Study Of Material Removal Mechanism In Nanomechanical Machining Of Organic Glass

The requirements on accuracy and surface finish of optical components arebecoming higher to satisfy the development of science and technology. As animportant manufacturing technique of micro-optical components, nanomechanicalmachining has been increasingly applied in optical engineering area. However, theimprovement of nanomechanical machining quality is severely constrainedwith the processing theory, especially with the lack of material removalmechanism.Organic material of PMMA is one of the most important materials for thehigh-precision optical components because of its attractive mechanical, optical andelectrical properties. It has become necessary for a better understanding and control ofthe processing behavior of PMMA. As a typical amorphous polymer, great differencesin physical and mechanical properties are found between PMMA and other materialssuch as metal and glass. The material removal mechanism in the nanomechanicalmachining process of PMMA has not yet been fully understood.The molecular dynamics (MD) which studies the fundamental particlecharacteristic can give unique explanations to some special phenomena that are hardlyexplained by macro theory. MD simulation is an important method in the research ofnanomechanical machining mechanism.In the paper, two models of PMMA of bead-spring model and full-atom modelare constructed. Molecular dynamic simulations are performed to study thenanoscratching behavior of the two models of PMMA.For the bead-spring model of PMMA, the pile-up height, forces and localtemperature in the nanoscratching process are analyzed. The material recoveryproperties under different temperature conditions are investigated. The effects ofdifferent scratching conditions are investigated from the perspective of pile-up height,scratching force and friction coefficient. Simulation results show that the polymermaterial in the scratching zone around the indenter can be removed in a plasticmanner as the local temperature in the scratching zone exceeds glass transitiontemperature Tg. The recovery of polymer can be more significant when thetemperature approaches or exceeds Tg. The tangential force, normal force and friction coefficient increase as the scratching depth increases. A larger scratching velocityleads to more material deformation and higher pile-up. The tangential force andnormal force are larger for a larger scratching velocity whereas the friction coefficientis almost independent of the scratching velocities studied. It is also found that strongerindenter/polymer interaction strength results in a larger tangential force and frictioncoefficient.For the full-atom model of PMMA, the change rules of selected molecular chainsare analyzed. Simulation results are compared and mutually verified with thebead-spring model from the perspective of forces and friction coefficients. Simulationresults reveal that inter-chain sliding and intra-chain change are two main deformationmechanisms governing the permanent deformation of PMMA. When the scratchingdepth is small, the main deformation mechanism is intra-chain change; as thescratching depth become larger, more and more inter-chain sliding happen. Thelong-range mobility of the polymer chains can be stronger and materials can beremoved more easily at higher bulk temperatures. As the scratching velocity increase,more pile-ups are formed with more polymer chains being removed. At strongerindenter/polymer interaction strength, more polymer chains adhere to the surface ofindenter, thus tangential force and friction coefficient are significantly increased.