Molecular Dynamics Investigation of Physical Phenomena in Microtribology and Ultraprecision Machining

采用基于Nose-Hoover方法的分子动力学模拟模型，研究了摩擦在纳米加工中的作用，该模型中限制解析区随刀具推进而移动。模拟了用边缘半径为3 nm的类金刚石刀具正交加工铜单晶（111）平面，其中未切削切屑厚度为1 nm，切削速度为20 m/s，切削方向为[101]。假设具有不同大小内聚能的莫尔斯型势来近似工具-工件界面和材料本身的摩擦。得到的主要结果如下：(1)纳米尺度金属加工中的切屑形成与宏观尺度加工中的切屑形成相似。随着界面结合能的增大，切屑厚度增大，切屑与刀具的接触长度增大，导致平均切削力增大，系统温度升高。此外，加工表面受到严重的破坏。(2)刀具磨损也考虑到碳原子组成的刀具具有降低的内聚能和服从牛顿运动方程。原子磨损是随着工件和刀具原子之间的相互扩散和磨损原子与刀具的重新粘附而进行的。(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区域的深度：最小值分别为2层和10层。(4)现有的空位、位错等原子缺陷增加了切削力，也扩大了工件内部的原子扰动，这是由于缺陷与切削刃产生的位错相互作用造成的。

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.