Processing Technology Basics Of Silicon Carbide Fresnel Microstructure Based On Laser Machining-ultraprecision Grinding

As a typical microstructured optical element,Fresnel microstructured optical element has the advantages of small size,light weight and compact structure.It is widely used in solar energy,projection display,photography,illumination optics,laser,infrared detection and terahertz antennas.In recently,the replication production technology represented by glass molding press has been developed to realize its mass and low-cost production.The key technology is the high-efficiency and high-precision manufacturing of the hard and brittle Fresnel microstructure mold.Due to the non-continuous surface profile of Fresnel microstructure,rapid and serious wear will occur on the sharp edge of the grinding wheel when ultra-precision grinding is used,which limits the further improvement of machining accuracy and efficiency.Pulsed laser machining technology has great potential and application prospects in the field of processing hard and brittle material microstructures.Laser ablation has the advantages of high efficiency,good controllability and no tool wear when machining hard and brittle materials,but the laser machined surface roughness is high and the ablation surface deterioration layer will affect the service performance of the elements.It is necessary to develop new technology to breakthrough the technology bottleneck of machining hard and brittle Fresnel microstructured mold.In this thesis,the two technologies of laser machining and ultraprecision grinding are combined and the laser machining-grinding process chain is proposed,in which the two processes are relayed to complete the rough machining and fine machining respectively.Focusing on the high-efficiency and high-precision machining goal of silicon carbide(Si C)Fresnel microstructure,which is a typical hard,brittle and complex element,this thesis conducts theoretical and experimental research about the laser machining mechanism of Si C,the laser machining process of Fresnel microstructure and the surface waviness of ground Fresnel microstructure.This thesis mainly includes the following aspects:(1)Through the laser ablation experiment of circle grooves,the damage threshold of Si C ceramics ablated by infrared sub-nanosecond pulse laser was analyzed,and the influence of laser defocusing distance on the circle groove microstructures was discussed.The material removal depth model for laser spiral milling of planar Si C workpieces was established and experimentally verified.Through single factor experiments,the influence of laser processing parameters on workpiece center residue,circumferential waviness profile error and material removal efficiency was analyzed and the laser processing parameters were optimized.The surface material composition of Si C after laser machining was analyzed based on Raman spectroscopy.The grindability of laser machined Si C surface was studied through single diamond scratching experiment.(2)For laser machining of Fresnel microstructure,a laser full-aperture scanning method with continuously varying feedrate was proposed.The trajectory of laser spot for machining Fresnel microstructure is planned based on the material removal depth model of laser ablation of Si C.Through single factor experiments,the influence of laser processing parameters on the production accuracy and efficiency of Si C Fresnel microstructure was analyzed and the process parameters were optimized.The surface material composition and sub-surface damage of laser machined Si C Fresnel microstructure were analyzed.(3)Based on grinding kinematics theory and microscopic material removal analysis,the generation mechanism and distribution trend of the surface waviness error in parallel grinding process were uncovered.A simulation prediction model of surface waviness topography and waviness feature values was established,and the influence of wave-shift value on waviness feature values was analyzed.According to the microscopic grinding wheel-workpiece contact model and the analysis of the grinding material removal behavior by the secondary grinding zone of the grinding wheel,the suppression mechanism of using non-integer rotation speed ratio upon the surface waviness error was revealed.Then the optimal matching strategy of process parameters was proposed to suppress the ground surface waviness error.(4)An experimental system of laser-grinding process chain combining subnanosecond pulse laser machining and ultra-precision grinding was developed.Based on the tantalum block truing method and acoustic emission monitoring technology,the experimental research of truing fine abrasive resin-bonded diamond grinding wheel was conducted.From the aspects of production accuracy,producton efficiency,surface integrity and sub-surface damage of Si C Fresnel microstructures,the two processing technologies of laser-grinding process chain and only ultra-precision grinding were compared and analyzed.