Research on magnetorheological finishing of aspheric optics for single-crystal silicon

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 mu m to 0.56 mu m, and roughness Rq reduced to 1.2 nm, verifying the excellent performance of MRF in infrared material processing.