Manipulation Of Tribological Properties Of Metals By Ultrashort Pulsed Laser Micro-/Nanotexturing

Friction and wear are very critical for the operation efficiency and service life of mechanical systems.Therefore,the manipulation of tribological properties of friction pairs,according to the actual condition,becomes an urgent demand.Surface texturing as a means for controlling tribological properties of mechanical components is well known for many years.Various technologies have been developed for surface texturing.Among them,laser surface texturing offers great advantages including fast processing time,clean to the environment,excellent control of the shape and size of the patterns and so on.However,the heat effect of the traditional long pulsed laser has a negative influence on surface properties of materials in the process of laser processing.The ultrashort pulsed laser technology can solve this problem,which is owing to its ultrashort pulse width and ultrahigh peak power that can minimize the melt ejection and heat-affected effects.Therefore,the ultrashort pulsed laser technology is one of the most promising ways to achieve micromachining in the field of tribological applications.In this study,we systematically investigated the formation mechanism of ultrashort pulsed laser-induced micro-/nanotextures on the stainless steel surface and the effect of laser processing parameters on the geometrical morphologies,chemical compositions,microstructures,mechanical properties and tribological properties of ultrashort pulsed laser textured surfaces,which is beneficial to the optimization design of the surface texture.The main achievements are summarized as below:Firstly,the tribological properties of micro-textured surfaces induced by femtosecond laser have been investigated.Regular arranged micro-grooved textures with different spacing were produced on the stainless steel surfaces by femtosecond laser surface texturing.The formation mechanism of femtosecond laser-induced micro-textures was also analyzed.The tribological properties of smooth surfaces and textured surfaces were investigated under both dry and starved lubrication condition in detail.Results show that the average friction coefficients and wear rates of textured surfaces first decrease and then increase with the increase of micro-grooves spacing under dry condition.When the spacing of micro-grooves is smaller than a certain value,the average fiction coefficients of textured surfaces are larger than those of the smooth surfaces.Then,the spacing is beyond this value,and the average friction coefficients of textured surfaces are less than those of the smooth surfaces.The average friction coefficients and wear rates of textured surfaces first decrease and then increase with the increase of micro-grooves spacing under starved lubrication condition as well.There exists an optimal micro-groove spacing at which the surface texture exhibits the best effects of the reduction of friction and wear.In addition,the sliding friction anisotropy of micro-grooved textures has also been found under both dry and starved lubrication condition.In summary,femtosecond laser surface texturing had a marked potential for modulating tribological properties if the micro-grooves were distributed in an appropriate manner.Secondly,the tribological properties of micro-/nanotextured surface induced by picosecond laser have been studied.The micro-grooved textures were formed on the stainless steel surface,accompanied with the appearance of nanostructures with grain sized of 80-400 nm,oxides thin film（Fe₂O₃ and Cr₂O₃）and martensite during picosecond laser surface texturing and quenching process.The tribological tests indicate that the average friction coefficients and wear rates of textured surfaces initially increase then decrease with the increase of spacing of micro-grooves.When the spacing of micro-grooves is smaller than a certain value,the average fiction coefficients and wear rates of textured surfaces are less than those of the smooth surfaces.While when the spacing is beyond this value,the average friction coefficients and wear rates of textured surfaces are larger than those of the smooth surfaces.The well-controlled friction and wear properties are attributed to the combined effects of induced micro-/nanostructures,martensitic transformation and oxides thin film in the local quenched zone of stainless steel samples.Thirdly,the tribological properties of femtosecond laser-induced periodic surface nanostructures have been investigated.Subwavelength periodic nanostructures were prepared on the stainless steel surface by femtosecond laser-induced self-formation technology.The effects of femtosecond laser processing parameters on the geometrical morphologies of surface nanostructures were systematically studied.Moreover,the formation mechanisms of femtosecond laser-induced periodic surface nanostructures were also analyzed.Results indicate that femtosecond laser-induced periodic surface nanostructures can reduce the friction and wear of stainless steel under both dry and starved lubrication condition.Meanwhile,the femtosecond laser-induced periodic surface nanostructures show the friction anisotropy under both dry and starved lubrication condition to some extent.Finally,the tribological properties of femtosecond laser-induced periodic surface nanostructures with atomic layer deposition coating have been studied.Uniform laser-induced periodic surface nanostructures were produced on the surfaces of stainless steel samples by femtosecond laser.These nanotextured surfaces were then deposited with highly conformal TiO₂coating by means of atomic layer deposition technology.The friction tests show that the average friction coefficients and the wear degree of the treated samples have been greatly reduced compared with untreated stainless steel samples.It is suggested that the synergistic effect of femtosecond laser-induced periodic surface nanostructures and atomic layer deposition TiO₂ coating had a more marked potential for reducing friction and wear.