| Quartz glass has a variety of brilliant properties including low-coefficient of thermal expansion,excellent spectral characteristics,good chemical stability and electric insulation,which enable quartz glass to be used widely in areas of aeronautics and astronautics,semicon doctor,laser,electrical equipment,medical equipment and other fields.As a typical hard and brittle material,due to its high hardness and low fracture toughness,traditional processing methods have not been able to meet the application requirements of quartz glass.Ultra-precision grinding process has the advantages of high processing accuracy,good surface quality,small damage,high efficiency and low cost,and is increasingly applied to the field of optical hard and brittle material processing.However,the grinding process inevitably introduces a certain degree of damage on the surface and subsurface of the quartz glass,resulting in poor imaging quality,low stability,and even endangering the safety of the entire system.Therefore,we must conduct a systematic study on the subsurface damage of the ground quartz glass after grinding to understand the damage form and depth,in order to guide the subsequent polishing process.In this paper,we first use different size(#400 #1500 #2000 #5000)of cup-type ceramic binder diamond grinding wheel to test the quartz glass on the vertical ultra-precision grinding machine to detect the grinding surface damage of quartz glass such as surface damage characteristics,surface roughness,subsurface damage forms,and subsurface crack depth effects.According to the experimental phenomena,the material removal method was summarized,and the grinding results were explained from the mechanism by comparing the depth of abrasive grain cutting of different size diamond grinding wheels and the brittle transition depth of quartz glass.Finally,based on the theory of indentation fracture mechanics,the mathematical relationship between the surface roughness PV and the subsurface crack depth SSD in the fragile domain of quartz glass is established.The main results are as follows:(1)With the decrease of abrasive grain size,the ground surface/sub-surface quality of the quartz glass becomes better.The surface roughness Ra of the quartz glass ground by the #400,#1500,#2000 and #5000 grinding wheels is respectively 274.0 nm,69.6 nm,3.4 nm,and 1.4 nm,and the subsurface damage depths is 5.73 ?m,3.64 ?m,0.98 ?m,and 0.004 ?m.Both the hardness and elastic modulus of the ground surface with different grain sizes are higher than that of quartz glass body itself.But with the decrease of subsurface damage depth,the hardness and elastic modulus of ground surface of quartz glass also reduce.(2)The material removal method for ground quartz glass of #400 and #1500 grinding wheels is brittle fracture removal.The material removal method of #2000 grinding wheel includes both brittle fracture removal and plastic flow removal,but the main method is plastic flow removal.The material removal method for #5000 grinding wheel is plastic flow.The surface damages type in the brittle field grinding are pits,micro-cracks and deep wear marks and the sub-surface damages types are in the form of micro-cracks.The surface damage in the plastic field grinding is micro-grinding and the sub-surface damage is in the form of plastic deformation of the material near the machined surface.(3)Based on the indentation fracture mechanics theory,the microscopic morphology of the workpiece surface and the formation mechanism of sub-surface microcracks in the brittle region were revealed.The mathematical formula was established among subsurface damage depth SSD,surface roughness PV value and the mechanical properties of the quartz glass.Several tests on the quartz glass ground in the brittle field under different parameters by #400 and #1500 diamond wheels were performed to verify the mathematical model.A quick and simple non-destructive testing method was raised,which provides a subsurface damage depth detection for the brittle field grinding. |
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