Studies On The Antibacterial Properties And Mechanism Of Microarc Oxidation Coating With Amorphous/Polycrystalline Bilayer On Ti6Al4V

Ti6Al4V titanium alloy is widely used in the field of shipbuilding and oceanography engineering due to its excellent mechanical properties and corrosion resistance,when Ti6Al4V alloy is in long-term service in water,it faces the problem of biological contamination caused by the adhesion of microorganisms,plants and animals,endowing the surface of Ti6Al4V alloy with antibacterial property is the necessary condition to suppress the formation of biological fouling.Microarc oxidation(MAO)is commonly used to modify the surface of titanium alloy,Ag,Cu and Zn are frequently added into the electrolyte to give the MAO coating with significant antibacterial effects,while Ag is expensive and not suitable for mass use,the Cu⁺,Cu²⁺and Zn²⁺dissolved from the MAO coating pollute the water ecological environment and lead to the genetic variation and deformity development of aquatic organisms,thus developing the environmentally friendly,non-toxic and harmless antibacterial surface has become a growing trend.Na₂WO₄,which has been widely used in the field of food and medicine,can be potentially applied in antibacterial case,compared with the traditional inorganic antibacterial matters(Cu,Ag and Zn),Na₂WO₄ has less adverse impacts on the biology and environment,and the literature manifest that the W containing coating displays significant antibacterial properties.On the other hand,a Ca,P and Al doped MAO coating on titanium alloy with excellent bactericidal and biological fouling resistance is also reported in literature,this MAO coating,which does not contain any fungicides,hardly has negative impacts on the ecological environment and aquatic organisms.Therefore,in this work,adding Na₂WO₄ into the electrolyte to prepare W containing antibacterial MAO coating is put forward,and exploring the fabrication of eco-friendly antibacterial MAO coating in Ca,P and Al containing electrolyte is also implemented,the composition,microstructure and antibacterial properties of two kinds of MAO coatings are studied,and their antibacterial mechanisms are deeply analyzed as well,the main results are presented as follows:(1)The MAO coatings with different W contents are prepared by doping 0,2,4,6 and 8g/L of Na₂WO₄ in Na₂SiO₃-(NaPO₃)₆-Na Al O₂basic electrolyte,the concentration of Na₂WO₄has obvious effects on the generation,composition,microstructure and antibacterial property of the MAO coating.Na₂WO₄ promotes the plasma discharge and accelerates the growth of MAO coating,W mainly exists in the forms of amorphous state in the MAO coating,the morphology and phase structure of MAO coating are hardly affected by the concentration of Na₂WO₄,the MAO coating prepared with 4 g/L Na₂WO₄ presents the optimal comprehensive performance.The representative MAO-0W and MAO-4W are used to conduct the antibacterial experiments,the results show that Na₂WO₄ plays a decisive role in the antibacterial properties of MAO coating,MAO-4W exhibits remarkable antibacterial efficiency for typical planktonic and adherent freshwater gram-negative Escherichia coli(E.coli)and gram-positive Staphylococcus aureus(S.aureus)and briny gram-negative Shewanella and gram-positive Bacillus,the microstructure of adhered bacteria and fluorescent images prove that MAO-4W can destroy the cell wall and membrane structure of adhered bacteria,resulting in the inactivation of adhesive bacteria.(2)P,Al,Al/Ca,Al/P,Ca/P and Ca/P/Al MAO coatings are prepared by regulating the compositions of EDTA-4Na-Ca(CH₃COO)₂-(NaPO₃)₆-Na Al O₂ electrolyte,the composition,microstructure and antibacterial property of MAO coatings are significantly affected by the components and concentration of electrolyte.The surface of all MAO coatings presents a porous volcanic crater-shaped morphology,while the roughness,hole size and density of the MAO coating are distinctly different,the electrolyte with higher concentration enhances the intensity of plasma discharge and facilitates the growth of MAO coating.TiO₂ is the main crystal phase in MAO coating prepared in the electrolyte without Al,while the MAO coating fabricated with Al containing electrolyte is primarily composed of Al₂TiO₅.The contents of Ca,P and Al in MAO coating are dependent on the concentration of electrolyte,the contents of Ca and P increase with the concentration of(NaPO₃)₆ and Ca(CH₃COO)₂,Ca and P mainly exist in the amorphous phase.On the contrary,the relative contents of Al and Ti decrease and mainly exist in the crystalline phase.With the increased concentration of electrolyte,the main crystalline phases in MAO coating change from Al₂TiO₅ to Al PO₄,Ca₃(PO₄)₂,TiO₂ and Al₂O₃.The antibacterial experiments of the six kinds of MAO coatings show that the Al/P,Ca/P and Ca/P/Al coatings display obvious antiseptic performance on the typical planktonic and adherent freshwater gram-negative E.coli and gram-positive S.aureus and briny gram-negative Shewanella and gram-positive Bacillus,however,P,Al and Al P/Ca coatings hardly have antibacterial effects,P is the necessary element,only if P cooperates with Al and Ca the MAO coating can exhibit the striking antimicrobial property.All the MAO coatings with different contents of Ca,P and Al show excellent antibacterial effects on planktonic and adherent E.coli,S.aureus,Shewanella and Bacillus,and altering the contents of Ca,P and Al almost has no effect on the bactericidal performance of MAO coatings.(3)By means of analyzing the microstructure,composition distribution and physicochemical property of W containing and Ca,P and Al containing MAO coatings,the antibacterial mechanism without light irradiation is disclosed to be related to the bilayer microstructure of amorphous outer layer and polycrystal inner layer in MAO coating.When the MAO coating is immersed in bacterial suspension,the amorphous outer layer absorbs the electron ejected from the bacterial membrane during the metabolic process,the absorbed electron is locally enriched in the amorphous layer,and induces the accumulation of hole in adjacent area,these holes with high oxidative potential directly kill the adherent bacteria,in the meantime,the induced holes also cause the generation of extracellular reactive oxygen species(ROS),the extracellular ROS destroy the bacterial cell walls,and further lead to the destruction of the cell membrane and cytoplasmic leakage.On the other hand,due to the potential discrepancy between the amorphous and crystal layers,the occurrence of micro-galvanic corrosion disturbs the suitable hydrogen proton(H⁺)concentration gradient required for bacterial metabolism,thus the normal activity of the bacteria is inhibited.In addition,the potential difference also exists between electron and hole enriched regions,micro-galvanic corrosion also occurs in these areas,further affects the H⁺concentration gradient.Moreover,the MAO coating can spontaneously stimulate the formation of more ROS in bacteria,and the amorphous substances dissolved from the surface layer adhere to the surface of the bacteria or enter into the bacteria when the bacterial structure is partially broken,inducing the generation of intracellular ROS,and the formed intracellular ROS directly cause the oxidation and decomposition of cytoplasmic components.Under the combined action of the above processes,the MAO coating eventually leads to the destruction of bacterial cell structure,cytoplasmic leakage and loss of bacterial respiratory activity.