Browsing by Subject "tribology"
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Item A tribological and biomimetic study of potential bone joint repair materials(2009-05-15) Ribeiro, RahulThis research investigates materials for bone-joint failure repair using tribological and biomimicking approaches. The materials investigated represent three different repairing strategies. Refractory metals with and without treatment are candidates for total joint replacements due to their mechanical strength, high corrosion resistance and biocompatibility. A composite of biodegradable polytrimethylene carbonate, hydroxyl apatite, and nanotubes was investigated for application as a tissue engineering scaffold. Non-biodegradable polymer polyimide combined with various concentrations of nanotubes was investigated as a cartilage replacement material. A series of experimental approaches were used in this research. These include analysis of material surfaces and debris using high-resolution techniques and tribological experiments, as well as evaluation of nanomechanical properties. Specifically, the surface structure and wear mechanisms were investigated using a scanning electron microscope and an atomic force microscope. Debris morphology and structure was investigated using a transmission electron microscope. The debris composition was analyzed using an X-ray diffractometer. Nanoindentation was incorporated to investigate the surface nanomechanical properties. Polytrimythelene carbonate combined with hydroxyapatite and nanotubes exhibited a friction coefficient lower than UHMWPE. The nanoindentation response mimicked cartilage more closely than UHMWPE. A composite formed with PI and nanotubes showed a varying friction coefficient and varying nanoindentation response with variation in nanotube concentration. Low friction coefficients corresponded with low modulus values. A theory was proposed to explain this behavior based on surface interactions between nanotubes and between nanotubes and PI. A model was developed to simulate the modulus as a function of nanotube concentration. The boronized refractory metals exhibited brittleness and cracking. Higher friction coefficients were associated with the formation of amorphous debris. The friction coefficient for boronized Cr (~0.06) under simulated body fluid conditions was in the range found in natural joints.Item Evaluation of Quasicrystal Al-Cu-Fe Alloys for Tribological Applications(2013-07-22) Nabelsi, NezarThis research investigated the tribological performance of a composite material, formed from an ultra high molecular weight polyethylene (UHMWPE) matrix and quasicrystalline Al-Cu-Fe alloy powders. An evaluation was conducted for the microstructure, material properties, and tribological performance of quasicrystalline materials formed from Al-Cu-Fe alloys. Arc melting was used as the fabrication technique for these alloys, and some samples were additionally heat treated in an argon environment. Vickers microhardness testing was done to make comparisons to wear rate behavior of the various alloys. Tribological studies were conducted using a linear pinon- desk configuration to evaluate friction and wear. Research indicated the annealed samples of Al-Cu-Fe that formed icosahedral quasicrystalline phases, where the quasicrystalline phase was most dominant of the observed alloys, displayed the greatest wear resistance and hardness. Abrasive wear was observed in each of the samples, as the brittle, hard nature of the quasicrystalline phase would not allow for the ductile adhesion. The addition of small amounts of Al-Cu-Fe quasicrystalline particles, crushed and pulverized from the arc-melted ingots, reduced the coefficient of friction and wear rate of UHMWPE, when added to the polymer.Item Geometric Nanoconfinement Effects on the Electronic and Mechanical Properties of Self-Assembled Molecular Systems(2014-08-20) Ewers, Bradley WilliamWith the ongoing research and development of nanoscale technologies and materials, it becomes increasingly important to understand how local environment influences molecular and material properties. An important factor in this regard is geometric nanoconfinement, for example, the restriction of molecules to nanostructure surfaces. The bulk or average characteristics of materials and molecules do not appropriately define their behavior in these circumstances, and highly localized measurement techniques developed to specifically identify the influence of confinement on their properties is essential to understanding their characteristics and behavior. In this dissertation, two forms of geometric confinement are considered in the context of different molecular properties. First, the role of radial confinement on the tribological properties of self-assembled monolayers (SAMs) is considered. SAMs are an excellent model lubricant for experimental studies of boundary lubrication, and they have been employed as boundary lubricant additives and surface coatings. The lubricated contacts of technologically relevant surfaces, however, consist of asperity interactions, and the summit curvature of these asperities can impact the critical cohesive forces from which the properties of the SAM are derived. Molecular dynamics simulation was employed to understand the influence of nanoscopic surface curvature, as well as surface coverage density, two factors which together contribute to the cohesive forces of SAMs, on their tribological properties. In particular their dissipative potential and effective surface protection were examined, as well as the influence of these factors on the contact mechanics of functionalized nanoasperity contacts. Another mode of geometric confinement studiedin this work is two-dimensional nanoconfinement of molecules and its influence on the mechanism of charge transport in molecular systems. Effective control of charge transport in molecules is essential for molecular modification of CMOS technologies, and is critical in controlling charge carrier dynamics in dye-sensitized photovoltaics. In this work, the size dependence of the electronic properties of thiol-tethered zinc porphyrin aggregateson the Au(111) surface was investigated. AFM nanolithography was used to confine these molecules within an alkanethiol matrix on the Au(111) surface, forming molecular islands of specific dimensions to investigate the relationship between island size and charge transport, demonstrating a shift from tunneling based charge transport to the more tunable and efficient charge hopping based transport.Item High Fidelity Simulation of Rotordynamic Morton Effect by Nonlinear Transient Approach(2014-08-06) Suh, JunhoThe present study is focused on accurate prediction of Morton effect problem including journal asymmetric heating and the corresponding long period amplitude oscillations using a nonlinear time transient rotordynamic simulation. For the analysis of the Morton effect problem, variable viscosity Reynolds equation and three-dimensional energy equation are coupled via temperature and viscosity, and solved simultaneously. Three-dimensional heat transfer equations of bearing and shaft are modeled by a finite element method, and thermally coupled with the fluid film via heat flux boundary condition. Asymmetric heat flux into the synchronously whirling rotor is solved by the orbit time averaged heat flux from fluid film to the spinning journal surface. The journal orbit is calculated by the nonlinear transient dynamic analysis of rotor-bearing system with variable time step numerical integration scheme. For the computation time reduction, modal coordinate transformation is adopted for dynamic and thermal transient analysis. This research explains how the thermal bow induced imbalance force develops in spinning journal with time, and how the vibration level is affected by the thermal bow vector. This dissertation is also focused on a new modeling method of three-dimensional thermo-elasto-hydro-dynamic cylindrical pivot tilting pad journal bearing (TPJB). For the computational efficiency, modal coordinate transformation is utilized in the flexible pad dynamic model, and pad dynamic behavior is represented only by means of modal coordinate. Fluid film thickness is calculated by a newly developed node based method, where pad arbitrary thermal and elastic deformation, and journal thermal expansion are taken into account simultaneously. This paper presents a new analysis method for a thermo-elasto-hydro-dynamic tilting-pad journal bearing system to reach a static equilibrium condition adopting nonlinear transient dynamic solver. In the nonlinear transient dynamic solver, physical and modal coordinates co-exist for computational efficiency, and transformation between modal and physical coordinate is performed at each numerical integration time step. Nonlinear time transient dynamic analysis and steady thermal analysis are combined to find the static equilibrium condition of the TPJB system, where the singular matrix issue of flexible pad finite element (FE) model is resolved.Item Micro- and nano- scale experimental approach to surface engineer metals(Texas A&M University, 2007-09-17) Asthana, PranayThis thesis includes two parts. The first part reviews the history and fundamentals of surface science and tribology. The second part presents the major research outcomes and contributions. This research explores the aspects of friction, wear, and surface modification for tribological augmentation of surfaces. An effort has been made to study these aspects through gaining insights by fundamental studies leading to specific practical applications in railroads. The basic idea was to surface engineer metals for enhanced surface properties. A micro- and nano- scale experimental approach has been used to achieve these objectives. Novel principles of nano technology are incorporated into the experiments. Friction has the potential to generate sufficient energy to cause surface reactions through high flash temperatures at the interface of two materials moving in relative motion. This allows surface modifications which can be tailored to be tribologically beneficial through a controlled process. The present work developed a novel methodology to generate a functional tribofilm that has combined properties of high hardness and high wear resistance. A novel methodology was implemented to distinguish sliding/rolling contact modes during experiments. Using this method, a super hard high-performance functional tribofilm with ????????????regenerative???????????? properties was formed. The main instrument used in this research for laboratory experiments is a tribometer, using which friction, wear and phase transformation characteristics of railroad tribo-pairs have been experimentally studied. A variety of material characterization techniques have been used to study these characteristics at both micro and nano scale. Various characterization tools used include profilometer, scanning electron microscope, transmission electron microscope, atomic force microscope, X-ray diffractometer, nanoindenter, and X-ray photon spectroscope. The regenerative tribofilms promise exciting applications in areas like gas turbines, automotive industry, compressors, and heavy industrial equipment. The outcome of this technology will be an economical and more productive utilization of resources, and a higher end performance.Item Surface Properties of Advanced Materials and Their Applications in Ballistics(2010-07-23) Yun, HuisungThis thesis research investigates the surface properties and performances of gold nanoparticles, microarc oxidation coating, and epitaxial nano-twinned copper film. The research aims to understand the critical behavior of material surfaces in order to facilitate design and development of new materials for tribological applications. The research will focus on improving of the gun barrel performances. Experimental approaches will be used for combining analysis with basic thermal energy transfer principles. Results obtained here will be used for developing new materials to be used in facilitating gun barrels. Experimental approach includes scanning calorimetry-thremogravimetric analysis, tribological testing, and potentiodynamic polarization. The fundamental understanding obtained here will be beneficial for the gun barrel design, manufacturing, and military technologies followed by the results of experiments with different three types of materials. The results of this research showed that the coatings with microarc oxidation and nano-twinned structure improved wear resistance from the tribological examinations and size of AuNPs affected their thermal behaviors measured by differential scanning calorimetry and thermogravimetric analysis method.Item The influence of hydrogen gas exposure and low temperature on the tribological characteristics of ti-6al-4v(2009-05-15) Gola, Ryan TravisThis research studies individual and combined effects of hydrogen gas exposure and low temperature on the tribological characteristics of Ti-6Al-4V. Experimental approaches include test system modification and tribological analysis. An existing ballon- disk tribometer was modified to allow liquid nitrogen to be constantly injected into an insulated test chamber to enable testing at low temperature. Twelve 3.8 cm diameter Ti-6Al-4V disks were manufactured and polished, then half were exposed to pure hydrogen gas at elevated temperature and pressure and the remaining disks were untreated. The testing was split in to four groups of three disks based on testing temperature and previous hydrogen exposure. A silicon nitride ball was used for all tests. Each group was tested at two normal loads, 10N and 20N, at the same linear speed. Group 1 was unexposed and tested at room temperature, Group 2 was unexposed and tested at low temperature, Group 3 was exposed and tested at room temperature and Group 4 was exposed and tested at low temperature. Average friction coefficients and the specific wear rate were calculated from the test data. Also high-resolution digital microscope imaging was used to observe and characterize the wear mechanisms of the four groups of samples. Results show that hydrogen exposure facilitated adhesive wear of the surface and that low temperature induced a slip-stick wear mechanism under higher loads, but not at lower loads and regardless of exposure to hydrogen gas. This research opens avenues for future investigation in effects of hydrogen and low temperature embrittlement on the tribological performance of materials. With the increasing interests in hydrogen energy, the present work established a foundation for future study.Item Tribochemical properties of metastable states of transition metals(2009-05-15) Kar, PrasenjitMechanical forces can be used to trigger chemical reactions through activating bonds and to direct the course of such reactions in organic materials, particularly in polymers. In inorganic materials, the small molecules present significant challenges in directing the reaction kinetics. This dissertation studied the dynamics and kinetics of oxidation of transitional metals, particularly on tantalum through mechanical forces. This is a new area of research in surface science. Experimentally using a combined electrochemical and mechanical manipulation technique, we compared the equilibrium and non-equilibrium oxidation processes and states of tantalum. An experimental setup was developed with an electrochemical system attached to a sliding mechanical configuration capable of friction force measurement. The surface chemistry of a sliding surface, i.e., tantalum, was controlled through the electrolyte. The mechanical force was fixed and the dynamics of the surface was monitored in situ through a force sensor. The formation of non-equilibrium tantalum oxides was found in fluid environments of hydrogen peroxide, acetic acid and deionized water. We found that the mechanical energy induced the non-stable state reactions leading to metal-stable oxides. Analytically, we compared the energy dispersion, reaction kinetics, and investigate physical chemical reactions. We proposed a modified Arrhenius equation to predict the effect of mechanical energy on non-spontaneous reaction under nonequilibrium conditions. At the end, we also propose a modified Pourbaix diagram known as the Kar-Liang diagram. The Kar-Liang diagram helps to understand the behavior of tantalum under non-equilibrium conditions. A complete understanding of the tribochemical reaction of tantalum is achieved through this dissertation. The dissertation contains six chapters. After the introduction and approach, oxidation of tantalum is discussed in Chapter IV, kinetics in Chapter V. The nonequilibrium Kar-Liang diagram is discussed in Chapter VI, followed by conclusion. This research has important impacts on the field of surface science in understanding the basics of mechanochemical reactions. The resulting theory is beneficial to understand chemical-mechanical planarization (CMP) and to optimize the current industrial processes in microelectronics in making integrated circuits.Item Tribological Properties of Nanoparticle-Based Lubrication Systems(2013-08-02) Kheireddin, BassemNew nanomaterials and nanoparticles are currently under investigation as lubricants or lubricant additives due to their unusual properties compared to traditional materials. One of the objectives of this work is to investigate the tribological properties of these materials in relation to surface topography. Chemical etching and metal evaporation methods were employed to prepare surfaces with various topographies. Surfaces were sheared with the use of a nanotribometer and characterized with an atomic force and scanning electron microscopes. For a system consisting of ZnS nanowires dispersed in dodecane sheared across ductile surfaces, it was found that the geometry of the nanowire relative to the surface topography plays a significant role. Moreover, for brittle surfaces, it was found that beyond a certain roughness the frictional properties remain unchanged. In addition, this work is also intended to explore novel lubricants with nanoparticle additives in efforts to control friction and wear. A system consisting of silica nanoparticles dispersed in ionic liquids was examined at various concentrations. It was found that an optimum concentration of nanoparticles exists and yields the best tribological properties. Such work represents an important step in understanding the tribological properties of nanoparticle lubricant additives in general; one that may ultimately provide the guidelines necessary for designing novel, low-friction, and wear-controlling nanoparticle-based lubrication systems that minimize energy and material losses due to friction.