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" Molecular-Scale Studies of Mechanical Phenomena at the Interface between Two Solid Surfaces: "
Piroozan, Nariman
Sahimi, Muhammad
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Latin Dissertation
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| Language of Document
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English
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| Record Number
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1106486
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TLpq2395712453
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| Main Entry
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Piroozan, Nariman
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Sahimi, Muhammad
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| Title & Author
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Molecular-Scale Studies of Mechanical Phenomena at the Interface between Two Solid Surfaces:\ Piroozan, NarimanSahimi, Muhammad
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| College
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University of Southern California
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| Date
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2019
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| student score
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2019
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| Degree
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Ph.D.
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| Page No
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82
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| Abstract
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Friction is an essential part of human experience, as it is a key component in our ability to gain traction to walk, stand, work and drive, and also save energy. While friction is certainly a necessary phenomenon, it presents one glaring problem. We need energy to overcome the resistance to motion that is caused by frictional forces, and too much of it causes excess cost, increased energy to perform work and, therefore, increased inefficiencies as well. It is the purpose of this Thesis, then, to present an advance towards two interconnected objectives in this regard. The first is a better understanding of the mechanical and energetic precursors that give rise to friction and, then, to present applicable results in this regard. These objectives are studied in this Thesis, including the tribological properties of silicon carbide, a material commonly used in high-temperature conditions, and the superlubric properties of graphene, an excellent solid lubricant, and its behavior coupled with a series of precious metals. The final chapter will study friction between sandstone surfaces, represented by quartz, one of the most common terrestrial minerals, and its frictional properties at subseismic sliding conditions. Sliding friction between two SiC surfaces is important, due to its relevance to many practical applications. It is also important to study whether kinetic friction at the nanoscale follows Coulomb's law. Since SiC exists both as an amorphous material and with a crystalline structure, the effect of surface roughness on the kinetic friction may also be significant. We report the results of extensive molecular dynamics simulation of sliding friction between surfaces of the two types of SiC over a wide range of sliding velocities. The amorphous SiC was generated by the reactive force field ReaxFF, which was also used to represent the interaction potential for the simulation of sliding friction. As the sliding velocity increases, bond breaking occurs at the interface between the two surfaces, leading to their roughening and formation of excess free volume. They reduce the kinetic friction force, hence resulting in decreasing the difference between kinetic friction in the amorphous and crystalline surfaces. The average kinetic friction force depends nonlinearly on the sliding velocity V, implying that Coulomb's law of friction is not satisfied by the surfaces that we study at the nanoscale. The average kinetic friction force Fₖ depends on V as, Fₖ ∝ ln V. The state of extremely low friction, known as superlubricity, has very important applications to the development of various types of materials, including those that are invaluable to the goal of reducing energy loss in mechanical systems, and those in complex gearing and bearing systems. One material that can produce very low friction is graphene that offers distinct properties as a solid-state lubricant, and can potentially be used as a coating material on surfaces. We have carried out extensive molecular dynamics (MD) simulations in order to study and compute the friction force between a graphene nanoribbon and an inert metal surface. The metal surfaces that we study are those of Au, Ag, and Pt, all of which are used in various instruments, as well as in various materials employed in the industry. Consistent with very recent experiments, the Au-graphene system exhibits superlubricity, but the Ag-graphene and Pt-graphene pairs manifest friction forces higher than that of Au-graphene, although they are still very small. The MD simulations indicate that the average friction force for graphene on an Au surface is approximately 1.5 pN, with the corresponding values being about 6 pN and 11 pN for, respectively, Ag and Pt surfaces. The frictional behavior of faults during seismic events, such as earthquakes, is a topic of profound importance. The experimentally-observed degradation of shear strengths in silicate rocks has been hypothesized to emanate from the phenomenon of “flash heating.” Using extensive MD simulations, we study this phenomenon at interstitial asperities for quartz crystals and present extensive results on the mechanical and thermal evolution of the material under shearing and various sliding velocities. Furthermore, we also present the evolution as a function of varying the thickness of the two layers sliding with respect to each other. We discover that with an increase in the sliding velocity, the frequency and intensity of flash heating events increase as well. This in turn destabilizes the crystalline structure at the interface between opposing quartz layers, forming an amorphous layer. At low slip rates, the heat generated is able to diffuse away appreciably through the material, resulting in a small temperature rise and, therefore, weak effect on the overall strength of the material. At higher slip rates, however, there is not enough time for the heat generated at the interface to dissipate. This in turn causes an increase in the overall interstitial temperature, which in turn causes decreased material strength, followed by sharply reduced frictional resistance.
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Mechanics
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