Study On The Monocrystal Silicon Nanometric Grinding By Molecular Dynamics Simulations

Ultra-precision grinding technology is at the forefront of modern manufacture, and plays an important role in developing our country's future IC industry. In ultra-precision, especially nanometric grinding process, chip removal takes place in a limited region containing only a few atoms or atomic layers. So some phenomena including energy dissipation, machined surface formation and chip removal, and so on, differ from those of general grinding process. It is extremely difficult to observe and measure various microscopic physical phenomena occurring in nanometric machining through experiments, nor can the conventional theory based on "continuum mechanics" explain these phenomena. And many facts have proven that molecular dynamics (MD) approach is a very effective tool for prediction and analysis of ultra-precision machining in theory, which provides a shortcut from micro phenomena to macro characteristics. MD method has many advantages, such as experimental test, security, advance study, reduction of experiments, etc. Since then, MD simulation has been applied to a wide range of fields, including physics, chemistry, biology, medicine and material, to name a few. It also has been introduced to machining in 1990's.Through profound research of basic MD theory and method, Debye model is introduced from solid-state physics for conversion between kinetic energy and temperature of the silicon atom, the grinding model of monocrystalline silicon are established. Based on the detail investigation of MD parallel algorithms, a new MD parallel algorithm in which the spatial domain is divided twice is developed according to the spatial decomposition, an atom transferring strategy based on "invariable sequence number" is designed and the concept of "atom relative list" is proposed by studying the listing method of atom neighbor list. The application of all these stratygies greatly simplies the programming, reduces the probability of error in programming and also saves the communication overhead and the simulation time. The lenovo shenteng 1800 server works out the newly developed MD parallel program and the results show that the simulation scale is expanded from several thousand atoms of serial program to one hundred thousand level compared with the MD serial programe. Accordingly the computing time can be decreased to 1/10 of serial computing time at most. And the parallel program also has good accelerator, parallel efficency and augmentability.Besides, the grinding processes are simulated with the help of MD approach. Nanometric grinding mechanism is analyzed from the viewpoint of instantaneous distribution of atoms, grinding force, potential energy between silicon atoms and depth of damage layers. It is found that subsurface damage of the monocrystal silicon is mainly concerned with the variation of potential energy between silicon atoms. On atomic scales, the depth of subsurface damage layer of monocrystal silicon is defined as the maximal thickness of the atomic layers with random array in the subsurface of monocrystal silicon in the direction of grinding depth. Furthermore, under the conditions of present simulations, it is discovered that the subsurface damage is mainly composed of the amorphous layers, no obvious dislocations are found and the sorption between silicon atoms and diamond atoms is occurred due to the surface effect of the single grit.Moreover, the effects of cutting edge radius, cut depth and grinding speed on grinding mechanism and subsurface damage are studied in response to simulation results of different grinding conditions from the viewpoint of theory. It is shown that cutting edge radius, cut depth and grinding speed have little effect on the mechanism of the nanometric grinding and there are some differences in the value of the grinding force, potential energy between silion atoms and the depth of subsurface damage layers. From the results of MD simulations, it is shown that when the cutting edge radius increases in the nanometric grinding process with the same cut-depth and grinding speed and the depth of damage layers will become larger, which accord with those of the conventional grinding process. Then, when cut depth rises, both the depth of damage layers will increase. When the grinding speed ranges between 20m/s and 200m/s, the depth of damage layers doesn't change much with the increase of the grinding speed under the same cutting edge radius and cut depth conditions. That means MD simulation is not sensitive to the change of the grinding speed, thus advancing the grinding speed properly can shorten the simulation time and enlarge the simulation scale. Finally, to enlarge the scale of the MD simulation, the MD parallel software is developed, which can simulate the monocrystal silicon grinding process with several hundred million atoms. A new hollowing-out data filtering algorithm is deveopped to realize the visualization of massive particles combining with the mapping method based on the spherical shape. The simulation of monocrystal silicon ultra-precision grinding process is carried out with the aid of the newly developed MD parallel software. Through comparison the MD simulation results and the results of the single grit ultra-precision grinding experiments and the nanometric scratching experiments by means of atomic force microscope (AFM), it is shown that the theoretical results are very similar to thoses of the experiments from the viewpoint of the depth of the groove, the hight of chip piled up on two sides of the abrasive grain, silicon surface topography after grinding and the main cutting force. That means MD simulation is very effective, reliable and successful to fulfill the investigation on nanometric machining mechanism.