- Email: [email protected]
Hydrogen release in UHMWPE upon He-ion bombardment
ARTICLE IN PRESS
Vacuum 78 (2005) 281–284 www.elsevier.com/locate/vacuum
Hydrogen release in UHMWPE upon He-ion bombardment A.M. Abdul-Kadera,,1, Andrzej Turosa,1, Jacek Jagielskia,1, Lech Nowickia, Renata Ratajczaka, Anna Stonerta, Mariam A. Al-Ma’adeedb a
The Andrzej Soltan Institute of Nuclear Studies, ul. Hoza 69, 00-681 Warsaw, Poland b Physics Department, Faculty of Science, Qatar University, Doha, State of Qatar
Abstract Ultrahigh molecular weight polyethylene (UHMWPE) due to its good wear resistance is the material of choice for the load-bearing surfaces of total joint implants. Unfortunately, failure is often observed caused by the UHMWPE fragility. As a consequence, the formation of microcracks leads to production of microparticles, which reduce the lifetime of the joint. The aim of this work was to improve micromechanical properties of UHMWPE by He-ion bombardment. Energetic ions are slowed down in material by interaction with the electronic system of target atoms. Such interaction producing excitation of atomic electrons or ionization leads to the cleavage of C–H bonds leading to the release of hydrogen. In this work, hydrogen release due to He-ion bombardment at energies ranging from 0.8 to 2.0 MeV was studied in situ by RBS. The data were analyzed using the statistical model of molecular recombination. The H-release cross-section and the saturation concentration were found to depend strongly on the electronic stopping power of the He-beam and exhibit quadratic dependence on the incident ion energy. Our data make it possible to extrapolate the H-release parameters to much lower energies where no RBS measurements are possible. The consistency of such a procedure was independently checked by the NRA measurements. To study the role of temperature on H-release some experiments were performed at 50 K. r 2005 Elsevier Ltd. All rights reserved. Keywords: Polymers; Irradiation; Hydrogen release
1. Introduction In the contemporary total joint arthroplasty, in which the total diseased joint is been replaced, metal-on-ultrahigh molecular weight polyethylene Corresponding author.
E-mail address: [email protected] (A.M. Abdul-Kader). Also with Institute of Electronic Materials Technology, Warsaw, Poland. 1
(UHMWPE) total joint replacement is an international standard [1]. Despite the recognized success, a major obstacle limiting the lifetime of implants is osteolysis induced by wear debris produced in the form of microparticles from the surface of the UHMWPE component. Microparticles are typically released from the microcracks generated by dynamical loads on the joint. Numerous efforts to solve the wear problem have revealed the necessity of UHMWPE modification, which is usually
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.01.039
ARTICLE IN PRESS A.M. Abdul-Kader et al. / Vacuum 78 (2005) 281–284
performed using different physical and chemical techniques leading to material crosslinking. The principal objective of this work was to improve micromechanical properties of UHMWPE through ion beam-induced structural and chemical modifications on a molecular level. The reported elsewhere [2] preliminary results were encouraging, however, little is known about the processes occurring during and after ion bombardment and the resulting structural transformation of polyethylene. The most promising results have been obtained after irradiation with He ions. Hydrogen escapes from polymers being subjected to a variety of energy-depositing processes such as thermal treatment, electron, photon or ion irradiation. Elucidation of the mechanism of surface layer transformation induced by hydrogen release upon ion bombardment seems to be an indispensable step for this project. Several studies of hydrogen release due to ion beam have been reported. Baumann et al. [3] fitted measured hydrogen concentration as a function of the dose with three different exponents attributed to three hydrogen release cross-sections. For analysis of hydrogen release from the amorphous C:H induced by ion irradiation, Adel et al. [4] proposed a statistical model based on the bulk molecular recombination. In order to release hydrogen atoms from the polymer two C–H bonds have to be cleaved and the liberated hydrogen atoms can escape subsequent trapping only if they recombine in the bulk and diffuse out as a H2 molecule. De Jong et al. [5] and Walker et al. [6] have developed further the model of Adel et al. scaling the release cross-section with the stopping power of incident beam. In this paper, we report the results of the study of hydrogen release from UHMWPE upon He-ion bombardment at energies ranging from 0.8 to 2.0 MeV. Hydrogen release at RT was measured using RBS. To study thermally activated H mobility, a number of RBS experiments were performed in situ at 50 K.
with He+-ion beam in the 0.8–2.0 MeV energy range from Van de Graaff accelerator Lech at the Institute for Nuclear Studies in Warsaw. Samples were covered with 75 nm of carbon layer to avoid beam-charging effects. A carbon sample has been used as a reference standard. Since polyethylene is usually very sensitive to ion bombardment, to study hydrogen release in great detail, instead of a conventional RBS arrangement, we have employed a large annular detector covering the solid angle of 0.16 srd with an average scattering angle of 1601. The energy resolution was 20 keV.
3. Results Fig. 1 shows typical RBS spectrum for RBS spectra for carbon reference standard and the UHMWPE samples bombarded with 1.25 1014 and 2.50 1016 He/cm2 at 2 MeV. One notes that the carbon signal increases with increasing He dose. This is attributed to the change of stopping power due to a release of hydrogen atoms from the measured sample. The calculated yield height ratio (yield in pure carbon signal over the yield in polyethylene signal) for the carbon reference sample and PE as a function of hydrogen content in PE is shown in Fig. 2. Based on this curve one easily can determine the PE composition. Hydrogen release curves for three different incident He energy: 0.8, 1.4, and 2.0 MeV are 600 Backscattering yield
282
400 300 200 100 0 20
2. Experimental The samples used were flat, rectangular wafers of 1 mm thickness. They were studied by the RBS
pure carbon 2.5x1015 (at/cm2) 5.0x1015 (at/cm2)
500
40
60
80
100
120
Channel number Fig. 1. RBS spectra for carbon reference sample and UHMWPE bombarded with different doses of 1.4 MeV He ions.
ARTICLE IN PRESS A.M. Abdul-Kader et al. / Vacuum 78 (2005) 281–284
shown in Fig. 3. The solid lines present the best fit to data using the molecular recombination model (MRM) developed by Adel et al. [2]. The model is based on the fact that atomic hydrogen is produced by cleavage of C–H bonds. Once formed, hydrogen atoms are usually trapped by neighboring dangling bonds and cannot diffuse over large distances. However, if two hydrogen atoms have a chance to encounter and recombine to form H2 molecule, this species can easily diffuse 1.0
CHx/C
0.9
0.8
0.7
0.6
0
20
40
283
out. According to MRM, the atomic density of the hydrogen r after being bombarded with ion to fluence D is given by rðDÞ ¼ ½1=rf þ ð1=r0 1=rf Þ expðkDÞ1 ,
(1)
where r0 ; rf and k are the initial and final hydrogen density, and the effective release crosssection, respectively. All these three parameters can be determined by fitting the experimental data. As pointed out by de Jong et al. [5] and Maree et al. [7] rf and k strongly depend on the electronic stopping power of incident ions. Fig. 4 shows rf and k as functions of the electronic stopping power of incident ions. There are important indications in the literature [5] that hydrogen mobility is largely suppressed at low temperatures. To check whether the hydrogen release from UHMWPE can be influenced by the irradiation temperature, RBS measurements were performed at 50 K. The corresponding release curves are shown in Fig. 5. Here again the solid lines indicate the MRM fit.
60
H concentration (at. %)
4. Discussion and conclusions Crosslinking of polyethylene has been performed for decades to improve its wear resistance [8]. It is usually accomplished by exposing to ionizing radiation, typically 60Co gamma rays. The
c
f
47
k
46
0.12
40
45 0.10 44 0.08
20
b
43 a
0.06 c 0.04
0 0
5
10
15
20
25
30
48
(at. %)
0.14
a
f
0.16
0.8 MeV 1.4 MeV 2.0 MeV
60
k (nm2)
H concentration (at. %)
Fig. 2. Calculated yield height ratio for the carbon reference sample and UHMWPE as a function of hydrogen content in polyethylene.
42 41
180
200
220
240
Stopping power (eV/nm)
Dose (1015 at/cm2) Fig. 3. Dose dependence of H-concentration at RT for different He-beam energies. Solid lines are fits according to MRM.
Fig. 4. Final hydrogen concentration rf and the effective release cross-section k as a function of the He-ion stopping power for energies: (a) 0.8 MeV, (b) 1.4 MeV and (c) 2 MeV. Solid lines show the quadratic fit.
ARTICLE IN PRESS A.M. Abdul-Kader et al. / Vacuum 78 (2005) 281–284
284
release induced by He-ion bombardment by two parameters: the final hydrogen density rf and the effective release cross-section k: This procedure makes it possible to extrapolate the H-release parameters to much lower energies, i.e. 100–300 keV, which are much more suitable for UHMWPE modification. Since at these energies no RBS measurements are possible, the consistency of such a procedure was successfully checked by the NRA measurements using the 15N profiling technique [9].
H concentration ( at. %)
70 60 50 40 30 RT 50 K warmed to 150 K warmed to RT
20 10 0
0
2
4
6 Dose
8 (1015
10
12
14
16
at/cm2)
Fig. 5. Dose dependence of H-concentration at 50 K due to in situ bombardment with 1.4 MeV He-beam. Solid lines are fits according to MMR. Open squares indicate hydrogen concentration after warming up to 150 K and RT, respectively.
crosslink density reaches the saturation value corresponding to a gel content of 90–100% after the dose of 100–150 kGy was absorbed. Due to the higher ionization produced by ion beams, polymers are much more sensitive to this type of radiation. Another advantage of ions is their limited, easily controlled range. Hence, by the judicious choice of implantation conditions, properties of the nearsurface region up to several micrometers thickness can be easily tailored. Light ions like H and He producing primarily ionization and little displacements, are at best suitable for this purpose. It is expected that producing harder PE on top of unmodified material shock resistance of such a structure can be significantly improved. With this respect the detailed knowledge of the mechanism leading to UHMWPE modification is indispensable. Using MRM, we have quantified H-
Acknowledgments The authors thank Prof. W. Wesch and the JULIA team at the Institute of Solid State Physics, University of Jena, Germany, for making possible RBS analysis at low temperatures.
References [1] Kurtz SM, Muratoglu OK, Evans M, Edidin AA. Biomaterials 1999;20:1659. [2] Turos A, Jagielski J, Pia˛ tkowska A, Bielin´ski D, S´lusarski L, Madi NK. Vacuum 2002;70:201. [3] Baumann H, Rupp Th, Bethge K, Koidl P, Wild Ch. E-MRS Conf Proc 1987;17:343. [4] Adel ME, Amir O, Kalish R, Feldman LC. J Appl Phys 1989;66:3248. [5] de Jong MP, Maas AJH, Van Ijzendoorn LJ, Klein S, de Voigt MJA. J Appl Phys 1997;82:1058. [6] Walker SR, Davies JA, Forster JS, Wallace SG, Kockelkoren AC. Nucl Instr and Meth B 1998;707:136. [7] Maree CHM, Vredenberg AM, Habraken FHPM. Mater Chem Phys 1996;46:198. [8] Beveridge C, Sabiston A. Mat Des 1987;8:263. [9] Turos A, Abdul-Kader AM, Posiadlo S, Strojek B, Strzalkowski I, Al-Madeed M. Vacuum, these proceedings.
Vacuum 78 (2005) 281–284 www.elsevier.com/locate/vacuum
Hydrogen release in UHMWPE upon He-ion bombardment A.M. Abdul-Kadera,,1, Andrzej Turosa,1, Jacek Jagielskia,1, Lech Nowickia, Renata Ratajczaka, Anna Stonerta, Mariam A. Al-Ma’adeedb a
The Andrzej Soltan Institute of Nuclear Studies, ul. Hoza 69, 00-681 Warsaw, Poland b Physics Department, Faculty of Science, Qatar University, Doha, State of Qatar
Abstract Ultrahigh molecular weight polyethylene (UHMWPE) due to its good wear resistance is the material of choice for the load-bearing surfaces of total joint implants. Unfortunately, failure is often observed caused by the UHMWPE fragility. As a consequence, the formation of microcracks leads to production of microparticles, which reduce the lifetime of the joint. The aim of this work was to improve micromechanical properties of UHMWPE by He-ion bombardment. Energetic ions are slowed down in material by interaction with the electronic system of target atoms. Such interaction producing excitation of atomic electrons or ionization leads to the cleavage of C–H bonds leading to the release of hydrogen. In this work, hydrogen release due to He-ion bombardment at energies ranging from 0.8 to 2.0 MeV was studied in situ by RBS. The data were analyzed using the statistical model of molecular recombination. The H-release cross-section and the saturation concentration were found to depend strongly on the electronic stopping power of the He-beam and exhibit quadratic dependence on the incident ion energy. Our data make it possible to extrapolate the H-release parameters to much lower energies where no RBS measurements are possible. The consistency of such a procedure was independently checked by the NRA measurements. To study the role of temperature on H-release some experiments were performed at 50 K. r 2005 Elsevier Ltd. All rights reserved. Keywords: Polymers; Irradiation; Hydrogen release
1. Introduction In the contemporary total joint arthroplasty, in which the total diseased joint is been replaced, metal-on-ultrahigh molecular weight polyethylene Corresponding author.
E-mail address: [email protected] (A.M. Abdul-Kader). Also with Institute of Electronic Materials Technology, Warsaw, Poland. 1
(UHMWPE) total joint replacement is an international standard [1]. Despite the recognized success, a major obstacle limiting the lifetime of implants is osteolysis induced by wear debris produced in the form of microparticles from the surface of the UHMWPE component. Microparticles are typically released from the microcracks generated by dynamical loads on the joint. Numerous efforts to solve the wear problem have revealed the necessity of UHMWPE modification, which is usually
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.01.039
ARTICLE IN PRESS A.M. Abdul-Kader et al. / Vacuum 78 (2005) 281–284
performed using different physical and chemical techniques leading to material crosslinking. The principal objective of this work was to improve micromechanical properties of UHMWPE through ion beam-induced structural and chemical modifications on a molecular level. The reported elsewhere [2] preliminary results were encouraging, however, little is known about the processes occurring during and after ion bombardment and the resulting structural transformation of polyethylene. The most promising results have been obtained after irradiation with He ions. Hydrogen escapes from polymers being subjected to a variety of energy-depositing processes such as thermal treatment, electron, photon or ion irradiation. Elucidation of the mechanism of surface layer transformation induced by hydrogen release upon ion bombardment seems to be an indispensable step for this project. Several studies of hydrogen release due to ion beam have been reported. Baumann et al. [3] fitted measured hydrogen concentration as a function of the dose with three different exponents attributed to three hydrogen release cross-sections. For analysis of hydrogen release from the amorphous C:H induced by ion irradiation, Adel et al. [4] proposed a statistical model based on the bulk molecular recombination. In order to release hydrogen atoms from the polymer two C–H bonds have to be cleaved and the liberated hydrogen atoms can escape subsequent trapping only if they recombine in the bulk and diffuse out as a H2 molecule. De Jong et al. [5] and Walker et al. [6] have developed further the model of Adel et al. scaling the release cross-section with the stopping power of incident beam. In this paper, we report the results of the study of hydrogen release from UHMWPE upon He-ion bombardment at energies ranging from 0.8 to 2.0 MeV. Hydrogen release at RT was measured using RBS. To study thermally activated H mobility, a number of RBS experiments were performed in situ at 50 K.
with He+-ion beam in the 0.8–2.0 MeV energy range from Van de Graaff accelerator Lech at the Institute for Nuclear Studies in Warsaw. Samples were covered with 75 nm of carbon layer to avoid beam-charging effects. A carbon sample has been used as a reference standard. Since polyethylene is usually very sensitive to ion bombardment, to study hydrogen release in great detail, instead of a conventional RBS arrangement, we have employed a large annular detector covering the solid angle of 0.16 srd with an average scattering angle of 1601. The energy resolution was 20 keV.
3. Results Fig. 1 shows typical RBS spectrum for RBS spectra for carbon reference standard and the UHMWPE samples bombarded with 1.25 1014 and 2.50 1016 He/cm2 at 2 MeV. One notes that the carbon signal increases with increasing He dose. This is attributed to the change of stopping power due to a release of hydrogen atoms from the measured sample. The calculated yield height ratio (yield in pure carbon signal over the yield in polyethylene signal) for the carbon reference sample and PE as a function of hydrogen content in PE is shown in Fig. 2. Based on this curve one easily can determine the PE composition. Hydrogen release curves for three different incident He energy: 0.8, 1.4, and 2.0 MeV are 600 Backscattering yield
282
400 300 200 100 0 20
2. Experimental The samples used were flat, rectangular wafers of 1 mm thickness. They were studied by the RBS
pure carbon 2.5x1015 (at/cm2) 5.0x1015 (at/cm2)
500
40
60
80
100
120
Channel number Fig. 1. RBS spectra for carbon reference sample and UHMWPE bombarded with different doses of 1.4 MeV He ions.
ARTICLE IN PRESS A.M. Abdul-Kader et al. / Vacuum 78 (2005) 281–284
shown in Fig. 3. The solid lines present the best fit to data using the molecular recombination model (MRM) developed by Adel et al. [2]. The model is based on the fact that atomic hydrogen is produced by cleavage of C–H bonds. Once formed, hydrogen atoms are usually trapped by neighboring dangling bonds and cannot diffuse over large distances. However, if two hydrogen atoms have a chance to encounter and recombine to form H2 molecule, this species can easily diffuse 1.0
CHx/C
0.9
0.8
0.7
0.6
0
20
40
283
out. According to MRM, the atomic density of the hydrogen r after being bombarded with ion to fluence D is given by rðDÞ ¼ ½1=rf þ ð1=r0 1=rf Þ expðkDÞ1 ,
(1)
where r0 ; rf and k are the initial and final hydrogen density, and the effective release crosssection, respectively. All these three parameters can be determined by fitting the experimental data. As pointed out by de Jong et al. [5] and Maree et al. [7] rf and k strongly depend on the electronic stopping power of incident ions. Fig. 4 shows rf and k as functions of the electronic stopping power of incident ions. There are important indications in the literature [5] that hydrogen mobility is largely suppressed at low temperatures. To check whether the hydrogen release from UHMWPE can be influenced by the irradiation temperature, RBS measurements were performed at 50 K. The corresponding release curves are shown in Fig. 5. Here again the solid lines indicate the MRM fit.
60
H concentration (at. %)
4. Discussion and conclusions Crosslinking of polyethylene has been performed for decades to improve its wear resistance [8]. It is usually accomplished by exposing to ionizing radiation, typically 60Co gamma rays. The
c
f
47
k
46
0.12
40
45 0.10 44 0.08
20
b
43 a
0.06 c 0.04
0 0
5
10
15
20
25
30
48
(at. %)
0.14
a
f
0.16
0.8 MeV 1.4 MeV 2.0 MeV
60
k (nm2)
H concentration (at. %)
Fig. 2. Calculated yield height ratio for the carbon reference sample and UHMWPE as a function of hydrogen content in polyethylene.
42 41
180
200
220
240
Stopping power (eV/nm)
Dose (1015 at/cm2) Fig. 3. Dose dependence of H-concentration at RT for different He-beam energies. Solid lines are fits according to MRM.
Fig. 4. Final hydrogen concentration rf and the effective release cross-section k as a function of the He-ion stopping power for energies: (a) 0.8 MeV, (b) 1.4 MeV and (c) 2 MeV. Solid lines show the quadratic fit.
ARTICLE IN PRESS A.M. Abdul-Kader et al. / Vacuum 78 (2005) 281–284
284
release induced by He-ion bombardment by two parameters: the final hydrogen density rf and the effective release cross-section k: This procedure makes it possible to extrapolate the H-release parameters to much lower energies, i.e. 100–300 keV, which are much more suitable for UHMWPE modification. Since at these energies no RBS measurements are possible, the consistency of such a procedure was successfully checked by the NRA measurements using the 15N profiling technique [9].
H concentration ( at. %)
70 60 50 40 30 RT 50 K warmed to 150 K warmed to RT
20 10 0
0
2
4
6 Dose
8 (1015
10
12
14
16
at/cm2)
Fig. 5. Dose dependence of H-concentration at 50 K due to in situ bombardment with 1.4 MeV He-beam. Solid lines are fits according to MMR. Open squares indicate hydrogen concentration after warming up to 150 K and RT, respectively.
crosslink density reaches the saturation value corresponding to a gel content of 90–100% after the dose of 100–150 kGy was absorbed. Due to the higher ionization produced by ion beams, polymers are much more sensitive to this type of radiation. Another advantage of ions is their limited, easily controlled range. Hence, by the judicious choice of implantation conditions, properties of the nearsurface region up to several micrometers thickness can be easily tailored. Light ions like H and He producing primarily ionization and little displacements, are at best suitable for this purpose. It is expected that producing harder PE on top of unmodified material shock resistance of such a structure can be significantly improved. With this respect the detailed knowledge of the mechanism leading to UHMWPE modification is indispensable. Using MRM, we have quantified H-
Acknowledgments The authors thank Prof. W. Wesch and the JULIA team at the Institute of Solid State Physics, University of Jena, Germany, for making possible RBS analysis at low temperatures.
References [1] Kurtz SM, Muratoglu OK, Evans M, Edidin AA. Biomaterials 1999;20:1659. [2] Turos A, Jagielski J, Pia˛ tkowska A, Bielin´ski D, S´lusarski L, Madi NK. Vacuum 2002;70:201. [3] Baumann H, Rupp Th, Bethge K, Koidl P, Wild Ch. E-MRS Conf Proc 1987;17:343. [4] Adel ME, Amir O, Kalish R, Feldman LC. J Appl Phys 1989;66:3248. [5] de Jong MP, Maas AJH, Van Ijzendoorn LJ, Klein S, de Voigt MJA. J Appl Phys 1997;82:1058. [6] Walker SR, Davies JA, Forster JS, Wallace SG, Kockelkoren AC. Nucl Instr and Meth B 1998;707:136. [7] Maree CHM, Vredenberg AM, Habraken FHPM. Mater Chem Phys 1996;46:198. [8] Beveridge C, Sabiston A. Mat Des 1987;8:263. [9] Turos A, Abdul-Kader AM, Posiadlo S, Strojek B, Strzalkowski I, Al-Madeed M. Vacuum, these proceedings.