Research On Removal Mechanism Of Sapphire Based On Molecular Dynamics Simulation

Sapphire is widely used in LED lighting,integrated circuits,aerospace and other fields due to its unique crystal structure and excellent physical and chemical properties,becoming an important industrial material.High-precision application require the surface quality of sapphire parts to have nano-level roughness and strictly controllable subsurface defects.Therefore,studying the deformation and removal mechanism of sapphire material has important significance for guiding its ultra-precision processing,but the anisotropy and hard-brittle characteristics of sapphire bring difficulties to this research.In this paper,MD（molecular dynamics）simulation has been used to simulate the nanoindentation and nano-scratching process of sapphire,and the corresponding experimental research has been carried out.The effects of crystal anisotropy,abrasive types and water media on the removal mechanism of sapphire have been investigated from various aspects of scratch load,surface morphology and sub-surface defects.The verification of MD simulation is the bottleneck that affects its engineering application.MD simulation and experiment of sapphire nanoindentation based on the Berkovich indenter have been carried out to study the anisotropy of hardness and verify the reliability of the simulation model.It is found that the loading angle of the Berkovich indenter will significantly affect the indentation hardness of the sapphire.Besides,the plastic deformation mechanism of sapphire with different crystallographic orientation has been explored through nanoindentation simulation based on the spherical indenter,and the relationship between the indentation morphology and its subsurface plastic deformation has been established.The anisotropy of the mechanical properties of sapphire is considered to be related to its dislocation/slip system.The atomic-scale analysis capabilities of MD simulation provide an effective method for exploring the influence of anisotropy on the material removal process.The MD simulations and experiments of nano-scratches along different directions have been carried out on different crystallographic plane of sapphire,and the effect of crystal anisotropy on the removal mechanism has been studied.It is found that the scratches on the A-plane along the[1^<sub>010]direction are easier to remove material,and the scratches on the C-and M-plane along different directions have similar material removal capabilities.Combining the morphology characteristics obtained by simulation and scanning electron microscope,the relationship between the brittle fracture characteristics on the scratched surface and its plastic deformation is established.The anisotropy of the subsurface defect form of sapphire scratches on different crystallographic plane along different directions have been detected by transmission electron microscope.It provides theoretical support for the optimization of the processing direction of sapphire grinding and scribing.The MD simulation has been used to study the influence of different abrasive particles and water media on the sapphire removal mechanism.According to the difference in the interaction relationship between atoms,the difference in the physical phenomena of the contact of different substances is revealed.The contact between alumina abrasive grains and sapphire is manifested as a greater attraction between atoms and a smaller repulsive force,which promotes the amorphization of the surface,thereby increasing the material removal rate.This research provides a theoretical basis for the matching of the physical properties of the sapphire abrasive grain processing abrasive and the workpiece material.In addition,according to the actual working conditions of sapphire processing,the MD simulation study on the influence of water medium on the removal mechanism of sapphire was carried out.Water molecules promote the amorphization of atoms on the processed surface,improve the plastic flow capacity of the sapphire surface,thereby improve the quality of the processed surface and the material removal rate.The water film makes the sub-surface stress distribution of the scratch more uniform,suppresses the slip defects caused by uneven stress distribution,and reduces the thickness of the sub-surface defect layer.