Deliquescent Mechanisms And Experimental Study On Machinability Based On Solution Theory Of KDP Crystals

KDP crystal, as an excellent non-linear optical crystal, is used widely in laser and non-linear optical fields such as the Inertial Confinement Fusion(ICF) solid laser driver, intense laser weapon and other advanced equipments, etc. However, it is difficult for KDP crystal to machine because of its properties of the softness, brittleness, deliquescence and sensitivity to the change of temperature.In order to explore a new method of ultraprecision machining of KDP crystals, the deliquescence mechanism of KDP crystals was studied, and on the basis of the obtained results the different processing methods are tested, and the surface quality was also analyzed by changing technological conditions.In this paper, firstly, the surface topography of KDP crystals deliquesced in different simulated conditions were investigated with Olympus MX40 light optical microscopy. The chemicial composition of deliquesced KDP crystals was analyzed on the basis of Raman spectrum method. The resultant of deliquescence was identified based on X-ray diffraction method. The 3D surface topography of deliquesced crystals were investigated by Olympus OLS3100 Confocal scanning laser microscope and VHX-500 three-dimensinoal digital microscope of KEYENCE company. The evaluating indicator of deliquescent degree of KDP crystal was given. The results show that there are a lot of dots on the surface of crystals at the beginning of deliquescence, howerer, the whole deliquescing precess, in which the part of KDP crystals were changed into the salt, is just the physical change without any chemical reactions. Each dot is a hump, which is accumulated with lots of single crystals with different directions. The sizes of these humps are about 10μm in diameter and 1μm in height. The etch pits with irregular contour on the surface of crystals will be obersived if the serious deliquescence occurred.Secondly, three factors, temperature, relative humidity and surface roughness, were considered as variables to simulate various environments. Under the simulated conditions the surface topography of KDP crystal are evaluated with Olympus MX40 light optical microscopy. The relationship of deliquescing velocity and these factors were studied. It is shown that KDP crystal will not deliquesce until the relative humidity reaches the critical values. As the relative humidity increase gradually, the deliquescing velocity will become faster and faster. The KDP crystals with large surface roughness will be deliquesced firstly. Finally, the new method to machine the KDP crystals was studied experimentally. Two solutions, de-ionized water and the mixed sol solution with alcohol and silocon dioxide powder, were used to polish the samples. The rate of materials removal(MRR) and surface quality were analyzed in different processing parameters. It is showed that the high MRR was obtained when using the de-ionized water, however, the surface quality that the surface roughness Ra is 50nm was not very good in the experiment. It is possible that the surface quality will be improved if the machining equipments and techonolgical conditions were improved. Forturnately, the roughness of surface Ra could reach 5nm when KDP crystals were machined with the polishing solution with alcohol and silocon dioxide powder, so it is feasible to machine soft and brittle KDP crystals.