Research On Surface/Subsurface Damage Of Silicon Carbide In Vibration-assisted Polishing

The silicon carbide(SiC)can been seen an ideal space mirror material due to its characteristics of high temperature resistance,wear resistance,stable chemical properties,and a higher specific stiffness.However,damage is inevitable,such as high wear rates of polishing tools,severe surface scratches and subsurface damage caused by brittle removal and high residual stress on the workpiece surface caused by stress concentration in the conventional polishing process,.Based on the difficulty of processing optical components in conventional processing,this paper uses vibration-assisted polishing(VAP)to improve the surface integrity of the workpiece after processing,and it is also of great significance to study the surface / subsurface damage mechanism of optical components.In this paper,the SiC ceramics is the main research goal.First,according to the characteristics of the flexible hinge structures,a suitable structure is selected to design a two-dimensional vibration-assisted polishing device(2D-VAPD).Then,based on the theoretical basis of the removal mechanism and kinematic characteristics of VAP,a subsurface damage depth(SSD)prediction model is established.In addition,VAP simulation is performed using 3D finite element simulation(FEM)technology.Combining model calculations and 3D FEM results,a series of VAP experiments of silicon carbide ceramic were designed to reveal the influence of process parameters on surface roughness and SSD.The main research contents of the thesis include:(1)VAP method is used to improve the surface integrity of optical elements by understanding the advantages of the vibration-assisted machining(VAM)method and processing characteristics of ceramic materials in optical elements.First,a 2D-VAPD was designed.The flexure hinges mechanism of the device are analyzed and calculated to obtain performance parameters such as the motion stroke and natural frequency of the device.Then,using the motion stroke and natural frequency as the objective function,the improved bacteria foraging algorithm is used to optimize the parameters of the device more quickly and accurately to ensure that the designed device has a better natural frequency and travel.Then,conduct experimental tests on the device,which are operated to verify that the actual performance of the device is consistent with theoretical calculations and FEM.(2)According to the kinematic characteristics of the vibration-assisted polishing device,combined with the removal mechanism of brittle materials and the fracture mechanics theory of ceramic materials,theoretical prediction models of different process parameters and subsurface damage depth(SSD)were established.By analyzing the influence of different process parameters on the subsurface damage depth of brittle material workpieces in vibration-assisted machining,the relationship between damage depth and process parameters,polishing head feed speed and abrasive apex angle)were obtained.Then,using the FEM software,a 3D model is established to simulate VAP SiC,which the mechanism of material deformation and damage during VAP is analyzed,as well as the effect of different processing parameters on the surface/subsurface fracture layer was investigated.Through the analysis of the simulation data,the changes of the surface/subsurface damage with the parameters of the lateral amplitude,vertical amplitude(nominal cutting depth),abrasive grain size,and processing frequency of VAP are discussed.The results show that the smaller vertical amplitude(nominal cutting depth)and the apex angle of the abrasive grains are conducive to improving the surface and subsurface quality.In contrast,with the increase of the feed rate of the abrasive grains the surface and subsurface quality is improved.(3)Through the experiment of variable cut depth conventional scratch experiment without vibration and vibration-assisted scratch experiment,the critical cutting depth and morphology of scratch grooves were detected after processing.The results show the VAM can increase the critical cutting depth of SiC ceramic material.Then,through comparison of surface residual stress and surface morphology of workpieces before and after conventional polishing experiments without vibration and VAP experiments,the experimental results show that the residual stress and surface roughness on the surface of the SiC workpiece after VAP is smaller than conventional polishing experiments without vibration.Finally,through observing the surface morphology and subsurface cracks of the workpiece after processing with different parameters,it is revealed that the process parameters of VAP on the effect of surface roughness and SSD.The experimental results verify the validity of the theoretical prediction model and finite element simulation results.