| It is well known that biomimetic superhydrophobic surface can improve the ability of anti-icing,corrosion resistance,self-cleaning and anti-reflection of material,which is a promising material surface protection technology.Aluminum and its alloy due to its performance advantage in aerospace,mould processing,fixture,ships and machinery manufacturing industries are involved,is the most widely used in industry of non-ferrous metal structural materials.Obtaining superhydrophobicity on the surface of aluminum alloy can not only solve the problems under harsh working conditions,but also broaden the application range of aluminum alloy materials.It is worth mentioning that the key to obtain superhydrophobicity lies in the preparation of micro-nano rough structure.Therefore,in this paper,7075 aluminum alloy was used as the test base,and based on optimized laser processing parameters,laser etching and chemical etching were combined to achieve efficient,flexible and controllable preparation of surface microtexture and large-area preparation of nanostructures.The effects of solid-liquid contact area fraction and chemical etching parameters on the surface morphology and hydrophobicity,the formation mechanism of micro/nano structure and the wettability transition mechanism were studied,and the durability of superhydrophobic surface was tested.Specific research results are as follows:From the perspective of both hydrophobicity and stability of the surface,the optimization of laser processing parameters was completed based on line etching test.Firstly,the single factor control variable method was used to study the influence of scanning times,scanning speed,pulse frequency and average power on the microgroove structure,and the primary selection of processing parameters was completed.Then the response surface method was used to design experiments to study the interaction of four processing parameters on the microgroove structure.By means of Optimization function in Design-Expert software,the regression model of groove depth was obtained with depth as the optimization index,and the optimal parameter combination for preparing superhydrophobic surface micro-nano structure was obtained as follows: scanning times is 2,scanning speed is 460 mm/s,pulse frequency is 835 k Hz,and average power is 21 W.Based on the optimized process parameters,the(super)hydrophobic 7075 aluminum alloy surface was prepared by laser etching method to construct the surface rough structure and stearic acid ethanol solution modification to reduce the surface energy.The effect of surface topography on surface wettability was studied by controlling the scanning spacing to control the surface topography.The relationship between the scanning spacing and surface topography,roughness and wettability was explored,and the transformation mechanism of surface wettability was analyzed.Results show that when the scanning interval is 20-80 μm,the surface is covered with "ridge"-like rough structures,and the "ridge" and "valley" structures are deposited with nano-papillae and nano-sheet,respectively,and the surface is superhydrophobic.The contact angle of surfaces with scan spacing of 20 and 80 μm reaches a maximum of154°,and the rolling angle is less than 10°.When the scanning spacing is 100-160 μm,the surface shows a grid structure consistent with the scanning trajectory of the laser beam,and the vertical crossing part is still "ridge" shaped,and the surface is hydrophobic.The infrared spectrum shows that the self-assembled monolayer film formed by esterification of stearic acid with surface hydroxyl group is the fundamental cause of surface wetness transition.Based on the design criteria of Cassie state superhydrophobic surface roughness,three kinds of microstructural surfaces,triangular prism,quadrilateral prism and cuboid,are built on the surface of aluminum alloy by laser etching method,and then dense nanostructures are prepared on the microstructural surface by etching with Na OH solution.The effects of chemical etching parameters on the morphology and wettability of different micro-nano structures surface are systematically studied.The hydrophobicity and composition changes of the surface at different stages of the preparation process were analyzed using the chemically etched surface as the control.The results show that the surface of the triangular prismatic microstructure has the best hydrophobic effect after Na OH etching,and the contact angle is up to 165°.The optimal chemical etching parameters are as follows: the etching time is 120 s,the etching temperature is 40 ℃,and the etching concentration is 0.6 mol/L,which confirms that reducing the solid-liquid contact area is an effective way to prepare the Cassie state superhydrophobic surface.The etching time,temperature and concentration of Na OH solution mainly affect the size of nanosheet and the formation of cluster.When the chemical etching parameter is too large,it is easy to cause corrosion damage to the original micro-nano structure.The composition and proportion of elements on the laser chemical etching and chemical etching superhydrophobic surfaces are basically the same,which excludes the possibility that the surface elements lead to the large difference in hydrophobicity between the two surfaces,and confirms the importance of micro/nano structure in the preparation of superhydrophobic surfaces.Based on the above research results,the mechanical stability,chemical stability,thermal stability and self-cleaning performance of the superhydrophobic surfaces prepared under the two processes were detected and analyzed.Results show that the mechanical stability of the laser processed superhydrophobic surface is slightly stronger than that of the laser-chemical composite processed superhydrophobic surface,because the anti-damage ability of the nanostructure is weak.The maintenance time of superhydrophobicity of two surfaces in different strength acid-base solutions is not the same.The maintenance time is only 20 or 30 min in strong acid-base solution,and 18 days in neutral solution.The temperature thresholds of the laser-processed surface and the laser-chemical composite surface are 150 ℃ and 250 ℃,respectively,and the surface instantly transforms into a superhydrophilic state when it exceeds the threshold. |
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