Microstructuring of Ti plates with femtosecond laser pulses is investigated in three different liquids. In these ambiences, complex microstructures with voids and islands are produced on the sample surfaces, whose feature sizes are controlled by the laser parameters. Through adopting supersaturated Hydroxyapatite suspension with higher incident laser fluences, it is for the first time to observe the firm deposition of biocompatible elements Ca-P on the microstructures. At lower laser fluence, only porous structure is present but without additional elements deposition. Both plasma-related ablation under the confinement of liquids and micro-bubbles striking are employed to discuss such structures formation. Tight combining elements Ca-P onto the structured surfaces provide a new way to improve the biocompatibility of body-embedded devices.
© 2009 OSA
Laser-induced surface microstructures on different materials have been attractive in recent years, not only because they benefit to exploring laser-matter interaction, but also provide great applications in the field of physics, chemistry and material science [1–5]. Ever since the first observation by Birnbaum in 1960s , surface microstructures formation under the radiation of intense laser beam has been reported on a variety of solid materials [7–10]. Although some mechanisms, including scattered light interference , self-organized plasma modification , secondary bubbles ablation in liquids  and so on, have been proposed, no single theory can be commonly accepted for different structuring cases.
For example, laser-induced periodic surface structure has been now mostly attributed to interference between the incident and scattered lights, wherein the ripple spacing was verified to depend closely on the incident angles [8,9,11]. However, such theory could not be employed simply to explain our previous report that the ripple’s periodicity could keep about 630 nm with varying incident angles . Recently, Guo et al. pointed out that the periodicity of ripples induced by femtosecond lasers on the metals could be shortened with increasing the number of laser shots . Anyway, previous studies suggest that the formation of surface microstructures should be related to the incident laser parameters and material properties.
In fact, the ambient medium parameters have also an important influence on the quality of laser treatments. For instance, Mazur reported that in the presence of SF6 gases quasi-ordered arrays of conical spikes could be formed spontaneously on silicon under the irradiation of femtosecond lasers . In water, high-density regular arrays of nanometer-scale rods were induced on silicon surface via femtosecond laser irradiation [17,18]. These phenomena indicate that laser-induced surface structures formation also depends on the surrounding environments.
Generally, the current femtosecond laser-induced surface morphology experiment in liquids has mostly employed de-ionized water as the surrounding medium [17,19,20]. During these experiments, nonlinear phenomena such as supercontinuum and plasma generation could be observed due to the interaction of water phase with the incoming high-fluence laser beam. Moreover, for the laser ablation at solid-liquid interface, spectroscopic study revealed that various chemical reactions between the species ablated from the solid target and the ambient liquid molecules could take place . Namely, not only the species did come from the solid target, but also the species originated from the liquid are involved in the interface plasma. So some new phenomena could be expected if the laser ablation of targets in the presence of biocompatible solutions.
In this letter, we investigate the ambient medium-related surface microstructures formation by femtosecond laser pulses when Ti alloy plate samples are submerged into liquid environments. In distilled water ambience, a new kind of micro-texture consisting of micro-islands and nano-voids is generated on the laser-exposed surfaces. The feature size of such structures can be controlled by both the incident laser fluences and the number of laser shots. In the supersaturated Hydroxyapatite (HA, Ca10(PO4)6(OH)2) suspension and the mixture solution of CaCl2 and Na3PO4, similar surface microstructures are also produced. The influence of solution concentration on the surface microstructures formation process is also studied. Both energy dispersive X-ray (EDX) spectra and X-ray diffraction (XRD) analyses reveal that bio-compatible elements of Ca and P (Ca-P) are deposited firmly onto the structured surfaces when the sample are ablated within supersaturated HA suspension. Finally underlying mechanisms for the formation of surface structures and the deposition of the elements Ca-P in liquid environments are discussed.
2. Experimental setup
In our experiments, a commercial chirped pulse amplification of Ti:sapphire laser system (HP-Spitfire, New Port Inc.) was employed as a light source, which operated at a repetition rate of 1 kHz and provided 50 fs laser pulse trains with the central wavelength of around 800 nm. The maximum energy of each amplified laser pulse was 2 mJ. The output laser beam was polarized horizontally and had a diameter of about 8 mm. A single shot auto-correlator (SSA, Positive Light Inc.) was used to monitor the time duration of the laser pulses in real time. Selection of pure titanium (Ti) plates as a sample in the experiments is due to their wide applications in space manufacturing, medical embedment and so on, wherein the modification of metal surface properties through generating microstructures is very important and much desirable.
After surfaces of the sample with a dimension of 10 × 10 × 1 mm3 were polished with SiC emery paper to remove the oxide layers, it was placed on the bottom of an open glass vessel filled with suspension. The height of the liquid layer above the targets was 2.5 mm. The vessel was mounted on a computer-controlled x-y-z translation stage (UTM100PPE1, New Port Inc.) to allow its displacement with pre-set scanning velocity. Alternatively, exposure of the Ti plate under the layer of liquid was performed by the stationary focal laser beam at the normal incidence angle through a 10×microscope objective with a numerical aperture of 0.25. The upper surface of the target was set 100 μm below the focal plane in air but about 760 μm above the focal plane in liquid, which was determined from a microscopic analysis of craters on the Ti plate. In this case, the diameter of laser spot on the target was estimated about 62 μm. A schematic diagram of our experimental setup is shown in Fig. 1 .
In order to investigate the influences of liquid media on the surface microstructuring of metals by femtosecond lasers, we adopted the distilled water and two different kinds of solutions, including supersaturated HA aqueous suspension and the admixture of CaCl2 and Na3PO4. The reason why we choose these two kinds of solutions is that their solutes have essential elements for human bones, so that the elements deposition on Ti surfaces might improve the biocompatibility of metals for the body embedment. In the supersaturated HA aqueous solution, the ions of Ca2+, PO4 3-, HPO4 2-, H2PO4 - as well as HA nanoparticles will dissolve in the solution, while in the admixture of CaCl2 and Na3PO4, Ca2+, Na+, PO4 3-, HPO4 2-, H2PO4 - and Cl- will dissolve in the solution.
All the experiments were carried out in a Class 1000 clean room. Before and after the experiments, the samples were cleaned by an ultrasonic cleaner with de-ionized water. The morphology of the laser-exposed metal surfaces was examined with the help of a scanning electron microscope (SEM, Hitachi S-4800). The deposition chemical elements and their structures were analyzed through the energy dispersive X-ray spectra and X-ray diffraction.
3. Experimental results
3.1 Formation of surface microstructures in distilled water
Firstly, we put some distilled water into the glass vessel to study the surface morphological evolution of the targets with varying femtosecond laser fluences and the sample scan speeds, the measured results are shown by SEM images in Fig. 2 . As it can be seen, for a given laser energy of 300 μJ, corresponding to the laser fluence of 9.9 J/cm2, numerous irregular-shaped small islands and tiny elliptical voids were generated on the laser-exposed Ti surface at the sample scan speed of 1.8 mm/s. In this case, the number of laser pulses partially overlapping on each spot reaches N = 35. The characteristic size of these islands, more or less connected spatially, ranges from 2.27 μm to 3.4 μm. And the average length of the void is about 880 nm. The estimated density of micro-islands on the Ti plate amounts to 3.4×107 cm−2. Particularly, the top surfaces of the micro-islands appear relatively flat and smooth.
When the sample scan speed was decreased to 0.8 mm/s with partially overlapped laser pulses of N = 80 at this given laser fluence, very similar microstructures could be observed on the laser-exposed surface. However, in this case the feature size of the micro-islands began to increase and the length of the elliptical voids is also enlarged to about 1.43 μm, which is bigger than those at 1.8 mm/s speed. Moreover, the estimated densities of micro-islands and micro-voids on the Ti plate also decreased at this time.
If the scan speed continued to reduce as small as 0.2 mm/s at the given laser fluence of 9.9 J/cm2, which corresponds to the partially overlapped laser pulses of N = 320, the laser-scribed region became much different from the above two cases. As shown by the corresponding image in Fig. 2, the micro-islands look like somewhat spikes. Voids between the micro-islands were pronounced resulting in the estimated density of islands and the void diameter on the Ti plate to be 1.2×107 cm−2 and 1.55 μm, respectively. Another interesting feature of such surface structures is that stratified patterns on the micro-islands and inside the micro-voids can be seen evidently. These observations could be understood as follows: when the scan speed decreased to be 0.2 mm/s, surface defects as a result of the ablation by previous pulses will increase the radiation absorption of the subsequent laser pulses. Moreover, the radiation absorption can also increase due to the varied incident angle of the laser beam at the pre-machined spot area. Therefore, the nonlinear interaction between the targets and the incident laser pulses will become much stronger under this situation.
When the laser fluence was reduced to 3.3 J/cm2 at the lower energy of 100 μJ, as shown by SEM images in the lower row of Fig. 2, laser-induced surface microstructures on the targets could still be seen at different scan speeds. Apparently, the evolution of surface morphologies with decreasing scan speeds or increasing pulse overlap is very similar to that at the higher laser fluence. However, at the same scan speed, both micro-islands and micro-voids dimensions are be closely related to the incident laser fluence, that is, the lower the laser fluence is, the smaller the microstructures become. The dependence of the micro-void size on the sample scan speed or the partially overlapped laser pulses for two different laser fluences is shown by Fig. 3(a) . For the same laser fluence, the length of voids decreases gradually with the increase of scan speed from 0.2 mm/s to 1.8 mm/s, or the decrease of the number of laser pulses from N = 320 to N = 35. While for a given scan speed, the higher laser fluence will induce bigger voids. Figure 3(b) describes the variation of the micro-islands density as the function of the laser fluence and the scan speed. Clearly, the density of the micro-islands gets larger as the scan speed increases or the number of overlapped laser pulses decreases.
3.2 Formation of surface microstructures in HA suspension
In order to investigate the influence of ambient solutions on the generation of surface microstructures, we replaced the distilled water by the supersaturated HA suspension with different concentrations. To obtain a longer time of laser irradiation, we reduced the sample scan speed to 0.2 mm/s. Figure 4(a) shows surface microstructures generated in HA suspension with a concentration of 0.02 g/ml by the laser fluence of 9.9 J/cm2. Similar to the structures formation in distilled water, the laser-exposed Ti surface was also covered by irregular-shaped small islands. However, in this case the micro-islands formation turned much sharper with bigger feature sizes. Correspondingly, the spatial arrangement density of these micro-islands was reduced as well.
As shown clearly by the zoom-in pictures of in Fig. 4(a), the entire surface of the micro-islands is studded with many tiny solid grains, which is different from the flat micro-islands generated in the case of distilled water. Interestingly, there are some deposition among the surface grains and the valleys. It should be noted that such deposited substances could still be observed even after 30 minutes’ washing by an ultrasonic cleaner, which suggests the bound force between these depositions and the microstructures is relatively high. Further analysis of these depositions will be given in the coming section 4. On the other hand, when the laser fluence was decreased to 3.3 J/cm2, such spikes could not be seen any more on the laser-exposed Ti surfaces within ambience of HA suspension. Instead, porous surface structures with the diameter of around 910 nm could be generated under this condition, which is similar to the observations in distilled water. However, as shown by Fig. 4(b), the generated pores seem to be smaller than those in distilled water. Particularly, the deposition of substances on the structured area was not found in this case, which indicates the higher laser energies should be very necessary for the substances deposition among the surface microstructures.
Figure 5 shows surface microstructures formation in HA suspension with a larger concentration of 0.04 g/ml, where the incident fluence of femtosecond laser was 3.3 J/cm2 and the sample was scanned at moving speed of 0.2 mm/s. Expectably, the generated porous surface is similar to the structure in Fig. 4(b), which suggests that the concentration of the solution has little influence on the microstructures formation. Moreover, the size of micro-cavity was found to remain about 910 nm even with different solute concentrations. Comparing with the results in Fig. 2, we can understand that the size of the micro-cavity on the targets is rather influenced by the incident laser fluence than by the solution concentration. Remarkably, although the higher concentration of the solution was employed, deposition of the particular substances could not be found in the regime of low laser fluence.
3.3 Formation of surface microstructures in the mixed solution of CaCl2 and Na3PO4
Figure 6 shows the typical surface structures on the Ti plate in the mixed solution of CaCl2 and Na3PO4 with a concentration of 0.018 g/ml, when the sample was scanned at the speed of 0.2 mm/s under the irradiation of femtosecond lasers with two different fluences. When the laser fluence was 9.9 J/cm2, the micro-islands structures were evidenced again, but with better-organized spatial distribution, and the shape of the micro-islands turns to be more regular. At the laser fluence of 3.3 J/cm2, micro-cavity structures could also be produced on the sample surfaces. Compared with the structures in Fig. 5, micro-cavities in this case had almost the same size of 910 nm. However, the deposition substances could not be found at this concentration, even with higher laser fluence of 9.9 J/cm2.
If the solution concentration was reduced to 0.008 g/ml and the sample was scribed by femtosecond laser with less fluence of 3.3 J/cm2, the porous surface microstructures on the laser-exposed region can be observed with the void size of 910 nm, which confirms that the size of micro-voids is insensitive to the variation of the solution concentration.
3.4 Chemical element deposition on the microstructured metal surfaces
In order to investigate chemical evolution of the metal surfaces structured by femtosecond lasers in the suspension of HA, we produced several large areas of these structures on different Ti plates, each of which has an area of 10 × 10 mm2. After the laser-structured samples were washed by the ultrasonic cleaner, their surface compositions were analyzed through EDX and XRD measurements. The obtained results are depicted by Fig. 7 , where two different laser fluences were employed. In the case of EDX measurements with peak signals corresponding to chemical components, multiple peaks standing in the case of laser treatment with the higher fluence of 9.9 J/cm2 suggests that two important chemical elements Ca and P have been deposited during the laser treatment. While for the laser fluence of 3.3 J/cm2, as shown in Fig. 7(a), only peaks of Ti and Oxygen elements are displayed in the curve, and signals relevant to both Ca and P elements seem to disappear, which indicates that little additional elements Ca-P are deposited on the micro-structured sample surfaces under this situation .
In the case of XRD measurement, distinct narrow peak signals represent diffraction intensities at different angles from the structured sample. Among them, information about Ti and CaHPO4 materials are marked by the solid cycles and squares, respectively. As shown in Fig. 7(b), the measured peaks at θ = 35°, 40° and 53° coincide in the position with Ti peaks, which cannot be employed to identify the production of CaHPO4 crystals. However, for the laser fluence of 9.9 J/cm2, there are three other distinct peaks appearing at the diffraction angles of θ = 27°, 33° and 59°. Through comparing them with the JCPDS (Joint Committee on Powder Diffraction Standards) data sheet, we can identify such signals from CaHPO4 rather than HA and other crystals. While for the laser fluence of 3.3 J/cm2, all diffraction peaks coincide in the position with Ti peaks, so that we could not recognize the existence of CaHPO4 crystal. These measurements provide another proof that the deposition of elements Ca-P in the form of CaHPO4 crystal could take place at the higher laser fluence of 9.9 J/cm2, while no elements Ca-P could be found to deposit into the structured surfaces at the lower laser fluence of 3.3 J/cm2. This result is consistent with the observations in Fig. 7(a).
It should be stressed that the deposited elements Ca-P could not be removed even after 30 minutes of ultrasonic cleaning, indicating a firm combination between the elements Ca-P and the structured surface. For a direct comparison, we submerged a flat and smooth Ti plate into the HA suspension with the supersaturated concentration for several days. With the water vaporizing naturally, the deposition of elements Ca-P on the sample surface could be seen by naked eyes. However, after washing by the ultrasonic cleaner, all these deposited chemical elements could be removed completely. This comparison suggests that it is the micro-roughness on the structured surfaces that makes elements Ca-P deposited so firmly that it cannot be washed out through ultrasonic cleaning. These behaviors provide good adaptive environments for the rapid osteoblast growing on Ti surface to improve its bioactivities, which can be supported by the previous studies [22–24]. However, in our experiments, the essential elements Ca-P were seeded directly into the metallic surfaces during laser microstructuring processes, rather than through conventional multi-step method to generate HA layers. This means that the production of bioactive layer with integration of microstructures and elements Ca-P deposition on the target could be achieved simply through femtosecond laser ablation in the supersaturated HA suspension. On the other hand, when the liquid was replaced by the unsaturated mixture solution of CaCl2 and Na3PO4, the island-void profiled microstructures could still be produced on the sample surfaces, but the firm deposition of the elements Ca-P was not evidenced, which is different from the observations in supersaturated HA suspension.
4. Discussion of underlying mechanisms
A simple scenario of surface microstructures formation on the metals immersed into liquid can be described as follows: In our experiments, when a femtosecond laser pulse strikes the solid target passing through the liquid, the radiation energy is known to be absorbed firstly to overheat the sample surface. Due to the incident laser fluence much higher than the damage threshold of the metal, the radiation-related ablation takes place at several picoseconds after the laser pulse, yielding to the ejection of ions and small clusters. During their subsequent cooling in the surrounding liquid, these ablated materials tend to coalesce into particles in the nanometer scale [25,26]. Basically, these ejected nanoparticles have relatively large kinetic energy due to the high speeds of several hundred meters per second , the collisions with the ambient molecules will result in the localized plasma generation. In our view, the plasma formation in liquid trigged by the ablative ejections can become intense enough to produce the secondary ablation of the target.
When the incident laser fluence is relatively small, such as 3.3 J/cm2 in our experiment, bubbles formation very close to the sample surface can be driven largely by the tensile thermoelastic stress of liquid, which arises from the temperature gradient between plasma and its surroundings [28,29]. Their subsequent mergence into cavitation bubbles and collapse occurring at 50-150 μs after the plasma initiation releases significantly energy to cause material moving away from the molten metallic surface. The observed elliptical nano-craters in our experiments might be attributed to the bubbles cavitation. In other words, the secondary plasma-related ablation of material results in the porosity formation on the sample surface. Since the bubble’s size is determined by the incident laser energy [28,29], in a certain range, the higher is the laser energy, the bigger the bubble will be, and the larger crater can be produced. This is consistent with our experimental observations in Fig. 2.
On the other hand, if the incident laser fluence becomes much higher, such as 9.9 J/cm2 in our experiment, the ablation of material was accompanied by the formation of hot plasma, which promotes the strong vaporization of water [13,30]. Under this circumstance, apart from the cavitation bubbles formation and collapse, the recoil pressure induced by the surrounding liquid vaporization began to play a dominant role , which results in the material expelling from the molten surface to form a pit. Since the scan speed of the sample is relatively small, the laser radiation is captured efficiently by the pit through its side walls reflection. In our experimental condition the average number of laser shots delivered into each laser spot is about 320 at the scan speed of 0.2 mm/s. Similar to the situations in Ref. 31, the formation of the adjacent pit is therefore inhibited until the laser spot crosses the first pit, and the cycle repeats. After the melt solidifies the micro-island in Fig. 4(a) and Fig. 6(a) can be created.
As concerning the firm deposition of elements Ca-P on the structured surface during femtosecond laser ablation, we have the following understanding: In the supersaturated HA suspension, since amount of water is vaporized by the incident higher laser fluences [13,30], some solutes will be separated through crystallization processes into the form of CaHPO4 crystals. In addition, the ejection of atoms and small clusters from the solid target leaves behind the fresh Ti atoms with active chemical properties on the sample surface. So the newly crystallized CaHPO4 substances are prone to have a firm combination with the fresh place of material ejection by the mechanical impingement from the bubbles burst. This is why we could still observe deposition of elements Ca-P among the surface microstructures even after the ultrasonic cleaning processes. In the case of lower laser fluence ablation, however, plasma-related water evaporation is not strong enough to result in the large burst of bubbles and the crystallization of CaHPO4. So that no elements Ca-P was identified on the nanoporous surface structures after ultrasonic washing processes. While for the mixture solution of CaCl2 and Na3PO4, since it was not saturated, the separation of CaHPO4 crystalline become difficult due to the insufficient water vaporization for some incident laser fluences. Moreover, weak burst of the cavitation bubbles from the low laser incidence is also defective to impinge the substance on the surface. As a result, no evidence of additional elements can be observed in our measurements.
In conclusion, we have experimentally studied the impact of liquid environments on surface microstructures formation on the Ti plate through line-scribing of femtosecond laser pulses. Unique surface structures have been generated on the samples within three different liquids surrounding. Particularly, in the supersaturated HA suspension, the firm deposition of elements Ca-P has been found on the microstructured surfaces for the higher incident laser pulse fluences but not for the lower laser incidence. While for the unsaturated mixture solution, this kind of bio-elements deposition has been not observed for both the low and high laser fluences. The plasma-related processes associated with the bubbles cavitation and the recoil pressure of vaporization could be possible mechanisms for the surface microstructures formation. Mechanical impingement from the bubbles burst could be responsible for the firm deposition of additional elements Ca-P on the structured surface. The above investigations provide a fast and convenient approach to generate surface microstructures with deposition of biocompatible elements Ca-P on them, which might improve the biocompatibility of the metallic devices for the interventional treatments.
This work is supported by National Natural Science Foundation of China (Grants No.10874092 and 60637020), RFDP of China (Grant No. 20070055066), Natural Science Foundation of Tianjin (Grant No. 09JCYBJC13900), and Research Plan of Hebei Education Department (Grant No. Z2008305).
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