Study On Polishing KDP Crystal With Micro Water Mist

KDP crystal (KH2PO4) is widely used in inertial confinement fusion because of its superior non-linear optical properties and good mechanical properties. However, KDP crystal is soft, easily deliquescent, strong hygroscopic and temperature sensitive, and has high precision requirements in application, which make it recognized as one of difficult to machine materials. So far, the single point diamond turning is the most effective methods in processing KDP crystal, and micro waviness and sub-surface damage may be produced in the processing which may affect the performance of the crystals. Hence, many other processing methods are introduced to KDP crystal machining, such as magnetorheological finishing, free abrasive dissolution polishing and ion beam polishing etc. which are all nearly free damage processing. However, these methods also have other disadvantages such as the subsequent clean-up and low polishing efficiency and so on.In this paper, a novel method polishing KDP crystal with micro water mist is proposed based on the solubility characteristics in water, which makes use of ultrasonic atomization water mist as polishing medium. And experiments studies were carried out combined with computer controlled optical surfacing (CCOS) technology. The following work was carried out in this paper:According to the solubility characteristics of KDP crystals, experiments dripping single water droplet onto the crystal’s surface were carried out to test the solubility properties. Through the experiments, the partial planarization mechanism was revealed and the basic principles of the micro mist dissolved polishing was put forward. A large amount of micro water mist flow onto the crystal’s surface, and a thin liquid layer is formed, making the material dissolved quickly. A nearly saturated solution layer is formed at the junction, which can effectively inhibit the further dissolution of crystals. Meanwhile, the materials in peak points are removed by friction between the polishing pad and the crystal, and a new surface is exposed, dissolved and removed continuously. However, valley points are retained for the presence of the saturated solution layer and the absence of the pad. Thus the peak points become lower and lower, and the surface is planarized at last.Combinind with CCOS technique, the basic principle of polishing KDP crystal with micro water mist was proposed and the polishing experiment system was established as well. Based on the classical Preston equation in polishing study, the removal function model was established by kinematic analysis of the polishing tool and the dwell time model was deduced as well.Experiments were also carried out to explore the connection between the polishing parameters and the surface quality. The results showed that the material removal rate increases with raising the water content of the micro water mist, while the surface roughness decreases first and then increases. Along with the raising of the polishing pressure and the polishing speed, the material removal rate increases as well. The surface roughness increases with the increasing of the polishing pressure firstly and then decreases, while increases with the decreasing of the polishing speed. The initial surface quality also has an impact on the quality of polished crystal surface. With the same conditions, smoother the initial surface more easily lower roughness surface obtained. To obtain better polished surface and taking the polishing efficiency into account, the polishing parameters should be optimized. Accordingly, the reasonable micro water mist generation amount, air pressure and the range or polishing speed were given in this paper in consideration of both quality and efficiency. With rational selected polishing parameters, the surface flatness after rough processing can be reduced by approximately 40%, reaching 64.945μm. While the final surface roughness reached 76.4nm rms, reducing by about 64 percent. All of these proved that the method polishing KDP crystal with micro water mist is rational and feasibility.