Study On Elastically Constrained And Dissociated Abrasive Polishing For Ultra-Smooth Surfaces

The ultra-smooth surface finishing is one of the most important branches of ultra-precision machining. With the development of science and technology and the ever-increasing needs in engineering application, the ultra-smooth surface finishing has aroused extensive attention in manufacturing industries. Especially in optical instrument, laser, aerospace, space exploration, LSI circuit, and other manufacturing industries, a substantial amount of research has been done to achieve ultra-smooth surfaces which includes chemical-mechanical polishing (CMP), elastic emission machining (EEM), float polishing, magnetorheological polishing, and others. The core ideas of these approaches are to thin the degenerating layer and reduce the surface roughness of machined surface through decreasing the removal rate of the polished workpiece. Up to now, the abrasive polishing has predominated over those finishing approaches to achieving the ultra-smooth surface, controlling the action behavior of small-sized abrasives on the polished surface is becoming one of the important areas of research. As such, an elastically constrained and dissociated abrasive polishing method is proposed in this dissertation.The proposed dissociated abrasive polishing method consists of the two different processes, the first is based on tool elastic constraint; and the second is based on fluid elastic constraint. With respect to the first process, polyamine or rubber is used as the material of polishing head, then the polishing head is immerged into the polishing fluid with uniformly suspended abrasives, suitable contact load is exerted between polishing head and the machined surface, polishing fluid enters into the contact zone with high-speed rotation of polishing head, workpiece material is removed by abrasives under the action of tool elastic constraint. With regard to the second process, a micro-split is kept between polishing head and machined surface, and abrasives enter into the micro-split and achieve the nano-scale erosion machining. These two processes can be mutually converted by adjusting the position of polishing head and workpiece, the rotational speed of polishing head, and other technological parameters. The theoretical and experimental studies on the elastically constrained and dissociated abrasive polishing process have been conducted in this dissertation. The creative researches are summarized as follows:1. An elastically constrained and dissociated abrasive polishing method is proposed, the action of abrasive on the machined surface with tool elastic constraint is theoretically studied. The machined surface marks ploughed by quasi-spheral and pyramid abrasive is mathematically modeled. The influencing characteristics of tool material, abrasive size, contact load, machined material and other technological parameters on polished surface qualities have been revealed.2. The action behaviors of abrasives under tool elastic constraint is studied, and as such, it is concluded that polished surface is generated by the ploughing and rolling mechanism of abrasives. As to ductile material, polished surface is generated by plastic deformation and flow, as result of ploughing and rolling. As to brittle material, polished surface is generated by ploughing marks. Under fluid elastic constraint, the kinetic energy of abrasives is provided with the daggling of high-speed fluid flow. The energy particles collide with the polished surface, but the energy of particles is not enough to destroy the material layer of machined surface. Therefore, the atoms of peaks of rough surface will be collided to generate the polished surface for plastic material; however, it is difficult to generate the novel polished surface since the removal rate of collision particles is too low. In the case of brittle material, for example K9 glass, machined surface is generated by the fatigue mechanism of fracture. As the peaks of machined surface are so brittle that they are easy to be removed by the collision of fluid particles, so the better quality of machined surface can be obtained.3. The removal mechanism of material with elastically constrained and dissociated abrasive polishing is explored. Under tool elastic constraint, plastic material is removed by micro-cutting, rolling and bonding of abrasives, but in the case of brittle material, if depth of cut exceeds the critical value, at which brittle or plastic mechanism of material removal may be converted, brittle material is removed by plastic cutting, otherwise by brittle cutting. Besides, abrasives roll on the peaks of machined surface such that material will be removed by fatigue mechanism of fractures. When nano-abrasives are used to polish brittle material, both plastic removal and rolling mechanism of material predominate over polishing process.4. In the case of dissociated abrasive polishing with fluid elastic constraint, material is removed by the collision action of energy particles. From quantum mechanics and Fermi energy theory of electrons in a metal, bonding energy of atoms and material removal energy of nano-abrasives are compared for plastic material, 45# steel and ZL109 aluminum. It is concluded that with fluid elastic constraint, material removal of surface layer at the atomic level predominates over the polishing process of plastic material From the characteristics of brittle material, e.g. K9 glass, when the energy of abrasives is not enough to cause the cracks of fatigue, the peaks of rough surface are dynamically collided by abrasives and removed by the cracks of fatigue, which is caused by the growth of initial transverse cracks and cracks in micro-structure of glass K9. Moreover, material of surface layer may also be removed at the atomic level. The removal rate of material caused by the fracture of fatigue is large, and material removal always occurs at the peaks of rough surface, these phenomena can be used to explain why dissociated abrasive polishing with fluid elastic constraint is suitable for brittle material to achieve ultra-smooth surface. Based on the above mechanism of material, this dissertation made a new conclusion different from abrasive erosion theory that when nano-abrasives collide with the polished surface, whatever brittle or plastic the polished material is, the smaller collision angle is, the better polishing effect will be achieved.5. An elastically constrained and dissociated abrasive polishing system is developed on a CNC machine tool, which can monitor and adjust the contact load and the gap between polishing head and machined surface. The formulation of polishing fluid and the measures to fabricate the high precision polishing head have been investigated; the preparation of machined workpiece is also discussed. The finishing procedures with elastically constrained and dissociated abrasive polishing are detailed, and the detection means of workpiece is also proposed with which three areas are uniformly sampled on machined surface to measure the surface morphology.6. Experimental studies on elastically constrained and dissociated abrasive polishing have been conducted. The influencing characteristics of technological factors on the proposed polishing process have been experimentally examined and verified. From the above experimental results, it is shown that the influencing factors on machined surface are, in turn, the granularity of abrasives, the rotational speed of polishing head, and the contact load. The experimental has been conducted to optimize technological factors so that optimum polishing effect can be achieved, it is shown that the influencing factors on polishing effect are, in turn, the polishing time, the rotational speed of polishing head, the concentration of polishing abrasives, and the properties of carrier fluid. The polishing experimental results between brittle and plastic material have been compared, it is shown that the roughness Ra of polished surface of brittle material can achieve 1.0 nm or less, the polishing effect of brittle material is far better than that of plastic material. From the experimental results of the different polished zones, it is shown that the polishing achieves better effect in the middle of polished zone, because of large dynamic pressure and small collision angle of abrasives. Using optimum technological parameters, two-step composited polishing experiment has been conducted, it is shown that increasing reasonablly the polishing time under tool elastic constraint can increase greately the polishing efficiency.