- Email: [email protected]
A study of the surface products on zinc-coated steel during laser ablation cleaning
Surface and Coatings Technology 137 Ž2001. 170᎐174
A study of the surface products on zinc-coated steel during laser ablation cleaning X. Zhoua,U , K. Imasaki a , H. Furukawaa , H. Umino b, K. Sakagishi b, S. Nakai c , C. Yamanakaa a
Institute for Laser Technology, 2-6 Yamada-oka, Suita, Osaka 565-0871, Japan b Atox Corporation, 1408 Takada, Kashiwa, Chiba 277-0861, Japan c Institute of Laser Engineering, Osaka Uni¨ ersity, 2-6 Yamada-oka, Suita, Osaka 565-0871, Japan Received 18 April 2000; received in revised form 13 October 2000; accepted 13 October 2000
Abstract A high-power short-pulsed Nd:YAG laser was focused onto carbon steel targets with a 13-m-thick Zn coating. The ablation effects, the behavior and the characteristics of the laser-induced products were studied. The ablation rate was defined as the ablated volume divided by consumed laser energy, and was used as a description of ablation cleaning ability. The dependence of ablation rate on laser fluence was plotted, and the optimum processing conditions were determined. The laser induced products consisted of plasma-vapor and liquid particles. The length of the laser-induced plasma-vapor was approximately 2 cm and its existence time wavered at approximately 4 s. It was observed that the moving direction of the liquid particles was in a small angular range that was tilted toward but symmetric to the incident laser. It was further noticed in this range that a higher moving velocity was obtained when the angle between the liquid particles and the incident laser beam was larger. The estimated highest moving velocity of the liquid particles was approximately 0.8 mrs. The analysis of the condensed particles confirmed that the laser-induced product on the target surface consisted of Zn and Fe, which mainly solidified from plasma-vapor and melt, respectively. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Laser ablation; Surface cleaning; Plasma-vapor; Liquid particles; Ablated product
1. Introduction Despite the great advancements made in the nuclear electrical power industry to control contamination in recent years, safety remains a major concern. As the number of facilities closing down continues to grow, there is an even more urgent need to find proper storage solutions for hazardous material in order to
U
Corresponding author. Tel.: q81-6-6879-8756; fax: q81-6-68798732. E-mail address: [email protected] y u.ac.jp ŽX. Zhou..
reduce the threat of radioactive pollution to humans and the natural environment. Nuclear power plants need to be cleaned thoroughly and non-threatening ways to convert facilities and materials must be found. Various effective methods of cleaning polluted facilities have been developed. A new laser ablation method to clean the surface of radioactively polluted facilities was studied in comparison with other methods, and laser ablation cleaning was shown to be potentially superior to all other methods w1x. Since laser beams act on opaque targets, a certain amount of their energy is absorbed and transformed into heat, thereby inducing a series of changes depen-
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 1 1 1 8 - X
X. Zhou et al. r Surface and Coatings Technology 137 (2001) 170᎐174
dent on the various laser parameters, the characteristics of the target materials and the environmental conditions. Studies show that when laser intensity is high enough, especially in the case of high-power short-pulse laser, the absorption of laser energy occurs rapidly and only in a very thin layer on the target surface. This thin layer is thus instantaneously evaporated and removed. Based on this mechanism, the application of high-power short-pulse laser is viewed as a highly attractive surface cleaning method for materials and objects that make up the coating of metals and their compounds w2,3x, paint on aircraft w4x and building structures such as metal, concrete and ceramic substrate w5x, and even dirt on artworks w6x. Further investigations into the properties of laser ablation cleaning and its possible application to nuclear facilities must be undertaken. In our study w7x, the various aspects of the application of this technique including laser optical fibers systems and product collection structure were discussed. It was pointed out that optical fiber possibly provided the best means to transport laser beams to far distances and perform surface cleaning under complicated circumstances. Also, the product collection effect is another key-point for the practical utilization of this technique. Based on these considerations, in this paper, a carbon steel plate with a 13-m electroplated Zn coating was selected as simulating sample Žat nuclear facilities, the inner surface contaminant mainly consisted of an oxide Fe 3 O4 layer of a few micrometers in thickness; our experiments showed that using the zinc coating instead of the Fe 3 O4 layer, did not alter the results ., and the behavior of the laser-induced plasmavapor and the liquid particles was diagnosed. Also, the cleaning ability of laser ablation and the characteristics of the ablated products were analyzed to confirm the feasibility and basic data for the design of a laser-fiber system and its practical utilization in the cleaning of nuclear facilities.
2. Experimental set-up An Nd:YAG laser with mode lock techniques was employed in this experiment. A laser beam with a wavelength of 1064 nm and a pulse width of 20᎐100 ps was produced and delivered into amplifiers, and an average power of 13 W was obtained. A laser beam was focused onto the target surface by a lens with a focal length of 250 mm. The substrate of the targets was made of carbon steel with an electroplated Zn coating 13-m-thick and was fixed on a digitally controlled X᎐Y translation stage. The defocusing distances were 2 ; 4 cm. The target scanning velocities were 2 ; 10 mmrs. A highspeed CCD video camera was set-up near the target
171
Fig. 1. The relationship of ablation rate to laser fluence.
surface to record the phenomena happening during laser ablation. The cross-section profile of the ablated ditches on the surface was measured by Dektak3. The area of the ditches was obtained and the ablated volumes were calculated. The characteristics of the ablated products were analyzed by SEM and EDX.
3. Results and discussion 3.1. Ablation rate Ablation rate is defined as ablated volume divided by consumed laser energy: R a s VrEs Vr Ž P⭈ t . s LAr P⭈
ž
L ¨
/ s¨ArP
Ž1.
where R a is the ablation rate, V and L are the ablated ditch volume and length, ¨ and t are the target scanning velocity and time, respectively, A is the cross-section area of the ablated ditch, E is the laser energy, and P is the average laser power. It is said that the ablation rate can be used to express the ability of material removal of laser ablation. Thus, at higher ablation rates, more coating material can be removed with less laser energy. The relationship of ablation rate to laser fluence ŽJrm2 . was studied. Due to the characteristics of both the coating and the substrate, there is a maximum ablation rate, which corresponds to optimal processing conditions Žsee Fig. 1.. With a 4.7= 10 6 Jrm2 laser fluence, the 13-m-thick Zn coating was completely ablated. Beyond this laser fluence, the laser beam irradiated onto the substrate. Compared to the substrate of carbon steel, a Zn coating is a more easily ablated material, so beyond the maximum ablation rate, more laser energy was consumed and ablation
172
X. Zhou et al. r Surface and Coatings Technology 137 (2001) 170᎐174
laser-induced plasma is approximately 4 s, as shown in Fig. 4a,b. Fig. 4b and Fig. 5a show the exact moment when the plasma disappears and when the presence of liquid particles is clearly observed. The motion of liquid product is due to the so-called piston effect w9x, namely the pressure of the plasma and vapor. The liquid particles were pressed to the two edges of the melting pool and sprayed out along a tilted scale of direction. In this scale, the moving velocity of the liquid particles with a larger tilting angle is higher than in a scale with a smaller tilting angle Žsee Fig. 5b.. If we look at the liquid particles with the highest moving speed, we notice that their moving distance from point A shown in Fig. 5a to point B shown in Fig. 5b is L s 12 mm, and the needed time duration is T D s 4 s. The corresponding real distance difference is L R s LrS s 3.2 mm Žhere, S is the scale of the picture photographed by CCD, and S s 3.75.. There-
Fig. 2. The picture and cross-section profile of the ablated ditch.
rates decreased. Fig. 2 shows the cross section profile of the ablated ditch on the target surface at the optimum point above measured by Dektak3. 3.2. Beha¨ ior of laser induced products The structure of the laser-induced products is controlled by laser fluence, which includes laser parameters and processing parameters. The stronger the laser intensity or the slower the target scanning velocity, the higher the laser fluence. Three typical structures of laser-induced products controlled by low, middle and high laser fluence respectively are shown in Fig. 3. When laser fluence is high enough, the induced product includes liquid particles in addition to plasma, which is present when laser fluence is low. The ‘middle fluence’ in Fig. 3b corresponding to the optimum condition is shown in Fig. 1 and the results are shown in Fig. 2. In this case, there is still some liquid product. Similarly to what was pointed out by Chichkov et al. for pico-second laser ablation regimes, surface evaporation can be considered as direct solid plasma or solid vapor transition, but there is a very thin liquid layer closely above the solid surface of the target w8x, especially for the easily ablated coating on the substrate with a high melting point. A report on nuclear power plants shows that radioactive contaminants include an oxide layer either on the inner wall of the facilities or within a few micrometers of the substrate w1x. Thus, a laser fluence slightly higher than the optimum point is necessary in order to completely decontaminate nuclear facilities. Further analysis shows that the existing time of the
Fig. 3. The influence of laser fluence on the induced products.
X. Zhou et al. r Surface and Coatings Technology 137 (2001) 170᎐174
173
Fig. 4. The generation of laser-induced plasma.
fore, the moving speed of these particles is S P s L RrT D s 0.8 mrs. 3.3. Analysis of the ablated products The SEM-EDX analysis results show that the ablated products consisted mainly of Zn particles that were spherical or deformed spherical in shape and less than 5 m in size, as shown in Fig. 6a. It is said the Zn coating transited from solid to plasma and vapor during laser ablation, then rapidly condensed from plasma and vapor to solid. Only a few Fe particles were present in the ablated products. Their form was spherical or irregular, and their size was within 5 m, as shown in Fig. 6b. They are regarded as directly condensed from plasma and vapor. Also, we found a few Fe particles with a size of approximately 50 m showing a trace of solidification from liquid Žsee Fig. 6c,d.. In Fig. 6d, the existence of Al was due to the inner material of the collector.
4. Conclusion Provided the laser parameters and processing parameters are appropriately selected, an optimum ablation effect can be achieved. In this case, the laser-induced products include plasma-vapor and liquid particles. Thus, the material removal mechanism is as follows: the coating is completely transferred to plasmavapor and decontaminated, and a very thin layer of substrate is evaporated and melted; then, the melt sprays out by recoil pressure resulting from the plasma-vapor. The plasma-vapor has an existence time
Fig. 6. The SEM and EDX analysis of the ablated products.
of approximately 0.004 s and a size of less than 2 cm; it doesn’t expand and produce a channel in the air along a laser beam. After the plasma-vapor condenses, tiny particles with a size of less than 5 m are generated. The liquid particles spray out at a speed of less than 0.8 mrs, and then solidify. Their size is approximately 50 m.
Acknowledgements This work is partially supported by Kansai Electric Power Co. Thanks shall be given to Dr K. Murai of Osaka National Research Institute, Dr S. Uchida of ILT, Prof. Y. Tsunawaki of Osaka Sanyo University and Prof. N. Ohigashi of Kansai University for their helpful insight and assistance. References
Fig. 5. The motion of the liquid particles.
w1x K Imasaki, Research on laser ablation cleaning of nuclear facilities, Report of Inst. for Laser Techn., Osaka, Japan, March 1999. w2x A.F. Semerok, C. Chaleard, V. Detalle et al., Laser ablation efficiency of pure metals with femtosecond, picosecond, and nanosecond pulses, SPIE 3343 Ž1998. 1049᎐1055. w3x J.L. Ocana, G. Nicolas, M. Autric et al., Study of ablation processes induced by UV lasers in metals and ceramics, SPIE 3343 Ž1998. 927᎐938. w4x A. Tsunemi, A. Endo, D. Ichishima, Paint removal from
174
X. Zhou et al. r Surface and Coatings Technology 137 (2001) 170᎐174
aluminum and composite substrates of aircraft by laser ablation using TEA CO 2 lasers, SPIE 3343 Ž1998. 1018᎐1022. w5x K. Liu, E. Garmire, Paint removal using lasers, Appl. Optics 34 Ž21. Ž1995. 4409. w6x S. Georgiou, V. Zafiropulos, V. Tomavi et al., Mechanistic aspects of excimer laser restoration of painted artworks, Laser Phys. ŽRussia. 8 Ž1. Ž1998. 307᎐312.
w7x X.L. Zhou, K. Imasaki, H. Umino et al., Laser ablation surface cleaning of nuclear facilities, SPIE 3887 Ž1999. 326᎐334. w8x B.N. Chichkov, C. Momma, S. Nolte et al., Femtosecond, picosecond and nanosecond laser ablation of solids, Appl. Phys. A63 Ž1996. 3. w9x M.V. Allmen, A. Blatter, Laser-Beam Interactions with Materials, Springer-Verlag, New York, 1995, p. 132.
A study of the surface products on zinc-coated steel during laser ablation cleaning X. Zhoua,U , K. Imasaki a , H. Furukawaa , H. Umino b, K. Sakagishi b, S. Nakai c , C. Yamanakaa a
Institute for Laser Technology, 2-6 Yamada-oka, Suita, Osaka 565-0871, Japan b Atox Corporation, 1408 Takada, Kashiwa, Chiba 277-0861, Japan c Institute of Laser Engineering, Osaka Uni¨ ersity, 2-6 Yamada-oka, Suita, Osaka 565-0871, Japan Received 18 April 2000; received in revised form 13 October 2000; accepted 13 October 2000
Abstract A high-power short-pulsed Nd:YAG laser was focused onto carbon steel targets with a 13-m-thick Zn coating. The ablation effects, the behavior and the characteristics of the laser-induced products were studied. The ablation rate was defined as the ablated volume divided by consumed laser energy, and was used as a description of ablation cleaning ability. The dependence of ablation rate on laser fluence was plotted, and the optimum processing conditions were determined. The laser induced products consisted of plasma-vapor and liquid particles. The length of the laser-induced plasma-vapor was approximately 2 cm and its existence time wavered at approximately 4 s. It was observed that the moving direction of the liquid particles was in a small angular range that was tilted toward but symmetric to the incident laser. It was further noticed in this range that a higher moving velocity was obtained when the angle between the liquid particles and the incident laser beam was larger. The estimated highest moving velocity of the liquid particles was approximately 0.8 mrs. The analysis of the condensed particles confirmed that the laser-induced product on the target surface consisted of Zn and Fe, which mainly solidified from plasma-vapor and melt, respectively. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Laser ablation; Surface cleaning; Plasma-vapor; Liquid particles; Ablated product
1. Introduction Despite the great advancements made in the nuclear electrical power industry to control contamination in recent years, safety remains a major concern. As the number of facilities closing down continues to grow, there is an even more urgent need to find proper storage solutions for hazardous material in order to
U
Corresponding author. Tel.: q81-6-6879-8756; fax: q81-6-68798732. E-mail address: [email protected] y u.ac.jp ŽX. Zhou..
reduce the threat of radioactive pollution to humans and the natural environment. Nuclear power plants need to be cleaned thoroughly and non-threatening ways to convert facilities and materials must be found. Various effective methods of cleaning polluted facilities have been developed. A new laser ablation method to clean the surface of radioactively polluted facilities was studied in comparison with other methods, and laser ablation cleaning was shown to be potentially superior to all other methods w1x. Since laser beams act on opaque targets, a certain amount of their energy is absorbed and transformed into heat, thereby inducing a series of changes depen-
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 1 1 1 8 - X
X. Zhou et al. r Surface and Coatings Technology 137 (2001) 170᎐174
dent on the various laser parameters, the characteristics of the target materials and the environmental conditions. Studies show that when laser intensity is high enough, especially in the case of high-power short-pulse laser, the absorption of laser energy occurs rapidly and only in a very thin layer on the target surface. This thin layer is thus instantaneously evaporated and removed. Based on this mechanism, the application of high-power short-pulse laser is viewed as a highly attractive surface cleaning method for materials and objects that make up the coating of metals and their compounds w2,3x, paint on aircraft w4x and building structures such as metal, concrete and ceramic substrate w5x, and even dirt on artworks w6x. Further investigations into the properties of laser ablation cleaning and its possible application to nuclear facilities must be undertaken. In our study w7x, the various aspects of the application of this technique including laser optical fibers systems and product collection structure were discussed. It was pointed out that optical fiber possibly provided the best means to transport laser beams to far distances and perform surface cleaning under complicated circumstances. Also, the product collection effect is another key-point for the practical utilization of this technique. Based on these considerations, in this paper, a carbon steel plate with a 13-m electroplated Zn coating was selected as simulating sample Žat nuclear facilities, the inner surface contaminant mainly consisted of an oxide Fe 3 O4 layer of a few micrometers in thickness; our experiments showed that using the zinc coating instead of the Fe 3 O4 layer, did not alter the results ., and the behavior of the laser-induced plasmavapor and the liquid particles was diagnosed. Also, the cleaning ability of laser ablation and the characteristics of the ablated products were analyzed to confirm the feasibility and basic data for the design of a laser-fiber system and its practical utilization in the cleaning of nuclear facilities.
2. Experimental set-up An Nd:YAG laser with mode lock techniques was employed in this experiment. A laser beam with a wavelength of 1064 nm and a pulse width of 20᎐100 ps was produced and delivered into amplifiers, and an average power of 13 W was obtained. A laser beam was focused onto the target surface by a lens with a focal length of 250 mm. The substrate of the targets was made of carbon steel with an electroplated Zn coating 13-m-thick and was fixed on a digitally controlled X᎐Y translation stage. The defocusing distances were 2 ; 4 cm. The target scanning velocities were 2 ; 10 mmrs. A highspeed CCD video camera was set-up near the target
171
Fig. 1. The relationship of ablation rate to laser fluence.
surface to record the phenomena happening during laser ablation. The cross-section profile of the ablated ditches on the surface was measured by Dektak3. The area of the ditches was obtained and the ablated volumes were calculated. The characteristics of the ablated products were analyzed by SEM and EDX.
3. Results and discussion 3.1. Ablation rate Ablation rate is defined as ablated volume divided by consumed laser energy: R a s VrEs Vr Ž P⭈ t . s LAr P⭈
ž
L ¨
/ s¨ArP
Ž1.
where R a is the ablation rate, V and L are the ablated ditch volume and length, ¨ and t are the target scanning velocity and time, respectively, A is the cross-section area of the ablated ditch, E is the laser energy, and P is the average laser power. It is said that the ablation rate can be used to express the ability of material removal of laser ablation. Thus, at higher ablation rates, more coating material can be removed with less laser energy. The relationship of ablation rate to laser fluence ŽJrm2 . was studied. Due to the characteristics of both the coating and the substrate, there is a maximum ablation rate, which corresponds to optimal processing conditions Žsee Fig. 1.. With a 4.7= 10 6 Jrm2 laser fluence, the 13-m-thick Zn coating was completely ablated. Beyond this laser fluence, the laser beam irradiated onto the substrate. Compared to the substrate of carbon steel, a Zn coating is a more easily ablated material, so beyond the maximum ablation rate, more laser energy was consumed and ablation
172
X. Zhou et al. r Surface and Coatings Technology 137 (2001) 170᎐174
laser-induced plasma is approximately 4 s, as shown in Fig. 4a,b. Fig. 4b and Fig. 5a show the exact moment when the plasma disappears and when the presence of liquid particles is clearly observed. The motion of liquid product is due to the so-called piston effect w9x, namely the pressure of the plasma and vapor. The liquid particles were pressed to the two edges of the melting pool and sprayed out along a tilted scale of direction. In this scale, the moving velocity of the liquid particles with a larger tilting angle is higher than in a scale with a smaller tilting angle Žsee Fig. 5b.. If we look at the liquid particles with the highest moving speed, we notice that their moving distance from point A shown in Fig. 5a to point B shown in Fig. 5b is L s 12 mm, and the needed time duration is T D s 4 s. The corresponding real distance difference is L R s LrS s 3.2 mm Žhere, S is the scale of the picture photographed by CCD, and S s 3.75.. There-
Fig. 2. The picture and cross-section profile of the ablated ditch.
rates decreased. Fig. 2 shows the cross section profile of the ablated ditch on the target surface at the optimum point above measured by Dektak3. 3.2. Beha¨ ior of laser induced products The structure of the laser-induced products is controlled by laser fluence, which includes laser parameters and processing parameters. The stronger the laser intensity or the slower the target scanning velocity, the higher the laser fluence. Three typical structures of laser-induced products controlled by low, middle and high laser fluence respectively are shown in Fig. 3. When laser fluence is high enough, the induced product includes liquid particles in addition to plasma, which is present when laser fluence is low. The ‘middle fluence’ in Fig. 3b corresponding to the optimum condition is shown in Fig. 1 and the results are shown in Fig. 2. In this case, there is still some liquid product. Similarly to what was pointed out by Chichkov et al. for pico-second laser ablation regimes, surface evaporation can be considered as direct solid plasma or solid vapor transition, but there is a very thin liquid layer closely above the solid surface of the target w8x, especially for the easily ablated coating on the substrate with a high melting point. A report on nuclear power plants shows that radioactive contaminants include an oxide layer either on the inner wall of the facilities or within a few micrometers of the substrate w1x. Thus, a laser fluence slightly higher than the optimum point is necessary in order to completely decontaminate nuclear facilities. Further analysis shows that the existing time of the
Fig. 3. The influence of laser fluence on the induced products.
X. Zhou et al. r Surface and Coatings Technology 137 (2001) 170᎐174
173
Fig. 4. The generation of laser-induced plasma.
fore, the moving speed of these particles is S P s L RrT D s 0.8 mrs. 3.3. Analysis of the ablated products The SEM-EDX analysis results show that the ablated products consisted mainly of Zn particles that were spherical or deformed spherical in shape and less than 5 m in size, as shown in Fig. 6a. It is said the Zn coating transited from solid to plasma and vapor during laser ablation, then rapidly condensed from plasma and vapor to solid. Only a few Fe particles were present in the ablated products. Their form was spherical or irregular, and their size was within 5 m, as shown in Fig. 6b. They are regarded as directly condensed from plasma and vapor. Also, we found a few Fe particles with a size of approximately 50 m showing a trace of solidification from liquid Žsee Fig. 6c,d.. In Fig. 6d, the existence of Al was due to the inner material of the collector.
4. Conclusion Provided the laser parameters and processing parameters are appropriately selected, an optimum ablation effect can be achieved. In this case, the laser-induced products include plasma-vapor and liquid particles. Thus, the material removal mechanism is as follows: the coating is completely transferred to plasmavapor and decontaminated, and a very thin layer of substrate is evaporated and melted; then, the melt sprays out by recoil pressure resulting from the plasma-vapor. The plasma-vapor has an existence time
Fig. 6. The SEM and EDX analysis of the ablated products.
of approximately 0.004 s and a size of less than 2 cm; it doesn’t expand and produce a channel in the air along a laser beam. After the plasma-vapor condenses, tiny particles with a size of less than 5 m are generated. The liquid particles spray out at a speed of less than 0.8 mrs, and then solidify. Their size is approximately 50 m.
Acknowledgements This work is partially supported by Kansai Electric Power Co. Thanks shall be given to Dr K. Murai of Osaka National Research Institute, Dr S. Uchida of ILT, Prof. Y. Tsunawaki of Osaka Sanyo University and Prof. N. Ohigashi of Kansai University for their helpful insight and assistance. References
Fig. 5. The motion of the liquid particles.
w1x K Imasaki, Research on laser ablation cleaning of nuclear facilities, Report of Inst. for Laser Techn., Osaka, Japan, March 1999. w2x A.F. Semerok, C. Chaleard, V. Detalle et al., Laser ablation efficiency of pure metals with femtosecond, picosecond, and nanosecond pulses, SPIE 3343 Ž1998. 1049᎐1055. w3x J.L. Ocana, G. Nicolas, M. Autric et al., Study of ablation processes induced by UV lasers in metals and ceramics, SPIE 3343 Ž1998. 927᎐938. w4x A. Tsunemi, A. Endo, D. Ichishima, Paint removal from
174
X. Zhou et al. r Surface and Coatings Technology 137 (2001) 170᎐174
aluminum and composite substrates of aircraft by laser ablation using TEA CO 2 lasers, SPIE 3343 Ž1998. 1018᎐1022. w5x K. Liu, E. Garmire, Paint removal using lasers, Appl. Optics 34 Ž21. Ž1995. 4409. w6x S. Georgiou, V. Zafiropulos, V. Tomavi et al., Mechanistic aspects of excimer laser restoration of painted artworks, Laser Phys. ŽRussia. 8 Ž1. Ž1998. 307᎐312.
w7x X.L. Zhou, K. Imasaki, H. Umino et al., Laser ablation surface cleaning of nuclear facilities, SPIE 3887 Ž1999. 326᎐334. w8x B.N. Chichkov, C. Momma, S. Nolte et al., Femtosecond, picosecond and nanosecond laser ablation of solids, Appl. Phys. A63 Ž1996. 3. w9x M.V. Allmen, A. Blatter, Laser-Beam Interactions with Materials, Springer-Verlag, New York, 1995, p. 132.