Research on the combined polishing technology for high-precision ZnS optical element fabrication

ZnS crystal is a multispectral infrared optical material with excellent optical and mechanical properties. Due to its large brittle properties, low hardness, and poor machining performance, Single Point Diamond Ultra Precision Turning Technology (SPDT) is the effective fabrication method for ZnS at present. However, the machining accuracy of the SPDT method is limited by the principle of "error copying", which cannot meet the application of high precision applications. Moreover, the traditional polishing method cannot achieve the high efficiency of surface shape accuracy and surface quality improvement of aspherical elements. In order to meet the demands of high precision surface shape and high surface quality of ZnS aspherical optical components, a new combined polishing technology which consists of Magnetorheological Finishing (MRF) and Small Tool Polishing (STP) technology is presented and researched in this paper. Firstly, the Magnetorheological Finishing method is used to achieve the deterministic convergence of surface figure error rapidly. The quantified relationship between process parameters and process defects is established through a series of experiments. Through the optimization of MRF polishing fluid and processing parameters, the surface quality is controlled from sharp deterioration while removing the cutting knife lines. Then STP method is used to improve the surface quality while maintaining the surface figure accuracy. This combined polishing technology is conducted through validation experiments on the ZnS experimental sample. The surface figure error is reduced by 3 times after polishing and the surface roughness is below Ra 1nm. This result validates the effectiveness of the combined polishing technology proposed in the paper.