Browsing by Author "Nicholls, J. R."
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Item Open Access Abrasive machining with MQSL.(Cranfield University, 2011-07) Morris, Tom; Stephenson, David J.; Walton, I.; Nicholls, J. R.Grinding and polishing of engineered components are critical aspects of the precision manufacturing of high performance, quality assured products. Elevated process temperatures, however, are a common and for the most part undesirable feature of the grinding process. High process temperatures increase the likelihood of microstructural change within the immediate subsurface layer and are detrimental to the strength and performance of the manufactured products. Increasing processing costs and tighter environmental legislation are encouraging industry to seek innovative fluid application techniques as significant savings in production can be achieved. In this context, and with sponsorship from three industrial partners, namely; Fives Cinetic, Fuchs Lubricants plc and Southside Thermal Sciences Ltd, and also from the Engineering and Physical Science Research Council (EPSRC), this research aimed to develop an understanding of Minimum Quantity Solid Lubrication (MQSL) as a method for abrasive machining, with particular reference to the control of surface temperatures. Improving the lubricity of Minimum Quantity Lubrication (MQL) fluids reduces the frictional source of process heat and controls the finish surface temperature. The application of effective solid lubricants is known as Minimum Quantity Solid Lubrication (MQSL). Molybdenum Disulphide (MoS2), Calcium Fluoride (CaF2), and hexagonal Boron Nitride (hBN) were compared against a semi-synthetic water soluble machining fluid (Fuchs EcoCool). A series of Taguchi factorial experimental trials assessed their performances through ANOVA (ANalysis Of VAriance) statistical method. The hBN produced the lowest grinding temperatures of the solid lubricants tested, although they still remained higher than those achieved using the EcoCool control. The reduction of the machining fluid enabled a Charged Coupled Device (CCD) sensor to be fitted into the grinding machine. The recorded movement in the emitted spectrum from the grinding chips was compared to experimental and modelled process temperatures. This showed that the wavelengths of the chip light correlated to the temperature of the finish grinding surface. This greatly contributed to determining the feasibility of constructing a non-destructive, non-invasive, thermally-adaptive control system for controlling grinding surface temperatures.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 Characterisation and Material Removal Properties of the RAP™ Process(Cranfield University, 2011) O'Brien, William John; Nicholls, J. R.; Shore, PaulThe Reactive Atom Plasma® (RAP) process is a plasma chemical etching process. RAP was developed at RAPT Industries as a process for removing subsurface damage from silicon carbide optics. The process is being investigated at Cranfield University as a novel method for the fine surface correction of large optics, with the aim of shortening the manufacturing period of the next generation of large telescopes. RAP offers material removal rates that are up to 10 times higher than those of ion beam figuring, the current state-of-the-art technique and the convenience in that it can be operated at atmospheric pressure. Cont/d.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 Development of novel coatings to resist fireside corrosion in biomass-fired power plants(Cranfield University, 2016-07) Orlicka, Dominika; Simms, Nigel J.; Nicholls, J. R.The emission of CO2 to the atmosphere from firing conventional fossil fuels has become a major concern for the power industry, due to the enhanced greenhouse effect and global warming predictions. The increasing worldwide demand for electricity production is another issue. The replacement of fossil fuels with increasing quantities of biomass is of interest as biomass is considered to be carbon neutral and is widely distributed. Unfortunately, due to its composition, the risk of fireside corrosion found on heat exchangers (super- heaters and re-heaters) is greater than in coal-fired plants. Consequently, biomass-fired power plants operate at lower steam temperatures and pressures, leading to their poorer efficiency. Biomass-fired power plants suffer from alkali chloride-induced corrosion, considered faster and more severe than alkali sulphate-based corrosion common in traditional coal-fired plants. The main aim of this project was to develop a range of novel coating compositions which would be resistant to fireside corrosion found on boiler tubes in biomass-fired power plants. To accomplish this, studies were carried out into salt stabilities, coating oxidation and deposit corrosion. Salt stability experiments have resulted in improved understanding of the evaporation and sulphidation behaviour of KCl, NaCl, K2SO4 and Na2SO4 at high temperatures in environments containing HCl and SO2. KCl was chosen as a deposit for coating screening. Two-target magnetron co-sputtering was successfully used to deposit a range of coating compositions. These coatings were analysed at 550°C in corrosion environments containing combinations of HCl, KCl and water vapour. The addition of gaseous HCl did not have a significant influence on the coating degradation compared to similar tests in air. Deposited KCl significantly increased the corrosion rate, whereas adding 10% moisture to the environment with KCl had little additional effect. The growth of either protective Cr2O3 or less protective mixed oxides was observed on the different coating compositions. The best performing coatings had compositions in the range: 26.2 – 79.4 at% Cr, 12.1 – 62.9 at% Fe, 8.5 – 10.9 at% Al.Item Open Access Effect of manufacturing parameters on TBC systems cyclic oxidation lifetime(Cranfield University, 2011-12) Chirivi, Laura; Nicholls, J. R.Aero-gas turbine engines have to meet reliability, durability and fuel e ciency requirements. High turbine inlet temperatures may contribute to minimise fuel consumption and, in turn, environmental impact of the engine. Over the past few years, new designs and engine optimisation have allowed increase of such temperatures at a rate of 15 C per year, with maximum operating temperatures currently exceeding 1650 C. Ceramic coatings (also known as Thermal Barrier Coatings or TBCs) in conjunction with advanced cooling technologies are adopted to protect stator vanes and high pressure turbine blades from excessive thermal loads. Nevertheless, even with these protections in place, such components may experience a continuous service temperature of 1050 C, and peak temperatures as high as 1200 C. Therefore, it is vital that engine rotating components are able to maintain their mechanical properties at high temperature, while being able to withstand thermal loads and having su cient oxidation resistance to preserve the integrity of the ceramic coating, and eventually reaching desired component lives. Such strict requirements can be met with the use of complex Thermal Barrier Coat- ing systems or TBC systems; these consist of a nickel-based superalloy component which is rst coated with an environmental resistant layer (identi ed as bond coat ) and then with a ceramic coating. As its name suggests, the bond coat must not only protect the metallic substrate against oxidation and/or corrosion but must also provide su - cient bonding of the ceramic top layer to the metallic substrate. This goal is achieved through the formation of a further layer between the bond coat and the ceramic. In gas turbine applications, such a layer (identi ed as Thermally Grown Oxide or TGO) is an alumina scale which is the result of the bond coat oxidation during the ceramic deposition. During engine service, several time and cycle related phenomena occur within the TBC system which eventually lead the system to failure by spallation of the top coat.Aero-gas turbine engines have to meet reliability, durability and fuel e ciency requirements. High turbine inlet temperatures may contribute to minimise fuel consumption and, in turn, environmental impact of the engine. Over the past few years, new designs and engine optimisation have allowed increase of such temperatures at a rate of 15 C per year, with maximum operating temperatures currently exceeding 1650 C. Ceramic coatings (also known as Thermal Barrier Coatings or TBCs) in conjunction with advanced cooling technologies are adopted to protect stator vanes and high pressure turbine blades from excessive thermal loads. Nevertheless, even with these protections in place, such components may experience a continuous service temperature of 1050 C, and peak temperatures as high as 1200 C. Therefore, it is vital that engine rotating components are able to maintain their mechanical properties at high temperature, while being able to withstand thermal loads and having su cient oxidation resistance to preserve the integrity of the ceramic coating, and eventually reaching desired component lives. Such strict requirements can be met with the use of complex Thermal Barrier Coat- ing systems or TBC systems; these consist of a nickel-based superalloy component which is rst coated with an environmental resistant layer (identi ed as bond coat ) and then with a ceramic coating. As its name suggests, the bond coat must not only protect the metallic substrate against oxidation and/or corrosion but must also provide su - cient bonding of the ceramic top layer to the metallic substrate. This goal is achieved through the formation of a further layer between the bond coat and the ceramic. In gas turbine applications, such a layer (identi ed as Thermally Grown Oxide or TGO) is an alumina scale which is the result of the bond coat oxidation during the ceramic deposition. During engine service, several time and cycle related phenomena occur within the TBC system which eventually lead the system to failure by spallation of the top coat.Aero-gas turbine engines have to meet reliability, durability and fuel e ciency requirements. High turbine inlet temperatures may contribute to minimise fuel consumption and, in turn, environmental impact of the engine. Over the past few years, new designs and engine optimisation have allowed increase of such temperatures at a rate of 15 C per year, with maximum operating temperatures currently exceeding 1650 C. Ceramic coatings (also known as Thermal Barrier Coatings or TBCs) in conjunction with advanced cooling technologies are adopted to protect stator vanes and high pressure turbine blades from excessive thermal loads. Nevertheless, even with these protections in place, such components may experience a continuous service temperature of 1050 C, and peak temperatures as high as 1200 C. Therefore, it is vital that engine rotating components are able to maintain their mechanical properties at high temperature, while being able to withstand thermal loads and having su cient oxidation resistance to preserve the integrity of the ceramic coating, and eventually reaching desired component lives. Such strict requirements can be met with the use of complex Thermal Barrier Coat- ing systems or TBC systems; these consist of a nickel-based superalloy component which is rst coated with an environmental resistant layer (identi ed as bond coat ) and then with a ceramic coating. As its name suggests, the bond coat must not only protect the metallic substrate against oxidation and/or corrosion but must also provide su - cient bonding of the ceramic top layer to the metallic substrate. This goal is achieved through the formation of a further layer between the bond coat and the ceramic. In gas turbine applications, such a layer (identi ed as Thermally Grown Oxide or TGO) is an alumina scale which is the result of the bond coat oxidation during the ceramic deposition. During engine service, several time and cycle related phenomena occur within the TBC system which eventually lead the system to failure by spallation of the top coat.This may have catastrophic consequences as the uncoated component would face temperatures higher than the melting point of the constituent metal. This is avoided by strict maintenance regimes based on the minimum expected life of the coating. While essential for safeguarding the aircraft, this approach prevents the TBC systems from being used to their full potential. This study investigates possible optimisation methods of the manufacturing process of TBC systems, with the aim of improving reproducibility in terms of time to failure, thereby extending their minimum life expectancy and reliability. Two di erent types of TBC systems are studied: a TBC system with a Platinum-di used bond coat and a TBC system with a Platinum-modi ed aluminide bond coat. The work focuses on the e ects due to modi cation of process parameters (varied within industrially accepted range) on the TBC systems lifetime in laboratory scale cyclic oxidation tests. Experimental results show that accurate monitoring of the metal substrate surface nish as well as of the Pt layer morphology and ceramic deposition temperature may result in a dramatic improvement in life expectancy of the system, up to sevenfold when compared to control samples, or threefold if compared to commercial coatings.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 Fabrication of Smart Intercalated Polymer - SMA Nanocomposite(Cranfield University, 2015-03-16) Anjum, Sadaf Saad; Nicholls, J. R.; Rao, JeffMimicking nature gives rise to many important facets of biomaterials. This study is inspired by nature and reports on the fabrication of an intercalated polymer-NiTi nanocomposite that mimics the structural order of urethral tissue performing micturition. PTFE is chosen due to its hydrophobicity, low surface energy, and thermal and chemical stability. NiTi has been selected as a prime candidate for this research due to its excellent mechanical stability, corrosion resistance, energy absorbance, shape memory and biocompatibility. Nanoscale engineering of intercalated nanocomposites is done by PVD sputtering PTFE and NiTi. FTIR spectroscopy confirms that PTFE reforms as polymer chains after sputtering. Suitable PVD sputtering parameters were selected by investigating their influence on deposition rates, microstructure and properties of PTFE and NiTi thin films. PTFE forms stable nanocomposite coatings with NiTi and displays favourable surface interactions, known as ‘intercalation’. Intercalated PTFE-NiTi films were fabricated as layered and co-sputtered thin films. Co-sputtered nanocomposites contained nearly one-third vacant sites within its internal microstructure because of intercalation while intercalation introduced minute pits in fibrous NiTi columns of layered nanocomposites. These pits allow PTFE to extend their chains and crosslinks, resulting in microstructural and functional changes in the thin films. Intercalated PTFE-NiTi nanocomposites offer a close match to the natural tissue in terms of responding to the fluid contact (wetting angle modifications), and allow the soft and hard matter to incorporate in one framework without any chemical reactions (intercalation). An intercalated microstructure in co-sputtered and layered nanocomposites was verified by EDS-SEM and EDS-TEM techniques. The functional responses were witnessed by changes in water contact angle (WCA) and coefficient of friction (CoF) values measured on the film surface. The WCA (99°) and CoF (0.1 – 0.2) of the intercalated nanocomposite (sample PNT12) were different to the NiTi (top layer). WCA and CoF indicate the internal microstructural interactions because of intercalation.Although the pseudoelastic behaviour of NiTi can provide additional fluid response but the difficulty is an absence of crystallinity in as-deposited NiTi, and the heat treatment that melts PTFE. However, DSC and XRD techniques were employed to find the optimum NiTi composition and transition temperatures for phase transformation related to pseudoelasticity. This study provides the basis to incorporate the shape memory (pseudoelasticity or thermal shape memory effect (shape memory effect)) features of NiTi into the intercalated nanocomposite in future. The intercalated PTFE-NiTi nanocomposite reveals a fascinating research precinct, having the response generating characteristics similar to that of natural tissue.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.