Research on key technology of ultra-precision machining of nickel-phosphorus modified metal mirrors

Metal mirrors fabricated from aluminium, beryllium, and similar alloys offer significant advantages, including ease of forming, high lightweighting potential, and integrated optical–mechanical properties, making them highly promising for optical systems in airborne detection, space astronomy, earth observation, and deep-space navigation. However, as these applications increasingly demand operation at visible, ultraviolet, and even X-ray wavelengths, the required surface figure accuracy and surface quality become exceptionally stringent. This presents a critical challenge because aluminium and beryllium substrates are inherently sensitive to machining damage, and directly machined surfaces typically fail to meet the demanding specifications of short-wavelength optical systems. To overcome this limitation, a common approach employs aluminum or beryllium substrates coated with a thick (tens to hundreds of microns) Nickel-Phosphorus (NiP) layer applied via electroless nickel plating. The NiP layer provides a dense, corrosion-resistant, ductile, and highly machinable surface modification. While ultra-precision turning is the standard method for machining NiP-coated mirrors, its achievable accuracy is fundamentally limited by the machine tool's "error replication principle," limiting its suitability for the most demanding short-wavelength applications. This study addresses this gap by introducing a novel sequential machining process: ultra-precision turning followed by magnetorheological polishing (MRF) and concluding with smoothing polishing. This combined approach leverages MRF to significantly enhance the surface figure accuracy initially achieved by turning. The subsequent smoothing and polishing step then refines the surface quality, effectively reducing roughness and micro-defects. Consequently, this integrated turning and polishing strategy demonstrably improves both the form accuracy and surface finish quality of NiP-coated metal mirrors, enabling their successful application in high-performance optical systems operating at short wavelengths.