Du fait de leur facilité de fabrication et de leurs propriétés physiques uniques, les nanofils (NFs) de semi-conducteurs présentent des potentialités d’application importantes elles pouvaient être comme briques élémentaires de nombreux dispositifs nano- et opto-électroniques. Différents procédés de fabrication ont été développés pour fabriquer et organiser ces nanofils en épitaxie sur silicium. Cependant, un des principaux problèmes réside dans le manque de reproductibilité des NFs produits naturellement. Pour obtenir un meilleur contrôle de leur périodicité, localisation, forme et taille, différents types de gravure ont été mis au point. Aujourd’hui, des incertitudes importantes persistent quant à leurs propriétés fondamentales, en raison d’un manque de corrélation entre les propriétés électroniques et optiques et les détails microscopiques (composition, structure, chimie ...etc.). L’objectif de ce travail est de développer deux types de procédés de fabrication : le premier "top-down" est basé sur la nanogravure directe par faisceau d’ions focalisés (FIB)de couches bi-dimonsionnelles de SiGe. Ce procédé permet de contrôler la taille des NFs, les déformations, et leur localisation précise. Il permet de fabriquer des réseaux de larges piliers. Les NFs réalisés par cette technique sont peu denses et de diamètre important. Le second procédé est de type "Bottom-Up" ; il s’appuie sur la croissance VLS à partir de catalyseurs métalliques (AuSi). Les NFs réalisés ont étudiés à l’échelle locale afin de mesurer la taille moyenne de contrainte ainsi que leur effet sur le confinement quantique et sur la structure de bande des NFs. / SiGe/Si core/shell nanowires (NWs) and nanodots (NDs) are promising candidates for the future generation of optoelectronic devices. It was demonstrated that the SiGe/Si heterostructure composition, interface geometry, size and aspect ratios can be used to tune the electronic properties of the nanowires. Compared to pure Si or Ge nanowires, the core-shell structures and exhibit extended number of potential configurations to modulate the band gap by the intrinsic strain. Moreover, the epitaxial strain and the band-offsets produce a better conductance and higher mobility of charge carriers. Recent calculations reported that by varying the core-shell aspect ratio could induce an indirect to direct band gap transition. One of the best configurations giving direct allowed transitions consists of a thin Si core embedded within wide Ge shell. The Germanium condensation technique is able to provide high Ge content (> 50%) shell with Si core whom thickness of core and shell can be accurately tuned. The aim of this work is to develop two types of synthesis processes: the first "top-down" will be based on direct nanoetching by focused ion beam (FIB) of 2D SiGe layer. This process allows the control of the size of NWs, and their precise location. The NWs achieved by this technique are not very dense and have a large diameter. The second processes called "bottom-up"; are based on the VLS growth of NWs from metal catalysts (AuSi). Grown NWs have been studied locally in order to measure the mean size and the strain and their effects on the quantum confinement and band structure of NWs.

Silicon Nanowire based Reconfigurable Field Effect Transistor (SiNW RFET) presents a solution to increase the system functionality beyond the limits of classical CMOS scaling in More-than-Moore era of semiconductor technology. They are not only spatially reconfigurable, i.e., the source and the drain can be interchangeable in design, but in such devices one can also control the primary charge carrier by controlling the voltage in the control gate. The two key morphological factors controlling reconfigurability are the structure and composition of the Schottky junctions, which serve as the location for Program and Control gates and radial strain induced by the self-limiting oxidation, which influences the carrier mobility resulting in symmetric p and n characteristic curves of an RFET. Despite its potential, in-depth nanoscale studies on the structural and compositional characterization of the key features controlling the reconfigurability are limited and thereby presents as a novel area of research. In this study, the composition and morphology of the Schottky junction and the radial strain profile due to self-limiting oxidation were studied using advanced imaging and sample preparation techniques like Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) imaging alongside with precise sample preparation methods like Focused Ion Beam (FIB) liftout techniques. For analysis of radial strain in nanowires that underwent self-limiting oxidation, a TEM lamella was taken of a cross-section of the NW. The lamella was kept at 200 nm thickness to preserve the strain state of the nanowire cross-section. It was observed that nanowires undergoing such oxidation have an omega (Ω) shaped oxide shell where the shell was discontinued at the spot where the nanowire was touching the substrate. Fast Fourier transform of the high-resolution image of such a NW crossection was used to calculate the strain profile. The strain is also found to be not radially uniform for such Ω shaped oxide shells. The strain profile shows a local maxima near the nanowire base where it touches the substrate then a minima approximately at the geometric center followed by the maximum strain at the area adjacent to the oxide shell thereby showing a sinusoidal profile. Theoretical simulations performed by Dr. Tim Baldauf further verified the nature of the sinusoidal strain that was observed experimentally. Similar simulations were done for different omega shell shapes, which yielded strain plots of similar sinusoidal strain plots, with the local maxima depending on the level of encapsulation of the NW by the shell. In the characterization of the Schottky junction, a TEM lamella was taken along the longitudinal direction of a nanowire, which was silicidized from both ends, similar to ones used in SiNW RFET devices. High resolution TEM micrographs and EDX (Energy dispersive X-Ray Spectroscopy) in the TEM along the Schottky junction showed a Ni rich phase and pure Si on either side of the junction. This participating phase was identified as NiSi2. However, the transition between the phases shows a gradient and in-situ experiments were designed to verify the sharpness of the junction. In in-situ silicidation experiments, Si nanowires with a thin native oxide shell were distributed on an electron transparent surface and were partially covered with Ni islands by shadow sputtering. The whole setup was then heated in a heating stage of a TEM and the Ni was allowed to disperse within the Si nanowires forming NiSi2. HRTEM (High Resolution TEM), EDX and EELS (Electron Energy Loss Spectroscopy) studies were performed on the silicidized samples for further ex-situ analysis. During the in-situ experiment, it was observed that Ni-phase interface is atomistically sharp and seldom progresses perpendicularly to the nanowire’s direction but through the closed packed planes of the NW. The interface velocity at different temperatures was used to calculate the activation energy of the silicidation process. The value of the activation energy indicates the Ni undergoing volume diffusion through the Ni-rich phase. The velocity of the interface was observed to be much higher in nanowires with smaller diameters than those with higher diameters, further proving the hypothesis. During the in-situ experiments, in around 10% of nanowires that underwent complete silicidation and held isothermally, the crystalline silicide phase was observed to partially or fully diffuse out of the nanowire core, leaving only a thin shell of Silicon oxide forming ultra-thin walled SiO2 nanotubes (NT). The onset and the time required for completion of the process varies in the nanowires depending on size of the nanowire, the distance and contact to the nearest Ni islands and presence of defects such as kinks and twists within the nanowire. In order to study the dynamics of the process, the velocity of the receding front was calculated for nanowires of two different diameters. They are found to be identical, indicating the volume flow rate of the process is directly proportional to the cross-sectional area. The voids were formed by the reduced diffusivity of Ni in Ni2Si phase in comparison to phases with lower percent of Ni. This indicates that the reason behind the phenomenon is coalition of Kirkendall voids and thus dependent on volume diffusion. From this study, it can be concluded that the extent of self-limiting oxidation and shape of the shell can influence the radial strain state. This can be used to manipulate the strain to tailor the electron and hole transfer characteristics within the RFET. A variety of factors including temperature, time, orientation and radius of the nanowires has been studied with respect to silicidation of a SiNW. The calculated activation energy can be used for precise process control over the location and morphology of Schottky junction. Although not directly related to SiNW RFET devices, the self-assembly of ultra-thin-walled SiO2 NT is a novel research area in itself, the findings of which can be applied in to design novel electronics and sensors.:TABLE OF CONTENTS Preface List of Abbreviations CHAPTER 1: Introduction and Motivation 1.1 Definition and History 1.2 Synthesis Routes 1.3 Properties and Applications 1.4 Nanoscale Electronics and Role of Si Nws 1.4.1 1.4.2 SiNW Reconfigurable Field Effect Transistor 1.5 Introduction to The Topic of The Thesis 1.6 Outline of The Thesis CHAPTER 2: Physical Basics and Previous Research: A Short Summary 2.1 Strain Measurement and Effects of Strain on on Nanoelectronics 2.1.1 Strain Analysis in Planar CMOS Structures 2.2 Silicidation and Schottky Junction 2.2.1 In-situ Silicidation 2.2.2 Silicon oxide nanotubes CHAPTER 3: Background of Instruments and Experimental Set-up 3.1 Scanning Electron Microscope 3.2 Transmission Electron Microscope 3.2.1 Imaging Techniques 3.2.2 TEM sample preparation 3.3 Focused Ion Beam CHAPTER 4: Strain in Nanowire 4.1 Goal of This Study 4.2 Strain in SiNW RFET Devices 4.3 Strain Analysis in SiNW Cross-section 4.3.1 Sample Preparation 4.3.2 Experimental Process 4.3.3 Results and Discussion 4.4 Conclusions CHAPTER 5: Schottky Junction 5.1 Crystallographic Data on Nickel Silicides 5.2 Formation of Silicides in 2-D Structures 5.2.1 Sample History 5.2.2 Sample Preparation 5.2.3 Results and Discussion 5.3 Formation of Silicides in 1-D Structures: Schottky Junction in NWs 5.3.1 Sample History 5.3.2 Sample Preparation 5.3.3 Results and Discussion 5.3.4 Shortcomings of The Lift-out Technique 5.4 In-situ Silicidation 5.4.1 Motivation 5.4.2 Sample Preparation 5.4.3 Experimental Procedure 5.4.4 Results and Discussions 5.4.5 Shortcoming of The Experiment 5.5 Self-assembling SiO2 Nanotubes 5.5.1 Sample Preparation 5.5.2 Experimental Process 5.5.3 Results and Discussion . 5.5.4 Post In-situ Experiment TEM Analysis 5.5.5 Conclusions CHAPTER 6: Conclusions and Outlook 6.1 Strain Analysis 6.2 Schottky Junction Studies Bibliography Acknowledgements

This thesis describes the study and characterization of secondary created structures in PN junction of silicon solar cells. Secondary created structure is the term which means the structures created for the purpose of suppressing the negative influence of local defects and edges of the solar cell. This means in particular laser notches used to isolate the edges. Furthermore, the secondary created structure means modification defective area using focused ion beam milling. Theoretical part of this work deals with short introduction to the topic of solar cells. There are mentioned the physical nature of the solar cell and the technology associated with the issue of solar cells. Experimental section begins with a description of the experimental methods. For diagnostic methods were used both electrical (UI characteristics, noise characteristics) and optical methods (measuring local radiation - CCD camera, thermal imager, lock-in thermography). Furthermore, there was also used a scanning electron microscope (SEM) equipped with technology using Focused Ion Beam (FIB). Sequentially there are presented individual results of characterization of created structures by laser. These partial results are incorporated into a comprehensive methodology developed for characterizing laser-created structures. The experimental part is finished by a presentation of the results of the research use of focused ion beam technology for sputtering defective areas of solar cells.

This work presents a new approach of fabricating arrays of electrodes, separated by sub-micrometer gaps allowing the systematic investigation of electric properties of organic semiconductors. The laterally stacked devices are fabricated by using a trench isolation technique for separating different electrical potentials, as it is known for micromachining technologies like Single Crystal Reactive Ion Etching and Metallization (SCREAM). The essential part of this process is the patterning of sub-micrometer trenches onto the silicon substrate in a single lithographic step. Afterwards, the trenches are refilled by SiO2 to allow the precise tuning of the electrode separation gap. The metal electrodes are formed via magnetron sputtering. This technological approach allows us to fabricate device structures with a transport channel length in the range of 100-250 nm by conventional photolithography. In this experiment, three different metals like Au, Co, and Ni were used as the electrode materials, while copper phthalocyanine, being deposited by thermal evaporation in high vacuum, was employed as the organic semiconductor under evaluation. The final aim has been study of spin transport through the organic channel in varied geometry.

In dieser Arbeit ist die Bildung von selbstorganisierten Nanostrukturen auf Galliumantimonid (GaSb) und Germanium (Ge) durch Ionenbeschuss untersucht worden. Zudem sind die auf Ge erhaltenen Lochstrukturen als Template für Silber- und Eisenschichten verwendet worden. Bei der Bestrahlung von GaSb mit Argonionen unter senkrechtem Ioneneinfall bilden sich hexagonal geordnete Punktstrukturen, während bei der Bestrahlung von Ge mit Galliumionen unter senkrechtem Ioneneinfall hexagonal geordnete Lochstrukturen entstehen. Dabei handelt es sich um zueinander inverse Muster. Für diese beiden Materialsysteme sind die Abhängigkeit der sich bildenden Strukturen von der Ionenenergie, dem Fluss, der Fluenz und dem Ioneneinfallswinkel untersucht, und die entstehenden Muster mit theoretischen Modellen verglichen worden. Bei der Bestrahlung von GaSb unter senkrechtem Ioneneinfall steigen charakteristische Länge und Höhe mit der Ionenenergie linear an, bis sie sättigen. Eine Variation des Einfallswinkels der Ar-Ionen führt zu hexagonal geordneten Punktstrukturen, geneigten Punktstrukturen und Rippeln auf GaSb. Das Aspektverhältnis steigt mit dem Winkel an, bis es für die Rippel wieder stark absinkt. Auf Ge bilden sich bei der Bestrahlung mit Ga-Ionen Lochstrukturen, deren Höhe linear mit der Ionenenergie ansteigt und deren charakteristische Länge mit dieser absinkt. Mit steigendem Ioneneinfallswinkel bilden sich aneinander gereihte Lochstrukturen sowie unregelmäßige Muster. Bei den Materialsystemen ist der Anstieg der Ordnung bis zu einer Sättigung mit der Fluenz ebenso wie der Anstieg der Höhe der Strukturen mit der Energie gemeinsam. Für schrägen Ioneneinfall werden gänzlich unterschiedliche Muster erhalten. Zudem ist ein Unterschied im Verhalten der charakteristischen Länge mit der Ionenenergie vorhanden. Ebenso wie die unterschiedlichen Aspektverhältnisse weist dies auf einen Unterschied im Musterbildungsprozess hin. Dieser wird auf Ge von der Vakanzdynamik dominiert, wohingegen auf GaSb das präferentielle Sputtern ausschlaggebend ist. Somit bestimmen die unterschiedlichen Materialeigenschaften von GaSb und Ge den Musterbildungsprozess. Um ein zeitlich entkoppeltes Bestrahlen zu betrachten, wird mit dem Strahl über die Ge-Oberfläche gerastert. Die erhaltenen Muster werden mit denen durch einen stehenden Ionenstrahl entstanden verglichen. Das Rastern des Ionenstrahls hat keinen Einfluss auf die entstehenden Muster. Zudem wird der Fluss bei Bestrahlung der Ge-Oberfläche über vier Größenordnungen variiert. Da der Fluss in allen Termen der Kontinuumsgleichungen enthalten ist, ist kein Einfluss auf die entstehenden Lochstrukturen vorhanden. Bei einer Bestrahlung von Ge mit Ge-Ionen bilden sich ebenfalls Lochstrukturen, die aber keine hexagonale Nahordnung aufweisen. Damit wird eine zweite Komponente, entweder im Substrat oder aus dem Ionenstrahl, benötigt, damit die Strukturen geordnet sind. Diese Lochstrukturen werden im Anschluss mit Silber und Eisen bedampft, um ein unterschiedliches Aufwachsen im Vergleich zu einer planaren Oberfläche zu untersuchen. Bei Verwendung einer vorstrukturierten Oberfläche steigt die Anzahl der Silbercluster aufgrund der größeren Oberfläche an. Eine Vorstrukturierung des Substrats verhindert eine Veränderung der Filmmorphologie eines nahezu geschlossenen Films durch Tempern. Bei Verwendung eines planaren Substrats bilden sich nach dem Tempern Cluster. Beim senkrechten Aufdampfen eines Eisenfilms folgt dieser der Oberfläche. Durch die erhöhte Rauigkeit aufgrund der Vorstrukturierung verändert sich die polare Magnetisierungskomponente. Beim Aufdampfen des Eisens unter streifendem Einfall bilden sich säulenartige Strukturen. Diese sind auf dem vorstrukturierten Substrat größer und haben im Mittel einen größeren Abstand. Diese Säulen weisen eine starke magnetische und magnetooptische Anisotropie auf, die im Anschluss untersucht wird. Die Verwendung des vorstrukturierten Substrats und somit die veränderte Größe der Säulen, beeinflusst die magnetischen Eigenschaften kaum.

Ce projet s’inscrit dans le cadre de la phase de conception d’un pont autoroutier dans la province de Québec au Canada. Une des variantes proposées comporte la construction d’un tablier en béton précontraint par encorbellements successifs. Il est prévu d’utiliser un béton à hautes performances avec fumée de silice et avec air entraîné pour le tablier et les piles. La résistance à la compression visée est de 40 MPa à trois jours, rendue possible par l’adjonction de fumée de silice. Une campagne d’essais a été réalisée, d’une part pour élaborer une composition qui réponde aux critères de performance et d’autre part afin de caractériser le comportement mécanique et viscoélastique du BHP. Un essai de retrait et de fluage d’une durée de 16 mois a été réalisé, conformément aux recommandations du document TC107-CSP de la RILEM. Le BHP a été testé dans des conditions endogènes et de séchage, à plusieurs échéances de chargement soit 3, 7, 28 et 90 jours après le bétonnage. Comme le béton formulé pour le projet possède des caractéristiques peu communes, il convient de s’assurer que le BHP présente des déformations acceptables et qui peuvent être correctement prédites par les modèles réglementaires. Le retrait endogène, le retrait total le fluage propre et le fluage total sont mesurés avec un comparateur manuel et avec une corde vibrante placée au coeur des éprouvettes. Le modèle B4 de base prédit correctement les deux types de retrait. Après 16 mois, le retrait endogène du BHP s’élève 350 μm/m tandis que le retrait de séchage atteint 340 μm/m. Le retrait de séchage et le fluage de dessiccation ne semblent pas avoir atteint un palier final à la fin des essais. Le modèle de l’Eurocode 2 Annexe B prédit le mieux les deux types de fluage lorsqu’on considère qu’il n’y a pas de fumée de silice dans le modèle. Trois modèles ont été ajustés et extrapolés à très long terme : Eurocode 2 Annexe B, fib 2010 et le modèle B4. Les paramètres intrinsèques au matériau dans les équations de ces modèles sont optimisés et présentés dans le but d’être intégrés dans les calculs d’analyse et de dimensionnement du pont. Le fib 2010 ne semble pas adapté pour modéliser convenablement le fluage de dessiccation. Le modèle B4 ajusté avec les mesures de pertes d’eau prédit le fluage le plus important à 50 ans. Il parait néanmoins difficile de conclure sur une valeur finale à très long terme sans avoir effectivement atteint la fin du séchage pendant l’essai.

Esta dissertação contempla o estudo sobre os principais aspectos que norteiam as propostas normativas com critérios de dimensionamento de estruturas de concreto armado reforçadas com Polímeros Reforçados com Fibras (FRP). Este trabalho procura avaliar e discutir as metodologias de cálculo sugeridas pelas publicações American Concrete Institute (ACI) \"Committee 440\" e Fédération Internationale du béton (FIB) e propor um procedimento para a análise e dimensionamento do reforço aos esforços de flexão e discutir a importância do esforço cortante com relação à flexão. Primeiramente, efetuou-se uma revisão bibliográfica sobre o estado da arte no que concerne ao dimensionamento de reforços com materiais compósitos de FRP. Em seguida, apresentou-se a análise da verificação de segurança de elementos reforçados à flexão segundo a perspectiva das publicações acima mencionadas: ACI440.2R (2017) e FIB Model Code (2010). Posteriormente, estabeleceu-se um estudo de análise comparativa entre os resultados experimentais encontrados na bibliografia, os previstos pelos referidos manuais para os modelos aqui estudados. Os valores máximos da capacidade resistente dos modelos serão confrontados e a importância do conhecimento das características dos materiais discutidos. Com tais informações sistematizaram-se os procedimentos de cálculo a se adotar em um projeto de reforço estrutural com sistemas compósitos de FRP, respeitando as propostas normativas que mais se adequam ao Brasil. / The present dissertation results from a study concerning the main design criteria for reinforced concrete structures through externally bonding Fibre-Reinforced Polymer (FRP) systems. This paper tries to evaluate and discuss the design methodologies proposed by the guidelines publications American Concrete Institute (ACI) \"Committee 440\" and Fédération Internationale du béton (FIB) and seek to propose a procedure for the analysis and design of FRP strengthened RC elements in bending and discuss the importance of shear stress regarding the bending efforts. Initially, a state-of-art concerning design criterias on the use of FRP composites is presented. Then, the analysis and procedures of the safety concept is exposed from the perspective of the publications: ACI440.2R (2017) and FIB Model Code 2010. After, it established a study of comparative analysis between the experimental results found in the bibliography and the ones predicted by international guidelines for beams and slabs. The maximum ultimate capacity of the strengthened element will be compared and confronted and the importance of the knowledge of the characteristics of the materials discussed. Finally, conclusions are drawn out and recommendations concerning the design and safety concepts are presented, respecting the guidelines proposals that are most appropriate to Brazil.

The controlled organization of individual molecules and nanostructures with nanoscale accuracy is of great importance in the investigation of single-molecule events in biological and chemical assays, as well as for the fabrication of the next generation optoelectronic devices. In this regard, the precise patterning of individual molecules into hierarchical structures has attracted substantial research interest in recent years. DNA has been shown to be an ideal structural material for this purpose, due to the specificity of its programmability and outstanding chemical flexibility. DNA origami can display a high degree of positional and precise binding sites, allowing for complex arrangements and the assembly of different nanoscale architectures. In this project, we present a novel platform based on the use of DNA scaffolds for the organization of individual nanomoieties (with nanoscale spatial control), and their selective immobilisation on surfaces for single-molecule investigations. In particular, semiconductor quantum dots (QDs), fluorescence molecules, linear small peptides, and structural proteins were tethered with single-molecule accuracy on DNA origami; their subsequent organization in array configuration on nanopatterned surfaces allowed us to fabricate and test different platforms for single-molecule studies. In particular, we developed a Focused Ion Beam (FIB) nanofabrication strategy and demonstrated its general applicability for the assembly of functionalised DNA nanostructures in highly uniform nanoarrays, with single-molecule control. In addition, we further explored this nanofabricated platform for biological investigations at the single-molecule level, from protein-DNA interactions to cancer cell adhesion studies with single-molecule control. Investigations have been carried out via fluorescence microscopy, scanning electron microscopy (SEM), Focused Ion Beam (FIB) and atomic force microscopy (AFM). By and large, combining the programming ability of DNA as a scaffolding material with a one-step lithographic process, we have developed a platform of general applicability for the fabrication of nanoscale chips that can be employed in a variety of single-molecule investigations.

This thesis presents the results of a study focussed on the elucidation of the mechanisms responsible for determining the structural integrity of carbon coatings on ductile substrates. Through elucidation of these mechanisms, two different coating systems are designed; a multilayered coating and a functionally graded coating. While concentrating upon carbon coatings, the findings of this study are applicable to a broad range of hard coatings on ductile substrates. The thesis concludes with a chapter outlining a brief study of the effects of gold coatings on silicon under contact load at moderate temperatures to complement the major part of the thesis. Carbon coatings with differing mechanical properties were deposited using plasma enhanced chemical vapour deposition (PECVD), filtered assisted deposition (FAD) and magnetron sputtering deposition methods. Combinations of these techniques plus variation of deposition parameters enabled composite multilayered and functionally-graded coatings to also be deposited. Substrates were ductile metals; stainless steel and aluminium. Characterisation of the coating mechanical properties was undertaken using nanoindentation and nano-scratch tests. The same techniques were used to induce fracture within the coatings to allow subsequent analysis of the fracture mechanism. These were ascertained with the assistance of cross-sectional imaging of indents prepared using a focussed ion beam (FIB) mill and transmission electron microscopy (TEM) using specimen preparation techniques also utilising the focussed ion beam mill. A two dimensional axisymmetric finite element model (FEM) was built of the coating systems using the commercial software package, ANSYS. Substrate elastic-plastic properties were ascertained by calibrating load-displacement curves on substrate materials with the finite element model results. Utilising the simulation of spherical indentation, the distribution of stresses and the locations for fracture initiation were ascertained using finite element models. This enabled determination of the influence such factors as substrate mechanical properties, residual stresses in the coatings and importantly the variation of elastic properties of the different coating materials. Based upon the studies of monolithic coatings, simulations were undertaken on multilayer and functionally-graded coatings to optimise design of these coating types. Based on the results of the modelling, multilayered and functionally graded coatings were then deposited and mechanical testing undertaken to confirm the models. Three major crack types were observed to occur as the result of the spherical nanoindentation on the coatings; ring, radial and lateral cracks. Ring cracks were found to initiate from the top surface of the film, usually at some distance from the edge of the spherical contact. Radial cracks usually initiated from the interface between the coating and the substrate directly under the symmetry axis of indentation and propagated outwards in a non symmetrical star-like fashion. Lateral cracks formed either between layers in the multilayer coatings or at the interface with substrate. Ring and radial cracks were found to form upon loading whereas lateral cracks formed upon both loading and unloading depending upon the crack driving mechanism. Pop-in events in the load displacement indentation curve were found to be indicative of the formation of ring cracks, while the formation of the other types of cracks was not signified by pop-ins but rather by variations in the slope of the curve. The substrate yield strength was found to influence the initiation of all crack systems while compressive stresses in the coating were seen to only influence the formation of ring and radial cracks. However, it was also noted that the initiation of one form of crack tended to then hinder the subsequent formation of others. In multilayer coatings, the lateral cracks were suppressed, as opposed to the monolayer coating system, but a ring crack was observed. This drawback in the multilayer system was successfully addressed by the design of a graded coating having the highest Young??s modulus at the middle of the film thickness. In this coating, due to the graded nature of the elastic modulus, the stresses at the deleterious locations (top surface and interface) were guided toward the middle of coating and hence increased the load bearing capabilities. The effect of substrate roughness upon the subsequent surface roughness of the coating and also upon the fracture process of the coating during indentation was also investigated. For the coatings deposited on rough substrates, the radial cracks were observed to form initially and this eventfully delayed the initiation of ring cracks. Also the number of radial cracks observed at the interface was found to be proportional to the distribution of the interfacial asperities. In summary, the study elucidated the fracture mechanisms of monolayer, multilayer and graded carbon coatings on ductile substrates under uniaxial and sliding contact loading. The effects of the yield strength, surface roughness of the substrate, along with the residual stress and elastic modulus of the coatings on the fracture of coatings were investigated. The study utilised finite element modelling to explain the experiments observations and to design coating systems.

In many engineering applications, including radar, sonar, communications and seismology, the direction of impinging signal wavefronts can be used to discriminate between competing sources. Often these source signals cover a wide bandwidth and conventional narrowband beamforming techniques are ineffective, since spatial resolution varies significantly across the band. In this thesis we consider the problem of beamforming for broadband signals, primarily when the spatial response remains constant as a function of frequency. This is called a frequency invariant beamformer (FIB).¶ Rather than applying the numerical technique of multi-parameter optimisation to solve for the beamformer parameters, we attempt to address the fundamental nature of the FIB problem. The general philosophy is to use a theoretical continuous sensor to derive relationships between a desired FI beampattern and the required signal processing structure. Beamforming using an array of discrete sensors can then be formulated as an approximation problem. This approach reveals a natural structure to the FIB which is otherwise buried in a numerical optimisation procedure.¶ Measured results from a microphone array are presented to verify that the simple FIB structure can be successfully implemented. We then consider imposing broadband pattern nulls in the FI beampattern, and show that (i) it is possible to impose an exact null which is present over all frequencies, and (ii) it is possible to calculate a priori how many constraints are required to achieve a null of a given depth in a FIB. We also show that the FIB can be applied to the problem of broadband direction of arrival (DOA) estimation and provides computational advantages over other broadband DOA estimators.¶ Through the theoretical continuous sensor approach, we show that the FIB theory can be generalised to the problem of designing a general broadband beamformer (GBB) which realizes a broadband angle-versus-frequency beampattern specification. Coupled with a technique for radial beampattern transformation, the GBB can be applied to a wide class of problems covering both nearfield beamforming (in which the shape of the impinging wavefront must be considered and farfield beamforming (which is simplified by the assumption of planar wavefronts) for a broadband beampattern specified over both angle and frequency.