The motivations for the research issues addressed in this thesis are based on the needs of the aerospace structural analysis and the design community. The specific focus is related to the characterization and shock induced chemical reactions of multi-functional structural-energetic materials that are also know as the reactive structural materials and their reaction capabilities. Usually motivation for selection of aerospace structural materials is to realize required strength characteristics and favorable strength to weight ratios. The term strength implies resistance to loads experienced during the service life of the structure, including resistance to fatigue loads, corrosion and other extreme conditions. Thus, basically the structural materials are single function materials that resist loads experienced during the service life of the structure. However, it is desirable to select materials that are capable of offering more than one basic function of strength. Very often, the second function is the capability to provide functions of sensing and actuation. In this thesis, the second function is different. The second function is the energetic characteristics. Thus, the choice of dual functions of the material are the structural characteristics and energetic characteristics. These materials are also known by other names such as the reactive material structures or dual functional structural energetic materials. Specifically the selected reactive materials include mixtures of selected metals and metal oxides that are also known as thermite mixtures, reacting intermetallic combinations and oxidizing materials. There are several techniques that are available to synthesize these structural energetic materials or reactive material structures and new synthesis techniques constitute an open research area. The focus of this thesis, however, is the characterization of chemical reactions of reactive material structures that involve two or more solids (or condensed matter). The subject of studies of the shock or thermally induced chemical reactions of the two solids comprising these reactive materials, from first principles, is a relatively new field of study. The published literature on ab initio principles or quantum mechanics based approach contains the ab initio or ab initio-molecular dynamics studies in related fields of a solid and a gas. One such study in the literature involves a gas and a solid. This is an investigation of the adsorption of gasses such as carbon monoxide (CO) on Tungsten. The motivation for these studies is to synthesize alternate or synthetic fuel technology by Fischer-Tropsch process. In this thesis these studies are first to establish the procedure for solid-solid reaction and then to extend that to consider the effects of mechanical strain and temperature on the binding energy and chemisorptions of CO on tungsten. Then in this thesis, similar studies are also conducted on the effect of mechanical strain and temperature on the binding energies of Titanium and hydrogen. The motivations are again to understand the method and extend the method to such solid-solid reactions. A second motivation is to seek strained conditions that favor hydrogen storage and strain conditions that release hydrogen easily when needed. Following the establishment of ab initio and ab initio studies of chemical reactions between a solid and a gas, the next step of research is to study thermally induced chemical reaction between two solids (Ni+Al). Thus, specific new studies of the thesis are as follows: 1. Ab initio Studies of Binding energies associated with chemisorption of (a) CO on W surfaces (111, and 100) at elevated temperatures and strains and (b) adsorption of hydrogen in titanium base. 2. Equations of state of mixtures of reactive material structures from ab initio methods 3. Ab initio studies of the reaction initiation, transition states and reaction products of intermetallic mixtures of (Ni+Al) at elevated temperatures and strains. 4. Press-cure synthesis of Nano-nickel and nano-aluminum based reactive material structures and DTA tests to study experimentally initiation of chemical reactions, due to thermal energy input.

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 160-167). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 160-167). Also available on the Internet.

The objective of this work is to evaluate the reaction initiation characteristics of quasi-statically compressed intermetallic-forming aluminum-based reactive materials upon impact initiation, consisting of equi-volumetric tantalum-aluminum, tungsten-aluminum, nickel-aluminum, and pure aluminum. A modified Taylor rod-on-anvil setup was employed to determine the reaction initiation threshold kinetic energy and actual energy for plastic deformation and subsequent reaction. Experimental sample remnants were recovered and examined through X-ray diffraction to determine reaction products.The overall results indicate that of the various intermetallic-forming systems investigated, Ta+Al was the most reactive and was the only system where any reaction products were retrieved. While all of the intermetallic systems reacted in air, only Ta+Al and W+Al reacted in vacuum environment suggesting differences in reaction mechanisms influencing the reactivity of intermetallic mixtures. Based on the threshold energy for onset of reaction it appears that the Ta-Al compacts show reaction conditions below those required for reaction of Al in air. This combined with the fact that Ta+Al compacts also react in vacuum implies that the Ta+Al undergoes anaerobic intermetallic reaction while the other systems react with the oxidation of Al. The effect of compact packing density on the kinetic energy threshold for reaction initiation were also evaluated. It was observed more densely packed Ta+Al and Ni+Al powder compacts react more easily than less densely packed samples. While the effect of packing density is not as obvious in the case of pure Al and W+Al powder compacts. Finally, a particle size effect is seen on Ni+Al on samples of < 92% density where coarser (+325 -200 mesh) equal-volumetric powder mixtures were observed to be more reactive than finer Ni+Al (-325 mesh).

The continuing quest for higher performance levels of modern gas turbine engines has been accompanied by the demand for higher engine operating temperatures. The use of Thermal Barrier Coatings (TBCs) enabled gas turbines to operate at higher temperatures by protecting the blade material (nickel superalloy) while operating in extreme environments. The TBC system typically consists of a bond coat for protection of the nickel–based superalloy against oxidation followed by a top coat consisting of a thermally insulating zirconia-yttria. In addition to the complex gradation in phases, the coatings are subjected to continuous oxidation with service exposure, mechanical loading on rotating parts, fatigue, thermal mis-match and temperature gradients. Hence, the study of failure mechanisms of TBCs become important in deciding operational reliability and service life of the coating. As there are many systems in which the operating temperatures are not high enough to warrant the use of the top coat (ceramic layer), the study of failure mechanisms in superalloys coated with only the bond coat continue to be of great interest. The present work concentrates on the fracture behavior of NiAl bond coats on nickel superalloy and seeks to evaluate the fracture toughness of the coating through the use of micro-machined samples. A review of the relevant literature indicated that while a considerable body of work exists on bulk tests of failure (spalling, splitting, etc.), not much has been reported in the open literature on the evaluation of basic quantities such as the toughness of the coating itself. The present thesis seeks to establish a protocol for the evaluation of toughness and crack propagation mechanisms in coatings through a combination of micro-sample testing that allows fracture to be correlated with location in the film and the use of an analytical model to quantitatively evaluate stress intensity factors in a bi-material system. A system of NiAl coating produced by pack aluminizing is studied for the fracture properties of the coating. Specimen geometries are optimized to enable micro-cracks to be machined and propagated in a low load testing system, such as a depth sensing indenter, so that crack lengths (and position relative to the interface) can be correlated with load. To enable linear elastic theory to be used, dimensions are determined that allow fracture before general yielding. A three point bending test using miniaturized micro-beam specimens of ~ 4 X 0.3 X 0.3 mm is found to be suitable for the above purpose. The technique is a challenging one that requires focused ion beam machining (FIB) along with careful handling and alignment of small samples. The coatings are characterized for their microstructure by electron microscopy to identify compositional variation across the thickness and to determine the thickness of the coating and inter diffusion zone (IDZ). The crack advancement is monitored with increments of loading and the stress intensity factor is evaluated using a program written in “MAPLE” for an edge crack subjected to bending in a bilayered material. Surprisingly, fracture in this system is found to be stable owing to a gradual increase in toughness from the coating surface to the interface. Such an increase from less than 2 to more than 9 MPa m0.5 may be due to the increasing Ni/Al ratio across the thickness of the bond coat. Crack branching is observed as the crack approaches the IDZ and the reasons for such behaviour are not fully understood. This work establishes the viability of this technique to determine fracture properties in highly graded coated systems and may be readily extended to more complex coating architectures and other forms of loading such as cyclic, mixed mode, etc.