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Effects of laser parameters on morphological change and surface properties of aluminum alloy in masked laser surface texturing
Journal of Manufacturing Processes 48 (2019) 260–269
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Effects of laser parameters on morphological change and surface properties of aluminum alloy in masked laser surface texturing
T
Seung Jai Won, Hong Seok Kim* Department of Mechanical and Automotive Engineering, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu, Seoul, 01811, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Masked laser surface texturing Laser shock loading Surface morphology Hardness Wettability
The masked laser surface texturing process was used to produce micro-pattern arrays. Using mesh grids as masks, the surface of the workpieces were selectively ablated and hundreds of micro-patterns were simultaneously generated by a single laser irradiation. The effects of laser energy intensity and number of laser pulses on surface morphologies and properties were investigated. It was found that it is more efficient to control the number of laser pulses than the laser energy intensity to form a uniform micro pattern array and to control the pattern shape. In addition, hardness values of the material surface can be selectively increased by adjusting laser parameters. When the laser energy intensity increased, the hardness of the hole region which was directly affected by the laser irradiation increased. When the surface was irradiated repeatedly by the multiple laser pulses, however, the increase in hardness was much pronounced in the masked bar region adjacent to the ablation zone. The structural changes of the patterned surface and the work hardening effect due to laser shock loading were superimposed to increase the hardness of the masked region. The contact angle decreased with increasing laser energy intensity and number of laser pulses. This is mainly due to an increase in surface heterogeneity at high laser energy intensities and an increase in bar width at multiple laser pulses condition.
1. Introduction With the rapid development of precision manufacturing technology [1–3], surface texturing (which produces the microscale shapes on the surface of the material) has attracted significant attention in various industries. By producing microscale dimples or shapes on the surface of the material, it is possible to change the surface properties such as hardness and wettability, and to improve the friction characteristic and wear resistance of the surface [4]. In addition, surface texturing is known to improve performance in battery efficiency [5], heat transfer performance [6], and osseointegration [7]. Therefore, surface texturing is a very effective technology for achieving desired surface properties without using expensive materials. Surface texturing has been conducted for various purposes using a variety of methods [8] including reactive ion etching (RIE), micro drilling, and electrochemical machining (ECM). Wang et al. [9] performed surface texturing using RIE on the surface of a SiC seal that can operate in high-temperature and high-pressure water. They showed that RIE texturing provided the SiC seal with an anti-seizure ability and low coefficient of friction. Pratap et al. [10] fabricated micro-dimple arrays on Ti-6Al-4 V titanium alloy using micro-tools of different geometries. They showed that surface texturing, texture geometry, and ⁎
initial surface conditions have significant influences on the wettability and surface hardness of the material. Chen et al. [11] presented a through-mask electrochemical micromachining process to produce micro hole and dimple arrays of several hundred micrometers on chrome-coated surface. The results showed that the tribological properties of mechanical components could be much enhanced in the case of square micro-dimples. These conventional methods, however, have various problems. RIEs and ECM entail complex procedures and are accompanied by chemical work, which adversely affects the environment. In the case of micro-drilling or machining, design and manufacture of the tool is very difficult and costly, and the chip must be effectively removed. Therefore, a more environmentally friendly and easy surface texturing method is required. In such a situation, laser-based surface texturing methods are attracting much attention because they are environmentally friendly [12] and inherently precise and repeatable. Etsion et al. [13] performed laser surface texturing on the SiC material. They reported that frictional performance of thrust bearings was much improved after surface texturing. Bathe et al. [14] textured the gray cast iron surface with different laser pulse durations and reported that femtosecond laser irradiation was a useful tool for improving wear characteristics. Yang et al. [15] fabricated line, rectangular, and circular dimple patterns on
Corresponding author. E-mail address: [email protected] (H.S. Kim).
https://doi.org/10.1016/j.jmapro.2019.10.034 Received 18 May 2019; Received in revised form 2 October 2019; Accepted 29 October 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
Journal of Manufacturing Processes 48 (2019) 260–269
S.J. Won and H.S. Kim
masked laser surface texturing process. A Q-switched Nd:YAG pulsed laser with 10 ns pulse width and 1064 nm wavelength was used as the energy source. The laser beam was focused on the surface of the material via two reflecting mirrors and a focusing lens. The diameter of the focused beam on the specimen was 1.8 mm. The visible He-Ne laser was installed and aligned with the Nd:YAG laser to accurately locate the laser beam spot on the specimen. Al1050 alloy (which has wide range of applications and good mechanical properties) was used as the specimen. This material was cut to 20 × 20 × 0.5 mm, polished, and fixed firmly on the 3-axis stage. Fig. 2 shows the experimental procedure for the masked laser surface texturing process. The mesh grid of Ni consists of bars with a width of 25 μm and square holes with a length of 100 μm. The overall diameter of the grid was 3 mm and contained approximately 200 repetitive microscale patterns. First, the mesh grid is placed and fixed on the top surface of the specimen. When the laser is subsequently irradiated, the bar portion of the grid acts as a mask and the material is selectively ablated. When the mesh grid is removed, the surface of the specimen can be divided into the bar region where the direct laser effect was not significant and the hole region where the ablation occurred. This process is simple and very efficient because more than 160 micro-patterns can be formed simultaneously on the metallic surface with only one laser beam irradiation. It is expected that micro-pattern arrays with different geometrical features will be generated according to the laser process conditions and the shape and size of the mesh grid. The geometrical features of the micro-pattern will also have a strong influence on the surface properties of the material. Hence, the laser energy intensity and the number of laser pulses were selected as major parameters and their effects on the morphologies and properties of the patterned surface were investigated. Three laser energy intensities were considered: 0.39, 3.9, and 7.8 GW/ cm2. Four different number of laser pulses were used: 1, 5, 10, and 20. The time interval between pulses in repeated irradiations was one second. The shape, roughness, hardness and contact angle of the specimens were analyzed after the masked laser surface texturing. Surface morphology was observed using a scanning electron microscope (SEM). A detailed 3D profile and surface roughness were measured using a confocal laser microscope (Carl Zeiss, LSM 800MAT). To measure the hardness, a micro-Vickers hardness tester (Future-Tech, FM-800) was used and the diamond tip was pressed against the workpiece surface for 5 s under an indentation load of 10 gf. The contact angle was measured by a contact-angle meter (KSV, CAM-200) by the Sessile-drop method using deionized water at the ambient temperature 20 °C and humidity 40 %. Three repetitions of the experiments were done to secure reliability.
Inconel 718 surfaces by effectively controlling the laser parameters such as scan speed and power. They showed that the variations in surface morphology and roughness were accompanied by modifications of surface wettability. In order to increase the efficiency of laser-based surface texturing processes, various novel laser processes in which a number of micro-pattern arrays are produced with a single laser irradiation have also been reported. Cardoso et al. [16] presented the direct laser interface patterning (DLIP) method, which can fabricate multiple dimples at a time by controlling the beam interference. They effectively generated various surface morphologies and contact angles on the Al2024 alloy surface by controlling laser parameters such as polarization and angle of incidence. Mao et al. [17] performed an indirect laser surface patterning process utilizing the laser-induced shock wave loadings to realize the patterning and strengthening effects simultaneously on the material surface. Using the grid as a micro-mold, multiple micro-pattern arrays could be generated with a single laser irradiation, and the improvement of hardness and wear resistance was achieved. In this study, a laser surface texturing process using a mesh grid as a mask was used to form hundreds of micro-patterns simultaneously with one laser irradiation. This method is simple, environmentally friendly, and economical because it uses a mesh grid alone to selectively ablate the surface of the material. In addition, since the surface patterning and the shock wave loading occur at the same time in this process, desired values of the surface properties such as hardness can be effectively realized by controlling the surface morphologies and laser parameters. However, systematic and detailed studies on this aspect are currently lacking in the present literature. Therefore, in this study, the influence of the major process parameters such as the laser energy intensities and the number of laser pulses on the morphologies and properties of the textured surface were investigated. It was found that it is more efficient to control the number of laser pulses than the laser energy intensity in order to form a uniform micro pattern array and to control the pattern shape. In addition, by adjusting the laser parameters, it was possible to selectively increase the hardness value of the masked bar region of the material or of the hole region directly affected by the laser irradiation. The change in hardness was considered to be caused by the structural changes of the surface and the superposition of the hardening effect due to the laser shock loading. The change in the contact angle could be also described quantitatively using the surface heterogeneity and the change in bar width of the patterned surface.
2. Experimental procedure Fig. 1 shows the schematic of the experimental equipment for the
3. Results and discussion 3.1. Surface morphologies according to laser energy intensity Fig. 3 shows the ablated surface of an aluminum specimen without a mesh grid at a laser intensity of 0.39 GW/cm2. It was confirmed that the surface layer of the material was roughened by melting and a rapid solidification process. Many pits and micro-sized holes were observed in the enlarged SEM picture. The profile measured in the ablation zone is shown in Fig. 3(b). Considering that the roughness Ra of the untreated specimen was about 0.05 μm, it can be seen that the profile of the ablated surface became very irregular. The roughness Ra of the ablated surface was about 0.35 μm. This is because the plasma generated by the laser ablation expanded and spattered the molten material rapidly. Fig. 4 shows the surface morphology of the textured specimen with a mesh grid at a laser intensity of 0.39 GW/cm2. The textured surface was well separated from the bar and hole regions. Compared with Fig. 3, several larger droplets were observed in the hole region. This is because the ablated area was divided into small pieces and the diffusion
Fig. 1. A schematic illustration of the masked laser surface texturing process. 261
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Fig. 2. Summary of experimental procedure.
the number of small droplets increased in the bar region. Additionally, the boundaries between the bar and the hole regions became difficult to distinguish. Fig. 5(b) shows the profile measured on the textured surface. As the energy increased, the average depth of the bar portion increased slightly to 8.7 μm. Also, the surface of the hole region became more irregular. It started to lose the planarity as more mass on the bottom of the hole melted and re-solidified under the influence of the laser plasma. The surface morphologies when the laser energy intensity was further increased to 7.8 GW/cm2 are presented in Fig. 6. Fig. 6(a) shows that for 60% of the ablated area, the bar and hole regions could not be clearly distinguished. This is because the part of the mesh grid placed on the specimen was melted and welded to the specimen surface due to excessive laser energy irradiation. Additionally, strong laser energy penetrating between the grid and the material had a strong thermal effect on the material. Fig. 6(b) shows the profile measurement results for the relatively well-separated area of the bar and hole regions. The
of the molten material was somewhat limited. Several small droplets were also observed in the bar region. It seems that some of the molten material by the laser was spattered from the hole region to the bar region with the expansion of the plasma. However, the effect of the laser on the masked bar region of the workpiece was not very significant. Fig. 4 (b) shows the profile measured on the textured surface. The average height of the bars is 7.3 μm from the lowest point of the hole region. The surface of the hole region was rough as the surface particles were subjected to rapid melting and re-solidification processes. However, the irregularity of the hole surface was reduced compared with Fig. 3(b) because the ablation area of the material was divided into small areas. Fig. 5(a) shows the SEM images of the textured surface with a mesh grid under increased laser intensity of 3.9 GW/cm2. As the laser energy increased, a large amount of material was ablated and larger sized droplets were observed in the hole region. Compared with Fig. 4(a), the spatter effect of the molten material increased in the bar region because
Fig. 3. Surface morphologies after laser ablation without a mesh grid (0.39 GW/cm2 laser intensity) (a) SEM images with different magnifications (b) Surface profile along the line. 262
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Fig. 4. Surface morphologies after masked laser surface texturing with 0.39 GW/cm2 laser intensity (a) SEM images with different magnifications (b) Surface profile along the line.
Fig. 5. Surface morphologies after masked laser surface texturing with 3.9 GW/cm2 laser intensity (a) SEM images with different magnifications (b) Surface profile along the line. 263
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Fig. 6. Surface morphologies after masked laser surface texturing with 7.8 GW/cm2 laser intensity (a) SEM images with different magnifications (b) Surface profile along the line.
shall be conducted into the geometric change of micro-patterns with detailed control of additional laser process parameters such as laser beam size and shape. 3.2. Surface morphologies according to the number of laser pulses In this section, changes in surface morphology were investigated by changing the number of laser pulses with the laser intensity fixed at 0.39 GW/cm2. Under this energy intensity condition, the mesh grid was damaged when the laser pulses were irradiated more than 30 times. Since the maximum number of laser irradiations in this study was 20, the damage of the mesh grid did not affect the experimental results. Fig. 8 shows the laser textured surface of the specimen after the laser beam was irradiated five times. Compared with Fig. 4, significant changes were observed from SEM pictures in both hole and bar regions. In the case of the bar region, the surface was observed to be more flexed and softened. It appears that the laser had repeated thermal effects on this region between the grid and the specimen. Unlike the bar shape in Fig. 4 (where the influence of the laser is limited), the thermal effects of the laser are expected to cause changes in the composition and grain structure of the bar region (see Fig. 8). In the case of the hole region, the droplet shape due to the rapid solidification was changed into a threadlike shape. This was considered to be the planarization of the hole surface caused by repeating the melting and re-solidification processes with repeated laser beam irradiations. The surface profile of the specimen is shown in Fig. 8(b). The micro-pattern array maintained regularity despite being exposed to the laser thermal effect repeatedly. The average depth of the bar was 12.8 μm, which was about 75% higher than that of the single irradiation case. The hole region did not lose planarity and the irregularity of the hole surface was significantly reduced by the aforementioned planarization effect of the repeated laser irradiations. Fig. 9 shows the laser textured surface when the number of laser pulses was increased to 10. SEM images show that the surface
Fig. 7. Surface roughness values (Ra) of the laser textured specimens at different laser intensities.
bar portion was often unable to maintain the shape of the peaks due to the increased laser effect. The surface of the hole region became more irregular and the surface roughness was significantly increased. Consequently, the general tendency to increase the surface roughness with increasing laser energy intensity was confirmed. As depicted in Fig. 7, the surface roughness Ra values for the laser textured surface were 1.7, 2.1, and 3.0 μm at laser intensities of 0.39, 3.9 and 7.8 GW/ cm2, respectively. In the case of 7.8 GW/cm2, it can be seen that surface irregularities caused large variation in roughness values. In the masked laser surface texturing process of this study, the laser energy needs to be adjusted appropriately to control the shape of micro-pattern array. An energy level of 3.9 GW/cm2 or less was adequate and the surface morphology could not be controlled beyond that value. Since the depth of the bars increased as the laser energy increased, further investigation
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Fig. 8. Surface morphologies after masked laser surface texturing with 5 laser pulses (0.39 GW/cm2 laser intensity) (a) SEM images with different magnifications (b) Surface profile along the line.
the thread shape in the hole region became slightly faded and the surface became more smooth and homogeneous. It appears that repeated laser irradiations caused the surface finishing effect with the surface being more planarized. Fig. 9(b) shows the measured surface profile of the textured specimen. The shape of the micro-pattern array
characteristics of the bar region is similar to those of the 5 laser-irradiation case. However, it was observed that the width of the bar increased. This was caused by the newly melted material accumulating on the bar side with repeated laser irradiations. The droplets in the hole region disappeared and were flattened. However, it was observed that
Fig. 9. Surface morphologies after masked laser surface texturing with 10 laser pulses (0.39 GW/cm2 laser intensity) (a) SEM images with different magnifications (b) Surface profile along the line. 265
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Fig. 10. Surface morphologies after masked laser surface texturing with 20 laser pulses (0.39 GW/cm2 laser intensity) (a) SEM images with different magnifications (b) Surface profile along the line.
was fairly regular and homogeneous. The average depth of the bars was 20.1 μm, which was a significant increase of more than 170% compared with the single irradiation case. In the hole region, the roughness of the surface profile was significantly reduced by the enhanced planarization effect. However, it was observed that the central part of the hole was slightly depressed off the plane. This is because the newly ablated material of the hole region was accumulated on the bottom of the bar side by the spatter effect and the material of the bar region melted down to the edge of the hole due to the increased thermal effect. Fig. 10(a) shows the SEM images of the laser textured specimen when the number of laser pulses was further increased to 20. As the accumulation of molten material increased, the width of the bars on the surface increased significantly. The change in bar width with the number of laser pulses is shown in Fig. 11. When the laser beam was irradiated 10 times, the increase of the bar width was 17.3%. When the laser beam was irradiated 20 times, the bar width further increased by 70% or more. Although increasing the number of laser pulses increases
the amount of material ablated by the laser, it appears that the ablated material is not effectively removed from the narrow hole region, but rather is deposited again near the bottom of the bar region. In the profile graph in Fig. 10(b), it can be seen that the bar of the previous peak shape changed into a triangular shape and the boundary between the bar and the hole regions become blurred. In the case of hole region, the planarization effect became more prominent. Consequently, the thread shape in the hole almost disappeared. However, some microscale holes were newly observed due to the accumulated thermal effect. Fig. 12 shows the change in height of the bar according to the number of laser pulses. When the number of laser pulses was less than 10, the height of the bar increased with the number of laser pulses. However, when the number of laser pulses was 20, the height of the bar decreased to 16 μm. This is because the bar region is exposed to a increased amount of laser energy as the number of laser pulses increases. When the number of laser pulses reaches a certain value or more, the
Fig. 11. Bar width as a function of number of laser pulses (0.39 GW/cm2 laser intensity).
Fig. 12. Bar height as a function of number of laser pulses (0.39 GW/cm2 laser intensity). 266
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Fig. 13. Surface roughness (Ra) of hole region as a function of number of laser pulses (0.39 GW/cm2 laser intensity).
Fig. 14. Hardness values of bar and hole regions at different laser intensities (single laser pulse).
erosion effect of the upper bar region appears to be larger than the deepening effect of the laser ablation. Fig. 13 shows the change in the surface roughness value of the hole region according to the number of laser pulses. The surface roughness decreased as the number of laser pulses increased due to the planarization effect caused by repeated laser irradiations. When the laser pulse was irradiated 5 times, the surface roughness of the hole region was 0.15 μm (48.7% lower than that of the single laser irradiation). In addition, the surface roughness when the laser was irradiated 10 times was further reduced to 0.07 μm (77% lower than that of single laser irradiation). In the case of 20 laser irradiations, the surface roughness was 0.07 μm, which was similar to the 10 laser irradiation case. After 10 laser irradiations, no further improvement in surface roughness (obtained from the planarization effect) was observed. In this section, morphological changes of the laser textured surface were observed according to the number of laser pulses. The height and width of bars, surface structure, and roughness values were found to vary significantly as the number of laser pulses increased. It was found that it is more efficient to control the number of laser pulses than the laser energy intensity to form a uniform micro pattern array and to control the pattern shape. In the case of controlling the number of laser pulses, it was possible to increase the depth of the micro-pattern and realize various pattern shapes without losing the homogeneity of the micro patterns. However, as shown in Fig. 10, some cracks were observed at the edge of the bar region when the laser was irradiated 20 times. This appears to be due to the excessive hardening effect of the surface. Therefore, in addition to the geometric aspects of the micropatterns, quality aspects including surface cracks and damage should be considered in future studies.
Fig. 15. Hardness values of bar and hole regions at different numbers of laser pulses (0.39 GW/cm2 laser intensity).
and the crystal size decreased, which was considered as the major cause of increased hardness. Therefore, it can be concluded that the increase in hardness of the laser patterned surface in Fig. 14 is mainly caused by the change in microstructure of the surface after the laser treatment rather than the geometrical characteristics of the micro-pattern itself. Fig. 15 shows the change of surface hardness with the change in the number of laser pulses. For the hole region, the hardness increases for the number of laser pulses of 5, 10, and 20 were 41.4%, 92.9%, and 128.7%, respectively. Since the laser intensity was fixed at 0.39 GW/ cm2, the total laser energy input for 10 and 20 laser pulses was identical to the energy conditions at 3.9 and 7.8 GW/cm2 in Fig. 14, respectively. Therefore, repeated laser irradiation was found to be a more effective way to increase hardness than to increase laser energy intensity. One of the most remarkable points in Fig. 15 was that in contrast to the results of Fig. 14, the increase in hardness was observed in the bar region as the number of laser pulses increased. The hardness increases with the number of laser pulses of 5, 10, and 20 were 102 %, 133 %, and 167 %, respectively. This increase was even greater than the increase in the hardness of the hole region directly affected by the laser. In the SEM images in Figs. 8–10, the surface characteristics of the bar region are shown to have changed depending on the influence of the repeated laser irradiation (compared with the bar regions in Figs. 4–6). However, it is not sufficient to explain the increase in the hardness of the bar region only by the defect density and crystal size described above because the hole region is much larger in laser energy input value than the bar region. As the laser beam repeats, the bar region of the specimen undergoes
3.3. Micro-hardness The micro-pattern array on the material surface greatly affects the surface properties of the material. Hardness is one of these mechanical properties. This study examined the effect of the micro-pattern array on micro-hardness under various process conditions The hardness was measured for the hole and bar regions. The change in hardness value at different laser intensities is shown in Fig. 14. Considering that the aluminum specimen before patterning had a Vickers harness number (VHN) of 45.18, the increase in hardness of the bar region after patterning was not very significant. This is because the direct effect of laser energy on the bar region is limited. However, the increase in hardness at the hole region was much greater. The increases were 25.5%, 46.7%, and 85.0% for 0.39, 3.9, and 7.8 GW/cm2 laser intensities, respectively. Butt et al. [18] reported that as the laser energy reaching the specimen increased, the defect density increased 267
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at an energy intensity of 0.39 GW/cm2, the contact angle increased by approximately 30%–86.4°. This contact angle change mechanism is described in the Cassie-Baxter model [21]. This model is expressed as follows:
repetitive loads from the grid. In addition, the plasma generated from the ablated area of the hole region expands rapidly and exerts a strong shock load in the direction of the bar. In the case of repetitive laser irradiations, it is judged that the above two phenomena repeatedly overlap and the hardness of the bar region is markedly increased. This is very similar to the shock peening process in which a steel ball impacts the material surface to produce compressive residual stresses. Work hardening and hardness increase due to laser shock loading can be also seen in many previous studies [19]. In conclusion, in the masked laser surface texturing of this study, it was confirmed that the increase of hardness was influenced by the microstructural change of the surface and the laser shock loading depending on the process condition (rather than the geometrical characteristics of the micro-patterns). Overall, increasing the number of laser pulses was effective in increasing the hardness, and it was possible to selectively harden the bar or hole regions of the surface by controlling the laser parameters.
{cosθCB = f1 cosθr − f2 }.
(1)
where θr is the contact angle of the reference surface, f1 is the area fraction of the water-solid surface, f2 is the area fraction of the water-air surface, and θCB is the contact angle of the Cassie-Baxter model. According to Eq. (1), the contact angle is determined by the ratio of f1 and f2. In this study, the irregularity of the surface after laser texturing was increased. Hence, it can be judged that the contact angle increased as the contact area f1 of the surface and water decreased and the contact area f2 of the air and water increased. A decrease in the contact angle with increasing laser intensity was associated with an increase in surface heterogeneity. As surface heterogeneity increases, water droplets in the Cassie-Baxter state become more difficult to maintain in terms of their shape and flow down into the valleys of the surface. Accordingly, the contact between water and air is reduced, which results in the contact angle being reduced. In particular, at laser intensity of 7.8 GW/cm2, the surface contact state of the specimen is explained using the Wenzel’s theory because the contact angle was more hydrophilic than the contact angle of the specimen before texturing. Wenzel's contact angle theory is expressed by the following equation [22]:
3.4. Contact angle The contact angle, which determines the wettability of the surface, is considered important in many engineering and industrial fields. A hydrophilic surface having a small contact angle is used for the improvement of antifogging and adhesive properties [20]. Hydrophobic or superhydrophobic surfaces with contact angles greater than 150 degrees are used to improve self-cleaning, anti-corrosion, and bacterial repellence [16]. Laser surface texturing is a technique that can effectively change the contact angle properties of the material surface. The contact angle is strongly affected by the shape of the micropattern and surface chemistry. Since the area of the ablated region in the specimens was the same regardless of the process conditions, this study investigated the change of the contact angle in terms of surface morphology. Since many researchers have reported that the contact angle of the laser textured surface changes with time [16], the contact angle was measured 30 days after the experiment. In addition, the contact angle was measured for the water droplet containing hundreds of repeating micro-patterns in order to exclude the influence of slight geometrical differences between the patterns and the sensitivity to the measurement position. The change in the contact angle of the textured surface with respect to the laser energy intensity is shown in Fig.16. The contact angle of the specimen before laser texturing was 63°. Therefore, it can be seen that
{cosθW = fcosθr }.
(2)
where θr is the contact angle of the flat reference surface, f is the ratio between the actual area and the apparent area, and θW is the contact angle from the Wenzel model. Eq. (2) shows that the hydrophilic surface becomes more hydrophilic as f increases. The change in the contact angle in Fig. 16 may also be related to the change in the surface roughness in Fig. 7. As the surface roughness increased, the contact angle decreased. Similar results were found in the study of Yilbas et al. [23]. Fig. 17 shows the change in the contact angle caused by repetitive irradiation by the laser. As the number of laser irradiations increased, the contact angle tended to decrease. However, in this case, the surface became much smoother as shown in Fig. 13. Therefore, it is difficult to relate the surface roughness to the change of the contact angle. Instead, the change in the contact angle appears to be related to the change in the width of the bar region shown in Fig. 11. As the width of the bar increased, the distance between the bars decreased and the amount of air trapped between the bars decreased. Consequently, the contact between water and air was reduced and the contact angle decreased according to Cassie-Baxter theory. Fig. 11 shows the bar width value was similar for the 5 and 10 laser pulse cases, and increased abruptly in the
Fig. 16. Variation of contact angle at different laser intensities (single laser pulse).
Fig. 17. Variation of contact angle at different numbers of laser pulses (0.39 GW/cm2 laser intensity). 268
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Acknowledgements
case of 20 laser pulses. Fig. 17 shows that the contact angle was also similar for 5 and 10 laser irradiations, and sharply decreased with 20 laser pulses. Jagdheesh et al. [24] reported that the irregularity of the micro-pattern array caused the Wenzel state transition. In the case of repetitive laser irradiation, the Wenzel state transition did not appear to occur as the regularity of the micro-patterns was relatively well maintained. Consequently, the contact angle of the surface treated specimen is found to be closely related to the surface roughness and the pattern geometry. Kam et al. [25] formed a conical surface structure with a laser and showed that the contact angle could range from super-hydrophilic (0°) to hydrophobic (113°) depending on processing conditions. It is expected that various wettable surfaces can be created by more finely adjusting the shape of the micro-pattern also by using the masked laser surface texturing process.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant NRF-2017R1D1A1B03035818). References [1] Wu SM, Ni J. Precision machining without precise machinery. CIRP Ann-Manuf Technol 1989;38:533–6. [2] Kim HS, Nam JS. Quantitative modeling and characterization of the size effects in microscale coining process of copper. Precis Eng 2018;51:490–8. [3] You JH, Kim HS. Formability and fracture of metallic sheet materials in microscale punching processes using laser-accelerated flyer impact. Int J Adv Manuf Technol 2018;96:3511–20. [4] Lorenzo-Martin C, Ajayi OO. Effect of sic particle impact nano-texturing on tribological performance of 304l stainless steel. Appl Surf Sci 2014;315:287–91. [5] Abbott M, Cotter JR. Optical and electrical properties of laser texturing for highefficiency solar cells. Prog Photovolt 2006;14:225–35. [6] Grabas B. Vibration-assisted laser surface texturing of metals as a passive method for heat transfer enhancement. Exp Therm Fluid Sci 2015;68:499–508. [7] Olsson R, Powell J, Palmquist A, Branemark R, Frostevarg J, Kaplan AFH. Production of osseointegrating (bone bonding) surfaces on titanium screws by laser melt disruption. J Laser Appl 2018;30:42009. [8] Ibatan T, Uddin MS, Chowdhury MAK. Recent development on surface texturing in enhancing tribological performance of bearing sliders. Surf Coat Technol 2015;272:102–20. [9] Wang X, Kato K. Improving the anti-seizure ability of sic seal in water with RIE texturing. Tribol Lett 2003;14:275–80. [10] Pratap T, Patra K. Mechanical micro-texturing of Ti-6Al-4V surfaces for improved wettability and bio-tribological performances. Surf Coat Technol 2018;349:71–81. [11] Chen XL, Qu NS, Hou ZB, Wang XL, Zhu D. Friction reduction of chrome-coated surface with micro-dimple arrays generated by electrochemical micromachining. J Mater Eng Perform 2017;26:667–75. [12] Maressa P, Anodio L, Bernasconi A, Demir AG, Previtali B. Effect of surface texture on the adhesion performance of laser treated Ti6Al4V alloy. J Adhesion 2015;91:518–37. [13] Etsion I. Improving tribological performance of mechanical components by laser surface texturing. Tribol Lett 2004;17:733–7. [14] Bathe R, Krishna VS, Nikumb SK, Padmanabham G. Laser surface texturing of gray cast iron for improving tribological behavior. Appl Phys A-Mater 2014;117:117–23. [15] Yang Z, Tian YL, Yang CJ, Wang FJ, Liu XP. Modification of wetting property of Inconel 718 surface by nanosecond laser texturing. Appl Surf Sci 2017;414:313–24. [16] Cardoso JT, Aguilar-Morales AI, Alamri S, Huerta-Murillo D, Cordovilla F, Lasagni AF, et al. Superhydrophobicity on hierarchical periodic surface structures fabricated via direct laser writing and direct laser interference patterning on an aluminium alloy. Opt Laser Eng 2018;111:193–200. [17] Mao B, Siddaiah A, Menezes PL, Liao YL. Surface texturing by indirect laser shock surface patterning for manipulated friction coefficient. J Mater Process Tech 2018;257:227–33. [18] Butt MZ, Javed A, Khaliq MW, Ali D, Bashir F. Impact of 1064 nm–10 ns pulsed laser on the surface morphology, structure, and hardness of Pd80Ni20 alloy. Int J Adv Manuf Technol 2017;90:1857–69. [19] Hoppius JS, Kukreja LM, Knyazeva M, Pohl F, Walther F, Ostendorf A, et al. On femtosecond laser shock peening of stainless steel ANSI 316. Appl Surf Sci 2018;435:1120–4. [20] Du QF AJ, Qin ZL, Liu JG, Zeng XY. Fabrication of superhydrophobic/superhydrophilic patterns on polyimide surface by ultraviolet laser direct texturing. J Mater Process Technol. 2018;251:188–96. [21] Cassie ABD, Baxter S. Wettability of porous surfaces. Trans Faraday Soc 1944;40:546–51. [22] Wenzel RN. Resistance of solid surfaces to wetting by water. Ind Eng Chem 1936;28:988–94. [23] Yilbas BS, Khaled M, Abu-Dheir N, Aqeeli N, Furquan SZ. Laser texturing of alumina surface for improved hydrophobicity. Appl Surf Sci 2013;286:161–70. [24] Jagdheesh R, Garcia-Ballesteros JJ, Ocana JL. One-step fabrication of near superhydrophobic aluminum surface by nanosecond laser ablation. Appl Surf Sci 2016;374:2–11. [25] Kam DH, Bhattacharya S, Mazumder J. Control of the wetting properties of an ANSI 316l stainless steel surface by femtosecond laser-induced surface modification. J Micromech Microeng 2012;22:105019.
4. Conclusions A masked laser surface texturing process was performed using a Nd:YAG pulsed laser and a mesh grid mask with a 25 μm bar width. By selectively ablating the aluminum surface, hundreds of micro-patterns were effectively created with a single laser irradiation. The influence of the laser energy intensity and the number of laser pulses on the surface morphology and properties of the micro-pattern array was investigated. The results are summarized as follows: (1) At low laser intensities (0.39 GW/cm2), well separated surface texturing of bar and hole regions was possible. In the hole region, irregular surface characteristics such as droplets were generated due to re-solidification of the molten material. As the laser intensity increased, the surface morphology of the hole region became very irregular and rough. (2) When the number of laser pulses increased, the surface was subjected to repeated re-solidification and planarization of the molten material to significantly reduce surface roughness. The deeper micro-pattern and various pattern shapes could be achieved without losing the homogeneity of the micro patterns. (3) Hardness values of the material surface can be selectively increased by adjusting laser parameters. The hardness increase of the hole region was observed when the laser intensity was increased due to the microstructural change of the material surface. As the number of laser pulses increased, a remarkable increase of the bar hardness value was observed, which can be interpreted as the superposition effect of the laser induced microstructure change and the laser shock loading due to the plasma expansion. (4) The contact angle of the laser textured surface decreased as the laser intensity and the number of laser pulses increased. Since the surface irregularity increased as the energy intensity increased, the contact angle can be quantitatively related to the surface roughness value. As the number of laser irradiations increased, the contact angle decreased because the contact area between air and water decreased as the width of the bar increased. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Effects of laser parameters on morphological change and surface properties of aluminum alloy in masked laser surface texturing
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Seung Jai Won, Hong Seok Kim* Department of Mechanical and Automotive Engineering, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu, Seoul, 01811, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Masked laser surface texturing Laser shock loading Surface morphology Hardness Wettability
The masked laser surface texturing process was used to produce micro-pattern arrays. Using mesh grids as masks, the surface of the workpieces were selectively ablated and hundreds of micro-patterns were simultaneously generated by a single laser irradiation. The effects of laser energy intensity and number of laser pulses on surface morphologies and properties were investigated. It was found that it is more efficient to control the number of laser pulses than the laser energy intensity to form a uniform micro pattern array and to control the pattern shape. In addition, hardness values of the material surface can be selectively increased by adjusting laser parameters. When the laser energy intensity increased, the hardness of the hole region which was directly affected by the laser irradiation increased. When the surface was irradiated repeatedly by the multiple laser pulses, however, the increase in hardness was much pronounced in the masked bar region adjacent to the ablation zone. The structural changes of the patterned surface and the work hardening effect due to laser shock loading were superimposed to increase the hardness of the masked region. The contact angle decreased with increasing laser energy intensity and number of laser pulses. This is mainly due to an increase in surface heterogeneity at high laser energy intensities and an increase in bar width at multiple laser pulses condition.
1. Introduction With the rapid development of precision manufacturing technology [1–3], surface texturing (which produces the microscale shapes on the surface of the material) has attracted significant attention in various industries. By producing microscale dimples or shapes on the surface of the material, it is possible to change the surface properties such as hardness and wettability, and to improve the friction characteristic and wear resistance of the surface [4]. In addition, surface texturing is known to improve performance in battery efficiency [5], heat transfer performance [6], and osseointegration [7]. Therefore, surface texturing is a very effective technology for achieving desired surface properties without using expensive materials. Surface texturing has been conducted for various purposes using a variety of methods [8] including reactive ion etching (RIE), micro drilling, and electrochemical machining (ECM). Wang et al. [9] performed surface texturing using RIE on the surface of a SiC seal that can operate in high-temperature and high-pressure water. They showed that RIE texturing provided the SiC seal with an anti-seizure ability and low coefficient of friction. Pratap et al. [10] fabricated micro-dimple arrays on Ti-6Al-4 V titanium alloy using micro-tools of different geometries. They showed that surface texturing, texture geometry, and ⁎
initial surface conditions have significant influences on the wettability and surface hardness of the material. Chen et al. [11] presented a through-mask electrochemical micromachining process to produce micro hole and dimple arrays of several hundred micrometers on chrome-coated surface. The results showed that the tribological properties of mechanical components could be much enhanced in the case of square micro-dimples. These conventional methods, however, have various problems. RIEs and ECM entail complex procedures and are accompanied by chemical work, which adversely affects the environment. In the case of micro-drilling or machining, design and manufacture of the tool is very difficult and costly, and the chip must be effectively removed. Therefore, a more environmentally friendly and easy surface texturing method is required. In such a situation, laser-based surface texturing methods are attracting much attention because they are environmentally friendly [12] and inherently precise and repeatable. Etsion et al. [13] performed laser surface texturing on the SiC material. They reported that frictional performance of thrust bearings was much improved after surface texturing. Bathe et al. [14] textured the gray cast iron surface with different laser pulse durations and reported that femtosecond laser irradiation was a useful tool for improving wear characteristics. Yang et al. [15] fabricated line, rectangular, and circular dimple patterns on
Corresponding author. E-mail address: [email protected] (H.S. Kim).
https://doi.org/10.1016/j.jmapro.2019.10.034 Received 18 May 2019; Received in revised form 2 October 2019; Accepted 29 October 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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masked laser surface texturing process. A Q-switched Nd:YAG pulsed laser with 10 ns pulse width and 1064 nm wavelength was used as the energy source. The laser beam was focused on the surface of the material via two reflecting mirrors and a focusing lens. The diameter of the focused beam on the specimen was 1.8 mm. The visible He-Ne laser was installed and aligned with the Nd:YAG laser to accurately locate the laser beam spot on the specimen. Al1050 alloy (which has wide range of applications and good mechanical properties) was used as the specimen. This material was cut to 20 × 20 × 0.5 mm, polished, and fixed firmly on the 3-axis stage. Fig. 2 shows the experimental procedure for the masked laser surface texturing process. The mesh grid of Ni consists of bars with a width of 25 μm and square holes with a length of 100 μm. The overall diameter of the grid was 3 mm and contained approximately 200 repetitive microscale patterns. First, the mesh grid is placed and fixed on the top surface of the specimen. When the laser is subsequently irradiated, the bar portion of the grid acts as a mask and the material is selectively ablated. When the mesh grid is removed, the surface of the specimen can be divided into the bar region where the direct laser effect was not significant and the hole region where the ablation occurred. This process is simple and very efficient because more than 160 micro-patterns can be formed simultaneously on the metallic surface with only one laser beam irradiation. It is expected that micro-pattern arrays with different geometrical features will be generated according to the laser process conditions and the shape and size of the mesh grid. The geometrical features of the micro-pattern will also have a strong influence on the surface properties of the material. Hence, the laser energy intensity and the number of laser pulses were selected as major parameters and their effects on the morphologies and properties of the patterned surface were investigated. Three laser energy intensities were considered: 0.39, 3.9, and 7.8 GW/ cm2. Four different number of laser pulses were used: 1, 5, 10, and 20. The time interval between pulses in repeated irradiations was one second. The shape, roughness, hardness and contact angle of the specimens were analyzed after the masked laser surface texturing. Surface morphology was observed using a scanning electron microscope (SEM). A detailed 3D profile and surface roughness were measured using a confocal laser microscope (Carl Zeiss, LSM 800MAT). To measure the hardness, a micro-Vickers hardness tester (Future-Tech, FM-800) was used and the diamond tip was pressed against the workpiece surface for 5 s under an indentation load of 10 gf. The contact angle was measured by a contact-angle meter (KSV, CAM-200) by the Sessile-drop method using deionized water at the ambient temperature 20 °C and humidity 40 %. Three repetitions of the experiments were done to secure reliability.
Inconel 718 surfaces by effectively controlling the laser parameters such as scan speed and power. They showed that the variations in surface morphology and roughness were accompanied by modifications of surface wettability. In order to increase the efficiency of laser-based surface texturing processes, various novel laser processes in which a number of micro-pattern arrays are produced with a single laser irradiation have also been reported. Cardoso et al. [16] presented the direct laser interface patterning (DLIP) method, which can fabricate multiple dimples at a time by controlling the beam interference. They effectively generated various surface morphologies and contact angles on the Al2024 alloy surface by controlling laser parameters such as polarization and angle of incidence. Mao et al. [17] performed an indirect laser surface patterning process utilizing the laser-induced shock wave loadings to realize the patterning and strengthening effects simultaneously on the material surface. Using the grid as a micro-mold, multiple micro-pattern arrays could be generated with a single laser irradiation, and the improvement of hardness and wear resistance was achieved. In this study, a laser surface texturing process using a mesh grid as a mask was used to form hundreds of micro-patterns simultaneously with one laser irradiation. This method is simple, environmentally friendly, and economical because it uses a mesh grid alone to selectively ablate the surface of the material. In addition, since the surface patterning and the shock wave loading occur at the same time in this process, desired values of the surface properties such as hardness can be effectively realized by controlling the surface morphologies and laser parameters. However, systematic and detailed studies on this aspect are currently lacking in the present literature. Therefore, in this study, the influence of the major process parameters such as the laser energy intensities and the number of laser pulses on the morphologies and properties of the textured surface were investigated. It was found that it is more efficient to control the number of laser pulses than the laser energy intensity in order to form a uniform micro pattern array and to control the pattern shape. In addition, by adjusting the laser parameters, it was possible to selectively increase the hardness value of the masked bar region of the material or of the hole region directly affected by the laser irradiation. The change in hardness was considered to be caused by the structural changes of the surface and the superposition of the hardening effect due to the laser shock loading. The change in the contact angle could be also described quantitatively using the surface heterogeneity and the change in bar width of the patterned surface.
2. Experimental procedure Fig. 1 shows the schematic of the experimental equipment for the
3. Results and discussion 3.1. Surface morphologies according to laser energy intensity Fig. 3 shows the ablated surface of an aluminum specimen without a mesh grid at a laser intensity of 0.39 GW/cm2. It was confirmed that the surface layer of the material was roughened by melting and a rapid solidification process. Many pits and micro-sized holes were observed in the enlarged SEM picture. The profile measured in the ablation zone is shown in Fig. 3(b). Considering that the roughness Ra of the untreated specimen was about 0.05 μm, it can be seen that the profile of the ablated surface became very irregular. The roughness Ra of the ablated surface was about 0.35 μm. This is because the plasma generated by the laser ablation expanded and spattered the molten material rapidly. Fig. 4 shows the surface morphology of the textured specimen with a mesh grid at a laser intensity of 0.39 GW/cm2. The textured surface was well separated from the bar and hole regions. Compared with Fig. 3, several larger droplets were observed in the hole region. This is because the ablated area was divided into small pieces and the diffusion
Fig. 1. A schematic illustration of the masked laser surface texturing process. 261
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Fig. 2. Summary of experimental procedure.
the number of small droplets increased in the bar region. Additionally, the boundaries between the bar and the hole regions became difficult to distinguish. Fig. 5(b) shows the profile measured on the textured surface. As the energy increased, the average depth of the bar portion increased slightly to 8.7 μm. Also, the surface of the hole region became more irregular. It started to lose the planarity as more mass on the bottom of the hole melted and re-solidified under the influence of the laser plasma. The surface morphologies when the laser energy intensity was further increased to 7.8 GW/cm2 are presented in Fig. 6. Fig. 6(a) shows that for 60% of the ablated area, the bar and hole regions could not be clearly distinguished. This is because the part of the mesh grid placed on the specimen was melted and welded to the specimen surface due to excessive laser energy irradiation. Additionally, strong laser energy penetrating between the grid and the material had a strong thermal effect on the material. Fig. 6(b) shows the profile measurement results for the relatively well-separated area of the bar and hole regions. The
of the molten material was somewhat limited. Several small droplets were also observed in the bar region. It seems that some of the molten material by the laser was spattered from the hole region to the bar region with the expansion of the plasma. However, the effect of the laser on the masked bar region of the workpiece was not very significant. Fig. 4 (b) shows the profile measured on the textured surface. The average height of the bars is 7.3 μm from the lowest point of the hole region. The surface of the hole region was rough as the surface particles were subjected to rapid melting and re-solidification processes. However, the irregularity of the hole surface was reduced compared with Fig. 3(b) because the ablation area of the material was divided into small areas. Fig. 5(a) shows the SEM images of the textured surface with a mesh grid under increased laser intensity of 3.9 GW/cm2. As the laser energy increased, a large amount of material was ablated and larger sized droplets were observed in the hole region. Compared with Fig. 4(a), the spatter effect of the molten material increased in the bar region because
Fig. 3. Surface morphologies after laser ablation without a mesh grid (0.39 GW/cm2 laser intensity) (a) SEM images with different magnifications (b) Surface profile along the line. 262
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Fig. 4. Surface morphologies after masked laser surface texturing with 0.39 GW/cm2 laser intensity (a) SEM images with different magnifications (b) Surface profile along the line.
Fig. 5. Surface morphologies after masked laser surface texturing with 3.9 GW/cm2 laser intensity (a) SEM images with different magnifications (b) Surface profile along the line. 263
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Fig. 6. Surface morphologies after masked laser surface texturing with 7.8 GW/cm2 laser intensity (a) SEM images with different magnifications (b) Surface profile along the line.
shall be conducted into the geometric change of micro-patterns with detailed control of additional laser process parameters such as laser beam size and shape. 3.2. Surface morphologies according to the number of laser pulses In this section, changes in surface morphology were investigated by changing the number of laser pulses with the laser intensity fixed at 0.39 GW/cm2. Under this energy intensity condition, the mesh grid was damaged when the laser pulses were irradiated more than 30 times. Since the maximum number of laser irradiations in this study was 20, the damage of the mesh grid did not affect the experimental results. Fig. 8 shows the laser textured surface of the specimen after the laser beam was irradiated five times. Compared with Fig. 4, significant changes were observed from SEM pictures in both hole and bar regions. In the case of the bar region, the surface was observed to be more flexed and softened. It appears that the laser had repeated thermal effects on this region between the grid and the specimen. Unlike the bar shape in Fig. 4 (where the influence of the laser is limited), the thermal effects of the laser are expected to cause changes in the composition and grain structure of the bar region (see Fig. 8). In the case of the hole region, the droplet shape due to the rapid solidification was changed into a threadlike shape. This was considered to be the planarization of the hole surface caused by repeating the melting and re-solidification processes with repeated laser beam irradiations. The surface profile of the specimen is shown in Fig. 8(b). The micro-pattern array maintained regularity despite being exposed to the laser thermal effect repeatedly. The average depth of the bar was 12.8 μm, which was about 75% higher than that of the single irradiation case. The hole region did not lose planarity and the irregularity of the hole surface was significantly reduced by the aforementioned planarization effect of the repeated laser irradiations. Fig. 9 shows the laser textured surface when the number of laser pulses was increased to 10. SEM images show that the surface
Fig. 7. Surface roughness values (Ra) of the laser textured specimens at different laser intensities.
bar portion was often unable to maintain the shape of the peaks due to the increased laser effect. The surface of the hole region became more irregular and the surface roughness was significantly increased. Consequently, the general tendency to increase the surface roughness with increasing laser energy intensity was confirmed. As depicted in Fig. 7, the surface roughness Ra values for the laser textured surface were 1.7, 2.1, and 3.0 μm at laser intensities of 0.39, 3.9 and 7.8 GW/ cm2, respectively. In the case of 7.8 GW/cm2, it can be seen that surface irregularities caused large variation in roughness values. In the masked laser surface texturing process of this study, the laser energy needs to be adjusted appropriately to control the shape of micro-pattern array. An energy level of 3.9 GW/cm2 or less was adequate and the surface morphology could not be controlled beyond that value. Since the depth of the bars increased as the laser energy increased, further investigation
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Fig. 8. Surface morphologies after masked laser surface texturing with 5 laser pulses (0.39 GW/cm2 laser intensity) (a) SEM images with different magnifications (b) Surface profile along the line.
the thread shape in the hole region became slightly faded and the surface became more smooth and homogeneous. It appears that repeated laser irradiations caused the surface finishing effect with the surface being more planarized. Fig. 9(b) shows the measured surface profile of the textured specimen. The shape of the micro-pattern array
characteristics of the bar region is similar to those of the 5 laser-irradiation case. However, it was observed that the width of the bar increased. This was caused by the newly melted material accumulating on the bar side with repeated laser irradiations. The droplets in the hole region disappeared and were flattened. However, it was observed that
Fig. 9. Surface morphologies after masked laser surface texturing with 10 laser pulses (0.39 GW/cm2 laser intensity) (a) SEM images with different magnifications (b) Surface profile along the line. 265
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Fig. 10. Surface morphologies after masked laser surface texturing with 20 laser pulses (0.39 GW/cm2 laser intensity) (a) SEM images with different magnifications (b) Surface profile along the line.
was fairly regular and homogeneous. The average depth of the bars was 20.1 μm, which was a significant increase of more than 170% compared with the single irradiation case. In the hole region, the roughness of the surface profile was significantly reduced by the enhanced planarization effect. However, it was observed that the central part of the hole was slightly depressed off the plane. This is because the newly ablated material of the hole region was accumulated on the bottom of the bar side by the spatter effect and the material of the bar region melted down to the edge of the hole due to the increased thermal effect. Fig. 10(a) shows the SEM images of the laser textured specimen when the number of laser pulses was further increased to 20. As the accumulation of molten material increased, the width of the bars on the surface increased significantly. The change in bar width with the number of laser pulses is shown in Fig. 11. When the laser beam was irradiated 10 times, the increase of the bar width was 17.3%. When the laser beam was irradiated 20 times, the bar width further increased by 70% or more. Although increasing the number of laser pulses increases
the amount of material ablated by the laser, it appears that the ablated material is not effectively removed from the narrow hole region, but rather is deposited again near the bottom of the bar region. In the profile graph in Fig. 10(b), it can be seen that the bar of the previous peak shape changed into a triangular shape and the boundary between the bar and the hole regions become blurred. In the case of hole region, the planarization effect became more prominent. Consequently, the thread shape in the hole almost disappeared. However, some microscale holes were newly observed due to the accumulated thermal effect. Fig. 12 shows the change in height of the bar according to the number of laser pulses. When the number of laser pulses was less than 10, the height of the bar increased with the number of laser pulses. However, when the number of laser pulses was 20, the height of the bar decreased to 16 μm. This is because the bar region is exposed to a increased amount of laser energy as the number of laser pulses increases. When the number of laser pulses reaches a certain value or more, the
Fig. 11. Bar width as a function of number of laser pulses (0.39 GW/cm2 laser intensity).
Fig. 12. Bar height as a function of number of laser pulses (0.39 GW/cm2 laser intensity). 266
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Fig. 13. Surface roughness (Ra) of hole region as a function of number of laser pulses (0.39 GW/cm2 laser intensity).
Fig. 14. Hardness values of bar and hole regions at different laser intensities (single laser pulse).
erosion effect of the upper bar region appears to be larger than the deepening effect of the laser ablation. Fig. 13 shows the change in the surface roughness value of the hole region according to the number of laser pulses. The surface roughness decreased as the number of laser pulses increased due to the planarization effect caused by repeated laser irradiations. When the laser pulse was irradiated 5 times, the surface roughness of the hole region was 0.15 μm (48.7% lower than that of the single laser irradiation). In addition, the surface roughness when the laser was irradiated 10 times was further reduced to 0.07 μm (77% lower than that of single laser irradiation). In the case of 20 laser irradiations, the surface roughness was 0.07 μm, which was similar to the 10 laser irradiation case. After 10 laser irradiations, no further improvement in surface roughness (obtained from the planarization effect) was observed. In this section, morphological changes of the laser textured surface were observed according to the number of laser pulses. The height and width of bars, surface structure, and roughness values were found to vary significantly as the number of laser pulses increased. It was found that it is more efficient to control the number of laser pulses than the laser energy intensity to form a uniform micro pattern array and to control the pattern shape. In the case of controlling the number of laser pulses, it was possible to increase the depth of the micro-pattern and realize various pattern shapes without losing the homogeneity of the micro patterns. However, as shown in Fig. 10, some cracks were observed at the edge of the bar region when the laser was irradiated 20 times. This appears to be due to the excessive hardening effect of the surface. Therefore, in addition to the geometric aspects of the micropatterns, quality aspects including surface cracks and damage should be considered in future studies.
Fig. 15. Hardness values of bar and hole regions at different numbers of laser pulses (0.39 GW/cm2 laser intensity).
and the crystal size decreased, which was considered as the major cause of increased hardness. Therefore, it can be concluded that the increase in hardness of the laser patterned surface in Fig. 14 is mainly caused by the change in microstructure of the surface after the laser treatment rather than the geometrical characteristics of the micro-pattern itself. Fig. 15 shows the change of surface hardness with the change in the number of laser pulses. For the hole region, the hardness increases for the number of laser pulses of 5, 10, and 20 were 41.4%, 92.9%, and 128.7%, respectively. Since the laser intensity was fixed at 0.39 GW/ cm2, the total laser energy input for 10 and 20 laser pulses was identical to the energy conditions at 3.9 and 7.8 GW/cm2 in Fig. 14, respectively. Therefore, repeated laser irradiation was found to be a more effective way to increase hardness than to increase laser energy intensity. One of the most remarkable points in Fig. 15 was that in contrast to the results of Fig. 14, the increase in hardness was observed in the bar region as the number of laser pulses increased. The hardness increases with the number of laser pulses of 5, 10, and 20 were 102 %, 133 %, and 167 %, respectively. This increase was even greater than the increase in the hardness of the hole region directly affected by the laser. In the SEM images in Figs. 8–10, the surface characteristics of the bar region are shown to have changed depending on the influence of the repeated laser irradiation (compared with the bar regions in Figs. 4–6). However, it is not sufficient to explain the increase in the hardness of the bar region only by the defect density and crystal size described above because the hole region is much larger in laser energy input value than the bar region. As the laser beam repeats, the bar region of the specimen undergoes
3.3. Micro-hardness The micro-pattern array on the material surface greatly affects the surface properties of the material. Hardness is one of these mechanical properties. This study examined the effect of the micro-pattern array on micro-hardness under various process conditions The hardness was measured for the hole and bar regions. The change in hardness value at different laser intensities is shown in Fig. 14. Considering that the aluminum specimen before patterning had a Vickers harness number (VHN) of 45.18, the increase in hardness of the bar region after patterning was not very significant. This is because the direct effect of laser energy on the bar region is limited. However, the increase in hardness at the hole region was much greater. The increases were 25.5%, 46.7%, and 85.0% for 0.39, 3.9, and 7.8 GW/cm2 laser intensities, respectively. Butt et al. [18] reported that as the laser energy reaching the specimen increased, the defect density increased 267
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at an energy intensity of 0.39 GW/cm2, the contact angle increased by approximately 30%–86.4°. This contact angle change mechanism is described in the Cassie-Baxter model [21]. This model is expressed as follows:
repetitive loads from the grid. In addition, the plasma generated from the ablated area of the hole region expands rapidly and exerts a strong shock load in the direction of the bar. In the case of repetitive laser irradiations, it is judged that the above two phenomena repeatedly overlap and the hardness of the bar region is markedly increased. This is very similar to the shock peening process in which a steel ball impacts the material surface to produce compressive residual stresses. Work hardening and hardness increase due to laser shock loading can be also seen in many previous studies [19]. In conclusion, in the masked laser surface texturing of this study, it was confirmed that the increase of hardness was influenced by the microstructural change of the surface and the laser shock loading depending on the process condition (rather than the geometrical characteristics of the micro-patterns). Overall, increasing the number of laser pulses was effective in increasing the hardness, and it was possible to selectively harden the bar or hole regions of the surface by controlling the laser parameters.
{cosθCB = f1 cosθr − f2 }.
(1)
where θr is the contact angle of the reference surface, f1 is the area fraction of the water-solid surface, f2 is the area fraction of the water-air surface, and θCB is the contact angle of the Cassie-Baxter model. According to Eq. (1), the contact angle is determined by the ratio of f1 and f2. In this study, the irregularity of the surface after laser texturing was increased. Hence, it can be judged that the contact angle increased as the contact area f1 of the surface and water decreased and the contact area f2 of the air and water increased. A decrease in the contact angle with increasing laser intensity was associated with an increase in surface heterogeneity. As surface heterogeneity increases, water droplets in the Cassie-Baxter state become more difficult to maintain in terms of their shape and flow down into the valleys of the surface. Accordingly, the contact between water and air is reduced, which results in the contact angle being reduced. In particular, at laser intensity of 7.8 GW/cm2, the surface contact state of the specimen is explained using the Wenzel’s theory because the contact angle was more hydrophilic than the contact angle of the specimen before texturing. Wenzel's contact angle theory is expressed by the following equation [22]:
3.4. Contact angle The contact angle, which determines the wettability of the surface, is considered important in many engineering and industrial fields. A hydrophilic surface having a small contact angle is used for the improvement of antifogging and adhesive properties [20]. Hydrophobic or superhydrophobic surfaces with contact angles greater than 150 degrees are used to improve self-cleaning, anti-corrosion, and bacterial repellence [16]. Laser surface texturing is a technique that can effectively change the contact angle properties of the material surface. The contact angle is strongly affected by the shape of the micropattern and surface chemistry. Since the area of the ablated region in the specimens was the same regardless of the process conditions, this study investigated the change of the contact angle in terms of surface morphology. Since many researchers have reported that the contact angle of the laser textured surface changes with time [16], the contact angle was measured 30 days after the experiment. In addition, the contact angle was measured for the water droplet containing hundreds of repeating micro-patterns in order to exclude the influence of slight geometrical differences between the patterns and the sensitivity to the measurement position. The change in the contact angle of the textured surface with respect to the laser energy intensity is shown in Fig.16. The contact angle of the specimen before laser texturing was 63°. Therefore, it can be seen that
{cosθW = fcosθr }.
(2)
where θr is the contact angle of the flat reference surface, f is the ratio between the actual area and the apparent area, and θW is the contact angle from the Wenzel model. Eq. (2) shows that the hydrophilic surface becomes more hydrophilic as f increases. The change in the contact angle in Fig. 16 may also be related to the change in the surface roughness in Fig. 7. As the surface roughness increased, the contact angle decreased. Similar results were found in the study of Yilbas et al. [23]. Fig. 17 shows the change in the contact angle caused by repetitive irradiation by the laser. As the number of laser irradiations increased, the contact angle tended to decrease. However, in this case, the surface became much smoother as shown in Fig. 13. Therefore, it is difficult to relate the surface roughness to the change of the contact angle. Instead, the change in the contact angle appears to be related to the change in the width of the bar region shown in Fig. 11. As the width of the bar increased, the distance between the bars decreased and the amount of air trapped between the bars decreased. Consequently, the contact between water and air was reduced and the contact angle decreased according to Cassie-Baxter theory. Fig. 11 shows the bar width value was similar for the 5 and 10 laser pulse cases, and increased abruptly in the
Fig. 16. Variation of contact angle at different laser intensities (single laser pulse).
Fig. 17. Variation of contact angle at different numbers of laser pulses (0.39 GW/cm2 laser intensity). 268
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S.J. Won and H.S. Kim
Acknowledgements
case of 20 laser pulses. Fig. 17 shows that the contact angle was also similar for 5 and 10 laser irradiations, and sharply decreased with 20 laser pulses. Jagdheesh et al. [24] reported that the irregularity of the micro-pattern array caused the Wenzel state transition. In the case of repetitive laser irradiation, the Wenzel state transition did not appear to occur as the regularity of the micro-patterns was relatively well maintained. Consequently, the contact angle of the surface treated specimen is found to be closely related to the surface roughness and the pattern geometry. Kam et al. [25] formed a conical surface structure with a laser and showed that the contact angle could range from super-hydrophilic (0°) to hydrophobic (113°) depending on processing conditions. It is expected that various wettable surfaces can be created by more finely adjusting the shape of the micro-pattern also by using the masked laser surface texturing process.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant NRF-2017R1D1A1B03035818). References [1] Wu SM, Ni J. Precision machining without precise machinery. CIRP Ann-Manuf Technol 1989;38:533–6. [2] Kim HS, Nam JS. Quantitative modeling and characterization of the size effects in microscale coining process of copper. Precis Eng 2018;51:490–8. [3] You JH, Kim HS. Formability and fracture of metallic sheet materials in microscale punching processes using laser-accelerated flyer impact. Int J Adv Manuf Technol 2018;96:3511–20. [4] Lorenzo-Martin C, Ajayi OO. Effect of sic particle impact nano-texturing on tribological performance of 304l stainless steel. Appl Surf Sci 2014;315:287–91. [5] Abbott M, Cotter JR. Optical and electrical properties of laser texturing for highefficiency solar cells. Prog Photovolt 2006;14:225–35. [6] Grabas B. Vibration-assisted laser surface texturing of metals as a passive method for heat transfer enhancement. Exp Therm Fluid Sci 2015;68:499–508. [7] Olsson R, Powell J, Palmquist A, Branemark R, Frostevarg J, Kaplan AFH. Production of osseointegrating (bone bonding) surfaces on titanium screws by laser melt disruption. J Laser Appl 2018;30:42009. [8] Ibatan T, Uddin MS, Chowdhury MAK. Recent development on surface texturing in enhancing tribological performance of bearing sliders. Surf Coat Technol 2015;272:102–20. [9] Wang X, Kato K. Improving the anti-seizure ability of sic seal in water with RIE texturing. Tribol Lett 2003;14:275–80. [10] Pratap T, Patra K. Mechanical micro-texturing of Ti-6Al-4V surfaces for improved wettability and bio-tribological performances. Surf Coat Technol 2018;349:71–81. [11] Chen XL, Qu NS, Hou ZB, Wang XL, Zhu D. Friction reduction of chrome-coated surface with micro-dimple arrays generated by electrochemical micromachining. J Mater Eng Perform 2017;26:667–75. [12] Maressa P, Anodio L, Bernasconi A, Demir AG, Previtali B. Effect of surface texture on the adhesion performance of laser treated Ti6Al4V alloy. J Adhesion 2015;91:518–37. [13] Etsion I. Improving tribological performance of mechanical components by laser surface texturing. Tribol Lett 2004;17:733–7. [14] Bathe R, Krishna VS, Nikumb SK, Padmanabham G. Laser surface texturing of gray cast iron for improving tribological behavior. Appl Phys A-Mater 2014;117:117–23. [15] Yang Z, Tian YL, Yang CJ, Wang FJ, Liu XP. Modification of wetting property of Inconel 718 surface by nanosecond laser texturing. Appl Surf Sci 2017;414:313–24. [16] Cardoso JT, Aguilar-Morales AI, Alamri S, Huerta-Murillo D, Cordovilla F, Lasagni AF, et al. Superhydrophobicity on hierarchical periodic surface structures fabricated via direct laser writing and direct laser interference patterning on an aluminium alloy. Opt Laser Eng 2018;111:193–200. [17] Mao B, Siddaiah A, Menezes PL, Liao YL. Surface texturing by indirect laser shock surface patterning for manipulated friction coefficient. J Mater Process Tech 2018;257:227–33. [18] Butt MZ, Javed A, Khaliq MW, Ali D, Bashir F. Impact of 1064 nm–10 ns pulsed laser on the surface morphology, structure, and hardness of Pd80Ni20 alloy. Int J Adv Manuf Technol 2017;90:1857–69. [19] Hoppius JS, Kukreja LM, Knyazeva M, Pohl F, Walther F, Ostendorf A, et al. On femtosecond laser shock peening of stainless steel ANSI 316. Appl Surf Sci 2018;435:1120–4. [20] Du QF AJ, Qin ZL, Liu JG, Zeng XY. Fabrication of superhydrophobic/superhydrophilic patterns on polyimide surface by ultraviolet laser direct texturing. J Mater Process Technol. 2018;251:188–96. [21] Cassie ABD, Baxter S. Wettability of porous surfaces. Trans Faraday Soc 1944;40:546–51. [22] Wenzel RN. Resistance of solid surfaces to wetting by water. Ind Eng Chem 1936;28:988–94. [23] Yilbas BS, Khaled M, Abu-Dheir N, Aqeeli N, Furquan SZ. Laser texturing of alumina surface for improved hydrophobicity. Appl Surf Sci 2013;286:161–70. [24] Jagdheesh R, Garcia-Ballesteros JJ, Ocana JL. One-step fabrication of near superhydrophobic aluminum surface by nanosecond laser ablation. Appl Surf Sci 2016;374:2–11. [25] Kam DH, Bhattacharya S, Mazumder J. Control of the wetting properties of an ANSI 316l stainless steel surface by femtosecond laser-induced surface modification. J Micromech Microeng 2012;22:105019.
4. Conclusions A masked laser surface texturing process was performed using a Nd:YAG pulsed laser and a mesh grid mask with a 25 μm bar width. By selectively ablating the aluminum surface, hundreds of micro-patterns were effectively created with a single laser irradiation. The influence of the laser energy intensity and the number of laser pulses on the surface morphology and properties of the micro-pattern array was investigated. The results are summarized as follows: (1) At low laser intensities (0.39 GW/cm2), well separated surface texturing of bar and hole regions was possible. In the hole region, irregular surface characteristics such as droplets were generated due to re-solidification of the molten material. As the laser intensity increased, the surface morphology of the hole region became very irregular and rough. (2) When the number of laser pulses increased, the surface was subjected to repeated re-solidification and planarization of the molten material to significantly reduce surface roughness. The deeper micro-pattern and various pattern shapes could be achieved without losing the homogeneity of the micro patterns. (3) Hardness values of the material surface can be selectively increased by adjusting laser parameters. The hardness increase of the hole region was observed when the laser intensity was increased due to the microstructural change of the material surface. As the number of laser pulses increased, a remarkable increase of the bar hardness value was observed, which can be interpreted as the superposition effect of the laser induced microstructure change and the laser shock loading due to the plasma expansion. (4) The contact angle of the laser textured surface decreased as the laser intensity and the number of laser pulses increased. Since the surface irregularity increased as the energy intensity increased, the contact angle can be quantitatively related to the surface roughness value. As the number of laser irradiations increased, the contact angle decreased because the contact area between air and water decreased as the width of the bar increased. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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