KDP晶体超精密切削中的跨尺度材料去除机理研究

KDP (Potassium dihydrogen phosphate) crystal is a superior optical material with larger nonlinear optical coefficient, wider transmission band and larger laser-induced damage threshold which makes it the best candidate for laser frequency converter, electro-optical modulator and optical switch. However, KDP crystal is simultaneously one of the most difficult-to-machining materials because of its soft, brittle and deliquescent property. It will fracture and lead to the surface defect such as pit and crack if the external load greater than the elastic strength. Free-abrasive machining technology (ELID, CMP, Lapping etc.) will embed the abrasive particles into the machined surface which is difficult to be removed. The potential defects can greatly deduce the laser-induced damage threshold which may result in the failure of the large solid-laser apparatus or high-energy laser weapons. Single point diamond turning (SPDT) is gradually developed for ultraprecision cutting of KDP crystal because SPDT can overcome the defects generated by free-abrasive machining and reduce the edge collapse and ensure the precise orientation between machining surface and crystal axis. Presently, ultraprecision machining of KDP crystal still lies in the empirical stage with little is known about the physical essence which constraint the further improving of machining quality. Ultraprecision cutting of KDP crystal is a complex multiscale and multilevel process originated at molecular and atomic scale and related with complex chemical-physical process. Further investigation of fracture/deformation mechanism will be the key factor about strengthen the technique control, improve the profile accuracy and scientize the machining process. This project aims at carrying out fundamental research on the ultraprecision machining of KDP crystal by means of discipline-crossing and multiscale method, build up discrete-continuum coupled model based on smooth transition of displacement and stress field which ensure the reasonable seamless link of physical process at different scale. This project will also analyze the mechanism of surface mechanical-geometrical character and the corresponding technique control method. Furthermore, this project will uncover the necessary conditions for brittle or ductile cutting and the transformation between these two machining mode. Further development of this project will be helpful about uncovering mechanism of deformation and migration at atomic scale, clarifying the complex interaction and synergetic behavior among atoms and molecules. This project also helpful about uncover the mechanism of defect-free and super-smooth surface generation, optimization the technique control and provide the theoretical basis and technical support for ultraprecision machining of functional crystal devices.

KDP晶体材料各向异性、软脆，外部载荷作用下制造表层/亚表层易于产生凹坑和裂纹，极难加工出纳米级光滑表面。深入研究KDP晶体材料断裂去除机制将是强化工艺过程主动控制、提高加工表面的面型精度、实现制造过程科学化的关键所在。单点金刚石切削工艺可以减少表层损伤和塌边，有益于提高激光损伤阈值，是目前最理想的KDP晶体加工方法。然而由于KDP晶体独特的物理化学特性以及加工过程的复杂性，目前对KDP晶体光学元件超精密加工技术的研究仍然处于半经验阶段，还没有形成国际公认的机理解释。本项目以深入理解和揭示KDP晶体超精密切削中的跨尺度材料去除机理为目标，构建应力场与位移场光滑过渡的离散-连续力学耦合区域模型并对KDP晶体超精密切削过程进行跨尺度数值解算；分析KDP晶体材料表层/亚表层空间组织结构的演变规律及应力特征；明确小尺度变形区域软脆材料塑性变形特征，在此基础上深度解析KDP晶体超精密切削中的脆-塑转变主控机制；研究刀具特征以及切削层参数之间的关联及其对材料去除、表面质量的影响。本项目的开展将有助于揭示KDP晶体材料无损伤超光滑表面生成机理，为功能晶体器件的超精密制造提供理论依据和技术支撑。

以KDP晶体为代表的高精度光学元件以其良好的非线性和激光损伤阈值在各种尖端设备中的应用日益广泛，尤其是在惯性约束聚变固体激光器以及强激光武器等尖端设备中。这些高性能光学系统要求晶体元件微观结构具有良好的完整性、较低的位错密度以及较小的残余应力。极高的设计要求对表面制造工艺提出了巨大挑战。深入理解并揭示超精密制造工艺环境下，软脆晶体材料几何-力学综合响应对于加强工艺过程主动控制，提高加工表面质量，实现制造过程的科学化，定量化具有重要的意义。.课题以深入理解和揭示KDP晶体超精密切削中的跨尺度材料去除机理为目标，构建了应力场与位移场连续变化的离散-连续力学耦合区域模型并进行KDP晶体超精密切削过程的数值解算；分析了KDP晶体材料表层/亚表层空间组织结构的跨尺度演变规律及应力特征；明确了KDP晶体超精密切削中的脆-塑转变主控机制及工艺调控方法；综合晶体材料各向异性及工艺系统动力学特性研究表面波纹度产生机制及其调控措施；阐明了工艺环境因素（温度和湿度）对KDP晶体材料去除过程的影响规律。.KDP晶体的超精密制造技术已超出传统机械制造研究的范畴，深入到纳米尺度表面和界面的设计、制造与控制的层次。传统的制造科学基本上建立在宏观体系热力学和动力学的理论基础之上，缺乏对微观尺度和极限制造条件下科学规律的认识。复杂光学晶体材料超精密制造并非对传统制造技术的局部改造，而是对制造极限的一种挑战，需要对其制造原理、制造工艺和制造装备进行深入细致的研究。KDP晶体材料的超精密制造研究涉及表面工程、光学工程、材料工程等诸多领域，属于前沿性学科交叉问题，该方向的科研进展将对复杂光学元件制造理论和水准产生较大影响。课题的开展将有助于掌握光学晶体高效、纳米级精度加工工艺技术的共性基础问题，实现工艺过程的主动优化调控，为实现具有自主知识产权的功能器件光滑近无损纳米级表面超精密加工技术提供理论支撑和技术支持。