Study On Subsurface Damage In Rotary Ultrasonic Grinding Of RB-SiC

With the development of space optical technology,the demand for large diameter space mirrors is increasing.However,the increasing diameter of mirror will directly lead to a significant increase in the weight of mirror body.Therefore,lightweight design and manufacture of mirror body should be carried out.Reaction bonded silicon carbide(RB-SiC)has many advantages,such as high specific stiffness,low coefficient of thermal expansion and low density,which is the preferred material for the fabrication of large diameter space mirrors.However,RB-SiC is prone to produce subsurface damage during machining because of high hardness and brittleness.Rotary ultrasonic grinding(RUG)technology is a composite processing technology combining traditional grinding with ultrasonic machining,which has good prospect in machining hard and brittle materials.In this paper,grinding experiments were carried out on the self-assembled RUG experimental platform using RB-SiC.With the subsurface damage as the object,the subsurface damage detection methods,ultrasonic vibration effects and the influence of processing parameters were studied,and the subsurface damage characteristics of RB-SiC in the RUG was summarized.The main research contents and conclusions are as follows:(1)A cross-sectional polishing method(taking a silicon as foil or taking a polyester as foil)and a bonded interface sectioning method were used to test the subsurface damage of RB-SiC in the RUG respectively.To determine the optimal test form,four kinds of subsurface damage evaluation indexes were used to analyze and compare the subsurface damage tested by the two methods mentioned above.The results demonstrate that the cross-sectional polishing method(silicon as foil)has higher detection accuracy,and the detection result is more in line with the actual situation.(2)The ultrasonic power single factor experiment and corresponding ordinary grinding experiment were conducted.The experimental results show that the introduction of ultrasonic vibration can effectively inhibit the subsurface damage,and the subsurface damage first decreases and then increases with the increase of ultrasonic power.Based on the indentation mechanics theory model of hard and brittle materials,considering the kinematics and dynamics characteristics of abrasive particles,the experimental results were analyzed,and the ultrasonic vibration effect was further explored.(3)The orthogonal experiment of RB-SiC ultrasonic assisted grinding with seven factors and three levels was carried out,and the processing parameters were systematically studied with the target of subsurface damage depth.The experimental results show that the influence of feed rate,abrasive grain size,grinding depth,ultrasonic power,spindle speed,ultrasonic frequency,and abrasive concentration on the subsurface damage depth decrease in order,in addition,the subsurface damage depth is positively correlated with grinding depth,feed rate and ultrasonic power,negatively correlated with spindle speed and abrasive grain size,first decrease and then increase with increasing ultrasonic frequency.Abrasive particle concentration has a relatively small influence on the subsurface damage.(4)The subsurface damage characteristics of RB-SiC ultrasonic assisted grinding were summarized as follows:(1)RB-SiC is mainly removed by brittle fracture;(2)the crack tends to expand along the grain boundary,while the grain boundary has the function of shielding crack extension;(3)the material removal on large-size SiC particles is caused by the extension of transverse cracks to the free surface or the intersection between cracks,while the removal of material on the mixing region consisting of small-size SiC particles and elemental free silicon is caused by the intersection of many micro transverse cracks or the extension of many micro transverse cracks to free surface;(4)the subsurface damages with significant influence,such as large pits and deeper median cracks,are mainly present on large-size SiC particles.In order to suppress the damage caused by grinding,not only reasonable processing methods and processing parameters should be selected,but also SiC particles should be refined as much as possible during material preparation.