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Functionalization and characterization of porous low-κ dielectrics.

Description: The incorporation of fluorine into SiO2 has been shown to reduce the dielectric constant of the existing materials by reducing the electrical polarizability. However, the incorporation of fluorine has also been shown to decrease film stability. Therefore, new efforts have been made to find different ways to further decrease the relative dielectric constant value of the existing low-k materials. One way to reduce the dielectric constant is by decreasing its density. This reduces the amount of polarizable materials. A good approach is increasing porosity of the film. Recently, fluorinated silica xerogel films have been identified as potential candidates for applications such as interlayer dielectric materials in CMOS technology. In addition to their low dielectric constants, these films present properties such as low refractive indices, low thermal conductivities, and high surface areas. Another approach to lower k is incorporating lighter atoms such as hydrogen or carbon. Silsesquioxane based materials are among them. However, additional integration issues such as damage to these materials caused by plasma etch, plasma ash, and wet etch processes are yet to be overcome. This dissertation reports the effects of triethoxyfluorosilane-based (TEFS) xerogel films when reacted with silylation agents. TEFS films were employed because they form robust silica networks and exhibit low dielectric constants. However, these films readily absorb moisture. Employing silylation reactions enhances film hydrophobicity and permits possible introduction of this film as an interlayer dielectric material. Also, this work describes the effects of SC-CO2 in combination with silylating agents used to functionalize the damaged surface of the ash-damaged MSQ films. Ashed MSQ films exhibit increased water adsorption and dielectric constants due to the carbon depletion and modification of the properties of the low-k material caused by interaction with plasma species. CO2 is widely used as a supercritical solvent, because of its easily accessible critical point, low ...
Date: May 2005
Creator: Orozco-Teran, Rosa Amelia

Materials properties of ruthenium and ruthenium oxides thin films for advanced electronic applications.

Description: Ruthenium and ruthenium dioxide thin films have shown great promise in various applications, such as thick film resistors, buffer layers for yttrium barium copper oxide (YBCO) superconducting thin films, and as electrodes in ferroelectric memories. Other potential applications in Si based complementary metal oxide semiconductor (CMOS) devices are currently being studied. The search for alternative metal-based gate electrodes as a replacement of poly-Si gates has intensified during the last few years. Metal gates are required to maintain scaling and performance of future CMOS devices. Ru based materials have many desirable properties and are good gate electrode candidates for future metal-oxide-semiconductor (MOS) device applications. Moreover, Ru and RuO2 are promising candidates as diffusion barriers for copper interconnects. In this thesis, the thermal stability and interfacial diffusion and reaction of both Ru and RuO2 thin films on HfO2 gate dielectrics were investigated using Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). An overview of Ru and RuO2/HfO2 interface integrity issues will be presented. In addition, the effects of C ion modification of RuO2 thin films on the physico-chemical and electrical properties are evaluated.
Date: May 2006
Creator: Lim, ChangDuk

Mechanical behavior and performance of injection molded semi-crystalline polymers.

Description: I have used computer simulations to investigate the behavior of polymeric materials at the molecular level. The simulations were performed using the molecular dynamics method with Lennard-Jones potentials defining the interactions between particles in the system. Significant effort was put into the creation of realistic materials on the computer. For this purpose, an algorithm was developed based on the step-wise polymerization process. The resulting computer-generated materials (CGMs) exhibit several features of real materials, such as molecular weight distribution and presence of chain entanglements. The effect of the addition of a liquid crystalline (LC) phase to the flexible matrix was also studied. The concentration and distribution of the second phase (2P) were found to influence the mechanical and tribological properties of the CGMs. The size of the 2P agglomerates was found to have negligible influence on the properties within the studied range. Moreover, although the 2P reinforcement increases the modulus, it favors crack formation and propagation. Regions of high LC concentration exhibit high probability of becoming part of the crack propagation path. Simulations of the tensile deformation under a uniaxial force have shown that the molecular deformation mechanisms developing in the material depend on several variables, such as the magnitude of the force, the force increase rate, and the level of orientation of the chains. Three-dimensional (3D) graphical visualization tools were developed for representation and analysis of the simulation results. These also present interesting educational possibilities. Computer simulations provide us information which is inaccessible experimentally. From the concomitant use of simulations and experiments, a better understanding of the molecular phenomena that take place during deformation of polymers has been established.
Date: August 2003
Creator: Simoes, Ricardo J. F.

Microstructural Evolution and Mechanical Response of Materials by Design and Modeling

Description: Mechanical properties of structural materials are highly correlated to their microstructure. The relationship between microstructure and mechanical properties can be established experimentally. The growing need for structural materials in industry promotes the study of microstructural evolution of materials by design using computational approaches. This thesis presents the microstructural evolution of two different structural materials. The first uses a genetic algorithm approach to study the microstructural evolution of a high-temperature nickel-based oxide-dispersion-strengthened (ODS) alloy. The chosen Ni-20Cr ODS system has nano Y2O3 particles for dispersion strengthening and submicron Al2O3 for composite strengthening. Synergistic effects through the interaction of small dispersoids and large reinforcements improved high-temperature strength. Optimization considered different weight factors on low temperature strength, ductility, and high temperature strength. Simulation revealed optimal size and volume fraction of dispersoids and reinforced particles. Ni-20Cr-based alloys were developed via mechanical alloying for computational optimization and validation. The Ni-20Cr-1.2Y2O3-5Al2O3 alloy exhibited significant reduction in the minimum creep rate (on the order of 10-9 s-1) at 800oC and 100 MPa. The second considers the microstructural evolution of AA 7050 alloy during friction stir welding (FSW). Modeling the FSW process includes thermal, material flow, microstructural and strength modeling. Three-dimensional material flow and heat transfer model was developed for friction stir welding process of AA 7050 alloy to predict thermal histories and extent of deformation. Peak temperature decreases with the decrease in traverse speed at constant advance per revolution, while the increase in tool rotation rate enhances peak temperature. Shear strain is higher than the longitudinal and transverse strain for lower traverse speed and tool rotation rate; whereas for higher traverse speed and tool rotation rate, shear and normal strain acquire similar values. Precipitation distribution simulation using TC-PRISMA predicts the presence of η' and η in the as-received AA 7050-T7451 alloy and mostly η in the friction ...
Date: May 2017
Creator: Dutt, Aniket Kumar

Microstructural Phase Evolution In Laser Deposited Compositionally Graded Titanium-Chromium Alloys

Description: A compositionally graded Ti-xCr (10≤x≤30 wt%) alloy has been fabricated using Laser Engineered Net Shaping (LENSTM) to study the microstructural phase evolution along a compositional gradient in both as-deposited and heat treated conditions (1000°C followed by furnace cooling or air cooling). The alloys were characterized by SEM BSE imaging, XRD, EBSD, TEM and micro-hardness measurements to determine processing-structure-property relations. For the as-deposited alloy, α-Ti, β-Ti, and TiCr2 (C15 Laves) phases exist in varying phase fractions, which were influential in determining hardness values. With the furnace cooled alloy, there was more homogeneous nucleation of α phase throughout the sample with a larger phase fraction of TiCr2 resulting in increased hardness values. When compared to the air cooled alloy, there was absence of wide scale nucleation of α phase and formation of ω phase within the β phase due to the quicker cooling from elevated temperature. At lower concentrations of Cr, the kinetics resulted in a diffusionless phase transformation of ω phase with increased hardness and a lower phase fraction of TiCr2. In contrast at higher Cr concentrations, α phase separation reaction occurs where the β phase is spinodally decomposed to Cr solute-lean β1 and solute-rich β2 resulting in reduced hardness.
Date: May 2016
Creator: Thomas, Jonova

Microstructure for Enhanced Plasticity and Toughness

Description: Magnesium is the lightest metal with a very high specific strength. However, its practical applicability is limited by its toughness and reliability. Mg, being HCP has low ductility. This makes the improvement of toughness a grand challenge in Mg alloys. Friction stir processing (FSP) is a thermomechanical technique used to effect microstructural modification. Here, FSP was utilized to affect the toughness of WE43 sheets through microstructural modification. Room temperature Kahn-type tests were conducted to measure the toughness of WE43 sheets. Microscopic techniques (SEM, TEM) was utilized to study the effect of various microstructural factors like grain size, texture, constituent particles, precipitates on crack initiation and propagation. Tensile properties were evaluated by mini-tensile tests. Crack growth in WE43 sheets was also affected by mechanics and digital image correlation (DIC) was utilized to study the plastic zone size. The underlying mechanisms affecting toughness of these sheets were understood which will help in formulating ways in improving it. WE43 nanocomposites were fabricated via FSP. Uniform distribution of reinforcements was obtained in the composites. Improved mechanical properties like that of enhanced strength, increased hardness and stiffness were obtained. But contrary to other metal matrix composites which show reduction in ductility with incorporation of ceramic reinforcements, the nanocomposites showed good strength-ductility combination. The composites were precisely characterized and mechanisms governing this property were studied. The nano-length of the reinforcements was observed to be the main criteria and the dislocation-particle interaction, the main reason behind the strength-ductility property.
Date: August 2016
Creator: Das, Shamiparna

Reactions and Interfacial Behaviors of the Water–Amorphous Silica System from Classical and Ab Initio Molecular Dynamics Simulations

Description: Due to the wide application of silica based systems ranging from microelectronics to nuclear waste disposal, detailed knowledge of water-silica interactions plays an important role in understanding fundamental processes, such as glass corrosion and the long term reliability of devices. In this dissertation, atomistic computer simulation methods have been used to explore and identify the mechanisms of water-silica reactions and the detailed processes that control the properties of the water-silica interfaces due to their ability to provide atomic level details of the structure and reaction pathways. The main challenges of the amorphous nature of the silica based systems and nano-porosity of the structures were overcome by a combination of simulation methodologies based on classical molecular dynamics (MD) simulations with Reactive Force Field (ReaxFF) and density functional theory (DFT) based ab initio MD simulations. Through the development of nanoporous amorphous silica structure models, the interactions between water and the complex unhydroxylated internal surfaces identified the unusual stability of strained siloxane bonds in high energy ring structure defects, as well as the hydroxylation reaction kinetics, which suggests the difficulty in using DFT methods to simulate Si-O bond breakage with reasonable efficiency. Another important problem addressed is the development of silica gel structures and their interfaces, which is considered to control the long term residual dissolution rate in borosilicate glasses. Through application of the ReaxFF classical MD potential, silica gel structures which mimic the development of interfacial layers during silica dissolution were created A structural model, consisting of dense silica, silica gel, and bulk water, and the related interfaces was generated, to represent the dissolution gel structure. High temperature evolution of the silica-gel-water (SGW) structure was performed through classical MD simulation of the system, and growth of the gel into the water region occurred, as well as the formation of intermediate range structural ...
Date: May 2016
Creator: Rimsza, Jessica M

Stable Nanocrystalline Au Film Structures for Sliding Electrical Contacts

Description: Hard gold thin films and coatings are widely used in electronics as an effective material to reduce the friction and wear of relatively less expensive electrically conductive materials while simultaneously seeking to provide oxidation resistance and stable sliding electrical contact resistance (ECR). The main focus of this dissertation was to synthesize nanocrystalline Au films with grain structures capable of remaining stable during thermal exposure and under sliding electrical contact stress and the passing of electrical current. Here we have utilized a physical vapor deposition (PVD) technique, electron beam evaporation, to synthesize Au films modified by ion implantation and codeposited ZnO hardened Au nanocomposites. Simultaneous friction and ECR experiments of low fluence (< 1x10^17 cm^-2) He and Ar ion implanted Au films showed reduction in friction coefficients from ~1.5 to ~0.5 and specific wear rates from ~4x10^-3 to ~6x10^-5 mm^3/N·m versus as-deposited Au films without significant change in sliding ECR (~16 mΩ). Subsurface microstructural changes of He implanted films due to tribological stress were analyzed via site-specific cross-sectional transmission electron microscopy (TEM) and revealed the formation of nanocrystalline grains for low energy (22.5 keV) implantation conditions as well as the growth and redistribution of cavities. Nanoindentation hardness results revealed an increase from 0.84 GPa for as-deposited Au to ~1.77 GPa for Au uniformly implanted with 1 at% He. These strength increases are correlated with an Orowan hardening mechanism that increases proportionally to (He concentration)1/3. Au-ZnO nanocomposite films in the oxide dilute regime (< 5 vol% ZnO) were investigated for low temperature aging stability in friction and ECR. Annealing at 250 °C for 24 hours Au-(2 vol%)ZnO retained a friction coefficient comparable to commercial Ni hardened Au of ~ 0.3 and sliding ECR values of ~35 mΩ. Nanoindentation hardness increases of these films (~2.6 GPa for 5 vol% ZnO) are correlated to ...
Date: May 2016
Creator: Mogonye, Jon-Erik

Structural, Thermal and Acoustic Performance of Polyurethane Foams for Green Buildings

Description: Decreasing the carbon footprint through use of renewable materials has environmental and societal impact. Foams are a valuable constituent in buildings by themselves or as a core in sandwich composites. Kenaf is a Southeast USA plant that provides renewable filler. The core of the kenaf is porous with a cell size in a 5-10 micrometer range. The use of kenaf core in foams represents a novel multiscalar cellular structural composite. Rigid polyurethane foams were made using free foaming expansion with kenaf core as filler with loadings of 5, 10 and 15 %. Free foaming was found to negatively affect the mechanical properties. An innovative process was developed to introduce a constraint to expansion during foaming. Two expansion ratios were examined: 40 and 60 % (decreasing expansion ratio). MicroCT and SEM analysis showed a varying structure of open and closed cell pores. The mechanical, thermal insulation, acoustic properties were measured. Pure PU foam showed improved cell size uniformity. Introducing kenaf core resulted in decreasing the PU performance in the free expansion case. This was reversed by introducing constraints. To understand the combined impact of having a mixed close cell and open cell architecture, finite element modeling was done using ANSYS. Models were created with varying percentages of open, closed, and bulk cells to encompass entire range of foam porosities. Net zero energy building information modelling was conducted using EnergyPlus was conducted using natural fiber composite skins. Environmental impacts for instance global warming potential, acidification, eutrophication, fossil fuel consumption, ozone depletion, and smog potential of the materials used in construction was studied using life cycle assessment. The results showed improvement on energy consumption and carbon footprint.
Date: December 2014
Creator: Nar, Mangesh

Structure Evolution and Nano-Mechanical Behavior of Bulk Metallic Glasses and Multi-Principal Element Alloys

Description: Bulk metallic glasses and multi-principal element alloys represent relatively new classes of multi-component engineering materials designed for satisfying multiple functionalities simultaneously. Correlating the microstructure with mechanical behavior (at the microstructural length-scales) in these materials is key to understanding their performance. In this study, the structure evolution and nano-mechanical behavior of these two classes of materials was investigated with the objective of fundamental scientific understanding of their properties. The structure evolution, high temperature nano-mechanical behavior, and creep of two Zr-based alloys was studied: Zr41.2Ti13.8Cu12.5Ni10.0Be22 (Vitreloy1) and Zr52.5Ti5Cu17.9Ni14.6All0 (Vitreloy105). Devitrification was found to proceed via the formation of a metastable icosahedral phase with five-fold symmetry. The deformation mechanism changes from inhomogeneous or serrated flow to homogenous flow near 0.9Tg, where Tg is the glass transition temperature. The creep activation energy for Vitreloy1 and Vitreloy105 were 144 kJ/mol and 125 kJ/mol, respectively in the range of room temperature to 0.75Tg. The apparent activation energy increased drastically to 192 kJ/mol for Vitreloy1 and 215 kJ/mol for Vitreloy105 in the range of 0.9Tg to Tg, indicating a change in creep mechanism. Structure evolution in catalytic amorphous alloys, Pt57.5Cu14.7Ni5.3P22.5 and Pd43Cu27Ni10P20, was studied using 3D atom probe tomography and elemental segregation between different phases and the interface characteristics were identified. The structure evolution of three multi-principal element alloys were investigated namely CoCrNi, CoCrFeMnNi, and Al0.1CoCrFeNi. All three alloys formed a single-phase FCC structure in as-cast, cold worked and recrystallized state. No secondary phases precipitated after prolonged heat treatment or mechanical working. The multi-principal element alloys showed less strain gradient plasticity compared to pure metals like Ni during nano-indentation. This was attributed to the highly distorted lattice which resulted in lesser density of geometrically necessary dislocations (GNDs). Dislocation nucleation was studied by low load indentation along with the evaluation of activation volume and activation energy. This was ...
Date: May 2017
Creator: Mridha, Sanghita

Study of Conductance Quantization by Cross-Wire Junction

Description: The thesis studied quantized conductance in nanocontacts formed between two thin gold wires with one of the wires coated by alkainthiol self assembly monolayers (SAM), by using the cross-wire junction. Using the Lorenz force as the driving force, we can bring the two wires in contact in a controlled manner. We observed conductance with steps of 2e2 / h. The conductance plateaus last several seconds. The stability of the junction is attributed to the fact that the coating of SAM improves the stability and capability of the formed contact.
Date: May 2004
Creator: Zheng, Tao

Supercritical Silylation and Stability of Silyl Groups

Description: Methylsilsesquioxane (MSQ) and organosilicate glass (OSG) are the materials under this study because they exhibit the dielectric constant values necessary for future IC technology requirements. Obtaining a low-k dielectric value is critical for the IC industry in order to cope time delay and cross talking issues. These materials exhibit attractive dielectric value, but there are problems replacing conventional SiO2, because of their chemical, mechanical and electrical instability after plasma processing. Several techniques have been suggested to mitigate process damage but supercritical silylation offers a rapid single repair step solution to this problem. Different ash and etch damaged samples were employed in this study to optimize an effective method to repair the low-k dielectric material and seal the surface pores via supercritical fluid processing with various trialkylchlorosilanes. Fourier transform infrared spectroscopy (FTIR), contact angle, capacitance- voltage measurements, and x-ray photoemission spectroscopy, dynamic secondary ion mass spectroscopy (DSIMS), characterized the films. The hydrophobicity and dielectric constant after exposure to elevated temperatures and ambient conditions were monitored and shown to be stable. The samples were treated with a series of silylating agents of the form R3-Si-Cl where R is an alkyl groups (e.g. ethyl, propyl, isopropyl). Reactivity with the surface hydroxyls was inversely proportional to the length of the alkyl group, perhaps due to steric effects. Contact angle measurements revealed that heating the films in ambient diminished hydrophobicity. Depth and surface profiling using (DSIMS) and (XPS) were utilized to develop a model for surface coverage.
Date: May 2006
Creator: Nerusu, Pawan Kumar

Surface Topography and Aesthetics of Recycled Cross-Linked Polyethylene Wire and Cable Coatings

Description: Our research focuses on re-using a waste a material, cross-linked polyethylene abbreviated XLPE, which is a widely used coating for wires. XLPE is strong and has excellent thermal properties due to its chemical structure - what leads to the significance of recycling this valuable polymer. Properties of XLPE include good resistance to heat, resistance to chemical corrosion, and high impact strength. A wire is usually composed of a metal core conductor and polymeric coating layers. One creates a new coating, including little pieces of recycled XLPE in the lower layer adjacent to the wire, and virgin XLPE only in the upper layer. Industries are often wasting materials which might be useful. Mostly, some returned or excess products could be recycled to create a new type of product or enable the original use. This method helps cleaning the waste, lowers the costs, and enhances the income of the manufacturing company. With the changing of the thickness of the outer layer, the roughness changes significantly. Moreover, different processing methods result in surfaces that look differently.
Date: December 2014
Creator: Xie, Wa

Synchrotron Radiation X-Ray Diffraction of Nickel-Titanium Shape Memory Alloy Wires during Mechanical Deformation

Description: Shape memory alloys (SMAs) are a new generation material which exhibits unique nonlinear deformations due to a phase transformation which allows it to return to its original shape after removal of stress or a change in temperature. It shows a shape memory effect (martensitic condition) and pseudoelasticity (austenitic condition) properties depends on various heat treatment conditions. The reason for these properties depends on phase transformation through temperature changes or applied stress. Many technological applications of austenite SMAs involve cyclical mechanical loading and unloading in order to take advantage of pseudoelasticity, but are limited due to poor fatigue life. In this thesis, I investigated two important mechanical feature to fatigue behavior in pseudoelastic NiTi SMA wires using high energy synchrotron radiation X-ray diffraction (SR-XRD). The first of these involved simple bending and the second of these involved relaxation during compression loading. Differential scanning calorimetry (DSC) was performed to identify the phase transformation temperatures. Scanning electron microscopy (SEM) images were collected for the initial condition of the NiTi SMA wires and during simple bending, SEM revealed that micro-cracks in compression regions of the wire propagate with increasing bend angle, while tensile regions tend to not exhibit crack propagation. SR-XRD patterns were analyzed to study the phase transformation and investigate micromechanical properties. By observing the various diffraction peaks such as the austenite (200) and the martensite (100), (110), and (101) planes, intensities and residual strain values exhibit strong anisotropy depending upon whether the sample is in compression or tension during simple bending. This research provides insight into two specific mechanical features in pseudoelastic NiTi SMA wires.
Date: December 2015
Creator: Zhang, Baozhuo

Synthesis and Characterization of Crystalline Assemblies of Functionalized Hydrogel Nanoparticles

Description: Two series monodispersed nanoparticles of hydroxylpropyl cellulose (HPC) and functionalized poly-N-isopropylamide (PNIPAM) particles have been synthesized and used as building blocks for creating three-dimensional networks, with two levels of structural hierarchy. The first level is HPC nanoparticles were made from methacrylated or degradable cross-linker attached HPC. These nanoparticles could be stabilized at room temperature by residual methacrylate or degradable groups are present both within and on the exterior of HPC nanoparticles. Controlled release studies have been performed on the particle and networks .The nearly monodispersed nanoparticles have been synthesized on the basis of a natural polymer of hydropropylcellulose (HPC) with a high molecular weight using the precipitation polymerization method and self-assembly of these particles in water results in bright colors. The HPC nanoparticles can be potential using as crosslinkers to increase the hydrogels mechanical properties, such as high transparency and rapid swelling/de-swelling kinetics. The central idea is to prepare colloidal particles containing C=C bonds and to use them as monomers - vinylparticles, to form stable particle assemblies with various architectures. This is accomplished by mixing an aqueous suspension of hydrogel nanoparticles (PNIPAM-co-allylamine) with the organic solvent (dichloromethane) to grow columnar crystals. The hydrogels with such a unique crystal structure behavior not only like the hydrogel opals, but also have a unique property: anisotropy.
Date: December 2005
Creator: Cai, Tong

Ternary Oxide Structures for High Temperature Lubrication

Description: In this research, a temperature dependent tribological investigation of selected ternary oxides was undertaken. Based on the promising results of previous studies on silver based ternary oxides, copper based ternary oxides were selected to conduct a comparative study since both copper and silver are located in the same group in the periodic table of the elements. Two methods were used to create ternary oxides: (i) solid chemical synthesis to create powders and (ii) sputtering to produce thin films. X-ray diffraction was used to explore the evolution of phases, chemical properties, and structural properties of the coatings before and after tribotesting. Scanning electron microscopy, Auger scanning nanoprobe spectroscopy, and X-ray photoelectron spectroscopy were used to investigate the chemical and morphological properties of these materials after sliding tests. These techniques revealed that chameleon coatings of copper ternary oxides produce a friction coefficient of 0.23 when wear tested at 430 °C. The low friction is due to the formation of copper tantalate phase and copper in the coatings. All sputtering coatings showed similar tribological properties up to 430 °C.
Date: August 2015
Creator: Gu, Jingjing

Thermomechanical Processing, Additive Manufacturing and Alloy Design of High Strength Mg Alloys

Description: The recent emphasis on magnesium alloys can be appreciated by following the research push from several agencies, universities and editorial efforts. With a density equal to two-thirds of Al and one-thirds of steel, Mg provides the best opportunity for lightweighting of metallic components. However, one key bottleneck restricting its insertion into industrial applications is low strength values. In this respect, Mg-Y-Nd alloys have been promising due to their ability to form strengthening precipitates on the prismatic plane. However, if the strength is compared to Al alloys, these alloys are not attractive. The primary reason for low structural performance in Mg is related to low alloying and microstructural efficiency. In this dissertation, these terminologies are discussed in detail. A simple calculation showed that the microstructural efficiency in Mg-4Y-3Nd alloy is 30% of its maximum potential. Guided by the definitions of alloying and microstructural efficiency, the two prime objectives of this thesis were to: (i) to use thermomechanical processing routes to tailor the microstructure and achieve high strength in an Mg-4Y-3Nd alloy, and (ii) optimize the alloy chemistry of the Mg-rare earth alloy and design a novel rare—earth free Mg alloy by Calphad approach to achieve a strength of 500 MPa. Experimental, theoretical and computational approaches have been used to establish the process-structure-property relationships in an Mg-4Y-3Nd alloy. For example, increase in strength was observed after post aging of the friction stir processed/additive manufactured microstructure. This was attributed to the dissolution of Mg2Y particles which increased the alloying and microstructural efficiency. Further quantification by numerical modeling showed that the effective diffusivity during friction stir processing and friction stir welding is 60 times faster than in the absence of concurrent deformation leading to the dissolution of thermally stable particles. In addition, the investigation on the interaction between dislocations and strengthening precipitate revealed that, ...
Date: May 2016
Creator: Palanivel, Sivanesh

Thermoplastic and Thermoset Natural Fiber Composite and Sandwich Performance

Description: The objective of this thesis is to investigate the effects of adding natural fiber (kenaf fiber, retted kenaf fiber, and sugarcane fiber) into polymer materials. The effects are obtained by considering three main parts. 1. Performance in thermoplastic composites. The effect of fiber retting on polymer composite crystallization and mechanical performance was investigated. PHBV/PBAT in 80/20 blend ratio was modified using 5% by weight kenaf fiber. Dynamic mechanical analysis of the composites was done to investigate the glass transition and the modulus at sub-ambient and ambient temperatures. ESEM was conducted to analyze fiber topography which revealed smoother surfaces on the pectinase retted fibers. 2. Performance in thermoset composites. The effect of the incorporation of natural fibers of kenaf and of sugarcane combined with the polyester resin matrix is investigated. A comparison of mechanical properties of kenaf polyester composite, sugarcane polyester composite and pure polyester in tensile, bending, dynamic mechanical thermal analysis (DMA) and moisture test on performance is measured.. 3. Performance in sandwich composites. The comparison of the performance characteristics and mechanical properties of natural fiber composites panels with soft and rigid foam cores are evaluated. A thorough test of the mechanical behavior of composites sandwich materials in tensile, bending and DCB is presented here.
Date: May 2014
Creator: Yang, Bing

Tribological Behavior of Spark Plasma Sintered Tic/graphite/nickel Composites and Cobalt Alloys

Description: Monolithic composites are needed that combine low friction and wear, high mechanical hardness, and high fracture toughness. Thin films and coatings are often unable to meet this engineering challenge as they can delaminate and fracture during operation ceasing to provide beneficial properties during service life. Two material systems were synthesized by spark plasma sintering (SPS) and were studied for their ability to meet these criteria. A dual hybrid composite was fabricated and consisted of a nickel matrix for fracture toughness, TiC for hardness and graphite for solid/self‐lubrication. An in‐situ reaction during processing resulted in the formation of TiC from elemental Ti and C powders. The composition was varied to determine its effects on tribological behavior. Stellite 21, a cobalt‐chrome‐molybdenum alloy, was also produced by SPS. Stellite 21 has low stacking fault energy and a hexagonal phase which forms during sliding that both contribute to low interfacial shear and friction. Samples were investigated by x‐ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x‐ray spectroscopy (EDS), and electron back‐scattered diffraction (EBSD). Tribological properties were characterized by pin on disc tribometry and wear rates were determined by profilometry and abrasion testing. Solid/self‐lubrication in the TiC/C/Ni system was investigated by Raman and Auger mapping. A tribofilm, which undergoes a stress‐induced phase transformation from polycrystalline graphite to amorphous carbon, was formed during sliding in the TiC/C/Ni system that is responsible for low friction and wear. TiC additions help to further decrease wear. Stellite 21 was also found to exhibit acceptably low friction and wear properties arising from the presence of Cr23C6 in the matrix and work hardening of the cobalt and chromium during sliding.
Date: December 2013
Creator: Kinkenon, Douglas