The Ultra-precision Numerical Control Polishing Method Based On Water Dissolution Principle For KDP Crystals

Potassium dihydrogen phosphate(KH2PO4,KDP)single crystal which possess unique excellent non-linear electro-optical properties,is currently the only material suitable for electro-optic switches and high power laser-frequency conversion applications in laser-induced inertial confinement fusion.However,KDP crystals are difficult to machine because of their inherent softness,brittleness,water solubility,as well as their strong anisotropy and temperature sensitivity.Single point diamond turning(SPOT)is currently the most important ultra-precision machining method for KDP crystals.However,during the SPDT process,the feeding pitch causes the formation of cutting marks(micro-waviness),which adversely affect the optical performance of the KDP element,especially the laser-induced damage threshold.How to remove the micro-waviness and achieve near-no mechanical damage are the problems urgent to be solved.Many new ultra-precision machining technologies(e.g.,ultra-precision grinding,magnetorheological finishing and ion beam figuring)have been developed for the processing of KDP crystals.However,the challenge of ultra-precision machining on KDP optical elements is still one of the bottlenecks in ICF system.Recently,the author's group has developed a abrasive-free water dissolution polishing method based on the water-soluble of KDP crystal,and a super smooth surface has been obtained on small KDP samples(18 mm×18 mm)by continuous polishing method.Based on these previous achievements,a ultra-precision numerical control water dissolution polishing method that combines the computer controlled optical surfacing technique was presented to solve the challenge of large size KDP optical elements machining in this article.The-"Mechanical-Water Dissolution" interaction selective removal mechanism of numerical control polishing method based on water dissolution principle for KDP crystals was revealed in this article.In the polishing area,in the valleys of the KDP surface where the polishing pad does not directly contact the KDP crystal,the water micells are stable and no dissolution should occur;meanwhile,at the points of contact between the polishing pad and crystal,the water micelles are deformed by the shear force applied between the polishing pad and KDP crystal,and the water droplets are released and are able to dissolve the asperities of the KDP crystal,thus the surface roughness in the polishing area decreases.Outside the polishing area,KDP crystal is protected by oil-based solvent and there is no dissolution.Besides,the initial surface-figure error is measured to determine the ideal amount of material to be removed at each point.In order to improve surface-figure error,the polishing tool moves along a pre-defined path and the dwell time of polishing tool at arbitrary point is controlled by computer to achieve the accurate removal followed pre-defined amount.The ultra-precision numerical control polishing method based on water dissolution principle for KDP crystals was presented,and a numerical control ultra-precision polishing system for KDP crystals was developed.To optimize the machining results of numerical control polishing method based on water dissolution principle for KDP crystals,static etch rate of "water in oil" polishing fluid under different conditions was investigated,and presented the conclusion that critical water content for corroding KDP surface of polishing fluid is ?*.When water content of polishing fluid is less than ?*,the surface quality will not be deteriorated even if the KDP surface is over-exposed to polishing fluid.Meanwhile,the increase of temperature will lead to a decrease in the stability of "water in oil" structure.To exactly explain the influence of main factors(e.g.,polishing speed,polishing pressure,water content,direction of revolution and rotation)on material removal rate,a tool influence function model was established and modified based on Preston equation,and its stability was verified by experimental results.Dwell time function and processing flow of ultra-precision numerical control polishing method based on water dissolution principle for KDP crystals were presented.The feature and processing result of raster path,spiral path,Hilbert path were investigated respectively,and raster path was selected to process KDP crystal in consideration of the convenience for computer controlling.Meanwhile,the optimized parameters to achieve lowest surface roughness were presented.By using numerical control polishing method based on water dissolution principle,surface roughness and micro-waviness on KDP and DKDP crystal samples after SPDT process(50 mm×50 mm)were significantly reduced from 6.974 nm,8.661 nm to 1.790 nm and 1.929 nm respectively,and the medium-high frequency error caused by micro-waviness was completely removed.After multiple polishing iterations,the surface figure error of 100 mm×100 mm KDP sample(valid dimensions=90%of the central region)was significantly reduced from 9.062 ? to 0.290 ?(Peak to Valley).The research results lay a foundation for further processing of large size samples applied in projects,and this new method is also applicable to the polishing of other water-soluble materials.