Fabrication Of Durable Superhydrophobic Surface With Long-term Service Performances On Alluminum Alloy

Metal and its alloys in some special service environment will face a series of serious problems such as corrosion,biological contamination,icing,etc.,which greatly limit the wide application of this kind of materials.In recent years,superhydrophobic surface has been widely used in the field of metal protection due to its unique surface properties.For example,metal-based superhydrophobic surface has shown potential application prospects in the fields of self-cleaning,fluid drag reduction,anti-ice and anti-fog,anti-biological fouling,anti-corrosion and so on.Superhydrophobic surface has received extensive attention from related researchers.In this research,method of anodic oxidation was used to fabricate the nanoporous structures on the surface of aluminum alloy,and the method of the vacuum-pressure impregnation was used to modify the nanoporous surface of the anodized aluminum alloy with the low-surface-energy PFDS/Ti O₂hydrophobic modifier.Thus,we successfully designed and constructed the superhydrophobic surface on aluminum alloy with excellent mechanical robustness.The mechanical stability,freezing-delay,anti-bacterial adhesion,anti-corrosion properties of the superhydrophobic surface have been researched and analyzed in this work.Besides,these properties of the superhydrophobic surface after abrasion have been tested to study the tolerability of the surface under abrasive conditions.The wetting mechanism of the aluminum-based bionic superhydrophobic surface and the action mechanism of other properties are also revealed in this work.The detailed research contents are as follows:（1）According to the Cassie-Baxter hydrophobic model,the pore size and porosity of the nano-porous structure in the anodic oxidation film are calculated and predicted,and the minimum porosity required for the aluminum alloy superhydrophobic surface is 37%in this model.In the process of anodic oxidation,when oxalic acid with a concentration of 5 wt%was used as electrolyte,the anodic oxidation of aluminum alloy surface was obtained at a constant pressure of 60 V for 60 min.The achieved film thickness is up to～50μm,the average pore diameter is up to～100nm,the average pore wall thickness is up to～16 nm,and the porosity is up to～45%.1wt%PFDS in the PFDS/Ti O₂hydrophobic modifier is the optimum addition amount,when the static contact angle of the hydrophobic surface is～160°,reaching the superhydrophobic state.In the filling process,the superhydrophobic surface is obtained by the secondary vacuum-pressure impregnation process.In this way,the filled surface has the uniform structure,and the filling of superhydrophobic modifier is sufficient.（2）The mechanical durability of the superhydrophobic surface is tested and evaluated in this work.The Vickers hardness of the superhydrophobic surface can reach more than 400 HV,whicis nearly 5 times higher than that of the aluminum alloy matrix.The hardness of the up-layer of the film is also more than 200 HV,which is enough to meet the requirements of the industrial application of aluminum alloy.The scratch test results shows that the film can resist the friction when the load is 40 N,which indicats that the film has a strong binding force with the aluminum alloy matrix.The wear resistance of superhydrophobic surface is evaluated by measuring the static contact angle of the film surface with different wear thickness.The experimental results shows that the static water contact angle is still above 150°and the superhydrophobic state is still maintained when the wear thickness is within～10μm.When the thickness loss is～15μm,the static contact angle is still greater than 120°and it remains hydrophobic.When the wear thickness reaches～20μm,the contact angle of the film drops to 67°suddenly,and the film completely changes to the hydrophilic state.Futher,based on the modified Cassie-Baxter wetting model and Gibbs interface energy model,a modified wetting model is established to analyze the nonlinear variation of static contact angle on the superhydrophobic surface.Based on the model,the hydrophobic mechanism and action principle of the superhydrophobic surface before and after abrasion are explained in this work.（3）The long-term service performance of aluminum alloy superhydrophobic surface was evaluated.By observing the typical Escherichia coli and Staphylococcus aureus in different growth status on the surface of the different samples,the anti-bacterial adhensive performance of the superhydrophobic surface was tested.Results show that surface without the hydrophobic modification had no anti-bacterial adhensive effects,and the surface after modification almost had no adhesion of bacterial colonies.It is indicated that the method of building a superhydrophobic film on the surface of aluminum alloy can significantly improve its anti-bacterial adhesion.The experimental results of the surface after abrasion showed that the film can still maintain anti-bacterial adhesive performance under wear conditions,indicating the long-term effectiveness of preventing bacteria from adhering to the superhydrophobic surface.By observing the freezing process of the water droplets under low temperature and large humidity respectively on the surface,the freezing-delay performance of the samples were evaluated.Results showed that the droplet on the surface without superhydrophobic modification started freezing at the 90^ths,and it started freezing at 241^ths on the superhydrophobic surface that delayed 151s than that of unmodified surface.It indicated that the superhydrophobic surface had a remarkable of freezing-delay perfaomance.The freezing-delay performances of the superhydrophobic surface before and after abrasion were tested to evaluate the long-term property.Results showed that the surface still had the freezing-delay performance after the thickness loss of 10μm,which proved the long-term effect of the superhydrophobic surface.By tesing the potentiodynamic polarization curves and electrochemical impedance spectroscopy of the samples with or without superhydrophobic modification,the corrosion-resistance properties of the samples were evaluated.It can be found that the superhydrophobic surface had high potential and small corrosion current density that could verify the good corrosion-resistance property.Further,the corrosion resistance mechanism of the superhydrophobic surface was explained by"air cushion"theory.To evalue the long-term property of the superhydrophobic surface,the potentiodynamic polarization curves of the worn superhydrophobic surfaces were tested.Compared with the unmodified surface,the worn superhydrophobic surface had higher self-corrosion potential and lower corrosion current density,which indicates the long-term corrosion-resistant performance of the superhydrophobic surface.