Study On Subsurface Damage And Material Removal In Multilayers By Using A Molecular Dynamics Study

In recent years,nano-multilayer films have been widely used in civil and military fields such as microelectronic mechanical systems,medical polymer materials,aerospace due to its unique physical structure and excellent mechanical properties in practical engineering.However,when the size of the nanocrystalline material reaches several nanometers,the nanocrystalline material exhibits rather different mechanical properties compared to those of the macroscopic machining process due to its nano-effect.Therefore,it is necessary to deeply study the material removal and sub-surface damage mechanism of nanomaterials in the micro-nano machining process in order to optimize the machining performance and improve the processing efficiency.Thereby,further improving the quality and reducing the cost in processing.However,there are some difficulties in the nano-scale machining process,such as controlling difficulty,limited measurement technology,and high research cost,which lead to some difficulties in the experiment,analysis and calculation of the nano-grinding process.At present,molecular dynamics simulation methods are usually used to simulate the micro-nano machining of nanomaterials.This simulation method is helpful to study the mechanical properties and deformation mechanism of nano-multilayer films under high-speed grinding.It is of high reference significance for the mechanical processing of actual micro-nano electromechanical system devices,which can shorten the ultra-precision micro-nano processing cycle.As for the well-attended nano-multilayer film materials,firstly we study its mechanical properties and its unique properties under special conditions,and further study the ultra-precision machining properties of materials based on molecular dynamics simulation.In order to achieve high-efficiency and low-subsurface damage or even non-destructive machining during high-speed grinding process,our work systematically studied the interaction between the tool and the workpiece as well as analyzed the influence of different processing parameters on the nano-machining process.We study the effects of surface integrity and material removal mechanism of multilayer films.The dislocation slip mechanism based on the interface of nano-multilayer films was also discussed.The main research contents and innovative work of this thesis are as follows:(1)Based on the molecular dynamics simulation method,the material removaland subsurface damage mechanism of Ni/Cu nano-multilayer film under high-speed grinding condition with diamond tool were studied.The dislocation slipping mechanism,surface morphology,grinding temperature and cutting force of the workpiece were studied when changing different machining parameters such as grinding speed,tool radius and depth of cut.The results show that during the stable high-speed grinding process of Ni/Cu nano-multilayer film,a higher grinding speed will produce more wear debris,higher machining temperature,lower subsurface damage,and greater cutting force.When the grinding speed is greater than 150 m/s,the subsurface damage of the workpiece slightly increase,which results in larger grinding temperature and a higher cutting force.Accelerating the nucleation and movement of dislocations.As the tool radius increases,the number of ISF atoms increases along with the increasing of defect structure of the workpiece.In other words,the interaction zone of the tool and the workpiece increases with a larger machining radius,and more dislocations nucleate and move within the tool-workpiece interaction zone,resulting in higher grinding temperature and larger grinding forces.The defect structure of the workpiece increase with the increasing of cutting depth,and a lower depth of cut can effectively improve the material removal rate of the Ni/Cu nano-multilayer film and reduce the subsurface damage.(2)The mechanisms of material removal and subsurface damage of bilayers built from two quite different materials,including a soft and ductile metal(Al)and a hard and brittle ceramic(Si),subjected to high speed grinding are investigated.By performing three-dimensional molecular dynamics(MD)simulations,the effects of the tool radius,depth of cut and grinding speed are thoroughly studied in terms of the workpiece deformation,dislocation movement,atomic trajectory,grinding temperature and average grinding force.The strength of metal-ceramics(Al-Si)bilayers is enhanced since the interface hinders the passage of dislocations.The interface in ceramics-metal(Si-Al)bilayers contributes to the ductility of the bilayers by increasing the movability of dislocations when gliding on it.The brittle to ductile transition of bilayers,which strongly depends on the interface debond energy,has a key role in controlling dislocation slipping.The investigation also reveals that the larger tool radius,higher grinding speed or deeper depth of cut result in more chipping volume and higher grinding temperature in both metal-ceramics and ceramics-metal bilayers.At the same machining parameters,the above changes in ceramics-metal(Si-Al)bilayers are more apparent than that in metal-ceramics(Al-Si)bilayers since Si is stiffer and has a higher yield strength than Al.