High Efficiency Controllable Precision Grinding Mechanisms Of Fused Silica

Aimed at the present problems in machining of fused silica, which are the low material removal efficiency and much difficulty in removing material in ductile mode, the dissertation focused on the high efficiency controllable precision grinding mechanisms of fused silica. The material removal mechanisms of fused silica duiring grinding process were investigated starting with the material mechanics response mechanisms and stress state in the grit-workpiece interacted region. The mechanics response mechanisms of fused silica were analyzed. The deformation mechanisms and cracks initiation mechanisms of fused silica at the room temperature and high temperature were studied by nanoindentation experiments. An analytical model for the elastic stress field during scratching optical glass was presented. The location and sequence of crack nucleation and its effects on the material removal mechanisms of fused silica were analyzed. A critical function for crack propagation for single grit scratching fused silica with original damage during grinding process was developed.Controllable grinding mechanisms of surface micro cracks on fused silica were investigated based on the critical function for crack propagation. High efficiency controllable grinding processes which were low damage full brittle grinding and ductile dry grinding of fused silicawere presented. The low damage brittle precision grinding was conducted on fused silica to decrease the micro cracks damage depth while the high efficiency controllable ductile dry was conducted to aquire a smooth surface of fused silica without micro cracks.The mechanics response mechanisms of fused silica were analyzed. The microstructure of the fused silica was not dense. In the case of hydrostatic pressure, it was densified and its plastic flow is difficult. The fracture toughness of fused silica was low, and the micro cracks were easily nucleated. Hence the ductile machining for fused silica was difficult. The deformation mechanisms and crack propagation mechanisms of fused silica and soda lime glass which was used for comparing were studied by the nanoindentation experiments at room temperature. It was indicated that the sink-in formed around the indentation, atom cavities piled up in the sink-in region to increase the microscopic defects density. It was easy for nucleation of borderline cracks around the indentation. Fused silica had a large densification and small plastic flow in the indentation experiments. Therefore, the strength of the Blister stress field was small and the radial crack' s formation was not easy. The molecular structure of soda lime glass was denser and easy to slide along the shear plane, hence its plasticity was better than fused silica. Material piled up around the indentation during nanoindentation experiments. Its strength of the Blister stress field was larger and easily nucleated radial cracks. The effect of temperature on the deformation mechanisms and crack propagation mechanisms of fused silica were studied by high temperature nanoindentation experiments. It was indicated that the molecular structure of fused silica at the high temperature was denser and easier for plastic flow. The strength of the Blister stress field was larger to increase the crack nucleation critical load. Therefore, ductile mode maching of fused silica is easier at the high temperature.An analytical model for the elastic stress field during single grit scratching optical glass was presented. The location and sequence of crack nucleation and its effects on the material removal mechanisms during single grit scratching fused slica and BK7 silicate glass whick was used for comparing were analyzed. Single grit scratching experiments were conducted for fused silica and BK7 glass. It was indicated that median cracks were formed first during scratching. The corresponding depth of the scratching determined the critical depth for brittle to ductile transition. Median crack's propagation depth determined the depth of micro cracks damage in the materials. Lateral cracks which were formed afterwards interacted with Hertzian cracks and radial cracks to induce material brittle removal. The open network molecular structure of fused silica was easy to propagate to micro cracks in the case of external stress. Therefore, the brittle to ductile transition critical scratching depth of fused silica was only 32 nm. For the BK7 glass, the metal ions filled the atom cavities of silicon dioxide network. The atoms were easy to glide along the slip plane. So the plasticity of the BK7 glass was greater than that of fused silica. The ductile to brittle transition critical depth of BK7 glass was 98 nm.A critical function for crack propagation for single grit scratching fused silica with original damage during grinding process was developed. Controllable grinding mechanisms of surface micro cracks on fused silica were investigated based on the critical function. The grinding mechanisms variated from ductile mode to low-load semi brittle mode, full brittle mode and high-load semi brittle mode in sequence with the increasing single grit scratching depth during the grinding process. When the feed rate of workpiece was low to 1 mm/min, the single grit grinding depth was only 1?2 nm,the fused silica was removed by crack-free ductile grinding. But the material removal efficiency was too low. When the fused silica was ground by full-brittle grinding, the grinding force was low with good self-sharpening of wheel.Full-brittle grinding was a feasible precision process which could obtain a grinding surface with a low surface roughness and small micro crack damage in the high material removal efficiency.The fused silica was dry ground by a high temperature resistant ceramic bond CBN wheel with a large grinding depth. The ductile machinability of fused silica was improved by the high grinding temperature which increased the ductile grinding depth of fused silica. A smooth crack-free surface of fused silica was obtained. The high effiency ductile grinding of fused slica was realized. The essential conditions of ductile dry grinding fused silica was presented which were a high grinding temperature and the single grit grinding depth was smaller than the brittle to ductile transition critical depth at the corresponding grinding temperature; the grinding depth was larger than the micro cracks depth indueed by the last process. A theorical model of dry grinding fused silica was established. The temperature of dry grinding fused silica was measured online by the infrared transmission measurement method to vertify the theorical model.