Study On Ultrasonic Assisted Grinding Of RB-SiC And Residual Strength

In order to improve the resolution of space mirror, the increasing diameter of mirror leads to a greatly increasing launching cost. The performance indexes of the optical system are severely related to the lightweight of mirror. As the preferred material for the space reflector, high hardness and brittleness of RB-SiC result in poor processing performance. A large grinding force will introduc damage into the surface layer in processing, which causes the decrease of mirror strength and even failure of the mirror. Therefore, in this paper, grinding experiments of RB-SiC were conducted with the independent research and development ultrasonic assisted system, comparing the two processing methods, with and without ultrosonic vibration. Combined with the normal grinding force, surface quality and subsurface damage, the residual strength of ground material was studied. The following conclusiongs were obtained:(1) The normal grinding force of ultrosonic assisted grinding (UAG) was studied systematically. Combined with the specific conditions, a model of normal grinding force was proposed and the orthogonal experiment of four factors and four levels was designed. Range anaysis showed that the influence of feed rate, grinding depth, ultrasonic vibration amplitude and spindle speed on grinding force decreased in order. The grinding force had negative correlation with ultrasonic vibration amplitude and spindle speed, and positive correlation with feed rate and grinding depth. Analysis of variance showed that the effect of feed rate on grinding force was highly significant. The coefficients and their confidence intervals of regression model were obtained and the regression results were highly significant.(2) Surface morphology, roughness and subsurface damage of the specimens with UAG and conventional grinding (CG) were observed and measured. The material removal mode of the two methods were both accompanied with plastic removal and brittle removal mode. Under the same processing method, the plastic deformation region ratio decreased with the increase of feed rate, and the surface roughness increased. While under the same condition with different method, the surface of UAG exhibited more tiny fractures. The sutface quality of UAG was better than that of CG, and the improvement effect was weakened with the increase of feed rate. The damage forms of the two methods on the ground surface were both collapse, surface crack and subsurface crack, and the most cracks existed in the SiC particles. The maximum subsurface damage depth produced by UAG was less than that of CG, and it was closely related to the normal grinding force.(3) Fracture morphology had nothing to do with the processing method, all of which were intergranular and transgranular fracture. The bending strength of ground material was lower than that of the original one, and it decreased as the feed rate increased. The UAG had less weakening effect on bending strength than CG, and the maximum crack depth was negatively correlated with the strength.In summary, UAG can reduce the grinding force, thus resulted in better surface intergrity and higher residual strength than CG.