Mechanical Properties And Optimization Of Ultra-Precision Cutting Process Of Cesium Iodide Crystal

Cesium iodide(CsI)scintillation crystal has excellent transformation efficiency from Xray or γ-ray to visible light and the light yield is high.It is widely used in y-ray spectroscopy,radiography,various detectors,nuclear medical imaging and other domains.However,because of the low hardness,nanoscale surface roughness for CsI crystal can’t be achieved by conditional slicing and polishing.Ultra-precision cutting method is an efficient way for soft and ductile crystals to acquire the optical surface quality.However,there is no relative research of the mechanical properties and ultra-precision cutting for CsI crystal at present.In this thesis,mechanical properties of different crystal planes of CsI crystal under different strain rates were studied.The impact for rake angle,machining parameters and anisotropy on surface roughness was studied,and the ultra-precision turning process of CsI was optimized.Furthermore,combined with the micro-textured single crystal diamond tools,the surface quality control of CsI crystal was revealed.Firstly,the mechanical properties of CsI crystal were studied in this thesis.The elastic constant for CsI crystal was simulated by the molecular dynamics software Materials Studio,the Young’s modulus is 10.08GPa and the Poisson’s ratio is 0.43.Then,three crystal planes which were(100),(110)and(111)of CsI crystal were obtained.The mechanical properties under quasi-static and high strain rates were acquired and analyzed by nano-indentation experiments and split Hopkinson pressure bar(SHPB)experiments,respectively.The experimental results indicated that the mechanical properties of CsI were anisotropic.Nanoindentation experiments showed that the Vickers hardness of(100),(110)and(111)crystal planes were 90MPa,106MPa and 116MPa and the Young’s modulus were 5.39GPa,12.22GPa and 11.98GPa,respectively.SHPB experiments acquired the strength limit for three crystal planes were 50MPa,23MPa and 18MPa,respectively.Secondly,the machining parameters for single point diamond ultra-precision turning(SPDT)process on different planes of CsI crystal was optimized.There was no obvious variation for cutting force and cutting temperature in ultra-precision turning CsI crystal.The influence law for tool rake angle,crystal orientation,cutting speed and other parameters on machined surface quality was studied.Therefore,the impact of them could be ignored.Experimental results indicated that there was obvious anisotropy for the surface roughness Sa after ultra-precision turning.When the same cutting parameters were adopted,there were significant differences for surface roughness between different crystal planes,and the surface roughness in the same crystal plane was also different after turning along different orientations.Among them,machined surface roughness of(100)plane was the lowest,which was below Sa 4nm.Tool rake angle had significant effect on surface roughness,diamond tool with 0°rake angle should be used in ultra-precision turning of CsI crystal,and surface roughness below Sa 10nm could be obtained on all crystal planes.The influence law for cutting parameters on surface roughness was obtained.The surface roughness would significantly increase with the increase of the feed rate and decrease with the increase of the rotation speed.However,the depth of cut had little impact on the surface roughness.When the rotational was 2000r/min,the feed speed was 2μm/r and the depth of cut was 2μm,the smooth surface with Sa<10nm can be obtained on any crystal.Finally,the impact for micro texture single crystal diamond tool on the surface quality of CsI was studied.The results showed that when the micro-texture was adopted in ultra-precision turning of CsI crystal,the surface roughness could be lower than that of the tool without microtexture.The main reason was that the parallel grooves micro-texture tool could effectively improve the phenomenon of chip accumulation and sticking on rake face.Among them,when the micro-texture that the distance from the cutting edge was 12.6μm was utilized,the machined surface roughness was the lowest.When the rotational speed was 2000r/min,the feed rate was 6μm/r and the depth of cut was 5μm,the mean surface roughness was Sa 5.45nm.Compared with the tool without micro-texture,the reduction was 30%.