Study On The Surface Layer Quality Of RB-SiC After Grinding

With the development of modern space optical technology, more demand will be placed on the large-aperture mirror. Silicon carbide ceramic is the third-generation space optical mirror material, which has important applications in the field of space optics. The primary characteristics silicon carbide offers relative to other traditional materials include low density, high specific stiffness, low coefficient of thermal expansion, and good thermal conductivity. Wherein the reaction bonded silicon carbide (RB-SiC) has been used in the preparation of space optical reflector, as complex structure can be sintered as long as green mirrors were prepared. Grinding with diamond wheels is the primary process used in achieving the desired tolerances and surface integrity. However, grinding would inevitably bring about surface layer damage such as micro-cracks, micro-fractures. RB-SiC is a special ceramic material, which consists of interconnected or interpenetrating networks of SiC and Si. Its damage mechanism is different from other ceramic materials. Therefore. it would be essential to understand the mechanism of material removal and also to evaluate the significance of the process parameters on the required responses. In this paper, the material deformation mechanism of RB-SiC was investigated using single-point diamond scratching experiment. The grinding surface layer damage characteristics during face grinding with diamond grinding wheel were researched. The effects of grinding conditions on the surface and subsurface damages were investigated and the associated material removal mechanism was discussed. The study can be summarized as follows:(1) The microscopic deformation and damage characteristic were investigated by single-point diamond scratching on RB-SiC workpiece. Under different axial loading conditions, RB-SiC material presented different deformation, including microscopic plastic deformation, micro-cracks, micro-fractures and crushing in large area.(2) Based on the observation and analysis of the grinding surface topographies and subsurface damages of RB-SiC substrates, the grinding characteristics and material removal mechanism of processing RB-SiC substrates were analyzed. The result shows that the material is removed in fracture mode by the extensions and intersections of large numbers of micro-cracks. In the SiC grains, the act of extrusion stress propagated the lateral cracks to intersect with the median cracks, which is the primary mechanism of the large SiC grains material removed in grinding RB-SiC. The intersection of intensive short lateral cracks with the free surface of the particle leads to chipping, which achieved to remove the material in the mixture region consisting of small SiC grains and Si.(3) The systematic grinding experiments of RB-SiC substrates were conducted, the influences of grinding depth, feed rate and spindle speed on subsurface damage depth and surface roughness were studied. The results indicated that the subsurface damage depth and surface roughness increased along with the rise of feed rate and grinding depth, and decreased with the rise of spindle speed. Both of these evaluation parameters are consistent in same trend.