Abstract
An efficient method for processing superhydrophobic surface is presented in this paper, which can realise the microstructure machining on a large area. First, the microgroove arrays were processed by high speed precision micromilling machine on the surface of the aluminium alloy. Then, the surface of the microgroove was ground by 1000 # electrostatic sand alumina water resistant abrasive belt. The machined surface topography was observed by SEM, which showed that the microgroove arrays were uniform. The contact angles of the water on the microgroove arrays were measured in different directions. In the parallel direction, the contact angle of microgroove arrays is 142±0·5°, whereas in the vertical direction, it arrives 160±0·6°. The above values of contact angle mean that a stable superhydrophobic metallic surface was prepared by the micromilling and grinding.
Introduction
In recent years, the superhydrophobic surface has increasingly important applications and broad prospects in the aerospace, optoelectronics, mechanical engineering, biomedical and defense fields due to its non-wetting, drag reduction, oxidation, corrosion resistant and self-cleaning properties.1–5 In such case, it is very significant and necessary to develop effective manufacturing methods for the superhydrophobic surfaces. The fabrication of the superhydrophobic surface has attracted wide attention and been an important subject of research.
It is hard to achieve the superhydrophobic property for a surface without low surface energy and proper roughness. Lots of methods6–11 for the fabrications of the superhydrophobic surface have been reported, such as chemical deposition, sol–gel method, nanoimprint technology, scanning probe, LIGA (Lithographie, Galanoformung and Abformung), self-assembly and biological manufacturing. Although the superhydrophobic surface can be manufactured using the above methods, the materials are limited to polymers, electrical conductor and colloidal materials, and the controllability and uniformity of the microstructures are poor. Furthermore, the microstructures have to be modified in order to achieve quite high contact angle. It should be noted that the coating is easy to wear and aging.12,13 Hence, their applications are greatly restricted.
Precision micromilling and abrasive belt grinding technologies14–17 are unique processing and have attracted much more concern due to its advantages, such as high precision and efficiency, simple processing, low cost and less limitations of the machined materials. The microstep, microconvex spherical array, and micropyramid arrays were successfully prepared on the surface of polymer of polyvinyl chloride, which were processed using three types of the micromilling cutter by Yang et al. 18 The straight grooves, rings, thin walled and face shape parts were fabricated using the micromilling technology with the two edge flat base end mill cutter (the diameter is 0·5 mm) by Sun et al. 19 In addition, the microwalls, microcolumns and microblades were performed with a low cost method, which use the milling processing. 20 Huang et al. 21 worked on the efficient forging blade edge abrasive belt grinding experiment for the aeroengine blade.
The formed superhydrophobic characteristics will have the significance on the hydrophilic surface, similarly to the superhydrophobic property of nanoscale microstructure arrays on biological surface such as lotus leave, pigeon feather, wing and leg of the cranefly.22–25 In this article, we proposed a method for the preparation of the superhydrophobic metal surface using the micromilling combined with the precision ultraprecision abrasive belt grinding technology. The aluminium alloy was chosen as the work material since it had widely been applied in the aerospace field, such as aircraft skin, rotor, propeller and siding. In addition, it is easy to be processed and have good corrosion resistance. First, the microgroove arrays were manufactured on the aluminium alloy surface using the precision micromilling technology. Second, this surface was treated by the abrasive belt grinding technology. Third, the integrity of the machined surface was evaluated by scanning electron microscopy (SEM). Finally, static contact angles were measured using contact angle measurement.
Experiments
Material and equipments
The material tested in this study is the aluminium alloy. The main experimental equipments include the self-developed miniature high speed precision micromilling machine (shown in Fig. 1) and the grinding device with 1000 # electrostatic sand alumina water resistant abrasive belt. The miniature high speed precision micromilling machine tool is mainly composed of numerical control system, high speed spindle, X-Y-Z precision motion stages, cooling system and micromilling cutter. The highest spindle speed is 100,000 rev min−1. The rotary precision of the spindle is 1 μm. The position precision of the worktable is ±0·5 μm/100 mm. The diameter of the solid carbide end mill used is 200 μm with two edges. The length of the cutting edge is 0·6 mm, as shown in Fig. 1.
Equipments: a miniature high speed precision micromilling machine; b,c images of milling cutter in different direction
The morphology and size of the machined surfaces were observed by SEM (model: JSM-6700 Fl). The surface roughness was measured by laser confocal microscope (model: ZEISS LSM 700). The static contact angle was measured on photos in an image analyser (optical contact angle measurement OCA20) at temperature 25±2°C. The liquid was deionised water. The volume of the deposited droplets was 5 μL. The mean of the contact angles was obtained by five times individual measurements.
Cutting parameters
In the precision micromilling process, the cutting parameters and processing route directly affect the integrity of the machined surface, dimensional accuracy and tool life. Reasonable cutting parameters can improve the dimensional accuracy of the microstructure, the quality of the surface, the tool life and the processing stability. So, an experiment was designed to select the optimal cutting parameters. The microgroove arrays were designed based on the tool parameters, the machining performance, as shown in Fig. 2. The structure parameters are as follows: the convex strip width (Fig. 2a) 95 μm, the groove width (Fig. 2b) 200 μm and the convex strip height (Fig. 2c) 110 μm.
Processing model of microgroove arrays: a convex strip width; b groove width; c convex strip height
Considering the processing efficiency, the tool life, the dimensional accuracy and the surface roughness, combined with the machine's performance, the milling parameters of the experiment were determined (the tool overhang L, the axial depth of cutting ap, the feedrate vf and the spindle speed n), as shown in Table 1.
Micromilling parameter selection
Experimental steps
The processing route was shown in Fig. 3. In order to process the microgroove arrays using the precision micromilling on the aluminium alloy surface, first, rough machining and semifinishing machining were completed using the precision milling machine on the sample surface. When the plane accuracy meets the precision micromilling requirements, the full radial cutting was performed through the microdiameter end mill on this surface with certain process parameters. Then, the samples were cleaned by an ultrasonic vibration cleaning method, successively soaked in acetone, ethanol and deionised water. Then, they were dried at 40°C. The morphology and the hydrophobic property of the machined surface were tested on the stage of the test instrument. Finally, the samples were ground with the 1000 # electrostatic sand alumina water resistant abrasive belt in the orthogonal direction of the groove structure. The precision ultraprecision abrasive belt grinding parameters are as follows: the belt speed was 4·5–5·5 mm s−1, and the grinding force 0·4–0·6 N. Using the same method, the samples were washed and dried, and then were measured.
Schematic representation of processing route
Results and discussion
Surface morphology
The surface morphology of the samples machined using the precision micromilling technology is shown in Fig. 4. In addition, the sizes of the samples are provided in Table 2.
Morphologies of samples after micromilling process: a sample 1; b sample 2; c sample 3; d sample 4
Size of samples/μm
Investigations of the surface morphology show that the burrs on the surface of samples 1 and 3 are more than those of samples 2 and 4. From Table 2, the actual sizes of the test sample 3 compared with the others are closer to the theoretical design values.
The precision micromilling parameters are as follows: the tool overhang L = 16 mm, the axial depth of cutting ap = 10 μm, the feedrate vf = 40 mm min−1 and the spindle speed n = 45 000 rev min−1. The composite structures with both micrometre scale and nanometre scale can be obtained under the above parameters. Table 3 exhibits the microgroove sizes (the convex strip width a, the groove width b and the convex strip height c) processed using the precision micromilling technology.
Dimensions of grooves manufactured by micromilling/μm
The SEM images of the microgroove arrays are shown in Fig. 5. The microgrooves manufactured by micromilling are uniform. There are also many milling marks at the bottom of the trench and curling burrs on the side edges, and the burrs on the convex strip are curved inwardly (Fig. 5a). The images of the convex strip surface treated with grinding are shown in Fig. 5b.
Images (SEM) of microgrooves: a microgroove arrays fabricated using precision micromilling technology; b microgroove arrays treated with grinding
The surface roughness was measured using laser scanning confocal microscope. Before grinding process, the surface roughness of the convex strip is 0·23–0·3 μm. After grinding process, the surface roughness of the convex strip is increased by 0·1–0·2 μm. In addition, we can see that the structures look like many spikes arranged regularly in the cross-sectional image of the microgroove arrays (Fig. 6). Considering the structures and the surface roughness of the surface treated with the grinding technology, the direction of the burrs on the convex strip is changed from the inward to the outward.
Cross-sectional image of microgrooves treated with abrasive belt grinding
Wettability
Wettability is to describe the phenomenon that a liquid droplet on a solid surface spreads out. It is determined by the chemical property and the microstructure of the surface and characterised by the contact angle. It implies ‘mostly non-wetting’ when the contact angle θ>90°, in other words, a hydrophobic surface is obtained. When the contact angle is further increased as θ>150°, it is the superhydrophobic surface.
The wettability of a flat solid surface by a liquid was quantified by the classical Young's equation
The micromilling and grinding operations do not change the material composition. Therefore, the hydrophobicity of the sample is dependent on the microstructures. The Cassie–Baxter equation is
The intrinsic contact angle of the aluminium alloy polished is 51°, which shows that it is hydrophilic surface. When the deionised water was dropped after 5 s, the contact angles were measured on the surface with microgroove arrays (Fig. 7). Figures 7a and b shows the contact angles on the samples’ surfaces in different directions after the micromilling processing without the grinding. From the experimental data, the wettability of the surfaces manufactured by the micromilling technology changes from the hydrophilicity into the hydrophobicity and the super hydrophobicity, and shows the obviously anisotropy. The contact angles of the surface ground by the abrasive belt were measured in the vertical and parallel directions of the microgrooves, as shown in Fig. 7c and d respectively. After the surface was ground by the abrasive belt, the contact angles are improved significantly and have large changed from the 90±0·3° and 154±0·2° to 137±0·4° and 160±0·7° respectively as shown in Table 4. In addition, the contact angle remains stable.
Images of water droplets: a parallel direction of groove structure fabricated by precision micromilling; b vertical direction of groove structure fabricated by precision micromilling; c,d treated with the grinding based on a and b respectively
Contact angles of samples/°
Owing to the low hardness of the aluminium alloy, the irregular burrs was curled up to inside and was produced along the lateral edges of the convex strip, while it was manufactured by the micromilling. Since the width of the groove is larger than that of the convex strip, the resistance force is too small for the flow of droplets along the grooves. When the microgrooves are treated with grinding process, the bending direction of the edge burrs will be changed, the width of the groove gets smaller and the contact area gets larger. The force along the grooves and surface roughness are improved, so the liquid can hardly completely get into grooves and exclude gas. Experimental results show that this method has a great inhibition on the lateral flow of water droplets along grooves and can increase the contact angle.
Taking the test results of sample 5 for example, substituting the measurement values before and after the grinding, and θe = 51° into equation (2), it can be calculated that the φs changes from 0·678 to 0·130 along the parallel direction and from 0·105 to 0·037 along the vertical direction. The above variations of the φs is produced by the changed surface roughness due to the micromilling and grinding operations. In addition, the grinding process has the effect on the direction of the burrs. As a result, the contact area between the droplet and the microstructures is increased. Meanwhile, it is more difficult for the water to get into the gaps expelling the gas, resulting in the increase in the contact angle.
Conclusion
Based on micromilling and precision ultraprecision abrasive belt grinding operations, a new method of manufacturing superhydrophobic surface is presented. The superhydrophobic surface with an array of microstructures is achieved on the surface of the hydrophilic aluminium alloy. After the micromilling process, when the geometry dimensions of the manufactured structures, the convex strip width, the groove width and the convex strip height are 35, 260 and 100 μm respectively, the contact angles of this sample are 90±0·3° and 154±0·2° in the parallel and vertical directions of grooves respectively. The wettability of the aluminium alloy surface with fabricated the microgroove arrays change from the hydrophilic to the hydrophobic. When those microgroove arrays are treated by abrasive belt grinding, the static contact angle in the parallel direction of the grooves increases to 142±0·5°, and in the vertical direction of the grooves to 160±0·6°.
Footnotes
Acknowledgements
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (grant no. 51275056), the Jilin Province Science Foundation for Youths (grant no. 201201123) and the Natural Science Foundation of Jilin Province of China (grant no. 201215139).