Investigation On Material Removal Mechanisms And Grinding Parameters Optimization Of Reaction Bonded Silicon Carbide

RBSiC material is a promising mirror material for space optical applications. In order to reduce the damage introduced in grinding process and enhance the grinding efficiency and the mechanics reliability, grinding mechanism, grinding parameters optimization and strength of annealed, polished and ground RBSiC material are studied.The structures, composition and physical properties of RBSiC are investigated. The experiment results show that RBSiC is a polycrystalline material composing ofα-SiC and Si. The filling of residual Si to the pores makes the products compact. The properties measurement results reveal that this RBSiC is appropriate for optical applications. The material has high Young's modulus, high bending strength and outstanding thermal properties. It also has high hardness and low fracture toughness and.According to the calculated critical grit cut depth, the removal mechanism is explored using scan electron microscope (SEM), Atom force microscope (AFM) and optical microscope technique. The removal mechanism includes brittle fracture and ductile cutting, presented as micro fracture, plowing, grains fracture and so on.According to the roughness measurement and SEM observation, the effect of grinding parameters on surface roughness and surface topography is investigated. The effect of grinding parameters on grinding force and specific energy is also analyzed. The analysis shows that the increaseing down feed leads to large grinding force, which makes surfaces rough and the fracture percents increased. There has no obvious relationship between worktable rotational speed and surface roughness. Burnishing can improve the surface quality, but it can also cause the extension of subsurface cracks induced in grinding process toward the surface. As a result, the surface gets worse. The combination of the total removal depth and the down feed has significant effect on surface roughness.The optimized grinding parameters obtained using orthogonal tests are as follow: down feed of 0.9μm/s, worktable rotational speed of 2.1 r.p.m, burnishing for 1 minute and truing/dressing the wheel after 0.1mm is removed.The surface/subsurface damage characteristics are analyzed. The subsurface damage consists of plastic distortion layer, chipping layer and crack layer. The damage layer increases with down feed.The ground surface residual stresses were determined using X-ray diffraction technique. The results show that residual stress has great direction dependency. The residual stress is high when the specimen was ground with its long axis paralleling to the worktable radius. Anneal and polishing can relax the compressive residual stress The finite element method was used to simulate the thermal and mechanical stress in grinding processing. The analysis reveals that the effects of mechanical load and thermal load decrease with the increasing down feeds.The effect of residual stresses and cracks on bending strength is assessed. The increase of down feed decreases the characteristic strength. The long straight cracks have less effect on strength of the specimens ground with their long axis paralleling to the worktable radius. The compressive residual stress has better control on the small cracks than the long cracks. After anneal at 800℃for one hour the specimens has small crack size, which makes its characteristic strength close to the ground one and larger than the polished specimens.The investigations of properties, removal mechanism and strength reliability provide valuable experimental data on material properties and grinding optimization of RBSiC optical materials.