Preparation of porous microstructures on NiTi alloy ...

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Preparation of porous microstructures on NiTi alloy surface with femtosecond laser pulses LIANG ChunYong1,3, YANG Yang2, WANG HongShui3, YANG JianJun2† & YANG XianJin1† 1

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China; Institute of Modern Optics, Nankai University, Key Laboratory of Opto-electronic Information Science and Technology, Education Ministry of China, Tianjin 300071, China; 3 School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China 2

Porous microstructures on Nickel-Titanium (NiTi) alloy surfaces were prepared by linearly polarized femtosecond lasers with moving focal point at a certain speed. It was found that various novel microstructures from feather-like ripples to cluster-like porous textures could be formed with increasing laser energy. Particularly, when the laser energy was 400 µJ, a periodic porous metal surface was generated. Measurement of X-ray diffraction showed that the grains on the sample surface were refined through femtosecond laser ablation processes, but the crystal structures still kept their original states. Analysis by X-ray photoelectron spectroscopy revealed that Ni/Ti on the sample surface was changed with an evident oxidization of titanium element under different laser energies. This investigation provides a new approach to improve the biocompatibility of NiTi-based implant devices. femtosecond laser, NiTi alloy, porous microstructures, grain refinement

1 Introduction NiTi alloy with the near proportion of atoms is well known for its shape memory effect, hyperelastic, high fatigue resistance and superior abradability. Because of its better biocompatibility compared with stainless steel[1], NiTi alloy usually becomes an ideal biological implant materials and has been widely used as the orthodontic arch wire, osteosynthesis staples, some devices for surgical correction of scoliosis, vena cave filter and stent, and so on[2,3]. Recent vitro studies[3,4] concluded that the cell culture based on NiTi alloy could be similar to that of pure titanium. This is unexpected since NiTi contained almost 50% of Ni atom. As it is known, Ni usually causes allergic and toxic effects despite its functional role in the human body. Recently, many researchers have focused their interests on porous NiTi alloy that has the similar structures ― with human bones[5 9]. Besides the intrinsic hyperelastic and good corrosion resistance, the porous structures of NiTi alloy have brought some new advantages including www.scichina.com | csb.scichina.com

adjustable mechanical properties, reduced weight and increased biocompatibility for biomedical applications[10]. Particularly, its porosity has an induction in the bone tissue growing, which makes the fixation of implant more safe and reliable. The typical methods of fabricating porous NiTi include hot isostatic press (HIP) [10], conventional sintering (CS) [11], self-propagating high-temperature synthesis (SHS)[12] and laser spray[13,14]. The methods of HIP, CS and SHS can obtain the bulk porous NiTi. While bulk porous NiTi samples possess attractive properties, they may not be strong enough for large load-bearing applications. In fact, with the increase of porosity, both the mechanical strength and the corrosion resistance of porous NiTi almost linearly decrease[15]. Although laser spray is a more convenient method to prepare the porous NiTi surface and can Received November 20, 2007; accepted November 28, 2007 doi: 10.1007/s11434-008-0149-0 † Corresponding author (email: [email protected]; [email protected]) Supported by the National Natural Science Foundation of China (Project No. 50471048) and Specialized Research Fund for the Doctoral Program of Higher Education of China (Project No. 20040056016)

Chinese Science Bulletin | March 2008 | vol. 53 | no. 5 | 700-705

2 Experimental In our experiments, a commercial Ti:sapphire femtosecond laser amplifier system was used, which (all from Spectra Physics Inc.) consists of a Ti:sapphire laser oscillator (Tsunami 3960) and a chirped pulse amplification unit (HP-Spitfire) pumped by a Evolution-30 operating at 1 kHz repetition rate. Laser pulse trains of 50 fs with the center wavelength of 800 nm and the maximum pulse energy of 2 mJ could be produced from the laser amplifier system. The pulse duration was measured continuously by an autocorrelator (SSA, Positive Light Inc.).

3 Results and discussion Some researches have proved that porous structures on NiTi surface would improve its biocompatibility[10]. So how to easily obtain the porous surface structures becomes crucial for NiTi alloy to realize its medical appli-

LIANG ChunYong et al. Chinese Science Bulletin | March 2008 | vol. 53 | no. 5 | 700-705

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The amplified laser beam was linearly polarized in the horizontal direction. According to the requirement of the particular experiment, the laser pulse energy could be attenuated through a variable neutral-density filter. All the experiments were carried out in ambient air in a Class 1000 clean room. NiTi plates of 10×10×2 mm mechanically ground with SiC emery paper were employed as samples. The sample was pasted on a microscope slide and then mounted on a computer-controlled three-axis (x-y-z) translation stage (UTM 100 PPE1, New Port Inc.) with a resolution of 1 µm. Figure 1 illustrates a schematic diagram of our experimental setup. First, the laser pulses were focused though a 10 × microscope objective (numerical aperture = 0.25) on the target surface at a normal incident angle, producing an estimated focal beam size around 10 µm. The type of laser micro-machining experiment we have performed was to scribe lines on the surface of NiTi targets with different laser energies through moving the sample at a speed of 0.8 mm/s along the y-direction, which is parallel to the laser beam polarization. The two consecutive scribing lines were spaced at 10 µm. After the laser ablation experiments, the deposited debris on the sample surfaces were removed by using an ultrasonic cleaner. Surface morphology of the samples was investigated by a scanning electron microscopy (SEM, HITACHI S-4300). The crystal structures of the sample were analyzed by X-ray diffraction (XRD), which conducted on a RIGAKUD/MAX2500 diffractometer with Cu Ka radiation, and the range of 2θ angles was from 20° to 80°. The surface components after the laser treatment were analyzed using X-ray photoelectron spectroscopy (XPS, Model PHI 5300, Physical Electronics) equipped with an aluminum anode (13 kV, 1486.6 eV) and a quartz monochromator, with take-off angle of 45°. PHI-Multipak software was used to integrate the experimental peaks. Elemental surface compositions were calculated from the integrated peak areas employing instrumental sensitivity factors as supplied by the manufacturer and expressed as atomic percentage.

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keep the mechanical property of the base[13,14], this method usually needs TiH2 as a vesicant which may have a negative effect on the bioactivity of NiTi. Recently Wong et al.[16] made an attempt to utilize Nd-YAG lasers to treat the NiTi alloy surface, and found that the oxidization depth increased. However, the porous metal surface was not generated on their expectations. Fortunately, femtosecond lasers have been established to be an excellent and universal tool for micro- and nanostructuring solid materials by direct ablative writing. Compared with conventional nanosecond laser pulses, femtosecond laser has great predominance for its rapid creation of vapor and plasma phase, negligible heat ― conduction, and absence of liquid phase[17 19]. As observed in previous studies, when a femtosecond laser beam focused stationarily on/into the sample, ripple structures could be formed. This phenomenon has been reported on a wide variety of materials such as metals, semiconductors, and dielectrics. Most important, the porous structures etched by femtosecond laser pulses will provide a new effective way to be applied in many aspects. In this work, a line-scribing experiment was performed on the surface of NiTi alloy block with the irradiation of Ti:sapphire femtosecond laser pulses (800 nm, 50 fs). Multiple porous surface microstructures were generated through translating the sample in the focal plane at 0.8 mm/s speed for several different laser energies. Their chemical evolution with different patterns was then analyzed by both X-ray diffraction and X-ray photoelectron spectroscopy. The results showed that the crystal structures on the sample surface still kept their original states, but the grains were refined and the ratio of Ni/Ti was also changed after the laser processing.

Figure 1 Schematic diagram of the experimental setup for the investigation of porous microstructures formation on the metal surface. E and S correspond to the direction of the laser polarization and the sample translation, respectively.

cations. Figure 2 shows SEM pictures of the surface morphology of the sample associated with different laser energies of 10 µJ (Figure 2(a), (b)), 100 µJ (Figure 2(c), (d)) and 400 µJ (Figure 2(e), (f)) respectively. As it is seen clearly, when the sample was irradiated with 10 µJ of femtosecond laser pulses, the feather-like ripple patterns with some micro-trenches and holes were formed on the surface. The orientation of these regular lines is perpendicular to the scan direction and the electric field of the laser beam. Since the spatial period of the ripples was measured to be 0.8 µm, which is in consistent with the moving step of the focal point, they are convinced to be from the pulse imprints. With the increase of the laser energy, the modification of the metal surface due to the laser ablation process was strengthened, and the laser-induced surface structures were found to change as well. For example, when the laser energy became 100 µJ, as shown in Figure 2(c), a unique pattern appeared on the metal surface, where two sets of periodic structures were formed: one is oriented perpendicularly to the scan direction with a high spatial frequency; the other is aligned along the scan direction with a low spatial frequency. Therefore, some deep micro-holes were generated in their overlapped areas. In this case, the high spatial frequency microstructures had a period about 630 nm, less than the incident laser wavelength. Their underlying mechanisms could attribute to the local field enhancement of transient surface air 702

nanoplasmas[20,21], resulting in a nonthermal ablation of the metal targets[22]. In fact, as the laser energy continued to increase up to 400 µJ, a more regular distribution of the porous structures was evident on the metal surface, which was surrounded by high spatial frequency microstructures. As shown in Figure 2(e), at this time, the diameter of the micro-holes was measured to be more than 1 µm, and the corresponding depth was increased as well. To investigate the structure evolution of the ablated metal surface by femtosecond laser pulses, we measured XRD spectra of the sample surfaces at different laser energies, as shown in Figure 3. Clearly, at different energies of the laser irradiation, the positions of the characteristic peak for the treated samples have not yet been moved. This suggests that the basic structure of the NiTi alloy surfaces does not change in spite of the laser treatment by using different energies. However, with increasing laser energy, the intensity of the characteristic peak of the NiTi alloys reduced gradually, and the corresponding full-width-at-half-maximum (FWHM) value became larger. It indicates that the grains on the sample surface have been refined after the laser irradiation. During the femtosecond laser machining, the specimen will pass through the heating, melting even gasification processes, then it rapidly cools down. Consequently, the crystal nucleus formation in the cooling stage does not have time to grow in time, resulting in the fine grains.


SPECIAL TOPIC ARTICLES OPTICS Figure 2 Various porous microstructures on the NiTi alloy surface after the laser treatment with different energies. (a) is for the laser energy of 10 µJ; (c) is for 100 µJ; (e) is for 400 µJ. (b), (d) and (f) are their respective Zoom-in pictures.

Figure 3 XRD spectra of the laser-treated NiTi alloy surfaces with different energies of 10, 100, and 400 µJ.

The XPS 2p spectra of Ti and Ni elements for the NiTi samples treated by femtosecond lasers with different energies are shown in Figures 4 and 5, respectively.

Figure 4 XPS 2p spectra of Ti element on NiTi alloy surface after treatment by femtosecond laser pulses with different energies. (a) is for the laser energy of 10 µJ; (b) is for 100 µJ; (c) is for 400 µJ.


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Table 1 Atomic percentage of elements on the NiTi alloy surface treated by femtosecond laser pulses Laser energy 10 µJ 100 µJ 400 µJ

Figure 5 XPS 2p spectra of Ni element on NiTi alloy surface after treatment by femtosecond laser pulses with different energies. (a) is for the laser energy of 10 µJ; (b) is for 100 µJ; (c) is for 400 µJ.

The quantivalence states and the atom content of the elements are shown in Table 1. It should be mentioned that the measured spectra have been corrected in respect of the background and scattering light through using the Liner method. From these two figures we can find that Ti element at the sample surface is essentially in the Ti4+ state (with corresponding 2p3/2 peaks at 459.2 eV, and 2p1/2 peaks at 465.0 eV), corresponding to the generation of TiO2 compound, while Ni element at the sample surface was oxidized greatly after the femtosecond laser irradiation, leading to an obvious change of Ni/Ti ratio. At the laser energy of 10 µJ, Ni3+/Ni was 1.70, and Ni/Ti was 0.376. When the laser energy increased to 100 µJ, Ni3+/Ni reached 5.06, and Ni/Ti increased to 0.857. However, when the laser energy continuously increased to as high as 400 µJ, Ni3+/Ni became decreased slightly (4.65) and the Ni/Ti was reduced to 0.368 as well. 1

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Ni/Ti 0.376 0.857 0.368

Ni3+/Ni 1.70 5.06 4.65

4 Conclusions In this study, porous microstructures on NiTi alloy surface were prepared by femtosecond lasers. Some conclusions are drawn as follows: (1) With the increase of the laser energy, the micro-holes formed on metal surface were enlarged and at the same time the ripples became shortened. When the laser energy was up to 400 µJ, a periodic porous surface was generated. (2) The grains on the sample surface were refined by femtosecond laser machining with different energies, but the crystal structure of the samples kept its original state. (3) The Ni/Ti on the surface was changed greatly and Ni was oxidized obviously when the samples were machined under different laser energies. At the laser energy of 400 µJ, the surface contained mainly titanium dioxides, together with a small amount of Ni mainly in its oxidized state. 6

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Science in China Series E: Technological S c i e n c e s EDITOR YAN Luguang Institute of Electrical Engineering Chinese Academy of Sciences Beijing 100080, China

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