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Mechanical Properties of Lubricants in Nano Powders Compaction Process Using Molecular Dynamics and Continuum Mechanics Methods

Palahang, Pezhman | 2023

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 56252 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Khoei, Amir Reza
  7. Abstract:
  8. The primary objective of this study entails the examination of the impact of lubricants intended for implementation in the compaction process of nano-powders, intending to mitigate friction at the nano scale during compaction. Molecular dynamics modeling is employed to scrutinize this phenomenon and derive the friction parameters within the framework of equations derived from continuum mechanics, based on data acquired from the nanoscale investigations. The manufacturing procedure of metallic components from metal powder compaction necessitates the application of immense pressures, thereby engendering notable friction between the powder particles and the confines of the mold. Consequently, the incorporation of lubricants in the form of thin film amidst the moving atomic surfaces serves to curtail direct contact between solid-to-solid interfaces, subsequently leading to a diminishment in the prevailing friction. In addition to this primary function, lubricants fulfill various secondary roles, encompassing the prevention of heat generation and temperature elevation, as well as corrosion protection, among others. This study initially employs non-equilibrium molecular dynamics simulations to investigate the viscosity, structural arrangement, and tribological properties of short straight, branched, and cyclic chain alkanes in confined systems. The viscosity of each chain type is calculated through simulations at various shear rates, and the results are compared with experimental data. The study finds that non-Newtonian viscosity shows no significant shear rate dependence at low pressures and shear rates less than 1010 (s-1). The Newtonian viscosity, determined using the Eyring model and averaging method, demonstrates the superior accuracy of the averaging approach under low pressures. Furthermore, the study explores film thickness alteration, mass density distribution, flow behavior, and friction phenomenon for the three chain types of alkanes confined between aluminum surfaces at high pressures and varying shear rates. The findings indicate that increasing pressure results in higher density peaks in the lubricant stratification, while higher shear rates lead to reduced variation in density peaks within the fluid layer, resulting in smoother density profiles. Moreover, the study reveals solid-like behavior at low shear rates and high pressures for cyclic-chain alkanes, while higher shear rates exhibit more liquid-like behavior. The study concludes that the friction coefficient increases linearly with a logarithmic shear rate for all alkanes, with a reduced slope observed at higher pressures, particularly for straight chain alkanes, eventually reaching insensitivity to the shear rate at a pressure of 1 GPa. The subsequent discussion revolves around the compaction of aluminum nano powder within a nickel mold, incorporating three distinct compression velocities of 2, 4, and 6 (Å/ps), as well as two primary structures: regular and irregular. Furthermore, two scenarios are considered, one involving the utilization of a lubricant and the other without its implementation. The selection of decane as the lubricant for the compression process is based on its linear chain structure, lower density and optimal molecular packing in the absorption layer, lower friction coefficient at high shear rates, and elevated pressures. The examination of the powder compaction process reveals three discernible stages, namely the transitional restacking stage, the plastic deformation stage, and the cold working and particle attrition stage. By scrutinizing the relative density-pressure and pressure-strain curves, it becomes evident that the irregular arrangement exhibits more favorable behavior during compaction, while the powder with a regular initial structure necessitates higher compaction pressures to attain the same relative density. The regular structure imposes greater shear and radial stresses on the mold wall, consequently inducing more pronounced plastic deformations within the mold. Furthermore, it is observed that an escalation in relative density corresponds to a sharp rise in the friction coefficient during the initial compaction stage until it reaches its maximum value. Subsequently, the friction coefficient gradually declines, and at higher pressures, it tends to approach the standard values associated with dynamic friction coefficient in metal-metal contact. Additionally, a slower compaction velocity allows for a more alteration in powder shape, resulting in a more uniform final product. Moreover, a slower compaction velocity facilitates enhanced dissipation of heat, whereas a swifter compaction speed leads to a significant temperature surge, particularly within the contact atoms layer. The utilization of a lubricant between the powder and the mold is found to reduce friction, thus leading to a 5-10% reduction in the final pressure required for powder compaction. Moreover, the implementation of a lubricant enables effective control of temperature increase within the mold wall, particularly within the contact atoms layer. The use of a lubricant ensures an almost uniform and consistent friction coefficient throughout the powder compaction process. Notably, the incorporation of lubricants in both cases of regular and irregular initial structures yields a substantial reduction in the friction coefficient, reaching a maximum value of 80% to 95% reduction in various instances
  9. Keywords:
  10. Friction ; Non-Equilibrium Molecular Dynamics ; Powder Compaction Process ; Non-Newtonian Fluids ; Lubricants ; Nanotribology

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