Mechanisms Of Laser Cleaning And Growth Inhibition Of Marine Microbial Film On The Surface Of Aluminum Alloys

In the marine equipment,material corrosion damage related to the adhesion of marine micro-organisms accounts for 70-80%of the total marine materials.Therefore,the problem of marine microbial fouling is one of the most urgent problems to be solved in the process of marine national defense and marine resource development.With the development of the lightweight structure of Chinese marine equipment and the coastalization of aerospace facilities,aluminum alloy materials have become more and more widely used in high-humidity and high-salt complex marine environments,such as high-speed and high-carrying hulls,marine helicopters landing platform support,large launch vehicle shell structure and shipborne weapon base,etc.This article focused on the corrosion of the marine microbial film on the surface of aluminum alloy after serving in the Bohai sea water environment,and carried out laser cleaning treatment of the marine microbial film on the aluminum alloy surface,that is,laser cleaning is used to remove marine microbial film and inhibit the attachment of marine micro-organisms.The ablation,peeling,cleaning,and defouling of the marine microbial film is realized.In addition,the specific micro-and nanostructures formed on the surface of the laser-cleaned aluminum alloy substrate provide an additional anti-adhesion/antibacterial effect,which provides the ability of anti-fouling for the sustainable cycle service of aluminum alloy in the marine environment.The research results provide a research foundation for the application of laser cleaning technology to the de fouling and antifouling of marine microbial film on aluminum alloy surface.In order to solve the fouling and corrosion problem of the marine microbial film on the aluminum alloy surface,the main composition of the marine microbial film growing on the aluminum alloy surface is firstly determined,which is a distributive continuous large area of flat extracellular polymeric substances(EPS layer)and random distributive oval barnacle base plates.Secondly,the laser cleaning characteristics of the marine microbial film are studied.The fiber laser cleaning method using nanosecond high-frequency pulses has successfully achieved the effect of cleaning and removal of the marine microbial film on the aluminum alloy surface.The optimized process parameters that are suitable for removing of the last marine film layer are pulsed width of 30 ns,pulsed frequency of 20 k Hz,and scanning speed of 4 m/s.The Al content on the substrate surface after laser cleaning is?90 wt.%.The nanosecond pulsed fiber laser with a wavelength of 1064nm is not selective for the cleaning and removal of molecular bonds that consists of proteins,polysaccharides,and oils.The residual particles on the substrate surface are inorganic salts(Na salt,Ca salt,and Cl salt)and various oxides.Finally,the relationship between the film removal thickness and the laser energy density that was used to remove of the film was established,and the laser cleaning threshold of marine microbial film was 1.21 J/cm²,and the damage threshold of aluminum alloy substrate was 5.5 J/cm².The process range of laser cleaning of marine microbial film is 1.5-5.5 J/cm²,the roughness of its laser-cleaned surface is 0.46-0.53?m,the thickness of the substrate damage is controlled less than 3?m,and the laser cleaning efficiency is less than 70×10^-6 m³/h.The substrate surface after laser cleaning is hydrophobicated,and it has superhydrophobic or strong hydrophobic ability.Under high magnification SEM,nano-particles with a size of?60-100 nm on the substrate surface were observed,which provides the antifouling feasibility for the aluminum alloy surface in service.In order to reveal the cleaning mechanism of the marine microbial film from the aluminum alloy surface,firstly,a high-speed camera was used to observe the morphology of the laser cleaning induced plasma plume,the dynamic behavior of ablation,peeling,and splashing of the EPS layer and the barnacle base plate.During the laser cleaning process of marine micro-organisms,organic vapor(lipid,phospholipid,protein,inorganic salt vapor,etc.),plasma and splashes are generated above the laser-cleaned surface,which together form the plasma plume.When the EPS layer is cleaned by laser,the transition steps of th e ablation contour edge are obvious,and the spatter is finely broken.When the laser energy density was small(1.38 J/cm²),the whole barnacle base plate was lifted and peeled off,then rotating and flying out at high speed under the action of shock wave/thermoelastic stress;when the laser energy density is large(4.14 J/cm²),the barnacle base plate is cut neatly,and the splash is finely broken.Therefore,the laser cleaning mechanisms of the EPS layer and the barnacle base plate are ablation vaporization and shock wave/thermoelastic vibration,respectively.Secondly,a fiber optic spectrometer was used to study the composition of the plasma.The migration and transformation law of the particles of the marine microbial film was described from the micro level,that is the fracture of the single chain of biological macromolecules(glycine,glucose sugar,uracil,glyceryl phosphate)caused by multiphoton absorption produces a large number of C,H,O,N,and other atoms,and then these atoms are ionized by photons to continue to produce ions,and the excitation and recombination between electrons and ions.Finally,the Comsol software was used to numerically simulate the direct vaporization process of the laser cleaning of the EPS layer.The calculation accuracy of temperature and topography profile indirectly proved that the cleaning mechanism of the EPS layer was laser ablation and vaporization.The shock wave pressure under different parameters was calculated.Compared the calculated shock wave pressure with the bending/tensile strength of the barnacle base plate,the result is consistent with the dynamic behavior of the base plate,which proved that the cleaning mechanism of the barnacle base plate was stripping and shattering under the action of shock wave.Based on the micro-and nano-scale physical structure obtained on the substrate surface after laser cleaning,the surfaces of the material with different functions were prepared by chemical treatment methods,including superhydrophilic,superhydrophobic,and ultra-slippery surfaces,they can achieve the antifouling purpose of the aluminum alloy after laser cleaning.Through the 14-day dynamic environmental bacterial liquid wettability test and the anti-SRB bacterial adhesion test,the antibacterial ability and growth inhibition mechanism of the different functional surfaces were studied.The ultra-smooth surface has the best wetting stability,and the hydrophobic angle remains almost unchanged;the super-hydrophilic surface becomes a general hydrophilic state,and the final hydrophilic angle is>?40°;the super-hydrophobic surface becomes a general hydrophobic state,gas-liquid area ratio has dropped significantly.The mos t excellent antibacterial surface is the ultra-smooth surface.The adhesion number of marine bacteria is 1.89×10³/mm²,which is 3 orders of magnitude lower than the adhesion density of bacteria on the original aluminum alloy surface.The antibacterial mechanism of the superhydrophilic surface is that the interaction of the electrostatic force between the aluminum nanostructure and the bacterial cell wall and the cell gravity both cause the cell wall to undergo elastic deformation.The peptidoglycan structure is destroyed,causing the cell wall to be pieced and perforated.This green bactericidal mechanism will not cause any changes in bacterial tolerance or genetic variation.The anti-adhesion mechanism of superhydrophobic surface is:the air layer stored between the nanostructures supports the bacterial cells and has the effect of repelling the ad-hesion of the cells to material surface.The anti-adhesion and antifouling mechanism of the super-smooth surface is:the liquid-like oil film flowing on the surface of the nanostructure cannot provide a firm attachment site for the bacterial cells.The cells undergo the process of contact,movement,sliding out,and finally detach from the material surface.