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Nano-machining Mechanism And Characterization Of Machined Crystalline Materials By Raman Spectroscopy

Posted on:2013-06-30Degree:MasterType:Thesis
Country:ChinaCandidate:F F XuFull Text:PDF
GTID:2251330392469977Subject:Instrument Science and Technology
Abstract/Summary:PDF Full Text Request
Full understanding of material removal mechanism in nanometric scale is helpfulin improving the machined surface quality, enhancing the machining accuracy,finding the ductile-brittle transition mechanism of brittle materials. Based on these,different methods could be used to modify the machinability of brittle materials,suppress the wear of diamond tool, improve the productivity, reduce themanufacturing costs, and finally promote the extensive application of ultra-precisionmachining in manufacturing free-form surfaces.The research results of this paper are shown as follows:(1) Influences of negative rake angle to the shearing plane are analyzed and therelationship between critical rake angle and coefficient of friction is found inconventional machining. Same method has been used to investigate influence of tooledge to the plastic flow of materials in front of the cutting tool. The stagnation pointof material in the tool edge has been calculated by the coefficient of friction betweenthe diamond tool and workpiece;(2) Size effects in the cutting process have been analyzed from the energyexhausting aspect. As the depth of cut decreases, materials under cutting needs moreexternal stress to initiate plasticity carriers like dislocations in the materials tomaintain continuous plastic deformation in the cutting process. The larger externalstress enlarges the friction between the tool and workpiece and makes most externalwork transformed to heat. It increases the cutting temperature which would soften themachined material and on contrast aggravate the wear of cutting tool;(3) Basic theories of tapper cutting and turning of brittle materials have beendiscussed. Ultra-precision machining experiment has been exerted on single crystalsilicon and gallium arsenide. Subsurface damage of them has been measured byRaman spectroscopy. Results show that the damage layer is constituted of aamorphous layer and a residual stress layer. The thickness of amorphous layer andresidual stress increases with the depth of cut. And the deeper depth of cut makes theresidual stress inhomogeneous. The crystalline Raman line of single crystal siliconwould split into two or three peaks due to the inhomogeneous residual stress. Theductile turned silicon surface has a thin amorphous layer which is less than5nm and slight residual compressive stress in it;(4) A fiber-coupled micro-Raman spectrometer is used to detect phasetransformation of silicon under cutting process. Results show that the single crystalsilicon would transform to amorphous phase which dominates the ductile deformationof silicon and suppress the fracture during machining. The amorphous silicon shouldhave some properties of amorphous materials like liquidmetal or polymers that thehardness decreases with the increase of temperature. Therefore the deeperductile-brittle transition point of silicon at the lager cutting speed could be explainedby the improved ductile deformation ability of amorphous silicon in high temperature.There exist two distinct atomic environments: one solidlike and the other liquidlike inamorphous silicon. The liquidlike component can be viewed as a ‘‘plasticity carrier’’in amorphous silicon, much like dislocations are ‘‘plasticity carriers’’ in crystallinematerials. And for this reason the removal of materials could be seen in the extrusionway, especially while the depth of cut is in the nanometric scale.
Keywords/Search Tags:Nano-mchining mechanism, Size effects, Single crystal silicon, Raman spectroscopy, Subsurface damage, Phase transformation, Dislocation
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