Browsing by Author "Nicholls, J. R."
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Item Open Access Advanced diffusion coatings for improved oxidation and corrosion performance(Cranfield University, 2001-08) Amperiawan, G.; Nicholls, J. R.Research to investigate advanced diffusion coatings for improved oxidation and hot corrosion resistance was conducted. The aim was to build on the performance of the standard aluminide on the nickel-base super alloy IN738LC. The main emphasis of this investigation was to examine the effects of adding small quantities of yttrium to the coating as well as to produce a platinum-modified aluminide diffusion coating which is accepted as the best industrial standard for diffusion aluminides at present. A pack diffusion process was used to produce the coatings in this current study. A standard aluminide coating was modified with a small YCI3 (0.5 Wt.%) addition to the pack, producing an aluminising-yttrising pack. In the platinum-modified altuninide coating process, a sample was coated with several microns of platinum using a sputtering technique prior to the aluminising process. The pack used in this platinum modified aluminide process was a standard aluminising pack. The performance of these modified coatings during oxidation and high temperature corrosion was evaluated. Promising results were obtained, which demonstrated that both platinum and yttrium modified aluminide coatings had beneficial effects on the oxidation performance of the coatings compared to the standard aluminide coating. However, they were found to work in different ways, to improve resistance to oxidation and hot corrosion resistance. The platinum modified aluminide extends the later stage of the coating life by reduces the amount of spall, whist yttrium modified aluminide reducing the amount of spall in the early stage of the coating life and increases the critical oxide thickness before the onset of spallation. From these results, the production of a combined platinum-yttrium modified aluminide coating would be expected to show improved oxidation and hot corrosion resistance throughout the life of the coating. This could be another direction for future work.Item Open Access Aluminide-based coatings for turbine blade internal cooling passages(Cranfield University, 2004-10) Long , K.; Nicholls, J. R.The development of aero-gas turbines is moving towards more efficient engines with higher pressure ratios and increased Turbine Entry Temperatures. This leads to increases in overall turbine blades temperatures which has resulted in the widescale development of turbine blades with film cooling and Thermal Barrier Coatings (TBCs) which reduce the metal temperature of the blade. The air used for film cooling is directed around the blade by internal passages within the blade, current engines are experiencing hot corrosion in areas of these internal passages, even with internal aluminide coatings. The trend for more efficient engines means that corrosion of the internal passages will become more common, coupled with the inability to inspect the internal passages of turbine blades in service, results in a requirement for an improved coating for the internal passages of turbine blades. The aim of this study was to develop a coating which provides improved corrosion and oxidation performance over a standard vapour aluminide on single crystal CMSX-4 turbine blades material. The coating needs to be compatible with the Rolls- Royce bond coat and the Rolls-Royce manufacturing strategy. The study investigated a number of additions which could be used to improve the performance of an aluminide coating. Silicon was selected as the optimum addition on the basis of performance and ease of deposition. The work then assessed the influence of various production parameters on the formation of a silicon-aluminide coating. It was possible to control the level of silicon deposited in the coating. Performance testing, using cyclic oxidation and salt recoat hot corrosion tests, of various silicon aluminides developed in this programme demonstrated at least a doubling in life compared with vapour aluminide coatings.Item Open Access Computer model to predict electron beam-physical vapour deposition (EB-PVD) and thermal barrier coating (TBC) deposition on substrates with complex geometry(Cranfield University, 2000-07) Pereira, Vitor Emanuel M. Loureiro S.; Nicholls, J. R.; Shaw, T. W.For many decades gas turbine engineers have investigated methods to improve engine efficiency further. These methods include advances in the composition and processing of materials, intricate cooling techniques, and the use of protective coatings. Thermal barrier coatings (TBCs) are the most promising development in superalloy coatings research in recent years with the potential to reduce metal surface temperature, or increase turbine entry temperature, by 70-200°C. In order for TBCs to be exploited to their full potential, they need to be applied to the most demanding of stationary and rotating components, such as first stage blades and vanes. Comprehensive reviews of coating processes indicate that this can only be achieved on rotating components by depositing a strain-tolerant layer applied by the electron beam-physical vapour deposition (EB-PVD) coating process. A computer program has been developed in Visual c++ based on the Knudsen cosine law and aimed at calculating the coating thickness distribution around any component, but typically turbine blades. This should permit the controlled deposition to tailor the TBC performance and durability. Various evaporation characteristics have been accommodated by developing a generalised point source evaporation model that involves real and virtual sources. Substrates with complex geometry can be modelled by generating an STL file from a CAD package with the geometric information of the component, which may include shadow-masks. Visualisation of the coated thickness distributions around components was achieved using OpenGL library functions within the computer model. This study then proceeded to verify the computer model by first measuring the coating thickness for experimental trial runs and then comparing the calculated coating thickness to that measured using a laboratory coater. Predicted thickness distributions are in good agreement even for the simplified evaporation model, but can be improved further by increasing the complexity of the source model.Item Open Access Cracking behaviour, failure modes and lifetime analysis of M320 abradable compressor seal coating(Cranfield University, 2012-10) Goergen, Sandra; Nicholls, J. R.; Rickerby, DavidMetco 320 is a AlSi-hBN-polyester abradable, used in the high pressure compressor of commercial gas turbines. The material response to cyclic heating and cooling, and the resulting changes in microstructure, as well as their associated failure mechanisms were investigated. It was found that the top surface layer of the abradable liner degrades over its lifetime. During thermal cycling hBN is removed from the material’s microstructure, which results in the degradation of the abradable and increased brittleness of the top surface. Furthermore, material cracking and delamination behaviour during service was successfully reproduced in the laboratory. The cracking and delamination observations made during overhaul, were replicated using cyclic water-quenching, but the spallation of abradable material did not occur. Investigations into material properties and their influence upon the abradable failure mechanics revealed, that soft M320 matched the observations made during engine overhauls. It could also be established, that the plasma spray process, grit blasting, surface treatment after deposition and the transient of the substrate affect the abradable’s performance and life-time, when heat cycled. Some service casings suffer from premature liner loss. These unscheduled overhauls are costly and their number is desired to be reduced, if possible eliminated. In order to control the material failures, the stresses introduced into the abradable seal during manufacturing need to be reduced, since this is one of main drivers for material cracking and delamination. Furthermore, it was established, that material at the top end of the hardness specification performed better in service. This is due to the fact, that more AlSi metal matrix is present in the microstructure and the hBN loss does not affect the material integrity as much as in soft material. 2D and 3D modelling showed temperature and strain profiles evolving during the quenching process. These show the areas of high strain, which are consistent with the crack initiation areas observed during testing. It can be concluded, that M320 abradable is a very complex material system, which is influenced by several parameters. This research project highlighted, how sensitive the failure modes are to changes in the material/substrate combination. Recommended is to increase the material hardness towards the upper end of the current specification (70 HR15Y), reduce the stresses in the substrate and the abradable material by means of annealing stages after grit blasting, and temperature control during plasma spraying. Furthermore, it would be beneficial to reduce the machining of the abradable’s surface after deposition, as well as carrying out further research into the failure modes of abradables.Item Open Access Degradation of Environmental Protection Coatings for Gas Turbine Materials(Cranfield University, 2008-12) Nalin, Laura; Simms, Nigel J.; Nicholls, J. R.Nowadays, problems of component materials reliability in gas and oil-fired gas turbines focus on assessing the potential behaviour of commonly employed coatings, in order to avoid expensive and unpredictable failure in service and producing new materials whose performance meets life time and manufacturing/ repairing requirements. This MPhil project has investigated the oxidative and corrosive degradation mechanisms for some of the alloy/coatings systems (CMSX-4, CMSX-4/ RT22, CMSX-4/ CN91 and CMSX-4/ “LCO22”), which are currently used for turbines blades and vanes, in order to achieve a better knowledge of materials behaviour and to improve models for the prediction of turbine components’ lives. To achieve this target the study has made use of realistic simulations of turbine exposure conditions in combined with pre- and post-exposure metrology of bar shape materials samples, while optical microscopy has been applied to describe the microstructural evolution during the exposure and the products of the degradation for the hot corrosion. For high temperature oxidation, over extended periods of time (up to 10,000 hours), the research has allowed to describe the morphological changes in respect of the exposure time and temperature and to determine the oxidation kinetics experienced by the alloy and coatings. A model has been presented for predicting θ- α-Al2O3 growth. Moreover, using NASA COSP spalling model, with rate constants values coming from this study, a comparison between experimental mass change data and prediction has been shown. The hot corrosion study has provided new quantitative metal loss data and observations that extend/validate an existing model for materials life prediction, based on defining the severity of the corrosion conditions through measures of gas composition and contaminant deposition flux.Item Open Access Design, manufacture, and high temperature behaviour of a-phase bondcoat for thermal barrier coating(Cranfield University, 2007) Carlin, Maxime; Nicholls, J. R.In order to improve jet engine efficiency and performance, manufacturers have been trying over the last five decades to increase the working temperature of gas turbines. This was achieved by improving materials performance and component design. The latter technological breakthrough is known as Thermal Barrier Coating (TBC), which consists of applying a ceramic insulating layer on the internally cooled parts of the turbine. This technology is now applied in military and civil aircraft engines, and allows temperature improvement up to 150°C. However, understanding degradation mechanisms and improvement in manufacturing still remain important activities in turbine development. This PhD thesis was founded by a turbine manufacturer, Snecma, with the aim of developing a new type of high temperature coating. The ceramic topcoat of TBC’s is currently deposited on typical binary platinum aluminide diffusion coating or NiCoCrAlY overlay, called bondcoat, which stands at the component/ceramic interface. In this work, a new kind of intermetallic was studied, a ternary compound of the Ni-Al-Pt system, called α.phase, and a manufacturing route to deposit it as an overlay coating was developed. The main result of this thesis is the achievement of a reliable, reproducible, and controlled manufacturing process of α-phase coatings. This process is based on sputtering multlilayers of pure metals, followed by the annealing of the layered coating. Produced coatings are thinner than commercial systems as they are richer in platinum (typically 5 m instead of 70 m), hence the so-called name of "low mass bondcoat". Such high temperature intermetallic coatings were characterised during this project (by XRD, SEM, EDS, FIB and TEM), as well as their isothermal and thermal cycled oxidation behaviour at high temperature. These systems were topped with a commercial ceramic layer in order to assess their potential as bondcoats for a full TBC system. Lifetimes are relatively promising, and failure modes, which will be described and discussed, are very specific compared to state of the art coatings. This specificity is proven to be due to the non conventional deposition route rather than to the new compound used as a bondcoat.Item Open Access Development of Coatings for Gas Turbines Burning Biomass and Waste-Fuels(Cranfield University, 2009-11) Bradshaw, A.; Nicholls, J. R.Worldwide, carbon dioxide emission reductions are in progress following the Kyoto Protocol implementation programme to mitigate climate change. More stringent reductions are expected to follow the present programme which ends in 2012. In addition to reducing carbon dioxide emissions, the major climate change mitigation policy is the elimination of waste. This project addresses both aspects, by facilitating the use of biomass and waste fuels in the gas turbines of highly efficient, integrated gasification combined cycle electricity generating units. Gases from the gasification of these fuels contain potentially damaging contaminants which, when combusted in gas turbines, will initiate hot corrosion. To resist hot corrosion, but still maximise gas turbine efficiency, the hot components of gas turbines require protective coatings. Five activities in this project required original research to meet the objectives. Firstly, to identify potentially damaging species in gasifier gases, which could remain after hot gas cleaning and, following combustion, initiate hot corrosion along the gas path of the gas turbine. Thermodynamic assessments, using MTDATA software, identified cadmium and lead species that could initiate hot corrosion in the gas turbine. The second research activity, involved Type II hot corrosion tests of the identified species on superalloys and typical commercial coatings. These tests simulated the same corrosion environment as in industrial high temperature gas turbine operation. Test results confirmed the thermodynamic assessments, with hot corrosion being initiated on all items tested, and was worse with lead and/or cadmium additions. The third research activity was to develop novel hot corrosion protective coatings. The approach was to develop the most economic coatings, which would provide comparable, or superior, hot corrosion performance to that provided by well proven commercial coatings already used with fossil fuel firing. From previous research at Cranfield, published literature, and after aluminising and silicon modified aluminising CVD trials, single-step silicon modified aluminising was adopted as the basis for novel coating development. The fourth research activity consisted of cyclic oxidation tests and, type II and type I hot corrosion tests, to assess the oxidation and hot corrosion protection provided by the novel coatings on IN738LC and CMSX-4 substrates. Cyclic oxidation tests at 950C and 1050C showed the novel coatings produced by CVD, at a soak temperature of 1050C and soak period of one hour, were superior for both substrates. Microstructurally, TCP phases were formed in CMSX-4 samples which could reduce mechanical strength in service. The TCP phases were observed in the high silicon containing coatings through a reaction with refractory metals diffusing outward from the CMSX-4. This was most noticeable in samples cyclically oxidised at 1050C for long times. Results of hot corrosion tests undertaken at 700C (type II) and 900C (type I) showed novel coatings on IN738LC samples to be more resistant than commercial coatings. Those on CMSX-4 samples had similar hot corrosion resistance to commercial coatings. The novel coatings provided high levels of hot corrosion resistance, which could be enhanced by improvements in deposition. The fifth research activity was to carry out EB-PVD TBC trials on an IN738LC turbine blade, which demonstrated that the novel coating provided an effective bond for the TBC. It is concluded that the novel, single-step silicon-aluminide coatings developed in this project, with identified improvements in quality, will provide effective hot corrosion resistance for gas turbines burning gasified biomass and waste fuels.Item Open Access Development of EB-PVD TBC'S : the role of deposition temperature and plasma assistance(Cranfield University, 1995-06) Jaslier, Yann; Nicholls, J. R.Gas turbine manufacturers have achieved continuingly improved engine efficiency and thrust-to-weight ratio by designing with increased Turbine Entry Temperature (TET). The protection of High Pressure Turbine (HPT) aerofoils with thin insulating ceramic coatings, referred to as Thermal Barrier Coatings (TBC's), has emerged as the next key technology to allow for further increases in TET. Electron Beam Physical Vapour Deposition (EB-PVD) is today's most promising processing route for the manufacture of TBC's applied on aerofoils. The purpose of this work was to generate a sound understanding of the relationship between the EB-PVD process and the structure of Zr02- 8wt%Y2O3 ceramic deposits, which could be exploited to achieve improved TBC performance. In particular, the role of deposition temperature and the potential benefits in using RF and DC plasma assistance of the EB-PVD process were investigated, together with their influence on the erosion performance of EB-PVD TBC's. The significance of particulate erosion as a degradation mode is assessed under conditions representative of the HPT environment. New explorable routes to achieve reduced thermal conductivity of EB-PVD TBC's are identified. It is shown that EB-PVD TBC's deposited at low temperature contain a massive content of microscopic voidage (-50%) which is responsible for their lack of thermal stability. The growth of EB-PVD TBC's at elevated deposition temperatures is explained in terms of dynamic sintering, whereby diffusion processes compete against the high rate arrival of vapour atoms to overcome the spontaneous defectiveness of the atomic build up. Modelling of the gas discharge physics has highlighted scope for improving the effectiveness of plasma assistance in causing ceramic structural damage, capable of modifying the coating thermal properties. The erosion rate of EB-PVD TBC's is shown to be controlled by their degree of plastic deformation upon particle impacts, which in turn depends on the ceramic column diameter and inherent porosity.Item Open Access Development of microtubular solid oxide fuel cells design, fabrication and performance.(2017-08) Camilleri, Alastair; Nicholls, J. R.Solid oxide fuel cells (SOFCs) are the most efficient energy conversion devices known. Many designs exist, with most current ones based on planar, tubular or so-called hybrid geometries. Tubular designs have many advantages over planar ones, including robustness and simpler sealing. They suffer from somewhat lower area-specific power density and considerably lower volume-specific power density. The miniaturization of tubular cells offers great improvement to both, and more besides. Pushing the boundaries of state-of-the-art manufacture to ever thinner films increases performance further, greatly advancing the long road to large scale commercialisation of SOFCs. This is only possible via the rigorous selection of materials and careful design – both for optimal performance and for mass manufacture. Previous work by the author established the potential of a novel anode fabrication route as well as showing that even un-optimized electron beam physical vapour deposition (EB-PVD) was capable of creating demonstrator cells. In this work these manufacturing processes receive at least two passes of optimization towards both reproducible fabrication and maximising microtubular SOFC performance. The former was achieved by creating statistically significant quantities to assess reproducibility and studying the underlying science, and the latter was investigated in three aspects: gas transport, electrical and electrochemical. The unique oxidation-reduction route creates robust, highly reproducible anodes with excellent through porosity offering as much as 5 orders of magnitude superior gas permeance to published sources. Nickel tubes (Ni200 5.9 mm OD, 125 μm wall thickness, 100 mm long) were oxidised in air at 1,100 for 42 h and reduced in pure hydrogen at four different temperatures. The extremes (400 °C and 1,000 °C) proved sufficiently promising that both were considered in subsequent stages of experiments and analysis for the final anode design. The morphology of the electrolyte (in particular with respect to gas-tightness) is a critical aspect of SOFC miniaturisation, and a challenge to achieve via mass-manufacture-friendly EB-PVD. The yttria-stabilized zirconia (YSZ) electrolyte deposition was optimized as far as proved possible with the available equipment. While results are more than encouraging there are a number of important concerns to be addressed in future to assure successful commercialization of the design. Accurately measuring gas permeance through the anode-electrolyte tube (sometimes called a half-cell) provides quantified justification. Finally a porous platinum cathode film 300 nm thick was successfully magnetron-sputtered onto the YSZ electrolyte at p Aᵣ100 mTorr, demonstrating the fabrication process and creating complete cells for electrical and electrochemical characterisation.Item Open Access Elevated Temperature Oxidation and Corrosion of a Titanium Aluminide Alloy(Cranfield University, 1997-10) Leggett, Jonathan; Nicholls, J. R.Titanium aluminides are being developed to expand the temperature capability of titanium alloys with maximum service temperatures around 700*C. These materials also have the ability to replace nickel superalloys with potential applications in the high pressure compressor, and in the 4th stages of the low pressure turbine. The above applications place these alloys in environments not previously considered. Within the compressor hot salt corrosion may be a problem with salt ingested from the atmospheric aerosol. While in the turbine the combination of salt ingestion,and SO, from the burning of fossil fuels, results in hot corrosion being a potential problem. In this study the individual effects of salt and So2 were investigated, with corrosion mechanisms being proposed using kinetic, metallographic and thermodynamic data. Understanding these effects enabled both the hot salt corrosion and hot corrosion behaviour of TiAl alloys to be evaluated. In air alone continuous alumina layers, within a mixed alumina/rutile scale, provide the oxidation resistance of TiAlNb alloys. Logarithmic kinetics operated for 100 hours at 700*C and for 13 hours at 750'C. Parabolic kinetics then operated out to 100 hours at 900*C. Mass gains ranged from 0.06 to 2.1mg/cO after 100 hours at 700 and 900'C respectively. This situation changes in bi-oxidant, air/S02, atmospheres where increased growth rates are linked to the formation of a continuous sulphide layer at the scale/substratien terface. Below 800'C logarithmic/parabolic kinetics operate. At and above 800*C initial logarithmic kinetics change to near linear/breakaway kinetics with spallation becoming a problem. Mass gains,after 100 hours, ranged from 0.2 mg/cM2 at 700"C up to 6.4 mg/cm2 at 900"C. The presence of low salt concentrations [<0.05mg/cm2] resulted in severe substrate degradation, with preferential attack down a2 lathes.The first 10-20 hours were shown to be the most important with low melting point salt mixtures spreading across the surface, increasing the rate of attack. The evolution of HCI/Cl2 during initial substrate attack leads to the Vapour Phase Transport of aluminium and manganes chlorides resulting in whisker growth over a porous rutile scale. The presence of salt modified the diffusion controlled kinetics under purely oxidising conditions. Chlorine was shown to promote the vapour phase transport mechanism which resulted in the initial accelerated logarithmic kinetics. A change to parabolic type kinetics occurred due to the loss of chlorine to the atmosphere. The mass gains, after 100 hours, ranged from 0.06 to 1.1rn g/cm2 between 500 and 800* C. The combination of salt deposits and S02 bearing environments resulted in severe substrate degradation. Salt played a dominant role during the early stages of corrosion, whilst low partial pressuresof S02 affected the later stages of corrosion. Non protective oxide scales were developed with low melting point MnSO4-Na2SO4 mixtures forming at salt deposits and a continuous sulphide layer at the scale/substrate interface. R apid scale growth resulted in severe scale spallation. The initial stages of hot corrosion followed rapid logarithmic type kinetics. Further increases in the corrosion rate where promoted by the formation of continuous sulphide layers at the scale/substrate interface.Parabolic kinetics, at this stage, were followed by linear growth rates once scale spalling occurred. Mass gains, after 100 hours, ranged from 0.52 to 3.89 mg/cm2between 650 and 800"C.Item Open Access Erosion resistance in metal - ceramic multilayer coatings for gas turbine compressor applications(Cranfield University, 1995-01) Goat, Christopher; Nicholls, J. R.The erosion resistance of 50 m metal-ceramic multilayer coatings has been investigated under impact conditions comparable to those in a gas turbine compressor cascade. lt was possible to improve upon the erosion resistance of Ti-6Al-4V by a significant margin. The influence of layer mechanical properties, layer thickness, ceramic content and coating process on erosion resistance has been studied over a range of impact conditions. The most suitable coating formulation for maximum erosion resistance changed with particle impact energy. Under low energy impact conditions (<55 joules) coatings with a high ceramic content demonstrated the highest erosion resistance. As particle impact energy increased, coatings with a high ceramic content perfonned poorly, and those containing a high volume fraction (50%) ductile metal layer, with thin metal and ceramic layers become more successful. Three principal damage types were observed: lateral fracture, tensile fracture and plastic definition. The most severe coating losses resulted from spallation due to lateral fracture. Coatings containing a high proportion of ductile metal with thin metal and ceramic layers were successful because such coatings had a high resistance to lateral fracture. Erosion resistance was greatest when the metal layer had a high yield strength and elastic modulus; such a combination of properties also resisted plastic definition. Scratch testing was investigated as a simple alterative technique for assessing coating erosion resistance. Repeated pass scratch testing generated similar damage modes to those of particle impact, but there was poor correlation between coating erosion rate and the threshold load for scratch damage.Item Open Access Investigation into the environmental assisted crack initiation mechanism of CMSX-4 in simulated aero engine environments at 450 - 550°C.(Cranfield University, 2023-03) Duarte Martinez, Fabian; Nicholls, J. R.; Gray, Simon; Castelluccio, Gustavo M.The aviation industry has continued to increase the efficiency of gas turbine engines, which are now designed to operate on a wide variety of flight routes. In general, the efficiency drive has led to components spending longer times at temperatures, where accelerated corrosion can occur. This has led to a complex degradation mechanism being identified in the lower shank region under the platform of single-crystal turbine blades. This research aims to understand the mechanism of crack initiation due to the synergistic effect of stress and high temperature corrosion environments on CMSX-4 in the lower operating temperature range, 450°C - 550°C, of an aero gas turbine blade. The first part of the investigation consisted in comparing the effect of different salt deposits in a 50 ppm SO₂ - air environment at 550°C. A 50 ppm SO₂ – air concentration was considered because the air going through the lower shank is fed directly from the compressor, and not from the combustor (which is the main source of sulphur). Characterisation of the resulting scales were carried out using scanning electron microscopy, energy dispersive spectroscopy and X- ray diffraction. Results from thermodynamic modelling are also presented. The first part of the investigation showed that CMSX-4 sample under an applied stress and no applied salt did not experience accelerated corrosion attack or crack formation when exposed to 50 ppm SO₂ - air in a 400-hour period. The same observation was made for a CMSX-4 sample under an applied stress and salted with CaSO₄. Sea salt caused accelerated corrosion attack with cracks up to 1.3 mm through the substrate formed after 400 hours of exposure. Further tests using NaCl salt in 50 ppm SO₂ – air showed that cracks can initiate after just 10 minutes of exposure at 550°C. Crack growth rates are significantly reduced after two hours of exposure within a 50-hour salt cycle. Cracks with NaCl in 50 ppm SO₂ – air have also been observed at temperatures as low as 450°C. When NaCl salt was applied to CMSX-4 and exposed to air only for 50 hours, the corrosion attack was reduced considerably, and the initiation of cracks is either suppressed or significantly delayed beyond a 50-hour period. Although this PhD has only focused on a 50-hour period, longer exposure times should be carried out to determine if air exposures delay crack initiation time, or if crack initiation is completely supressed. This thesis has therefore shown that the interaction of stress, NaCl and a sulphur- containing environment are critical to cause early crack initiation in single crystal nickel-based superalloys in the temperature range 450 - 550°C. The effect of having small concentrations of moisture in the gaseous environment or as inclusions retained in the salt are still to be investigated. A working hypothesis is that that the interaction of alkali chlorides with a sulphur-containing atmosphere is the trigger to a self-sustaining cycle where metal chloride formation, vaporisation and oxidation leads to high amounts of H₂ formed at the scale/alloy interface. Potentially, the H₂ formed at the alloy/scale interface may dissociate into atomic hydrogen, and lead to hydrogen embrittlement. For further verification of this hypothesis, a set of tests have been suggested.Item Open Access Investigation into the manipulation of the properties of Indium Tin Oxide (ITO) coatings(Cranfield University, 2008) Atterbury, Clair; Nicholls, J. R.; Hatchett, PhilThis thesis investigates the manipulation of the properties of Indium Tin Oxide (ITO) coatings. This is carried out with a combination of Experimental and Theoretical work. The coating of ITO onto a glass substrate was both theoretically modelled and the practical work analysed to observe the effects. Observation of the effects on the output parameters when depositing a single layer of ITO via Electron beam evaporation onto a glass substrate multiple times with varying conditions was carried out. The amount of ITO required to produce optimum % transmission and the deposition conditions required to provide <20 7/▢ and <100 7/▢ were investigated. This study then considered the addition of a single layer of an additional coating both theoretically and practically to maximise the %T for the wavelength ranges under consideration. From this, the ideal refractive index for the additional coating to maximise the %T for the ranges was deduced. Progression was then made to consider multiple layers. Theoretical work carried out on the addition of extra layers and the deduction of the optimal refractive index implied that overall, Cryolite would produce the best average %T across the ranges considered. In addition to this, the results of ITO deposition via Evaporation and sputtering were examined to determine the difference the technique used has upon the coating produced.Item Open Access A langmuir multi-probe system for the characterization of atmospheric pressure arc plasmas(Cranfield University, 2003-04) Fanara, C.; Nicholls, J. R.The 'high-pressure' atmospheric (TIG) arc plasma is studied by means of a multi-Langmuir probe system. In order to determine the appropriate regime of operation, definitions of the plasma parameters for the description of the argon arc are considered and evaluations are presented. A description of the probe basic techniques is followed by an in-depth discussion of the different regimes of probe operation. The emphasis is put on atmospheric and flowing (arc) regimes. Probe sheath theories are compared and “Nonidealities” like cooling due to plasma-probe motion and probe emission mechanisms are then described. The extensive literature review reveals that the existing probe theories are inappropriate for a use in the TIG arc, because of ‘high’ pressure (atmospheric), broad range of ionization across the arc, flowing conditions, and ultimately, to the uncertainty about onset of Local Thermodynamical Equilibrium. The Langmuir probe system is built to operate in floating and biased conditions. The present work represents the first extensive investigation of electrostatic probes in arcs where the experimental difficulties and the primary observed quantities are presented in great detail. Analysis methodologies are introduced and experimental results are presented towards a unified picture of the resulting arc structure by comparison with data from emission spectroscopy. Results from different measurements are presented and comparison is made with data on TIG arcs present in literature. Probe obtained temperatures are lower than the values obtained from emission spectroscopy and this ‘cooling’ is attributed to electron-ion recombination. However, it is believed that probes can access temperatures regions not attainable by emission spectroscopy. Only axial electric potential and electric field are obtained because of the equipotential-probe requirement. Estimations of the sheath voltage and extension are obtained and a qualitative picture of the ion and electron current densities within the arc is given.Item Open Access Low mass platinum aluminide bondcoat for thermal barrier coating(2001) Saint-Ramond, Bertrand; Nicholls, J. R.During the last 30 years, Thermal Barrier Coating systems (TBCs) have been extensively used to protect the hottest part of aero-engines. They can extend significantly the lifetime of high pressure turbine blades and combustor walls by decreasing the superalloy substrate temperature by up to several hundreds o f degree C. TBCs are duplex systems consisting of a thermal insulative ceramic toplayer and an intermediate metallic bondcoat layer, whose function is to protect the substrate against corrosion and oxidation and to promote the ceramic adherence by forming an alumina scale at the interface with the ceramic. The lifetime of the TBCs is however limited by chemical, mechanical and thermal stresses in the coatings due to bondcoat oxidation and the mismatch of thermal expansion coefficient (CTE) between the ceramic, the bondcoat and the substrate. The bondcoat consideration is therefore of a substantial importance for the TBCs lifetime extension, and the present work has been focused on the development of a novel and innovative intermetallic overlay bondcoat, having a much thinner thickness than conventional bondcoats, acting as a diffusion barrier for substrate harmful elements, and promoting the formation of a pure, slow-growing and adherent alumina scale. The low-mass bondcoat system has been based on a 3-15 microns thick PtAh intermetallic layer, with the ternary addition of a reactive element (Hafnium, Zirconium, or Yttrium). Aluminium and Platinum are deposited sequentially by the sputtering process (Physical Vapor Deposition). The bondcoat is thus a multi-layer coating, and the layers react one with another exothermically by diffusion after a subsequent heat treatment at a relatively low temperature. The temperature of reaction between the layers and the stability of the obtained intermetallics has been studied by using Differential Thermal Analysis. Different platinum aluminides have been developed as bondcoats and the number of layer has been varied (up to 350 layers) in order to study the influence on the coating structure. Finally, the most successful systems have been cyclically tested to be compared to industrial bondcoats systems. These experimentations have led to the development of a highly controllable bondcoat deposition and formation process. Different morphologies and compositions can be accurately obtained by varying the individual layer thickness and Al/Pt thickness ratio within the coatings. A reactive element, which consists of either zirconium, yttrium or hafnium has been introduced into the aluminium layer by sputtering co-deposition and it has been therefore demonstrated the possibility of improving the efficiency of the low-mass bondcoat by adding such an element evenly through the coating. Whatever the composition or its structure, the low-mass bondcoat is adherent to the substrate and does not interact with the substrate during the deposition and the formation process. The bondcoat is thermally stable for a significant time of aging at 700°C, 900°C and 1100°C, but do not withstand cyclic oxidation testing better than industrial bondcoats. Nevertheless, to really assessed the potential of the low mass bondcoat, a cyclic oxidation test has to be performed after ceramic topcoat deposition, which would modify the local stress gradients on the thermally grown oxide, during cooling.Item Open Access Manufacture of novel intermetallic bond coats from the electroplating of ionic liquids(2010) Craig, Mark; Nicholls, J. R.; Robinson, M. J.Gas turbine engines for both aerospace and power generation are constantly being revised to improve running efficiency and performance. Gas turbine engines essentially consist of three distinct regions: compressor, combustion and turbine sections with the combustor and turbine sections required to experience higher and higher temperatures in pursuit of efficiency gains. Stage 1 high pressure turbine blades (buckets) are located closest to the combustion zone and experience extremely high temperatures. Further, turbine blades experience high centrifugal force whilst in operation and therefore engine designers must take into consideration both the mechanical effects o f operation and the high temperatures associated with engine use. Environmental resistant coating systems are therefore employed to allow the design of the base material (nickel-based superalloys) to be biased towards mechanical properties (high creep resistance). Nickel-platinum-aluminide coatings are the diffusion coating of choice for both aero-and industrial turbines with the platinum being typically deposited by electroplating on the nickel alloys, followed by heat treating to form a platinised enriched area which is then aluminised by insertion into a chemical vapour deposition (CVD) retort and reacting with an aluminium halide at elevated temperature. The CVD process is utilised as it is relatively easy to form desirable intermetallics though this route. The electrodeposition of aluminium from aqueous media is not possible as water undergoes hydrolysis before the reduction potential of aluminium is reached. Ionic liquids are an alternative method o f depositing aluminium via electroplating without the need o f water as the electrolyte. Ionic liquids have numerous benefits including a wide electrochemical window and have low toxicity. In comparison to the CVD process, they are multiuse and can be easily recycled/reused as the ionic liquid itself is not consumed within the plating process. Electroplating aluminium from ionic liquids to form a dense coating onto nickelbased superalloys is therefore proposed within this thesis as an alternative novel approach to achieving desirable nickel aluminide intermetallic coatings after post processing heat treatment. Furthermore, the post heat treatment may be done within either a traditional CVD-type regime or with a new and novel low temperature heat treatment regime developed as part of this thesis - ICON. Both heat treatments form β-NiAl. The heat treatment using CVD-type parameters forms coatings akin to those produced using a CVD route, whereas the ICON coating shows improved chemical homogeneity and a smaller interdiffusion zone - both o f which are shown to offer superior coating oxidation performance. Aluminium electrodeposited on CMSX4 heat treated with CVD-type parameters shows excellent cyclic oxidation data which is at least equal to, if not greater than those produced using traditional methods.Item Open Access The Mechanism of Dross Formation on Aluminium and Aluminium Magnesium Alloys(Cranfield University, 1989-12) Impey, S. A.; Stephenson, David J.; Nicholls, J. R.Metal loss is an unavoidable consequence of the large scale melting of aluminium and its alloys. The objective of such processing must be to minimise losses, both from an economic viewpoint and to ensure optimum quality of cast and wrought products. Aluminium losses during melting and casting are primarily due to the formation of dross, a mixture of oxide and melt. Many of the commercially important aluminium alloys contain appreciable levels of magnesium (up to 5%) which can result in enhanced oxidation rates that give rise to particular problems in recycling. Results are presented from a study aimed at reducing melt loss through a knowledge of the mechanism by which dross is formed. Work has centred on an understanding of the early stages in oxide scale growth, a study of growth kinetics and subsequent breakdown of these initial scales to form dross. In humid atmospheres, the amorphous oxide covering both aluminium and aluminium-magnesium at 750°C provides a highly effective barrier protecting the molten metal. In the absence of water vapour, oxide crystal development in aluminium-magnesium alloys is dominated by magnesium, and is extremely rapid in comparison with pure aluminium. Despite the different oxides formed, the manner of crystal formation at the 'amorphous' oxide-melt interface at 750°C on both aluminium and aluminium-magnesium is comparable. Nucleation and growth of crystals in the 'amorphous' film generates high stresses which result in failure of the surface oxide. Scanning electron microscopy has shown that the localised failure of this protective oxide film results in exudations forming on the melt surface, the size and number of which increase with exposure time. These exudations would appear to be the onset of dross formation. Parallel studies of the wetting characteristics of aluminium to alumina have shown that the reported non-wetting is due to the presence of the thin alumina film on the melt surface. Once broken, wetting of the alumina takes place and accounts for the exudation of molten metal through the surface oxide and hence dross formation.Item Open Access Modelling the erosion of electron beam physical vapour deposited thermal barrier coatings(Cranfield University, 2001) Wellman , Richard; Nicholls, J. R.Since the introduction of electron beam (EB) physical vapour deposition (PVD) thermal barrier coatings (TBCs) and their application to moving components in the hot gas stream, erosion has become a prime concern. EB PVD TBCs, due to their unique columnar microstructure are far more strain tolerant than their plasma sprayed (PS) counter parts and can thus be used under more exacting operating conditions. It is under these operating conditions that erosion of the coated components is of primary importance. The main aim of this project was the development of a computer model capable of predicting the erosion rate of EB PVD TBCs under various different conditions. I order to do this it was first necessary to determine the erosion mechanisms of EB PVD TBCs as well as their mechanical properties. Steady state erosion and single impact studies together with SEM were used to determine the erosion mechanisms, while nano indentation techniques were used to obtain the hardness and the Youngs Modulus of the EB PVD TBC. Literature searches contributed to the understanding of erosion principles and factors affecting erosion. All these findings were then used in the development of a Monte-Carlo type computational erosion model capable of predicting the erosive wear rate of EB PVD TBCs under various conditions. The model which has been developed' is capable of predicting the erosion rate of EB PVD TBC to within 30%, so long as the erosion falls within a certain defined mechanism, which can easily be checked against a erosion map, which has been drawn.Item Open Access Multilayer TiB2/X hard coatings by sputtering deposition(1998-10) Da Silva, Maria de Fatima Oliveira Vales; Hancock, P.; Nicholls, J. R.Titanium diboride has been investigated as a potential candidate for aerospace structures, cutting tools, surface coatings of first-wall components and diffusion barriers in integrated circuit metallization. Titanium diboride is a very stable hard refractory compound but its brittleness is the main drawback. It was possible to lessen the TiB2 brittleness by producing TiB2/X coating designs by the multi-target RF magnetron sputtering process. X is the metal layer (Al, Ti, NiCr, Mo) in the composite system. The influence of the composition wavelength and volume fraction of ceramic has been studied over a range of sputtering conditions. The most suitable multilayer coating design (TiB2/NiCr) on steel substrate, for maximum hardness (18.81GPa) and elastic modulus (304.6GPa) was found to be with a composition wavelength of 50nm and volume fraction of ceramic of 75%. The greatest improvement of the elastic modulus measured by nanoindentation was found to be for a TiB2/Al two-layer coating design either on steel or on aluminium substrate, giving 36.2% and 40% improvement above the rule of mixtures respectively, when compared with TiB2 coatings deposited under the same sputtering conditions. Several pieces of three-point bent apparatus were designed for measuring the inplane elastic modulus of the coatings. The three-point bent test by nanoindenter shows promise as a method for measuring the in-plane elastic modulus on uncoated beams. A comparison between traditional and non-traditional methods of measuring mechanical properties of the coatings was performed in this study. The nanoindentation technique was found to be an appropriate method to measure the mechanical properties of multilayer coating designs.Item Open Access New, low mass, bond-coat technology for thermal barrier coating(Cranfield University, 2005-10) Silva, Manuel; Nicholls, J. R.To remain competitive, gas turbine manufacturers must aim for continuingly improved engine efficiencies and thrust-to-weight ratio. This has resulted in the design of gas turbines with increased turbine entry temperature (TET). Thermal barrier coatings (TBCs) are the most promising systems, which thermally protect engine components and allow their use at higher engine gas temperature by potentially reducing metal surface temperature by up to 150°C. The TBC system consists of a metallic bondcoat and a thermally insulating strain-tolerant ceramic top coat. The bondcoat is a critical part of the system; its failure has a major impact on the lifetime of the TBC. The purpose of this work is the development of a novel and innovative bondcoat with reduced weight, also called "low-mass" bondcoat. This new class of bond coat consisted of a thin (2.5 to 8 J..lm thick) coating containing successive layers (from 9 to 163) of aluminium and platinum. The layers react with one another exothermically by diffusion after a subsequent heat-treatment at a relatively low temperature (700°C), to form an intermetallic bond coat. In this thesis, the manufacture and optimisation of the low-mass bond coat TBC are presented and discussed. Deposition prerequisites along with good deposition practice were defined in order to produce successfully the low-mass bond coat in a clean environment. Stable working parameters were established, among which a roughness working window, as the substrate initial roughness appears to be a key parameter for coating adherence. The structure of the individual as deposited layers were characterised, which allowed to determine the surface temperature during deposition (between 150°C and 350°C). This was well below the temperature above which the exothermic reaction is triggered (400°C). High-multilayered bondcoats (PtAI, PtAh, Pt2Ah stoichiometries) were successfully manufactured, characterised and integrated in a TBC system, among which the thinnest bond coat for THC ever made (51 layers for a 2.5 J..lm thick PtAh). The low-mass bond coat TBC system presented a singular structure consisting of a dense intermetallic layer overlaid by a composite structure of Ah03 precipitates within a (Ni,Pt)xAly matrix. Furthermore the TGO, thermally grown oxide, formed and grew with a typical equiaxed granular structure. This novel TBC system was tested along with commercial coatings under thermal cyclic oxidation, aiming to simulate the thermal cycles induced by the operating aircraft gas turbine .. Regarding to the thickness and the aluminium reservoir of the low mass bond coats, the performances are outstanding, confirming the potentiality of this new type of TBC systems. A degradation mechanism was proposed based on FIB and SEM observations along with chemical analysis. The outstanding performance of the low mass bond coat TBC system is thought to be due to the very specific manufacturing process and its influence on the alumina scale growth under the TBC.