| With the increasingly widespread application of optoelectronic devices, the demand for high surface quality of the optical elements is increasing. This needs to maximally guarantee in the quality of optical elements in the machining process. With the continuous improvement of product performance, the machining accuracy, surface quality and surface integrity of hard brittle material devices are also increasingly demanding. Polishing is the key of ultra-precision machining technology for hard brittle materials, so the claim to the polishing quality and polishing efficiency should be improved. As a new type of smart materials, under the effect of magnetic field, the magnetorheological (MR) fluid mixed with polished abrasives behaves as Bingham liquid, a viscoelastic cluster MR effect polishing pad between the surface of polishing plate and workpiece is formed to ultra-precision polish. Then an accommodate-sinking effect (AS effect) of MR effect pad on abrasives is presented in polishing process. It can reduce or eliminate the machined surface defects caused by non-uniformity of abrasive size. Modeling of the abrasive accommodate-sinking effect of MR effect pad is carried out in this paper. And relevant experiments and measurement set-up are designed to research mechanism of the abrasive accommodate-sinking effect, performance characteristics of MR effect pad and polishing forces, and material removal mechanism.Experiments of polishing K9glass and single crystal silicon based on MR fluid with or without doping larger abrasives are conducted, and the surface roughness and morphology of the workpieces being machined are investigated, it is found that polished surface based on the fluid mixed1.8μm diamond into0.6μm diamond is better than polished surface based on the fluid of uniform1.1μm diamond. The surface roughness and morphology of the workpieces demonstrate that abrasive accommodate-sinking effect is prominent. Then, a set of experiments were conducted by adding the rogue particles to polish the free-damage K9glasses and to study the influence of the rogue particles and carbonyl iron particles (CIPs), and the machining parameters on AS effect. The surface roughness of the machined surface was measured and adopted as the parameter of evaluating the AS effect, and a Keyence VHX-600microscope was used to investigate the machined surface. Experimental results indicated that when the size and concentration of rogue particles were no more than the threshold value, a well AS effect on abrasives aided in obtaining a super smooth surface. The maximal contribution was made by a machining gap on AS effect, followed by the rotational speeds of workpiece and polishing plate, while the least contribution was from the flow rate of MR fluid. The recommended machining parameters for obtaining a super smooth surface are a machining gap of1.2mm, the rotational speeds of60r/min and350r/min for polishing plate and workpiece respectively, and MR fluid flow rate of1000ml/min.The property of MR effect pad has a decisive influence on AS effect, so experiments are conducted to measure the viscoelasticity and the force distribution of MR effect pad. The MR effect pad generated in the different condition is solidified by adding epoxy resin glue Keyence, then VHX-600microscope is used to investigate internal characteristics of the solidified pad and its hardness is measured by shore durometer. The particle distribution of MR effect pad is studied by means of these two ways. The method to mesure force distribution of MR effect pad is presented. And accurate measurement of positive pressure and shear force acting between MR effect pad and the probe is achieved by this method. The trend curves along X axis and Z axis are obtained by experiments. Finally, the particle distribution and force distribution of MR effect pad are consistent with the results of magnetic field simulation.Knowledge of forces acting is important to understand the mechanism of material removal for MR effect pad. The systematic experimental investigations are carried out to measure the forces (normal force and tangential force) acting between the workpiece surface and MR effect pad in magnetorheological polishing process by means of a3-component dynamometer. The machining parameters and workpiece material respond on the forces are studied in the present study.The main conclusions are summarized as follows:(1) It is noted that the maximal tangential force Ft value is32.25N and the maximal normal force Fn value is62.35N, and the ratio between them is about0.46-0.77.(2) Both forces (normal force and tangential force) decrease with increase in machining gap; however, both forces increase with increase in magnetic field strength and CIPs concentration. Both forces increase with increase in abrasives concentration, at the abrasives concentration of5%, then they decrease with further increase in abrasives concentration.(3) Both forces continuously and marginally increase with increase in polishing plate speed, fluid flow rate and feed distance and feed rate of the workpiece, finally they get stabilized with further increase in polishing plate speed, fluid flow rate and feed distance and feed rate of the workpiece.(4) The most contribution is observed of that the machining gap and magnetic field strength on the forces developed on the workpiece surface, followed by CIPs and abrasives concentration, the fluid flow rate, the polishing plate speed, while the least contribution is noticed by the feed distance and feed rate of workpiece.(5) The forces from the cluster MR effect pad and the ratio Ft/Fn increase with the increase in the hardness of the workpieces. The forces acting on the harder workpieces have the characteristic with low normal pressure and high shear force, and this is beneficial to improve the quality of super-smooth planarization polishing.The hard brittle AIN substrate, thin SrTiO3substrate and SiC substrate polished by cluster MR effect polishing pad is researched. Then the result of the abrasive accommodate-sinking effect, the mode of material removal and the generation of machined surface are studied. The material removal mechanism, combination effect of positive pressure and shear force on abrasive, deformational behavior of hard brittle materials and the feature of machined surface morphology is investigated in this paper. The polishing process to obtain the high surface quality and material removal rate and the optimal combination effect of positive pressure and shear force on abrasive is presented. Experimental result demonstrates that cluster MR effect plane polishing these hard brittle substrates is feasible and effective. Under the well abrasive accommodate-sinking effect, the ultra smooth machined surfaces of AIN substrate (Ra0.0290μm), thin SrTiO3substrate (Ra3.8nm&RMS0.973nm) and SiC substrate (Ra0.4nm) can be obtained. |
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