Surface modification of ion implanted ultra high molecular weight polyethylene

Nuclear Instruments and Methods in Physics Research B 169 (2000) 26±30

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Surface modi®cation of ion implanted ultra high molecular weight polyethylene Jingsheng Chen a, Fuying Zhu a, Haochang Pan a, Jianqing Cao a, Dezhang Zhu Hongjie Xu a, Qing Cai b, Jingen Shen b, Lihua Chen b, Zhengrui He b a

a,*

,

Laboratory of Nuclear Analysis Techniques, Shanghai Institute of Nuclear Research, Academia Sinica, P.O. Box 800-204, Shanghai 201800, People's Republic of China b Shanghai Secondary Medical University, Shanghai 200025, People's Republic of China

Abstract The surface modi®cation has been studied for the ultra high molecular weight polyethylene (UHMWPE) implanted ‡ ‡ ‡ 14 by 80 keV N‡ to 5 ´ 1015 ions/cm2 . Elastic recoil 2 , C3 H8 (40 keV N , 22 keV C ) with ¯uences ranging from 1 ´ 10 detection (ERD) and X-ray photoelectron spectroscopy (XPS) have been employed to characterize the modi®ed surface of the samples. ERD results show that the high energy edge of ERD spectra shifts in the lower energy direction with the increase of implantation ¯uency, indicating that a hydrogen de®cient surface layer is formed after implantation. XPS result shows that injected nitrogen atoms assist in crosslinking by forming chemical bonds with the polymer chains. KyowA's DF-PM reciprocating tester has been used to measure the wear property before and after implantation. The ‡ results show that the wear-resistance of samples after N‡ 2 , C3 H8 implantation has been improved by 68 and 47.5 times, respectively. Some interpretations are given to explain the observed phenomena. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Implantation; Wear resistance; Ultra-high molecular weight polyethylene

1. Introduction Polymeric solid materials have been applied to many ®elds ranging from everyday life to low- and high-technology engineering due to their many unique advantages such as light weight, moldability, ability to form complicate shapes, corrosion resistance, versatile electronic properties, and low *

Corresponding author. Tel.: +86-21-5955-3998; fax: +8621-5955-3021. E-mail address: [email protected] (D. Zhu).

manufacturing cost. Applications of the materials have been, however, generally limited due to their inherent properties such as very softness, etc. Though radiation on polymers using UV-light, c-rays, or electron beams has been used for surface modi®cation of these materials, most investigations were focused on the changes in electronic or optical properties [1±4]. This may be related to very small improvement of mechanical properties of polymers by such radiation source. Lee et al. [5] have reported that the surface of Kapton, Te¯on, Tefzel and Mylar revealed substantial

0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 0 1 1 - 2

J. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 26±30

improvement in surface smoothness, hardness and wear resistance after energetic heavy ion implantation. In particular, B, N, C triple-beam implanted Kapton showed over 30 times larger hardness than unimplanted materials, making it more than three times harder than stainless steel. They proposed that radiation-induced crosslinking played an important role in improvement of mechanical properties of polymers when a charged particle passes through a polymeric material. Ultra high molecular weight polyethylene (UHMWPE), ±(CH2 )n ±, is generally used in total joint replacement. However, the material for longterm using is not available. The wear and wear debris generated by the articulation of current design of orthopaedic device is the single most common cause of the failure [6,7]. The wear results in loosening of the prosthesis, in¯ammation of tissue, and the need for revision surgery. So it is necessary to minimize the overall amount of wear and to increase the using life of prosthesis. The purpose of this work is to improve the wear properties of UHMWPE by energetic ion implantation. 2. Experimental UHMWPE used has an average molecular weight of 5 millions g/mol and density of 0.937 g/ cm3 . UHMWPE was implanted by 80 keV N‡ 2, ‡ ‡ C 3 H‡ 8 (40 keV N , 22 keV C ) with the ¯uency ranging from 1 ´ 1014 to 5 ´ 1015 ions/cm2 by Z-200 ion implantation machine. The samples for N‡ 2 implantation were mechanically polished and the roughness was about 0.12 lm. Unfortunately, the samples for C3 H‡ 8 implantation were not polished and its roughness was about 0.84 lm. During implantation, the pressure of the chamber was 6.6 ´ 10ÿ4 Pa. The beam current density was less than 0.1 mA/cm2 . The sample holder was cooled by ¯owing water and the temperature of the specimen was lower than 100°C. After implantation, elastic recoil detection (ERD) described by LÕEcuyer et al. [8] was employed to analyze the hydrogen pro®les of the samples. The incident angle of 2.5 MeV 4 He‡ ions was ®xed at 75°, and the recoil hydrogen was re-

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corded at 30° with respect to the incident beam by a surface barrier detector, in front of which an Al foil with the thickness of 11 lm was located to absorb scattered He particles. Since a certain ¯uency of energetic ion beam can lead to an obvious surface damage of the sample [9], we changed measurement area four times during the data acquisition. X-ray photoelectron spectroscopy (XPS), PHI-5702 multi-technique system (America), was used to study the nature of chemical bonding. Prior to XPS analysis, the surface contamination was removed by 1 keV Ar‡ ion etch. KyowA's DF-PM reciprocating tester was used to measure wear rate of the samples. During the test, the GCr15 steel ball of 3 mm in diameter was slid on the samples in a single direction under a load of 100 g. The roughness of the ball was less 0than 0.1 lm and their Vickers microhardness was 7.1 GPa (for detailed compositions for the steel, see [10]). The sliding speed and the track length were 2.47 mm/s and 5 mm, respectively. The N‡ 2, C3 H‡ 8 -implanted samples experienced 200 and 100 sliding circles, respectively. The wear track volume was calculated by the width across the wear track determined by an optical microscopy. For each track, a new steel ball was used. Moreover, for all samples, the above wear tests were run three times and the results obtained showed good agreement with one another. 3. Results 3.1. ERD analysis Figs. 1 and 2 show the ERD spectra of UHMWPE samples implanted with various ¯u‡ ency of N‡ 2 and C3 H8 , respectively. From Fig. 1, the thickness of the modi®cation layer and measurable depth of the sample are estimated to be 359 and 1001 nm, respectively. The projected range of 80 keV N‡ 2 in the sample calculated by TRIM program is 279.6 ‹ 44.6 nm. The experimental result is nearly consistent with theoretical simulation. In addition, ERD spectrum is usually normalized by charge collection from the sample for conductor and semiconductor materials, but it is not available for insulating materials. It is

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J. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 26±30

Fig.1. The ERD spectra of N‡ 2 -implanted UHMWPE samples for various ¯uences.

increasing ¯uence. The reduction ratio of H in the surface region can be calculated according to the area variation from ERD spectra of the unimplanted and the implanted samples, respectively. It is obvious that the areas begin to change at 759 channel for N‡ 2 -implanted samples. Therefore, compared to the unimplanted sample, the hydrogen content of samples implanted with the ¯uences of 1 ´ 1014 , 5 ´ 1014 and 1 ´ 1015 ions/cm2 decreased by 7%, 15% and 29%, respectively. The surface color of the samples also varies from white to light brown with the increasing ¯uence. In addition, for C3 H‡ 8 -implanted samples, at near surface region hydrogen content of the samples implanted with the ¯uence of 1 ´ 1014 , 5 ´ 1014 and 1 ´ 1015 decreased by 12%, 19% and 16%, respectively (Fig. 2). When the ¯uence reaches 5 ´ 1015 ions/ cm2 , the surface is damaged and the surface color becomes black brown. 3.2. XPS spectra

Fig. 2. The ERD spectra of C3 H‡ 8 -implanted UHMWPE samples for various ¯uences.

considered that the ion implantation cannot a€ect hydrogen content of the sample at the depth exceeding the thickness of the modi®cation layer. So ERD spectra of UHMWPE (insulator) are normalized by the yield at the low energy part of the spectra. For N‡ 2 -implanted samples, it is clearly found that the high energy edge of ERD shifts in the lower energy direction with increasing of the ¯uence (Fig. 1), which indicates that at near surface region hydrogen content decreases with the

The XPS N1s results for the sample N‡ 2 implanted with a ¯uence of 1 ´ 1015 ions/cm2 are shown in Fig. 3. A small N1s peak at 399.1 eV is found. Sjostrom et al. [11] have reported calculations of two model systems in which a N atom is surrounded by sp2 hybridized carbon atoms or sp3 hybridized carbon atoms. N1s binding energy of N±sp3 C and N±sp2 C are 398.4 and 400.25 eV, respectively. It has been reported that N1s peaks position for ±C¹N (nitrile) is at 399.4 eV [12], and the binding energy for ±N¹C (isonitrile) would be expected at a lower value due to the higher electron density at N atom. So the peak at 399.1 eV may be the overlap of the two peaks (398.4 and 400.25 eV) or correspond to ±N¹C (isonitrile). In any way, the injected nitrogen atoms form chemical bonds with the polymer chains, and do not form precipitates by self-clustering. The C1s binding energy of the unimplanted and implanted samples is 284.6 eV and the peaks are rather featureless. 3.3. Wear rate The wear volume of UHMWPE before and ‡ after N‡ 2 , C3 H8 implantation are shown in Table 1.

J. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 26±30

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¯uency of 1 ´ 1015 ions/cm2 , including acute systemic toxicity test, pyrogen test, cytotoxity test and micronucleus test, show that the material after implantation is suitable for the total joint replacement. This will be discussed in detail in a separate paper. 4. Discussion

Fig. 3. The XPS spectra of N1s and C1s for the N‡ 2 -implanted 2 sample with the ¯uency of 1 ´ 1015 N‡ 2 ions/cm .

The wear volume has decreased after implantation. For the N‡ 2 -implanted samples, the wear volume decreases with the increase of the ¯uence. When the ¯uence is up to 1 ´ 1015 ions /cm2 , the wear volume is the smallest and decreases by 68 times compared with the unimplanted UHMWPE. For C3 H‡ 8 -implanted samples, the wear volume is the smallest at the ¯uency of 1 ´ 1014 ions /cm2 and decreases by 47.5 times. In addition, the creature compatibility tests of UHMWPE implanted with a

‡ N‡ 2 or C3 H8 implantation of UHMWPE induces dehydrogenation. The XPS result shows that the injected nitrogen atoms form chemical bonds with the polymer chains. Ion implantation results in improvement of wear-resistance. It is well known that when a charged particle passes through a polymeric material, it loses its energy mainly by electronic and nuclear stopping [13]. The electronic processes result in activated chemical complexes, which then react or dissociate leading to radical formation, molecular emission, crosslinking, branching or grafting and chainscission. The nuclear processes mainly induce bond breakage or chain-scission by displacing atoms from polymer chain, producing recoil atoms and molecular scission products. So the dehydrogenation can be attributed to the electronic and nuclear processes during irradiation. Dehydrogenation from the polymer chain produces dangling bonds. The dangling bonds may be contacted with the implanted atoms or other dangling bonds, which cause the form of crosslinking. Mechanical properties of irradiated polymer such as hardness and wear-resistance is largely related to the degree of crosslinking [14,15]. So the improvement of ‡ wear-resistance of UHMWPE after N‡ 2 or C3 H8 implantation is due to the crosslinking which reduces the sliding between molecules.

Table 1 Wear volume of UHMWP before and after implantation Sample

The wear volume (mm3 ´ 10ÿ4 ) C3 H‡ 8 implantation

The wear volume (mm3 ´10ÿ4 ) N‡ 2 implantation

Unimplanted Implanted (1 ´ 1014 ions/cm2 ) Implanted (5 ´ 1014 ions/cm2 ) Implanted (1 ´ 1015 ions/cm2 )

190 4.00 8.00 68

230.4 38.35 34.32 3.39

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J. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 26±30

5. Conclusion ‡ 1. N‡ 2 and C3 H8 implantation induces dehydrogenation. This can be attributed to the electronic and nuclear processes during irradiation. 2. The XPS results show that the injected nitrogen atoms form chemical bonds with the polymer chains. 3. Wear-resistance of UHMWPE after N‡ 2 and C 3 H‡ 8 implantation has been improved by 68 and 47.5 times, respectively. These are due to the formation of crosslinking of UHMWPE which reduces the sliding between molecules.

Acknowledgements This work is supported by Shanghai Research Center for Applied Physics and Shanghai Science and Technology Development Foundation. Authors thank Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science for Wear and XPS measurement. References [1] E.T. Arakawa, M.W. Williams, J.C. Ashley, L.R. Painter, J. Appl. Phys. 52 (1981) 3579.

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