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Investigation of interaction mechanisms in laser drilling of thermoplastic and thermoset polymers using different wavelengths
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 66 (2006) 1289–1296 www.elsevier.com/locate/compscitech
Investigation of interaction mechanisms in laser drilling of thermoplastic and thermoset polymers using different wavelengths O. Yalukova *, I. Sa´rady Lulea˚ University of Technology, Department of Applied Physics and Mechanical Engineering, SE-971 87 Lulea˚, Sweden Received 4 August 2005; accepted 6 November 2005 Available online 22 December 2005
Abstract In this paper a comparison of the percussion drilled holes in fiber reinforced polymer and non-reinforced thermoplastic sheets using three wavelengths, 1064, 532 and 266 nm is presented. At near infrared and visible wavelengths, 1064 and 532 nm, most of the bulk thermoplastic and thermoset polymers are partially transparent. The degree of transparency is dependent upon the degree of crystallinity. The most amorphous thermoplastics, such as PMMA have the highest transparency. In comparison, the fibre reinforced thermosets – graphite fibre reinforced epoxy – have low transparency. If the IR or visible laser beam is strongly focused, it can cause thermally induced degradation and sometimes burning and other damages (cracking, boiling, etc.) around the focused laser spot and the interaction zone. By using the fourth harmonics ultraviolet light (266 nm) of a diode-pumped and AOQ-switched Nd:YAG-laser with pulse duration of about 100 ns, the hole drilling process was significantly improved and the risk for thermal damage has been significantly reduced for both materials. Around the irradiated spot or drilled holes the material is influenced. This was investigated by both scanning electron microscopy and optical microscopy. There was clear evidence of a change in the interaction mechanism if UV light with high enough photon energy was used. By using UV light, bond breaking rather than thermal material removal occurred, i.e., a change from thermal to photo-chemical dissociation or photoablation becomes the dominant interaction mechanism. 2005 Elsevier Ltd. All rights reserved. Keywords: E. Laser drilling; Ultraviolet; Visible; Near infrared beam; Thermoplastic and thermoset polymers; D. Optical microscopy; D. Scanning electron microscopy (SEM); A. Polymers
1. Introduction When using short wavelengths and short pulse duration, the photon/material interaction will occur at a shallow absorption depth. To be able to create small features and to drill small holes with good quality, it is preferable to have short pulse duration, typically in the order of 100 ns or less. This will give pulses with high peak power (above 5 kW). The dominant physical process involved in laser drilling can be thermal or photo-chemical, depending mainly on the wavelength of the laser beam. Shorter wavelengths (in *
Corresponding author. Tel.: +46920491771; fax: +46920492228. E-mail address: [email protected] (O. Yalukova).
0266-3538/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.11.002
the UV range) have higher photon energy and lead to photo-chemical reactions. Longer wavelengths in the infrared range have lower photon energy leading to thermal reactions. There are primarily two processes that occur in the UV laser/material interaction, i.e., photo-chemical ablation and photo-thermal ablation. In polymers, the mechanism of UV laser/material interaction can be explained as follows: the absorbed UV photons directly break the chemical bonds in a so called photo-chemical reaction. Thermal ablation, which also can be used in case of some thermoplastics, is the other possible mechanism. There is a threshold in photon energy or critical wavelength required to achieve bond breaking: The average
O. Yalukova, I. Sa´rady / Composites Science and Technology 66 (2006) 1289–1296
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dipole < or hydrogen bonds), with energies between 0.2 and 20 kJ/mol. The function of these secondary bonds is to keep the material in solid state (If the secondary bonds are removed, a thermoplastic polymer will become a highly viscous liquid). The degree of transparency is dependent upon the degree of crystallinity. The most amorphous polymers, such as polymethyl methacrylate, PMMA, are the most transparent. Polycarbonate, PC is often partially crystalline; therefore, its transparency is lower than for PMMA. The thermoplastic polymers are often influenced by or are soluble in some solvents (e.g., acetone). Thermosets or network polymers consist of densely crosslinked networks, where the chain segments between crosslinks have low mobility. The chain molecules are linked together by strong, covalent bonds even between the chains, forming a continuous network. Such a network can neither be dissolved nor become molten. The reason for this is that the covalent bonds prevent chain segments from slipping past each other [4]. Therefore, thermosets are difficult to drill or cut using infrared lasers, without severe carbonisation or pyrolysis, where the matrix is heavily damaged. The only reasonable possibility is if the wavelength of the laser is in the ultraviolet with corresponding photon energy higher than the binding energy of the covalent bonds.
binding energy for one C–C bond is about 347 kJ/mol and for one C–H bond is about 414 kJ/mol. These values correspond to photon energies between 3.6014 and 4.2967 eV, or, in the wavelength domain, between 344 and 288 nm. To generate these wavelengths, excimer lasers, frequency converted copper vapour lasers (CVL) or frequency converted diode pumped solid state lasers (DPSSL) can be used [1]. All of these have their advantages and disadvantages. The excimers can generate the shortest wavelengths and pulse durations, but have in general poor beam quality and are often restricted to be used in combination with mask projection. This is often combined with low pulse repetition frequency and therefore, slower machining or material removal rates. Several polymers are under investigation of interaction with excimer laser radiation, were observed in Ref. [2]. The CVLs have excellent beam quality and therefore are focusable to diffraction limited spot sizes. They can be readily frequency doubled and wavelengths of 255 and 289 nm can be generated. However, both excimers and CVLs have high investment and operating cost and require a high degree of maintenance. Therefore, such lasers are often limited to use in highly specialised companies and laboratories. For general use, the DPSSL in combination with frequency conversion based on non-linear optical crystals promise a lower costs and reliable alternative. The main advantage of DPSSLs is their need of minimum of maintenance. The disadvantage of the DPSSL is that the pulse duration is often in the range of 100 ns. Therefore, even in the UV it is hard to avoid some thermal effects affecting the processed material. To completely avoid thermal influences, the pulse duration should be in the sub-nanosecond range [3]. In the present paper two kinds of polymers, thermoplastics and thermosets were used as target materials. To perform percussion drilling of above targets the fundamental wavelength, 1064 nm, the second harmonic, 532 nm and the fourth harmonic, 266 nm wavelengths were used.
3. Experimental set up and procedure 3.1. The laser used The laser which was used for the experiments is a special version of a ROFIN RSY 200D/SHG continuously diodepumped Nd:YAG one, with a dual-rod, symmetric resonator and birefringence compensation. This laser can deliver a continuous IR beam at 1064 nm with a maximum power of about 200 W in multimode. Interchangeable apertures can be used to change the beam quality from multimode (M2 = 12) to near Gaussian or low-order mode (M2 = 1.5–3), depending upon the inserted mode aperture and diode pumping current. Using the smallest apertures, the power is restricted to about 20 W, and the laser operates in the low order, close to TEM00 mode. The laser is normally pulsed using double acousto-optical Q-switches, emitting pulses with a duration of about 100 ns and a repetition rate of 0.1–20 kHz. To generate the second harmonics at 532 nm, intra-cavity frequency doubling is used, with a constant-temperature
2. The target materials Polymer materials are classified in two major categories, thermoplastics and thermosets (or network polymers). The thermoplastic polymers consist of individual chain molecules [4]. The monomers forming the chains are bonded with strong covalent bonds 40–500 kJ/mol. The bonds between the chains are of a weaker type (van der Waals <
Table 1 Specification of the average power and pulse energies used in the experiments k (nm)
Pulse energy (mJ)
Average power (W)
Spot diameter (lm)
Irradiance (kW/cm2)
Photon energy (eV)
IR: 1064 Green: 532 UV: 266
4.8 2.8 0.5
4.8 2.8 0.5
60 40 20
170 223 159
1.165 2.331 4.662
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Fig. 1. Set up for the experiments with the 1064 nm (IR) and the 532 nm (green) beam. 1 – diode-pumped Nd:YAG laser with acousto-optical Qswitching and with an intracavity second harmonic generation crystal (SHG) (two separate beam paths can be selected), 2 – beam expanding telescope, 3 – the target, M1, M2 – mirrors, HR for both wavelengths, L – air spaced achromatic lens, f = 100 mm, AR/AR for the respective wavelength.
(25 C) angle-tuned LBO crystal (7 · 7 · 20 mm). The direction of polarisation for this beam is vertical and the average power in multimode is in excess of 80 W. At low order or close to TEM00 mode, using the smallest apertures, the power is limited to about 10 W. To generate the fourth harmonic UV beam at 266 nm, extra-cavity frequency doubling of the low order, 532 nm,
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beam with a constant-temperature (25 C) angle-tuned BBO crystal (5 · 5 · 7 mm) is used. The maximum average UV power which could be generated with this crystal using the 10 W green beam as source is about 2.5 W. In all cases, the average power was measured using a Spectra-Physics 407A power meter (max range 30 W). The diode pumping power (at a current level of 39 A), the mode limiting apertures and the repetition frequency of 1 kHz were kept constant for all three wavelengths. A pulse frequency of 1 kHz was chosen to provide a narrow pulse shape (pulse duration of 100 ns). At this repetition rate, the duty cycle of the laser will be extremely low (A 0.1 ls pulse duration gives a 999.9 ls pause between the pulses), which will lower the thermal load on the target. Details of the average power and pulse energies used in the experiments are given in Table 1. The laser drilling was first performed at k = 1064 nm. The experimental set up is shown in Fig. 1. The light from the Nd:YAG laser (1) is guided via mirrors M1 and M2, through the beam expanding telescope (2). The beam was focused by an air-spaced achromatic lens L with focal length f = 100 mm to a spot size of about 60 lm. The IR beam was circularly polarised, due to a phase retardation plate of k/4, inserted in the beam path. The set up used to perform experiments at 532 nm was the same as for the 1064 nm IR, shown in Fig. 1. The beam is deflected by mirrors M1 and M2 (both are highly reflective for 532 nm). After mirror M2 the beam was expanded by a telescope (2) and then focused using an air-spaced achromatic lens L with focal length f = 100 mm to a spot size of about 40 lm on the front side of the target. The lens has an anti-reflective coating on both sides. The beam was also
Fig. 2. Set up for the experiments at 266 nm (UV). 1 – diode-pumped Nd:YAG laser with acousto-optical Q-switching and with intracavity second harmonic generation crystal (SHG), emitting 532 nm pulsed beam, 2 – extracavity fourth harmonic generation crystal (FHG), 3 – beam dumps, 4 – beam expanding telescope, (10·), 5 – the target, M1 – mirror, HR for 532 nm, M2, M3, M4 – harmonic separation mirrors (HT for 532 nm, HR for 266 nm), L1 – focusing lens, f = 250 mm, AR/AR for 266 and 532 nm, L2 – collimating lens, f = 250 mm, AR/AR for 266 and 532 nm, L3 – air spaced achromatic lens, f = 100 mm, AR/AR for 266 nm.
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circularly polarised by inserting another phase retardation plate of k/4. The set up used to perform experiments at 266 nm is shown in Fig. 2. The light from the Nd:YAG laser (1) with wavelength k = 532 nm, is guided via the mirror M1 (high reflective for 532 nm). The lens L1 (f = 250 mm) focuses the beam into the extracavity fourth harmonic generation crystal (FHG) (2), to get a high beam intensity for efficient conversion. The beam is collimated by lens L2 (f = 250 mm), reflected by M2, M3 and M4, harmonic separation mirrors, which are highly transmissive for 532 nm and highly reflective for 266 nm. The green light is then dumped in the beam dumps (3). The k = 266 nm beam passes the expanding telescope (4), and is finally focused by an air-spaced achromatic lens L3 (f = 100 mm) on the front side of the target (5). The spot size was of the order of 20 lm. The samples used in the experiments at all three wavelengths were clamped in a fixture, which was placed on
an x–y–z micro-positioning stage (13 · 13 · 13 mm). This made it possible to fine tune the focusing of the beam (xdirection) and to move the samples between individually irradiated spots in the y–z plane. Typically arrays of 10 · 10 spots were irradiated by the focused beams with a spacing between the spots of 0.5 and 1 mm.
Fig. 3. (a) A 2 mm thick PMMA sheet, irradiated with a focused beam at 1064 nm, by 10,000 pulses. (b) A 2 mm thick PC sheet irradiated with a focused beam at 1064 nm, by 10,000 pulses.
Fig. 4. (a) A 2 mm thick PMMA sheet, irradiated with a focused beam at 532 nm, by 2000 pulses. (b) A 2 mm thick PC sheet, drilled with a focused beam at 532 nm, by 2000 pulses.
4. Results 4.1. Thermoplastic polymers: PMMA and PC PMMA and PC are transparent or translucent in the near IR and the visible part of the spectrum. Therefore, no real hole drilling could be observed at these wavelengths: However, the strongly focused beam caused thermal deterioration – cracking and bubble forming/boiling – inside the materials. These defects could not be observed from the surface by scanning electron microscopy (SEM), but could be seen by using optical microscopy (OM) on polished, sectioned samples Fig. 3–5 (samples cut in the
O. Yalukova, I. Sa´rady / Composites Science and Technology 66 (2006) 1289–1296
Fig. 5. (a) A 2 mm thick PMMA sheet, drilled with a focused beam at 266 nm, by 5000 pulses. (b) A 2 mm thick PC sheet, drilled with a focused beam at 266 nm, by 5000 pulses.
x–y plane parallel to the irradiation). The number of pulses used for all the samples are listed in Table 2. The PMMA and PC samples, 2 mm thick sheets, were irradiated by about 10,000 pulses of the 1064 nm beam. The pulse energy was 4.8 mJ and the photon energy 1.165 eV. The total energy input is about 48 J, over a period of about 10 s. The integrated irradiation time was about 1 ms. In the case of PMMA (Fig. 3(a)) most of the energy is transmitted through the sample, but as a result of the irradiation, crack formation, some sub-surface inho-
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Fig. 6. Glass fibre reinforced epoxy composite (1.5 mm thick) irradiated with a focused beam at 1064 nm, by 3000 pulses: (a) entrance side, (b) exit side.
mogeneities and some thermal damage occurred. In the case of PC (Fig. 3(b)), significant thermal influence and bubble formation can be seen. During processing, burning of the front surface could be observed. At 532 nm about 2000 pulses were needed to influence the samples. The pulse energy used was 2.8 mJ and total energy input is 5.6 J, over about 2 s. The integrated time of irradiation was only 0.2 ms. The PMMA sheet (Fig. 4(a)) could not be drilled, but some inhomogenities and thermal degradation could be
Table 2 The number of pulses used for all the samples Material/thickness (mm)
PC, 2 PMMA, 2 Glass fibre reinforced epoxy composite, 1.5 Graphite fibre reinforced epoxy composite, 1.1
Number of pulses/wavelength 1064 nm
532 nm
266 nm
10 000 10 000 3000 1500
2000 2000 1000 1000
5000 5000 10 000 10 000
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observed. This indicates that most of the energy transmitted through the sample. The PC samples (Fig. 4(b)) could be drilled using the 532 nm beam (2000 pulses); the thermal damage which can be observed was much less than for the 1064 nm irradiated samples. Irradiation of polymers using UV light with energy density greater than 10 J/cm2 leads to photo-ablation. The absorption is strongly dependent on the applied wavelength. In this case, the energy density was in excess of 150 J/cm2. Both PMMA and PC sheets could be drilled using the 266 nm UV beam (Fig. 5(a) and (b)), resulting in drilled holes of excellent quality. To create a hole, some 5000 pulses of 0.5 mJ each were needed, which corresponds to a total energy input of 2.5 J, over about 5 s. The integrated time of irradiation/hole was 0.5 ms. PMMA and PC consist of monomer chains. The main forces holding the monomer units together are the carbon–carbon bonds along the chains. The strength of a C– C bond is approximately 3.6 eV [5]. The photon energy of the UV beam is 4.662 eV, which enables it to break the covalent C–C and C–H bonds by photo-chemical disso-
The thermoset material used for the experiments was an epoxy, which can be influenced to a varying extent by infrared, visible and ultraviolet light. High-intensity infrared radiation can cause thermal degradation and ‘‘burning and carbonisation’’ or pyrolysis will always occur. When irradiated with green light, the thermal influence is still dominant. Ultraviolet light with a wavelength of 266 nm is capable of breaking the C–C and C–H covalent bonds, so the thermal effects decrease significantly. Two types of reinforced epoxy composites used for the experiments, glass fibre reinforced and carbon fibre reinforced, with a thickness of 1.5 and 1.1 mm respectively. The presence of the reinforcing fibres makes laser processing much more complicated. The fibres will conduct heat in
Fig. 7. Graphite fibre reinforced epoxy composite (1.1 mm thick) irradiated with a focused beam at 1064 nm, by 1500 pulses: (a) entrance side, (b) exit side.
Fig. 8. Glass fibre reinforced epoxy composite (1.5 mm thick) irradiated with a focused beam at 532 nm, by 1000 pulses: (a) entrance side, (b) exit side.
ciation. Therefore, a minimum of thermal damage could be observed around the holes, which confirms the change in interaction mechanism: the so-called ‘‘cold ablation’’ will occur. 4.2. Thermoset polymers
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the direction of their axes which will damage the epoxy matrix. The thermal deterioration is especially pronounced when the target is irradiated with the infrared beam and decreases with decreasing wavelength. Both fibre reinforced epoxy composites were irradiated using a focused 1064 nm beam. The pulse energy was 4.8 mJ and the photon energy was 1.165 eV. For the glass fibre reinforced epoxy, the total number of pulses needed to create a hole was around 3000, with an energy input of 14.4 J over 3 s. The integrated irradiation time was 0.3 ms. In Fig. 6(a), and (b) the entrance and exit sides of glass fibre reinforced epoxy composite samples irradiated with the IR beam are shown. Burning of the epoxy and thermal damage (breaking) of the glass fibres can be observed. In Fig. 7(a) and (b), entrance and exit sides of graphite fibre reinforced epoxy composite samples irradiated by 1500 pulses are shown. The total energy input is 7.2 J, over a period of 1.5 s. The integrated time of irradiation was 0.15 ms. Burning of the epoxy and thermal damage (breaking) of the graphite fibres can be observed.
Fig. 10. Glass fibre reinforced epoxy composite (1.5 mm thick) irradiated with a focused beam at 266 nm, by 10,000 pulses: (a) entrance side, (b) exit side.
Fig. 9. Graphite fibre reinforced epoxy composite (1.1 mm thick) irradiated with a focused beam at 532 nm, by 1000 pulses: (a) entrance side, (b) exit side.
When irradiating both of the fibre reinforced composites with focused 532 nm beam, 1000 pulses were needed to create a drilled-through hole. The thermal influence was significantly reduced, there was no burning but some crack formation in the epoxy matrix could be observed. The epoxy matrix is quite crack sensitive when influenced by thermal and mechanical stresses. Figs. 8 and 9 show glass and graphite fibre reinforced epoxy composites, respectively: (a) entrance side; (b) exit side. The pulse energy was 2.8 mJ and the total energy input 2.8 J over 1 s. The integrated irradiation time was 0.1 ms. Results of irradiating both thermoset polymers using 10 000 pulses of a focused UV beam at 266 nm are presented in Fig. 10 and Fig. 11, respectively; (a) entrance side, (b) exit side. The pulse energy was 0.5 mJ, the total energy input is 5 J over 10 s. The integrated irradiation time was 1 ms. The quality of the drilled holes was significantly improved, which confirms that the UV beam can photo chemically decompose the epoxy matrix and break both glass and graphite fibres. The thermal influence around the drilled holes is minimised.
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too low and can only lead to thermal degradation of thermoplastic polymers. The 266 nm beam has a photon energy of 4.662 eV, which is higher than the binding energies of the C–C and C–H covalent bonds and therefore photochemical bond breaking will be possible. In the case of fibre reinforced epoxy, the thermal degradation of the matrix and fibres was severe both at the infrared and green wavelengths. The thermal damage effects are more pronounced for the carbon fibre reinforced composite than for the glass fibre reinforced epoxy, because the carbon fibres conduct heat much better than glass fibres. The strong absorption of UV light in the glass and graphite fibres will cause the fibre to break. At the same time, the C–C and C–H covalent bonds which bind the epoxy matrix can be photo chemically dissociated. This will result in quite clean drilled holes for both types of fibre reinforced composites sheets. The results achieved indicate that using even higher average UV power at 266 nm and higher repetition frequency contour cutting of epoxy and polyester polymer sheets reinforced with different fibres would be possible. Higher power outputs could be achieved using a longer BBO crystal, increasing the average power of the present laser in the UV up to 10 W. This power level should also be enough to cut polymer sheets of up to 2 mm thickness. References
Fig. 11. Graphite fibre reinforced epoxy composite (1.1 mm thick) irradiated with a focused beam at 266 nm, by 10,000 pulses: (a) entrance side, (b) exit side.
5. Conclusions Three different wavelengths, 1064, 532 and 266 nm, were used to investigate the interaction of laser beams with thermoplastic and fibre-reinforced thermoset (epoxy) polymers. The experiments show clearly that hole drilling in the thermoplastic PMMA and PC can only be performed using the UV beam. The photon energy at 1064 and 532 nm is
[1] Glover ACJ, Coutts DW, Ramsay DJ, Karabaic DF, Brown DJW, Piper JA. The frequency-doubled copper vapour laser – an alternative source for high repetition rate micro-machining. In: Proceedings of the ICALEOÕ93, vol. 77, Orlando, FL; 1993. p. A-253. [2] Breuer J, Metev S, Sepold, G. Photolytical interaction of excimer laser radiation with polymers. In: Proceedings of industrial and microelectronics applications EUROPTOÕ94. Vienna, Austria, SPIE vol. 2207, p. 566–576. [3] Schaeffer RD. Micro-machining using UV lasers. Laser solutions, course #1. In: Proceedings of the ICALEOÕ03, Jacksonville, FL; 2003. [4] Berglund LA. Introduction to polymer engineering, Compendium. Lulea˚ University of Technology; 1997. [5] Garrison BJ, Srinivasan R. Microscopic model for the ablative photodecomposition of polymers by far-ultraviolet radiation (193 nm). Appl Phys Lett 1984;44:849–51.
Investigation of interaction mechanisms in laser drilling of thermoplastic and thermoset polymers using different wavelengths O. Yalukova *, I. Sa´rady Lulea˚ University of Technology, Department of Applied Physics and Mechanical Engineering, SE-971 87 Lulea˚, Sweden Received 4 August 2005; accepted 6 November 2005 Available online 22 December 2005
Abstract In this paper a comparison of the percussion drilled holes in fiber reinforced polymer and non-reinforced thermoplastic sheets using three wavelengths, 1064, 532 and 266 nm is presented. At near infrared and visible wavelengths, 1064 and 532 nm, most of the bulk thermoplastic and thermoset polymers are partially transparent. The degree of transparency is dependent upon the degree of crystallinity. The most amorphous thermoplastics, such as PMMA have the highest transparency. In comparison, the fibre reinforced thermosets – graphite fibre reinforced epoxy – have low transparency. If the IR or visible laser beam is strongly focused, it can cause thermally induced degradation and sometimes burning and other damages (cracking, boiling, etc.) around the focused laser spot and the interaction zone. By using the fourth harmonics ultraviolet light (266 nm) of a diode-pumped and AOQ-switched Nd:YAG-laser with pulse duration of about 100 ns, the hole drilling process was significantly improved and the risk for thermal damage has been significantly reduced for both materials. Around the irradiated spot or drilled holes the material is influenced. This was investigated by both scanning electron microscopy and optical microscopy. There was clear evidence of a change in the interaction mechanism if UV light with high enough photon energy was used. By using UV light, bond breaking rather than thermal material removal occurred, i.e., a change from thermal to photo-chemical dissociation or photoablation becomes the dominant interaction mechanism. 2005 Elsevier Ltd. All rights reserved. Keywords: E. Laser drilling; Ultraviolet; Visible; Near infrared beam; Thermoplastic and thermoset polymers; D. Optical microscopy; D. Scanning electron microscopy (SEM); A. Polymers
1. Introduction When using short wavelengths and short pulse duration, the photon/material interaction will occur at a shallow absorption depth. To be able to create small features and to drill small holes with good quality, it is preferable to have short pulse duration, typically in the order of 100 ns or less. This will give pulses with high peak power (above 5 kW). The dominant physical process involved in laser drilling can be thermal or photo-chemical, depending mainly on the wavelength of the laser beam. Shorter wavelengths (in *
Corresponding author. Tel.: +46920491771; fax: +46920492228. E-mail address: [email protected] (O. Yalukova).
0266-3538/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.11.002
the UV range) have higher photon energy and lead to photo-chemical reactions. Longer wavelengths in the infrared range have lower photon energy leading to thermal reactions. There are primarily two processes that occur in the UV laser/material interaction, i.e., photo-chemical ablation and photo-thermal ablation. In polymers, the mechanism of UV laser/material interaction can be explained as follows: the absorbed UV photons directly break the chemical bonds in a so called photo-chemical reaction. Thermal ablation, which also can be used in case of some thermoplastics, is the other possible mechanism. There is a threshold in photon energy or critical wavelength required to achieve bond breaking: The average
O. Yalukova, I. Sa´rady / Composites Science and Technology 66 (2006) 1289–1296
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dipole < or hydrogen bonds), with energies between 0.2 and 20 kJ/mol. The function of these secondary bonds is to keep the material in solid state (If the secondary bonds are removed, a thermoplastic polymer will become a highly viscous liquid). The degree of transparency is dependent upon the degree of crystallinity. The most amorphous polymers, such as polymethyl methacrylate, PMMA, are the most transparent. Polycarbonate, PC is often partially crystalline; therefore, its transparency is lower than for PMMA. The thermoplastic polymers are often influenced by or are soluble in some solvents (e.g., acetone). Thermosets or network polymers consist of densely crosslinked networks, where the chain segments between crosslinks have low mobility. The chain molecules are linked together by strong, covalent bonds even between the chains, forming a continuous network. Such a network can neither be dissolved nor become molten. The reason for this is that the covalent bonds prevent chain segments from slipping past each other [4]. Therefore, thermosets are difficult to drill or cut using infrared lasers, without severe carbonisation or pyrolysis, where the matrix is heavily damaged. The only reasonable possibility is if the wavelength of the laser is in the ultraviolet with corresponding photon energy higher than the binding energy of the covalent bonds.
binding energy for one C–C bond is about 347 kJ/mol and for one C–H bond is about 414 kJ/mol. These values correspond to photon energies between 3.6014 and 4.2967 eV, or, in the wavelength domain, between 344 and 288 nm. To generate these wavelengths, excimer lasers, frequency converted copper vapour lasers (CVL) or frequency converted diode pumped solid state lasers (DPSSL) can be used [1]. All of these have their advantages and disadvantages. The excimers can generate the shortest wavelengths and pulse durations, but have in general poor beam quality and are often restricted to be used in combination with mask projection. This is often combined with low pulse repetition frequency and therefore, slower machining or material removal rates. Several polymers are under investigation of interaction with excimer laser radiation, were observed in Ref. [2]. The CVLs have excellent beam quality and therefore are focusable to diffraction limited spot sizes. They can be readily frequency doubled and wavelengths of 255 and 289 nm can be generated. However, both excimers and CVLs have high investment and operating cost and require a high degree of maintenance. Therefore, such lasers are often limited to use in highly specialised companies and laboratories. For general use, the DPSSL in combination with frequency conversion based on non-linear optical crystals promise a lower costs and reliable alternative. The main advantage of DPSSLs is their need of minimum of maintenance. The disadvantage of the DPSSL is that the pulse duration is often in the range of 100 ns. Therefore, even in the UV it is hard to avoid some thermal effects affecting the processed material. To completely avoid thermal influences, the pulse duration should be in the sub-nanosecond range [3]. In the present paper two kinds of polymers, thermoplastics and thermosets were used as target materials. To perform percussion drilling of above targets the fundamental wavelength, 1064 nm, the second harmonic, 532 nm and the fourth harmonic, 266 nm wavelengths were used.
3. Experimental set up and procedure 3.1. The laser used The laser which was used for the experiments is a special version of a ROFIN RSY 200D/SHG continuously diodepumped Nd:YAG one, with a dual-rod, symmetric resonator and birefringence compensation. This laser can deliver a continuous IR beam at 1064 nm with a maximum power of about 200 W in multimode. Interchangeable apertures can be used to change the beam quality from multimode (M2 = 12) to near Gaussian or low-order mode (M2 = 1.5–3), depending upon the inserted mode aperture and diode pumping current. Using the smallest apertures, the power is restricted to about 20 W, and the laser operates in the low order, close to TEM00 mode. The laser is normally pulsed using double acousto-optical Q-switches, emitting pulses with a duration of about 100 ns and a repetition rate of 0.1–20 kHz. To generate the second harmonics at 532 nm, intra-cavity frequency doubling is used, with a constant-temperature
2. The target materials Polymer materials are classified in two major categories, thermoplastics and thermosets (or network polymers). The thermoplastic polymers consist of individual chain molecules [4]. The monomers forming the chains are bonded with strong covalent bonds 40–500 kJ/mol. The bonds between the chains are of a weaker type (van der Waals <
Table 1 Specification of the average power and pulse energies used in the experiments k (nm)
Pulse energy (mJ)
Average power (W)
Spot diameter (lm)
Irradiance (kW/cm2)
Photon energy (eV)
IR: 1064 Green: 532 UV: 266
4.8 2.8 0.5
4.8 2.8 0.5
60 40 20
170 223 159
1.165 2.331 4.662
O. Yalukova, I. Sa´rady / Composites Science and Technology 66 (2006) 1289–1296
Fig. 1. Set up for the experiments with the 1064 nm (IR) and the 532 nm (green) beam. 1 – diode-pumped Nd:YAG laser with acousto-optical Qswitching and with an intracavity second harmonic generation crystal (SHG) (two separate beam paths can be selected), 2 – beam expanding telescope, 3 – the target, M1, M2 – mirrors, HR for both wavelengths, L – air spaced achromatic lens, f = 100 mm, AR/AR for the respective wavelength.
(25 C) angle-tuned LBO crystal (7 · 7 · 20 mm). The direction of polarisation for this beam is vertical and the average power in multimode is in excess of 80 W. At low order or close to TEM00 mode, using the smallest apertures, the power is limited to about 10 W. To generate the fourth harmonic UV beam at 266 nm, extra-cavity frequency doubling of the low order, 532 nm,
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beam with a constant-temperature (25 C) angle-tuned BBO crystal (5 · 5 · 7 mm) is used. The maximum average UV power which could be generated with this crystal using the 10 W green beam as source is about 2.5 W. In all cases, the average power was measured using a Spectra-Physics 407A power meter (max range 30 W). The diode pumping power (at a current level of 39 A), the mode limiting apertures and the repetition frequency of 1 kHz were kept constant for all three wavelengths. A pulse frequency of 1 kHz was chosen to provide a narrow pulse shape (pulse duration of 100 ns). At this repetition rate, the duty cycle of the laser will be extremely low (A 0.1 ls pulse duration gives a 999.9 ls pause between the pulses), which will lower the thermal load on the target. Details of the average power and pulse energies used in the experiments are given in Table 1. The laser drilling was first performed at k = 1064 nm. The experimental set up is shown in Fig. 1. The light from the Nd:YAG laser (1) is guided via mirrors M1 and M2, through the beam expanding telescope (2). The beam was focused by an air-spaced achromatic lens L with focal length f = 100 mm to a spot size of about 60 lm. The IR beam was circularly polarised, due to a phase retardation plate of k/4, inserted in the beam path. The set up used to perform experiments at 532 nm was the same as for the 1064 nm IR, shown in Fig. 1. The beam is deflected by mirrors M1 and M2 (both are highly reflective for 532 nm). After mirror M2 the beam was expanded by a telescope (2) and then focused using an air-spaced achromatic lens L with focal length f = 100 mm to a spot size of about 40 lm on the front side of the target. The lens has an anti-reflective coating on both sides. The beam was also
Fig. 2. Set up for the experiments at 266 nm (UV). 1 – diode-pumped Nd:YAG laser with acousto-optical Q-switching and with intracavity second harmonic generation crystal (SHG), emitting 532 nm pulsed beam, 2 – extracavity fourth harmonic generation crystal (FHG), 3 – beam dumps, 4 – beam expanding telescope, (10·), 5 – the target, M1 – mirror, HR for 532 nm, M2, M3, M4 – harmonic separation mirrors (HT for 532 nm, HR for 266 nm), L1 – focusing lens, f = 250 mm, AR/AR for 266 and 532 nm, L2 – collimating lens, f = 250 mm, AR/AR for 266 and 532 nm, L3 – air spaced achromatic lens, f = 100 mm, AR/AR for 266 nm.
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circularly polarised by inserting another phase retardation plate of k/4. The set up used to perform experiments at 266 nm is shown in Fig. 2. The light from the Nd:YAG laser (1) with wavelength k = 532 nm, is guided via the mirror M1 (high reflective for 532 nm). The lens L1 (f = 250 mm) focuses the beam into the extracavity fourth harmonic generation crystal (FHG) (2), to get a high beam intensity for efficient conversion. The beam is collimated by lens L2 (f = 250 mm), reflected by M2, M3 and M4, harmonic separation mirrors, which are highly transmissive for 532 nm and highly reflective for 266 nm. The green light is then dumped in the beam dumps (3). The k = 266 nm beam passes the expanding telescope (4), and is finally focused by an air-spaced achromatic lens L3 (f = 100 mm) on the front side of the target (5). The spot size was of the order of 20 lm. The samples used in the experiments at all three wavelengths were clamped in a fixture, which was placed on
an x–y–z micro-positioning stage (13 · 13 · 13 mm). This made it possible to fine tune the focusing of the beam (xdirection) and to move the samples between individually irradiated spots in the y–z plane. Typically arrays of 10 · 10 spots were irradiated by the focused beams with a spacing between the spots of 0.5 and 1 mm.
Fig. 3. (a) A 2 mm thick PMMA sheet, irradiated with a focused beam at 1064 nm, by 10,000 pulses. (b) A 2 mm thick PC sheet irradiated with a focused beam at 1064 nm, by 10,000 pulses.
Fig. 4. (a) A 2 mm thick PMMA sheet, irradiated with a focused beam at 532 nm, by 2000 pulses. (b) A 2 mm thick PC sheet, drilled with a focused beam at 532 nm, by 2000 pulses.
4. Results 4.1. Thermoplastic polymers: PMMA and PC PMMA and PC are transparent or translucent in the near IR and the visible part of the spectrum. Therefore, no real hole drilling could be observed at these wavelengths: However, the strongly focused beam caused thermal deterioration – cracking and bubble forming/boiling – inside the materials. These defects could not be observed from the surface by scanning electron microscopy (SEM), but could be seen by using optical microscopy (OM) on polished, sectioned samples Fig. 3–5 (samples cut in the
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Fig. 5. (a) A 2 mm thick PMMA sheet, drilled with a focused beam at 266 nm, by 5000 pulses. (b) A 2 mm thick PC sheet, drilled with a focused beam at 266 nm, by 5000 pulses.
x–y plane parallel to the irradiation). The number of pulses used for all the samples are listed in Table 2. The PMMA and PC samples, 2 mm thick sheets, were irradiated by about 10,000 pulses of the 1064 nm beam. The pulse energy was 4.8 mJ and the photon energy 1.165 eV. The total energy input is about 48 J, over a period of about 10 s. The integrated irradiation time was about 1 ms. In the case of PMMA (Fig. 3(a)) most of the energy is transmitted through the sample, but as a result of the irradiation, crack formation, some sub-surface inho-
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Fig. 6. Glass fibre reinforced epoxy composite (1.5 mm thick) irradiated with a focused beam at 1064 nm, by 3000 pulses: (a) entrance side, (b) exit side.
mogeneities and some thermal damage occurred. In the case of PC (Fig. 3(b)), significant thermal influence and bubble formation can be seen. During processing, burning of the front surface could be observed. At 532 nm about 2000 pulses were needed to influence the samples. The pulse energy used was 2.8 mJ and total energy input is 5.6 J, over about 2 s. The integrated time of irradiation was only 0.2 ms. The PMMA sheet (Fig. 4(a)) could not be drilled, but some inhomogenities and thermal degradation could be
Table 2 The number of pulses used for all the samples Material/thickness (mm)
PC, 2 PMMA, 2 Glass fibre reinforced epoxy composite, 1.5 Graphite fibre reinforced epoxy composite, 1.1
Number of pulses/wavelength 1064 nm
532 nm
266 nm
10 000 10 000 3000 1500
2000 2000 1000 1000
5000 5000 10 000 10 000
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observed. This indicates that most of the energy transmitted through the sample. The PC samples (Fig. 4(b)) could be drilled using the 532 nm beam (2000 pulses); the thermal damage which can be observed was much less than for the 1064 nm irradiated samples. Irradiation of polymers using UV light with energy density greater than 10 J/cm2 leads to photo-ablation. The absorption is strongly dependent on the applied wavelength. In this case, the energy density was in excess of 150 J/cm2. Both PMMA and PC sheets could be drilled using the 266 nm UV beam (Fig. 5(a) and (b)), resulting in drilled holes of excellent quality. To create a hole, some 5000 pulses of 0.5 mJ each were needed, which corresponds to a total energy input of 2.5 J, over about 5 s. The integrated time of irradiation/hole was 0.5 ms. PMMA and PC consist of monomer chains. The main forces holding the monomer units together are the carbon–carbon bonds along the chains. The strength of a C– C bond is approximately 3.6 eV [5]. The photon energy of the UV beam is 4.662 eV, which enables it to break the covalent C–C and C–H bonds by photo-chemical disso-
The thermoset material used for the experiments was an epoxy, which can be influenced to a varying extent by infrared, visible and ultraviolet light. High-intensity infrared radiation can cause thermal degradation and ‘‘burning and carbonisation’’ or pyrolysis will always occur. When irradiated with green light, the thermal influence is still dominant. Ultraviolet light with a wavelength of 266 nm is capable of breaking the C–C and C–H covalent bonds, so the thermal effects decrease significantly. Two types of reinforced epoxy composites used for the experiments, glass fibre reinforced and carbon fibre reinforced, with a thickness of 1.5 and 1.1 mm respectively. The presence of the reinforcing fibres makes laser processing much more complicated. The fibres will conduct heat in
Fig. 7. Graphite fibre reinforced epoxy composite (1.1 mm thick) irradiated with a focused beam at 1064 nm, by 1500 pulses: (a) entrance side, (b) exit side.
Fig. 8. Glass fibre reinforced epoxy composite (1.5 mm thick) irradiated with a focused beam at 532 nm, by 1000 pulses: (a) entrance side, (b) exit side.
ciation. Therefore, a minimum of thermal damage could be observed around the holes, which confirms the change in interaction mechanism: the so-called ‘‘cold ablation’’ will occur. 4.2. Thermoset polymers
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the direction of their axes which will damage the epoxy matrix. The thermal deterioration is especially pronounced when the target is irradiated with the infrared beam and decreases with decreasing wavelength. Both fibre reinforced epoxy composites were irradiated using a focused 1064 nm beam. The pulse energy was 4.8 mJ and the photon energy was 1.165 eV. For the glass fibre reinforced epoxy, the total number of pulses needed to create a hole was around 3000, with an energy input of 14.4 J over 3 s. The integrated irradiation time was 0.3 ms. In Fig. 6(a), and (b) the entrance and exit sides of glass fibre reinforced epoxy composite samples irradiated with the IR beam are shown. Burning of the epoxy and thermal damage (breaking) of the glass fibres can be observed. In Fig. 7(a) and (b), entrance and exit sides of graphite fibre reinforced epoxy composite samples irradiated by 1500 pulses are shown. The total energy input is 7.2 J, over a period of 1.5 s. The integrated time of irradiation was 0.15 ms. Burning of the epoxy and thermal damage (breaking) of the graphite fibres can be observed.
Fig. 10. Glass fibre reinforced epoxy composite (1.5 mm thick) irradiated with a focused beam at 266 nm, by 10,000 pulses: (a) entrance side, (b) exit side.
Fig. 9. Graphite fibre reinforced epoxy composite (1.1 mm thick) irradiated with a focused beam at 532 nm, by 1000 pulses: (a) entrance side, (b) exit side.
When irradiating both of the fibre reinforced composites with focused 532 nm beam, 1000 pulses were needed to create a drilled-through hole. The thermal influence was significantly reduced, there was no burning but some crack formation in the epoxy matrix could be observed. The epoxy matrix is quite crack sensitive when influenced by thermal and mechanical stresses. Figs. 8 and 9 show glass and graphite fibre reinforced epoxy composites, respectively: (a) entrance side; (b) exit side. The pulse energy was 2.8 mJ and the total energy input 2.8 J over 1 s. The integrated irradiation time was 0.1 ms. Results of irradiating both thermoset polymers using 10 000 pulses of a focused UV beam at 266 nm are presented in Fig. 10 and Fig. 11, respectively; (a) entrance side, (b) exit side. The pulse energy was 0.5 mJ, the total energy input is 5 J over 10 s. The integrated irradiation time was 1 ms. The quality of the drilled holes was significantly improved, which confirms that the UV beam can photo chemically decompose the epoxy matrix and break both glass and graphite fibres. The thermal influence around the drilled holes is minimised.
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too low and can only lead to thermal degradation of thermoplastic polymers. The 266 nm beam has a photon energy of 4.662 eV, which is higher than the binding energies of the C–C and C–H covalent bonds and therefore photochemical bond breaking will be possible. In the case of fibre reinforced epoxy, the thermal degradation of the matrix and fibres was severe both at the infrared and green wavelengths. The thermal damage effects are more pronounced for the carbon fibre reinforced composite than for the glass fibre reinforced epoxy, because the carbon fibres conduct heat much better than glass fibres. The strong absorption of UV light in the glass and graphite fibres will cause the fibre to break. At the same time, the C–C and C–H covalent bonds which bind the epoxy matrix can be photo chemically dissociated. This will result in quite clean drilled holes for both types of fibre reinforced composites sheets. The results achieved indicate that using even higher average UV power at 266 nm and higher repetition frequency contour cutting of epoxy and polyester polymer sheets reinforced with different fibres would be possible. Higher power outputs could be achieved using a longer BBO crystal, increasing the average power of the present laser in the UV up to 10 W. This power level should also be enough to cut polymer sheets of up to 2 mm thickness. References
Fig. 11. Graphite fibre reinforced epoxy composite (1.1 mm thick) irradiated with a focused beam at 266 nm, by 10,000 pulses: (a) entrance side, (b) exit side.
5. Conclusions Three different wavelengths, 1064, 532 and 266 nm, were used to investigate the interaction of laser beams with thermoplastic and fibre-reinforced thermoset (epoxy) polymers. The experiments show clearly that hole drilling in the thermoplastic PMMA and PC can only be performed using the UV beam. The photon energy at 1064 and 532 nm is
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