- Email: [email protected]
Plasma immersion ion implantation of poly(tetrafluoroethylene)
Surface and Coatings Technology 177 – 178 (2004) 483–488
Plasma immersion ion implantation of poly(tetrafluoroethylene) T.L. Schillera,b,*, D. Sheejac, D.R. McKenzied, D.G. McCullocha, D.S.P. Lauc, S. Burnb, B.K. Tayc a
Department of Applied Physics, RMIT University, G.P.O. Box 2476V, Melbourne 3001, Australia b CSIRO, Manufacturing and Infrastructure Technology, Highett, Vic. 3019, Australia c Nanyang Technological University, Singapore, Singapore d School of Physics (A28), University of Sydney, Sydney, NSW 2006, Australia
Abstract Plasma immersion ion implantation (PIII) has been used with a filtered cathodic arc to implant copper and carbon ions into poly(tetrafluoroethylene) (PTFE). The PTFE substrates for the copper implantation were placed perpendicular to the plasma beam, whilst those for carbon implantation were oriented parallel to the drift velocity of the beam to minimise the deposition of low energy ions. Electrodes in the form of a backing plate and a mask with holes were used to apply the pulsed bias from the PIII supply. X-ray photoelectron spectroscopy has shown that there is a structural change in the PTFE induced by both the copper and carbon implantation. Raman spectroscopy of the carbon implanted samples showed the presence of an amorphous carbon peak, which remained even after cleaning the surface to remove loosely bound carbon. This shows that there is both implantation and deposition of the carbon occurring. In the case of copper, this method resulted in well-adhered films. The implanted PTFE has been examined for changes in wear resistance. Both copper and carbon modified surfaces showed improvements in wear resistance. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: PTFE; Surface modification; Ion implantation; Cathodic arc; Wear
1. Introduction Many polymers are attractive as materials for biomedical applications as they combine a high strength to density ratio and ease of formability with a low surface reactivity and a good biocompatibility. However, polymers show a higher wear rate than other candidate materials, such as metals and ceramics. Poly(tetrafluoroethylene) (PTFE) has excellent thermal and chemical properties, a low coefficient of friction and a low dielectric constant. Hence, PTFE has been used extensively as an encapsulation material for components in harsh environments. However, the inherent chemical inertness of PTFE makes chemical surface modification difficult and makes it unsuitable for metallisation because of its poor adhesion qualities. In this study, plasma immersion ion implantation (PIII) w1,2x and plasma deposition were applied to PTFE to study the effect of ion modification on the surface *Corresponding author. Tel.: q61-3-9252-6603; fax: q61-3-92526253. E-mail address: [email protected] (T.L. Schiller).
structure and wear characteristics. Two basic types of modification were investigated, using a filtered cathodic arc plasma as a source of ions. The first involved the modification of PTFE using PIII with a C plasma. This allowed the investigation of the effects on the structure and properties of PTFE without introducing a different chemical species. The second type of modification involved the use of a Cu plasma with an initial deposition. In this case we were particularly interested in investigating the possibility of producing Cu films onto the PTFE surfaces with good adhesion. Plasma processing and surface modification of polymers using ion beams has received significant attention w3–6x. Typically, energetic ions modify the polymer by causing chain scission, which reduces crystallinity w7,8x. It is known that ion modification results in cross-linking in many polymers w7–10x. However, there have been few studies of the effects of ion irradiation on PTFE. It has been found that ion beam modification improves the adhesion properties of PTFE w11–14x, and it has been shown that both the increase in the surface roughness and degradation of the polymer chain are attributed
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0257-8972(03)00916-2
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T.L. Schiller et al. / Surface and Coatings Technology 177 – 178 (2004) 483–488
Fig. 1. Schematic diagram of the cathodic arc deposition system with the PIII attached.
to the improvement. In this work, we investigate both changes in structure and mechanical properties of PTFE following plasma treatment. 2. Experimental PTFE sheets of thickness 0.5, 1.0, 3.0 and 4.0 mm were cut into 40=40-mm2 samples. They were then placed in an ultrasonic bath for 10 min with ethanol then acetone for 10 min. They were then dried in an oven at 100 8C in atmospheric pressure for 2 h, to remove excess moisture. Two similar sets of filtered cathodic arc equipment fitted with PIII, one at Nanyang Technological University, Singapore, and the other at the University of Sydney were used in this work. The apparatus employed in this project incorporates both filtered cathodic arc and PIII. A schematic of the Sydney University apparatus is shown in Fig. 1. The cathodic arc deposition system consists of a cathode, at one end of the chamber, connected using a duct with the substrate holder at the other. The PIII power supply is attached to the substrate holder as indicated by Fig. 1. A high negative pulse bias is applied to the substrate at a voltage of up to 15 kV while it is immersed in plasma. In PIII, during each pulse, a plasma sheath is formed, which accelerates the plasma ions towards the substrate. The PIII process can achieve implantation into the substrate causing interface mixing, as well as a modification of the material deposited onto the substrate. The walls of the duct are surrounded by a series of coils which when supplied with a current act together as a curved magnetic solenoid
filter for reducing the macroparticle content of the plasma. The magnetic filter duct has several power sources for controlling the currents in the coils to give control of the magnetic field generated w15x. The pulse generator has the capability of producing pulses of up to 30 kV, with durations of 10–60 ms and a repetition rate of up to 1 kHz. In the case of carbon implantation, a graphite target cathode was employed and specimens were prepared with PIII voltages of either 10 or 15 kV with a pulse width of 10 ms and a frequency of 500 Hz for periods of 3, 5 and 10 min. The substrate was attached to a metal substrate holder and positioned parallel to the drift velocity of the plasma beam at the exit of the filter duct. This parallel configuration was used for carbon implantation, to minimise deposition on the surface. The effect of placing a metal mesh over the substrate and connected to the holder was also examined with the aim of improving implantation efficiently by preventing charge build-up on the polymer surface. In the case of copper implantation, a Cu target cathode was used with a PIII voltage of 5 kV with a frequency of 100 and 200 Hz and the same pulse length as was used for carbon. A perpendicular configuration of the specimen holder relative to the plasma beam was used in this case to give both implantation and deposition. The adhesion of copper films deposited onto the PTFE substrates was determined by testing with adhesive tape. Visible Raman spectroscopy, with 514 nm line of Arq laser as the excitation source, was used to study the microstructure of the films. The laser was focused onto 2–3-mm diameter, with 20-mW output power. X-ray photoelectron spectroscopy (XPS) was performed on a VG Microlab 310F with a dual AlyMg anode unmonochromated X-ray source operated at a power of 300 W and 15 kV excitation voltage. The sample was tilted such that the electron analyzer normal to the sample surface collected the escaping electrons. The analyzed area is determined by the electrostatic lens and slits of the analyzer and in this case was approximately a rectangular area of 5=1 mm2. All survey spectra were acquired in a constant analyzer energy (CAE) mode at a pass energy of 100 eV at 1 eV steps between data points. All single element scans were performed in CAE mode at a pass energy of 20 and 0.1 eV steps. Charge correction was done by assuming that the adventitious carbon peak present in all samples was located at 284.5 eV. Wear was assessed in this study by using a specially designed pin-on-disk apparatus w16x in which the specimen is rotated above a fixed PTFE pin creating a circular wear track. The cylindrical pins (6-mm diameter) were made from commercial PTFE. One end of the pin was machine-finished to a 24.8-mm radius tip. A brushless DC-servomotor (3556K024B, Minimotor SA, Croglio, Switzerland) was used to revolve the specimen.
T.L. Schiller et al. / Surface and Coatings Technology 177 – 178 (2004) 483–488
Fig. 2. Raman spectra of (a) virgin PTFE; (b) PTFE sample treated with 15 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 3 min; (c) PTFE sample treated with 15 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 10 min and (d) PTFE sample treated with 10 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 15 min with the sample surface covered with a steel grid. In each of the treated samples, the peaks corresponding to virgin sample have been subtracted.
The rotational speed was approximately 2000 rpm. This was controlled by a four-quadrant pulse-width modulation servo amplifier (BLD 5606-SH4P, Minimotor SA, Croglio). A 50-mm long titanium extension shaft transmitted the motor shaft revolution to the disk via a cylindrical sample holder, which was screw-attached to the end of the extension shaft in axial alignment. The method also provides a measurement of the friction coefficient of the samples.
485
Fig. 3. XPS spectra in the vicinity of the carbon peaks for (a) PTFE; (b) PTFE sample treated with 15 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 10 min and (c) PTFE sample treated with 10 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 10 min.
characteristic of amorphous carbon. This peak is more prominent in the 10-min sample shown in Fig. 2c. A similar spectrum was obtained for the sample, which was covered with the steel grid (Fig. 2d). The presence of the grid was found to reduce the occurrence of arcing across the specimen due to charge build-up on the polymer surface.
3. Results and discussion 3.1. Carbon treatment The structural changes near the surface of the samples that had been subjected to modification by the carbon plasma were investigated using XPS and Raman spectroscopy. Prior to Raman and XPS analysis, the specimens were thoroughly cleaned in order to remove loosely bound carbon from the surface. Fig. 2a–d show the Raman spectra of the virgin, and two of the C treated samples and one sample that was covered with a steel grid. The virgin spectrum contains sharp peaks, which are characteristic of PTFE w17x. In each of the treated samples, the spectra have been processed to remove most of the peaks corresponding to the virgin sample by a simple subtraction. The resulting spectrum of the sample treated for 3 min (Fig. 2b) shows a broad asymmetric peak at approximately 1500 cmy1, which is
Fig. 4. XPS spectra in the vicinity of the fluorine peaks for (a) virgin PTFE; (b) PTFE sample treated with 15 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 10 min and (c) PTFE sample treated with 10 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 10 min.
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Fig. 3 shows the XPS spectra for the virgin (Fig. 3a) and two carbon treated samples (Fig. 3b and c). The peak corresponding to the CF2 bond within the polymer backbone is clearly seen at approximately 293 eV and decreases in intensity by approximately 80% as the exposure time increases. The peak at approximately
284.5 eV, which occurs in all the spectra, corresponds to C–C bonding. This peak could be due to either adventitious carbon or the build up of carbon in the surface. Given that the intensity of this peak increases with treatment time, this increase in intensity can be attributed to the build of carbon due to implantation.
Fig. 5. Wear track (a) and corresponding pin (b) for virgin PTFE, wear track (c) and corresponding pin (d) for PTFE sample treated with 15 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 10 min, wear track (e) and corresponding pin (f) PTFE sample treated with 10 kV Cu ions at 600 Hz pulsing frequency 10-ms pulse width for 10 min.
T.L. Schiller et al. / Surface and Coatings Technology 177 – 178 (2004) 483–488
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attributed to the increase in hardness of the polymer surface, or an increase in the surface roughness. Further studies need to be carried out to determine if the increase in wear resistance is due to the hardening of the PTFE surface. 3.2. Cu deposition and implantation
Fig. 6. XPS spectra in the vicinity of the carbon peaks for (a) virgin PTFE; (b) PTFE sample treated with 5 kV Cu ions at 200 Hz pulsing frequency 10-ms pulse width for 12 min and (c) PTFE sample treated with 5 kV Cu ions at 100 Hz pulsing frequency 10-ms pulse width for 10 min.
There were also weak peaks between 285 and 290 eV in the modified samples due to CF3 and CF, which indicates a break-up of the polymer chains and the creation of branching shorter chain molecules. In all three samples, the dominant fluorine peak (Fig. 4) occurs at 690 eV, which corresponds to CF2 bonding and this reduces in intensity with exposure time. A small peak was also observed at approximately 686 eV after exposure, which can be seen as a skewness of the peak at lower binding energies. This corresponds to other fluorine bonding arrangements (CF3 and CF). All three samples also showed some oxygen at a level, which increased with exposure time to the plasma. This may be due to trace oxygen contamination of the plasma. The wear testing results for the virgin (a and b) and one of the carbon treated (c and d) are shown in Fig. 5. Shown in each case is a typical section of the wear track on the sample and the tip of the pin used in the test. The wear track width decreases from approximately 2 mm in the case of the virgin sample to 1.25 mm for the C modified sample. This indicates that there is an increase in wear resistance probably due to an increase in surface hardness caused by the irradiation. In the case of the pins, which are untreated in all cases, slightly more wear is observed following modification, this may be compensated for by the reduction in wear of the disc, giving lower wear overall for the treated surfaces. The coefficient of friction was found to be 0.35 for the virgin PTFE sample and 0.30 following carbon treatment. Therefore, the wear results seem to contradict the friction measurement; however, greater wear may be
A control Cu deposition was carried out without the PIII power supply operating. A film of 50 nm thickness was deposited, followed by a further deposition with the PIII operating (two-step process). Films were also deposited onto the substrate with the PIII operating throughout the deposition process. The PIII treatment in either procedure improved the adhesion compared to the control deposition; this was tested using Scotch娃 tape to observe the level of adhesion of the copper film to the substrate. The best adhesion was found using the two-step process. The presence of the copper layer enables implantation to occur more readily without the build up of surface charge. The implantation depth is also limited by the presence of the copper layer, but interface mixing will still occur and this is likely to promote adhesion. Fig. 6 shows the XPS spectra in the vicinity of the carbon peak for the virgin and two copper treated samples. The peak corresponding to the CF2 bond within the polymer backbone at approximately 293 eV is almost completely masked by the Cu film. The peak at approximately 284.5 eV occurs in all the spectra corresponds to C–C bonding. There are also weak peaks between 285 and 293, which may be due to CF3 and CF indicating a break-up of the polymer chains. The XPS spectrum of copper (Fig. 7) shows a main peak at 933
Fig. 7. XPS spectra in the vicinity of the copper peaks for (a) PTFE sample treated with 5 kV Cu ions at 100 Hz pulsing frequency 10ms pulse width for 10 min and (b) PTFE sample treated with 5 kV Cu ions at 200 Hz pulsing frequency 10-ms pulse width for 12 min.
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eV, which can be attributed to copper metal. The shoulder on the high-energy side is due to a peak at approximately 935 eV which we attribute to surface hydroxide groups. There was no evidence for Cu–C bonding which has been observed previously at approximately 931–932 eV w18x. The wear track and pin are shown in Fig. 5e and f. The wear track is similar to that observed in the case of C irradiation, which again indicates that the surface has been hardened by the treatment. The pin shows more wear than either the virgin or C implanted sample. This increase in pin wear is due to the presence of a copper film on the surface of the polymer. 4. Conclusion We have used a combined cathodic and PIII system to modify the surface of PTFE with C and Cu plasmas. We found for both species that significant increases in wear resistance can be achieved. In the case of Cu, this combined method resulted in well-adhered films, shown with adhesive tape test. There was no evidence for Cu–C bonding. Raman spectroscopy of the carbon modified samples showed the presence of an amorphous carbon peak, which remained even after cleaning the surface to remove any loosely bound carbon. This shows that both implantation and deposition of the carbon occur. It was found that the use of a metal grid on the surface of the polymer improves the efficiency of plasma treatment by minimising charge build up. Acknowledgments We would like to thank N. Fujisawa of Ventracor Pty. Ltd. for use of the wear test apparatus, also S.H.N. Lim and M. Glenn of RMIT and T.W. Oates of Sydney University for their contributions to this study. We also
acknowledge the support of the Australian Research Council (ARC). References w1x A. Anders, Surf. Coat. Technol. 156 (2002) 3–12. w2x A. Anders, Surf. Coat. Technol. 93 (1997) 158–167. w3x M. Ikeyama, Y. Hayakawa, M. Tasawa, et al., Thin Sold Films 281–282 (1996) 529–532. w4x L. Zhang, K. Takata, T. Yasui, H. Tahara, T. Yoshikawa, Mater. Chem. Phys. 54 (1998) 98–101. w5x N. Inagaki, S. Tasaka, K. Narushima, K. Teranishi, J. Appl. Polym. Sci. 83 (2002) 340–348. w6x T.W.H. Oates, D.R. McKenzie, M.M.M. Bilek, Surf. Coat. Technol. 156 (2002) 332–337. w7x A. Oshima, Y. Tabata, H. Kdoh, T. Seguchi, Radiat. Phys. Chem. 45 (1995) 269–275. w8x Y. Tabata, S. Ikeda, A. Oshima, Nucl. Instrum. Meth. Phys. Res. B 185 (2001) 169–174. w9x A. Oshima, A. Udagawa, Y. Morita, Radiat. Phys. Chem. 62 (2001) 39–45. w10x U. Lappan, U. Geißler, L. Haußler, ¨ D. Jenichen, G. Pompe, K. Lunkwitz, Nucl. Instrum. Meth. Phys. Res. B 185 (2001) 178–183. w11x J.P. Badey, E. Espuche, Y. Jugnet, B. Chabert, T.M. Duc, Int. J. Adhes. Adhes. 16 (1996) 173–178. w12x Y. Fujinami, H. Hayashi, A. Ebe, O. Imai, K. Ogata, Mater. Chem. Phys. 54 (1998) 102–105. w13x G. Mesyats, Y. Klyachkin, N. Gavrilov, A. Kondyurin, Vacuum 52 (1999) 285–289. w14x L. Guzman, B.Y. Man, A. Miotello, M. Adami, P.M. Ossi, Thin Solid Films 420–421 (2002) 565–570. w15x S.H.N. Lim, D.G. McCulloch, S. Russo, M.M.M. Bilek, D.R. McKenzie, Nucl. Instrum. Meth. Phys. Res. B 190 (2002) 723–727. w16x N. Fujisawa, N.L. James, R.N. Tarrant, D.R. McKenzie, J.D. Woodard, Swain, Wear 254 (2003) 111–119. w17x B.H. Stuart, B.J. Briscoe, Spectrochim. Acta, Part A 50 (1994) 2005–2009. w18x D. Popovici, J.E. Klemberg-Sapieha, G. Czeremuszkin, E. Sacher, M. Meunier, L. Martinu, Microelectron. Eng. 33 (1997) 217–221.
Plasma immersion ion implantation of poly(tetrafluoroethylene) T.L. Schillera,b,*, D. Sheejac, D.R. McKenzied, D.G. McCullocha, D.S.P. Lauc, S. Burnb, B.K. Tayc a
Department of Applied Physics, RMIT University, G.P.O. Box 2476V, Melbourne 3001, Australia b CSIRO, Manufacturing and Infrastructure Technology, Highett, Vic. 3019, Australia c Nanyang Technological University, Singapore, Singapore d School of Physics (A28), University of Sydney, Sydney, NSW 2006, Australia
Abstract Plasma immersion ion implantation (PIII) has been used with a filtered cathodic arc to implant copper and carbon ions into poly(tetrafluoroethylene) (PTFE). The PTFE substrates for the copper implantation were placed perpendicular to the plasma beam, whilst those for carbon implantation were oriented parallel to the drift velocity of the beam to minimise the deposition of low energy ions. Electrodes in the form of a backing plate and a mask with holes were used to apply the pulsed bias from the PIII supply. X-ray photoelectron spectroscopy has shown that there is a structural change in the PTFE induced by both the copper and carbon implantation. Raman spectroscopy of the carbon implanted samples showed the presence of an amorphous carbon peak, which remained even after cleaning the surface to remove loosely bound carbon. This shows that there is both implantation and deposition of the carbon occurring. In the case of copper, this method resulted in well-adhered films. The implanted PTFE has been examined for changes in wear resistance. Both copper and carbon modified surfaces showed improvements in wear resistance. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: PTFE; Surface modification; Ion implantation; Cathodic arc; Wear
1. Introduction Many polymers are attractive as materials for biomedical applications as they combine a high strength to density ratio and ease of formability with a low surface reactivity and a good biocompatibility. However, polymers show a higher wear rate than other candidate materials, such as metals and ceramics. Poly(tetrafluoroethylene) (PTFE) has excellent thermal and chemical properties, a low coefficient of friction and a low dielectric constant. Hence, PTFE has been used extensively as an encapsulation material for components in harsh environments. However, the inherent chemical inertness of PTFE makes chemical surface modification difficult and makes it unsuitable for metallisation because of its poor adhesion qualities. In this study, plasma immersion ion implantation (PIII) w1,2x and plasma deposition were applied to PTFE to study the effect of ion modification on the surface *Corresponding author. Tel.: q61-3-9252-6603; fax: q61-3-92526253. E-mail address: [email protected] (T.L. Schiller).
structure and wear characteristics. Two basic types of modification were investigated, using a filtered cathodic arc plasma as a source of ions. The first involved the modification of PTFE using PIII with a C plasma. This allowed the investigation of the effects on the structure and properties of PTFE without introducing a different chemical species. The second type of modification involved the use of a Cu plasma with an initial deposition. In this case we were particularly interested in investigating the possibility of producing Cu films onto the PTFE surfaces with good adhesion. Plasma processing and surface modification of polymers using ion beams has received significant attention w3–6x. Typically, energetic ions modify the polymer by causing chain scission, which reduces crystallinity w7,8x. It is known that ion modification results in cross-linking in many polymers w7–10x. However, there have been few studies of the effects of ion irradiation on PTFE. It has been found that ion beam modification improves the adhesion properties of PTFE w11–14x, and it has been shown that both the increase in the surface roughness and degradation of the polymer chain are attributed
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0257-8972(03)00916-2
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T.L. Schiller et al. / Surface and Coatings Technology 177 – 178 (2004) 483–488
Fig. 1. Schematic diagram of the cathodic arc deposition system with the PIII attached.
to the improvement. In this work, we investigate both changes in structure and mechanical properties of PTFE following plasma treatment. 2. Experimental PTFE sheets of thickness 0.5, 1.0, 3.0 and 4.0 mm were cut into 40=40-mm2 samples. They were then placed in an ultrasonic bath for 10 min with ethanol then acetone for 10 min. They were then dried in an oven at 100 8C in atmospheric pressure for 2 h, to remove excess moisture. Two similar sets of filtered cathodic arc equipment fitted with PIII, one at Nanyang Technological University, Singapore, and the other at the University of Sydney were used in this work. The apparatus employed in this project incorporates both filtered cathodic arc and PIII. A schematic of the Sydney University apparatus is shown in Fig. 1. The cathodic arc deposition system consists of a cathode, at one end of the chamber, connected using a duct with the substrate holder at the other. The PIII power supply is attached to the substrate holder as indicated by Fig. 1. A high negative pulse bias is applied to the substrate at a voltage of up to 15 kV while it is immersed in plasma. In PIII, during each pulse, a plasma sheath is formed, which accelerates the plasma ions towards the substrate. The PIII process can achieve implantation into the substrate causing interface mixing, as well as a modification of the material deposited onto the substrate. The walls of the duct are surrounded by a series of coils which when supplied with a current act together as a curved magnetic solenoid
filter for reducing the macroparticle content of the plasma. The magnetic filter duct has several power sources for controlling the currents in the coils to give control of the magnetic field generated w15x. The pulse generator has the capability of producing pulses of up to 30 kV, with durations of 10–60 ms and a repetition rate of up to 1 kHz. In the case of carbon implantation, a graphite target cathode was employed and specimens were prepared with PIII voltages of either 10 or 15 kV with a pulse width of 10 ms and a frequency of 500 Hz for periods of 3, 5 and 10 min. The substrate was attached to a metal substrate holder and positioned parallel to the drift velocity of the plasma beam at the exit of the filter duct. This parallel configuration was used for carbon implantation, to minimise deposition on the surface. The effect of placing a metal mesh over the substrate and connected to the holder was also examined with the aim of improving implantation efficiently by preventing charge build-up on the polymer surface. In the case of copper implantation, a Cu target cathode was used with a PIII voltage of 5 kV with a frequency of 100 and 200 Hz and the same pulse length as was used for carbon. A perpendicular configuration of the specimen holder relative to the plasma beam was used in this case to give both implantation and deposition. The adhesion of copper films deposited onto the PTFE substrates was determined by testing with adhesive tape. Visible Raman spectroscopy, with 514 nm line of Arq laser as the excitation source, was used to study the microstructure of the films. The laser was focused onto 2–3-mm diameter, with 20-mW output power. X-ray photoelectron spectroscopy (XPS) was performed on a VG Microlab 310F with a dual AlyMg anode unmonochromated X-ray source operated at a power of 300 W and 15 kV excitation voltage. The sample was tilted such that the electron analyzer normal to the sample surface collected the escaping electrons. The analyzed area is determined by the electrostatic lens and slits of the analyzer and in this case was approximately a rectangular area of 5=1 mm2. All survey spectra were acquired in a constant analyzer energy (CAE) mode at a pass energy of 100 eV at 1 eV steps between data points. All single element scans were performed in CAE mode at a pass energy of 20 and 0.1 eV steps. Charge correction was done by assuming that the adventitious carbon peak present in all samples was located at 284.5 eV. Wear was assessed in this study by using a specially designed pin-on-disk apparatus w16x in which the specimen is rotated above a fixed PTFE pin creating a circular wear track. The cylindrical pins (6-mm diameter) were made from commercial PTFE. One end of the pin was machine-finished to a 24.8-mm radius tip. A brushless DC-servomotor (3556K024B, Minimotor SA, Croglio, Switzerland) was used to revolve the specimen.
T.L. Schiller et al. / Surface and Coatings Technology 177 – 178 (2004) 483–488
Fig. 2. Raman spectra of (a) virgin PTFE; (b) PTFE sample treated with 15 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 3 min; (c) PTFE sample treated with 15 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 10 min and (d) PTFE sample treated with 10 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 15 min with the sample surface covered with a steel grid. In each of the treated samples, the peaks corresponding to virgin sample have been subtracted.
The rotational speed was approximately 2000 rpm. This was controlled by a four-quadrant pulse-width modulation servo amplifier (BLD 5606-SH4P, Minimotor SA, Croglio). A 50-mm long titanium extension shaft transmitted the motor shaft revolution to the disk via a cylindrical sample holder, which was screw-attached to the end of the extension shaft in axial alignment. The method also provides a measurement of the friction coefficient of the samples.
485
Fig. 3. XPS spectra in the vicinity of the carbon peaks for (a) PTFE; (b) PTFE sample treated with 15 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 10 min and (c) PTFE sample treated with 10 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 10 min.
characteristic of amorphous carbon. This peak is more prominent in the 10-min sample shown in Fig. 2c. A similar spectrum was obtained for the sample, which was covered with the steel grid (Fig. 2d). The presence of the grid was found to reduce the occurrence of arcing across the specimen due to charge build-up on the polymer surface.
3. Results and discussion 3.1. Carbon treatment The structural changes near the surface of the samples that had been subjected to modification by the carbon plasma were investigated using XPS and Raman spectroscopy. Prior to Raman and XPS analysis, the specimens were thoroughly cleaned in order to remove loosely bound carbon from the surface. Fig. 2a–d show the Raman spectra of the virgin, and two of the C treated samples and one sample that was covered with a steel grid. The virgin spectrum contains sharp peaks, which are characteristic of PTFE w17x. In each of the treated samples, the spectra have been processed to remove most of the peaks corresponding to the virgin sample by a simple subtraction. The resulting spectrum of the sample treated for 3 min (Fig. 2b) shows a broad asymmetric peak at approximately 1500 cmy1, which is
Fig. 4. XPS spectra in the vicinity of the fluorine peaks for (a) virgin PTFE; (b) PTFE sample treated with 15 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 10 min and (c) PTFE sample treated with 10 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 10 min.
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Fig. 3 shows the XPS spectra for the virgin (Fig. 3a) and two carbon treated samples (Fig. 3b and c). The peak corresponding to the CF2 bond within the polymer backbone is clearly seen at approximately 293 eV and decreases in intensity by approximately 80% as the exposure time increases. The peak at approximately
284.5 eV, which occurs in all the spectra, corresponds to C–C bonding. This peak could be due to either adventitious carbon or the build up of carbon in the surface. Given that the intensity of this peak increases with treatment time, this increase in intensity can be attributed to the build of carbon due to implantation.
Fig. 5. Wear track (a) and corresponding pin (b) for virgin PTFE, wear track (c) and corresponding pin (d) for PTFE sample treated with 15 kV C ions at 500 Hz pulsing frequency 10-ms pulse width for 10 min, wear track (e) and corresponding pin (f) PTFE sample treated with 10 kV Cu ions at 600 Hz pulsing frequency 10-ms pulse width for 10 min.
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attributed to the increase in hardness of the polymer surface, or an increase in the surface roughness. Further studies need to be carried out to determine if the increase in wear resistance is due to the hardening of the PTFE surface. 3.2. Cu deposition and implantation
Fig. 6. XPS spectra in the vicinity of the carbon peaks for (a) virgin PTFE; (b) PTFE sample treated with 5 kV Cu ions at 200 Hz pulsing frequency 10-ms pulse width for 12 min and (c) PTFE sample treated with 5 kV Cu ions at 100 Hz pulsing frequency 10-ms pulse width for 10 min.
There were also weak peaks between 285 and 290 eV in the modified samples due to CF3 and CF, which indicates a break-up of the polymer chains and the creation of branching shorter chain molecules. In all three samples, the dominant fluorine peak (Fig. 4) occurs at 690 eV, which corresponds to CF2 bonding and this reduces in intensity with exposure time. A small peak was also observed at approximately 686 eV after exposure, which can be seen as a skewness of the peak at lower binding energies. This corresponds to other fluorine bonding arrangements (CF3 and CF). All three samples also showed some oxygen at a level, which increased with exposure time to the plasma. This may be due to trace oxygen contamination of the plasma. The wear testing results for the virgin (a and b) and one of the carbon treated (c and d) are shown in Fig. 5. Shown in each case is a typical section of the wear track on the sample and the tip of the pin used in the test. The wear track width decreases from approximately 2 mm in the case of the virgin sample to 1.25 mm for the C modified sample. This indicates that there is an increase in wear resistance probably due to an increase in surface hardness caused by the irradiation. In the case of the pins, which are untreated in all cases, slightly more wear is observed following modification, this may be compensated for by the reduction in wear of the disc, giving lower wear overall for the treated surfaces. The coefficient of friction was found to be 0.35 for the virgin PTFE sample and 0.30 following carbon treatment. Therefore, the wear results seem to contradict the friction measurement; however, greater wear may be
A control Cu deposition was carried out without the PIII power supply operating. A film of 50 nm thickness was deposited, followed by a further deposition with the PIII operating (two-step process). Films were also deposited onto the substrate with the PIII operating throughout the deposition process. The PIII treatment in either procedure improved the adhesion compared to the control deposition; this was tested using Scotch娃 tape to observe the level of adhesion of the copper film to the substrate. The best adhesion was found using the two-step process. The presence of the copper layer enables implantation to occur more readily without the build up of surface charge. The implantation depth is also limited by the presence of the copper layer, but interface mixing will still occur and this is likely to promote adhesion. Fig. 6 shows the XPS spectra in the vicinity of the carbon peak for the virgin and two copper treated samples. The peak corresponding to the CF2 bond within the polymer backbone at approximately 293 eV is almost completely masked by the Cu film. The peak at approximately 284.5 eV occurs in all the spectra corresponds to C–C bonding. There are also weak peaks between 285 and 293, which may be due to CF3 and CF indicating a break-up of the polymer chains. The XPS spectrum of copper (Fig. 7) shows a main peak at 933
Fig. 7. XPS spectra in the vicinity of the copper peaks for (a) PTFE sample treated with 5 kV Cu ions at 100 Hz pulsing frequency 10ms pulse width for 10 min and (b) PTFE sample treated with 5 kV Cu ions at 200 Hz pulsing frequency 10-ms pulse width for 12 min.
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eV, which can be attributed to copper metal. The shoulder on the high-energy side is due to a peak at approximately 935 eV which we attribute to surface hydroxide groups. There was no evidence for Cu–C bonding which has been observed previously at approximately 931–932 eV w18x. The wear track and pin are shown in Fig. 5e and f. The wear track is similar to that observed in the case of C irradiation, which again indicates that the surface has been hardened by the treatment. The pin shows more wear than either the virgin or C implanted sample. This increase in pin wear is due to the presence of a copper film on the surface of the polymer. 4. Conclusion We have used a combined cathodic and PIII system to modify the surface of PTFE with C and Cu plasmas. We found for both species that significant increases in wear resistance can be achieved. In the case of Cu, this combined method resulted in well-adhered films, shown with adhesive tape test. There was no evidence for Cu–C bonding. Raman spectroscopy of the carbon modified samples showed the presence of an amorphous carbon peak, which remained even after cleaning the surface to remove any loosely bound carbon. This shows that both implantation and deposition of the carbon occur. It was found that the use of a metal grid on the surface of the polymer improves the efficiency of plasma treatment by minimising charge build up. Acknowledgments We would like to thank N. Fujisawa of Ventracor Pty. Ltd. for use of the wear test apparatus, also S.H.N. Lim and M. Glenn of RMIT and T.W. Oates of Sydney University for their contributions to this study. We also
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