目的 探索基于成形金刚石刀具的高精度氟化钙（CaF2）柱面微透镜阵列（Cylindrical Microlens Array,CMLA）加工方法,评估不同辅助工艺与刀具几何精度对表面质量与成形精度的影响规律。方法 基于数值计算对CaF2的物理参数进行分析,验证其各向异性与软脆特性。采用成形金刚石刀具,结合超声振动辅助切削与飞刀切削2种工艺,在CaF2基板上制备CMLA。通过白光干涉仪与三维轮廓仪测试不同工艺下的表面粗糙度与轮廓精度,并考察加工参数及刀具磨损特性。在此基础上,优化刀具轮廓精度以提升微结构成形质量。结果 数值计算表明,CaF2晶体的杨氏模量为110.04 GPa,泊松比小于1/3,硬度为5.71 GPa,显示其软脆性特征;弹性各向异性因子均位于0~1之间,表明其存在弱各向异性。实验结果显示,飞刀切削可获得较低表面粗糙度和稳定轮廓精度;刀具几何误差在加工中被直接复制至工件表面,是轮廓误差的主要来源。通过修正非圆弧面刀具形状后,工件轮廓误差（Root-Mean-Square,RMS）由200 nm降至16.8 nm,验证了刀具轮廓精度对微结构成形精度的决定性作用。磨损分析表明,刀具尖端因长行程切削产生显著磨损,而中段磨损较轻,主要由刃口重复切削及局部热-力摩擦作用引起。结论 成形金刚石刀具结合飞刀切削的工艺为实现CaF2 CMLA高精度、低损伤加工提供关键工艺依据。研究结果为CaF2及其他脆性光学晶体的精密微结构制备提供了实验支持,对高端光学元件制造具有重要参考价值。
Abstract
Fabrication of high-precision cylindrical microlens arrays (CMLA) on single-crystal calcium fluoride (CaF2) substrates is critical for advanced lithographic illumination systems. However, the intrinsic soft-brittle characteristics and weak elastic anisotropy of CaF2 often induce brittle fracture and surface damage during ultra-precision machining, thereby limiting the achievable form accuracy and surface quality of microstructures. This study systematically investigates the coupling relationship between profile accuracy and surface integrity in the diamond machining of CaF2 CMLA, with the particular emphasis on the effects of material properties, auxiliary cutting strategies, tool geometric accuracy, and tool wear evolution. First, the fundamental mechanical properties of CaF2 are analyzed based on its elastic constants. The calculations show that CaF2 exhibits a Young's modulus of 110.04 GPa, a Poisson's ratio lower than 1/3, and a hardness of 5.71 GPa, confirming its typical soft-brittle nature. The elastic anisotropy factor ranges between 0 and 1, indicating relatively weak anisotropy compared with many crystalline optical materials. These characteristics imply that maintaining stable ductile-mode material removal during conventional cutting is challenging. To address these limitations, two auxiliary ultra-precision machining techniques, ultrasonic elliptical vibration cutting (EVC) and fly-cutting, are comparatively investigated. Experimental results show that EVC can locally improve surface finish by introducing intermittent cutting and reducing cutting forces. Nevertheless, the periodic reversal of tool motion in the elliptical trajectory inevitably generates scratches and defects at the groove bottom of the microlens structures. In contrast, fly-cutting consistently produces superior surface quality across the entire CMLA surface. This advantage is primarily attributed to the significantly reduced tool-workpiece contact time and the suppression of instantaneous cutting forces, which effectively mitigate brittle fracture during material removal. A systematic parametric investigation of the fly-cutting process is subsequently performed by varying spindle speed and feed rate. Within the investigated parameter range, the root-mean-square (RMS) profile error remained nearly constant at (208.2±5.76) nm, while the radius error is maintained at (0.23±0.04)%. The negligible influence of process parameters on form accuracy indicates that the dominant source of shape error originates from the direct replication of tool geometric inaccuracy onto the workpiece surface. This "direct replication" mechanism is further verified through post-processing correction of non-circular diamond tools. After tool contour calibration, the workpiece form error decreased dramatically from approximately 200 nm to an RMS value of 16.8 nm, with a peak-to-valley (PV) value of 72.4 nm, demonstrating that tool contour accuracy is the key factor governing microstructure formation accuracy. In addition, the evolution of surface roughness with cumulative cutting distance is investigated to evaluate the influence of tool wear on machining stability. The results reveal a distinct three-stage behavior: an initial stable stage with surface roughness below 2 nm for cutting distances shorter than 8 m, followed by a rapid degradation region between 12 and 18 m, and subsequently another relatively stable stage. This deterioration is mainly attributed to the progressive accumulation of localized tool wear. Post-machining characterization of V-shaped diamond tools reveals a highly non-uniform wear distribution, with severe wear concentrated at the tool tips, the primary cutting regions in multilayer cutting; while the central regions exhibit minimal wear. These results demonstrate that once the dominant geometric errors originating from tool fabrication are eliminated, tool wear, particularly in high-stress regions such as the tool tips, becomes the primary factor limiting process stability and surface quality. Consequently, ultra-precision machining of CaF2 CMLA exhibits a characteristic transition from a geometry-error-dominated regime to a wear-dominated regime. The findings confirm that the combination of form diamond tools and fly-cutting provides an effective and reliable approach for achieving high-precision, low-damage fabrication of CaF2 CMLA, offering valuable experimental guidance for the precision machining of CaF2 and other brittle optical crystals used in advanced optical systems.
关键词
氟化钙 /
脆性光学材料 /
柱面微透镜阵列 /
成形刀具切削 /
轮廓精度
Key words
calcium fluoride (CaF2) /
brittle optical materials /
cylindrical microlens array /
form tool cutting /
profile accuracy
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基金
国家自然科学基金重点项目（U24B2061）; 2023年中国湖南省十大重点科技项目（2023GK1060）
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