cu

ng

f Ma

1003

ty, S

Applied Surface Science 256 (2010) 39413945

Wear resistance

lasm

HM

nd w

MW

yge

tch

reat

Contents lists available at ScienceDirect

Applied Surfa

journa l homepage: www.e ls1. Introduction

The use of articial implants for replacement of the hip, knee,and other articular joints has become very common in light ofimproving life quality and increasing life expectancies [1]. Theultra-high molecular weight polyethylene (UHMWPE) has thecharacteristics of notably high abrasion resistance, high impactstrength, low friction, excellent toughness, ease of fabrication,biocompatibility and bio-stability [2], therefore, UHMWPE hasbeen used as a bearing material in hip and knee total jointreplacement. However, this soft material has a high materials lossin relative motions between the friction partners. The biologicalreaction to these debris particles is called osteolysis, resulting in agradual loss of the bone tissue surrounding the stem of the implantand thus the loosening of the prosthesis [3,4]. Plasma treatment issuggested here as an efcient and environmentally friendlyprocess which has become a frequently used method to modifypolymer surfaces without changing the bulk composition.

Argon (Ar) plasma has been often used for crosslinking ofmolecules on a polymer surface [5,6]. The structural changes ofpolymer surface caused by plasma ions could induce improve-ments in hardness, wettability, surface chemical activity, cross-linking and other properties [3]. In our current investigation,

microwave electron cyclotron resonance (MWECR) plasma sourcehas been used to modify the surface of UHMWPE samples.Compared with radio frequency glow discharge (RFGD) plasmafrequently used in polymer surface modication, the MWECRplasma has signicant advantages, such as high ion density, lowsubstrate temperature, and independent control of ion energyduring the treatment. These ensure it a better surface modicationof the polymer compared with the conventional plasma surfacemodication technique [7]. In this paper, the effect of argon plasmatreatment on the surface properties of UHMWPE is investigated.

2. Experiment details

2.1. Materials

Themedical grade UHMWPE in this studywasmade by GermanPloy Hi Solidur Deutschland GmbH MediTECH Division Company,the detail properties are shown in Table 1. The UHMWPE was cutinto 12 mm diameter discs with 3 mm thickness and the discsamples were ground and polished to reduce the surfaceroughness.

2.2. Experimental procedure

Microwave electron cyclotron resonance (MWECR) plasmamodication technique was used to modify the polymer surfaces.The schematic diagram of the MWECR source is shown in Fig. 1,which has been reported by Xie et al. [8]. The microwave power

* Corresponding author. Tel.: +86 28 87601149; fax: +86 28 87601149.

** Corresponding author. Tel.: +86 512 67784226; fax: +86 512 68659962.

E-mail addresses: yxleng@263.net (Y.X. Leng), jyct@163.com (Y. Jin).

0169-4332/$ see front matter 2010 Elsevier B.V. All rights reserved.Surface modication of ultra-high moleargon plasma

Hengjun Liu a, Yanan Pei a, Dong Xie a,b, Xingrui DeaKey Laboratory for Advanced Technologies of Materials, Ministry of Education, School ob School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 6c Interventional Therapy Department, The Second Afliated Hospital, Soochow Universi

A R T I C L E I N F O

Article history:

Received 21 November 2009

Received in revised form 15 January 2010

Accepted 15 January 2010

Available online 25 January 2010

Keywords:

UHMWPE

Argon plasma

Surface modication

A B S T R A C T

In this work, argon (Ar) p

been used to modify the U

wettability, anti-scratch a

comparing with native UH

should come from the ox

improvement of anti-scra

UHMWPE by Ar plasma tdoi:10.1016/j.apsusc.2010.01.054lar weight polyethylene (UHMWPE) by

a, Y.X. Leng a,*, Yong Jin c,**, Nan Huang a

terials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China

1,China

uzhou 215004, China

a generated by microwave electron cyclotron resonance (MWECR) has

WPE in order to increase the wear resistance. The results showed that the

ear resistance of UHMWPE treated by the Ar plasma had been improved,

PE. The FTIR and XPS spectra indicated the improvement of wettability

n based functional groups generated on the surface of UHMWPE. The

and wear resistance may come from the enhancement of crosslinking of

ment.

2010 Elsevier B.V. All rights reserved.

ce Science

evier .com/ locate /apsusc

TayHighlight

TayHighlight

TayHighlight

TayHighlight

TayHighlight

TayHighlight

TayHighlight

TayHighlight

TayHighlight

Table 1Physical properties of UHMWPE.

Molecular

weight

Density

(g cm3)Elastic

modulus (MPa)

Water

absorption (%)

9.2106 0.9270.944 700

Fig. 4. SEM images of scratch deformation for UHMWPE: (a) untreated (b) Argonplasma treated. (Normal load varied from 0.1 to 10 mN and the scratching direction

is from right to left.).

H. Liu et al. / Applied Surface Science 256 (2010) 39413945 3943after argon plasma treatment as indicated in FTIR. The resultsindicate the hydrophilic transformation of the treated polymersurfaces.

The UHMWPE samples were immersed in the inert argonplasma and etched by the plasma, then the free radicals wereproduced at or near the UHMWPE surface which can interact toform the cross-links and unsaturated groups with the chainscission. When inert gas plasma treated polymer samples wereexposed to the atmosphere after the treatment, the activatedsurfaces readily adsorbed the moisture and the radicals reactedwith the oxygen in the air, which was indicated by the broad bandat 3300 and 15001747 cm1. The band around 3300 cm1 wasdue to the OH groups and the broadening could be due to itsintermolecular or intramolecular reactions [9]. The band around15001747 cm1 may present the oxygen based groups, whichcontain C55O stretch absorption [10] in the range of 17501600 cm1 and COO asymmetrical stretching [10] in the spectralrange of 16501560 cm1.

3.2. Nano-scratching analysis

The scratch tests were conducted using a dynamic loading from0.1 to 10 mNat a constant scratch velocity of 0.3 mm/s for a scratchdistance of 300 mm. Fig. 3 shows the scratch penetration depth of

Fig. 3. Scratch penetration depth as a function of progressively applied normal load.untreated and treated UHMWPE. It can be observed that there is adifference in the penetration depths for the two samples as theload increases. The penetration depth of argon-treatedUHMWPE isshallower than that of untreated UHMWPE under the same load.FTIR results have shown that the crosslinking of UHMWPE chainsincreases after plasma treatment. The increased crosslinking mayenhance the anti-scratch capacity and surface hardness ofUHMWPE. The process of crosslinking is discussed in the following.The active species in the plasma, i.e., electron and ions, bombardthe surface macromolecular chains of UHMWPE samples, whichlead to the breakage of CH and CC bondings and more freeradicals are produced on the surface [11]. In consequence,additional crosslinking among radicals occurs between themolecular chains which enhances the action force of themolecularchains. Then the surface hardness of UHMWPE increases whichimproves the anti-scratch capacity of UHMWPE.

The different SEM images of scratch deformation for theuntreated and plasma treated samples are shown in Fig. 4. InFig. 4(a), it can be seen a clear damage of polymer along both sidesof the scratch for the untreated sample. The plastic deformation ofuntreated UHMWPE is severe and a pile of polymer at the end ofthe scratch track by a process of plastic deformation and partialdelamination [12] is shown in Fig. 4(a). That is because the nativeUHMWPE is soft, therefore, it could easily be damaged by loadingforce. In contrast, for Ar plasma treated sample there are crackpropagation and brittle spalling at the beginning of the scratchtrack and yield deformation at the end of the scratch track, whichare shown in Fig. 4(b). These scratch deformation differences canbe attributed to the increased surface hardness of UHMWPE afterargon plasma treatment. The anti-scratch capacity of UHMWPEhas improved by plasma treatment.

3.3. Wear resistance of UHMWPE

The changes in surface chemical structures and the improve-ment in anti-scratch of UHMWPE after plasma treatment will havesome effects on its tribological properties, such as wear mecha-nism and wear resistance. The pin-on-disk test was used toevaluate these properties. Fig. 5 shows the friction coefcientcurves of UHMWPE samples. The friction process is divided intothree phases, i.e., the initial phase, running-in phase and stablephase. The reason for the rapidly decreasing of friction coefcient

TayHighlight

TayHighlight

TayHighlight

TayHighlight

TayHighlight

TayHighlight

TayHighlight

in the initial stage is the obvious water absorption of UHMWPEwhen immersed into calf serum. The shear strength of UHMWPEdecreases by the swelling behavior of molecular chains caused bywater absorption, and the absorbed water and proteins have thefunction of a lubricant. As shown in Fig. 5, the untreated sampleshows a low friction coefcient (the mean is 0.085). After argon

plasma treatment, the mean friction coefcient of treated sampleincreases to 0.201. This was the result of the increased crosslinkingof the molecular chains, as discussed both in FTIR and nano-scratch. The increased crosslinking restricted the sliding ofmolecular chains and caused an increased friction coefcient ofthe argon plasma treated UHMWPE.

Fig. 6 shows the SEM images of wear tracks of both untreatedand plasma treated samples. The different wear morphology ofwear tracks suggests a change in wear mechanism before and afterplasma treatment. The wear tracks of untreated UHMWPE(Fig. 6(a1)) show severe furrows with eventual separation ofparticles from the bulk which is coursed by the hard convex partsof CoCrMo pin. Fig. 6(a2) shows ridges of raised and frayedpolymer that are highly directional. The plastic deformation inFig. 6(a2) is called crazing damage in polymers [13]. A mass ofplastic deformation may lead to microcracks in subsurface ofUHMWPE. Followed with microcrack growth, eventual fatiguespalling from the bulk occurs on the surfacewhich can be observedin Fig. 6(a2). The wear morphology of Ar plasma treated sample inFig. 6(b1 and b2) shows a different wear mechanism and the wearis relatively less than that of untreated sample. Plastic deformationand fatigue spalling caused by cyclic shear loading are greatlyreduced after plasma treatment. It is reasonable that two keyfactors are responsible for this wear behavior: more crosslinkingamong molecular chains and the increased surface hardness afterplasma treatment. The enhancement of crosslinking increasesshear strength of UHMWPEwhich restricts the removal of materialfrom bulk [14], while the increased surface hardness improves itsresistance to plastic deformation. So it is noted that the wearresistance of UHMWPE improves after argon plasma treatment.

Fig. 5. The friction coefcient of UHMWPE samples (a) untreated (b) Argon plasmatreated.

H. Liu et al. / Applied Surface Science 256 (2010) 394139453944Fig. 6. SEM images of wear tracks on the samples (a1)(a2) untreated (b1)(b2) Argon plasma treated.

TayHighlight

TayHighlight

TayHighlight

TayHighlight

TayHighlight

TayHighlight

TayHighlight

TayHighlight

4. Conclusions

In this paper, the feasibility and validity of using MWECR Arplasma treatment tomodify the surface of UHMWPE are discussed.Ar plasma treatment greatly improves surface wettability due tothe formation of polar groups (mainly carbonyl, carboxyl andhydroxyl groups) as demonstrated by FTIR spectroscopy. Theactive electrons and ions in the plasma bombard the surface, breakCH and CC bondings and produce free radicals. Additionalcrosslinking among radicals occurs between the molecular chainswhich enhances the action force of molecular chains and increasethe surface hardness. Anti-scratch and wear resistance of Arplasma treatment samples are improved compared to the initialUHMWPE. Thewear test results show a high friction coefcient butlow wear rate after Ar plasma modication. It is concluded fromwear morphologies that the resistance to plastic deformation andfatigue spalling of UHMWPE has enhanced after Ar plasmatreatment.

Acknowledgements

The authors would like to thank Prof. Linmao Qian, Prof. JinWang and Prof. Yong Wang for technical assistance and their help

in discussing this paper. This work is supported by the NationalHigh Technology Research and Development Program of China(863 Program #2006AA02A135), the Program for New CenturyExcellent Talents in University (NCET-06-0800) and NationalNatural Science Foundation of China (#50802077).

References

[1] C.J. DeFrances, M.J. Hall, 2002 National hospital discharge survey, Adv. Data 342(2004) 129.

[2] G. Lewis, J. Biomater. Res. Appl. Biomater. 38 (1997) 5574.[3] H.C. Amstutz, P. Campbell, N. Kossovsky, I.C. Clarke, Clin. Orthop. Res. 276 (1992)

718.[4] W.J. Maloney, R.L. Smith, J. Bone Joint Surg. 77A (1995) 14481461.[5] Y.G. Yao, X.S. Liu, Y.F. Zhu, J. Appl. Polym. Sci. 48 (1993) 5765.[6] Y.G. Yao, X.S. Liu, Y.F. Zhu, J. Adhesion Sci. Technol. 7 (1993) 6375.[7] N. Wang, Master degree thesis of Nanjing University of Science and Technology,

2005, 3336.[8] D. Xie, H.J. Liu, X.R. Deng, Y.X. Leng, N. Huang, Appl. Surf. Sci. 256 (2009) 284288.[9] S. Guruvenket, G.M. Rao,M. Komath, A.M. Raichur, Appl. Surf. Sci. 236 (2004) 278

284.[10] M.R. Sanchis, V. Blanes, M. Blanes, D. Garcia, R. Balart, Eur. Polym. J. 42 (2006)

15581568.[11] L.S. Shi, L.Y. Wang, Y.N. Wang, Eur. Polym. J. 42 (2006) 16251633.[12] B.J. Briscoe, E. Pelillo, S.K. Sinha, Polym. Eng. Sci. 36 (24) (1996) 2996.[13] L. Wenbo, L. Wenxian, Y. Tingqing, W. Xiayu, Acta Mechanica Solida Sinica 2

(2004) 171174.[14] Ch.J. Schwartz, S. Bahadur, S.K. Mallapragada, Wear 263 (2007) 10721080.

H. Liu et al. / Applied Surface Science 256 (2010) 39413945 3945

TayHighlight

Surface modification of ultra-high molecular weight polyethylene (UHMWPE) by argon plasmaIntroductionExperiment detailsMaterialsExperimental procedureCharacterisation methods

Results and discussionChanges in chemical structureNano-scratching analysisWear resistance of UHMWPE

ConclusionsAcknowledgementsReferences