金刚石刀具机械研磨过程中材料的去除机理_英文_.pdf

【机械类毕业论文中英文对照文献翻译】金刚石刀具机械研磨过程中材料的去除机理

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【机械类毕业论文中英文对照文献翻译】金刚石刀具机械研磨过程中材料的去除机理,机械类毕业论文中英文对照文献翻译,机械类,毕业论文,中英文,对照,文献,翻译,金刚石,刀具,机械,研磨,过程,材料,去除,机理
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第 7 卷 第 1 期 2009 年 1 月 纳 米 技 术 与 精 密 工 程 Nanotechnology and Precision Engineering Vol.7 No.1 Jan. 2009 收稿日期:2008-01-17. 基金项目:国家自然科学基金资助项目(50705025). 作者简介:李增强(1980 ),男,博士研究生.通讯作者:孙 涛,教授,. Material Removing Mechanism for Mechanical Lapping of Diamond Cutting ToolsMaterial Removing Mechanism for Mechanical Lapping of Diamond Cutting Tools LI Zeng-qiang,ZONG Wen-jun,SUN Tao,DONG Shen (Center for Precision Engineering, Harbin Institute of Technology, Harbin 150001, China) Abstract:The material removing mechanism for mechanical lapping of diamond cutting tools was illuminated at the at-omistic scale. In lapping process, phase transformation of the lapping region was the main reason for the material removal. Thus a three-dimensional model of a specimen of the diamond monocrystal and rigid diamond grit was built with the aid of the molecular dynamics(MD)simulation. The force between all of the atoms was calculated by the Tersoff potential. After that, lapping with a certain cutting depth of 1.5 lattice constants was simulated. By monitoring the positions of atoms within the model, the microstructure in the lapping region changes as diamond transformed from its diamond cubic structure to amorphous carbon were identified. The change of structure was accomplished by the flattening of the tetrahedron structure in diamond. This was verified by comparing the radial distribution functions of atoms in the lapping and un-lapping regions. Meanwhile, the debris produced in lapping experiment was analyzed by XRD(X-ray diffraction). The results show that the phase transformation happens indeed. Keywords:diamond cutting tools; mechanical lapping; material removing mechanism; molecular dynamics simulation 金刚石刀具机械研磨过程中材料的去除机理 李增强,宗文俊,孙 涛,董 申 (哈尔滨工业大学精密工程研究所, 哈尔滨 150001) 摘 要:从原子量级揭示了金刚石刀具机械研磨过程中材料的去除机理研磨过程中, 研磨区域原子的相变是材料去除的主要原因 使用分子动力学方法建立了金刚石刀具和刚性金刚石磨粒的三维模型 采用 Tersoff 势函数计算了所有原子之间的作用力, 并以 1.5 个晶格的切深进行了研磨过程模拟 通过观测模型内部原子位置的变化发现,研磨区域刀具原子的晶胞在研磨过程中被压扁, 发生了从金刚石体心立方结构向无定形碳结构的转变 计算了研磨区域原子研磨前后的径向分布函数, 进一步验证了该相变过程 同时, 使用 X 射线衍射方法分析了研磨实验过程中产生的碎屑, 分析结果表明研磨过程中确实有相变发生 关键词:金刚石刀具; 机械研磨; 材料去除机理; 分子动力学仿真 中图分类号:TG580.1 文献标志码:A 文章编号:1672-6030(2009)01-0031-05 It is an important way to turn the optical surface with natural diamond cutting tools to obtain high accu-racy. The processed work-pieces surface has lower sur-face roughness and residual stress, and smaller meta-morphic region than those machined in usual ways. Diamond is the most important material to make cutting tools in the ultra-precision machining, for it is an ideal brittle solid with the greatest hardness and resistance to plastic deformation of any material and has very high dimensional homogeneity. The sharpening method of diamond cutting tools is the key technology to obtain sharp cutting radius, good surface quality and small geometric tolerance1. There are many sharpening methods such as lapping, ion beam sputtering, thermal chemistry polishing, plasma polishing, oxide etching and laser erosion, etc. The most common and effective method is lapping2. The mechanism of the material removal in lapping has a lot of statements, such as the 32 纳 米 技 术 与 精 密 工 程 第 7 卷 第 1 期 micro-cleavage theory3, the thermal abrasion theory4,electro-abrasion theory5 and theory of fracture taking place in the hard direction6,etc. However,these ex-planations are only satisfactory in the particular situa-tion.The explanation accepted by most people is that the hybridized orbit of the carbon converts from sp3 to sp2 in lapping,as demonstrated by van Bouwelen7,Grillo8,Hird and Field9. As yet,few man has veri-fied it at the atomistic level. The extremely powerful technique of molecular dynamics(MD)simulation involves solving the classi-cal many-body problem in contexts relating to the study of matter at the atomistic level. Since there is no alterna-tive approach capable of handling this broad range of problems at the required level of detail,molecular dy-namics methods have been proved indispensable in both pure and applied research,as demonstrated by Rapa-port10. Molecular dynamics analysis is an effective method in studying indentation,adhesion,wear and friction,surface defects and nano-cutting at the atom-istic scale. Nowadays,MD analysis has already been employed to investigate the AFM-based nanolithogra-phy process using an AFM tool11 and atomic surface modification in monocrystalline silicon12. Therefore, it is an efficient way to approach the mechanism of the material removal in lapping using molecular dynamics simulation. From all the above,this study will focus on the material removing mechanism in diamond mechanical lapping using three-dimensional MD simulation. And the microcosmic phenomena in mechanical lapping will be presented and discussed. 1 Methods 1.1 Simulation modeling At the beginning,the mechanical lapping process of diamond cutting tools is introduced. The scaife used was made from a grey cast iron and was medium “striped”(radial grooves to hold diamond grit).It was prepared for use by applying a film of olive oil to the surface,before a few carats of graded diamond grits were rubbed evenly into it. With the scaife running at a high speed,a diamond cutting tool was lapped by ap-plying a load. In this process,the diamond grit was fixed in the scaife. So,the process belongs to the fixed abrasive polishing category13. Therefore,a model of a specimen of the diamond monocrystal and rigid dia-mond grit was built,as shown in Fig.1. Fig.1 Molecular dynamics simulation model of mechanical lapping of diamond cutting tools The crystal lattice of the specimen and the grit be-longed to the diamond cubic system. The lattice con-stant of this system was 0.356 67 nm, which was repre-sented as a. The control volume of the specimen must be large enough to eliminate boundary effects.Taking this into consideration, an optimum control volume was chosen based on an iterative process of increasing the control volume size until further increases did not affect the displacements and velocities of the atoms due to lapping. An optimum size of 50a15a30a was ob-tained, consisting of 183,930 atoms. Moreover, the pe-riodic boundary condition was used in the z-direction to reduce the effects of the simulation scale. The specimen included three kinds of atoms ,namely :boundary atoms, thermostat atoms and Newtonian atoms.To re-strict the rigid-body motion of the specimen, the bound-ary atoms in the left and bottom layers of the specimen that were fixed in space were used to contain the New-tonian atoms.Thermostat atoms were also used to ensure reasonable outward heat conduction away from the con-trol volume.Thermostat atoms and the Newtonian atoms obey the Newtons second law.The top surface of the specimen was(100)surface, which was exposed to the grit.The spherical diamond grit had a radius of 8a,consisting of 17,116 atoms.And it slid on the specimen with the depth of h. Before carrying out the molecular dynamics simulation on the lapping of diamond, it is important to ensure that the chosen potential function gives a reliable result for the simulation. Tersoff potential was used in the present simulation to dictate the interaction among the diamond atoms in this simulation14. The parameters in Tersoff potential for carbon were as follows :A=1,393.6 eV, B=347.6 eV,=34.879 nm1, =22.119 nm1,=1.572,4107,n=0.727,51 ,c=380,49 ,d= 4.384, h=0.570 58, R=0.18 nm, and S=0.21 nm. Posi-2009 年 1 月 李增强等:金刚石刀具机械研磨过程中材料的去除机理(英文) 33 tions and velocities of the atoms were determined by the Verlet method as demonstrated by Maekawa and Itoh15. To simulate lapping under room-temperature conditions, the diamond atoms were arranged in a per-fect diamond cubic structure with the lattice parameters equal to their equilibrium values at an ambient tempera-ture of 293 K. The ambient temperature was maintained by scaling the velocities of the thermostat atoms at every special time step.In this simulation, the 0.5 fs was selected as the time step to obtain a high accuracy. This simulation was calculated by the Lammps software16, and visualized by the VMD software17. The velocity of the lapping was 100a with 1.5a in cut-ting depth and 40a in lapping length. Before the simula-tion, the specimen had been relaxed for 10 000 time steps in order to maintain the thermal balance. 1.2 Experiment The test apparatus of lapping experiment is shown in Fig.2.The abrasive used was diamond grit with an average radius of 0.1 m.They were coated on the scaife in a ring with a radius of 120 mm.The diamond cutting tool was fixed on the arm by a special fixture.Then, the tool was lapped with the scaife running at 3 000 r/min(ca.38 m/s), under a load of 5 N which was ob-tained by adjusting the place of the weight. The debris was collected after 30 min lapping.Thereafter, the XRD studies were carried out by SHIMADZU XRD-6000. Fig.2 Schematic diagram of the lapping apparatus 2 Results and discussions 2.1 Molecular dynamics analysis The 3D view and cross-section view of the simula-tion are shown in Fig.3. The crystal lattices near the diamond grit are distorted when the diamond grit cuts into the specimen.The region including these crystal lattices is half-ellipse in shape.The region is under the diamond grit and a bit left to the center o. And the major axis of the ellipse is in the same direction as the compo-sition of forces. Furthermore,this region moves left as the diamond grit slides. As shown in Fig.4,A1+A2A3,where O1O2 represents the surface of the workpiece.It shows that the removal materials do not pole up on both sides of the groove completely.Some materials are removed and form chips. It is a cutting process. Whereas,the exist-ing A1 and A2 show that ploughing also occurs.So this state is the cutting state accompanied by ploughing. (a)3D view of debris forming (b)A cross-section view of the specimen shown ,in(a)(point o is the center of the grit) Fig.3 Microstructure of specimen after the grit sliding A1, A2Section area of the bulge materials; A3Section area of the groove Fig.4 Section of the grooves in the longitudinal direction There are three key points in lapping,as shown in Fig.5. Firstly,atoms near the diamond grit are forced to make some displacement from their initial posi-tion.The crystal lattices including these atoms distort a little.The boundary between the distorted lattices and the perfect lattices is along the diamond(111)surface(the black lines)as shown in Fig.5(a).The displace-ments of the atoms become bigger and bigger along with the diamond grit sliding left.More and more atoms deviate from their initial position.The lattices including these atoms distort seriously.The phase transformation that the diamond cubic diamond transforms into amor-phous graphite starts on a few atoms(in the dark circles)at the end of this moment.That is to say that the hybridized orbit converts from sp3 to sp2. Secondly,the lattices below the diamond grit have the worst distortion and the boundary faceting along the(111)surface ex-tend to the deeper layer,as shown in Fig.5(b). More atoms transform from diamond cubic diamond to amor-34 纳 米 技 术 与 精 密 工 程 第 7 卷 第 1 期 phous graphite , especially those in the dark cir-cle.Besides, some atoms are taken away by the diamond grit.Thirdly, some lattices revert a little with the force minimizing, as shown in Fig.5(c). However, the atoms which have the phase transformation cannot revert to their initial phase,especially those in the dark circle. Therefore,the groove is to the left on the surface of the diamond specimen. (a)Point o to be about to cross section A (b)Point o on section A (c)Point o passed section A Fig.5 Scattergrams of atoms in longitudinal section A in different states 2.2 Bond formation From the simulation, it is found that the phase transformation is due to the flattening of the tetrahedron structure in diamond cubic diamond, as shown in Fig.6. The position transformation at progressive time steps is demonstrated in Fig.7. Fig.6 Crystal cell of the diamond crystal lattice taken out from the circular region in Fig.5(a) As shown in Fig.7(a), the tetrahedron is de-formed when the grit slides close. And the deforma-tion is serious when the grit cuts into section A,as shown in Fig.7(b). The tetrahedron is flattened a little. Soon after,the tetrahedron deforms badly,as shown in Fig.7(c).Its four vertexes are almost on a plane and some bonds are broken. At the same time the phase transformation is accomplished. Fig.7 Change of the tetrahedron marked in Fig.6 when the grit slides 2.3 Pair correlation function The pair correlation functions of the specimen and the chip are shown in Fig.8 and Fig.9 respectively.The curve in Fig.8 is syllabified to a lot of clear peaks,which are the same as the diamonds radial distribution fuction(RDF). However, there are only two peaks in Fig.9, and the peaks are continued, which illuminates that amorphous exists in debris atoms. Therefore, it is sure that the phase transformation takes place in lap-ping. Fig.8 Pair correlation function of specimen atoms Fig.9 Pair correlation function of debris atoms 2.4 XRD Fig.10 shows the X-ray diffraction(XRD) analy-sis of the debris produced in the lapping experiment. It demonstrates that the amorphous carbon,small dia-mond particles or chips and Fe-C compositions(like 2009 年 1 月 李增强等:金刚石刀具机械研磨过程中材料的去除机理(英文) 35 Fe7C3 and Fe5C2)exist together in the debris. Conse-quently,the amorphous carbon is produced in lapping,which corresponds to the simulation result. Fig.10 XRD analysis of the debris produced in the experiment 3 Conclusions (1)A three-dimensional MD model about the at-oms of diamond cutting tools and diamond grit is built by using the molecular dynamics. Lapping at a special cutting depth is simulated. (2)The boundary of the transformation zone is regular , faceting along(111)surface. The micro-cleavage only occurs inside this boundary. (3)Interaction between the diamond grit and dia-mond specimen leads to a phase transformation event. An amorphous transformation appears as the grit slides. And it is expounded from the comparison between the bond formatting and pair correlation function. Moreover,it has also been proved in the lapping ex-periment. References: 1 Yuan Z J, Yao Y X, Zhou M,et al. Lapping of single crystal diamond toolsJ CIRP Annals-Manufacturing Technology, 2003, 52(1): 285-288. 2 Uegami K, Tamamura K, Jang K K. Lapping and frictional properties of diamond, and characteristics of diamond cutting toolJ Journal of Mechanical Work-ing Technology, 1988, 17(8): 147-155. 3 Tolkowsky M. Research on the Abrading, Grinding or Polishing of DiamondDLondon: City and Guilds College, University of London, 1920. 4 Bowden F P, Tabor D. Physical Properties of DiamondMOxford: Clarendon Press
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