Molecular Dynamics Investigation of Physical Phenomena in Microtribology and Ultraprecision Machining

采用基于Nose-Hoover方法的分子动力学仿真模型，研究了摩擦在纳米加工中的作用，该模型中的受限分析区域随着刀具的前进而移动.在切削方向为[101]、切屑厚度为1 nm、切削速度为20 m/s的条件下，模拟了用刃口半径为3 nm的类金刚石刀具对铜单晶的（111）面进行正交切削。采用具有不同内聚能大小的莫尔斯型势来模拟工具-工件界面以及材料本身的摩擦力，得到的主要结果如下：（1）纳米尺度金属切削中切屑的形成与宏观尺度切削中观察到的切屑形成相似。随着界面结合能的增加，切屑厚度和切屑与刀具的接触长度均变大，导致平均切削力和系统温度增加。此外，加工表面受到严重损伤。(2)刀具磨损也被建模，考虑到工具的碳原子，具有降低的凝聚能和服从牛顿的运动方程。原子磨损的过程是工件和刀具原子之间的相互扩散和磨损原子重新粘附到刀具上的重复过程。(3)界面电位的形状和刀具磨损分布也影响最小切削厚度和位移工件原子存在的区域的深度：两个和十个原子层分别是它们的最小值。(4)存在的原子级缺陷如空位和位错增加了切削力，也扩大了工件中的原子级扰动，这是由于缺陷与从切削刃中出现的位错之间的相互作用。

The role of friction in nano-scale machining has been investigated using a new molecular dynamics simulation model based on the Nose-Hoover method, in which the restricted analytical region moves together with the tool advancement. Orthogonal machining of a (111) plane of a copper single crystal by a diamond-like tool with 3 nm edge radius has been simulated, where the uncut chip thickness and the cutting speed was set at 1 nm and 20 m/s, respectively, and the cutting direction was taken to be [101]. Morse type potentials with various magnitudes of the cohesion energy were postulated to approximate the friction at the tool-workpiece interface as well as the materials themselves.The major results obtained are as follows :(1) Chip formation in the nano-scale metal machining is similar to that observed in macro-scale machining. With increase of the bond energy at the interface, both chip thickness and contact length between the chip and tool become larger, leading to increases in the average cutting forces and the temperature in the system. Besides, the machined surface is subjected to severe damages.(2) Tool wear has also been modelled by taking into account the tool consisting of the carbon atoms which have a reduced cohesion energy and obey Newton's equation of motion. The atomistic wear proceeds with the repetition of inter-diffusion between the workpiece and tool atoms and re-adhesion of the worn atoms to the tool.(3) The shape of interface potential and the tool wear distributions also affect the minimum thickness of cut and the depth of the region where the displaced workpiece atoms exist : two and ten atomic layrs are their minimums, respectively.(4) Existing atomistic defects such as vacancies and dislocations increase the cutting force and enlarge the atomistic disturbance in the workpiece as well, which is due to the interaction between the defects and the dislocations emerging from the cutting edge.