Polishing technique for potassium dihydrogen phosphate crystal based on magnetorheological finishing

Potassium dihydrogen phosphate (KDP) crystal is an extremely valuable and irreplaceable nonlinear optical crystal that is widely used as frequency converter and electric-optical switch in various areas such as inertial confinement fusion (ICF) laser systems. However, it is very difficult to machine KDP crystals because they are thermally sensitive, prone to fracture, hygroscopic and easily scratched. The typical process used for machining KDP crystals is single-point diamond turning (SPDT). Though ultra-precision flatness fly-cutting machines are employed to machine large-aperture KDPs, small-scale ripple errors persist, and the cutting accuracy is limited by machine tool imprecision. Magnetorheological finishing (MRF) has been developed for KDP crystal processing, but controlling the surface integrity of KDP crystal is a key associated problem, due to the low hardness of the crystal and water solubility. In this paper, a new type of oil-based magnetorheological fluid (MR fluid) is developed, and the removal mechanism for KDP crystals is studied. The roughness control and the defect-removing mechanism are also investigated. Using the MR fluid, surface integrity controlling techniques and other relevant technologies, a large-aperture KDP crystal is successfully polished. For a KDP crystal of 410 mm x 410 mm, PV value is greatly reduced from 4.36 lambda to 1.32 lambda RMS from 0.698 lambda to 0.138 lambda and roughness is reduced to Rq 2 nm. Results show that large-aperture KDP crystal can be successfully polished by the MRF machine, indicating that the MR fluid designed has superior effectiveness, longer-term stability and an excellent surface integrity. (C) 2018 The Authors. Published by Elsevier Ltd.