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Surface hardening of titanium by pulsed Nd:YAG laser irradiation at 1064- and 532-nm wavelengths in nitrogen atmosphere
Applied Surface Science 257 (2010) 691–695
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface hardening of titanium by pulsed Nd:YAG laser irradiation at 1064- and 532-nm wavelengths in nitrogen atmosphere Naofumi Ohtsu a,∗ , Misao Yamane a , Kenji Kodama b , Kazuaki Wagatsuma c a b c
Instrumental Analysis Center, Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507, Japan EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 12 April 2010 Received in revised form 8 July 2010 Accepted 8 July 2010 Available online 16 July 2010 Keywords: Laser nitridation Laser wavelength Depth profile Nanoindentation
a b s t r a c t The surface hardness of titanium modified by laser irradiation at different wavelengths in nitrogen atmosphere was investigated. Further, surface characteristics such as morphology, chemical state, and chemical composition in the depth direction were also studied. The size and depth of the craters observed in the laser-irradiated spots increased monotonically with an increase in the laser power. Furthermore, the crater formed by the 532-nm laser was deeper than that formed by the 1064-nm laser for the same laser power. Laser power beyond a certain threshold value was required to obtain a titanium nitride layer. When the laser power exceeds the threshold value, a titanium nitride layer of a few tens of nanometers in thickness was formed on the substrate, whereas a titanium oxide layer containing small amounts of nitrogen was formed when the laser power is below the threshold value. Thus, it was shown that laser irradiation using appropriate laser parameters can successfully harden a titanium substrate, and the actual hardness of the titanium nitride layer, measured by nanoindentation, was approximately five times that of an untreated titanium surface. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titanium and its alloys have low density, good corrosion resistance, good biocompatibility, and high specific strength. However, their frictional wear resistance is sometimes not adequate for many applications [1]. Surface modification is one of the most effective techniques for improving the wear resistance of titanium; in particular, surface nitridation resulting from the diffusion of nitrogen is widely used in the industry. Heretofore, surface nitridation has been achieved by heat treatment in nitrogen atmosphere (gas nitriding process) [2–4] or by exposure in a nitrogen plasma (plasma nitriding process) [5–7]. These processes can form a homogeneous nitride layer on a titanium surface having a relatively large area, but the modified area is not controllable. Recently, George et al. reported that a surface layer containing nitrogen can be formed on a titanium substrate by laser irradiation in a nitrogen atmosphere [8,9]. Surface treatment using laser irradiation has several advantages; for example, the modified surface is localized in an area of the order of micrometers and can be controlled by rasterizing the laser beam. Consequently, nitridation by laser irradiation has gained considerable attention as a new process that can modify
∗ Corresponding author. Tel.: +81 157 26 9563; fax: +81 157 26 9563. 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.07.025
a localized area. This process is expected to be applied to produce various microdevices. In the laser nitridation process, the nitride layer is produced as a result of complex effects such as the ablation of the naturally formed oxide, melting of the substrate, reaction of the substrate material with the nitrogen gas, and chemical and physical reactions occurring in the gas plasma because of the laser breakdown. These phenomena are affected by the laser irradiation conditions; accordingly, it is considered that the characteristics of the irradiated surface vary depending on the laser parameters [10]. In a previous study [11], the authors used X-ray photoelectron spectroscopy (XPS) to carefully analyze a surface film on titanium formed by laser irradiation at different laser powers and wavelengths. The study showed that a titanium nitride layer is formed on the surface when the laser power exceeds a certain threshold value. Those experimental results showed that a hard surface layer might be obtained by laser irradiation under appropriate laser conditions. In this study, we focus on the actual hardness of the laser-nitrided surface. The primary objective of the present study is to obtain the hardness profiles of a titanium surface modified by laser irradiation at different wavelengths in nitrogen atmosphere. The mechanical properties of titanium surfaces depend on several characteristics of the surface film such as the morphology, thickness and so on [12]. Accordingly, we also investigated the surface characteristics such
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N. Ohtsu et al. / Applied Surface Science 257 (2010) 691–695
as morphology, chemical state, and chemical composition in the depth direction, in order to discuss the effects of laser conditions on the hardness profiles.
culated from load–displacement curves using the Oliver and Pharr method [13]. Considering the surface roughness, the indentations were repeated seven times and the average of these results was used for the calculations.
2. Experimental procedures 3. Results and discussion A pure titanium plate (99.99% purity) with dimensions of 10 mm × 10 mm × 1 mm, which was mechanically polished with a diamond paste and then ultrasonically washed in ethanol, was fabricated as the substrate. The titanium substrate was transferred to the reaction chamber having the base pressure less than 2.0 × 10−6 Pa, and nitrogen gas was then introduced into the chamber through a variable leak valve at a pressure of 13.3 kPa. A Nd:YAG Q-switched laser (Minilite I, Continuum, Inc., U.S.A.) 1064-nm and 532-nm (SHG mode) wavelength outputs with a (4 ± 1) ns pulse width was used for nitridation. The laser energy per pulse was varied from 4.8 ± 0.5 mJ to 19.8 ± 0.5 mJ, which was determined with a laser-power meter. The laser beam was manually shot with a trigger switch at a repetition rate of ca. 0.8 Hz, focused using a lens, introduced into the reaction chamber through a synthetic quartz window, and finally irradiated on the titanium substrate under nitrogen atmosphere. The laser shots were fixed at 50 shots. The size and depth of the craters formed by the laser irradiation were estimated by optical microscopy and laser scanning microscopy analysis (VF-7500, Keyence Co., Ltd., Japan), respectively. Microstructural observation was conducted by scanning electron microscopy (SEM; JSM-5800, JEOL, Japan) at an operation voltage of 10 kV. The changes in the chemical state and composition of the surface in the depth direction were analyzed by Auger electron spectroscopy (AES; ESCA1600, Ulvac-Phi, Japan) along with argon-ion etching. The energy spectrum of the Auger electrons was analyzed with a concentric hemispherical analyzer (CHA). The acceleration voltage of the electron probe and Ar ions for the etching were set to 10 kV and 3.0 kV, respectively. Load–displacement curves of the surface were measured by using a nanoindenter (UMIS2000, CSIRO, Australia) with a Berkovich diamond indenter tip. The indentation load was varied from 20 N to 2 mN to achieve different penetration depths. The hardness of the surface was cal-
3.1. Surface characteristics of laser-irradiated titanium surface Optical views of the laser-irradiated spots drastically changed depending on the laser power and wavelength. Typical images obtained by optical microscopy are shown in Fig. 1. A round-shaped central zone of the crater appeared after the laser irradiation. The size of the crater increased when using the high-power laser and, concomitantly, a surrounding zone observed in the boundary region was discolored. It has been reported that the color of a titanium surface depends on the thickness of the surface oxide layer [14,15]. A golden color corresponding to a relatively thin layer changes into purple through brown with an increase in the thickness. Therefore, the change in color, as shown in Fig. 1, indicates that the thickness of the surface oxide at the boundary region increased with an increase in the laser power. Furthermore, it should be noted that, at the same laser power, the surface oxide at the boundary regions formed by the 532-nm laser was thicker than that formed by the 1064-nm laser. In Fig. 2, the depth of the crater is plotted against the laser power. The depth increases monotonically with an increase in the laser power; at the same power, the crater formed by the 532nm laser was deeper than that formed by the 1064-nm laser. The craters were formed due to the vaporization of the titanium substrate; therefore, their depth was determined principally by the laser energy absorbed on the surface. It is likely that the energy distributed on the substrate increases with the laser power. Accordingly, the differences in the crater depths indicate that the laser energy distributed on the surface changes depending on the laser wavelength. This result is consistent with the results reported in other literatures [11,16,17]. Microstructures at the laser-irradiated spot formed by the 532nm laser at 15 mJ were observed by SEM (Fig. 3). The surface
Fig. 1. Optical views around the laser irradiated spot observed by optical microscopy. (a) 4.8 mJ at 1064 nm, (b) 9.2 mJ at 1064 nm, (c) 19.7 mJ at 1064 nm, (d) 4.6 mJ at 532 nm, and (e) 9.4 mJ at 532 nm.
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Fig. 2. Laser power dependence for the depth of the crater observed in laser irradiated spot.
morphology of the crater appeared to be smooth, whereas small pits of submicrometer size were scattered in the crater. Similar smooth surfaces containing small pits were also observed in the craters formed under the other laser conditions. Li et al. reported that small pits appearing after the laser irradiation correlated with the formation of surface metallographic defects, such as microinclusions and small visible scratches during a polishing process [18]. They also reported that the number of pits could be decreased with a defect-eliminating treatment such as chemical etching [18]. Fig. 3 shows that the boundary region contained many microcracks, which was probably due to the solidification of melted titanium at a high cooling rate [19]. The changes in the chemical composition in the depth direction at the laser-irradiated spots, measured by AES, are shown in Fig. 4. The Auger electron peak corresponding to N KLL around 377 eV overlapped with the Ti L3 M23 M23 peak around 387 eV, as shown in
Fig. 4. AES depth profiles of the laser irradiated spot formed by different wavelength and power laser. (a) 1064 nm at 20 mJ, (b) 532 nm at 7 mJ, and (c) 1064 nm at 7 mJ.
Fig. 5. We estimated the chemical composition of the surface from a quantitative evaluation proposed by Johansson et al. [20]. In the case of the 1064-nm laser at 20 mJ and the 532-nm laser at 7 mJ, the depth profile obtained can be roughly divided into three
Fig. 3. SEM images around the laser irradiated spot formed by 15.1-mJ laser at 532 nm. (a) Boundary region, (b) crater area, and (c) enlargement of the crater area.
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Fig. 6. Typical load–displacement curves of a titanium plate before and after the laser irradiation. The maximum applied load was 2 mN.
Fig. 5. AES differential spectra containing Ti LMM and N KLL transition lines for the laser irradiated spot after sputtering for: (a) 100 s and (b) 800 s.
regions, as shown in Fig. 4(a) and (b). The topmost layer (labeled as “Layer 1”) contained oxygen, nitrogen, and titanium, which corresponds to an oxynitride layer having a TiOx Ny -like structure, as reported in several literatures [21–23]. The lower surface layer (labeled as “Layer 2”) consisted of titanium and nitrogen. AES differential spectra corresponding to the lower surface layer were compared with standard spectra for stoichiometric titanium nitride (TiN) [24], as shown in Fig. 5(a). Spectral shapes obtained using the 1064-nm laser at 20 mJ and the 532-nm laser at 7 mJ are similar to those of TiN; further, the peak energies corresponding to the N KLL transition agree with those for TiN. Therefore, the chemical state of the lower surface layer was identified to be titanium nitride. The thickness of the nitride layer was estimated to be a few tens of nanometers. The inner region below the surface layer (labeled as “Layer 3”) showed strong titanium and weak nitrogen intensities. AES differential spectra of the inner region almost coincided with the standard titanium spectrum, as shown in Fig. 5(b), indicating that the chemical state was metallic titanium. Low nitrogen intensity observed in the inner region was considered to be background. In contrast, in the case of the 1064-nm laser at 7 mJ, the depth profile can be roughly divided into two regions, as shown in Fig. 4(c). The surface layer (labeled as “Layer I”) contained titanium, nitrogen, and a high concentration of oxygen. In the AES spectrum of the surface layer, the N KLL peak was hardly observed, and the peak corresponding to the Ti L3 M23 M45 transition (solid arrow) shifted toward the lower energy side as compared to that for TiN, as shown in Fig. 5(a). These results indicate that a titanium nitride layer cannot be obtained by using a 1064-nm laser at 7 mJ. The surface layer probably consisted of a titanium oxide layer containing a small amount of oxygen [21]. The inner region below the surface layer (labeled as “Layer II”) consisted of titanium and oxygen. The AES peak around 387 eV, corresponding to the Ti L3 M23 M45 transition, shifted slightly toward the negative energy (dotted arrow), as shown in Fig. 5(b). This negative shift was likely due to the for-
mation of titanium oxide such as TiO [25], indicating that oxygen diffused into the deeper region under this laser condition. In our previous study, we found that a laser power beyond a certain threshold value is required for forming a titanium nitride [11]. The threshold powers, estimated from XPS analysis, for the 1064- and 532-nm lasers were 14 mJ and 6 mJ, respectively. It is clear that the laser power of 20 mJ at 1064 nm and 7 mJ at 532 nm exceeded the required threshold power; this was confirmed by the titanium nitride layers observed in the AES profiles. These analytical results obtained from the AES analysis are completely consistent with the results from our previous XPS analysis [11]. 3.2. Hardness profile of laser-irradiated surface Typical load–displacement curves of a titanium plate before and after the laser irradiation are shown in Fig. 6. It is clear that tip penetration for the laser-irradiated titanium plate was shallow, as compared to the non-irradiated titanium plate. Hardness profiles before and after the laser irradiation, which were calculated from the load–displacement curves, are shown in Fig. 7. The laser powers exceeded the threshold values for obtaining a titanium nitride layer. The hardness at a sampling depth of 10 nm for the non-irradiated sample is approximately 5 GPa, which is almost equal to that reported in a previous paper [26]. The hardness in a depth range of 10–50 nm is almost constant. On the other hand, the hardness of the laser-irradiated sample at a depth
Fig. 7. Hardness profiles of a titanium plate before and after the laser irradiation, obtained by nanoindentation.
N. Ohtsu et al. / Applied Surface Science 257 (2010) 691–695
of 10 nm is approximately 27 GPa at both the laser wavelengths. This value is approximately five times that of the non-irradiated titanium plate. We found that the hardness gradually decreases with increasing depth and reaches approximately 13 GPa at a depth of 30 nm. In the region beyond 30 nm, the hardness is almost constant. 3.3. Effects of surface characteristics on hardness profiles The nanoindentation test clearly shows that a titanium surface can be successfully hardened by laser irradiation by using a laser with power exceeding the threshold value. The hardness profile obtained by using the 1064-nm laser at 20 mJ almost coincided with that of the 532-nm laser at 7 mJ. The thickness of the hardened region is estimated to be approximately 20 nm in both the profiles, which almost corresponds to that of the nitride layer estimated from the AES profile (shown in Fig. 4). The hardness of the laserirradiated surface, measured by nanoindentation, is almost equal to that of a titanium nitride layer prepared by DC reactive sputtering [27]. These results indicate that the hard surface originated from the formation of a titanium nitride layer. When a titanium surface is irradiated by a laser, a new surface is formed due to the absorption of laser energy. The energy distributed on the substrate is indicated by the target surface reflectivity, which changes depending on the wavelength. For example, the reflectivity of Cu is 0.97 and 0.61 at 1064- and 532-nm wavelengths, respectively [28]. At the 532-nm wavelength, 39% of the laser energy penetrated into the substrate, resulting in heating, melting, and vaporization. On the other hand, only 3% of the laser energy can be used for the substrate reaction at the 1064-nm wavelength. Accordingly, at the same laser power, the crater was deeper when using the 532-nm laser. The difference in wavelengths also affected the characteristics of the laser-induced plasma, such as plume size, maximum temperature, and laser adsorption in the plasma [29]; however, these effects are not pronounced in the obtained hardness profiles. Consequently, we conclude that laser irradiation at the wavelength of 532 nm is more beneficial for hardening a titanium surface because a titanium nitride layer is obtained at a lower power. 4. Conclusions In the present study, we measured the hardness of a titanium surface modified by 1064- and 532-nm laser irradiation in nitrogen atmosphere. We also investigated the surface characteristics such as morphology, chemical state, and chemical composition in the depth direction. Our conclusions are as follows: (1) The size and depth of the craters increase monotonically with an increase in the laser power; concomitantly, the thickness of the oxide layer surrounding the crater also increases. The crater depth and the oxide layer thickness in the case of the 532-nm laser were larger than those in the case of the 1064-nm laser at the same laser power.
such as TiO is formed at the upper and lower surface regions, respectively. (3) Laser nitridation using optimal laser parameters can harden a titanium surface. The modified surface depth almost equal to the thickness of the titanium nitride layer. When using the 1064-nm laser at 20 mJ or the 532-nm laser at 7 mJ, the hardened region is approximately 20 nm in thickness, and the hardness of the surface is approximately five times that of an untreated titanium surface. Under the present experimental conditions, laser irradiation at the wavelength of 532 nm is more beneficial for hardening a titanium surface. Acknowledgements The authors gratefully acknowledge Dr. H. Kato and Dr. S. Sato of Institute for Materials Research, Tohoku University for their help in hardness measurement using nanoindenter. The authors also acknowledge Dr. M. Kawamura from Kitami Institute of Technology for providing AES standard spectra. This work was performed under the inter-university cooperative research program of the Laboratory for Analytical Science, Institute for Materials Research, Tohoku University and was supported by a Grant-in-aid from the Suzuki Foundation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
(2) Laser power exceeding a certain threshold value is required to obtain a titanium nitride layer. By using the 1064-nm laser at 20 mJ or the 532-nm laser at 7 mJ, a titanium nitride layer of few tens of nanometers in thickness is formed on the substrate. When the 1064-nm laser is used at 7 mJ, a titanium oxide layer containing small amounts of nitrogen and titanium oxide
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[25] [26] [27] [28] [29]
M. Niinomi, Biomaterial 24 (2003) 2673. A. Zhecheva, S. Malinov, W. Sha, Surf. Coat. Technol. 201 (2006) 2467. H. Shibata, K. Tokaji, T. Ogawa, C. Hori, Int. J. Fatigue 16 (1994) 370. M. Nakai, M. Niinomi, T. Akahori, N. Ohtsu, H. Nishimura, H. Toda, H. Fukui, M. Ogawa, Mater. Sci. Eng. A 486 (2008) 193. P. Saillard, A. Gicquel, J. Amouroux, Surf. Coat. Technol. 45 (1991) 201. N. Renevier, P. Collignon, H. Michel, T. Czerwiec, Surf. Coat. Technol. 111 (1999) 128. V. Fouquet, L. Pichon, M. Drouet, A. Straboni, Appl. Surf. Sci. 221 (2004) 248. E. Gyorgy, A. Perez del Pino, P. Serra, J.L. Morenza, Appl. Surf. Sci. 186 (2002) 130. E. Gyorgy, A. Perez del Pino, P. Serra, J.L. Morenza, Surf. Coat. Technol. 173 (2003) 265. M.W. Turner, P.L. Crouse, L. Li, Appl. Surf. Sci. 253 (2007) 7992. N. Ohtsu, K. Kodama, K. Kitagawa, K. Wagatsuma, Appl. Surf. Sci. 256 (2010) 4522. O.R. Shojaei, A. Karimi, Thin Solid Films 332 (1998) 202. W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. A. Pérez del Pino, P. Serra, J.L. Morenza, Thin Solid Films 415 (2002) 201. A. Pérez del Pino, J.M. Fernández-Pradas, P. Serra, J.L. Morenza, Surf. Coat. Technol. 187 (2004) 106. C. Barnett, E. Gahoon, J.R. Almirall, Spectrachim. Acta Part B 63 (2008) 1016. Q. Wang, P. Jander, F.-C. Begemann, R. Noll, Spectrochim. Acta Part B 63 (2008) 1011. H. Li, S. Costil, V. Barnier, R. Oltra, O. Heintz, C. Coddet, Surf. Coat. Technol. 201 (2006) 1383. E.C. Santos, M. Morita, M. Shiomi, K. Osakada, M. Takahashi, Surf. Coat. Technol. 201 (2006) 1635. B.O. Johansson, J.-E. Sundgren, J.E. Greene, A. Rockett, S.A. Barnett, J. Vac. Sci. Technol. A 3 (1985) 303. N. Ohtsu, K. Kodama, K. Kitagawa, K. Wagatsuma, Appl. Surf. Sci. 255 (2009) 7351. F. Esaka, K. Furuya, H. Shimada, M. Imamura, N. Matsubayashi, H. Sato, A. Nishijima, A. Kawana, H. Ichimura, T. Kikuchi, J. Vac. Sci. Technol. A 15 (1997) 2521. A. Rizzo, M.A. Signore, L. Mirenghi, T. Di Luccio, Thin Solid Films 517 (2009) 5956. M. Kawamura, Y. Abe, H. Yanagisawa, K. Sasaki, Thin Solid Films 287 (1996) 115. J.L. Solomon, W.L. Baun, Surf. Sci. 51 (1975) 228. G.B. de Souza, C.E. Foerster, S.L.R. de Silva, F.C. Serbena, C.M. Lepienski, C.A. dos Santos, Surf. Coat. Technol. 191 (2005) 76. P. Patsalas, C. Charitidis, S. Logothetidis, Surf. Coat. Technol. 125 (2000) 335. L.M. Cabalin, J.J. Laserna, Spectrochim. Acta Part B 53 (1998) 723. A. Bogaerts, Z. Chen, Spectrochim. Acta Part B 60 (2005) 1280.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface hardening of titanium by pulsed Nd:YAG laser irradiation at 1064- and 532-nm wavelengths in nitrogen atmosphere Naofumi Ohtsu a,∗ , Misao Yamane a , Kenji Kodama b , Kazuaki Wagatsuma c a b c
Instrumental Analysis Center, Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507, Japan EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 12 April 2010 Received in revised form 8 July 2010 Accepted 8 July 2010 Available online 16 July 2010 Keywords: Laser nitridation Laser wavelength Depth profile Nanoindentation
a b s t r a c t The surface hardness of titanium modified by laser irradiation at different wavelengths in nitrogen atmosphere was investigated. Further, surface characteristics such as morphology, chemical state, and chemical composition in the depth direction were also studied. The size and depth of the craters observed in the laser-irradiated spots increased monotonically with an increase in the laser power. Furthermore, the crater formed by the 532-nm laser was deeper than that formed by the 1064-nm laser for the same laser power. Laser power beyond a certain threshold value was required to obtain a titanium nitride layer. When the laser power exceeds the threshold value, a titanium nitride layer of a few tens of nanometers in thickness was formed on the substrate, whereas a titanium oxide layer containing small amounts of nitrogen was formed when the laser power is below the threshold value. Thus, it was shown that laser irradiation using appropriate laser parameters can successfully harden a titanium substrate, and the actual hardness of the titanium nitride layer, measured by nanoindentation, was approximately five times that of an untreated titanium surface. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titanium and its alloys have low density, good corrosion resistance, good biocompatibility, and high specific strength. However, their frictional wear resistance is sometimes not adequate for many applications [1]. Surface modification is one of the most effective techniques for improving the wear resistance of titanium; in particular, surface nitridation resulting from the diffusion of nitrogen is widely used in the industry. Heretofore, surface nitridation has been achieved by heat treatment in nitrogen atmosphere (gas nitriding process) [2–4] or by exposure in a nitrogen plasma (plasma nitriding process) [5–7]. These processes can form a homogeneous nitride layer on a titanium surface having a relatively large area, but the modified area is not controllable. Recently, George et al. reported that a surface layer containing nitrogen can be formed on a titanium substrate by laser irradiation in a nitrogen atmosphere [8,9]. Surface treatment using laser irradiation has several advantages; for example, the modified surface is localized in an area of the order of micrometers and can be controlled by rasterizing the laser beam. Consequently, nitridation by laser irradiation has gained considerable attention as a new process that can modify
∗ Corresponding author. Tel.: +81 157 26 9563; fax: +81 157 26 9563. 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.07.025
a localized area. This process is expected to be applied to produce various microdevices. In the laser nitridation process, the nitride layer is produced as a result of complex effects such as the ablation of the naturally formed oxide, melting of the substrate, reaction of the substrate material with the nitrogen gas, and chemical and physical reactions occurring in the gas plasma because of the laser breakdown. These phenomena are affected by the laser irradiation conditions; accordingly, it is considered that the characteristics of the irradiated surface vary depending on the laser parameters [10]. In a previous study [11], the authors used X-ray photoelectron spectroscopy (XPS) to carefully analyze a surface film on titanium formed by laser irradiation at different laser powers and wavelengths. The study showed that a titanium nitride layer is formed on the surface when the laser power exceeds a certain threshold value. Those experimental results showed that a hard surface layer might be obtained by laser irradiation under appropriate laser conditions. In this study, we focus on the actual hardness of the laser-nitrided surface. The primary objective of the present study is to obtain the hardness profiles of a titanium surface modified by laser irradiation at different wavelengths in nitrogen atmosphere. The mechanical properties of titanium surfaces depend on several characteristics of the surface film such as the morphology, thickness and so on [12]. Accordingly, we also investigated the surface characteristics such
692
N. Ohtsu et al. / Applied Surface Science 257 (2010) 691–695
as morphology, chemical state, and chemical composition in the depth direction, in order to discuss the effects of laser conditions on the hardness profiles.
culated from load–displacement curves using the Oliver and Pharr method [13]. Considering the surface roughness, the indentations were repeated seven times and the average of these results was used for the calculations.
2. Experimental procedures 3. Results and discussion A pure titanium plate (99.99% purity) with dimensions of 10 mm × 10 mm × 1 mm, which was mechanically polished with a diamond paste and then ultrasonically washed in ethanol, was fabricated as the substrate. The titanium substrate was transferred to the reaction chamber having the base pressure less than 2.0 × 10−6 Pa, and nitrogen gas was then introduced into the chamber through a variable leak valve at a pressure of 13.3 kPa. A Nd:YAG Q-switched laser (Minilite I, Continuum, Inc., U.S.A.) 1064-nm and 532-nm (SHG mode) wavelength outputs with a (4 ± 1) ns pulse width was used for nitridation. The laser energy per pulse was varied from 4.8 ± 0.5 mJ to 19.8 ± 0.5 mJ, which was determined with a laser-power meter. The laser beam was manually shot with a trigger switch at a repetition rate of ca. 0.8 Hz, focused using a lens, introduced into the reaction chamber through a synthetic quartz window, and finally irradiated on the titanium substrate under nitrogen atmosphere. The laser shots were fixed at 50 shots. The size and depth of the craters formed by the laser irradiation were estimated by optical microscopy and laser scanning microscopy analysis (VF-7500, Keyence Co., Ltd., Japan), respectively. Microstructural observation was conducted by scanning electron microscopy (SEM; JSM-5800, JEOL, Japan) at an operation voltage of 10 kV. The changes in the chemical state and composition of the surface in the depth direction were analyzed by Auger electron spectroscopy (AES; ESCA1600, Ulvac-Phi, Japan) along with argon-ion etching. The energy spectrum of the Auger electrons was analyzed with a concentric hemispherical analyzer (CHA). The acceleration voltage of the electron probe and Ar ions for the etching were set to 10 kV and 3.0 kV, respectively. Load–displacement curves of the surface were measured by using a nanoindenter (UMIS2000, CSIRO, Australia) with a Berkovich diamond indenter tip. The indentation load was varied from 20 N to 2 mN to achieve different penetration depths. The hardness of the surface was cal-
3.1. Surface characteristics of laser-irradiated titanium surface Optical views of the laser-irradiated spots drastically changed depending on the laser power and wavelength. Typical images obtained by optical microscopy are shown in Fig. 1. A round-shaped central zone of the crater appeared after the laser irradiation. The size of the crater increased when using the high-power laser and, concomitantly, a surrounding zone observed in the boundary region was discolored. It has been reported that the color of a titanium surface depends on the thickness of the surface oxide layer [14,15]. A golden color corresponding to a relatively thin layer changes into purple through brown with an increase in the thickness. Therefore, the change in color, as shown in Fig. 1, indicates that the thickness of the surface oxide at the boundary region increased with an increase in the laser power. Furthermore, it should be noted that, at the same laser power, the surface oxide at the boundary regions formed by the 532-nm laser was thicker than that formed by the 1064-nm laser. In Fig. 2, the depth of the crater is plotted against the laser power. The depth increases monotonically with an increase in the laser power; at the same power, the crater formed by the 532nm laser was deeper than that formed by the 1064-nm laser. The craters were formed due to the vaporization of the titanium substrate; therefore, their depth was determined principally by the laser energy absorbed on the surface. It is likely that the energy distributed on the substrate increases with the laser power. Accordingly, the differences in the crater depths indicate that the laser energy distributed on the surface changes depending on the laser wavelength. This result is consistent with the results reported in other literatures [11,16,17]. Microstructures at the laser-irradiated spot formed by the 532nm laser at 15 mJ were observed by SEM (Fig. 3). The surface
Fig. 1. Optical views around the laser irradiated spot observed by optical microscopy. (a) 4.8 mJ at 1064 nm, (b) 9.2 mJ at 1064 nm, (c) 19.7 mJ at 1064 nm, (d) 4.6 mJ at 532 nm, and (e) 9.4 mJ at 532 nm.
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Fig. 2. Laser power dependence for the depth of the crater observed in laser irradiated spot.
morphology of the crater appeared to be smooth, whereas small pits of submicrometer size were scattered in the crater. Similar smooth surfaces containing small pits were also observed in the craters formed under the other laser conditions. Li et al. reported that small pits appearing after the laser irradiation correlated with the formation of surface metallographic defects, such as microinclusions and small visible scratches during a polishing process [18]. They also reported that the number of pits could be decreased with a defect-eliminating treatment such as chemical etching [18]. Fig. 3 shows that the boundary region contained many microcracks, which was probably due to the solidification of melted titanium at a high cooling rate [19]. The changes in the chemical composition in the depth direction at the laser-irradiated spots, measured by AES, are shown in Fig. 4. The Auger electron peak corresponding to N KLL around 377 eV overlapped with the Ti L3 M23 M23 peak around 387 eV, as shown in
Fig. 4. AES depth profiles of the laser irradiated spot formed by different wavelength and power laser. (a) 1064 nm at 20 mJ, (b) 532 nm at 7 mJ, and (c) 1064 nm at 7 mJ.
Fig. 5. We estimated the chemical composition of the surface from a quantitative evaluation proposed by Johansson et al. [20]. In the case of the 1064-nm laser at 20 mJ and the 532-nm laser at 7 mJ, the depth profile obtained can be roughly divided into three
Fig. 3. SEM images around the laser irradiated spot formed by 15.1-mJ laser at 532 nm. (a) Boundary region, (b) crater area, and (c) enlargement of the crater area.
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Fig. 6. Typical load–displacement curves of a titanium plate before and after the laser irradiation. The maximum applied load was 2 mN.
Fig. 5. AES differential spectra containing Ti LMM and N KLL transition lines for the laser irradiated spot after sputtering for: (a) 100 s and (b) 800 s.
regions, as shown in Fig. 4(a) and (b). The topmost layer (labeled as “Layer 1”) contained oxygen, nitrogen, and titanium, which corresponds to an oxynitride layer having a TiOx Ny -like structure, as reported in several literatures [21–23]. The lower surface layer (labeled as “Layer 2”) consisted of titanium and nitrogen. AES differential spectra corresponding to the lower surface layer were compared with standard spectra for stoichiometric titanium nitride (TiN) [24], as shown in Fig. 5(a). Spectral shapes obtained using the 1064-nm laser at 20 mJ and the 532-nm laser at 7 mJ are similar to those of TiN; further, the peak energies corresponding to the N KLL transition agree with those for TiN. Therefore, the chemical state of the lower surface layer was identified to be titanium nitride. The thickness of the nitride layer was estimated to be a few tens of nanometers. The inner region below the surface layer (labeled as “Layer 3”) showed strong titanium and weak nitrogen intensities. AES differential spectra of the inner region almost coincided with the standard titanium spectrum, as shown in Fig. 5(b), indicating that the chemical state was metallic titanium. Low nitrogen intensity observed in the inner region was considered to be background. In contrast, in the case of the 1064-nm laser at 7 mJ, the depth profile can be roughly divided into two regions, as shown in Fig. 4(c). The surface layer (labeled as “Layer I”) contained titanium, nitrogen, and a high concentration of oxygen. In the AES spectrum of the surface layer, the N KLL peak was hardly observed, and the peak corresponding to the Ti L3 M23 M45 transition (solid arrow) shifted toward the lower energy side as compared to that for TiN, as shown in Fig. 5(a). These results indicate that a titanium nitride layer cannot be obtained by using a 1064-nm laser at 7 mJ. The surface layer probably consisted of a titanium oxide layer containing a small amount of oxygen [21]. The inner region below the surface layer (labeled as “Layer II”) consisted of titanium and oxygen. The AES peak around 387 eV, corresponding to the Ti L3 M23 M45 transition, shifted slightly toward the negative energy (dotted arrow), as shown in Fig. 5(b). This negative shift was likely due to the for-
mation of titanium oxide such as TiO [25], indicating that oxygen diffused into the deeper region under this laser condition. In our previous study, we found that a laser power beyond a certain threshold value is required for forming a titanium nitride [11]. The threshold powers, estimated from XPS analysis, for the 1064- and 532-nm lasers were 14 mJ and 6 mJ, respectively. It is clear that the laser power of 20 mJ at 1064 nm and 7 mJ at 532 nm exceeded the required threshold power; this was confirmed by the titanium nitride layers observed in the AES profiles. These analytical results obtained from the AES analysis are completely consistent with the results from our previous XPS analysis [11]. 3.2. Hardness profile of laser-irradiated surface Typical load–displacement curves of a titanium plate before and after the laser irradiation are shown in Fig. 6. It is clear that tip penetration for the laser-irradiated titanium plate was shallow, as compared to the non-irradiated titanium plate. Hardness profiles before and after the laser irradiation, which were calculated from the load–displacement curves, are shown in Fig. 7. The laser powers exceeded the threshold values for obtaining a titanium nitride layer. The hardness at a sampling depth of 10 nm for the non-irradiated sample is approximately 5 GPa, which is almost equal to that reported in a previous paper [26]. The hardness in a depth range of 10–50 nm is almost constant. On the other hand, the hardness of the laser-irradiated sample at a depth
Fig. 7. Hardness profiles of a titanium plate before and after the laser irradiation, obtained by nanoindentation.
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of 10 nm is approximately 27 GPa at both the laser wavelengths. This value is approximately five times that of the non-irradiated titanium plate. We found that the hardness gradually decreases with increasing depth and reaches approximately 13 GPa at a depth of 30 nm. In the region beyond 30 nm, the hardness is almost constant. 3.3. Effects of surface characteristics on hardness profiles The nanoindentation test clearly shows that a titanium surface can be successfully hardened by laser irradiation by using a laser with power exceeding the threshold value. The hardness profile obtained by using the 1064-nm laser at 20 mJ almost coincided with that of the 532-nm laser at 7 mJ. The thickness of the hardened region is estimated to be approximately 20 nm in both the profiles, which almost corresponds to that of the nitride layer estimated from the AES profile (shown in Fig. 4). The hardness of the laserirradiated surface, measured by nanoindentation, is almost equal to that of a titanium nitride layer prepared by DC reactive sputtering [27]. These results indicate that the hard surface originated from the formation of a titanium nitride layer. When a titanium surface is irradiated by a laser, a new surface is formed due to the absorption of laser energy. The energy distributed on the substrate is indicated by the target surface reflectivity, which changes depending on the wavelength. For example, the reflectivity of Cu is 0.97 and 0.61 at 1064- and 532-nm wavelengths, respectively [28]. At the 532-nm wavelength, 39% of the laser energy penetrated into the substrate, resulting in heating, melting, and vaporization. On the other hand, only 3% of the laser energy can be used for the substrate reaction at the 1064-nm wavelength. Accordingly, at the same laser power, the crater was deeper when using the 532-nm laser. The difference in wavelengths also affected the characteristics of the laser-induced plasma, such as plume size, maximum temperature, and laser adsorption in the plasma [29]; however, these effects are not pronounced in the obtained hardness profiles. Consequently, we conclude that laser irradiation at the wavelength of 532 nm is more beneficial for hardening a titanium surface because a titanium nitride layer is obtained at a lower power. 4. Conclusions In the present study, we measured the hardness of a titanium surface modified by 1064- and 532-nm laser irradiation in nitrogen atmosphere. We also investigated the surface characteristics such as morphology, chemical state, and chemical composition in the depth direction. Our conclusions are as follows: (1) The size and depth of the craters increase monotonically with an increase in the laser power; concomitantly, the thickness of the oxide layer surrounding the crater also increases. The crater depth and the oxide layer thickness in the case of the 532-nm laser were larger than those in the case of the 1064-nm laser at the same laser power.
such as TiO is formed at the upper and lower surface regions, respectively. (3) Laser nitridation using optimal laser parameters can harden a titanium surface. The modified surface depth almost equal to the thickness of the titanium nitride layer. When using the 1064-nm laser at 20 mJ or the 532-nm laser at 7 mJ, the hardened region is approximately 20 nm in thickness, and the hardness of the surface is approximately five times that of an untreated titanium surface. Under the present experimental conditions, laser irradiation at the wavelength of 532 nm is more beneficial for hardening a titanium surface. Acknowledgements The authors gratefully acknowledge Dr. H. Kato and Dr. S. Sato of Institute for Materials Research, Tohoku University for their help in hardness measurement using nanoindenter. The authors also acknowledge Dr. M. Kawamura from Kitami Institute of Technology for providing AES standard spectra. This work was performed under the inter-university cooperative research program of the Laboratory for Analytical Science, Institute for Materials Research, Tohoku University and was supported by a Grant-in-aid from the Suzuki Foundation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
(2) Laser power exceeding a certain threshold value is required to obtain a titanium nitride layer. By using the 1064-nm laser at 20 mJ or the 532-nm laser at 7 mJ, a titanium nitride layer of few tens of nanometers in thickness is formed on the substrate. When the 1064-nm laser is used at 7 mJ, a titanium oxide layer containing small amounts of nitrogen and titanium oxide
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[25] [26] [27] [28] [29]
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