Magnetorheological finishing (MRF) is usually used to remove subsurface defects (SSD) of workpieces to improve the laser-induced damage threshold due to its low stress. Although MRF has been widely used to remove surface defects and SSD, the evolution and removal mechanism of surface scratches under MRF have not been fully elucidated. A systematic study is conducted on the scratch removal process and removal ability of MRF. First, an experimental study on the removal of scratches with different depths is carried out, and the effect of the scratch depth on the removal efficiency is analyzed. Second, when the scratches are removed, the processing is performed in two directions: parallel and perpendicular to the scratch. Thereby, the relationship between the scratch removal efficiency (SRE) and the processing direction is analyzed. Finally, based on the scratch removal process and mechanism, an innovative method for efficient scratch removal using a sacrificial layer is proposed. The effectiveness of this method is verified by experiments. This method can significantly improve the SRE of MRF.
Single-crystal silicon is a typical infrared optical material, commonly machined by single-point diamond turning (SPDT) with micro-level figure accuracy and nano-level roughness. However, the tool marks, surface damage and middle-frequency error left by the diamond turning process may greatly affect the imaging quality. Magnetorheological finishing (MRF) is a deterministic, sub-aperture polishing technology that is very helpful in improving both surface nano-roughness and surface figure and can be used to polish silicon materials. Although the feasibility of MRF for single-crystal silicon has been proved, there are still some problems such as low material removal rate and uncontrollable surface integrity. In this study, the MRF mechanism for single-crystal silicon was explored, and the preparation method of MR fluid was optimised. An experiment was performed to machine a large-aperture single-crystal silicon aspheric surface on an MRF machine developed by China Academy of Engineering Physics. After polishing for several times, the figure accuracy PV improved from 5.9 μm to 0.56 μm, and roughness Rq reduced to 1.2 nm, verifying the excellent performance of MRF in infrared material processing.
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