具有优良电光非线性特性的磷酸二氢钾(KH₂PO₄，KDP)晶体，在大功率激光器特别是惯性约束核聚变中具有不可替代的作用。然而鉴于KDP晶体软脆、各向异性、极易潮解、对温度敏感等特点，同时工程应用上对其极高精度(面型精度PV≤λ／6，表面粗糙度RMS≤5 nm)、高表面完整性(无损伤、无污染)和使役性能(激光损伤阈值LIDT≥15J／㎝²)的要求，对于大尺寸KDP晶体表面超精密加工及其表面完整性的研究一直是国内外的热点。 先前学者研究的重心主要集中在KDP晶体的可加工性方面，通过采用先进的光学制造技术，目前已能够获得超精密、近无损伤的光学表面。然而，加工后的表面污染和运输贮藏过程中的二次污染问题已经成为制约这些先进光学制造技术加工出的超精密光学元件通向工程化应用的关键环节，因此，超精密加工后光学表面的残留物污染问题不容忽视。本研究团队前期在水溶解超精密抛光KDP晶体方面开展了大量的工作，已能够获得低损伤高质量的光学表面，但还没有对该方法加工后晶体的表面残留物污染问题进行研究。基于此，本论文针对新型的水溶解超精密抛光方法加工后的超光滑KDP晶体开展了表面残留物分析及清洗方法的研究，主要内容和结论如下： 本文首先分析了水溶解超精密抛光后KDP晶体的表面残留物对加工后晶体表面质量、光学透过率、激光损伤阈值以及后续镀膜膜层的影响。表面残留物覆盖在已加工晶体超光滑表面引起表面形貌参数改变，使得仪器测量的表面粗糙度RMS和表面峰谷值PV均有增加，这会引起光散射，降低晶体的光学透过率；表面残留物可使KDP晶体对紫外至红外光波段的光学透过率降低8％-10％；表面残留物能够降低已加工晶体的激光损伤阈值，特别是损伤阈值的稳定性，降幅达到了89％；表面残留物能够与KDP晶体后续喷涂的防潮膜层中的甲基硅氧烷成分发生化学反应，破坏薄膜的完整性和致密性，引起晶体潮解和损伤。水溶解超精密抛光后KDP晶体表面残留物的这些影响严重阻碍了晶体后续的工程应用，必须将其去除。 通过研究水溶解超精密抛光KDP晶体这一机械物理交互过程，确认了抛光区域内晶体材料不与抛光液中的化学组分发生反应变化，从而揭示了抛光表面残留物的形成机理，是抛光液中的水分溶解晶体材料和抛光液中化学物质混合形成的液体；观察抛光后KDP晶体表面有机残留物的状态，发现其具有强烈的疏水性，以液膜和液珠形式随机分布在晶体表面，使用红外光谱仪分析得到残留物的化学成分包含苯环、醚键、甲基、亚甲基和羟基等化学基团；进一步分析得到表面残留物在KDP晶体超光滑表面的吸附主要是物理吸附和弱化学作用吸附，吸附机理是表面残留物成分分子与KDP晶体表面分子之间存在色散力、诱导力和氢键作用。 根据分子间吸附模型，计算了不同含水量抛光液和表面残留物在晶体表面的润湿功，确定了残留物性质及所处的状态；提出了基于相似相溶原理的表面残留物清洗方法，根据KDP晶体具有亲水易潮解的特性，选择了非水基且不具有亲水性的清洗溶剂，考虑到克服残留物成分分子和晶体表面之间的氢键作用，在清洗溶剂中加入短链醇作为清洗助剂，提升清洗效果，同时引入超声波的空化作用来增强清洗力，实现对表面残留物的精密清洗；研制了专用的清洗液，并对清洗液的溶解性能、表面张力特性、润湿特性和挥发性能进行了表征；开展了小尺寸样件的清洗试验，通过红外光谱分析和原子力显微镜观察，证明了研制的专用清洗液性质优于其他的清洗液，能够完全去除KDP晶体水溶解超精密加工后表面的残留物且不发生材料侵蚀和清洗残留，露出抛光后的高质量表面，获得良好的清洗效果。 研制了针对水溶解超精密抛光后KDP晶体的专用多工位超声喷淋清洗装置，能够实现被清洗工件的上料、超声清洗、喷淋清洗和烘干下料四工位的自动清洗过程。提出了目视法结合表面质量参数指标的清洗效果评价方法，在100×100㎜²尺寸的样件上进行了清洗试验，研究了工艺参数对清洗效果的影响，并以此制定了最优的清洗工艺：在超声清洗功率为400 W，超声清洗时间为5 min，喷淋清洗压强为0.1 MP，喷淋清洗时间为3 min，清洗温度为25-30℃，烘干时间大于10 min的条件下，能够去除KDP晶体表面水溶解超精密抛光后的残留物，露出高质量的原始表面(RMS 2.185 nm，PV 16.097 nm)，获得最佳清洗效果。 关键词：KDP晶体；清洗；表面残留物；水溶解超精密抛光

 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/㎝², 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 features, 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 ㎜² 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). Key Words: KDP crystal; Cleaning; Surface residue; Water dissolution ultra-precision polishing