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NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 2490–2493 www.elsevier.com/locate/nimb
Polymer processing by a low energy ion accelerator A. Lorusso a,*, L. Velardi e, V. Nassisi a, F. Paladini b, A.M. Visco c, N. Campo c, L. Torrisi d, D. Margarone d, L. Giuffrida d, A. Raino` e a
Laboratorio di Elettronica Applicata e Strumentazione (LEAS), Department of Physics, University of Salento, INFN, Via Provinciale Lecce-Monteroni, C.P. 193, 73100 Lecce, Italy b Laboratorio di Dinamica Nonlineare (NDL), Department of Physics, University of Salento, Via Provinciale Lecce-Monteroni, 73100 Lecce, Italy c Department of Ch. Ind. and Ing. of Materials, University of Messina, Ctr. Di Dio, 98166 S. Agata, Messina, Italy d Department of Physics, INFN, University of Messina, Ctr. Papardo 31, 98166 S. Agata, Messina, Italy e Department of Physics, University of Bari, Via Amendola, 70126 Bari, Italy Available online 7 March 2008
Abstract Ion implantation is a process in which ions are accelerated toward a substrate at energies high enough to bury them just below the surface substrate in order to modify the surface characteristics. Laser-produced plasma is a very suitable and low cost technique in the production of ion sources. In this work, a laser ion source is developed by a UV pulsed laser of about 108 W/cm2 power density, employing a C target and a post ion acceleration of 40 kV to increase the ion energy. In this work, we implanted C ions on ultra-high-molecularweight-polyethylene (UHMWPE) and low-density polyethylene (LDPE). We present the preliminary results of surface property modifications for both samples. In particular, we have studied the modifications of the surface micro-hardness of the polymers by applying the ‘‘scratch test” method as well as the hydrophilicity modifications by the contact angle measurements. Ó 2008 Elsevier B.V. All rights reserved. PACS: 52.38.r; 52.38.Mf; 52.50.Dg; 52.70.m; 52.75.d Keywords: Laser-produced plasma; Ion implantation; Ultra-high-molecular-weight-polyethylene; Low-density polyethylene; Hydrophilicity; Micro-hardness
1. Introduction Polymer materials are very suitable due to their uses in many fields such as industry, engineering, etc. The polymers are widely utilized also in the biomedical field because some of them have good and important biocompatibility. For instance, ultra-high-molecular-weight-polyethylene (UHMWPE) is employed in many prosthesis devices for its high degree of homogeneity, high mechanical stability and good elastic properties similar to the hard tissue and the bone [1,2]. Nevertheless, UHMWPE suffers serious alterations especially when it is submitted to high mechan*
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ical dynamical stresses, high temperature and high corrosive environment giving rise to the wear of UHMWPE. Generally, the problem of wear involves the first layer of materials and it is important to investigate about the technique which could improve the wear resistance and the quality of the materials. It is known in literature that ion implantation is a good tool to modify the surface characteristics of the materials, such as hardness improving [3]. Laser-produced plasma is a good and versatile technique to generate in easy way an ion source which can be employed for ion implantation. The advantage to utilize this technique with respect to the conventional ones consists in the production of high doses of ions from any solid target working at laser power density of the order of 108– 109 W/cm2 [4–6]. Furthermore, this technique has the
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advantage to operate at room temperature avoiding stray diffusion of the implanted ions inside the substrate bulk. In this work, a laser ion source obtained by a C target is developed by utilizing a post DC accelerating voltage of 40 kV in order to extract and accelerate the ions. Our preliminary results showed the surface property modifications of UHMWPE and low-density polyethylene (LDPE) induced by C ion implantation at different ion doses. 2. Experimental set-up 2.1. Ion implantation The laser-produced plasma was created in a stainlesssteel vacuum chamber (106 mbar) by an excimer laser (KrF of 248 nm wavelength) which was able to provide a laser beam of fluence of 4.5 J/cm2 and pulse duration of 23 ns, with a resulting irradiance of about 2 108 W/cm2 onto the solid targets. The laser operated at 1 Hz repetition rate. The implantation chamber consisted of a plasma generation chamber (GC), 26.5 cm long, and a removable expansion chamber (EC), which allows an initial free expansion of the plasma before the ion extraction. The target support (T) was a stem (2 cm in diameter) mounted on the GC by an insulating flange (IF) and kept at positive high voltage, in DC mode (see Fig. 1). The EC was indispensable to avoid arcs between T and ground. It is an almost hermetic cylinder, 18.5 cm long, 9.0 cm in diameter, having an hole (1.0 cm in diameter) necessary for the ion extraction worked on its basis-electrode. On its lateral surface an optical hole was bored, as inlet port for the laser beam, and it was closed by a thin quartz window to avoid the plasma outgoing. A grounded electrode (GE) in front of the EC allowed either to generate an intense electric field or to support the substrate to be implanted. The GE was an aluminium pierced disc fixed at 1.3 cm from the perforated EC electrode. The implantation zone area was 0.78 cm2.
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during the implantation process in order to avoid the formation of a large crater which could change the plasma characteristics. The UHMWPE substrates were produced by utilizing a Ticona UHMWPE resin GUR 1020, with an average molecular weight of 2–4 106 g/mol, density of 0.930 g/ cm3, melting point at 138 °C without calcium stearate addiction. Pellets of low-density polyethylene (density of 0.922 g/cm3, melting point at 113 °C) were supplied by Polimeri Europa. UHMWPE and LDPE sheets (1 mm thick) were obtained by compression molding in a laboratory press. Here, UHMWPE powder was kept in a box at 200 °C for about 15 min at 20 MPa pressure and LDPE pellets were kept in a box at 180 °C for about 15 min at 20 MPa pressure. 2.3. Ion diagnostic The overall ion fluence was estimated by using a FC located just behind the GE at about 24 cm from the target (see Fig. 1). It consisted of an aluminium collector equipped by a suppression ring (SR) connected to a negative bias voltage for stopping the secondary electron emission. Fig. 2 reports the comparison of the ion current signals performed without any accelerating voltage (open square) and with 40 kV accelerating voltage (black circle). Both signals are normalized to the maximum value of the current. It is possible to observe the shift in time of the ion current signal due to the accelerating field. By the results obtained in our previous work [7], we can state that the whole charge carried by the laser-produced plasma is practically due to the ions with charge +1, obtaining a value of about 1.7 1011 ions/cm2 per laser pulse. The corresponding peak values of the extracted current density exceeded 10 mA/cm2. It is noteworthy that generally the ion dose is about 10% of the total ablated
2.2. Materials A 99.99% pure carbon was used as laser beam target, in the form of a little thin disc. It was suitably rotated
Fig. 1. Experimental apparatus: GC: generating chamber; EC: expansion chamber; T: target; IF: insulting flange; GE: ground electrode; DT: drift tube; FC: Faraday cup; OSC: oscilloscope; P: polyethylene.
Fig. 2. Current signal of C ions collected by FC. Open squares: free expansion current signal; black circle: accelerated current signal at 40 kV voltage.
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material (atoms, clusters, molecules and charged component) [7]. 2.4. Surface property modification study Surface property modification of polymers after C ions implantation were studied. In particular, the hydrophilicity modification was studied by the sessile drop technique by performing contact angle measurements [8,9]. A micro-syringe was employed to deposit 1 ll drop of distilled water (at 20 °C temperature) on the polymer surface. The contact angle h was measured by a CCD camera. A frame-grabber connected to the computer guides a fast-camcorder that recorded one sequence of images during the deposition of the droplet on the sample. All of the frames were analyzed using our software based on the droplet contour reconstruction by B-spline method for measuring contact angle by the following equation: H h ¼ 2arctan ; r where H is the height of the drop and r is the radius of the drop base. Besides, the modification of polymers micro-hardness was carried out by applying the ‘‘scratch test” method. The scratch tests were performed at room temperature by a Profiler Tencor-P10. This instrument was employed with two different tip forces: a higher one (50 mg) to cause a stress pressure imprint on the polymer surface, and a lower one (1 mg) to analyze the imprint profile.
decrease of contact angle with the increase of the laser shots, which means an improving of the hydrophilicity on implanted ion dose. Besides, at the same ion dose, the UHMWPE is characterized by a more evident improvement of the hydrophilicity than LDPE. The increasing of hydrophilicity due to the ion implantation could be ascribed to the chemical morphology modification of polymers and in particular to the formation of various kinds of oxidized groups such as hydroxyl and carbonyl functions [10]. About the micro-hardness modification by the ion implantation, Fig. 4 shows two examples of crater profiles obtained performing the scratch test on no-implanted UHMWPE (Fig. 4(a)) and implanted UHMWPE at the lowest ion dose (Fig. 4(b)). The resulting scratch width measurements at FWHM of the crater profiles give information about the micro-hardness of polymer: the implanted UHMWPE shows a scratch width reduction of about 85% respect to the no-implanted one. Therefore, the smaller scratch indicates a lower tip deep in the sample surface, i.e. an increment of the surface micro-hardness occurred in the C implanted UHMWPE substrate. At the higher ion doses a further improvement of the micro-hardness was not observed. On the contrary, the scratch test performed on LDPE polymers has not pointed out any significant increment of the micro-hardness after the C ion implantation.
3. Results and discussion The polymers were implanted with different C ion doses obtained by 2000, 4000, 6000 laser shots with a repetition rate of 1 Hz. Fig. 3 gives the contact angle modification as a function of laser shots. Both UHMWPE and LDPE data show a
Fig. 3. Variation of contact angle as a function of the number of laser shots.
Fig. 4. Crater profiles obtained for the scratch test: (a) no-implanted UHMWPE; (b) implanted UHMWPE.
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4. Conclusion In conclusion, this work shows the realization of a laser ion source with the interesting application of ion implantation for material processing in order to modify the surface characteristics. In particular, UHMWPE and LDPE polymers show an enhancement of hydrophilicity after C ion implantation which depends also on the ion dose. Furthermore, the improvement of micro-hardness of UHMWPE confirms that ion implantation is a good tool to increase the material biocompatibility when such a polymer is utilized as material for prosthesis realization. References [1] A. Valenza, A.M. Visco, L. Torrisi, N. Campo, Polym. J. 45 (2004) 1707.
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[2] L. Torrisi, A.M. Visco, A. Valenza, Rad. Eff. Def. Solids 158 (2003) 621. [3] R. Ghisleni, L. Shao, D.A. Lucca, V. Doan, M. Nastasi, J. Dong, A. Mehner, Nucl. Instr. and Meth. B 261 (2007) 708. [4] B. Qi, Y.Y. Lau, R.M. Gilgenbach, Appl. Phys. Lett. 78 (2001) 706. [5] F. Belloni, D. Doria, A. Lorusso, V. Nassisi, Nucl. Instr. and Meth. B 240 (2005) 40. [6] A. Lorusso, F. Belloni, D. Doria, V. Nassisi, J. Krasa, K. Rohlena, J. Phys. D: Appl. Phys. 39 (2006) 294. [7] V. Nassisi, A. Pedone, Rev. Sci. Instrum. 74 (2003) 68. [8] W.A. Zisman, Contact angle, Wettability and adhension, in: R.F. Gould (Ed.), Advances in Chemistry Series, Vol. 43, American Chemical Society, Washington, DC, 1964, p. 1. [9] W. Wong, K. Chan, K.W. Yeung, K.S. Lau, J. Mater. Process Technol. 132 (2003) 114. [10] M. Dadsetan, H. Mirzadeh, N. Sharifi, Radiat. Phys. Chem. 56 (1999) 597.
