Investigation On The Mechanism And Approaches Of Controllable Dissolution Micro-Nano Polishing For KDP Crystal

Potassium dihydrogen phosphate(KDP)crystal has become a key optical material in high-end equipment in the fields of energy,national defense,etc due to its excellent electro-optic properties and nonlinear optical properties.Its role and status are irreplaceable.However,KDP crystal has a series of difficult to machine characteristics,such as easy deliquescence,high brittleness,low hardness,and strong anisotropy,which brings great challenges to its ultraprecision machining.The subsurface damage in traditional machining seriously restricts the improvement of laser-induced damage threshold,which is a key performance index.Exploring new techniques of nearly nondestructive ultraprecision machining for KDP crystal has become one of the research hotspots of experts and scholars at home and abroad.The micro-nano deliquescence ultraprecision machining based on the water dissolution characteristic of KDP crystal is a new mechanism with significant advantages proposed by the author's team,and has been recognized and valued by more and more scholars and engineering application departments.Based on the micro-nano deliquescence ultraprecision polishing approach using microemulsion developed by the author's team previously,this paper firstly proposed a concept of reversal growth of crystals,according to the characteristics and mechanisms of crystallization,dissolution,and growth of crystals.It was pointed out that the so-called reversal growth of crystals,from the point of view of material transport,had the orderly and controllable characteristics highly similar to the growth process,which is different from the disorderly and random dissolution process in the natural state.On this basis,a mechanism of controllable dissolution micro-nano polishing for KDP crystal was proposed.Through the mechanical synergy of the controllable dissolution of KDP crystal in undersaturated aqueous solution and the polishing pad,controllable micro-nano removal of the crystal material was achieved without introducing contaminants.To verify the feasibility of controllable dissolution micro-nano polishing of KDP crystal,the main factors affecting the material removal of controllable dissolution were experimentally studied,and the effects of KDP solution concentration,temperature,and fluidity,etc.on material removal efficiency of KDP crystal controllable dissolution were given.Based on the above feasibility study,an investigation on surface planarization approach for KDP crystal controllable dissolution was carried out.Controllable dissolution polishing experiments were conducted for macro processing textures on the surface of the KDP crystal workblank.After 4 min of the controllable dissolution polishing,the macro groove with a width of 4.6 mm and a depth of 0.2 mm on the surface of KDP crystal was removed.The average rate of groove depth reduction was 52.5 ?m/min.On this basis,the surface planarization mechanism of KDP crystal controllable dissolution was proposed.Before the polishing process,the surface of the KDP crystal in the static polishing solution was covered by a high concentration solution layer.The controllable dissolution was constrained,which led to a small material removal rate.During the polishing process,the peaks of the surface micro profile of KDP crystal were exposed to the high-speed flow of polishing liquid.The high concentration solution layer nearby the peaks disappeared,which led to a significant increase in the controllable dissolution rate of the crystal material located at the peaks.Meanwhile,with the help of the polishing pad,a relatively big material removal rate was achieved.By contrast,the environment of the valleys of the surface micro profile of KDP crystal was relatively occlusive,greatly reducing the fluidity of the polishing fluid and the mechanical action of the polishing pad.Consequently,the high concentration solution layer near the valleys was retained,which led to a smaller material removal rate compared with that of the peaks.Thus,the distances between the peaks and the valleys decreased,and the surface planarization was thus achieved.To realize the mechanism,surface planarization tests were carried out.Through 1 min of controllable dissolution polishing,the surface planarization of KDP crystal with micro texture and brittle damage was achieved,and the RMS surface roughness was reduced rapidly from 605.5 nm to 23.9 nm,with a reduction of 96%.Based on the above polishing results,a good planarization effect of the controllable dissolution micro-nano polishing approach for surface processing morphologies of KDP crystal with different scales was verified.Through further experimental analysis,the material removal mechanism of controllable dissolution polishing was confirmed,and the synergistic effect of polishing pad on the surface planarization of controllable dissolution was revealed.To clarify the influences of process conditions and process parameters on the controllable dissolution polishing effect for KDP crystal,experimental studies on controllable dissolution polishing process were carried out.Using the surface quality of KDP crystal and the stability of the polishing process as criteria,by conducting comparative experiments for different types of polishing pads,the IC1000 polishing pad was determined to be the most suitable for controllable dissolution polishing.It was experimentally confirmed that the polishing effects of different crystal faces were similar under the same process conditions.The differences of RMS surface roughness and material removal rate were not more than 6.1%and 7.2%respectively.On this basis,the influences of the concentration and temperature of the controllable dissolution polishing liquid on the polishing effect were studied.The effects of polishing parameters such as polishing time,polishing pressure,and the rotational speed of polishing disk on the surface roughness and material removal rate of KDP crystal were given.Furthermore,a controllable dissolution polishing process based on a concentration control strategy was developed.Through 2 min of controllable dissolution polishing,the RMS surface roughness of KDP crystal decreased rapidly from 5702.4 nm to 17.1 nm,the surface shape error decreased from 49.8 ?m to 4.9 ?m,and the average material removal rate was as high as 51.7 ?m/min.Therefore,it was confirmed that the controllable dissolution polishing was an extremely efficient controllable micro-nano removal processing approach.In addition,to improve the stability and the economy of the controllable dissolution polishing process,investigations on the performance characterization and the service life of the polishing liquid were carried out.It was revealed that the developed controllable dissolution polishing liquid was easy to regenerate.To further improve the surface quality of KDP crystal after controllable dissolution polishing and obtain the crystal surface with nanometer-scale roughness,a novel controllable dissolution ultraprecision polishing method for KDP crystal based on ultrasonic atomized water film was proposed,which uses ultrasonic atomization technique and the scratching effect of a polishing pad to produce water film on the crystal surface.Through changing the supply rate of micro water mist,the concentration of KDP liquid film transformed from water film is indirectly controlled to trigger the controllable dissolution of the crystal.Meanwhile,the surface planarization ultraprecision machining can be achieved under the mechanical action of the polishing pad.A micro water mist polishing test device was developed.To verify the feasibility of this method,the surface planarization of controllable dissolution based on ultrasonic atomized water film was studied.Through experiments and analysis,the influence laws of process parameters of controllable dissolution polishing based on ultrasonic atomized water film on the polishing effect of KDP crystal were given.A composite process of controllable dissolution ultraprecision polishing based on ultrasonic atomized water film was developed.Through employing the controllable dissolution ultraprecision polishing based on ultrasonic atomized water film to further machine the KDP crystal surface obtained by the controllable dissolution micro-nano polishing approach,the RMS surface roughness was reduced by 70.2%in 7 min,and finally reached 5.3 nm.The average material removal rate was 1.2 ?m/min.Hence,high quality and high-efficiency ultraprecision machining of the KDP crystal was achieved.It can be predicted that the comprehensive application of controllable dissolution micro-nano polishing approach and controllable dissolution ultraprecision polishing method based on ultrasonic atomized water film will be a new high-efficiency precision machining technique without mechanical damage and surface contamination,which is suitable for water-soluble crystals such as KDP.The research results have important academic value and engineering application prospect for enriching and developing ultraprecision machining theory and technique.