Multi-Scale Modeling Study On The Nanometric Cutting Process Of Calcium Fluoride

Single-crystal CaF₂ is a material with very excellent optical properties. In civilian use, it is the most ideal lens material for ultra-violet photolithography systems. In military use, it is an ideal material for the optical components of UV-VIS-NIR combined electro-optical sensor systems and ultra-violet high power laser weapons. Traditional mechanical polishing may leave abrasive particles embedded in the surface of CaF₂, which will lead to a dramatic decrease in the laser-induced damage threshold of the material. Therefore, the single-point diamond turning of CaF₂ has caught much interest in the recent years.In this study, the hierarchical approach of multi-scale modeling was adopted to study the nanometric cutting process of calcium fluoride. First, the mechanism of chip-formation during the nanometric cutting process of CaF₂ was studied by molecular dynamics simulations. Then molecular dynamics simulations were performed to obtain the mechanical properties of CaF₂ under high strain rates. And the results were fitted with reported stress-strain relationship of CaF₂ under low strain rates to get rate dependent constitutive formulation of CaF₂. Then finite element method was employed to study the ductile-mode cutting process of CaF₂, using the constitutive formulation obtained above. Finally, fly cutting experiments of CaF₂ was performed to analyze the influence of cutting speed upon the surface roughness of CaF₂.The results of molecular dynamics simulations show that the orientation relationship between cutting direction and slip system {100}<011> has great influence on the mechanism of chip-formation. When their orientation relationship favors slipping of atoms on the slip system, slipping on the slip system becomes the main mechanism of chip-formation. Otherwise, amorphous phase transformation and shearing in the amorphous region will be responsible for chip formation. Larger negative rake angle leads to greater amorphous phase transformation and forces more material to flow towards the two sides of the cutter. When cutting in the orientation of (111)<112>, the rake angle of -20? will be most favorable for slipping on the slip system. This corresponds very well with the report that the rake angle of -20? is most suitable for machining on the (111) plane, which has been concluded from experiments. The results of FEM simulations show that larger negative rake angle and larger cutting edge radius lead to lower tensile stress in the cutting region, but at the same time make it difficult for the material to flow towards the upper surface of the specimen by shearing. This will also lead to extrusion of the material towards the front-lower direction. And as a result tangential cutting force will first increase with the increase of negative rake angle and cutting edge radius, and then starts to decrease with them. The tensile stress in the cutting region will increase with cutting depth at first, and then becomes stable when it reaches a certain extent. The specific cutting force increases rapidly with the decrease of cutting depth, showing an obvious size effect. Within the range of cutting speeds adopted in the simulations, cutting forces grows slowly with cutting speed, which corresponds quite well with the reports of experiments. In the FEM simulations, cutting speed has little influence on the tensile stress. And the results of fly cutting experiments show that cutting speed has little influence on the surface roughness of the machined surface, under the cutting conditions and cutting speeds adopted. This verifies the validity of the simulation results to some extent.