Molecular Dynamics Simulation Study Of Diamond Wire Sawing Mechanism In Monocrystalline Silicon

At present,the application of solar energy is developing rapidly because of its advantages such as no risk of depletion,high reliability,low operating cost and no pollution.Solar cell is the system of photovoltaic conversion equipment,and its main raw material of semiconductor is silicon.Monocrystalline silicon occupies a dominant position in the global PV industry due to its advantages of mature material preparation and high photoelectric conversion efficiency.Slicing is the first process of preparing PV cell substrate.The sawing quality plays an important role in the breakage rate of silicon wafer and subsequent texturing effect,which directly determines the production cost of the whole silicon-based solar cell as well as the cell quality.In recent years,diamond wire saws have gradually replaced free abrasive wire saws and become the mainstream technology for monocrystalline silicon slicing due to its advantages of high cutting efficiency,low kerf loss,high precision of slice shape,and environmental friendliness.In this paper,the mechanism of diamond wire sawing of monocrystalline silicon materials is investigated from the atomic scale by using molecular dynamics simulation method.The deformation mechanism of monocrystalline silicon during diamond wire sawing at the atomic scale is simulated.Several aspects are studied in depth,including the removal mechanism of silicon atoms under the diamond wire sawing process,the stresses on silicon crystals and their phase changes,cutting forces and temperatures,and the quality of the processed surface.The main elements are as follows.(1)A simulation model of nanoscale single-crystal silicon diamond wire cutting was established using the molecular dynamics method,and the mechanism of single-crystal silicon diamond wire cutting processing was investigated in depth using crystal structure analysis methods and visualization and analysis of material microstructure changes.The removal patterns of single crystal silicon material from bottom and side sufaces of the diamond grain are analyzed.The stress distribution and phase transition in the workpiece during the cutting process are investigated,and the critical criteria for the occurrence of phase transition under stress are also found.This means that when the hydrostatic stress is greater than about 5.6 GPa and the von Mises stress is greater than about 10 GPa,the single-crystal silicon crystal with a diamond structure will undergo a phase transition.The surface quality of the machined surfaces was also analyzed,and surface craters and microcracks were found to be caused by uneven distribution of stresses on the workpiece atoms within the crystal due to anisotropy.Finally,the generation and distribution of cutting forces and temperatures during machining process were explained and discussed.(2)The effect of changing the three main machining parameters such as diamond grain radius,wire speed and feed rate on the sawing mechanism and quality was simulated and analyzed.Firstly,when changing only the diamond grain radius,it is found that the surface quality of the bottom machined surface of silicon workpiece under the diamond grain decreases gradually with increasing the grain radius,while the thickness of the damage layer on the side surface decreases first,but when the grain radius increases beyond a certain limit,the thickness of the damage layer gradually becomes larger.Secondly,only change the diamond wire speed,all the cutting phenomenon changes can be explained,the faster the diamond wire cutting alignment speed,the better the cut surface quality,but more than a certain speed,the depth of cut is very low,but can not achieve the purpose of material removal,and the damage becomes larger.Finally,when changing the feed rate of diamond wire,it is found that the lower the feed rate,the more the material removal mode is mainly ductile mode,and the better the quality of the cut surface is obtained,but at the same time the cutting efficiency is reduced,and the amount of material removal per unit time is decreased.Less than a certain rate,the depth of cut is extremely low,and the material removal is also not achieved,and the damage becomes larger.Through the study of the effect of the above-mentioned parameter changes on the machining quality,it was found that the machining quality of the diamond wire cutting process can be optimized by adjusting various parameters.In the present study,it was found that the best surface quality and better efficiency were obtained when the RTScenter was kept between 1 and 1.5(3)The effect of vacancy defects on the yield strength and anisotropy of the material was simulated for single-crystal silicon materials.The results show that the yield strength of monocrystalline silicon decreases exponentially with the increase in the size of vacancy defects in all three major crystal directions.However,when comparing the yield strengths between the three main crystal directions of single-crystal silicon,the[1 0 0] crystal direction has the lowest yield strength,while the [1 1 1] crystal direction has the highest yield strength and the highest resistance to the decay of a single vacancy defect.Also,the effect of the number of vacancy defects contained in single crystalline silicon on diamond wire cutting was modeled to simulate and analysis.It is found that as the number of vacancy defects increases,the structural strength of the single crystal silicon decreases and the material removal mode tends to be more ductile.At the same time,the thickness of the amorphous silicon damage layer on the surface of the workpiece machined by diamond grains increases and the surface quality decreases.This paper simulates and deeply analyzes the diamond wire cutting process of single crystal silicon through molecular dynamics simulation,and understands the processing mechanism of single crystal silicon workpiece wire cutting,which provides important theoretical guidance for optimizing the processing process of single crystal silicon diamond wire cutting and improving the processing accuracy to obtain high quality component processing surface.