerical integration method, thus repre-sented the unrecovered plastic deformation energy. It was noticedthat due to the viscoelastic effect of the polymeric material, therecovering process might be slower than the unloading time; thusthe plastic deformation energy might be overestimated [54]. Theplastic deformation energy for each of the seven materials wascalculated and summarized in Fig. 8. For nanocomposites rein-forced with each type of CNFs (P, T1, and T2), the plastic defor-mation energy of the one with 3 wt. % CNFs was always lowerthan that with 0.5 wt. % CNFs. It was believed that the tougherthe material, the larger the indentation deformed depth, and thusthe higher the plastic deformation energy [54]. Even reinforcedwith 0.5 wt. % of pristine CNFs, the HDPE nancompositesshowed a higher plastic deformation energy or more toughnessthan the neat HDPE. Enhancements in the ductility of UHMWPE/MWCNT and HDPE/CNF nanocomposites have been reported ina number of studies [56–58]. The large ductility increase in PEwas most likely attributed to the secondary crystal formation dueto the presence of nanofillers to enhance the chain mobility [56].The addition of thick-silane treated CNFs at each concentrationlevel, 0.5 wt. % or 3 wt. %, led to larger energy absorption duringplastic deformation and thus toughened the HDPE further, com-pared with pristine CNFs and thin-silane treated CNFs at the sameconcentration level. It is speculated in this study that such tough-ening ability of thick-silane treated CNFs resulted from the 3D

Fig. 9 Relationship between the wear rate (y) and plastic defor-mation energy (x) from the indentation testing; a and b are con-stants where a 5 1.966 310�7 and b 5 1.5127 310�7

Fig. 10 Optical microscopy of the worn surfaces and wear debris of three materials: neatHDPE ((a) and (b)), T1-05 ((c) and (d )), and P-3 ((e) and (f ))

041003-6 / Vol. 3, NOVEMBER 2012 Transactions of the ASME

Downloaded From: http://nanoengineeringmedical.asmedigitalcollection.asme.org/ on 08/16/2013 Terms of Use: http://asme.org/terms

network of the cross-linked silane coating which might haveworked as a buffer spring to absorb more deformation energy[34]. The correlations of the seven materials’ wear rates (bothSWRs and SWVs) and their corresponding plastic deformationenergy are plotted in Fig. 9. Power functions were found to fitthese correlations well. The wear rates of the samples seemed tobe inversely proportional to their plastic deformation energy.Budinski [55] studied the abrasion resistance of a wide variety ofplastics (21 materials) including polystyrene, epoxy, HDPE, andpolytetrafluoroethylene etc., through a three-body abrasion test,and concluded that the plastic that deformed easily was less likelyto form material removal by scratching or fracture. Therefore, thereason for T1-05 to have the highest wear resistance, or the lowestwear rate, might be that it was relatively easier to reach plastic de-formation during sliding contact with the steel ball since it had thehighest plastic deformation energy (Fig. 8).

3.4 Wear Mechanisms. Figure 10 shows optical micro-graphs of the wear tracks ((a), (c), (e)) and wear debris ((b), (d),and (f)) from the three samples (neat, T1-05, and P-3) in whichthe T1-05 and P-3 had the lowest and the highest SWV, respec-tively. The wear track of the neat HDPE exhibited areas of deeppits in Fig. 10(a). It is speculated that these pits began with smallcracks transverse to the sliding direction due to repeated stressingand unstressing of the shear force, and eventually the subsurfaceover the microcrack was exfoliated out. This indicated a largercomponent of fatigue wear. The wear track of T1-05 possessedthe smoothest surface and, compared with the deep pits of the neatHDPE, showed smaller and shallower defects transverse to thesliding direction, as shown in Fig. 10(c). This suggests that themajor wear mechanism in T1-05 may have been fatigue wear,same as the neat HDPE, but to a lesser degree. It was believedthat CNFs could resist the nucleation of cracks by acting as stress-transfer “bridges” and even if the cracks were nucleated, CNFscould block the crack from propagating further [59]. Therefore,these smaller and shallower defects of T1-05 indicated the largecontribution of silanized CNFs to the nucleation and propagationof microcrack in the fatigue wear. However, with much moreCNFs, P-3 exhibited worse wear with wider and deeper scratchinggrooves on the wear track in Fig. 10(e). Such scratching groovesare typical of the abrasive wear mechanism. Although the wornsurfaces of all specimens had more or less abrasive wear patterns,the scratching grooves, P-3 obviously exhibited the most severeabrasive wear, making abrasive wear its primary wear mode. Thepotential reason for the severe abrasive wear of P-3 may be itsinferior capability to resist plastic deformation as discussed inSec. 3.3 since, as shown in Fig. 8, P-3 had the lowest plastic defor-mation energy.

The wear debris of neat HDPE and T1-05 showed comparableflake shapes in Figs. 10(b) and 10(d). Obviously, T1-05 produced

smaller debris compared with the neat HDPE. This phenomenonwas consistent with the smaller and shallower defects on the wornsurface in Fig. 10(c). However, the debris from P-3 was observedto be very large in Fig. 10(f) and can be described as thin films,possibly due to the severe abrasive wear during which the exfoli-ated debris from each scratching groove aggregated or linked to-gether. The wear modes of the neat HDPE, T1-05, and P-3 arefurther illustrated in Fig. 11. In Fig. 11(a), the microcracks formedin the neat HDPE sample under cyclic strain were followed by therelatively large wear debris particles exfoliated out, leaving deeppits on the wear track. However, due to the resistance of thick-silane CNFs to the nucleation and extension of cracks, the secondwear mode demonstrated in Fig. 11(b) occurring to the T1-05nanocomposite had shallower pits and smaller wear debris par-ticles. Due to the lowest ductility of P-3, the severe abrasive wearshown in Fig. 11(c) took place with the formation of large thinfilm-shaped wear debris.

4 Conclusions

In this study, the effect of concentration and surface modifica-tion of CNFs on the tribological properties of the HDPE/CNFsnanocomposites under dry sliding condition was investigatedusing a pin-on-disc wear simulator. The addition of CNFs at0.5 wt. % loading level not only decreased the COF but alsoimproved the wear resistance of the nanocomposites compared tothe neat HDPE. Higher loading level of CNFs (3 wt. %) resultedin higher wear rates possibly due to nonuniform dispersion andsmall aggregates of CNFs. The wear resistance of the nanocom-posties with thick-silane treated CNFs were proven to be betterthan those with pristine and thin-silane treated CNFs due toimproved interfacial bonding between the CNFs and the HDPEmatrix. The wear rates (both SWR and SWV) of thick-silanetreated CNF/HDPE at 0.5 wt. % concentration were lower thanthose of the neat HDPE by about 68%. It was found that the wearrates of the samples were inversely proportional to their plasticdeformation energy obtained from the indention tests. The moreplastic deformation energy absorbed, the more easily the materialwould deform plastically when in sliding contact with the hard as-perity of the steel ball, and thus the less material removal wouldbe produced during the wear process. In addition, it was revealedthat P-3 went through severe abrasive wear, while the major wearmechanism of the neat HDPE and T1-05 seemed to be fatiguewear. In summary, this work shows that thick-silane treated CNFsat 0.5 wt. % loading level have the potential to serve as promisingreinforcements for HDPE for improved wear resistance in tribol-goical applications.

Acknowledgment

This material is based upon work supported by the NationalScience Foundation under Grant Nos. 0900181 and 0856510.

References[1] Zoo, Y., An, J., Lim, D., and Lim, D., 2004, “Effect of Carbon Nanotube Addi-

tion on Tribological Behavior of UHMWPE,” Tribol. Lett., 16, pp. 305–309.[2] Suh, N. P., Mosleh, M., and Arinez, J., 1998, “Tribology of Polyethylene

Homocomposites,” Wear, 214, pp. 231-236.[3] Anderson, J. C., 1982, “High Density and Ultra-High Molecular Weight Polye-

thenes: Their Wear Properties and Bearing Applications,” Tribol. Int., 15, pp.43–47.

[4] Sahebian, S., Zebarjad, S. M., Sajjadi, S. A., Sherafat, Z., and Lazzeri, A.,2007, “Effect of Both Uncoated and Coated Calcium Carbonate on FractureToughness of HDPE/CaCO3 Nanocomposites,” J. Appl. Polym. Sci., 104, pp.3688–3694.

[5] Guermazi, N., Elleuch, K., Ayedi, H. F., Fridrici, V., and Kapsa, P., 2009,“Tribological Behaviour of Pipe Coating in Dry Sliding Contact With Steel,”Mater. Des., 30, pp. 3094–3104.

[6] Mourad A.-H. I., Fouad, H., and Elleithy, R., 2009, “Impact of Some Environ-mental Conditions on the Tensile, Creep-Recovery, Relaxation, Melting andCrystallinity Behaviour of UHMWPE-GUR 410-Medical Grade,” Mater. Des.,30, pp. 4112–4119.

Fig. 11 Illustrations of the wear mechanisms of (a) neat HDPE,(b) T1-05, and (c) P-3

Journal of Nanotechnology in Engineering and Medicine NOVEMBER 2012, Vol. 3 / 041003-7

Downloaded From: http://nanoengineeringmedical.asmedigitalcollection.asme.org/ on 08/16/2013 Terms of Use: http://asme.org/terms

[7] Charnley, J., 1960, “Anchorage of the Femoral Head Prosthesis to the Shaft ofthe Femur,” J. Bone Joint Surg. Br., 42, pp. 28–30.

[8] Unsworth, A., 1995, “Recent Developments in the Tribology of Artificial Join-ts,” Tribol. Int., 28, pp. 485–495.

[9] Fouad, H., and Elleithy, R., 2011, “High Density Polyethylene/Graphite Nano-Composites for Total Hip Joint Replacements: Processing and In Vitro Charac-terization,” J. Mech. Behav. Biomed. Mater., 4, pp. 1376–1383.

[10] Chowdhury, S., Mishra, A., Pradhan, B., and Saha, D., 2004, “Wear Character-istic and Biocompatibility of Some Polymer Composite Acetabular Cups,”Wear, 256, pp. 1026–1036.

[11] Willert, H., and Buchorn, G., 1991, Ultra-High Molecular Weight Polyethyleneas Biomaterial in Orthopaedic Surgery, P. Eyerer, ed., Hogrefe and HuberPublishers, Toronto, Canada.

[12] Barbour, P., Stone, M., and Fisher, J., 1999, “A Study of the Wear Resistanceof Three Types of Clinically Applied UHMWPE for Total Replacement HipProstheses,” Biomaterials, 20, pp. 2101–2106.

[13] Kim, S., 2008, “Changes in Surgical Loads and Economic Burden of Hip andKnee Replacements in the US: 1997–2004,” Arthritis Rheum., 59, pp. 481–488.

[14] Westby, M. D., and Backman, C. L., 2010, “Patient and Health ProfessionalViews on Rehabilitation Practices and Outcomes Following Total Hip andKnee Arthroplasty for Osteoarthritis: A Focus Group Study,” Health Serv. Res.,10, p. 119.

[15] Ingham, E., and Fisher, J., 2000, “Biological Reactions to Wear Debris in TotalJoint Replacement,” Proc. Inst. Mech. Eng., 214, pp. 21–36.

[16] Sochart, D., 1999, “Relationship of Acetabular Wear to Osteolysis and Loosen-ing in Total Hip Arthroplasty,” Clin. Orthop. Relat. Res., 363, pp. 135–150.

[17] Vermes, C., Roebuck, K., Chandrasekaran, R., Dobai, J., Jacobs, J., and Glant,T., 2000, “Particulate Wear Debris Activates Protein Tyrosine Kinases and Nu-clear Factor Kappa BETA, Which Down-Regulates Type I Collagen Synthesisin Human Osteoblasts,” J. Bone Miner. Res., 15, pp. 1756–1765.

[18] Charlesby, A., 1952, “Cross-Linking of Polythene by Pile Radiation,” Proc. R.Soc. London, Ser. A, 215, pp. 187–214.

[19] Deboer, J., and Pennings, A., 1982, “Crosslinking of Ultra-High MolecularWeight Polyethylene in the Melt by Means of 2,5-dimethyl-2,5-bis(tert-butyl-dioxy)-3-hexyne: 2. Crystallization Behaviour and Mechanical Properties,”Polymer, 23, pp. 1944–1952.

[20] Atkinson, J., and Cicek, R., 1984, “Silane Crosslinked Polyethylene for Pros-thetic Applications. II. Creep and Wear Behavior and a Preliminary MoldingTest,” Biomaterials, 5, pp. 326–335.

[21] Xiong, D., 2005, “Friction and Wear Properties of UHMWPE CompositesReinforced With Carbon Fiber,” Mater. Lett., 59, pp. 175–179.

[22] Cao, S., Liu, H., Ge, S., and Wu, G., 2011, “Mechanical and TribologicalBehaviors of UHMWPE Composites Filled With Basalt Fibers,” J. Reinf. Plast.Compos., 30, pp. 347–355.

[23] Tong, J., Ma, Y., and Jiang, M., 2003, “Effects of the Wollastonite Fiber Modi-fication on the Sliding Wear Behavior of the UHMWPE Composites,” Wear,255, pp. 734–741.

[24] Johnson, B., Santare, M., Novotny, J., and Advani, S., 2009, “Wear Behavior ofCarbon Nanotube/High Density Polyethylene Composites,” Mech. Mater.,41(10), pp. 1108–1115.

[25] Galetz, M., Blass, T., Ruckdaschel, H., Sandler, J., Altstadt, V., and Glatzel, U.,2007, “Carbon Nanofibre-Reinforced Ultrahigh Molecular Weight Polyethylenefor Tribological Applications,” J. Appl. Polym. Sci., 104(6), pp. 4173–4181.

[26] Wright, T., Astion, D., Bansal, M., Rimnac, C., Green, T., Insall, J., and Robin-son, R., 1988, “Failure of Carbon Fiber-Reinforced Polyethylene Total Knee-Replacement Components. A Report of Two Cases,” J. Bone Joint Surg. Am.,70, pp. 926–932.

[27] Sui, G., Zhong, W., Ren, X., Wang, X., and Yang, X., 2009, “Structure, Me-chanical Properties and Friction Behavior of UHMWPE/HDPE/Carbon Nano-fibers,” Mater. Chem. Phys., 115, pp. 404–412.

[28] Tan, E., and Lim, C., 2006, “Mechanical Characterization of Nanofibers—AReview,” Compos. Sci. Technol., 66, pp. 1102–1111.

[29] Elias, K., Price, R., and Webster, T., 2002, “Enhanced Functions of Osteoblastson Nanometer Diameter Carbon Fibers,” Biomaterials, 23, pp. 3279–3287.

[30] Price, R., Waid, M., Haberstroh, K., and Webster, T., 2003, “Selective BoneCell Adhesion on Formulations Containing Carbon Nanofibers,” Biomaterials,24, pp. 1877–1887.

[31] Webster, T., Waid, M., McKenzie, J., Price, R., and Ejiofor, J., 2004, “Nano-Biotechnology: Carbon Nanofibres as Improved Neural and OrthopaedicImplants,” Nanotechnology, 15, pp. 48–54.

[32] Smart, S., Cassady, A., Lu, G., and Martin, D., 2006, “The Biocompatibility ofCarbon Nanotubes,” Carbon, 44, pp. 1034–1047.

[33] McKenzie, J., Waid, M., Shi, R., and Webster, T., 2004, “Decreased Functionsof Astrocytes on Carbon Nanofiber Materials,” Biomaterials, 25, pp.1309–1317.

[34] Wood, W., Kumar, S., and Zhong, W. H., 2010, “Synthesis of Organosilane-Modified Carbon Nanofibers and Influence of Silane Coating Thickness on thePerformance of Polyethylene Nanocomposites,” Macromol. Mater. Eng., 295,pp. 1125–1135.

[35] Liu, T., Wood, W., and Zhong, W. H., 2011, “Sensitivity of Dielectric Proper-ties to Wear Process on Carbon Nanofiber/High-Density PolyethyleneComposites,” Nanoscale Res. Lett., 6, p. 7.

[36] Colaco, R., Gispert, M., Serro, A., and Saramago, B., 2007, “An Energy-BasedModel for the Wear of UHMWPE,” Tribol. Lett., 26, pp. 119–124.

[37] Jahangiria, M., Hashempourb, M., Razavizadehb, H., and Rezaieb, H. R., 2012,“Application and Conceptual Explanation of an Energy-Based Approach for theModeling and Prediction of Sliding Wear,” Wear, 274-275, pp. 168–174.

[38] Huq, M., and Celis, J., 2002, “Expressing Wear Rate in Sliding Contacts Basedon Dissipated Energy,” Wear, 252, pp. 375–383.

[39] Fouvry, S., Liskiewicz, T., Kapsa, P., Hannel, S., and Sauger, E., 2003, “AnEnergy Description of Wear Mechanisms and Its Applications to OscillatingSliding Contacts,” Wear, 255, pp. 287–298.

[40] Liskiewicz, T., and Fouvry, S., 2005, “Development of a Friction EnergyCapacity Approach to Predict the Surface Coating Endurance Under ComplexOscillating Sliding Conditions,” Tribology Int., 38, pp. 69–79.

[41] Ramalho, A., and Miranda, J., 2006, “The Relationship Between Wear and Dis-sipated Energy in Sliding Systems,” Wear, 260, pp. 361–367.

[42] Dowson, D., and Harding, R., 1982, “The Wear Characteristics of UltrahighMolecular-Weight Polyethylene Against a High-Density Alumina CeramicUnder Wet (Distilled Wear) and Dry Conditions,” Wear, 75, pp. 313–331.

[43] Allen, C., Bloyce, A., and Bell, T., 1996, “Sliding Wear Behaviour of IonImplanted Ultra High Molecular Weight Polyethylene Against a Surface Modi-fied Titanium Alloy Ti-6Al-4V,” Tribology Int., 29, pp. 527–534.

[44] Ganesh, B. K. C., Ramaniah, N., and Chandrasekhar Rao, P. V., 2012, “Effectof Heat Treatment on Dry Sliding Wear of Titanium-Aluminum-Vanadium (Ti-6Al-4V) Implant Alloy,” J. Mech. Eng. Res., 4, pp. 67–74.

[45] Lavrakas, V., 1957, “Textbook Errors: Guest Column. XII: The LubricatingProperties of Graphite,” J. Chem. Educ., 34, p. 240.

[46] Ho, S., Carpick, R., Boland, T., and Laberge, M., 2002, “Nanotribology ofCoCr-UHMWPE TJR Prosthesis Using Atomic Force Microscopy,” Wear, 253,pp. 1145–1155.

[47] Jahangiri, M., Hashempour, M., Razavizadeh, H., and Rezaie, H. R., 2012,“A New Method to Investigate the Sliding Wear Behaviour of Materials Based onEnergy Dissipation: W–25 wt.%Cu Composite,” Wear, 274–275, pp. 175–182.

[48] Huq, M., and Celis, J., 1997, “Reproducibility of Friction and Wear Results inBall-on-Disc Unidirectional Sliding Tests of TiN-Alumina Pairings,” Wear,212, pp. 151–159.

[49] Mohrbacher, H., Celis, J., and Roos, J., 1995, “Laboratory Testing of Displace-ment and Load-Induced Fretting,” Tribology Int., 28, pp. 269–278.

[50] Bhushan, B., 2001, Modern Tribology Handbook, CRC Press LLC, Boca Raton,FL.

[51] Wood, W., Maguire, R., and Zhong, W. H., 2011, “Improved Wear and Me-chanical Properties of UHMWPE-Carbon Nanofiber Composites Through anOptimized Paraffin-Assisted Melt-Mixing Process,” Composites Part B, 42, pp.584–591.

[52] Sahoo, N., Rana, S., Cho, J., Li, L., and Chan, S., 2010, “Polymer Nanocompo-sites Based on Functionalized Carbon Nanotubes,” Prog. Polym. Sci., 35, pp.837–867.

[53] von Recum, A. F., ed., 1986, Handbook of Biomaterials Evaluation, Macmillan,Publishing Co., New York.

[54] Cenna, A., Allen, S., Page, N., and Dastoor, P., 2003, “Modelling the Three-Body Abrasive Wear of UHMWPE Particle Reinforced Composites,” Wear,254, pp. 581–588.

[55] Budinski, K., 1997, “Resistance to Particle Abrasion of Selected Plastics,”Wear, 203, pp. 302–309.

[56] Ruan, S., Gao, P., Yang, X., and Yu, T., 2003, “Toughening High PerformanceUltrahigh Molecular Weight Polyethylene Using Multiwalled Carbon Nano-tubes,” Polymer, 44, pp. 5643–5654.

[57] Lozano, K., Yang, S., and Jones, R., 2004, “Nanofiber Toughened PolyethyleneComposites,” Carbon, 42, pp. 2329–2331.

[58] Ruan, S., Gao, P., and Yu, T., 2006, “Ultra-Strong Gel-Spun UHMWPE Fibers Re-inforced Using Multiwalled Carbon Nanotubes,” Polymer, 47, pp. 1604–1611.

[59] Chen, Y., and Qiao, P., 2011, “Crack Growth Resistance of Hybrid Fiber-Reinforced Cement Matrix Composites,” J. Aerosp. Eng., 24, pp. 154–161.

041003-8 / Vol. 3, NOVEMBER 2012 Transactions of the ASME

Downloaded From: http://nanoengineeringmedical.asmedigitalcollection.asme.org/ on 08/16/2013 Terms of Use: http://asme.org/terms