Conventional liquid lubricants are faced with limitations under extreme cyclic operating conditions, such as in applications that require lubrication when changing from atmospheric pressure to ultrahigh vacuum and ambient air to dry nitrogen (e.g., satellite components), and room to elevated (>500°C) temperatures (e.g., aerospace bearings). Alternatively, solid lubricant coatings can be used in conditions where synthetic liquid lubricants and greases are not applicable; however, individual solid lubricant phases usually perform best only for a limited range of operating conditions. Therefore, solid lubricants that can adequately perform over a wider range of environmental conditions are needed, especially during thermal cycling with temperatures exceeding 500°C. One potential material class investigated in this dissertation is lubricious oxides, because unlike other solid lubricant coatings they are typically thermodynamically stable in air and at elevated temperatures. While past studies have been focused on binary metal oxide coatings, such as ZnO, there have been very few ternary oxide and no reported quaternary oxide investigations. The premise behind the addition of the third and fourth refractory metals Ti and Zr is to increase the number of hard and wear resistant phases while maintaining solid lubrication with ZnO. Therefore, the major focus of this dissertation is to investigate …

Solid state lithium ion electrolytes are important to the development of next generation safer and high power density lithium ion batteries. Perovskite-structured LLT is a promising solid electrolyte with high lithium ion conductivity. LLT also serves as a good model system to understand lithium ion diffusion behaviors in solids. In this thesis, molecular dynamics and related atomistic computer simulations were used to study the diffusion behavior and diffusion mechanism in bulk crystal and grain boundary in lithium lanthanum titanate (LLT) solid state electrolytes. The effects of defect concentration on the structure and lithium ion diffusion behaviors in LLT were systematically studied and the lithium ion self-diffusion and diffusion energy barrier were investigated by both dynamic simulations and static calculations using the nudged elastic band (NEB) method. The simulation results show that there exist an optimal vacancy concentration at around x=0.067 at which lithium ions have the highest diffusion coefficient and the lowest diffusion energy barrier. The lowest energy barrier from dynamics simulations was found to be around 0.22 eV, which compared favorably with 0.19 eV from static NEB calculations. It was also found that lithium ions diffuse through bottleneck structures made of oxygen ions, which expand in dimension by 8-10% …

Nickel-base superalloys have been emerged as materials for gas turbines used for jet propulsion and electricity generation. The strength of the superalloys depends mainly from an ordered precipitates of L12 structure, so called gamma prime (γ’) dispersed within the disorder γ matrix. The Ni-base alloys investigated in this dissertation comprise both model alloy systems based on Ni-Al-Cr and Ni-Al-Co as well as the commercial alloy Rene N5. Classical nucleation and growth mechanism dominates the γ’ precipitation process in slowed-cooled Ni-Al-Cr alloys. The effect of Al and Cr additions on γ’ precipitate size distribution as well as morphological and compositional development of γ’ precipitates were characterized by coupling transmission electron microscopy (TEM) and 3D atom probe (3DAP) techniques. Rapid quenching Ni-Al-Cr alloy experiences a non-classical precipitation mechanism. Structural evolution of the γ’ precipitates formed and subsequent isothermal annealing at 600 °C were investigated by coupling TEM and synchrotron-based high-energy x-ray diffraction (XRD). Compositional evolution of the non-classically formed γ’ precipitates was determined by 3DAP and Langer, Bar-on and Miller (LBM) method. Besides homogeneous nucleation, the mechanism of heterogeneous γ’ precipitation involving a discontinuous precipitation mechanism, as a function of temperature, was the primary focus of study in case of the Ni-Al-Co …

Bioactive glasses are a class of synthetic inorganic material that have wide orthopedics, dentistry, tissue engineering and other biomedical applications. The origin of the bioactivity is closely related to the atomic structures of these novel glass materials, which otherwise lack long range order and defies any direct experimental measurements due to their amorphous nature. The structure of bioactive glasses is thus essential for the understanding of bioactive behaviors and eventually rational design of glass compositions. In this dissertation, molecular dynamics (MD) and reverse monte carlo (RMC) based computer simulations have been used to systematically study the atomic structure of three classes of new bioactive glasses: strontium doped 45S5 Bioglass®, ZnO-SrO containing bioactive glasses, and Cao-MgO-P2O5-SiO2 bioactive glasses. Properties such as ionic diffusion that are important to glass dissolution behaviors are also examined as a function of glass compositions. The accuracy of structure model generated by simulation was validated by comparing with various experimental measurements including X-ray/neutron diffraction, NMR and Raman spectroscopy. It is shown in this dissertation that atomistic computer simulations, when integrated with structural and property characterizations, is an effective tool in understanding the structural origin of bioactivity and other properties of amorphous bioactive materials that can lead to …

Understanding the relationships between microstructures and properties of materials is a key to developing new materials with more suitable qualities or employing the appropriate materials in special uses. In the present world of material research, the main focus is on microstructural control to cost-effectively enhance properties and meet performance specifications. This present work is directed towards improving the fundamental understanding of the microscale deformation mechanisms and mechanical behavior of metallic alloys, particularly focusing on face centered cubic (FCC) structured metals through a unique computational methodology called three-dimensional dislocation dynamics (3D-DD). In these simulations, the equations of motion for dislocations are mathematically solved to determine the evolution and interaction of dislocations. Microstructure details and stress-strain curves are a direct observation in the simulation and can be used to validate experimental results. The effect of initial dislocation microstructure on the yield strength has been studied. It has been shown that dislocation density based crystal plasticity formulations only work when dislocation densities/numbers are sufficiently large so that a statistically accurate description of the microstructure can be obtainable. The evolution of the flow stress for grain sizes ranging from 0.5 to 10 µm under uniaxial tension was simulated using an improvised model by integrating …