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Abstract: As thin films are reduced in thickness, allotropic phase transformations to structures that are not the equilibrium phase in the standard state can be stabilized. These polymorphic phase transformations have been referred to as pseudomorphism. Many of these pseudomorphic phases have been serendipitously discovered. For the first time, the use of a classical thermodynamic model has been developed in the prediction of phase stability in Zr/Nb and Ti/Nb multilayered thin film structures. The classical thermodynamic model predicts that in regions of high volume fractions of Nb, the lower volume fraction, or alternatively, the thinner Zr or Ti layer, can be stabilized as a bcc phase rather then an hcp phase. The pseudomorphic phase is stabilized by a reduction in the interfacial free energy. An outcome of the classical thermodynamic model is a new type of phase stability diagram, referred to as a biphase diagram, in predicting which combinations of length scale and volume fraction will stabilize the pseudomorphic or bulk equilibrium phases. The change in hcp to bcc phase stability in Zr and Ti has been confirmed by transmission and reflection x-ray diffraction and electron diffraction. In each case, the Zr or Ti layer adopted a lattice parameter similar to its high temperature beta-bcc lattice parameter. An O-lattice construction, a nearest-neighbor-bond model, and a van der Merwe model have been used to estimate the contributing structural and chemical contributions to the hcp-bcc interfacial free energy reduction value. The Zr/Nb values match well to experimentally determined interfacial free energies that can be calculated from the slopes of the stability boundaries on the biphase diagram. Atom Probe Tomography (APT) results indicated a significant interdiffusion of up to 15 at.% Nb into the Ti layers that helped to facilitate the hcp-bcc transition in Ti. Refinement of the free energy calculations using the APT results brought the predicted and experimental interfacial free energy values in closer agreement for Ti/Nb. The successful prediction of bcc Zr or Ti in volume fraction rich Nb multilayers was used in the prediction and confirmation by x-ray and electron diffraction of a novel bcc to hcp phase stability change for Nb for each multilayer system. The hcp Nb phase has adopted independent hcp lattice parameters from either Zr or Ti. In either the Zr/Nb or Ti/Nb system, the hcp Nb lattice parameters were found to be equivalent. Finally, a series of Ti-8at.%V/Nb multilayers were sputter deposited. The addition of V, a bcc-stabilizer in Ti alloys, has been shown to allow for controlled facilitation of the hcp-bcc phase stability in the Ti layer. This result opens up the possibility for predictive phase engineering of multilayers at a specific layer thickness.
This study highlights the role of nitrogen vacancies and defect structures in engineering hard coatings with enhanced phase stability and mechanical properties for high temperature applications. Titanium aluminum nitride (Ti,Al)N based materials in the form of thin coatings has remained as an outstanding choice for protection of metal cutting tools due to its superior oxidation resistance and high-temperature wear resistance. High-temperature spinodal decomposition of metastable (Ti,Al)N into coherent c-TiN and c-AlN nm-sized domains results in high hardness at elevated temperatures. Even higher thermal input leads to transformation of c-AlN to w-AlN, which is detrimental to the mechanical properties of the coating. One mean to delay this transformation is to introduce nitrogen vacancies. In this thesis, I show that by combining a reduction of the overall N-content of the c-(Ti,Al)Ny (y < 1) coating with a low substrate bias voltage during cathodic arc deposition an even more pronounced delay of the c-AlN to w-AlN phase transformation is achieved. Under such condition, age hardening is retained until 1100 ?C, which is the highest temperature reported for (Ti,Al)N films. During cutting operations, the wear mechanism of the cathodicarc-deposited c-(Ti0.52Al0.48)Ny with N-contents of y = 0.92, 0.87, and 0.75 films are influenced by the interplay of nitrogen vacancies, microstructure, and chemical reactions with the workpiece material. The y = 0.75 coating contains the highest number of macroparticles and has an inhomogeneous microstructure after machining, which lower its flank and crater wear resistance. Age hardening of the y = 0.92 sample causes its superior flank wear resistance while the dense structure of the y = 0.87 sample prevents chemical wear that results in excellent crater wear resistance. Heteroepitaxial c-(Ti1-x,Alx)Ny (y = 0.92, 0.79, and0.67) films were grown on MgO(001) and (111) substrates using magnetron putter deposition to examine the details of their defect structures during spinodal decomposition. At 900 ?C, the films decompose to form coherent c-AlN- and c-TiN- rich domains with elongated shape along the elastically soft <001> direction. Deformation maps show that most strains occur near the interface of the segregated domains and inside the c-TiN domains. Dislocations favorably aggregate in c-TiN rather than c-AlN because the later has stronger directionality of covalent chemical bonds. At elevated temperature, the domain size of (001) and (111)- oriented c-(Ti,Al)Ny films increases with the nitrogen content. This indicates that there is a delay in coarsening due to the presence of more N vacancies in the film. The structural and functional properties (Ti1-x,Alx)Ny are also influenced by its Al content (x). TiN and (Ti1-x,Alx)Ny (y = 1, x = 0.63 and x = 0.77) thin films were grown on MgO(111) substrates using magnetron sputtering technique. Both TiN and Ti0.27Al0.63N films are single crystals with cubic structure. (Ti0.23,Al0.77)N film has epitaxial cubic structure only in the first few atomic layers then it transitions to an epitaxial wurtzite layer, with an orientation relationship of c-(Ti0.23,Al0.77)N(111)[1-10]??w-(Ti0.23,Al0.77)N(0001)[11-20]. The w-(Ti0.23,Al0.77)N shows phase separation of coherent nm-sized domains with varying chemical composition during growth. After annealing at high temperature, the domains in w-(Ti0.23,Al0.77)N have coarsened. The domains in w-(Ti0.23,Al0.77)N are smaller compared to the domains in c-(Ti0.27,Al0.63)N film that has undergone spinodal decomposition. The results that emerged from this thesis are of great importance in the cutting tool industry and also in the microelectronics industry, because the layers examined have properties that are well suited for diffusion barriers.
Materials structures with large surface area-to-volume ratios can exhibit size dependent physical and chemical properties that are different than their bulk form. These changes are often related to the material adopting a different crystallographic phase. Often these phase transformations are serendipitously observed with the criteria for their stability difficult to ascertain. This work elucidates the underpinnings of phase stability behavior in the nanoscale regime by providing a systematic study using Ti/bcc multilayered thin film architectures. The influences of lattice misfit, layer thickness, composition and chemical intermixing on the phase stability are determined. In situ thin film growth stresses of these materials are measured and correlated to the interfacial stress evolution to help rationalize the stability behavior. X-ray and electron diffraction have been employed to determine the phase with atom probe tomography used to characterize the chemical compositions within the materials and across the interfaces. This work will delineate how intrinsic film stress drives compositional intermixing across such interfaces which can thermodynamically promote phase transformations.
Metal Oxide-Based Thin Film Structures: Formation, Characterization and Application of Interface-Based Phenomena bridges the gap between thin film deposition and device development by exploring the synthesis, properties and applications of thin film interfaces. Part I deals with theoretical and experimental aspects of epitaxial growth, the structure and morphology of oxide-metal interfaces deposited with different deposition techniques and new developments in growth methods. Part II concerns analysis techniques for the electrical, optical, magnetic and structural properties of thin film interfaces. In Part III, the emphasis is on ionic and electronic transport at the interfaces of Metal-oxide thin films. Part IV discusses methods for tailoring metal oxide thin film interfaces for specific applications, including microelectronics, communication, optical electronics, catalysis, and energy generation and conservation. This book is an essential resource for anyone seeking to further their knowledge of metal oxide thin films and interfaces, including scientists and engineers working on electronic devices and energy systems and those engaged in research into electronic materials. Introduces the theoretical and experimental aspects of epitaxial growth for the benefit of readers new to the field Explores state-of-the-art analysis techniques and their application to interface properties in order to give a fuller understanding of the relationship between macroscopic properties and atomic-scale manipulation Discusses techniques for tailoring thin film interfaces for specific applications, including information, electronics and energy technologies, making this book essential reading for materials scientists and engineers alike
The MRS Symposium Proceeding series is an internationally recognised reference suitable for researchers and practitioners.
The last hundred years have been full of scientific discoveries leading to technological advances, such as, computers, smart phones, etc. Most of the advances would not have been possible without new discoveries within the vast field of materials science. The specific area within materials science covered in this thesis is multicomponent nitride alloys, which are commonly used as thin films in industrial applications, e.g., as hard wear-resistant coatings for cutting-tools or as part of intricate electronic components in mobile telecommunication devices. The core of this thesis is towards the fundamental understanding of existing, and the discovery of new, nitride alloys using theoretical tools. Knowledge about the quantum mechanics of the alloys was gained using density functional theory, alloy theory, and thermodynamics investigating piezoelectricity, phase stability, and surface diffusion. The focus of the piezoelectricity research is on piezoelectric properties of both ordered and disordered nitrides. The exploration of disordered wurtzite nitrides revealed important aspects of the nitride alloying physics and the implications for their piezoelectric response, in addition to the discovery of interesting alloy candidates and their synthesis, e.g., YxIn1-xN. For the ordered nitrides, novel TMZnN2 (TM = Ti, Zr, Hf) structures with high piezoelectric responses have been predicted as stable. The focus of the piezoelectricity research is on piezoelectric properties of both ordered and disordered nitrides. The exploration of disordered wurtzite nitrides revealed important aspects of the nitride alloying physics and the implications for their piezoelectric response, in addition to the discovery of interesting alloy candidates and their synthesis, e.g., YxIn1-xN. For the ordered nitrides, novel TMZnN2 (TM = Ti, Zr, Hf) structures with high piezoelectric responses have been predicted as stable. The thermodynamic stability of novel alloys with interesting properties is investigated in order to determine if equilibrium or non-equilibrium synthesis is feasible. The studies consist of ternary phase diagrams of TM-Zn-N, mixing enthalpies for disordered YxAl1-xN and YxIn1-xN that can be used to predict possible synthesis routes and guide experiments. In addition, mixing enthalpies for strained ScxAl1-xN/InyAl1-yN superlattices show that the stability of certain phases and, therefore, the crystalline quality can be improved by modifying in-plane lattice parameters through higher indium content in the InAlN layers. Surface diffusion is studied because it is an important factor during thin film growth with, for example, physical vapor deposition. It is the main atomic transport mechanism and, thus, governs the structure development of thin films. Specifically, the research is focused on diffusion on the surfaces of disordered alloys, and in particular Ti, Al, and N adatom diffusion on TiN and TiAlN surfaces. The investigations revealed that Ti adatom mobilities are dramatically reduced in the presence of Al in the surface layer on the TiN and Ti0.5Al0.5N(0 0 1) surfaces, while Al adatoms are largely unaffected. Furthermore, the reverse effect is found on the TiN(1 1 1) surface, Al adatom migration is reduced while Ti adatom migration is unaffected. In addition, it is shown that neglecting the magnetic spin polarization of Ti adatoms will locally underestimate the binding energies and the diffusion path, e.g., underestimating the stability of TiN(0 0 1) bulk sites.
This book presents a summary of the topic of supercooling during crystallization in condensed films. While recent findings are mainly published in English, the foundational classical results were originally published in Russian, with limited accessibility to general readers. The present work is based on a 2019 Ukrainian monograph, "Temperature Stability of the Supercooled Liquid Phase in Condensed Films," which has been extensively revised and expanded. The book includes a detailed analysis of the thermodynamics of supercooled fluids, with updated and expanded sections. Additionally, new results on the supercooling of indium-lead (In-Pb) alloys in contact with amorphous molybdenum and fusible metals in contact with nanocrystalline layers are presented. These layers occupy a middle ground between amorphous (carbon, molybdenum, as-deposited germanium films) and polycrystalline (copper, silver, aluminum) substrates. The book gives particular attention to the peculiarities of contracted geometry conditions, which are natural for multilayered structures and can occur through fusible component segregation at grain boundaries. The analysis of new data has prompted a rethinking of the role of the more refractory layer's microstructure on the crystallization processes of metastable melts. The book includes a thorough discussion of these findings, highlighting the crucial role of the microstructure in the crystallization process. This book is a valuable resource for researchers and students interested in crystallization in thin-film metallic systems. This comprehensive study provides a detailed and authoritative analysis of the thermodynamics of supercooled fluids, and the impact of microstructure on the crystallization processes of metastable melts, making it an essential addition to any academic library.
The terms phase transitions and phase transformations are often used in an interchangeable manner in the metallurgical literature. In Phase Transformations, transformations driven by pressure changes, radiation and deformation and those occurring in nanoscale multilayers are brought to the fore. Order-disorder transformations, many of which constitute very good examples of continuous transformations, are dealt with in a comprehensive manner. Almost all types of phase transformations and reactions that are commonly encountered in inorganic materials are covered and the underlying thermodynamic, kinetic and crystallographic aspects elucidated. Shows readers the advancements in the field - due to enhanced computing power and superior experimental capability Drawing upon the background and the research experience of the authors, bringing together a wealth of experience Written essentially from a physical metallurgists view point