Analysis Of Surface Residue On KDP Crystal Machined By Water Dissolution Polishing And The Cleaning Method

Potassium dihydrogen phosphate(KH2PO4,KDP)crystal,which possesses excellent nonlinear electro-optic property,plays an important and irreplaceable role in high power laser systems especially for inertial confinement fusion facilities.However,due to its soft-brittle,anisotropic,easily deliquescent and thermally sensitive characteristics,as well as the particular stringent engineering requirements of surface accuracy(wave front distortion PV≤λ/6,surface roughness RMS≤5 nm),surface integrity(free of damage and pollution)and the most important,laser performance(laser induced damage threshold LIDT≥15 J/cm2),research on the ultra-precision machining technology of large aperture KDP crystal and the processed surface integrity has long been the worldwide hot topic.Previous studies mainly concentrated on the machinability of KDP crystal,and ultra-precision surface with no or less damage can be obtained so far by adopting advanced optical manufacturing technologies.However,problem of residual surface pollution after machining process and secondary pollution from crystal transportation and storage comes out,which becomes the key link that constrains the processed ultra-precision optical components stepping into engineering application and can not be neglected.Plenty of research work on the water dissolution ultra-precision polishing of KDP crystal has been done by our research team earlier,and high quality surface with little damage is acquired.However,surface residual pollution issues after this processing method have not been studied.Based on this,this paper conducts research work on the surface residue analysis on KDP crystal surface machined by water dissolution ultra-precision polishing and the removal method,the main research contents and results are as follows:The influences of surface residue on KDP crystal after water dissolution utra-precision polishing on the machined surface quality,optical transmittance,LIDT and subsequent coating film were firstly analyzed.Polishing residue covered on the machined super smooth surface and changed the surface morphology fearures,which caused the instrumentally measured RMS surface roughness and PV values to increase.And this would increase light scattering and decrease the optical trasmittance for KDP crystal.Surface residue can bring down the optical trasmittance for machined KDP crystal by 8%-10%in ultraviolet to near infrared wave band.Surface residue was proved to be able to decrease the LIDT of polished KDP crystal,especially for LIDT stability with a magnitude of 89%.Surface residue can react with siloxane which is the main chemical composition of the subsequent moisture-proof protective coatings,and damage the integrity and compactness of the film.The consequence is the deliquescence and damage for KDP crystal.Surface residue on KDP crystal after water dissolution utra-precision polishing severely hampers the following enginnering application of machined crystal and has to be removed.By studying deeply into the mechanical-physical interaction process of water dissolution ultra-precision polishing of KDP crystal,the surface residue formation mechanism was revealed and confirmed,it is a complex compound of dissolved material and reacted chemicals of polishing fluid produced after polishing.Through thorough observation,the residual oil-like compound showed characteristic of strong hydrophobicity,distributed randomly on the polished super smooth surface in self-wrapped marbles and thin films.Fourier transformed infrared(FTIR)spectrometer results indicated that the residual compounds were comprised of chemical groups like benzene,methyl,methylene,hydroxyl etc.Further investigations demonstrated that the polishing residue attached to the polished super smooth surface through physical adsorption and weak chemical reaction adsorption,the adsorption mechanism is the dispersion force,induction force and hydrogen bond between residue compound molecule and crystal surface molecule.According to the adsorption force model between molecules,the wetting works of polishing fluid with various water concentration and residual fluid on polished KDP crystal were calculated,which help to understand the characteristic of the residual fluid.A cleaning method was proposed based on the principle of the dissolution in the similar chemical structure.Nonaqueous and hydrophobic solvent was selected as the cleaning agent because of the water solubility and deliquescence property of KDP crystal.Short chain alcohol was added as the auxiliary to overcome the hydrogen bonding force and improve the cleaning effect.Ultrasonic assisted cavitation was also employed to strengthen the cleaning force in consideration of stubborn residue.Special cleaning agents were prepared and their solubility,surface tension,wettability and volatility were characterized.Cleaning experiments on small size samples were carried out,FTIR and atomic force microscopy(AFM)results showed that special developed cleaning agent had excellent performance over other selected solvents,and it can remove the surface residue without material erosion and leftovers,and finally bring out the super smooth surface,achieving great cleaning effect.A special developed multi-station automatic ultrasonic cleaning and rinsing equipment for large aperture KDP crystal was designed and fabricated,which can realize the functions of automatic feeding,ultrasonic cleaning,rinsing and unloading of KDP crystal.A new method for evaluating the cleaning effect based on the visual observations and surface quality specifications was proposed.Cleaning experiments were performed on 100 × 100 mm2 sized KDP crystal to investigate the process parameters on final cleaning effect.Finally,the optimal cleaning process was made as follows:ultrasonic cleaning power of 400 W,ultrasonic cleaning for 5 minutes,rinsing for 3 minutes under rinsing pressure of 0.1 MPa,cleaning temperature at 25-30 ℃,longer than 10 minutes of drying time.The established cleaning process can eliminate the residue after water dissolution ultra-precision polishing and reveals the real polished super smooth surface(RMS 2.185 nm,PV 16.097 nm).