Research On Electrochemical Machining Of Surfaces With Extreme Wettability On Mold Steel GCr15 Substrates

Surfaces with extreme wettability,as a special phenomenon of bionics,have been widely concerned due to a wide range of applicaitons in self-cleaning,anti-icing,drag reduction,corrosion resistance,water-oil separation and liquid directional transportation.Steel,as a kind of alloy,has become the most widely used metallic materials due to low price and reliable mechanical properties.Bearing steel is widely used in various manufacturing industries because of its high hardness,good wear resistance,good thermal processing performance and good contact fatigue.Therefore,the research on the processing of fabrication of superhydrophobic coatings on mold steel has considerable industrial significance with the fast development of steel and mold industry.To date,various methods can be used for fabricating extreme wettability steel surfaces.However,there are problems such as limited fabricating methods,environmental pollution,high cost,complex process and others.Based on problems,we used the characteristic of passivation of steel and proposed an electrochemical methods using neutral electrolyte to fabricate surfaces with extreme wettability on GCr15 mold steel substrate.We designed special fixture to get turbulent electrolyte flow for smooth electrochemical processing and took GCr15 and brass as anode and cathode,respectively.Economical and environmental friendly neutral sodium nitrate?NaNO₃?was used for electrochemical processing to form passive films with micro rough structures on GCr15 substrates that required.Ethanol solution of fluoroalkylsilane?FAS?was used to low surface energy modification of GCr15 surfaces.After that,superhydrophobic surface can obtained.Scanning electron microscopy?SEM?was used to characterize microstructure.Passive films on substrates,which has micro/nano binary rough structures composed of rugged plateau structures and rupture-like shapes,would become superhydrophobic after FAS modification.X-ray photoelectron spectroscopy?XPS?,X-ray diffraction?XRD?,energy-dispersive spectroscopy?EDS?and Fourier transform infrared spectrophotometry?FTIR?were used to characterize chemical composition and crystal structure of original surfaces or surfaces with passive films after electrochemical processing.It indicates that crystal structures are unchanged after electrochemical processing.Moreover,passive film is generally considered to be composed of two or more layers of compound films.The surface layer of it is composed of hydroxide and oxyhydroxide in the form of Fe[II]and Cr[II],while the inner layer is mainly compact Cr₂O₃.The surfaces showing superhydrophobicity have FAS on them,while the untreated surface does not.Under optimum parameters(i=5 A·cm^-2 and t=90 s),the samples still showed high superhydrophobicity with a water contact angle of 161.8°after exposure to air for one year under normal ambient conditions.This indicates that the superhydrophobic surfaces fabricated via the ECM methods have good durability to the environment.In addition,we studied parameters of electrochemical processing?current density and electrochemical processing time?on the micro morphologies and wettability of substrates.The results show that passive films that were composed of micro/nano binary rough structures are important factors affecting the surface wettability.The current density has great influence on the micro morphologies.Under low current density,micro morphologies show almost micro-scale structures while rupture-like nano-scale structures appear with current density increasing,micro/nano binary rough structures are formed at last.Moreover,the current density has great influence on the wettability.The superhydrophobicity could be obtained while i?3 A·cm^-2.Additionally,processing time has little effect on surface wettability under i=5 A·cm^-2 proper current density,and superhydrophobic surfaces with more than 150°water contact angles can be prepared within just 2 s.Moreover,practical applications were also considered.The superhydrophobic GCr15surfaces has better durability with good water contact angles after more than one year storing under normal situations and stronger anti-icing characteristics than normal.The different size and kind of steel were researched via this electrochemical machining.Meanwhile,it is valid for application for water-oil separation.Porous thin GCr15 slice with point array pore structure can be fabricated via laser machining.After electrochemical machining,the samples show superhydrophilicity?superoleophobicity under water?and can be applied for oil-water separation,the efficiency of which can reach 97%and more.