Abrasive-Free Water-Dissolution Polishing And Processing Mechanism For KDP Crystal

KH2PO4 (KDP) crystal is a unique excellent nonlinear optical materical, used for laser frequency conversion and electrooptic switch in Inertial Confinement Fusion (ICF). KDP crystal is extremely hygroscopic, soft (Mohs hardness 2.5), which is considered to be difficult to machine. So far, KDP crystal is mainly processed by Single Point Diamond Turning, but there are micro-scale ripples on the finished surface, which degrade the quality of laser beam in system. Aiming at the machining difficulties and technical challenge, a new processing method, abrasive-free water-dissolution polishing, is developed in the dissertation according to the hygroscopy of KDP crystal, which provides a new approach for fabricating other hygroscopic crystals.The dissolution rate of KDP crystal in water is about 69.09μm/min according to the experiment, thus it is difficult to control material removal rate when processing KDP crystal using conventional aqueous slurry. Water-solubility becomes a disadvantage for ultra-precision processing. In this work, using water-solubility of KDP crystal, a new abrasive-free water-dissolution polishing has been developed. By dissolution action of abrasive-free polishing fluid and mechanical action of pad in polishing process, water molecule dissolves KDP crystal to remove the material on the surface. The key of this new method is how to control material removal rate by controlling the water content and stability of microemulsion. Carrier liquid long-chain alcohol A and surfactant S were selected to formulate abrasive-free polishing fluid AFPF-1. Long-chain alcohol A/surfactant S/water ternary phase diagram was plotted to formulate polishing fluids with different water content. The polishing fluid is colorless, transparent, nontoxic, noncorrosive and stable. The measurements of conductivity, viscosity, nuclear magnetic resonance and dynamic light scatter were engaged to characterize the polishing fluid AFPF-1. It is concluded that KDP polishing fluid is water-in-oil microemulsion with droplet diameter lower than 10nm. The experiments shows that the static etch rate and the material removal rate during polishing using the polishing fluid AFPF-1 can be controlled lower than 10nm/min and larger than 600nm/min respectively.It is concluded from the tribology analysis on KDP polishing that polishing fluid flow rate, fluid temperature and polishing pressure affect the friction force, which changes little with platen speed; with increase of polishing time, friction force decreases at first and then remains constant. Material removal rate rises up with increase of friction force and the temperature of pad goes up due to tribology heat cumulation. It was measured that the coefficient of friction ranged from 0.2 to 0.4. According to Stribeck curve, the lubrcating state between pad and KDP crystal is boundary lubrication, the contact state is protruded region contact and polshing fluid film thickness is 1～50nm. There is no abrasive in the polishing fluid and material removal derives from KDP crystal dissolution, therefore, corrosion wear is main wear mechanism in KDP abrasive-free water-dissolution polishing.The model and mechanism of material removal during KDP polishing were constructed. The material removal is composed of material dynamic etch and the material removal due to dissolvent-mechanical mutual action. The latter is main factor to removal KDP crystal during polishing. The process of KDP material removal is as follows:before polishing, KDP surface is protected by oil phase not to dissolve; during polishing, water droplet locating on the protruded region of KDP surface is exposed to the shear action between pad and surface to deform, and then water molecule broken away from surfactant micelle dissolves the material on the protruded region; at the same time, the recessed region is not suffered from friction force until it contacts with pad; finally, the dissolved material is taken away by the polishing fluid and smooth surface is achieved. The X-ray Photoelectron Spectroscopy experiment showed that the valence state of K, P,0 on the polished surface does not change and the component on the surface is still KH2PO4.Finally, the KDP crystal polishing experiments were carried out to research the effects of different parameters including water content in polishing fluid, fluid temperature, fluid flow rate, polishing pressure and platen speed on material removal rate and surface roughness, which are different on the different planes. The flatness of KDP crystal decreases from 22.53μm to 2.54μm by abrasive-free water-dissolution polishing. According to polishing experiments, the best processing parameters was obtained. After processing KDP crystal with these parameters, surface roughness (rms) decreases from 475.06nm to 1.70nm in 20min.The experiments of abrasive-free polishing fluid life-span were carried out. This water-in-oil abrasive-free polishing fluid can be circularly used with stable surface roughness until viscosity is higher than 32mPa-s. The cleaning process for polished surface was also researched and ethanol (>99.7) was chosen as cleaning solvent. With this cleaning method, damage-free smooth surface was obtained.