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Laser transmission joint between PET and titanium for biomedical application
Journal of Materials Processing Technology 210 (2010) 1767–1771
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Laser transmission joint between PET and titanium for biomedical application Xiao Wang, Pin Li, Zhenkai Xu, Xinhua Song, Huixia Liu ∗ School of Mechanical Engineering, Jiangsu University, Xuefo Road 301, Zhenjiang 212013, China
a r t i c l e
i n f o
Article history: Received 27 January 2010 Received in revised form 28 April 2010 Accepted 8 June 2010
Keywords: Laser transmission joint Implantable device Bond strength SEM/EDS XPS
a b s t r a c t Laser transmission joint between biocompatible, dissimilar materials have the potential for application in biomedical and their encapsulation process. This process may involve photochemical reaction, and alter the chemical compositions of the interface and chemical bonds form at the interface. Understanding the laser joint at material interfaces is essential for the advancement in the laser joining application. This paper is devoted to laser transmission joint between 0.1 mm thick PET films and 0.1 mm thick Titanium. We have found processing conditions for successful joining of titanium with PET using near-infrared diode lasers. Laser joint samples were tested in microtester under tensile loading to determine joint strengths. The joint strength was found to be 65.46–90 MPa. The PET/titanium interfaces thus obtained were studied by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy coupled with energy dispersive spectroscopy (SEM/EDS) and microscopy techniques. The results give evidence for the formation of Ti–C chemical bonds in a sharp interfacial region between the two sides. These chemical bonds are believed to be responsible for the observed mechanical strength of the joints. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the biomedical sector, new implantable microsystems have the potential to significantly impact the future treatment of disease such as pacemakers, cochlea implants, neural stimulation, and implantable drug delivery systems. These implants including functional elements such as electrodes need to be joint (encapsulated) with biocompatible materials, similar or dissimilar in nature to adapt different environments in the human body, as be pointed out by Herfurth et al. (2004). The encapsulation material must meet specific biocompatibility requirements. The group of metal materials includes titanium, platinum, gold and stainless. Non-metals play an even more important role based on their biocompatibility include glass, sapphire, silicon and polymers, as be pointed out by Bauer et al. (2004). Conventional joining techniques such as the use of adhesives have several drawbacks such as they often lack long-term stability, shrink during curing, and sometimes do not meet biocompatibility requirements by Sultana (2008). High heat input during soldering or brazing may potentially damage the implant electronics that are being packaged. These limitations can be overcome by laser transmission joining technique. Recently, Newaz et al. (2008) and Georgiev et al. (2009a) have found successful joining between polymer and titanium using this technique. The feasibility of laser transmission joining polyimide to titanium and polyimide to Ti-
∗ Corresponding author. Tel.: +86 511 88797998; fax: +86 511 88780276. E-mail address: [email protected] (H. Liu). 0924-0136/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2010.06.007
coated borosilicate glass has been investigated by Mian et al. (2005) and Georgiev et al. (2005), and the finite element modeling of the transmission joining of titanium and polyimide has been developed by Mahmood et al. (2007) and Dhorajiya et al. (2010). This process may involve photochemical reaction, alter the chemical compositions and chemical bonds form at the interface were also performed by Sultana et al. (2008) and Georgiev et al. (2009b). Understanding the laser joint at material interface is essential for the advancement in the laser joining application. This paper is devoted to laser transmission joining between 0.1 mm thick polyethylene terephthalate (PET) film and 0.1 mm thick titanium foil using near-infrared diode laser. PET is known to be transparent and the transparency of PET is about 90% in the near-infrared region as be pointed out by Katayama and Kawahito (2008). Laser joint samples were tested in microtester and observed by microscopy techniques. The PET/titanium interfaces thus obtained were studied by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy coupled with energy dispersive spectroscopy. 2. Samples preparation and fabrication Polyethylene terephthalate (PET) film 0.1 mm thick was laserjoined to 0.1 mm Ti foil (99.6% purity) by means of continuous wave radiation from a near-infrared diode laser (Compact 130/140, Dilas Corporation) emitting a maximum power of 130 W, wavelength 980 ± 10 nm, minimum spot size 800 m. It is mentioned here that the diode laser had super-Gaussian type intensity distributions. Both of the samples have the same dimension of
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Fig. 1. Schematic of sample dimensions.
Fig. 4. Schematic diagram of a sample with multiple joint lines for XPS experiment.
as the interface detachment or any burning mark was found in the sample. Further investigation was conducted to determine a suitable parameter window for laser joining between PET and titanium. For a power range between 3.5 and 6 W, joint samples are achieved at scanning speeds ranging from 15 to 400 mm/min. 3. Experimental methods of sample characterization
Fig. 2. Schematic diagram of laser transmission micro-joining technique.
W 10 mm × L 30 mm × H 0.1 mm, the size and configuration of the prepared PET/Ti sample is shown in Fig. 1. The surface of titanium and PET was ultrasonically cleaned with acetone and then dried with hot gas before joining. No additional surface treatment was performed on them. A 3-axis laser motion system that manipulates the samples under the stationary beam was used to prepare the samples. This system provides a repetitive positioning accuracy of 0.01 mm. The laser transmission micro-joining process schematically shown in Fig. 2 was utilized in this case to create bonds between titanium and PET film. The ideal material combination to be joined using this process includes one absorbent (in this case, titanium) and transparent (in this case, PET) part, as be described by Mian et al. (2006) and Liu et al. (2009). The laser energy penetrates the transparent part and is absorbed by the absorbing part, so that the heat is induced directly at the interface. The temperature at the interface exceeds the melting point of PET and is sufficient to create chemical bonds between titanium and PET. A laser beam was first focused on the titanium surface at the interface, and then it is defocused by the desired distance to get a spot with a diameter of 900 m. The defocusing of the beam controls the joint width and reduces overheating and burning of the polymer by lowering the intensity in the center of the laser spot. A clamping pressure of 1 MPa was used in the joining procedure to hold the sample between two K9 glass plates (6 mm thick). Fig. 3 shows the picture of laser joint sample using a laser power of 5 W and a scanning speed of 100 mm/min. No observable damage, such
The samples were experimentally characterized by mechanical tensile testing, optical microscopy, SEM/EDS and XPS. A brief description of the characterization procedure is given below. 3.1. Mechanical tensile tests Tensile testing is performed using a multi-axis microtester which is general-purpose micromechanical testing instrument, fully controlled by a PC computer. The samples were loaded in the fixture of the testing machine and were subjected to uniaxial tension (in-plane) by turning on the motor that moves in direction perpendicular to the laser joint. The samples were tested at a stage speed of 0.05 mm/s. The software collects the load displacement date. The failure loads obtained for all samples were normalized by the corresponding measured bond lengths to account for any joint length variations. 3.2. Microscopy Six samples were studied by optical microscopy to study the morphologies in the joint areas using model XTZ-FG (Shanghai Optical Instruments Factory) optical microscopy. 3.3. SEM/EDS SEM images and EDS spectra of the tensile test separated Ti side of the PET/Ti joint sample was obtained in JEOL (Japan) JSM-7001F thermal field emission SEM and Oxford (British) INCA ENERGY 350 X-ray EDS. To avoid charging effects the part was sputter-coated with a thin Au layer before taking the SEM images. 3.4. XPS
Fig. 3. The joint sample using a laser power of 5 W and a scanning speed of 100 mm/min.
In order to study the chemical bond formation during the process of laser joining, a PET/Ti sample of the type shown in Fig. 4 was prepared at laser power of 5 W and laser speed of 100 mm/min for XPS measurements. The sample was separated into its PET and Ti parts by peeling in air and the titanium side was loaded immediately in the UHV analysis chamber of a Kratos Axis Ultra DLD X-ray photoelectron spectroscopy with a Monochromated Al K␣ X-ray source, 15 kV, 10 mA, wide 160 eV. A relatively large analysis area was used that averaged over several joint lines and inter-joint regions following the procedure outline in Georgiev et al. (2004). This approach was found to provide a good signal-to-noise ratio,
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Fig. 5. Test data for laser transmission joint PET to titanium: (a) the effect of laser power on bond strength and (b) the effect of scanning speed on bond strength.
Fig. 6. Optical micrographs of six samples.
and it insured that the signal is origination from a known area of the peeled joint.
4. Results and discussion 4.1. Microscopy and mechanical tensile test results All of the fabricated samples using different parameter settings were loaded in tension until failure. The corresponding joint lengths were measured prior to test the samples to calculate the failure bond strength. In Fig. 5a the effect of laser power on bond strength is shown and demonstrates that the bond strength increases with lager powers unless the power is larger than 5 W. It is believed that the PET may have been degenerated or burnt at the high intensity power. In Fig. 5b the effect of scanning speed on bond strength is shown and demonstrates that the bond strength decreases with higher scanning speeds unless the scanning speed is lower than 50 mm/min. When the joint samples were prepared using higher scanning speeds, the laser had less time to interact with the titanium surface that may have resulted in poor chemical bonds between titanium and PET. It is thus clear that the laser power and scanning speed play an important role in attaining highest possible bond strength. At a constant scanning speed of 80 mm/min, good bonds are achieved for a reasonably wide laser power range. The optical micrographs of six samples, which were fabricated with a constant scanning speed of 80 mm/min and different power levels between 3.5 and 6 W, are shown in Fig. 6 and demonstrate that when the
joint samples were prepared using larger laser power, the PET have been degenerated or burnt at the joint area. For the investigated parameter ranges, power levels between 3.5 and 5.5 W and speeds up to 400 mm/min, the maximum bond strength of 90 MPa is achieved using 4.5 W and a scanning speed of 80 mm/min. As shown in the process parameter diagram in Fig. 7, good bonds are achieved for a reasonably wide speed range at a constant power level. Therefore, the joint process appears to
Fig. 7. The process parameter diagram for laser transmission joint PET to titanium.
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Fig. 8. SEM image and EDS spectrum taken from the bond area of the titanium side.
be very forgiving of small deviations from the fixed parameter settings. 4.2. SEM/EDS results Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy was used to establish the interface in which the chemical bond formation occurred during the laser joining process. The titanium side of the peel tested PET/Ti joint sample was studied. In Fig. 8 SEM images and the EDS spectrum taken from the bond area of the titanium side of the peel tested PET/Ti joint sample is shown. The SEM images show that the titanium side of the laser joint contains the interface and the PET residue. The EDS spectrum which is taken from the small square area bordered with white lines indicates the presence of Ti, C and O. These results showed that the chemical bond formation between the carbon and titanium might be possible in the bond area. 4.3. XPS results XPS studies on the PET/Ti interface were conducted to further investigate the chemistry of the joint formation and research the established bond mechanism. Both low-resolution (survey) spectra and high-resolution (multiplex) spectra were collected from the titanium part of the sample. A pass-energy of 40 eV was used in the case of the high-resolution spectra. The binging energy scales were adjusted using the C1s line at 284.8 eV. In Fig. 9 the low-resolution (survey) spectrum taken from the titanium part is shown. The spectrum shows the presence of oxygen and carbon on the titanium surface. Fig. 10 shows a schematic diagram of the titanium side of a PET/Ti sample after peeling off the
Fig. 9. Low-resolution (survey) XPS spectrum taken from the Ti side of the sample.
Fig. 10. Schematic diagram of the titanium side of a PET/Ti sample: (a) metallic Ti; (b) layer of native TiO2 ; (c) layer containing Ti–C bonds formed during the laser joining; (d) layer of PET residue.
polymer side. On the titanium side of the PET/Ti sample, regions with a layer of PET polymer residue (Fig. 10d) were found. In Fig. 11 C1s spectra taken from the Ti side of the laser joint (no sputtering) are shown. The spectrum can be resolved into five peaks with some at relatively high binding energy indicating environment. The peak at 284.8 eV is most likely due to C–C bond. The peak at 287.4 and 289.6 eV are likely due to carbonyl groups that form part of the PET chain as depicted in Fig. 10d. The new weak peak at 281.7 eV is very close to the reported carbon line in Ti–C by Ramqvist et al. (1969), and it is therefore justified to conclude that the interface contains certain amount of Ti–C bonds as depicted in Fig. 10c. In Fig. 12 Ti2p spectrum taken from the Ti side of the laser joint (no sputtering) are shown. A Ti2p peak binding energy of 454.9 eV might be corresponding to Ti–C. It is not easy to discriminate the Ti–O from the Ti–C contribution in spectrum Ti2p as pointed out by Luthin et al. (2001).
Fig. 11. C1s XPS spectra taken from the Ti side of the PET/Ti sample.
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Acknowledgments We would like to acknowledge the assistance of associate professor Kangmin Chen in the SEM/EDS measurements. We also acknowledge the assistance of Physical & Chemical analysis section of Shanghai institute of measurement and testing technology in the XPS measurements. References
Fig. 12. Ti2p XPS spectra taken from the Ti side of the PET/Ti sample.
These results of the Figs. 8, 11 and 12 might give evidence for the formation of Ti–C type of chemical bonds between the PET polymer and Ti foil which is believed to be responsible for the observed mechanical strength of the joints. 5. Conclusions Laser joints between PET and titanium were fabricated by a novel laser joint technique. The bond strength of the laser bonds was measured. The results show that the laser power and scanning speed play an important role in attaining highest possible bond strength. The optical micrographs of six joint samples have acquired using optical microscopy and demonstrate that when the joint samples were prepared using larger laser power, the PET have been degenerated or burnt at the joint area. Good bonds are achieved for a reasonably wide speed range at a constant power level. The fabricated samples were studied using scanning electron microscopy coupled with energy dispersive spectroscopy. Chemical bond formation was studied by examination of the interface with XPS. The results might give evidence for the formation of Ti–C type chemical bonds which is believed to be responsible for the observed bond strength.
Dhorajiya, A.P., Mayeed, M.S., Auner, G.W., et al., 2010. Finite element thermal mechanical analysis of transmission laser microjoining of titanium and polyimide. Journal of Engineering Materials and Technology 132, 0110041–011004-9. Georgiev, D.G., Bairda, R.J., Newaz, G., et al., 2004. An XPS study of laser-fabricated polyimide titanium interfaces. Applied Surface Science 236, 71–76. Georgiev, D.G., Sultana, T., Mian, A., et al., 2005. Laser fabrication and characterization of sub-millimeter joints between polyimide and Ti-coated borosilicate glass. Journal of Materials Science 40 (21), 5641–5647. Georgiev, G.L., Sultana, T., Baird, R.J., et al., 2009a. Laser bonding and characterization of Kapton FN Ti and Teflon FEP Ti system. J Mater Sci 44, 882–888. Georgiev, G.L., Baird, R.J., McCullen, E.F., et al., 2009b. Chemical bond formation during laser bonding of Teflon® FEP and titanium. Applied Surface Science 255, 7078–7083. Herfurth, H.J., Witte, R., Heinemann, S., et al., 2004. Joining challenges in the packaging of BIOMEMS. In: Proceedings of the 23rd International Congress on Applications of Lasers and Electro-Optics. Bauer, I., Russek, U.A., Herfurth, H.J., et al., 2004. Laser micro-joining of dissimilar and biocompatible materials. Process of SPIE, Bellingham 5339, 454–464. Katayama, S., Kawahito, Y., 2008. Laser direct joining of metal and plastic. Scripta Materialia 59, 1250–1274. Liu, H., Li, P., Xing, A., et al., 2009. Laser transmission welding of thermoplastic polyurethane films. Chinese Journal of Lasers 36 (Suppl. 1), 156–160 (in Chinese). Luthin, J., Plank, H., Roth, L., et al., 2001. Ion beam-induced carbide formation at the titanium–carbon interface. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 182, 216–218. Mahmood, T., Misn, A., Amin, M.R., et al., 2007. Finite element modeling of transmission laser microjoining process. Journal of Materials Processing Technology 186, 37–44. Mian, A., Newaz, G., Vendra, L., et al., 2005. Laser bonded microjoints between titanium and polyimide for applications in medical implants. Journal of Materials Science: Materials in Medicine 16 (3), 229–237. Mian, A., Mahmood, T., Auner, G., et al., 2006. Effects of laser parameters on the mechanical response of laser irradiated micro-joints. Materials Research Society 926 (4). Newaz, G., Sultana, T., Nusier, S., et al., 2008. Miniaturized samples for bond strength and hermetic sealing evaluation for transmission laser joints. Journal of Laser Micro/Nanoengineering 3 (3), 186–195. Ramqvist, L., Hamrin, K., Johansson, G., et al., 1969. Charge transfer in transition metal carbides and related compounds studied by ESCA. Journal of Physics and Chemistry of Solids 30, 1835–1847. Sultana, T., 2008. Bond quality and failure mode assessment for polymer–metal and polymer–glass transmission laser joints, Wayne State University Dissertation. Sultana, T., Grigor, L., Greg, A., et al., 2008. XPS analysis of laser transmission microjoint between poly (vinylidene fluoride) and titanium. Applied Surface Science 255, 2569–2573.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Laser transmission joint between PET and titanium for biomedical application Xiao Wang, Pin Li, Zhenkai Xu, Xinhua Song, Huixia Liu ∗ School of Mechanical Engineering, Jiangsu University, Xuefo Road 301, Zhenjiang 212013, China
a r t i c l e
i n f o
Article history: Received 27 January 2010 Received in revised form 28 April 2010 Accepted 8 June 2010
Keywords: Laser transmission joint Implantable device Bond strength SEM/EDS XPS
a b s t r a c t Laser transmission joint between biocompatible, dissimilar materials have the potential for application in biomedical and their encapsulation process. This process may involve photochemical reaction, and alter the chemical compositions of the interface and chemical bonds form at the interface. Understanding the laser joint at material interfaces is essential for the advancement in the laser joining application. This paper is devoted to laser transmission joint between 0.1 mm thick PET films and 0.1 mm thick Titanium. We have found processing conditions for successful joining of titanium with PET using near-infrared diode lasers. Laser joint samples were tested in microtester under tensile loading to determine joint strengths. The joint strength was found to be 65.46–90 MPa. The PET/titanium interfaces thus obtained were studied by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy coupled with energy dispersive spectroscopy (SEM/EDS) and microscopy techniques. The results give evidence for the formation of Ti–C chemical bonds in a sharp interfacial region between the two sides. These chemical bonds are believed to be responsible for the observed mechanical strength of the joints. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the biomedical sector, new implantable microsystems have the potential to significantly impact the future treatment of disease such as pacemakers, cochlea implants, neural stimulation, and implantable drug delivery systems. These implants including functional elements such as electrodes need to be joint (encapsulated) with biocompatible materials, similar or dissimilar in nature to adapt different environments in the human body, as be pointed out by Herfurth et al. (2004). The encapsulation material must meet specific biocompatibility requirements. The group of metal materials includes titanium, platinum, gold and stainless. Non-metals play an even more important role based on their biocompatibility include glass, sapphire, silicon and polymers, as be pointed out by Bauer et al. (2004). Conventional joining techniques such as the use of adhesives have several drawbacks such as they often lack long-term stability, shrink during curing, and sometimes do not meet biocompatibility requirements by Sultana (2008). High heat input during soldering or brazing may potentially damage the implant electronics that are being packaged. These limitations can be overcome by laser transmission joining technique. Recently, Newaz et al. (2008) and Georgiev et al. (2009a) have found successful joining between polymer and titanium using this technique. The feasibility of laser transmission joining polyimide to titanium and polyimide to Ti-
∗ Corresponding author. Tel.: +86 511 88797998; fax: +86 511 88780276. E-mail address: [email protected] (H. Liu). 0924-0136/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2010.06.007
coated borosilicate glass has been investigated by Mian et al. (2005) and Georgiev et al. (2005), and the finite element modeling of the transmission joining of titanium and polyimide has been developed by Mahmood et al. (2007) and Dhorajiya et al. (2010). This process may involve photochemical reaction, alter the chemical compositions and chemical bonds form at the interface were also performed by Sultana et al. (2008) and Georgiev et al. (2009b). Understanding the laser joint at material interface is essential for the advancement in the laser joining application. This paper is devoted to laser transmission joining between 0.1 mm thick polyethylene terephthalate (PET) film and 0.1 mm thick titanium foil using near-infrared diode laser. PET is known to be transparent and the transparency of PET is about 90% in the near-infrared region as be pointed out by Katayama and Kawahito (2008). Laser joint samples were tested in microtester and observed by microscopy techniques. The PET/titanium interfaces thus obtained were studied by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy coupled with energy dispersive spectroscopy. 2. Samples preparation and fabrication Polyethylene terephthalate (PET) film 0.1 mm thick was laserjoined to 0.1 mm Ti foil (99.6% purity) by means of continuous wave radiation from a near-infrared diode laser (Compact 130/140, Dilas Corporation) emitting a maximum power of 130 W, wavelength 980 ± 10 nm, minimum spot size 800 m. It is mentioned here that the diode laser had super-Gaussian type intensity distributions. Both of the samples have the same dimension of
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Fig. 1. Schematic of sample dimensions.
Fig. 4. Schematic diagram of a sample with multiple joint lines for XPS experiment.
as the interface detachment or any burning mark was found in the sample. Further investigation was conducted to determine a suitable parameter window for laser joining between PET and titanium. For a power range between 3.5 and 6 W, joint samples are achieved at scanning speeds ranging from 15 to 400 mm/min. 3. Experimental methods of sample characterization
Fig. 2. Schematic diagram of laser transmission micro-joining technique.
W 10 mm × L 30 mm × H 0.1 mm, the size and configuration of the prepared PET/Ti sample is shown in Fig. 1. The surface of titanium and PET was ultrasonically cleaned with acetone and then dried with hot gas before joining. No additional surface treatment was performed on them. A 3-axis laser motion system that manipulates the samples under the stationary beam was used to prepare the samples. This system provides a repetitive positioning accuracy of 0.01 mm. The laser transmission micro-joining process schematically shown in Fig. 2 was utilized in this case to create bonds between titanium and PET film. The ideal material combination to be joined using this process includes one absorbent (in this case, titanium) and transparent (in this case, PET) part, as be described by Mian et al. (2006) and Liu et al. (2009). The laser energy penetrates the transparent part and is absorbed by the absorbing part, so that the heat is induced directly at the interface. The temperature at the interface exceeds the melting point of PET and is sufficient to create chemical bonds between titanium and PET. A laser beam was first focused on the titanium surface at the interface, and then it is defocused by the desired distance to get a spot with a diameter of 900 m. The defocusing of the beam controls the joint width and reduces overheating and burning of the polymer by lowering the intensity in the center of the laser spot. A clamping pressure of 1 MPa was used in the joining procedure to hold the sample between two K9 glass plates (6 mm thick). Fig. 3 shows the picture of laser joint sample using a laser power of 5 W and a scanning speed of 100 mm/min. No observable damage, such
The samples were experimentally characterized by mechanical tensile testing, optical microscopy, SEM/EDS and XPS. A brief description of the characterization procedure is given below. 3.1. Mechanical tensile tests Tensile testing is performed using a multi-axis microtester which is general-purpose micromechanical testing instrument, fully controlled by a PC computer. The samples were loaded in the fixture of the testing machine and were subjected to uniaxial tension (in-plane) by turning on the motor that moves in direction perpendicular to the laser joint. The samples were tested at a stage speed of 0.05 mm/s. The software collects the load displacement date. The failure loads obtained for all samples were normalized by the corresponding measured bond lengths to account for any joint length variations. 3.2. Microscopy Six samples were studied by optical microscopy to study the morphologies in the joint areas using model XTZ-FG (Shanghai Optical Instruments Factory) optical microscopy. 3.3. SEM/EDS SEM images and EDS spectra of the tensile test separated Ti side of the PET/Ti joint sample was obtained in JEOL (Japan) JSM-7001F thermal field emission SEM and Oxford (British) INCA ENERGY 350 X-ray EDS. To avoid charging effects the part was sputter-coated with a thin Au layer before taking the SEM images. 3.4. XPS
Fig. 3. The joint sample using a laser power of 5 W and a scanning speed of 100 mm/min.
In order to study the chemical bond formation during the process of laser joining, a PET/Ti sample of the type shown in Fig. 4 was prepared at laser power of 5 W and laser speed of 100 mm/min for XPS measurements. The sample was separated into its PET and Ti parts by peeling in air and the titanium side was loaded immediately in the UHV analysis chamber of a Kratos Axis Ultra DLD X-ray photoelectron spectroscopy with a Monochromated Al K␣ X-ray source, 15 kV, 10 mA, wide 160 eV. A relatively large analysis area was used that averaged over several joint lines and inter-joint regions following the procedure outline in Georgiev et al. (2004). This approach was found to provide a good signal-to-noise ratio,
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Fig. 5. Test data for laser transmission joint PET to titanium: (a) the effect of laser power on bond strength and (b) the effect of scanning speed on bond strength.
Fig. 6. Optical micrographs of six samples.
and it insured that the signal is origination from a known area of the peeled joint.
4. Results and discussion 4.1. Microscopy and mechanical tensile test results All of the fabricated samples using different parameter settings were loaded in tension until failure. The corresponding joint lengths were measured prior to test the samples to calculate the failure bond strength. In Fig. 5a the effect of laser power on bond strength is shown and demonstrates that the bond strength increases with lager powers unless the power is larger than 5 W. It is believed that the PET may have been degenerated or burnt at the high intensity power. In Fig. 5b the effect of scanning speed on bond strength is shown and demonstrates that the bond strength decreases with higher scanning speeds unless the scanning speed is lower than 50 mm/min. When the joint samples were prepared using higher scanning speeds, the laser had less time to interact with the titanium surface that may have resulted in poor chemical bonds between titanium and PET. It is thus clear that the laser power and scanning speed play an important role in attaining highest possible bond strength. At a constant scanning speed of 80 mm/min, good bonds are achieved for a reasonably wide laser power range. The optical micrographs of six samples, which were fabricated with a constant scanning speed of 80 mm/min and different power levels between 3.5 and 6 W, are shown in Fig. 6 and demonstrate that when the
joint samples were prepared using larger laser power, the PET have been degenerated or burnt at the joint area. For the investigated parameter ranges, power levels between 3.5 and 5.5 W and speeds up to 400 mm/min, the maximum bond strength of 90 MPa is achieved using 4.5 W and a scanning speed of 80 mm/min. As shown in the process parameter diagram in Fig. 7, good bonds are achieved for a reasonably wide speed range at a constant power level. Therefore, the joint process appears to
Fig. 7. The process parameter diagram for laser transmission joint PET to titanium.
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Fig. 8. SEM image and EDS spectrum taken from the bond area of the titanium side.
be very forgiving of small deviations from the fixed parameter settings. 4.2. SEM/EDS results Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy was used to establish the interface in which the chemical bond formation occurred during the laser joining process. The titanium side of the peel tested PET/Ti joint sample was studied. In Fig. 8 SEM images and the EDS spectrum taken from the bond area of the titanium side of the peel tested PET/Ti joint sample is shown. The SEM images show that the titanium side of the laser joint contains the interface and the PET residue. The EDS spectrum which is taken from the small square area bordered with white lines indicates the presence of Ti, C and O. These results showed that the chemical bond formation between the carbon and titanium might be possible in the bond area. 4.3. XPS results XPS studies on the PET/Ti interface were conducted to further investigate the chemistry of the joint formation and research the established bond mechanism. Both low-resolution (survey) spectra and high-resolution (multiplex) spectra were collected from the titanium part of the sample. A pass-energy of 40 eV was used in the case of the high-resolution spectra. The binging energy scales were adjusted using the C1s line at 284.8 eV. In Fig. 9 the low-resolution (survey) spectrum taken from the titanium part is shown. The spectrum shows the presence of oxygen and carbon on the titanium surface. Fig. 10 shows a schematic diagram of the titanium side of a PET/Ti sample after peeling off the
Fig. 9. Low-resolution (survey) XPS spectrum taken from the Ti side of the sample.
Fig. 10. Schematic diagram of the titanium side of a PET/Ti sample: (a) metallic Ti; (b) layer of native TiO2 ; (c) layer containing Ti–C bonds formed during the laser joining; (d) layer of PET residue.
polymer side. On the titanium side of the PET/Ti sample, regions with a layer of PET polymer residue (Fig. 10d) were found. In Fig. 11 C1s spectra taken from the Ti side of the laser joint (no sputtering) are shown. The spectrum can be resolved into five peaks with some at relatively high binding energy indicating environment. The peak at 284.8 eV is most likely due to C–C bond. The peak at 287.4 and 289.6 eV are likely due to carbonyl groups that form part of the PET chain as depicted in Fig. 10d. The new weak peak at 281.7 eV is very close to the reported carbon line in Ti–C by Ramqvist et al. (1969), and it is therefore justified to conclude that the interface contains certain amount of Ti–C bonds as depicted in Fig. 10c. In Fig. 12 Ti2p spectrum taken from the Ti side of the laser joint (no sputtering) are shown. A Ti2p peak binding energy of 454.9 eV might be corresponding to Ti–C. It is not easy to discriminate the Ti–O from the Ti–C contribution in spectrum Ti2p as pointed out by Luthin et al. (2001).
Fig. 11. C1s XPS spectra taken from the Ti side of the PET/Ti sample.
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Acknowledgments We would like to acknowledge the assistance of associate professor Kangmin Chen in the SEM/EDS measurements. We also acknowledge the assistance of Physical & Chemical analysis section of Shanghai institute of measurement and testing technology in the XPS measurements. References
Fig. 12. Ti2p XPS spectra taken from the Ti side of the PET/Ti sample.
These results of the Figs. 8, 11 and 12 might give evidence for the formation of Ti–C type of chemical bonds between the PET polymer and Ti foil which is believed to be responsible for the observed mechanical strength of the joints. 5. Conclusions Laser joints between PET and titanium were fabricated by a novel laser joint technique. The bond strength of the laser bonds was measured. The results show that the laser power and scanning speed play an important role in attaining highest possible bond strength. The optical micrographs of six joint samples have acquired using optical microscopy and demonstrate that when the joint samples were prepared using larger laser power, the PET have been degenerated or burnt at the joint area. Good bonds are achieved for a reasonably wide speed range at a constant power level. The fabricated samples were studied using scanning electron microscopy coupled with energy dispersive spectroscopy. Chemical bond formation was studied by examination of the interface with XPS. The results might give evidence for the formation of Ti–C type chemical bonds which is believed to be responsible for the observed bond strength.
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