The current thesis takes a step forward into understanding the intergranular stress corrosion cracking (IGSCC) by applying a relatively new micro-mechanical technique to look at the crack growth rate of individual grain boundaries in 304 stainless steel (SS) and to measure fracture toughness for different grain boundaries in Ni Alloy 600. In addition, a model is tested and proposed that could predict crack initiation in 304 SS. Pentagonal cross-section cantilevers 5 μm wide by 25 μm long were milled at individual grain boundaries in both 304 SS and Ni Alloy 600. The cantilevers milled in 304 SS were tested in-situ in a customised stage, using the nanoindenter. Crack growth rate was measured for two different cantilevers to be approximately 40 μm/s (K = 1.1 MPa(m)^(1/2)) and 120 μm/s (K = 1.7 MPa(m)^(1/2)). Cantilevers were milled in Ni Alloy 600 for three different samples: samples that were exposed to simulated pressurized water reactors (PWR) environment for 4500 h, for 1500 h and un-oxidised samples. The fracture toughness calculated for the fractured cantilevers in samples that were exposed for 4500 h was measured to be between 0.73 and 1.82 MPa(m)^(1/2). No intergranular fracture occurred in the samples that were exposed for 1500 h and in the un-oxidised samples. The grain boundary misorientation was measured for the tested cantilevers but no direct correlation was observed between the misorientation angle and the fracture toughness. A Schmid-modified grain boundary stress (SMGBS) model previously used to study the intergranular behaviour of irradiated 316L steel in supercritical water was applied to predict crack initiation in 304 stainless steel. The model was successfully applied and accurately predicted crack initiation. To extend the model, sensitisation was also included. In addition, different areas of the specimen, including the initiation site were analysed using High resolution electron backscatter diffraction (HR-EBSD) technique to measure the geometrically necessary dislocations (GNDs) density. It was observed that the boundary average GNDs is lower for the intact boundaries and higher for the cracked grain boundaries.

The mechanical response of geomaterials is highly influenced by geometrical and material heterogeneity. To date, most modelling practices consider heterogeneity qualitatively and the choice of input parameters can be subjective. In this study, a novel approach to combine detailed micro-scale characterization with modelling of heterogeneous geomaterials is presented. The influence of micro-scale heterogeneity and microcracks on the mechanical response and brittle fracture of a crystalline rock was studied using numerical and experimental tools. An existing Combined Finite-Discrete element (FEM/DEM) code was extended to suit heterogeneous, discontinuous, brittle rocks. By conducting grid micro-indentation and micro-scratch tests, the Young's modulus and fracture toughness of the constituent phases of the rock were obtained and used as accurate input parameters for the numerical models. The models incorporated the exact phase mapping obtained from a MicroCT-scanned specimen and the existing microcrack density obtained from thin section analysis. The results illustrated that by incorporating accurate micromechanical input parameters and the intrinsic rock geometric features, the numerical simulations could more accurately predict the mechanical response of the specimen, including the fracture patterns and tensile strength. The numerical simulations illustrated that microstructural flaws such as microcracks should be included in the models to more accurately reproduce the rock strength. In addition, the differential elastic deformations caused by rock heterogeneity altered the stress distribution in the specimen, creating zones of local tensile stresses, in particular, on the boundaries between different mineral phases. As a result, heterogeneous models exhibited rougher fracture surfaces. MicroCT observations emphasized the influence of heterogeneity and, in particular, biotite grains on the fracture trajectories in the specimens. Favourably oriented biotite flakes and cleavage splitting significantly deviated the cracks. The interaction of the main crack with perpendicular cleavage planes of biotite caused strong crack deviation and termination. Considering heterogeneity and the strength degradation caused by microcracks, the simulations captured reasonably accurate mechanical responses and failure mechanisms for the rock, namely, the nonlinear stress-strain relationships. The insights presented in this study improve the understanding of the role of heterogeneity and microstructure on damage and mechanical behaviour of brittle rock.

The mechanical response of geomaterials is highly influenced by geometrical and material heterogeneity. To date, most modelling practices consider heterogeneity qualitatively and the choice of input parameters can be subjective. In this study, a novel approach to combine detailed micro-scale characterization with modelling of heterogeneous geomaterials is presented. The influence of micro-scale heterogeneity and microcracks on the mechanical response and brittle fracture of a crystalline rock was studied using numerical and experimental tools. An existing Combined Finite-Discrete element (FEM/DEM) code was extended to suit heterogeneous, discontinuous, brittle rocks. By conducting grid micro-indentation and micro-scratch tests, the Young's modulus and fracture toughness of the constituent phases of the rock were obtained and used as accurate input parameters for the numerical models. The models incorporated the exact phase mapping obtained from a MicroCT-scanned specimen and the existing microcrack density obtained from thin section analysis. The results illustrated that by incorporating accurate micromechanical input parameters and the intrinsic rock geometric features, the numerical simulations could more accurately predict the mechanical response of the specimen, including the fracture patterns and tensile strength. The numerical simulations illustrated that microstructural flaws such as microcracks should be included in the models to more accurately reproduce the rock strength. In addition, the differential elastic deformations caused by rock heterogeneity altered the stress distribution in the specimen, creating zones of local tensile stresses, in particular, on the boundaries between different mineral phases. As a result, heterogeneous models exhibited rougher fracture surfaces. MicroCT observations emphasized the influence of heterogeneity and, in particular, biotite grains on the fracture trajectories in the specimens. Favourably oriented biotite flakes and cleavage splitting significantly deviated the cracks. The interaction of the main crack with perpendicular cleavage planes of biotite caused strong crack deviation and termination. Considering heterogeneity and the strength degradation caused by microcracks, the simulations captured reasonably accurate mechanical responses and failure mechanisms for the rock, namely, the nonlinear stress-strain relationships. The insights presented in this study improve the understanding of the role of heterogeneity and microstructure on damage and mechanical behaviour of brittle rock.

UV-LIGA is a microfabrication process realzed by material deposition through microfabricated molds. UV photolithography is conducted to pattern precise thick micro molds using UV light sensitive materials, mostly SU-8, and electroforming is performed to fabricate micro metallic structures defined by the micro molds. Therefore, UV-LIGA is a bottom-up in situ material-addition process. UV-LIGA has received broad attention recently than LIGA – a micro molding fabrication process using X-ray to pattern the micro molds. LIGA is an expansive and is limited in access. In comparing to LIGA, the UV-LIGA is a cost effective process, and is widely accessible and safe. Therefore, it has been extensively used for the fabrication of metallic micro-electro-mechanical-systems (MEMS). The motivation of this research was to study micro mechanical systems fabricated with nano-structured metallic materials via UV-LIGA process. Various micro mechanical systems with high-aspect-ratio and thick metallic structures have been developed and are presented in this desertation. A novel micro mechanical valve has been developed with nano-structured nickel realized with UV-LIGA fabrication technique. Robust compact valves are crucial for space applications where payload and rubstaness are critically concerned. Two types of large flow rate robust passive micro check valve arrays have been designed, fabricated and tested for robust hydraulic actuators. The first such micro valve developed employs nanostructured nickel as the valve flap and single-crystal silicon as the substrates to house inlet and outlet channels. The Nano-structured nickel valve flap was fabricated using the UV-LIGA process developed and the microchannels were fabricated by deep reactive etching (DRIE) method. The valves were designed to operate under a high pressure (>10MPa), able to operate at high frequencies (>10kHz) in cooperating with the PZT actuator to produce large flow rates (>10 cc/s). The fabricated microvalves weigh 0.2 gram, after packing with a novel designated valve stopper. The tested results showed that the micro valve was able to operate at up to 14kHz. This is a great difference in comparison to traditional mechanical valves whose operations are limited to 500 Hz or less. The advantages of micro machined valves attribute to the scaling laws. The second type of micro mechanical valves developed is a in situ assembled solid metallic (nickel) valves. Both the valve substrates for inlet and outlet channels and the valve flap, as well as the valve stopper were made by nickel through a UV-LIGA fabrication process developed. Continuous multiple micro molds fabrication and molding processes were performed. Final micro mechanical valves were received after removing the micro molds used to define the strutures. There is no any additional machining process, such as cutting or packaging. The alignment for laminated fabrication was realized under microscope, therefore it is a highly precise in situ fabrication process. Testing results show the valve has a forward flow rate of19 cc/s under a pressure difference of 90 psi. The backward flow rate of 0.023 cc/s, which is negligible (0.13%). Nano-structured nickel has also been used to develop laminated (sandwiched) micro cryogenic heater exchanger with the UV-LIGA process. Even though nickel is apparently not a good thermal conductor at room temperature, it is a good conductor at cryogentic temerpature since its thermal conductivity increases to 1250 W/k·m at 77K. Micro patterned SU-8 molds and electroformed nickel have been developed to realize the sandwiched heat exchanger. The SU-8 mold (200mm x 200mm x50mm) array was successfully removed after completing the nickel electroforming. The second layer of patterned SU-8 layer (200mm x 200mm x50mm, as a thermal insulating layer) was patterned and aligned on the top of the electroformed nickel structure to form the laminated (sandwiched) micro heat exchanger. The fabricated sandwiched structure can withstand cryogenic temperature (77K) without any damages (cracks or delaminations). A study on nanocomposite for micro mechanical systems using UV-LIGA compatible electroforming process has been performed. Single-walled carbon nanotubes (SWNTs) have been proven excellent mechanical properties and thermal conductive properties, such as high strength and elastic modulus, negative coefficient of thermal expansion (CTE) and a high thermal conductivity. These properties make SWNT an excellent reinforcement in nanocomposite for various applications. However, there has been a challenge of utilizing SWNTs for engineering applications due to difficulties in quality control and handling – too small (1-2nm in diameter). A novel copper/SWNT nanocomposite has been developed during this dissertational research. The goal of this research was to develop a heat spreader for high power electronics (HPE). Semiconductors for HPE, such as AlGaN/GaN high electron mobility transistors grown on SiC dies have a typical CTE about 4~6x10-6/k while most metallic heat spreaders such as copper have a CTE of more than 10x10-6/k. The SWNTs were successfully dispersed in the copper matrix to form the SWNT/Cu nano composite. The tested composite density is about 7.54 g/cm3, which indicating the SWNT volumetric fraction of 18%. SEM pictures show copper univformly coated on SWNT (worm-shaped structure). The measured CTE of the nanocomposite is 4.7 x 10-6/°C, perfectly matching that of SiC die (3.8 x 10-6/°C). The thermal conductivity derived by Wiedemann-Franz law after measuring composit's electrical conductivity, is 588 W/m-K, which is 40% better than that of pure copper. These properties are extremely important for the heat spreader/exchanger to remove the heat from HPE devices (SiC dies). Meanwhile, the matched CTE will reduce the resulted stress in the interface to prevent delaminations. Therefore, the naocomposite developed will be an excellent replacement material for the CuMo currently used in high power radar, and other HPE devices under developing. The mechanical performance and reliability of micro mechanical devices are critical for their application. In order to validate the design & simulation results, a direct (tensile) test method was developed to test the mechanical properties of the materials involved in this research, including nickel and SU-8. Micro machined specimens were fabricated and tested on a MTS Tytron Micro Force Tester with specially designed gripers. The tested fracture strength of nanostructured nickel is 900±70 MPa and of 50MPa for SU-8, resepctively which are much higher than published values.

The demands for high density, fine pitch interconnections in electronics systems has seen solder-based approaches for such interconnections miniaturized to the scale of tens of micro meters. At such a small scale, such 'micro joints' may contain only one or a few grains and the resultant mechanical behaviour may not be that for a polycrystalline aggregate, but rather for a single crystal. Since the ~-Sn matrix of SnAgCu solder has a contracted body-centred tetragonal (BCT) structure, such a solder grain is expected to demonstrate a considerably anisotropic behaviour. In such cases the reliability of a Phfree solder is strongly dependent on the local microstructural features, such as the size and orientation of the grains. This thesis presents the investigation of the evolution of microstructure within a joint or at the interface and, the influence of such microstructural features on the meso-scale mechanical behaviour of the Ph-free solder. It includes Evolution of the interface between a molten solder and the Cu substrate To form a joint, the solder alloy is heated and molten, wetting a solid under-bump metallization. After solidification, layers of brittle intermetallic compounds (IMCs) are formed at the interface. In this project, facilities were set up to obtain interfacial reactants at an arbitrary moment of the liquid/solid reaction. Formation and evolution ~ during reflow of SnCu IMCs at the interface between the molten SnAgCu alloy and the Cu UBM was captured and presented for the first time. Formation of phases and IMCs with the body of a liquid SnAgCu solder during solidification The formation behaviour of basic components for a SnAgCu grain (including Sn dendrites, AIDSn and Cu6Sns IMCs) during solidification was investigated. Relationships between the growth behaviour of these components and their internal lattice orientation were studied. The characteristic growth and coupling of AIDSn IMCs and the Sn matrix to form eutectics has been elaborated and presented in this study for - 1- the first time. Based on the results, the forming process of a eutectic SnAgCu grain under the non-equilibrioum solidification condition was illustrated; and major factors that determine the lattice-orientation, size and substructure of the grain were discussed. Meso- and Micro- scale mechanical behaviour of a SnAgCu solder joint To study the size effect on the microstructure, and subsequently, the meso-scale mechanical behaviour, solder joints were manufactured with varying geometries. Shearing tests were performed on these meso-scale joints. The results first demonstrated that the anisotropic characteristics of a SnAgCu grain play an important role in the mechanical behaviour of both a meso-scale solder joint and the adjacent interfacial IMCs. To further investigate the micro-scale deformation and damage mechanisms, micro-mechanical tests were preformed within a SnAgCu grain. Constitutive equations for a SnAgCu grain Based on the experimental results, a crystal model was established to describe the local microstructure-dependent mechanical behaviour. The constitutive equation was implemented by means of the finite element approach, and applied in solder joints of a Flip Chip (FC) package by a multi-scale method. To describe the crystal behaviour at the higher temperature, the model was improved to account for deformations due to vacancy diffusion and thermal expansion. This model was integrated by an implicit approach, and implemented in a full three dimension (3D) finite element (FE) model.

Dans un turboréacteur civil, le contact aube/disque de la turbine basse pression est soumis à un cyclage thermomécanique dû aux dilatations et déplacements différentiels des pièces pendant les phases de vol. Ce cyclage implique des micro-mouvements alternés relatifs, c’est-à-dire du fretting, à l’interface aube/disque. Le fretting à l’étude ici concerne un contact céramique-métal, en vue de comprendre le comportement tribologique des futures aubes revêtues de céramique en remplacement des pièces métalliques traditionnelles. Dans un premier temps, différents revêtements sont comparés vis-à-vis de leur tenue à l’usure par fretting à la température de fonctionnement des pièces (700°C). L’alliage métallique du contrecorps est celui du clinquant protégeant le disque, à savoir l’alliage base cobalt HS25. Pour le contact HS25/céramique le plus prometteur, le taux d’usure très bas ainsi que le faible frottement observés sont associés à la formation d’une glaze layer. La glaze layer est un troisième corps formé à partir des débris d’usure qui apparaît dans des contacts frottant à haute température. Traditionnellement observée pour des contacts métal/métal, sa formation pour un contact métal/céramique est nouvelle. La glaze layer est alors étudiée en détail. D’un point de vue tribologique, on établit sa cinétique et ses conditions de formation en température et en fonction des paramètres tribologiques, afin de garantir une usure faible en conditions de vol. D’un point de vue morphologique, la glaze layer est caractérisée comme étant un matériau nanostructuré amorphe et cristallin, formé à partir de débris à la fois métalliques et oxydés. Enfin, la glaze layer nanostructurée est associée à un comportement mécanique ductile dans son domaine de stabilité, alors que les débris sont fragiles en conditions d’usure forte. La corrélation des angles d’étude de la glaze layer permet alors de connaître les modalités de sa formation, en vue d’anticiper la protection d’un contact vis-à-vis de l’usure grâce à la création d’une glaze layer. / In a civil turbojet motor, the blade/disk contact in the low pressure turbine undergoes thermomechanical cycling due to relative displacements between parts during the different flight phases. This cycling results in reciprocating micro-movements named “fretting” at the blade/disk interface. This study focuses on a ceramic versus metallic contact under fretting, aimed at describing the tribological behavior of developing ceramic-coated blades to replace phased-out metallic parts. Firstly, different ceramic coatings are compared regarding their wear resistance under fretting at in-flight temperature (700°C). The counterbody is the HS25 (cobalt-based alloy) protecting foil of the disk. The most favorable ceramic/metallic tribocouple evidences a very low wear rate as well as low friction that match the formation of a glaze layer. The glaze layer is a third body formed from wear debris in high temperature rubbed contacts. Such tribofilm has been commonly observed in metallic/metallic interfaces but its occurrence in a ceramic/metallic contact is new. Then the glaze layer is precisely characterized. Tribologically speaking, its kinetics and formation conditions are determined over temperature and tribological parameters, in order to ensure low wear under flight conditions. Morphologically, the glaze layer is a nanostructured amorphous and crystalline sintered from both metallic and oxidized worn debris. Finally, the nanostructured glaze layer is mechanically described as a ductile material in its stability domain, whereas debris from severe wear are brittle. The correlation of morphological, physico-chemical and mechanical studies enlighten the glaze layer formation criteria, with the aim of predicting glaze layer occurrence, hence wear protection for a given contact.

Ziel dieser Arbeit ist es, eine Verbindung zwischen der Mikrostruktur teilkristalliner Polymere und derer mechanischen Eigenschaften auf der Mikro- und Nanometerskala aufzubauen. Dazu wurden Methoden der Rasterkraftmikroskopie verwendet um sowohl orts- als auch zeitaufgelöst Kristallisations-, Deformations- und Diffusionsprozesse in der Mikrostruktur von elastomerem Polypropylen (ePP) abzubilden. Die mechanischen Eigenschaften wurden simultan mit Mikrozugversuchen bestimmt. So konnte beispielsweise ein Zusammenhang zwischen abnehmender Kristall-Kristall-Distanz und einem Ansteigen des Elastizitätsmoduls während der Kristallisation nachgewiesen werden. Weiterhin war es möglich die Veränderung der nano-mechanischen Eigenschaften während der Kristallisation einzelner kristalliner Lamellen in deren direkter Umgebung mit MUSIC-mode Rasterkraftmikroskopie zu untersuchen. Laterale Querexpansion (auxetisches Verhalten) konnte bei uniaxialen Zugversuchen für die Kreuzschraffur-Struktur elastomeren Polypropylens auf der Größenskala einiger Mikrometer nachgewiesen werden. Zusätzlich wurde eine Orientierungsabhängigkeit dieses Effekts beobachtet. Außerdem wurde die Diffusion einzelner Kristalle in der Mikrostruktur von ePP beobachtet. Die Heterogenität dieser Diffusion lässt auf eine kristallin-amorph Grenzschicht um alle Kristalle schließen.

Ziel dieser Arbeit ist es, eine Verbindung zwischen der Mikrostruktur teilkristalliner Polymere und derer mechanischen Eigenschaften auf der Mikro- und Nanometerskala aufzubauen. Dazu wurden Methoden der Rasterkraftmikroskopie verwendet um sowohl orts- als auch zeitaufgelöst Kristallisations-, Deformations- und Diffusionsprozesse in der Mikrostruktur von elastomerem Polypropylen (ePP) abzubilden. Die mechanischen Eigenschaften wurden simultan mit Mikrozugversuchen bestimmt. So konnte beispielsweise ein Zusammenhang zwischen abnehmender Kristall-Kristall-Distanz und einem Ansteigen des Elastizitätsmoduls während der Kristallisation nachgewiesen werden. Weiterhin war es möglich die Veränderung der nano-mechanischen Eigenschaften während der Kristallisation einzelner kristalliner Lamellen in deren direkter Umgebung mit MUSIC-mode Rasterkraftmikroskopie zu untersuchen. Laterale Querexpansion (auxetisches Verhalten) konnte bei uniaxialen Zugversuchen für die Kreuzschraffur-Struktur elastomeren Polypropylens auf der Größenskala einiger Mikrometer nachgewiesen werden. Zusätzlich wurde eine Orientierungsabhängigkeit dieses Effekts beobachtet. Außerdem wurde die Diffusion einzelner Kristalle in der Mikrostruktur von ePP beobachtet. Die Heterogenität dieser Diffusion lässt auf eine kristallin-amorph Grenzschicht um alle Kristalle schließen.

In the absence of a fusion neutron source, research on the structural integrity of materials in the fusion environment relies on current fission data and simulation methods. Through investigation of the Fe-Cr system, this detailed study explores the challenges and limitations in the use of currently available radiation sources for fusion materials research. An investigation of ion-irradiated Fe12%Cr using nanoindentation with a cube corner, Berkovich and spherical tip, and micro-cantilever testing with two different geometries, highlighted that the measurement of irradiation hardening was largely dependent on the type of test used. Selected methods were used for the comparison of Fe6%Cr irradiated by ions and neutrons to a dose of 1.7dpa at a temperature of 288°C. Micro-cantilever tests of the Fe6%Cr alloy with beam depths of 400 to 7000nm, identified that size effects may significantly obscure irradiation hardening and that these effects are dependent on radiation conditions. Irradiation hardening in the neutron-irradiated alloy was approximately double that of the ion-irradiated alloy and exhibited increased work hardening. Similar differences in hardening were observed in an Fe5%Cr alloy after ion-irradiation to a dose of 0.6dpa at 400°C and doses rates of 6 x 10^-4dpa/s and 3 x 10^-5dpa/s. Identified by APT, it was shown that increased irradiation hardening was likely to be caused by the enhanced segregation of Cr observed in the alloy irradiated with the lower dose rate. These observations have significant implications for future fusion materials research in terms of the simulation of fusion relevant radiation conditions and micro-mechanical testing.