Laser Induced Periodic Nanotextures Biology Essay
Youqiang Xing, Jianxin Deng *, Yunsong Lian, Xiuting Feng
Department of Mechanical Engineering, Shandong University, Jinan 250061, PR China
Department of Mechanical Engineering, Shandong University, No. 17923 Jingshi Road, 250061 Jinan, Shandong Province,
Tel: +86-531-88399769; E-mail address: firstname.lastname@example.org (J. Deng)
Laser-induced periodic nanotextures (ripples) on Al2O3/TiC ceramic formed by femtosecond laser pulses were investigated. Pulse energy, scanning speed and the number of overscans were studied for the formation of ripples. The evolution of surface morphology, surface roughness and ablation depth with different parameters was measured by scanning electron microscopy (SEM), white lighting interferometer and atomic force microscopy (AFM). The experimental results indicate that the nanotextures are dependent on the pulse energy and scanning speed, and the optimum parameter for the formation of continuous and clear ripples is 1.75 μJ with 500 μm/s or 2 μJ with 500 μm/s. Finally, the mechanism of the formation of laser-induced periodic surface nanotextures on Al2O3/TiC ceramic surface was discussed.
Keywords: Femtosecond pulsed laser; ablation; nanotextures; Al2O3/TiC ceramic
In recent years, laser processing techniques have rapidly developed. In particular, femtosecond pulsed lasers have been established to be a useful tool for precision material microprocessing because of their ultra-fast time duration and ultra-high peak laser intensity with a relatively low pulse energy. Generally, in this regime of modification [1-4]: (1) the heat transfer to the material is drastically reduced; (2) the undesirable defects on the samples are reduced; (3) the transition from solid to vapor phase is quite possible; and (4) high precision of material modification is ensured. Due to these advantages, laser-induced periodic surface textures have been used on many materials: semiconductor [5-7], ceramic [8-9], metal [10-12], and film [13-15].
It has been known that micro/nanotexture is an effective way to improve friction properties of solid surfaces for many years. Many scholars have fabricated various kinds of micro/nanotextures on solid surfaces, resulting in significant improvement of their tribological properties. Enomoto and Sugihara  developed novel cutting tools with micro/nanoscale textures on their surfaces, the results showed that the textured surface significantly improved the lubricity and anti-adhesive properties at the tool-chip interface. Mo et al  fabricated a nanotexture on H-passivated Si surface by current-induced local anodic oxidation and investigated its nanotribological properties by a colloidal probe. Results showed that the nanotexture exhibited low adhesion and reduced friction force strongly.
Alumina (Al2O3) ceramic is widely used in engineering applications: cutting tools, mechanical seals, engine components and bearings [18-21] due to its excellent intrinsic characteristics, such as low density, high hardness, high melting point, high wear resistance and good chemical inertness. In order to improve the properties of ceramic, many advanced approaches were developed in the past years. Wang et al  used a CO2 laser to treat an Al2O3-based ceramic aiming at modifying its microstructure and improving its erosion resistance. Deng et al.  developed the Al2O3 ceramic composites with the additions of CaF2 solid lubricants to reduce the surface friction in machining processes. Shen et al.  developed Al2O3 planted with Cu nanoparticles by ion implantation to modify surface performance. Cappelli et al.  studied ceramic surface modifications induced by pulsed laser treatment. However, surface nanotexturing by femtosecond pulsed laser may be an effective surface modification on Al2O3 ceramic, the nanoprocessing regimes for Al2O3 ceramic require further optimization of the process parameters and they have been reported in few literatures.
The aim of this work is to investigate the possibility of applying a femtosecond pulsed laser to surface nanotexturing of the Al2O3/TiC ceramic. The evolution of surface morphology, surface roughness and ablation depth was studied with different laser machining parameters: pulse energy (laser fluence), scanning speed and the number of overscans. The results reported in the paper are important in the field of surface nanotexturing by femtosecond pulsed laser of Al2O3/TiC ceramic.
2. Experimental details
The samples used were commercial hot-pressed Al2O3/TiC ceramic (Zibo Dongtai Co., Ltd., China), the main component and mechanical properties of the samples were listed in Table 1. The surfaces of the samples were finished by grinding and polishing to the roughness less than Ra 0.05 μm, ultrasonically cleaned by alcohol and then dried.
The femtosecond laser pulse is generated by a Ti: sapphire regenerative amplified laser system (Coherent Inc., USA), the wavelength, pulse width and repetition rate of the laser are 800 nm, 120 fs and 500 Hz, respectively. A single femtosecond laser pulse with a beam diameter of 6 mm was selected and the beam was focused on the sample surfaces by a lens with focal lengths of 20 cm to give a spot diameter of about 5 μm at the focal plane. An attenuator was used to obtain a proper energy of the pulse. The Al2O3/TiC ceramic sample was placed on a three-dimensional XYZ stage with a precision of 100 nm, the incident angle of the laser beam with respect to the sample surface was near normal and the whole experiments can be monitored by a charge-coupled device (CCD) in real time. All experiments were performed in air condition under atmospheric pressure. Fig. 1 shows the diagram of the experimental setup.
Experiments were carried out with various pulse energies (Ep) ranging from 0.75 to 3 μJ, and scanning speeds ranging from 130 to 1500 μm/s, the experimental parameters of femtosecond laser are given in Table 2. The surface morphologies of the Al2O3/TiC ceramic samples before and after femtosecond laser irradiation were observed by scanning electron microscopy (SEM, QUANTA FEG 250, USA), atomic force microscopy (AFM, Nanoscope IIIa, USA) and white lighting interferometer (Wyko NT9300, USA). At the same time, energy dispersive X-ray (EDX, X-MAX50, UK) analysis was used to investigate the sample surface composition.
Fig. 2 (observed at the centers of laser beams irradiated on the sample surfaces) shows the SEM micrographs of Al2O3/TiC ceramic samples after irradiation with 0.75 and 1μJ pulse energy with 1 overscan. It can be seen that for 0.75 μJ pulse energy, few nanotextures were produced on the surface at scanning speed of 130 and 250 μm/s (the lower scanning speeds in the current studies). Under an irradiation with 1 μJ pulse energy, the femtosecond laser irradiation resulted in non-continuous ripples on the surface at scanning speed of 130 μm/s; while few ripples can be seen on the surface at scanning speed of 250 μm/s, and no any morphological changed at scanning speed of 500 μm/s.
SEM micrographs of Al2O3/TiC ceramic samples after irradiation with 1.5-3 μJ pulse energy and 1 overscan are shown in Fig. 3. It can be seen that with 1.5-2 μJ pulse energy at scanning speed of 130 and 250 μm/s, the ripples were fabricated, but they were irregular and vague, the ablation of the surface was serious; at scanning speed of 500 μm/s, the ripples were observed, they were clear and regular; but in some case, they were different. In case of the pulse energy of 1.5 μJ, it can be seen that some bumps were not characterized by ripples, which was Al2O3 conformed by EDX of point A; while with the increasing pulse energy up to 2 μJ, some micropores appeared on the sample surfaces. It also can be seen that the ripples were oriented perpendicular to the polarization of the incident laser beam, and the periodicity of the ripples was about 700-800 nm. In addition, we observed that the periodicity of ripples did not change with increasing pulse energy. At high scanning speeds (1000 and 1500 μm/s), ripples were non-continuous and they were unclear compared to those observed at lower scanning speeds (500 μm/s). With a pulse energy of 2.5 μJ, the laser generated continuous ripples at relatively higher scanning speeds (500-1500 μm/s), while they were non-continuous and vague ripples at low scanning speeds (130 and 250 μm/s), and more micropores can be seen on the textured surfaces. With higher pulse energy of 3 μJ, we can’t see any regular ripples regardless of scanning speed (130-1500 μm/s), the surface periodic ripples were replaced by a chaotic deterioration of ripples and granular structures.
Fig. 4 shows AFM images of two-dimensional surface topographies (a), three-dimensional surface topographies (b) and profiles (c) of the ripples after irradiation with 1.75 μJ (8.91 J/cm2) pulse energy and 500 μm/s scanning speed with 1 overscan. According to Fig. 4 (b) and (c), it can be seen that ripples were arranged on the surfaces evenly, the depth of ripples was about 150 nm, and the periodicity of the ripples was about 750 nm.
Fig. 5 shows the surface roughness of samples after irradiation with different pulse energies and scanning speeds with 1 overscan. It was shown that the surface roughness of the samples after irradiation with laser was higher than that of pristine surface after polishing. The pulse energy and scanning speed had an important influence on the surface roughness of samples. Obviously, the surface roughness increased with the increasing pulse energy, and reduced with the increasing scanning speed.
Irradiation of the Al2O3/TiC ceramic samples was performed in air, therefore, the element content on the surface may be changed. Monitoring of the sample surface constituents detected by EDX, including oxygen (O), aluminum (Al), titanium (Ti), carbon (C), molybdenum (Mo), nickel (Ni) and wolfram (W) before and after the irradiation of laser with different pulse energies at 500 μm/s scanning speed and 1 overscan, is shown in Table 3. The results showed that the content of oxygen increased approximately with the increasing pulse energy from 0 to 1.75 μJ, and then decreased with the increasing pulse energy from 2 to 2.5 μJ, while it was up to a maximum of 30.92 % with 3 μJ; the content of Al decreased with the pulse energy from 2 to 3 μJ.
The evolution of sample surface morphologies after irradiation with 1.75 and 2 μJ pulse energy at scanning speed of 500 μm/s and different overscans (1-6) is shown in Fig. 6. It can be seen that for the low number of overscans (1-2), the irradiated surfaces of the samples were characterized by periodic ripples, and there were some bumps on the surfaces; for the high number of overscans (3-6), the non-continuous ripples were formed on the surfaces, and a large number of micropores and granular structures were observed on the surfaces.
Fig. 7 shows the development of surface roughness with the number of overscans. It can be seen that the surface roughness increased with the increasing number of overscans. The surface roughness of samples irradiated with 2 μJ pulse energy was larger compared with that of 1.75 μJ, which meant that high pulse energy led to a large surface roughness.
The ablation depth of samples irradiated with different pulse energies ranging from 0.75 to 3 μJ and the number of overscans ranging from 1 to 6 were studied and plotted in Fig. 8. It showed that the ablation depth increased with the increase of pulse energy at scanning speed of 500 μm/s and 1overscan (Fig. 8 (a)). Fig. 8 (b) showed that the ablation depth as a function of the number of overscans, it increased linearly with the increase of the number of overscans for 1.75 and 2 μJ at scanning speed of 500 μm/s, and the ablation depth of 2 μJ was larger than that of 1.75 μJ at the same scanning speed and number of overscans.
Fig. 9 shows the AFM images of three-dimensional surface topographies of samples after laser irradiation with different pulse energies, scanning speeds and overscans. It was shown that the low laser pulse energy (0.75 μJ) was not enough for producing ripples, see Fig. 9 (a). As the pulse energy increases to 1.5 μJ, the ripples appeared on the sample surfaces, while the ripples were not clear and regular, as it was shown in Fig. 9 (b); with the pulse energy of 1.75 and 2 μJ (Fig. 9 (c) and (d)), the femtosecond laser irradiation resulted in the fabrication of continuous line structures; as the pulse energy increased to 3 μJ (Fig. 9 (e), the chaotic and deep ripples were formed. For high scanning speed, the ripples were shallow and not clear (Fig. 9 (f) and (g)); for the high number of overscans, a chaotic deterioration of the surface nanotextures were formed, and the textures were deeper.
Laser-induced periodic nanotextures (ripples) on many materials have been reported for many years. It is generally accepted that the laser-induced ripples are formed in different mechanisms, such as interference effects, second-harmonic generation, self-organization [26-28]. The laser-induced ripples are oriented perpendicular to the polarization of the incident laser beam, parallel ripples with an orientation parallel to the plane of incidence of the ablating laser beam or even quadratic patterns which have been reported on many materials in the current studies [29-31].
In this paper, we studied the effects of pulse energy, scanning speed and the number of overscans on the formation of periodic nanotextures induced by a femtosecond pulsed laser on Al2O3/TiC ceramic. It can be seen that ripples were formed in different experiments, they were oriented perpendicular to the polarization of the incident laser beam in all experiments, and the periodicity of the ripples was about 750 nm, approximately in accordance with the laser wavelength or smaller than the laser wavelength, which did not change with varieties of pulse energies and scanning speeds. These have been reported in many literatures and the general mechanism is the interference of incident laser with the surface electromagnetic wave [32-34]. It also can be seen that the pulse energy and scanning speed had an important effect on the formation of periodic ripples and the ripples can be formed by various pulse energies and scanning speeds (see Figs. 2, 3 and 9), also there was an optimum parameter for the formation of ripples. In our studies, 1.75 or 2 μJ pulse energy, 500 μm/s scanning speed with 1 or 2 overscans were the best parameters for the formation of regular, clear and continuous ripples. The ripples were not formed for low pulse energy of 0.75 μJ regardless of the scanning speed, which was due to the laser fluence according to 0.75 μJ pulse energy was lower than the damage threshold fluence. For low pulse energy at high scanning speed and high pulse energy at low scanning speed, no regular and continuous ripples can be formed and some bumps of Al2O3 can be seen on the sample surface. It can be explained that in case of low pulse energy at high scanning speed, the total energy deposition was not high enough to induce the surface defect that was necessary for the initiation of interference between the incident and the surface electromagnetic wave. Conversely, in case of high pulse energy at low scanning speed, the maximum total energy was deposited on the sample surface, large amount of materials were removed  and some solid Al2O3 particles were evaporated directly, which produced more micropores on the sample surface. For higher pulse energy (3 μJ) or higher number of overscans (Figs. 3 (c), 6 and 9), a chaotic deterioration of ripples and granular structures were formed regardless of scanning speed, which can be explained that the deposited energy on samples was more over the target threshold energy for ablation, and excessive materials were violently ejected, which was consistent with an energy accumulation effect reported by Li . Of course, pulse energy deposited on the sample surfaces increased with the increase of pulse energy and the reduction of scanning speed, which led to a large surface roughness (Fig. 5).
The element content on the surface changed (see Table 3) can be explained that the oxidation occurred of sample surface after laser irradiation with the increasing pulse energy from 0 to 1.75 μJ; for high pulse energy (2 and 2.5 μJ), the oxygen and aluminum content decreased due to the removal of Al2O3 from the sample surface, which produced the micropores on the sample surfaces and it was in line with the Fig. 3 (c) and (d); while the strong oxidation occurred and more Al2O3 were removed by the high pulse energy of 3 μJ, which led to the high oxygen content and low aluminum content.
It also can be seen that micropores and granular structures were formed by low pulse energy with the high number of overscans (Fig. 6) or high pulse energy (Fig. 3 (c)). The surface roughness and ablation depth increased with the increase of pulse energy and the number of overscans, and decreased with the increase of scanning speed (Figs. 5,7 and 8). This is because that high pulse energy, low scanning speed and high number of overscans resulted in high total laser energy deposition per area.
In this paper, we investigated femtosecond pulsed laser nanotexturing of Al2O3/TiC ceramic with different parameters. The results show that ripples are formed by the interference of the incident laser with the surface electromagnetic wave excited by the incident laser, and the ripples are oriented perpendicular to the polarization of the incident laser beam regardless of pulse energy and scanning speed. The pulse energy and scanning speed have an important effect on the formation of ripples and there is an optimum parameter for the formation of ripples. Ripples are not formed by low pulse energy at high scanning speeds, or high pulse energy at low scanning speeds. With low pulse energy at high scanning speeds, there are some bumps of Al2O3 on the surface, while some mircopores produced on the surface with high pulse energy at low scanning speeds. The continuous and clear ripples can be obtained by using 1.75 or 2 μJ pulse energy, 500 µm/s scanning speed with 1or 2 overscans in our experiments. A high pulse energy, low scanning speed and high number of overscans result in a large surface roughness and deep ablation depth.
This work is supported by the National Natural Science Foundation of China (51075237), the Taishan Scholar Program of Shandong, the Specialized Research Fund for Doctoral Program of Higher Education (2011013113002), and the Independent Innovation Foundation of Shandong University (2011JC001).