| Calcium fluoride material has become an irreplaceable lithography system lens and infrared window material due to its excellent optical properties.However,it is difficult to obtain a high quality surface with traditional processing.In order to obtain a high-quality calcium fluoride surface,it is necessary to study the removal mechanism of calcium fluoride materials,and it is necessary to develop a nano-scratch device,which can realize ultra-precision processing of calcium fluoride materials.Based on the principle of nano-scratch,this paper combines the precision piezoelectric drive and the flexible hinge structure to design a thermoadjustable thermal-assisted nano-scratch device that can sense the cutting normal force and the tool tip displacement at the same time;with the help of FBMM analytical modeling Methods Analytical modeling and parameter selection of force-sensing unit flexure hinges;commercial software Solidworks and Workbench were used to complete the modeling and basic performance simulation of the thermally assisted nano-scratch device;basic performance tests were performed on the prototype and used under different conditions A nano-scratch experiment was performed on single crystal calcium fluoride materials to explore the brittle-plastic transition mechanism of the material.Experimental results show:1)The stroke of the nano-scratch device is 29 μm,the first-order resonance frequency is 1223 Hz,the displacement resolution is 35 nm,the thermal assist module can make the tool tip temperature reach 215 ℃,and the force sensing resolution is 20 m N;2)Scratch experiments on calcium fluoride(100)crystal plane found that brittle removal cracks can be divided into two types,parallel cracks and micro-ridge cracks.The critical depth of the brittle-plastic transition in each direction of the crystal plane and the crack morphology show periodic changes with the change of the scratch direction.In addition,it was found that the brittle damage on the scratched surface was distributed asymmetrically,and some scratches showed brittle cracks on one side while the other side was smooth at the same cutting depth.This is because the cleavage surface is distributed asymmetrically along the scratch direction.Under the same cutting stress,the dislocation and rotation difficulty of the crystal lattice on both sides of the scratch are different,resulting in the scratch surface at the same cutting depth.Different brittle plastic state;3)In the calcium fluoride(100)crystal plane,the maximum cut depth of the brittle-plastic transition limit obtained by the experiment with a 0° rake angle tool is 396 nm,and the minimum is 42 nm.Compared with the 0° rake angle tool,the limit depth of cut obtained by the experiment with the-30° rake angle tool is significantly reduced,and the maximum and minimum values are reduced to 158 nm and 10 nm,respectively.This is because the negative rake angle increases the hydrostatic pressure in the scratch area,and the larger hydrostatic pressure promotes the formation of cracks;4)The nano-scratch experiment was carried out in the free-cutting direction of the calcium fluoride(111)crystal plane at 20℃,50℃ and 80℃ respectively.The critical depths of brittleplastic transition were 552.20 nm,731.401 nm,1241.41 nm,and 80 respectively.The critical depth at ℃ is 2.25 times that at 20℃,which is caused by the enhancement of plastic slip by external thermal energy.It can be seen that thermal assistance has a positive effect on plastic cutting of materials. |
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