Study On Femtosecond Laser Micro/Nanostructuring On443Ferritic Stainless Steel Surface

With the advantages of high processing resolution, minimized heat affectedzone and high processing quality, femtosecond (fs) laser machining has becomeone of the best methods for materials micro/nanomachining. Fs laser surfacemicro/nanostructuring can induce a variety of micro/nanostructures on solids’surface. It is one of the most important application areas of fs lasermicro/nanomachining, and it is also an important method to fabricate functionalmaterials with special surface properties, such as superhydrophobic surface,anti-corrosion, anti-icing, anti-reflection surface, etc. Thus, fs laser surfacemicro/nanostructuring has important application potential in sensor, solarabsorber, turbine blade, aerofoil, radar, communication and so on.However, the morphological optimal control of fs laser surfacemicro/nanostructuring and exact formation mechanisms of fs laser-inducedsurface micro/nanostructures are not yet fully understood and lack systematicstudy. Understanding exact formation mechanisms of fs laser-induced surfacemicro/nanostructures is crucial for fs laser surface micro/nanostructuring tocontrollably fabricate functional surfaces.In this thesis, the study of fs laser micro/nanostructuring on TTS443ferriticstainless steel surface in air, water and methanol was conducted. A variety of fslaser-induced surface micro/nanostructures were formed on443stainless steelsurface via single or multiple raster scans and their formation mechanisms werediscussed, which would help lay a foundation for further study on controllablefabrication of function surfaces via fs laser surface micro/nanostructuring. Themain research content of this thesis is as following:1. The influence of laser processing parameters in air and the influence ofenvironmental media when using same laser processing parameters on themorphology of fs laser micro/nanostructuring on443stainless steel surface werestudied. And the control of the morphology of fs laser micro/nanostructuring on 443stainless steel was discussed.2. Linked nanostructure-textured mound-shaped microstructures (termedN-mounds) were induced by fs laser on443stainless steel surface in air viamultiple raster scans. The formation mechanism of the linked N-mounds isdiscussed. With increasing number of raster scans, four stages of the linkedN-mounds’ formation are identified, that is, the formation of fs laser-inducedperiodic surface structures, the generation of precursor protrusions on theFLIPSSs, the development of random N-mounds, the formation of linkedN-mounds. The growth of N-mounds and the formation of linked N-mounds arealso attributed to preferential ablation of valleys between N-mounds.3.443stainless steel was ablated by fs laser via multi-scans in air, water andmethanol using same laser processing parameters. The formation mechanisms offs laser-induced surface micro/nanostructures in water and methanol werediscussed. In water, fs laser induced periodic surface structures were formed andtheir formation can be explained by the interference between incident laser andsurface plasma polaritons. In methanol, with the increase of scanning number,ripples-textured micro protrusions formed, which was the result ofmulti-ablation evolution and caused by inhomogeneous absorbed laser intensitydistribution due to the diffraction of surface roughness. The difference in surfacemorphology obtained in different environmental media was caused by thedifference in physiochemical properties between environmental media.4. The influence of environmental media on ablation rate of443stainlesssteel under fs laser single raster scan and multiple raster scans in air, water, andmethanol was investigated. Meanwhile, the development of ablation rate withthe change of fs laser-induced surface morphology in the three environmentalmedia is comparatively studied. The results show that environmental media aswell as fs laser-induced morphology control the ablation rate with the increasingnumber of raster scans. Under single raster scanning, the ablation rate is higherin liquid than in air due to the confinement of plasma, laser-induced shockwaves,and bubble-related mechanical forces. However, under multiple raster scans, thevariation in ablation rate with the increasing number of raster scans in these three environmental media is complicated and is largely determined by thesurface morphology induced by previous fs laser ablation.