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The PhD thesis investigates toughening of polymer nanocomposites in terms of processing, characterization and simulation. Two different nanocomposite systems were addressed, which are polypropylene/nano calcium carbonate (PP/nCC) and polycarbonate/Polyhedral Oligomeric Silsesquioxane(PC/POSS). The first part of the thesis focuses on processing and characterization of the PP/nCC. Batch-process and continuous liquid assisted extrusion were performed. PP nanocomposite containing 0~3wt.% nCC were prepared by those method and further moulded by compression moulding. Impact resistance of the samples were obtained by falling weight impact tester. The dispersion of nCC in the samples prepared from different methodology was compared using SEM. As liquid was involved during processing, DSC and TGA were used to determine the amount of residual liquid phase in the product. The impact resistance of PP/nCC is peaked at the continuous liquid assisted processed sample which contain 2.0 wt.% nCC powder. For this loading of nCC, A roughly 300% increment compare to the pure PP is obtained on energy absorption during FWIT test. Among all process technique based on extrusion, SEM results indicate a better quality of dispersion is achieved by continuous liquid-assisted process compared with directly dry mixing and batch-processed. DSC and TGA results indicate the final products are free from residual liquid phase. DSC results also indicate that the nCC powder promote the crystallization of PP. TGA results indicate that the thermostability of PP is enhanced by introduce well dispersed nCC powder. Toughening mechanism is concluded by analysis of the fracture SEM image. The second part focus on the characterization and numerical simulation of PC/POSS nanocomposite. The aim is to investigate how POSS molecule interact with PC and affect its performance in terms of the tensile properties and toughness. Three grade of PC containing 0 to 0.3 wt. % of POSS is melt blended. The impact resistance of the PC/POSS samples are obtained by FWIT. The tensile properties of the samples are obtained by tensile tester. Fracture surfaces are observed by SEM. Depends on grade, a 30%~150% increment in energy absorption is obtained for PC/POSS nanocomposite. Molecular dynamics simulations are used to answer two questions. The first one is how the POSS particle interact with PC chains. As the PC used contains Si-OH group, possibility of the hydrogen bond formation is investigated. However, the result indicates the hydrogen bond is hardly formed between POSS and PC chains. The reason maybe the stereo effect. On the other hand, after comparing with different structure of POSS, the organic shell part of POSS turns to be the dominate factor of the PC-POSS interaction. The second question is how the POSS affected mechanical performance of PC. Tensile, compression, shear and impact simulation are conducted for PC and PC/POSS system. During tensile simulations, the present of the POSS increase the elongation at yield, which agree with the experiment. During compression simulations, POSS molecules turn to have a stereo effect by its high stiffness. During shearing simulations, POSS molecules absorb energy and deform. During impact simulations, PC/POSS system absorb more energy by interfacial debonding. As suggested by previous research that POSS improved the sustainability of the polymer matrix, Simulations have been conducted to mimic the oxidization at low earth orbit. The result indicates POSS increase the stability of the PC in such condition.

Key to the safe operation of nuclear reactors is the understanding of materials degradation due to neutron damage. Ion implantation is often used as a surrogate for nutron damage when screening nuclear candidate materials. Ion implantation results in a thin damage layer, the mechanical properties of which are often difficult to determine. In this study a micromechanical test regime is developed in a model material, 6H single crystal silicon carbide (SiC). This test technique is then applied to gold ion irradiated zirconium nitride (ZrN). Micromechanical test samples are often prepared using a focused ion beam. However, ion beam milling has the potential to damage the crystal structure of a material and introduce residual stress. Therefore, a range of cutting strategies were used to assess the effects of focused ion beam cutting on the modulus and strength of SiC cantilevers. The effects of sample size were also explored. Gallium ion milling resulted in amorphisation of the surface of the SiC crystal micro cantilevers. The thickness of the amorphous zone was then reduced using low voltage cleaning. Low voltage cleaning did not, however, result in increased mechanical performance as other unintended consequences such as cantilever edge rounding occurred. SiC exhibited a plastic deformation threshold of around 0.3 × 0.3 µm but did not exhibit a significant size effect. Nanoindentation was used as a benchmark test to compare to the mechanical properties gathered during micro bend testing. Under indentation conditions, a size effect was identified in hardness and modulus but not in fracture toughness. Modulus results from indentation, and micro bend testing was comparable when ion damage was accounted for. Hot pressed ZrN samples were ion implanted with gold ions. Microstructural characterisation, nanoindentation and micromechanical tests were performed in the ion implanted zone. Microstructural characterisation identified a dual phase microstructure consisting of ZrN and Zr2ON2. The implanted layer consisted of implanted gold ions followed by a network of dislocations centred around a depth of 1.20 µm. High-resolution electron backscatter diffraction (HR-EBSD) identified that tensile surface stresses and compressive subsurface stress had been introduced. Nanoindentation linked ion implantation to increased hardness and no modification in modulus. Micromechanical testing indicated a reduction in modulus and strength. This work highlighted the need to understand sample size effect and ion damage on micro mechanical tests if they are to be used for screening nuclear materials.

There are thousands of technical papers, reports and other documents which seek to identify the key factors affecting the performance of Grade 91 steel. Yet, as evidenced by the recent reduction in stress allowable values for Grade 91 steel and the introduction of a Type II specification in the ASME B&PV Code, considerable uncertainty still exists regarding the performance, composition and heat treatment. Knowledge regarding the creep performance of weldments is even more limited and opinions on damage development even more diverse. This point is pertinent whether consideration is given to laboratory cross-weld creep behavior or whether the assessment considers thick section components operating under high pressure and temperature. In part, this additional complexity is because the welding thermal cycle introduces a heat affected zone (HAZ). Although it is widely recognized that the HAZ is linked to creep failures of welds which is likely to be life-limiting in structures, there is no agreement over basic approaches such as the correct methods to characterize the microstructures formed, how to classify the structures and the specifics of the region where creep failure occurs. For example, published work states that the locations for failure should be described as the soft-zone, the subcritical heat affected zone (SCHAZ), the over tempered zone (OTZ), the intercritical heat affected zone (ICHAZ) or the fine-grained heat affected zone (FGHAZ). It is thus apparent that for cross-weld behavior in Grade 91 steel these descriptions cannot be simultaneously and technically accurate.

The whole thesis is divided into 8 chapters. Chapter 1 introduces some background information of the polylactic acid (PLA), poly(caprolactone) (PCL) and electrospinning which are the main polymers and technique used for this PhD project. Chapter 2 is a comprehensive literature review related to the project, which firstly introduces nanofibres and various methods to fabricate nanofibres, followed by introducing different setups of electrospinning and the main factors (concentration, molecular weight and solution properties) influencing electrospinning results. Next, previous studies on electrospun, PLA-based, PCL-based and PLA/PCL blend fibres are reviewed. The final part is a critical review of porous polymeric fibres fabricated by different mechanisms and various applications in which electrospun porous fibres show improved performance (tissue engineering, drug delivery and oil spill clean-up & oil/water separation). Chapter 3 is the experimental methods part of the project, which includes preparation of spinning solutions, electrospinning conditions and parameters, and a variety of techniques to characterise spinning solutions and the resultant fibres. Chapters 4, 5 and 6 are results and discussion of the whole PhD project. Chapter 4 is focused on electrospinning of PLA, PCL and PLA/PCL nonporous fibres by using different solvent systems including acetone (AC) or AC/dimethylformamide (DMF), AC/ethanol (EtOH) and AC/dimethyl sulphoxide (DMSO). Also, the effect of blending ratio and applied voltage on fibre morphology and diameter have been studied. In addition, PLA incorporating magnetic particles (Fe3O4) has been electrospun into randomly oriented or aligned fibres. Chapter 5 investigates electrospinning of porous PLA fibres by careful selection of solvents, such as single solvent Chloroform (CHL) and DMSO or binary solvents: (CHL/EtOH and CHL/DMSO). The controlled surface and internal porosity can be attributed to mechanisms including breath figure, non-solvent induced phase separation (NIPS) and vapour induced phase separation (VIPS). Chapter 6 investigates fabrication of electrospun porous PLA, PCL and PLA/PCL blend fibres by the NIPS mechanism at given conditions (10kV, CHL/DMSO 80/20 %v/v). Properties, including shear viscosity of solutions, morphologies, phase inversion, thermal, mechanical and in vitro degradation of porous fibres are also studied in detail. Chapter 7 concisely summaries the results and some discussion of the whole thesis. Chapter 8 suggests some future work that can be done.

Interest and investment in additive manufacturing (AM) has been growing exponentially in recent years. Many AM processes for polymers and metals are nearing maturity, however for ceramics there are fewer processes available and they are in their early stages of development, with a significantly narrower material selection. AM offers benefits of light-weighting and increased efficiency through complex functional designs not possible by conventional manufacturing techniques whilst reducing material waste and tooling costs for different components. Ceramic heaters that exhibit positive temperature coefficient of resistance (PTCR) could be fabricated by AM for improved use in automotive and aerospace applications. This project demonstrates the feasibility of using AM to fabricate PTCR heating elements in various shapes and sizes including honeycomb lattice structures with the same material extrusion equipment - thus signifying the flexibility and suitability of AM to manufacture complex heater geometries. The project work traverse through an entire power-to-product processing excursion to achieve a functional prototype PTCR ceramic heater. Barium titanate (BT) and lanthanum/manganese doped barium titanate (BT) based PTCR functional heater elements/structures were fabricated with desirable electrical properties for the first time using additive manufacturing by material-extrusion (robocasting). 3D printed components of varying size and shape, and prototype honeycomb lattices with high density were achieved with comparable and in some cases superior electrical properties. Aqueous, less organic containing (2.5 wt% additives versus 10-30 wt% added typically), eco-friendly ink formulations were developed with suitable rheological properties for 3D printing. For BT prints, the sintered densities of the 3D ceramic parts were found to be >99% TD, this is the highest reported value so far. The rheology of pastes required for robocasting was investigated in detail to understand the effect of dispersant, solids loading and viscofier contents. The rheological characteristics studied included yield stress, viscosity, shear stress, storage modulus and loss modulus. Different BT based PTCR mixtures were investigated through 3D printing, calcination, sintering along with conventional pressing & sintering for comparison. Effect of printing parameters on print quality and print microstructure were investigated. Drying of the prints was investigated as a parameter to allow printing of a wider range of pastes with different viscosity and yield stress. Yield stress of paste formulations and controlled drying were identified as key contributors to the successful 3D fabrication via robocasting. The microstructure, electrical properties and heating characteristics of the printed PTCR components were studied in detail and their thermal stability evaluated using infrared imaging and benchmarked against commercial PTCR heating element. The heating behaviour of the solid and porous 3D printed components was demonstrated to be similar, paving the way for light weight ( ̴47% reduction in weight) heaters suitable for automotive, aeronautical and even astronautical applications and the technique can also lead to significant materials savings during device fabrication. The general applicability of the robocasting-based AM technique augers well for its use to manufacture complex shaped advanced ceramics for demanding applications, for example, in automotive (heaters as described here), healthcare (biomedical implants - personalised dental and hip/knee implants), defence (high frequency, high temperature conformal antenna structures) and energy (battery components) sectors.

This thesis is based on a collection of papers and patents published and filed between 2010 and 2017 and added as appendixes 1 to 16. The objective of the work was to study the influence of certain reaction parameters to achieve a profitable industrial scale production of ultra-high molecular weight polyethylene UHMWPE with reduced number of entanglements, aiming to establish the: most cost-effective chemicals to use for highest amount of product per batch with a targeted product morphology to achieve best processability in continuous solid state processing (chapter 3); targeted molecular weight of the product to achieve desired performances in the commercial article (chapter 4); addition of functionalities to improve product synthesis and processability as well as to increase performances and renovate manufacturing of new articles (chapter 5). In this work most of the issues on synthesis upscaling have been addressed and some solutions presented have allowed the first successful upscaling to 500 L reactor in the synthesis of nascent disentangled UHMWPE. A systematic study of mechanical properties vs molar mass vs reaction conditions is presented, proving a direct correlation between the three. Such correlation was present even in the preliminary data for thermal conductivity of thin films of UHMWPE. Moreover, novel routes to prepare UHMWPE composites by benefitting the unique processability of nascent disentangled UHMWPE have been described. In addition, the synthesis of disentangled UHMWPE from supported catalysts and the preliminary tests in absence of solvent media have the potential to unlock the upscaling to regular continuous gas phase reactor.

The use of fillers in materials engineering has long been used to improve material properties such as strength, stiffness and toughness. However, these enhancements tend to occur at the detriment to optical transparency in transparent polymeric materials. Nano-sized fillers or nanofillers are thought to maintain the transparency of such materials due to their small size. Moreover because of their smaller size, nanofillers have larger interfacial areas and can therefore provide increased interaction sites (providing fine dispersion in the medium). In this work, the nanofiller trisilanol-phenyl polyhedral oligomeric silsesquioxane (TSP-POSS) was added to polycarbonate (PC) to create a series of polymer nanocomposites for the intended use in transparent, lightweight armour. A variety of techniques were used to characterise the structural, morphological, viscoelastic, optical and mechanical properties of the polymer nanocomposites. A preliminary study into the effect of TSP-POSS on the physical aging behaviour of polycarbonate was also carried out. The effect of TSP-POSS on polycarbonate depended on its loading in the matrix and the type of strain it was subjected to. Increases in toughness were seen in the nanocomposites in comparison to the unmodified polymer and is thought to be due to the formation of a stretchy hydrogen bonded network between TSP-POSS and the polycarbonate chains. Spectroscopy and microscopy techniques suggest TSP-POSS is uniformly dispersed at the nano- or molecular-scale within the polymer matrix, and therefore indicative of high compatibility between the nanofiller and the polymer. Moreover, TSP-POSS can induce one of two toughening mechanisms in the polymer matrix; at low loadings (0.1-0.3wt%) ductility of the matrix increases to form stretched fibrils, at higher loadings (0.5-1wt%), potential nanoscale TSP-POSS aggregates are observed, which appear to be involved in plastic void growth. Both changes increase the energy absorbing capabilities of the polymer matrix. Crucially, it was found that the optical transparency was maintained at all nanofiller loadings. However, at higher strain rates the toughness enhancements of TSP-POSS disappear. This could be due to the higher impact energy which overcomes the interactions between TSP-POSS and the polymer chains, particularly if these involve weak bonds e.g. hydrogen. Therefore, the TSP-POSS/PC nanocomposite studied in this work would be more suitable for protective applications involving low velocity projectiles or even as lightweight, backing materials for energy absorbing applications.

Grade 92 steel is a commonly used material for steam pipe and tube applications in the power generation industry. The advantages of Grade 92 steel include excellent creep resistance and sufficient corrosion resistance, which provide a long service lifetime expectancy for components. Grade 92 steel is typically heat treated by a normalisation process performed at approximately 1100°C followed by a tempering process at approximately 750°C. The resulting microstructure of Grade 92 steel is composed of a tempered martensitic matrix with secondary precipitates distributed both on lath and grain boundaries and within laths and grain interiors. Welds in thick-section steam pipes made from Grade 92 steel are typically fabricated by a gas tungsten arc root process followed by a multi-pass submerged arc fill process. Post weld heat treatment (PWHT) is then performed on as-fabricated welds, helping to relieve residual stress and stabilise the martensitic microstructure. The microstructures in the HAZs of these complicated welds have not to date been fully understood. There is a lack of systematic microstructural investigations to define the different regions of the microstructure across the HAZ as a function of the welding process.

Ni-based alloys must be considered for specific components in advanced ultra supercritical (A-USC) plants in order to cope with the conditions required for high efficiency energy conversion plants at temperatures of ~760°C and pressures of 35 MPa. For large, complex components such as valves and casings, casting is the main route for their commercial production. Haynes(H) 282 and Inconel (IN) 740H are two possible candidate alloys, however, both of these alloys were originally developed for wrought applications rather than cast applications, and consequently there are limited data about the cast microstructure and its associated challenges for these alloys. This project has focused on large castings manufactured from H282 andIN740H and aims to produce comprehensive data for their cast versions, including their microstructure and associated issues, and this has then led to consideration of the development of a new composition Ni-based alloy suitable for the manufactureof large castings.

Dismantleable structural adhesion is required to facilitate the recycling and re-use of critical materials and to enable flexible manufacturing. There are a number of reported ways to achieve dismantleable adhesion. These include: the incorporation of functional fillers such as thermally expandable microspheres (TEMs) and chemical foaming agents (CFAs) in the adhesive phase; and designing joints which on the application of a potential di?erence lead to disbonding. These approaches have shown > 90 % reduction in initial adhesion levels but have limitations in terms of their applicability in industrially-useful bonding systems. The current Project will introduce a number of novel methods for achieving dismantleable adhesion with aluminium-to-aluminium and carbon fibre reinforced polymer (CFRP)-to-CFRP joints. These methods are based upon: microwave triggerable reactive fillers within the adhesive phase; ohmic heating of a semiconductive adhesive; and thermally decomposable interphases. Expandable graphite (EG) exfoliates upon heating, which can be triggered when exposed to microwaves. When the EG is added to the adhesive phase and microwaved, a solid piece of adhesive forms a flaky powder. When exposed to microwaves, CFRP-to-CFRP joints bonded using an adhesive containing EG failed within 40 s. Semi-conductive adhesives were produced which underwent ohmic heating when a constant current was applied to them. Aluminium-to-aluminium joints were manufactured which were subsequently bonded using an adhesive which had functionalised carbon fibre added. Static load tests were carried out with a 196 N load; when 50 A was passed through a joint containing 30 wt% carbon fibre in the adhesive, the sample failed in 3 s. Thermally decomposable interphases were developed for dismantling aluminiumto-aluminium bonded joints. The aluminium substrate was anodised and then subsequently sealed in boiling water to form a hydrated layer on the surface. A solgel layer was then applied to the hydrated oxide and joints were manufactured. On heating, the hydrated layer released water which initiated the reverse condensation reaction of the sol-gel, causing a failure at the interphase. In all the above cases, the novel approaches have proven to be extremely effective with initial adhesion levels of 15 MPa and above. They either reduce to zero joint strength after triggering, or, fail in static load tests within a few seconds. This Thesis will provide a scientific understanding of the novel methods used and for the observed reduction in joint strengths.

This project is basically an investigation of graphene and carbon nanotube (CNT) based material's electrical properties. In its first part, graphene (G) and graphene oxide (GO)/carbon nanotubes (CNTs) hybrid films were successfully fabricated as high-performance electrode materials for an energy storage application using a simple water solution casting method and with an assistance of strong ultra-sonication. This was done with different contents of G, GO, single-wall CNT (SWCNT), multi-wall CNT (MWCNT) and multi-wall CNT with a hydroxyl group (MWCNT-OH). The films with MWCNTs showed well interconnected layered structures at the nanoscale range where GO worked as support insulated plates for the CNTs. The electrical properties were investigated in an alternating circuit (AC) which revealed a linear relationship between the dielectric constant and the weight percent of the CNTs. By increasing the CNT contents, the dielectric constant of the G/MWCNT and GO/MWCNT films raised almost linearly and their resistivity reduced. On the other hand, the dielectric constant was found to be decreased as the frequency went up. The maximum special capacitance reached 142 F/g in G (40wt %) /MWCNTs (60wt %) in which the dielectric constant reached 9.98 x107/g in the same film. In comparison, GO/SWCNT and G/SWCNT were found to be not applicable to be used as a capacitor system using the water solution casting method which resulted in a bad dispersion. G/SWCNT and GO/SWCNT films did not form layered structures leading to a very low dielectric constant. On the other hand, the dimension and the thickness of the film influence the capacitor performance and the conductivity. Shorter and thicker film can make a huge difference. Nonlinear behaviour of the dielectric constant with voltage was observed in both ofG/CNT or GO/CNT hybrid films. At some voltages, the dielectric constant reached to peak or valley. Obviously, it is quite dependent on the voltage loaded. In the second part, a well-dispersed MWCNT/HDPE nanocomposite powder was successfully prepared by coating the MWCNTs on the surface of the matrix particles (HDPE). The volume resistivity of the nanocomposites was investigated relating to the temperature and stress influences. Besides, the reproducibility of the nanocomposites was studied in this project and several conclusions could be drawn: Firstly, the average electrical resistivity for the MWCNT/HDPE nanocomposite sheets with the MWCNT contents of 0.1 wt%, 0.5 wt%, and 1.0 wt% were 792.64 kO.mm, 111.67 kO.mm, and 9.953 kO.mm respectively, which indicated that the 1.0 wt% MWCNT/HDPE nanocomposite showed the best electrical conductivity. Furthermore, the results of the temperature electrical conductivity measurements revealed that with rising of temperature, the electrical resistivity for the MWCNT/HDPE nanocomposites increases due to the widening of the distances between the conductive nanofillers. In addition, the heat treatment could effectively improve the reproducibility of the MWCNT/HDPE nanocomposites, especially the nanocomposite with the MWCNT content of 1.0 wt%, as it has been found that the voids in the nanocomposite sheets were excluded during the heat treatment. Finally, the results of the tensional electrical resistivity measurements showed that the initial electrical resistivity for the MWCNT/HDPE nanocomposites increased with the increase of the applied tensional stress which caused the widening of the distances between the conductive nanofillers and some conductive networks to be damaged. Additionally, the reproducibility of the 1.0 wt% MWCNT/HDPE nanocomposites was better than that of the 0.5 wt% MWCNT/HDPE nanocomposite. It was found that the MWCNT/HDPE nanocomposite sheets exhibited 'viscoelastic' behaviour of the electrical recovery in which the electrical resistivity could not totally recover after relaxation.

The aim of this research project was to obtain a deep and quantified understanding of the effects of chemical composition and heat treatment on the microstructures of Grade 92 parent metals, and in turn, their corresponding creep behaviour. Comprehensive microstructural characterisation on the virgin and creep-tested samples was carried out in this study to give valid and reliable data in order to correlate the microstructural changes with creep behaviour, which can provide steel manufacturers with useful information to optimise the manufacturing route for fabrication. Predicted long term creep behaviour based on the numerical extrapolations of short term data has been shown to be overestimated due to series of microstructural degradations. In particular, creep cavitation, which is equally important compared to other microstructural degradation mechanisms during creep exposure, was investigated by using a wide range of complementary microscopy techniques including field emission gun scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM) on focused ion beam (FIB) prepared thin foil lift-out samples with energy dispersive X-ray spectroscopy (EDS), dual beam serial sectioning with post image 3D reconstruction on the cavities and particles, and combined in-lens SEM examination and EDS analysis. BN, MnS and Al2O3 inclusion particles were observed to be the preferential cavity nucleation sites. Within these three types of inclusion particles, BN was the most frequent particle observed to be associated with cavities.

Martensitic 9-12% Cr steels have been the key materials used to increase the efficiency of ultra-super-critical (USC) power plants, offering a good combination of high steam oxidation resistance and high creep strength. Martensitic 9-12% Cr steels have been the key materials used to increase the efficiency of ultra-super-critical (USC) power plants, offering a good combination of high steam oxidation resistance and high creep strength. The addition of boron was found to stabilize the martensitic laths by decreasing the coarsening rate of M23C6 carbides, which act as pinning points in the microstructure and lead to an improved creep behaviour in these new enhanced martensitic steels. The aim of this research project was to obtain a deep and quantified understanding of microstructural changes in boron-containing high Cr ferritic-martensitic power plant steels, as a function of pre-service heat treatment, stress, time and temperature. The creep strength, which is the main design criteria for this class of alloys, depends on the stability of the microstructure, which consists of tempered martensite and a fine dispersion of carbide precipitates. During this study a comprehensive microstructural characterisation on the virgin and creep-tested samples was undertaken in order to provide valid and reliable data to correlate the microstructural changes with creep behavior. This can provide steel manufacturers with useful information to optimise the manufacturing route for fabrication. Advanced characterisation techniques have been employed to investigate the effect of normalizing and tempering on the microstructure of a MARBN steel, and the stability of second phase particles after pre-service heat treatment has been investigated. It has been observed that the normalizing temperature has a significant impact on the size of prior austenite grain boundaries, which increase significantly at higher normalizing temperature. Early precipitation of Laves phase has been observed after tempering. The processing conditions were observed to have a significant impact on the area percentage of this phase. It has been identified that normalizing temperature doesn't seem to significantly affect the precipitation of Laves, whereas the tempering temperature and time were found to influence the precipitation of Fe₂W precipitates. Furthermore, the tempering temperature was observed to have a direct impact on the number of M₂₃C₆ and MX carbonitrides precipitated during tempering. In fact, a reduction of M₂₃C₆ particles and an increase of MX carbonitrades has been observed by increasing the tempering temperature. The effect of manufacturing process on the microstructural evolution and creep life of two modified FB2 steels, with the same chemical composition but different manufacturing process, was studied using scanning and transmission electron microscopy (SEM and TEM), focused ion beam microscopy (FIB), energy dispersive x-ray spectroscopy (EDS), electron backscattered diffraction (EBSD), X-ray diffraction (XRD) and thermodynamic equilibrium calculations. Investigations have proven that the additional melting stage before the pre heattreatment underwent by one of the two steels has significantly affected the boron nitride (BN) characteristics in the material. In fact, it was observed that the amount of BN particles is much higher in the steel that hasn't experienced the re-melting process (sample C). In turn, the faster evolution rate of the M₂₃C₆ particles in the C sample during the exposure to the creep conditions correlates very well with the amount of BN particles observed in the original conditions. It is believed that more boron is trapped in the BN phase in the C sample, leaving less boron in the solid solution to stabilize the M₂₃C₆ particles from coarsening and in turn influence the population of creep resisting VN particles. Analysis of the gauge sections using a combined in-lens SEM/EDS approach showed that cavities containing BN particles were also present within the C steel, while no cavities were detected in the P steel, which is the steel that has experience the re-melting stage. Hence, it is believed that the significantly shorter creep life of the C steel compared to the P sample is determinated by a combined effect of the higher BN particle number density and the formation of cavities containing BN particles within the gauge area, which contribute to the real fracture. The lower BN content in the P sample, as well as the absence of any cavities, instead, could result in an increased concentration of soluble boron available to stabilize the M₂₃C₆ particles from coarsening during creep, as well as more nitrogen, which allowed a more stable MX distribution, resulting in the observed increased creep life. Advanced characterisation techniques have also been employed to understand the reasons why a modified St13 steel failed the ultrasonic test performed by the manufacturer for a first quality check before pre-heat treatment. Bright field reflected light microscopy revealed a very heterogeneous microstructure with large grains (sized up to 4 mm) mixed with area with much smaller grains. A combination of selected area diffraction and STEM-EDS analysis was used to identify the large VN "decorating" the grain boundaries, which have been identified as the possible reasons of the ultrasonic test failing. Ni enriched regions were also detected in the modified St13 steels investigated, and TEM diffraction confirmed their fcc crystal structure to be consistent with γ austenite. An orientation relationship, corresponding to Kurdjumov-Sachs (K-S), has also been identified between the bcc martensitic matrix and the fcc retained austenite. The study of the evolution of Ni-rich regions has also shown that the exposure to the creep conditions significantly accellerates the coarsening of the Ni enriched regions in the modified St13 steel investigated.

Stainless steels have been used for decades in nuclear power stations, particularly in pipe and tube applications. For these applications, welding is often required to join sections of components or pipes/tubes due to their large sizes and lengths. These components may be required to withstand the harsh operating conditions inside nuclear power stations (high temperatures and pressures) for durations of more than 25 years. During long term ageing at high temperatures, the microstructures of materials are known to evolve and change as a function of time at temperature. These changes must be characterised and understood to ensure the safe and economic operation of nuclear power stations. This thesis investigates five different stainless steels and their response to long term isothermal ageing in the context of advanced gas-cooled reactors. The five different materials include three base materials: A wrought type-321 stainless steel and two cast Nb-stabilised stainless steels. The other two materials are two welds made from welding each cast steel to the wrought type-321. The materials were aged at both 650°C and 750°C for up to 15,000 hours in an inert atmosphere. All five materials were characterised using optical microscopy, scanning electron microscopy, focussed ion beam imaging and transmission electron microscopy. A displacive form of ferrite was observed in the wrought type-321 material after ageing for 1000 hours at 750°C, which was found to increase in volume fraction during long term thermal ageing. Sigma phase was also detected during ageing at both temperatures along with secondary intergranular Ti(C,N) particles in the wrought material. An extensive sigma phase transformation took place in both of the cast materials, with a large proportion of delta ferrite transforming to sigma phase gradually during ageing. This sigma phase transformation was accelerated in both cast materials at 750°C compared to at 650°C. The sigma phase regions displayed a tendency to crack after longer ageing times, which is a significant concern with regard to the mechanical properties of these alloys during long term thermal ageing. Both welds exhibited a large scale martensitic transformation during long term ageing, which grew gradually inwards to the centre of the weld bead from the base materials. The martensitic regions were localised, with several small regions being present in the same parent austenite grain. The martensitic regions maintained a consistent orientation relationship with the parent austenite grains, along with being chemically indifferent to the parent austenite. The martensitic regions were found to increase in size at longer ageing times at both temperatures. The welds both also exhibited sigma phase precipitation after 1000 hours at 650°C and 500 hours at 750°C with particles found to increase in size at later ageing times with the general size of the sigma phase particles consistently being larger at 750°C than at 650°C when compared at the same ageing time.

There exists a broad range of automotive and marine coatings which cater for aesthetic considerations as well as protecting against a number of potentially deleterious environmental factors. Polyurethanes (PU) paint coatings are particularly widely used in this application. It is also possible to optimise the aesthetic and protective properties of these coatings yet further through the application of an additional paint protection scheme (PPS). Such PPS layers are often siloxane based thin films which are commonly applied due to the particularly demanding performance requirements in this automotive and marine sectors. A range of experimental methods and chemical analysis techniques have been used to study and establish the formulation and baseline performance characteristics of one such, commercially available siloxane based PPS. Orbitrap, gas chromatography (GCMS) and tandem mass (MS/MS) spectroscopy techniques have shown the siloxane based PPS polymers within the present study to contain amino, alkoxy and hydroxyl functionalities, as well as methyl pendants. Atomic force microscopy (AFM), glossmeter measurements, spectrophotometry, water contact angle analysis (WCA) and X-ray photoelectron spectroscopy (XPS) have been used to characterise the effect of the PPS on a polyurethane paint coating. Results show that the existing system deposits: ~20 at.% of a Si-O based coating. This alters the surface wettability, from water contact angles and surface free energy (SFE) values of ~65° and ~56 mN/m, to ~90° and ~31 mN/n respectively. In addition, this coating increases surface gloss from ~64° to ~75° at 60° measurement angle; and decreases surfaces roughness (Ra) from ~60 to ~30 nm. Attempts to modify and enhance the wettability, gloss and stiffness of this existing coating, using a number of polymers and fillers were generally unsuccessful. However, an increase in surfaces stiffness of ~38% was observed through the use of 0.25 wt.% carboxylic functionalised nanodiamond. In recognition of the difficulty in augmenting the existing siloxane PPS coating, an entirely new coating and matrix was developed using an oxygen/argon pretreatment and a fluoroalkyl silane (FAS) coating, with sole emphasis on two critical performance characteristics: superhydrophobicity and anti-biofouling. It was found that the oxygen/argon plasma treatment increased both the surface roughness (Ra) and SFE of the PU paint coating from approximately ~60 to ~320 nm, and, ~52 to ~80 mN/m respectively. It was also found that the plasma process created a multiscale roughened texture through the process of differential ablation between the PU polymer and the barium sulphate solid content, present in the paint as an extender, and other additives. In addition, the process also imparted favourable polar groups into the PU surface from the ionised and radical oxygen species in the plasma. When the FAS coating was subsequently applied to the PU without prior plasma treatment, there was a significant increase in water contact angles. This parameter increased from approximately 60° on untreated PU to around 130° with FAS applied. In this case, the SFE decreased to ~7.5 mN/m and showed 42.0 at.% fluorine present as indicated by XPS. However, subsequently applying the FAS polymer after plasma pretreatment takes advantage of the known synergistic relationship that exists between surface roughness and low surface free energy coatings. The two processes combined to create superhydrophobicity with a surface that exhibited water contact angles up to 153.1°. With this optimised process, the apparent SFE was reduced to 0.84 mN/m with a more highly fluorinated surface present. In this case 47.2 at.% surface fluorine was observed by XPS, with a coating thickness of ~26nm. The coating has also been shown to exhibit excellent resistance to a salt spray environment compared to the siloxane based coating, as well as provide a an improvement in the PU's anti-biofouling capability against staphylococcus epidermidis. In addition to the changes in SFE, the plasma treatments also alters levels of surfaces gloss and colour. After exposure to 600s of plasma gloss levels are shown to reduce from ~50 to ~21 (at 60°), with a small but significant increase in the lightness and yellowness of the surface.

A series of graphene based conductive and anticorrosion coatings were developed in this project. Multi-layer coating consists of EPD pristine graphene coating, PU/graphene primer and PU/graphene topcoat was developed. A simple mechanical-chemical approach was suggested to fabricate graphene with low cost and high efficiency. XRD was used to characterize the exfoliation efficiency of graphite. TEM was used to examine the size of the graphene sheets. SEM was used to characterize the surface morphology of the coatings. The particle size of all the carbon materials used was characterised by Malvern particle sizer. FTIR and XPS were used to characterize the chemical composition of the graphene powder and the coatings fabricated. MDSC and FTIR were used to monitor the cure dynamic of PU.The proposed mechanical-chemical approach was cost effective and suitable for large-scale production of graphene. XRD results indicated that the graphite layers were exfoliated efficiently and TEM results confirmed the existence of graphene. EPD was used to deposit graphene and PP10 graphite on steel substrates. With fine control of the EPD conditions and thermal treatment, electrical conductive coatings on steel substrates were produced successfully. The best electrical conductivity was 10 times higher than the electrical conductivity than steel substrate. The anticorrosion properties of the EPD coatings were not good due to their porous nature. Therefore, a polymeric protective coating is needed toimprove the anti-corrosion properties. Hybrid filler was adopted in the PU nanocomposites and the performance of the nanocomposites reinforced by hybrid filler was the best. The conductive mechanism of the nanocomposites was proposed. From the results of FTIR and MDSC, graphene had catalytic effect and steric hindrance effect on the cure of PU where catalytic effect was more obvious at high cure temperature and steric hindrance effect was more dominated with high graphene loading. GO also showed catalytic effect and steric hindrance effect. In addition, the functional groups on the GO surface can participate in the reaction with PU. Therefore, the reaction mechanism was altered. From the results, the addition of excessive amount of filler can significantly affect the cure behaviour of two-part PU and even reduce the crosslink density and weaken its mechanical properties.

In this thesis a non-equilibrium polymer melt with a heterogeneous distribution of entanglement density is investigated. This melt is achieved on melting of Ultra-High Molecular Weight Polyethylene (UHMWPE) which has been synthesised in specific conditions to yield a nascent material with a low amount of entanglements compared to its equilibrium state. In time the melt equilibrates by formation of entanglements, continuously increasing its elastic modulus, until the equilibrium state is reached, having a homogeneous distribution of entanglements. Step strain and step stress rheology on the non-equilibrium melts show that relaxation times and viscosity increase during equilibration following a power law dependence on the instantaneous elastic modulus. In this way, information of the elastic modulus increase can be used to transfer relaxation modulus or creep compliance, of experiments started out of equilibrium, to an effective time domain where time translational invariance is regained and equilibrium viscoelastic behaviour is observed. To increase the range of available characterisation techniques, deuterated UHMWPEs with an initially low amount of entanglements are synthesised, as well as block copolymers consisting of blocks of protonated and deuterated UHMWPE.

The reactor pressure vessel (RPV) is a key component within a nuclear reactor, as it contains fission products and acts as a primary pressure boundary. The hostile conditions these components are subjected to means that the initial mechanical properties and microstructure at the start of life need to be well understood to enable design and safety allowances for their evolution in service to be made. RPVs have historically been manufactured from large thick section forgings, produced from ASME SA508 type steels and fabricated through submerged arc welding. Where the welded joints are paramount to maintaining the integrity and safe operation of the nuclear plant. However, through the collaboration of the Generation IV initiative [1] new materials and joining processes are being researched and trialled to prolong the reactor life. The effect of forging prior to heat treatment is a key feature to understand and it is important to determine what happens to the microstructure following the forming process prior to heat treatment. Through the use of a trial forging it was possible to establish that the effect of forging was not uniform across the whole component, with areas of high and low stress and strain identified. However, the effect of location within the forging was found to have less of an effect on the final properties achieved, compared to the effect of heat treatment. Heat treatment is carried out following the forging process to achieve the mechanical properties required for the application of reactor pressure vessels. However, to determine what happens to both the mechanical properties and microstructure and to investigate the relationship between the two, sections of the trial forging were subjected to different cooling rates and tempering times. It was found that air cooling leads to the formation of ferrite throughout the microstructure and this has a detrimental effect on the mechanical properties achieved. However, water quenching the component does not give the material time for ferrite grains to form and instead results in a lath structure of bainite. Water quenching combined with a tempering heat treatment that results in a Larson Miller Parameter of 19 was found to give preferred properties that are substantially higher than that of the other thermal treatment conditions examined. Narrow gap tungsten inert gas (NG-TIG) welding has also been investigated as a replacement for submerged arc welding of reactor pressure vessels. It was found that the NG-TIG weld provided a more refined microstructure in the heat affected zone and fusion regions of the samples compared to the heterogeneous microstructure achieved with submerged arc welding. This also contributed to the substantially improved mechanical properties, including increased ultimate tensile strength and hardness in each region of the weld microstructure. This research has established an understanding of an SA508 Grade 3 forging and the effect heat treatment and cooling rates have on the microstructure and mechanical properties that can be achieved with this grade of material. A new welding technique for the fabrication of reactor pressure vessels has also been determined to be an improved process, in which the microstructural refinement and enhanced properties achieved will potentially help to prolong the life of the RPV in the next generation of nuclear reactors.

The aim of this study is to ultimately improve the multi-hit capability of armoured constructs. To gain an understanding of how different armoured layers respond to impact, and distribute the stress within the armour pack. In this study an investigation into possible adhesives and surface treatments for armoured constructs is conducted, first at a laboratory level, followed by full scale ballistic testing. A joint will always fail at its weakest link, so understanding and manipulating these interfaces will ultimately improve the multi-hit capability of vehicular armour. Furthermore an investigation into whether an interphase is formed with the adjoining substrates has been carried out. The adhesives and interphase layers help determine the distribution of stresses within an armour pack. A two part epoxy with and without nano-fillers, an elastic two-part adhesive and a silicone were used in this study. The interphase study was completed using Fourier Transform Infrared Spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC).

This thesis studies the microstructural evolution of welded austenitic stainless steels that are commonly used in power plant applications. This work has been funded by EDF Energy and will be used to aid with the plant life extension work for the Advanced Gas Cooled Reactor nuclear power plants currently operating in the UK. Extending the life of power plants puts demands on the steel pipe work, as the steels could be exposed to high temperatures for many years past their originally planned decommissioning date. This could cause changes in microstructure which may result in worsening of mechanical properties The plant life extension will help to meet the rising demand for energy in the UK, but the effect on materials must be understood in order to achieve this. Various grades of austenitic stainless steels were artificially aged at temperatures of 650°C and 750°C for up to 15,000 hours and then analysed using a variety of techniques. This analysis showed differences in the microstructural evolution between the alloy grades, as well as between the parent and weld metals of the same alloying grade. The 321 and 347 grade austenitic stainless steels primarily formed the brittle intermetallic sigma phase during thermal ageing. Whereas the 316 grade stainless steel showed a more complex precipitation sequence due to differences in the chemical composition. The size of the particles was measured using ion beam imaging as it provided excellent contrast between the matrix and the intermetallic particles. The particles formed in all of the alloys rapidly grew during the first 1,000 hours after which the growth rate decreased. The Larson Miller Parameter was used to relate the equivalent ageing times between ageing at different temperatures. It was found that to make the Larson Miller Parameter fit for the 316H parent metal a constant of 10 for the area fraction and 15 for the particle size was required. Whereas, the 316L, 321 and 347 alloys fit better with a constant of 5.81 due to having a different precipitation sequence to the 316H alloy. A low temperature ferrite phase was found to form in the 321 and 347 grade austenitic stainless steels in the unaged condition of both the parent and weld metals. In the parent metal the ferrite phase was characterised using Electron Back Scatter Diffraction, while in the weld metals Energy Dispersive Xray Spectroscopy was also used to aid in the differentiation between the alpha and delta ferrite. The presence of alpha ferrite was unusual for these steels as they were expected to be primarily austenitic. As such, various techniques were used to investigate the behaviour of this phase, this included dilatometry, Differential Scanning Calorimetry, high temperature X-ray diffraction and various heat treatments. It was found that ferrite dissolved at between 800°C and 900°C but re-formed again on cooling at around 200°C. Analysing the chemical composition of these steels showed that the nitrogen content was particularly low, as nitrogen is a strong austenite stabiliser it was thought to be the cause of the austenite instability. It was also found that, during thermal ageing, the area fraction of ferrite increased. This was postulated to be caused by the diffusion of nitrogen into the sigma phase which reduces the stability of the austenite, which can then transform to ferrite during cooling.

Highly extent exfoliated and intercalated silicone rubber (SR) nanocomposites based on natural montmorillonite (Cloisite Na+) and organically modified montmorillonite (Cloisite 30B and Cloisite 20A) were successfully prepared by melt-mixing technique. As indicated by the X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis, intercalation and exfoliation of the clay particles in the nanocomposites was achieved at less than 8 parts per hundred (phr) rubber by weight, irrespective of the initial interlayer spacing of the nanoclay particles. Cloisite 30B was spontaneously transformed into exfoliated nanostructures during the vulcanisation stage as suggested by X-ray diffraction (XRD), fourier transforms infra-red spectroscopy (FTIR) and differential scanning calorimeter (DSC) results. Overall, the use of the nanoclays in silicone rubber improved the Young's modulus, tensile strength and elongation at break by more than 50% as compared to the neat SR. However, the Cloisite Na+yielded outstanding mechanical properties with low hysteresis at the same loading of the exfoliated Cloisite 30B and intercalated Cloisite 20A organoclays, even though the filler had a tendency to produce mixture intercalated/exfoliated nanostructures in the SR matrix. It was concluded that the tensile strength and Young's modulus in this particular rubber nanocomposite are dominated bythe degree of crosslink density instead of filler dispersion. In addition, incorporation of the nanoclays in SR altered the wear mechanism of silicone rubber where addition of the fillers minimised the weight loss during abrasion testing, particularly at low filler loading (< 6phr). Moreover, this study showed that the SR/C30B nanocomposite exhibited excellent heat resistance in comparison to the other two nanocomposites and neat SR as revealed by higher retention strength. The thermal stability of the rubber in air was strongly dependent on the clay morphology and increased in the following order: highly intercalated/exfoliated SR/Na+MMT < highly intercalated SR/C20A < highly exfoliated SR/C30B. Furthermore, the performance of SR Abstractiiinanocomposites containing precipitated silica (PS)/montmorillonite (MMT) hybrid fillers were investigated as a function of PS/MMT hybrid filler weight ratio. Interestingly, the stress-strain properties of the nanocomposites with the hybrid fillers showed a large improvement at high deformation in comparison with the properties of those nanocomposites containing the PS and MMT fillers. The resistance to ageing of SR/PS/MMT was improved by gradually adding amounts of PS filler into the PS/MMT hybrid filler system. Finally, the comparative study demonstrated that the SR containing precipitated silica (PS)/multiwall carbon nanotube (MWMT) hybrid filler significantly improved stiffness of the rubber but accompanied by a small improvement in the tensile strength and elongation at breakwith regard to the SR/PS/MMT. In spite of that, the inclusion of PS/MWNT gave adverse effects to the cure efficiency of the SR compounds which is a drawback from processing point of view.

Environmental and regulatory constraints on the global energy market are driving the need to improve conventional power generation methods to meet commitments made to reduce carbon emissions set out in the Kyoto Protocol. To maintain security of supply and ensure competitive energy prices, fossil fuels will play a vital role in the energy market over the next century, however, in order to comply with regulations it is necessary to improve on current conventional power generation technologies. Improvements in security of supply and environmentally deleterious emissions can be realised via increases in plant efficiency. A major route to achieving a considerable increase in efficiency in conventional coal fired power station is via elevating the operating conditions of the working steam to benefit from enhanced thermal cycle efficiencies. It has been proposed that increasing the operating steam parameters from the current typical levels of ~540°C/~190 bar to ~700°C/~300 bar could lead to an increase in plant efficiency of up to 15%. The major challenges to increasing steam parameters need to be met by advanced materials. Increasing the steam temperature and pressure to the suggested levels would push traditional materials beyond their acceptable oxidation and creep resistances. One strategy to accommodate the proposed steam conditions is to substitute traditional steel based materials at key regions of the steam cycle for alternative nickel-based superalloys.

High velocity oxygen fuel (HVOF) thermally sprayedWC/Co/Cr coating is considered as an electroplated hard Cr (EHC) coating replacement and Al-based coatings including electrodeposited Al (EDAl), flame sprayed Al (FSAl) and SermeTel 962 are considered as electrodeposited Cd (EC) coating replacements. This is due to the environmental issues surrounding the continued use of the latter two systems. All these coatings were characterised in terms of composition, morphology, microstructure, porosity and corrosion resistance (electrochemical linear polarisation resistance). Surface analytical techniques used included scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and focused ion beam scanning electron microscopy (FIBSEM). The feasibility of these coatings to replace EHC and EC coatings has also been identified.

The research and development of more efficient gas turbines has been one of the central focuses in materials design and characterisation for the past 20 years. High-temperature materials capable of maintaining their properties at elevated temperatures and under load are required for a wide range of components such as aircraft gas turbine engines, steam turbines, and industrial gas turbines. The operating temperature for gas turbine blades used for aerospace, marine and power plant applications, has been continuously increased in order to improve fuel efficiency. Nickel-based superalloys with various types of high temperature coatings have been widely and successfully used for such applications.

Environmental and economical demands are driving the need for the UK and the rest of the world to develop higher efficiency, next generation coal-fired plant. Higher efficiency energy recovery will ultimately result in a combination of cheaper energy for the consumer and enabling industry to meet emission targets. The intention is to increase operating parameters from 550°C and 17.5 MPa to 700°C and 30 MPa resulting in more efficient energy generation. This increase in operating parameters will also increase the demand on power plant materials, resulting in the use of some existing materials to be no longer viable. It is for this reason that nickel based alloys have been considered for areas of the plant which will experience high temperatures, pressure and will also require enhanced creep resistance. Nickel based alloys will be used in high temperature, high pressure components such as steam headers. This research focuses on one of the candidate materials for this area of plant, the alloy Nimonic 263. The ultimate aim of the research is to study the microstructural evolution with respect to time at a range of high temperatures to help assess the suitability of the alloy. To fully assess the alloy's candidacy it must be studied in each product form in which it is likely to be used, and for this reason this research has considered Nimonic 263 as a forged, welded and cast alloy.

Industrial gas turbine blades are typically made from coated Ni-based superalloys for power generation plant. The operation environments for these blades are very aggressive, comprising high temperatures (> 900°C) and potential chemical contaminants leading to oxidation and corrosion. An MCrAlX (where M is Ni and/or Co) coating is usually applied directly on the top of the alloy surface and forms an oxide scale (typically alumina) that can protect the substrate against further oxidation. However, it is widely accepted that the current generation of MCrAlX coatings only provide limited protection and need significant improvement. The MCrAlX coatings are usually applied by using either low pressure plasma spraying (LPPS) or an HVOF spraying process is increasingly used to deposit MCrAlX coatings onto gas turbine blades as an alternative method, which is more economically efficient. The main aim of this project is to design a new composition MCrAlX coating for oxidation resistant applications as well as to demonstrate the compatibility of the HVOF method to the LPPS process. A detailed experimental study of typical commercial MCrAlX coatings was carried out, this included the examination of the as-received powder particles before spraying and partially and fully melted powder particles captured during the HVOF spraying process, through to the as-sprayed and finally the fully aged coating prior to entry into service. It has been shown that as-manufactured powders consist of dendritic structures. Powder particles were found to be deformed differently after the HVOF spraying process, and dendritic structures were only observed on the barely deformed particle surface and the average grain size within these differently deformed particles decreased as the deformation degree increases. Yttrium (Y) was found at the outer surface and internal boundaries within the powder particles both before and after the spraying process. Vacuum ageing homogenised the microstructure within the as-sprayed coating, and the phases started coarsening upon further ageing in the air, and a β depletion zone formed beneath the coating surface after a short term air ageing. A set of six coatings with very similar compositions sprayed either by LPPS or HVOF methods have been examined in three different ageing conditions: the as-received condition, and after short and long term ageing at high temperature, to determine the differences between the LPPS and HVOF spraying method and their effects on coated systems. The general microstructural evolution upon ageing for both LPPS and HVOF sprayed coatings were found to be similar to each other. Before pre-service heat treatments, the HVOF as-sprayed coating contained a lot of poorly melted particles, and pores were found around the splats, enriched in Y and O. The pre-service treatment carried out after the LPPS spraying process decreased the porosity level of the coating and promoted a fine grain structure across the coating. Upon ageing, the grain size and inter-diffusion zone (IDZ) were much bigger in the long-term aged HVOF coating than in the LPPS coating. Y was found to diffuse both outwards to the sub-surface and into the oxide scale, and into the IDZ area. It was typically associated with O, even in the as-sprayed coating. After ageing, Y-containing particles were found to grow and merge together in the sub-surface area whereas they only grew in size, rather than coalescing in the IDZ area. However, there were some differences in the particles found in the LPPS and HVOF sprayed coatings including particle morphology, distribution and the exactnature of the phase. Three new compositions of MCrAlX coatings were developed based on a systematic study of current coatings which including coating failure mechanisms, typical mechanical properties (Coefficient of Thermal Expansion, CTE and Ductile Brittle Transition Temperature, DBTT) and coating composition studies. A new algorithm for CTE calculation for coatings has also been developed. Coatings with these new compositions were subsequently produced by a commercial powder manufacturer and then sprayed onto CMSX4 substrates using an HVOF spray gun. Theses coatings were found to consist of a similar microstructure to that within the Praxair powders. There was no clear correlation between the oxide scale thicknesses with the addition of TBC. However, after 3,000 hours ageing at 1000°C, the TBC was found to spall off from coating C that contained 6 wt.% of Ta. Results suggested that the other two coatings retain a moderate amount of the beta phase after 10,000 hours ageing at 880°C.This project has established an understanding of current MCrAlX coatings in terms of microstructural evolution before and throughout the spraying process. In addition, an extensive study focused on the LPPS and HVOF sprayed coatings was carried out and it v has been demonstrated there is no significant difference between these two spraying processes. A least two of the three coatings designed in this project have shown potential for application in gas turbine blades.

There is a need to increase the thermal efficiency of coal fired power plants. High chromium ferritic steels have historically been used to manufacture steam pipes, tube and headers. Over the last forty years there has been a continuous development of the 9 - 12 wt. % chromium martensitic alloys which has allowed the service temperature to be increased from 510°C - 550°C1. There is now a high demand for Ultra Super Critical coal fired power plants which operate at 650°C and hence have an increased thermal efficiency. The aim of this research project was to develop a MarBN steel (Martensitic Steel Strengthened with Boron and Nitrides) with an optimised composition in order to allow MarBN to be used at 650°C. In order for MarBN to be used within high temperature applications it is vital to understand how microstructural changes can have an impact on their creep strength. This is a key area that the present research has focused upon. The long term creep strength of 9 wt. % chromium ferritic steels is derived from the stability of the microstructure over its service life, which has a direct relationship to the stability of the precipitates and the lath structure within the microstructure.

Ni-based superalloys are widely used in aerospace and power generation as gas turbine components due to their excellent high temperature performance. These components are subjected to a combination of elevated temperature, high stress and aggressive environments during service, which can cause significant degradation of the materials. This microstructural and chemical degradation deteriorates the mechanical properties such as tensile strength and creep resistance, which can lead to failure in the components and reduce their service life. When a superalloy power plant gas turbine component reaches the end of its predicted life, the user typically has two options: replace the component or attempt to extend its remaining life.

A number of novel water-dispersible and solution polyurethanes were prepared via the prepolymer mixing process. The principal diisocyanate, polyols and emulsifier used in these studies were isophorone diisocyanate (IPDI), polytetrahydrofuran diol (PTHF), cyclohexane dimethanol (CHDM) and 2,2-bis(hydroxymethyl) propionic acid (DMPA), respectively. The carboxylic acid groups of the emulsifier were neutralised with either an organic base (triethylamine) or an inorganic base (sodium or potassium hydroxide). The dispersion molecular weight was built up through chain extension by use of aliphatic diamine chain extenders (hydrazine monohydrate, carbodihydrazide or adipic dihydrazide). The dispersions studied remained stable throughout the period of the research. Structure-property studies were carried out on polyurethanes of similar composition prepared in solution (THF) and in water. The effect of the degree of chain extension on the molecular weight of the dispersions, in addition to the physical properties and morphology, were investigated for hydrazine monohydrate, carbodihydrazide and adipic dihydrazide derived systems. Variations in the polyol, polyol molecular weight, ionic component and the diisocyanate used and their effect on the polyurethane properties were also investigated. The effects of the neutralising counter-ion and the degree of neutralisation were investigated in addition to a study of the effect of the hard segment content for samples prepared with a PTHF soft segment of molecular weight 2000. Each dispersion was thoroughly examined and characterised according to their appearance, pH, viscosity and particle size. The DMPA content is well documented as being a primary factor with respect to the dispersion stability, but also significantly affects the pH, viscosity and particle size of the dispersion. Hence, for purposes of the studies described here the DMPA component remained constant so that this could be neglected from the discussion of the results obtained. In a subsequent examination of the polyurethane films, differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), Fourier transform infra-red spectroscopy (FT-IR), tensile testing and solvent resistance were employed for the study of the solid state morphology and physical properties of the cast films. The results obtained suggest that the advantages of water-based polyurethane dispersions over more conventional solvent-borne systems lies in the molecular weight (chain extension) achievable in water. Hence, this was found to affect significantly the solid state properties of comparable water-borne and solvent-borne systems. Variation of the polyol and its molecular weight, and the diisocyanate types resulted in polyurethanes of considerably different properties and morphologies. It was discovered that synergistic properties could be achieved from the use of isocyanate blends. The degree of neutralisation and the counter-ion employed contributed to a range of colloidal properties. Hard segment content was also found to be contributory to the differing properties and morphologies encountered for PTIlF2000 based polyurethanes.