Simulation And Experimental Study Of Nanosecond Pulsed Laser Micromachining Of Funtional Surface Microstructure

Nanosecond pulsed laser-based fabrication of functional surface microstructure is a non-traditional processing method with low cost and high efficiency that has emerged in recent years.However,the nanosecond pulsed laser has a significant thermal diffusion phenomenon,and the formed heat affected zone greatly reduces the processing quality,which in turn has a great influence on the functionality of the microstructures.Therefore,the mechanism and processing of nanosecond pulsed laser micromachining are studied by means of finite element simulation and experimental verification.The surface texture of stainless steel with friction reduction characteristics and the surface texture of single crystal silicon with anti-reflection properties are prepared.A high-precision finite element simulation model of nanosecond pulsed laser micromachining was established for stainless steel and single crystal silicon.The model involves the evaluation of temperature distribution and processing width,including the following aspects:(1)heat transfer model consists of heat conduction,heat convection and heat radiation;(2)thermal physical parameters of the material change with temperature;(3)thermal evaporation-induced material removal;(4)Pulsed lasers have a Gaussian space and temporal distribution.The as-established model provided the technical foundation for following study of nanosecond pulse laser processing mechanism by finite element method.Two-dimensional and three-dimensional finite element simulation studies of nanosecond pulsed laser micromachining on stainless steel were carried out for fabricating microstructures that regulate frictional properties.In the two-dimensional finite element simulation of single-pulse laser processing,the changes of ablation region and heat-affected region of the dimples at different times in one pulse period were studied.The ablation morphology,temperature field distribution and cooling rate of the dimples under different energy densities were studied.In the three-dimensional simulation of multi-pulse laser machining,the formation mechanism of micro-grooves by multi-pulse laser processing was studied,and the results of nanosecond pulse laser micromachining experiments under the same laser parameters were compared to verify the correctness of the finite element simulation results.The surface microstructure composed of aligned groove microstructures on polished SUS420J2 stainless steel sample was processed by nanosecond pulsed laser,and the linear reciprocating friction and wear test was carried out to study the friction properties of surface textures of stainless steel under lubricating oil lubrication conditions,which shows a significant reduction in friction coefficient as compared to untextured surface.Two-dimensional and three-dimensional finite element simulations of nanosecond pulsed laser micromachining of single crystal silicon were carried out for fabricating microstructures with light-trapping function.In the two-dimensional finite element simulations of single-pulse laser processing,the effect of laser energy density on ablated dimple morphology was studied.In the three-dimensional finite element simulations of multi-pulse laser processing,the effects of laser energy density and scanning speed on the ablation morphology of micro-grooves were studied,and the results of nanosecond pulse laser processing experiments under the same laser parameters were compared and verified,which demonstrate the high accuracy of the finite element simulation results.High-precision micro-grooves are fabricated on the polished single crystal silicon sample by nanosecond pulse laser processing,and the hemispherical reflectivity test was performed by using a spectrophotometer.The reflectivity of the surface microstructure of single crystal silicon at different light wavelengths shows considerable anti-reflection function,which is comparable to the reflectivity of the surface microstructures of single crystal silicon obtained by picosecond and femtosecond pulse laser processing.Therefore,this thesis studied the nanosecond pulse laser micromachining mechanism of stainless steel and single crystal silicon.Furthermore,the processing parameters were optimized to reduce the heat affected zone and improve the surface quality,so as to prepare the textures on stainless steel with friction reduction and textures on single crystal silicon with anti-reflective function.The research results of this thesis have important theoretical significance and practical value for improving the technology level of functional surface microstructure fabrication by nanosecond pulse laser processing.