Study On Liquid-film-contacting Deliquescent Polishing For KDP Crystals Based On Surface Modification By Using Of Cold Plasma

High quality and efficiency machining of KDP crystal is an urgent need of some major projects like Inertial Confinement Fusion.KDP crystal is currently recognized as one of the most difficult machining materials.At present,deliquescent polishing is a very potential ultra-precision machining method.This method uses hydrous polishing media and there are no problems such as abrasive particles embedding in the surface and micro-waviness on the surface after being processed.However,the existing deliquescent polishing has the problem of uneven deliquescence caused by retention of droplets,which leads to the low surface quality and limits the improvement of it.In order to solve this problem,liquid-film-contacting deliquescent polishing was proposed.In this paper,the polishing principle and material removal mechanism were studied in detail.The main contents are as follows:(1)Liquid-film-contacting deliquescent polishing of KDP crystals based on real-time surface modification by using of cold plasma was proposed and the liquid-film-contacting deliquescent polishing device was developed.The inherent wettability of KDP crystals are the cause of the problem of droplet retention and cold plasma hydrophilic modification for KDP crystals can achieve the transformation of droplet retention to liquid-film-contact.Nitrogen cold plasma can quickly modify KDP crystals to super hydrophilic.The hydrophilic modification treatment of KDP crystals by nitrogen cold plasma is reversible: the surface roughness and the micro structure of KDP crystal after treatment of nitrogen cold plasma change little;there are no new substance generated on the surface;the contact angle of treated KDP crystals will gradually return to the level before treatment.(2)Through etch experiments,the material removal characteristics of KDP crystals in hydrous media were studied.The etch rate of KDP crystal in hydrous media can be accurately controlled by controlling the properties of hydrous media.The etch rate of the KDP crystal surface in the micro mist after the treatment of plasma is less affected by water volume of output than before,which is conducive to keeping the polishing process smoothly.After the treatment of nitrogen cold plasma,the ability to absorb droplets in the micro mist of KDP crystals is enhanced,and the intensity of slippage of the water layer near the crystal surface inside the liquid film would be weakened.At this time,the overall liquidity of the liquid film would be weakened and the flow velocity gradient would be increased,which facilitates the preferential removal of high-point materials by the polishing system.(3)The friction and wear experiments under different parameter conditions were completed,and the frictional property of KDP crystal at the polished interface was studied.By adjusting the polishing parameters,the magnitude of the frictional force on the KDP crystal can be precisely controlled.Adding nitrogen cold plasma in the polishing interface does not change the friction between KDP crystal and polishing pad.The boundary lubrication is formed between KDP crystal and polishing pad.The wear mode of material is mainly corrosion wear,meanwhile,adhesion wear and abrasive wear occur at the same time.(4)Through polishing experiments,the influence of nitrogen cold plasma and various polishing parameters on the polishing effect was analyzed,and the superiority of the liquid-film-contact deliquescent polishing method was verified.The surface roughness of KDP crystal can be reduced and the material removal rate can be increased by adding plasma to the process of micro mist deliquescent polishing.By adjusting plasma discharge parameters and polishing parameters,different surface qualities of KDP crystals can be processed,while the material removal rate can be precisely controlled.It was found that the method of liquid-film-contacting deliquescent polishing could effectively remove the micro pits caused by the retention of droplets compared with the existing micro mist deliquescent polishing method and the surface roughness(rms)has dropped from 18.362 nm to 7.626 nm,and the PV value has dropped from 109.928 nm to 61.526 nm.