Preparation And Wear Resistance Of Hydrophobic Surface By Nitrogen Assisted Laser Texturing

Due to its high strength and light weight,Ti-6Al-4V alloy is widely used in the aviation field.However,surface poor hydrophobicity and low wear resistance limit the service life of the material.How to imporve the wear resistance and hydrophobicity of Ti-6Al-4V alloy surface has always been one of the hot issues.Based on the study of biological surface properties,microstructures are closely related to surface wetting properties,which are widely used to adjust surface wettability of metal materials.In addition,comparing with traditional processing methods,laser processing technology has become an effective means for preparing microstructures on the surface of materials because of its simple and rapid processing,easy large-area preparation and industrial realization and high reproducibility.In order to improve the surface hydrophobicity of aviation grade Ti-6Al-4V alloy,a variety of micro-and nano-scale texture were prepared on the surface of Ti-6Al-4V alloy by laser surface texturing and nitrogen assisted laser texturing.Finally,the tests on hydrophobic properties and wear properties were carried out to demonstrate that nitrogen assisted laser texturing is an effective method for preparing well wear resistant and hydrophobic surfaces.The finite element simulation software Ansys Fluent was used to simulate the moving process of droplets on the micro-textured surface and the contact angle was calculated.The simulation results showed that as the droplets were deposited statically,the contact angles increased with the increase of areal density of the microtexture,the trend of which was in good agreement with the theoretical value based on the Cassie-Baxter model.When the droplets impacted on the textured surface,the impact velocity of droplets has a greater impact on the contact angles of the droplets on the textured surface.As a result,it could be inferred that the relatively small micro-pits diameter?40?m?and the relatively large micro-pits areal density?66%?are more conducive to avoid the wettability transition?from Cassie state to Wenzel state?and maintain the high hydrophobicity better for textured surface.Laser texturing experiments were divided into two parts:laser surface texturing and nitrogen assisted laser texturing.Firstly,the effect of the laser process parameters including the amount of defocus?spot diameter?and the number of single-point pulse?cumulative laser fluence?on the diameter and depth of the micro-pits were discussed and then it was realized to control the micro-pit size precisely.Further,laser-induced periodic surface structure?LIPSS?was successfully prepared on the surface of Ti-6Al-4V alloy by adjusting the laser fluence and scanning speed with height of500⁶00 nm and periodicity of 1000¹100 nm.And then,the typical micro/nano hierarchical texture were obtained by combining the LIPSS and micro-pit array.Finally,when the nitrogen assisted laser texturing was introduced to prepare texture under the same laser process parameters,agglomerated spheroidal structure and a dense layer,which composed of TiO₂ and TiN compound were found and distributed on the surfaces of micro-pit array and LIPSS uniformly.The investigation of surface hydrophobic properties were discussed in terms of the effect of texture on surface hydrophobicity and the effect of surface chemical composition on surface wettability transition.The contact angle of untreated surface was about 71.6°and the contact angle of all textured surfaces after natural aging?storage for 1 month?varied from 91.2°to 144.58°.Firstly,the contact angles of textured surface showed a significant growth trend with the increase of surface texture surface density,which was in good agreement with the trend of Cassie theoretical values and simulation values.The contact angles of nitrogen assisted laser texturing surface were slightly larger than the laser textured surfaces.On the one hand,the increase of the areal density improved the surface roughness that amplified the surface wettability.On the other hand,TiN replaced the part of TiO₂ during nitrogen assisted laser texturing process,which could reduce surface free energy and improve hydrophobicity.At the same time,the hierarchical texture could increase the contact angle furtherly,which was mainly because that the hierarchical texture could minimize solid-liquid contact area,then increased the gas-liquid contact and mad the surface maintain high hydrophobic properties ultimately.The increase of surface oxygen and the production of TiO₂ were the main reasons for the initial surfaces with hydrophilicity.The increase of adsorption of carbon and organic compounds including surface polar carbon compounds?C-O and C=O?and non-polar carbon compounds?C-C/C-H?in surfaces were the main reason for the natural aging surfaces with well hydrophobicity,which was based on the results of EDS and XPS analysis.However,it was proved that low temperature annealing treatment could accelerate the transition from hydrophilicity to hydrophobicity,which was because the removal of some hydroxyl groups and the chemical adsorption of some non-polar organic chains happened more rapidly than which happened during natural aging process.Under the current experimental conditions,the microhardness of the nitrogen assisted laser texturing surfaces were significantly enhanced compared to the laser textured surface.The width and depth of wear track in nitrogen assisted laser texturing surfaces were reduced significantly,which was because of the formation of high-hardness dense layer.Under the condition of load friction,the coefficient of friction and the amounts of wear debris decreased significantly and the integrity of the hierarchical texture was better.Therefore,it was proved that the technology of nitrogen assisted laser texturing was an effective method for preparing wear-resistant hydrophobic surfaces.