Study On Fabrication And Mechanisms Of Superhydrophobic Surface Of Marine Steel Using Femtosecond Laser Ablation

With the further increase of the application of superhydrophobic surfaces,the rapid preparation of stable superhydrophobic surfaces has gradually become one of the research hotspots.The study shown that the key factors that determine the hydrophobicity of the material surface are the microstructure and surface energy of the material surface.Femtosecond laser processing technology as a green and clean processing means has been widely used in material surface processing.Laser direct writing as the most direct processing means,can be processed a variety of complex direct writing structures.However,the relationship between the direct writing structure of femtosecond laser fabrication and processing parameters is still unclear.In addition,the relationship between heat accumulation and surface morphology has not been considered in the existing studies on the mechanism of self-organizing structure.The metal surface after femtosecond laser ablation is usually hydrophilic/superhydrophilic.The surface can change from hydrophilic/superhydrophilic to hydrophobic/superhydrophobic after natural transformation in the air,but the transformation time is longer.Low temperature annealing can be used to achieve rapid wettability conversion,but the mechanism is still unclear.In this thesis,femtosecond laser ablation is used to prepare superhydrophobic surface of steel and explore the mechanism of rapid wettability transformation through low temperature annealing.The LSFL,ellipse,strip,cone,hemispherical and square structures were fabricated by femtosecond laser direct writing technology.The relationships among laser processing parameters,surface morphology and wettability were investigated.In the prepared direct writing structures,the surface contact angle of the square structure was the largest,CA = 155°,the rolling angle was about 15°,and the wettability was between the superhydrophobic "lotus leaf effect" and the superhydrophobic "rose petal effect".The contact angle of the LSFL surface was the smallest,CA= 93.6°,but it was still higher than that of the matrix(75.9°).The self-organized structures of LIPSS,lamellar and strip were prepared by femtosecond laser ablation,and the relationship between laser repetition frequencies and surface morphology was investigated.A heat accumulation model was established to simulate the heat accumulation temperature of the sample surface at different repetition frequencies,as to explain the formation mechanism of the melt on the surface of the self-organized structure.In addition,the maximum surface contact angle of the prepared self-organized structure was 154° and the rolling angle was approximately 20°.The surface wettability of the laser ablated sample was rapidly transformed from hydrophilic to superhydrophobic by annealing at 100 ℃ for 6 h.The changes of chemical composition on the surface of substrate,laser treated and annealed samples were detected by XPS.The results show that the rapid change of wettability from hydrophilic to superhydrophobic is mainly due to the removal of polar substances(groups)and the adsorption of non-polar substances(groups)on the sample surface.