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