Rozprawy doktorskie na temat „Turbine engines materials”

This thesis starts out with a brief description of gas turbine engines and information on railroad locomotives being the gas-turbine electric locomotives with some comparison of the diesel-electric locomotives in the introduction. Section 1.1 is the research problem looking at the older gas turbine electric locomotives in the 1950’s that ran on the rail and the problems they suffered. In section 1.2 titled the purpose of the study takes a look at newer gas turbine locomotives that were being consider or has been built with improvements since the 1950’s. The objective of the study being section 1.3 looks at the advantages of new gas turbines engines. Section 1.4 titled the research questions discusses better materials and methods of gas turbine engines. Chapter 2 is the literature review looking at the fuel oil specifications being number 4, number 5, and number 6. This chapter also talks about the used of distillates, types of distillates, composition of distillates, specifications for distillates, residual fuel oil and fuel oil quality dealing with the firing of gas turbine engines. Section 2.3 of chapter 2 being titled power generation looks at power plant gas-turbine engines and the power they produce. Chapter 3, titled the proposed methodology looks at setting up an experiment using a gas-turbine engine and a diesel-electric engine to compare the advantages of along with the disadvantages. Section 3.1 is titled data collected, within this section is discussion on the data collected from the experiment and improvements that could be made to the gas turbine engines. The end of chapter 3, section 3.2 titled data analyzing, talks about possible the results collected, calculations done, improvements made and rerunning another experiment with the improvements made. Chapter 4 discuss the types of materials using in building the compressor and turbine blades. Last, but not least is chapter 5 which discusses the actual experiment using the gas turbine simulator for aircrafts and how to apply it to the railroad locomotives. After the conclusion which discusses the results, is the appendix a being gas tables, appendix b being trial run 1 and appendix c being trial run 2.

The research carried out as part of this EngD is aimed at understanding the high temperature materials used in modern gas turbine applications and providing QinetiQ with the information required to assess component performance in new propulsion systems. Performance gains are achieved through increased turbine gas temperatures which lead to hotter turbine disc rims and blades. The work has focussed on two key areas: (1) Disc Alloy Assessment of High Temperature Properties; and (2) Thermal Barrier Coating Life Assessment; which are drawn together by the overarching theme of the EngD: Lifing of Critical Components in Gas Turbine Engines. Performance of sub-solvus heat treated N18 alloy in the temperature range of 650°C to 725°C has been examined via monotonic and cyclically stabilised tensile, creep and strain controlled low cycle fatigue (LCF) tests including LCF behaviour in the presence of a stress concentration under load-control. Crack propagation studies have been undertaken on N18 and a particular super-solvus heat treatment variant of the alloy LSHR at the same temperatures, in air and vacuum with 1s and 20s dwell times. Comparisons between the results of this testing and microstructural characterisation with RR1000, UDIMET® 720 Low Interstitial (U720Li) and a large grain variant of U720Li have been carried out. In all alloys, strength is linked to a combination of γ' content and grain size as well as slow diffusing atoms in solid solution. High temperature strength improves creep performance which is also dependent on grain size and grain boundary character. Fatigue testing revealed that N18 had the most transgranular crack propagation with a good resistance to intergranular failure modes, with U720Li the most intergranular. Under vacuum conditions transgranular failure modes are evident to higher temperature and ΔK, with LSHR failing almost completely by intergranular crack propagation in air. For N18 significant cyclic softening occurs at 725°C with LCF initiation occurring at pores and oxidised particles. An apparent activation energy technique was used to provide further insights into the failure modes of these alloys, this indicating that, for N18 with 1s dwell, changes in fatigue crack growth rates were attributed to static properties and for LSHR, with 20s dwell in air, that changes were attributed to the detrimental synergistic combination of creep and oxidation at 725°C. Microchemistry at grain boundaries, especially M23C6 carbides, plays an important role in these alloys. Failure mechanisms within a thermal barrier coating (TBC) system consisting of a CMSX4 substrate, PtAl bond coat, thermally grown oxide (TGO) layer and a top coat applied using electron beam physical vapour deposition have been considered. TGO growth has been quantified under isothermal, two stage temperature and thermal cyclic exposures. An Arrhenius relation was used to describe the TGO growth and produce an isothermal TGO growth model. The output from this was used in the QinetiQ TBC Lifing Model. Thermo-mechanical fatigue test methods were also developed including a novel thermocouple placement permitting substrate temperature to be monitored without disturbing the top coat such that the QinetiQ TBC Lifing Model could be validated. The importance of material, system specific knowledge and performance data with respect to a particular design space for critical components in gas turbine engines has been highlighted. Data and knowledge regarding N18, LSHR and TBC systems has been added to the QinetiQ’s databank enhancing their capability for providing independent advice regarding high temperature materials particularly in new gas turbine engines.

In the aeronautics industry, the propulsion systems stand among the most advanced and critical components. Over the last 50 years, gas turbine aeroengines were subjected to intensive research to increase efficiency and reduce weight, noise and harmful emissions. Together with design optimization, breakthrough in materials science for structural applications triggered the development of the most advanced gas turbine engines. For low temperatures, basically ahead of the combustion section, lightweight Ti alloys are preferred for their good mechanical properties. For high temperatures instead, Ni-based superalloys exhibit outstanding properties up to very high temperatures despite a rather high material’s density. Research have focused on enhancing to the maximum the potential of materials in gas turbine engines. According to the application, the components experience various mechanical and environmental constraints. Special designs, manufacturing process, material compositions and protective coatings have been developed to push the limits of advanced materials. Nowadays, the attention is focused on innovative materials to replace the existing Ti and Ni based alloys leading to substantial benefits. Light weight composite materials in particular were found very attractive to replace some components’ Ti alloys. At higher temperatures, it is of great interest to replace Ni-based superalloys by materials with lower density and/or higher temperatures applications, which in turn would lead to substantial weight reduction and increase efficiency. At the highest temperatures range, in particular in the combustion chamber and high pressure turbine sections, ceramic based materials offer promising balance of properties. Research are dedicated to overcome the drawbacks of ceramics for such structural applications, and in particular their brittle fracture behavior, by addition of reinforcing fibers. At lower temperatures range, TiAl based intermetallics emerged as very promising materials at half the density of Ni-based superalloys. Significant weight reduction could be achieved by the introduction of TiAl based alloys for rotating components of the compressor and low pressure turbine. 2nd generation γ-TiAl alloys were lately introduced in GE’s GEnx and CFM’s LEAP engines. The present work concerns the fabrication by the additive manufacturing technique Electron Beam Melting of 3rd generation γ-TiAl alloys for high temperatures application in gas turbine aeroengines. EBM, building parts layer by layer according to CAD, offers many advantages compared to other manufacturing processes like casting and forging. Reported by Avio, 2nd generation γ-TiAl alloys have been successfully fabricated by EBM. To increase the material’s potential, the production of 3rd generation γ-TiAl alloys Ti-(45-46)Al-2Cr-8Nb was therefore studied. The optimization of the EBM parameters led to high homogeneity and very low post-processing residual porosity ≤ 1%. The fine equiaxed microstructure after EBM could be tailored towards the desired mechanical properties by simple heat treatment, from equiaxed to duplex to fully lamellar. In particular, a duplex microstructure composed by about 80 % lamellar grains pinned at grain boundaries by fine equiaxed grains was obtained after heat treatment slightly over the α transus temperature. The study showed that addition of a higher amount of Nb significantly increased the oxidation resistance of the material, thus increasing the application temperature range of these γ-TiAl alloys.

The discovery of coherent g-g’ microstructure in Co-Al-W and Co-Al-Mo-Nb/Ta systems represents new possibilities for turbine engines materials. Their chemical and mechanical properties are similar to commercially successful Ni base superalloys. The present work aims at understanding the oxidation behavior of ‘W’ free γ’ strengthened Co-based superalloys. To understand the oxidation kinetics, mass change of the alloys was recorded with time during isothermal exposure at elevated temperature (650-950°C) in static air. To elucidate the oxidation mechanism in these alloys, in-depth analysis of kinetic data along with the characterization of the oxide scales have been carried out. The high temperature isothermal oxidation behavior of the basic Co-30Ni-10Al-5Mo-2Ta (at%) alloy has been studied in a static atmosphere to provide benchmark information on the oxidation behavior. Further, a 2 atom% Ti was added to this alloy to evaluate the consequences of Ti addition. Additionally, systematic addition of Cr (5at% and 8at%) in Co-30Ni-10Al-5Mo-2Ta-2Ti (at%) is evaluated. In the series ‘Mo’ free Co-30Ni-10Al-2Nb-x (x: 8 and 12) Cr-y (y: 2 and 4)Ti (at%) alloy system possess superior high temperature properties. Thus, the effect of Ti and Cr on this alloy system is studied from the point of view of environmental degradation through oxidation. Three different types of oxide layers were observed. The outer layer comprises of cobalt rich oxides followed by mixed oxides in the middle and inner layers. Oxides of Ta, Al, Mo, Ti, Cr, Nb were detected in the middle layer. Phase transformation of alumina oxide from amorphous to crystalline α-Al2O3 is observed with the addition of Ti and Cr in the base alloy. All the alloys show the γ-γ’ microstructural stability at the oxide-matrix interface during oxidation. No oxidation induced secondary phase is observed at the alloy side of the oxide-matrix interface as compared to the formation of Co3W phase in ‘W’ containing alloys.

Indiana University-Purdue University Indianapolis (IUPUI)
This study seeks to improve the understanding of inlet conditions of a large rotor-stator cavity in a turbofan engine, often referred to as the drive cone cavity (DCC). The inlet flow is better understood through a higher fidelity computational fluid dynamics (CFD) modeling of the inlet to the cavity, and a coupled finite element (FE) thermal to CFD fluid analysis of the cavity in order to accurately predict engine component temperatures. Accurately predicting temperature distribution in the cavity is important because temperatures directly affect the material properties including Young's modulus, yield strength, fatigue strength, creep properties. All of these properties directly affect the life of critical engine components. In addition, temperatures cause thermal expansion which changes clearances and in turn affects engine efficiency. The DCC is fed from the last stage of the high pressure compressor. One of its primary functions is to purge the air over the rotor wall to prevent it from overheating. Aero-thermal conditions within the DCC cavity are particularly challenging to predict due to the complex air flow and high heat transfer in the rotating component. Thus, in order to accurately predict metal temperatures a two-way coupled CFD-FE analysis is needed. Historically, when the cavity airflow is modeled for engine design purposes, the inlet condition has been over-simplified for the CFD analysis which impacts the results, particularly in the region around the compressor disc rim. The inlet is typically simplified by circumferentially averaging the velocity field at the inlet to the cavity which removes the effect of pressure wakes from the upstream rotor blades. The way in which these non-axisymmetric flow characteristics affect metal temperatures is not well understood. In addition, a constant air temperature scaled from a previous analysis is used as the simplified cavity inlet air temperature. Therefore, the objectives of this study are: (a) model the DCC cavity with a more physically representative inlet condition while coupling the solid thermal analysis and compressible air flow analysis that includes the fluid velocity, pressure, and temperature fields; (b) run a coupled analysis whose boundary conditions come from computational models, rather than thermocouple data; (c) validate the model using available experimental data; and (d) based on the validation, determine if the model can be used to predict air inlet and metal temperatures for new engine geometries. Verification with experimental results showed that the coupled analysis with the 3D no-bolt CFD model with predictive boundary conditions, over-predicted the HP6 offtake temperature by 16k. The maximum error was an over-prediction of 50k while the average error was 17k. The predictive model with 3D bolts also predicted cavity temperatures with an average error of 17k. For the two CFD models with predicted boundary conditions, the case without bolts performed better than the case with bolts. This is due to the flow errors caused by placing stationary bolts in a rotating reference frame. Therefore it is recommended that this type of analysis only be attempted for drive cone cavities with no bolts or shielded bolts.

Aero engines are one of the most complex machines on this planet and have propelled the necessity of advanced material technologies. Health monitoring of the engine is performed by a variety of sensors and amongst these strain sensor is very important as it aids in evaluating the stresses experienced by the body. Unlike conventional foil gauge which tends to debond under hostile environments in the engine like high rpm of blades, temperature, mass flow, etc, thin film based strain gauges are likely to exhibit better adhesion on the substrates. The usage of Ni-Cr thin films in strain gauge sensor has been proven for static application, wherein the substrate does not experience the fluctuating loads. Material and mechanical aspects of thin films for design and development of thin film based strain gauge sensor for aero engine application was taken up as a research work. One of the objectives of the work was to characterize the Ni-Cr thin films with varying composition deposited by sputter deposition process and characterize the films for its microstructural features and mechanical properties. The correlation of these properties is performed and amongst the film compositions investigated the film with alloy composition of Ni-Cr:80-20 at% exhibits the most distinct columnar structure, highest electrical resistivity (2.037 μΩm), hardness (5.8 GPa) and the modulus (180 GPa). This Ni-Cr: 80-20 at% film exhibits no surface cracks when loaded in the elastic region of the titanium alloy GTM-Ti-64. Resistance to deformation under the action of externally applied load on a body results in stress within the body. In single or multilayer film stacking the stress experienced in the film by virtue of substrate deformation needs to be investigated quantitatively. The substrate stresses are transferred to the films by shear stresses at the interface. In order to measure the surface strain by change in the electrical resistance of the gage it is important to quantitatively evaluate the stresses in the films. Are these stresses very high to cause delamination and film cracking or are these stresses too less to be measured. In order to understand the stress evolution and transfer mechanism, an analytical approach, numerical simulation and experimental validation were performed. Thin film strain gauge device architecture has been engineered such that an insulating layer of alumina is deposited on substrate and a sensing layer is deposited on the insulating layer to avoid thermal mismatch and maintain the strain compatibility. A alumina of 45 micron thick alumina layer was successfully deposited on Titanium alloy (GTM-Ti-64) by sputter deposition without any edge delamination and microcracks. Finite Element Analysis (FEA) results showed that the axial and shear stress profiles at the Ti alloy-alumina interfaces for both single and multilayer architecture are similar and higher when compared with the stresses in alumina-NiCr. The shear stress profile for single layer and multilayer architecture follows the modified shear lag model with peak shear stresses at the extremes and peak axial stress at the centre of the film. The axial stresses in the alumina film is found to be significant in both FEA and validated by experimental findings with film fracture strength of 814 MPa. Similarly, the shear stresses were found to be minimal by FEA studies and the experimental finding suggests the film fracture under tensile mode. Complete strain transfer was observed from substrate to these thin films under both tensile and vibratory fatigue, suggesting proper adhesion of the alumina film on the Ti alloy substrate. The maximum strain compatibility of thin film alumina on Ti alloy substrate was found to be 0.22 %. A Goodman correction for the fatigue data under axial mode was performed and on combining the entire fatigue data for R = -1 linear fit was observed across all the data points wherein the Basquin equation was considered for data analysis and the fatigue strength coefficient and exponent are found to be 872.56 and -0.054 for alumina thin film on Ti alloy substrate. Thin film strain gauges (TFSG) with these characteristics were deposited on the compressor rotor blade of one of the typical aero engines. Thick contact pads and a new bonding technique are used for taking the lead wires. The entire multilayer structure with wire bonding was tested under static and dynamic (vibratory fatigue) conditions and TFSG exhibited a reproducible strain when compared against foil based strain gauge under both tension and compression. TFSG device was tested for a duration of 2200 seconds with a blade vibration frequency of 406 Hz i.e. 8.9x105 cycles. During the entire test duration, TFSG successfully measured strains from the aero engine blade.

Pt-aluminide (PtAl) coatings form an integral part of thermal barrier coating (TBC) systems that are applied on Ni-based superalloy components operating in the hot sections of gas turbine engines. These coatings serve as a bond coat between the superalloy substrate and the ceramic yttrium stabilized zirconia (YSZ) coating in the TBC system and provide oxidation resistance to the superalloy component during service at high temperatures. The PtAl coatings are formed by the diffusion aluminizing process and form an integral part of the superalloy substrate. The microstructure of the PtAl coatings is heavily graded in composition as well as phase constitution. The matrix phase of the coating is constituted of the B2-NiAl phase. Pt, in the coating, is present as a separate PtAl2 phase as well as in solid solution in B2-NiAl. The oxidation resistance of the PtAl bond coat is derived from the B2-NiAl phase. At high temperatures, Al from the B2-NiAl phase forms a regenerative layer of alumina on the coating surface which, thereby, lowers the overall oxidation rate of the superalloy substrate. The presence of Pt is beneficial in improving the adherence of the alumina scale to the surface and thereby enhancing the oxidation resistance of the coating. However, despite its excellent oxidation resistance, the B2-NiAl being an intermetallic phase, renders the PtAl coating brittle and imparts it with a high brittle-to-ductile-transition-temperature (BDTT). The PtAl coating, therefore, remains prone to cracking during service. The penetration of these cracks into the substrate is known to degrade the strain tolerance of the components. Evaluation of the mechanical behavior of these coatings, therefore, becomes important from the point of views of scientific understanding as well as application of these coatings in gas turbine engine components. Studies on the mechanical behavior of coatings have been mostly carried on coated bulk superalloy specimens. However, since the coating is brittle and the superalloy substrate more ductile when compared to the coating, the results obtained from these studies may not be representative of the coating. Therefore, it is imperative that the mechanical behavior of the coating in stand-alone condition, i.e. the free-standing coating specimen without any substrate attached to it, be evaluated for ascertaining the true mechanical response of the coating. Study of stand-alone bond coats involves complex specimen preparation techniques and challenging testing procedures. Therefore, reports on the evaluation of mechanical properties of stand-alone coatings are limited in open literature. Further, no systematic effort has so far been made to examine important aspects such as the effect of temperature and strain rate on the tensile behavior of these coatings. The deformation mechanisms associated with these bond coats have also not been reported in the literature. In light of the above, the present research study aims at evaluating the tensile behavior of free-standing PtAl coatings by the micro-tensile testing technique. The micro-tensile testing method was chosen for property evaluation because of its inherent ability to generate uniform strain in the specimen while testing, which makes the results easy to interpret. Further, since the technique offers the feasibility to test the entire graded PtAl coating in-situ, the results remain representative of the coating. Using the above testing technique, the tensile behavior of the PtAl coating has been evaluated at various temperatures and strain rates. The effect of strain rate on the BDTT of the coating has been ascertained. Further, the effect of Pt content on the tensile behavior of these coatings has also been evaluated. Attempts have been made to identify the mechanisms associated with tensile deformation and fracture in these coatings. The thesis is divided into nine chapters. Chapter 1 presents a brief introduction on the operating environment in gas turbine engines and the materials that are used in the hot sections of gas turbine engines. The degradation mechanisms taking place in the superalloy in gas turbine environments and the need for application of coatings has also been highlighted. The basic architecture of a typical thermal barrier coating (TBC) system applied on gas turbine engine components has been presented. The constituents of the TBC system, i.e. the ceramic YSZ coating, MCrAlY overlay as well as diffusion aluminide bond coats and, the various techniques adopted for the deposition of these coatings have been described in brief. Chapter 2 presents an overview of the literature relevant to this study. This chapter is divided into four sub-chapters. The formation of diffusion aluminide coatings on Ni-based superalloys has been described in the first sub-chapter. Emphasis has been laid on pack cementation process for the formation of the coatings. The fundamentals of pack aluminizing process, including the thermodynamic and kinetic aspects, have been mentioned in brief. The microstructural aspects of high activity and low activity plain aluminide and Pt-aluminide coatings have also been illustrated. The techniques applied for the mechanical testing of bond coats have been discussed in the second sub-chapter. The macro-scale testing techniques have been mentioned in brief. The small scale testing methods such as indentation, bend tests and micro-tensile testing have also been discussed in the context of evaluation of mechanical properties of bond coats. Since the matrix in the aluminide bond coats is constituted of the B2-NiAl phase, a description of the crystal structure and deformation characteristics of this phase including the flow behavior, ductility and fracture behavior has been mentioned in the third sub-chapter. In the fourth sub-chapter, reported literature on the tensile behavior and brittle-to-ductile-transition-temperature (BDTT) of diffusion aluminide bond coats has been discussed. In Chapter 3, details on experiments carried out for the formation of various coatings used in the present study and, their microstructural characterization, are provided. The method for extraction of stand-alone coating specimens and their testing is discussed. The microstructure and composition of the various coatings used in the present study are discussed in detail in Chapter 4. Unlike in case of bulk tensile testing, for which standards on the design of specimens exist, there are no standards available for the design of micro-tensile specimens. Therefore, as part of the present research work, a finite element method (FEM)-based study was carried out for ascertaining the dimensions of the specimens. The simulation studies predicted that failure of the specimens within the gage length can be ensured only when certain correlations between the dimensional parameters are satisfied. Further, the predictions from the simulation study were validated experimentally by carrying out actual testing of specimens of various dimensions. Details on the above mentioned aspects of specimen design are provided in Chapter 5. The PtAl coatings undergo brittle fracture at lower temperatures while ductile fracture occurs at higher temperatures. Further, the coatings exhibit a scatter in the yielding behavior at temperatures in the vicinity of BDTT. Therefore, the BDTT, determined as the temperature at which yielding is first observed in the stress-strain curves, may not be representative of the PtAl coatings. In Chapter 6, a method for the precise determination of BDTT of aluminide bond coats, based on the variation in the plastic strain to fracture with temperature, has been demonstrated. The BDTT determined by the above method correlated well with the variation in fracture surface features of the coating and was found representative of these coatings. In Chapter 7, the effect of temperature and strain rate on the tensile properties of a PtAl bond coat has been evaluated. The temperature and strain rate was varied between room temperature (RT)-1100°C and 10-5 s-1-10-1 s-1, respectively. The effect of strain rate on the BDTT of the PtAl bond coat has been examined. Further, the variation in fracture surface features and mechanism of fracture with temperature and strain rate are illustrated. The micro-mechanisms of deformation and fracture in the coating at different temperature regimes have also been discussed. The coating exhibited brittle-to-ductile transition with increase in temperature at all strain rates. The BDTT was strain rate sensitive and increased significantly at higher strain rates. Above BDTT, YS and UTS of the coating decreased and its ductility increased with increase in the test temperature at all strain rates. Brittle behavior occurring in the coating at temperatures below the BDTT has been attributed to the lack of operative slip systems in the B2-NiAl phase of the coating. The onset of ductility in the coating in the vicinity of BDTT has been ascribed to generation of additional slip systems caused by climb of dislocations onto high index planes. The coating exhibited two distinct mechanisms for plastic deformation as the temperature was increased from BDTT to 1100°C. For temperatures in the range BDTT to about 100°C above it, deformation was controlled by dislocations overcoming the Peierls-Nabarro barrier. Above this temperature range, non-conservative motion of jogs by jog dragging mechanism controlled the deformation. The transition temperature for change of deformation mechanism also increased with increase in strain rate. For all strain rates, fracture in the coating at test temperatures below the BDTT, occurred by initiation of cracks in the intermediate single phase B2-NiAl layer of the coating and subsequent inside-out propagation of the cracks across the coating thickness. Ductile fracture in the coating above the BDTT was associated with micro-void formation throughout the coating. The effect of Pt content on the tensile behavior of PtAl coating, evaluated at various temperatures ranging from room temperature (RT) to 1100°C and at a nominal strain rate of 10-3 s-1, is presented in Chapter 8. Irrespective of Pt content in the coating, the variation in tensile behavior of the coating with temperature remained similar. At temperatures below BDTT, the coatings exhibited linear stress-strain response (brittle behavior) while yielding (ductile behavior) was observed at temperatures above BDTT. At any given temperature, the elastic modulus decreased while the strength increased with increase in Pt content in the coating. On the other hand, the ductility of the coating remained unaffected with Pt content. The BDTT of the coating also increased with increase in Pt content in the coating. Addition of Pt did not affect the fracture mechanism in the coating. Fracture at temperatures below BDTT was caused by nucleation of cracks at the intermediate layer and their subsequent inside-out propagation. At high temperatures, fracture occurred in a ductile manner comprising void formation, void linkage and subsequent joining with cracks. The deformation sub-structure of the coating did not get affected with Pt incorporation. Short straight dislocations were observed at temperatures below BDTT, while, curved dislocations marked by jog formation were observed at temperatures above BDTT. The factors controlling fracture stress and strength in the PtAl coatings at various temperatures have also been assessed. The overall summary of the present research study and recommendations for future studies are presented in the last chapter, i.e. Chapter 9.

Pt-aluminide (PtAl) coatings form an integral part of thermal barrier coating (TBC) systems that are applied on Ni-based superalloy components operating in the hot sections of gas turbine engines. These coatings serve as a bond coat between the superalloy substrate and the ceramic yttrium stabilized zirconia (YSZ) coating in the TBC system and provide oxidation resistance to the superalloy component during service at high temperatures. The PtAl coatings are formed by the diffusion aluminizing process and form an integral part of the superalloy substrate. The microstructure of the PtAl coatings is heavily graded in composition as well as phase constitution. The matrix phase of the coating is constituted of the B2-NiAl phase. Pt, in the coating, is present as a separate PtAl2 phase as well as in solid solution in B2-NiAl. The oxidation resistance of the PtAl bond coat is derived from the B2-NiAl phase. At high temperatures, Al from the B2-NiAl phase forms a regenerative layer of alumina on the coating surface which, thereby, lowers the overall oxidation rate of the superalloy substrate. The presence of Pt is beneficial in improving the adherence of the alumina scale to the surface and thereby enhancing the oxidation resistance of the coating. However, despite its excellent oxidation resistance, the B2-NiAl being an intermetallic phase, renders the PtAl coating brittle and imparts it with a high brittle-to-ductile-transition-temperature (BDTT). The PtAl coating, therefore, remains prone to cracking during service. The penetration of these cracks into the substrate is known to degrade the strain tolerance of the components. Evaluation of the mechanical behavior of these coatings, therefore, becomes important from the point of views of scientific understanding as well as application of these coatings in gas turbine engine components. Studies on the mechanical behavior of coatings have been mostly carried on coated bulk superalloy specimens. However, since the coating is brittle and the superalloy substrate more ductile when compared to the coating, the results obtained from these studies may not be representative of the coating. Therefore, it is imperative that the mechanical behavior of the coating in stand-alone condition, i.e. the free-standing coating specimen without any substrate attached to it, be evaluated for ascertaining the true mechanical response of the coating. Study of stand-alone bond coats involves complex specimen preparation techniques and challenging testing procedures. Therefore, reports on the evaluation of mechanical properties of stand-alone coatings are limited in open literature. Further, no systematic effort has so far been made to examine important aspects such as the effect of temperature and strain rate on the tensile behavior of these coatings. The deformation mechanisms associated with these bond coats have also not been reported in the literature. In light of the above, the present research study aims at evaluating the tensile behavior of free-standing PtAl coatings by the micro-tensile testing technique. The micro-tensile testing method was chosen for property evaluation because of its inherent ability to generate uniform strain in the specimen while testing, which makes the results easy to interpret. Further, since the technique offers the feasibility to test the entire graded PtAl coating in-situ, the results remain representative of the coating. Using the above testing technique, the tensile behavior of the PtAl coating has been evaluated at various temperatures and strain rates. The effect of strain rate on the BDTT of the coating has been ascertained. Further, the effect of Pt content on the tensile behavior of these coatings has also been evaluated. Attempts have been made to identify the mechanisms associated with tensile deformation and fracture in these coatings. The thesis is divided into nine chapters. Chapter 1 presents a brief introduction on the operating environment in gas turbine engines and the materials that are used in the hot sections of gas turbine engines. The degradation mechanisms taking place in the superalloy in gas turbine environments and the need for application of coatings has also been highlighted. The basic architecture of a typical thermal barrier coating (TBC) system applied on gas turbine engine components has been presented. The constituents of the TBC system, i.e. the ceramic YSZ coating, MCrAlY overlay as well as diffusion aluminide bond coats and, the various techniques adopted for the deposition of these coatings have been described in brief. Chapter 2 presents an overview of the literature relevant to this study. This chapter is divided into four sub-chapters. The formation of diffusion aluminide coatings on Ni-based superalloys has been described in the first sub-chapter. Emphasis has been laid on pack cementation process for the formation of the coatings. The fundamentals of pack aluminizing process, including the thermodynamic and kinetic aspects, have been mentioned in brief. The microstructural aspects of high activity and low activity plain aluminide and Pt-aluminide coatings have also been illustrated. The techniques applied for the mechanical testing of bond coats have been discussed in the second sub-chapter. The macro-scale testing techniques have been mentioned in brief. The small scale testing methods such as indentation, bend tests and micro-tensile testing have also been discussed in the context of evaluation of mechanical properties of bond coats. Since the matrix in the aluminide bond coats is constituted of the B2-NiAl phase, a description of the crystal structure and deformation characteristics of this phase including the flow behavior, ductility and fracture behavior has been mentioned in the third sub-chapter. In the fourth sub-chapter, reported literature on the tensile behavior and brittle-to-ductile-transition-temperature (BDTT) of diffusion aluminide bond coats has been discussed. In Chapter 3, details on experiments carried out for the formation of various coatings used in the present study and, their microstructural characterization, are provided. The method for extraction of stand-alone coating specimens and their testing is discussed. The microstructure and composition of the various coatings used in the present study are discussed in detail in Chapter 4. Unlike in case of bulk tensile testing, for which standards on the design of specimens exist, there are no standards available for the design of micro-tensile specimens. Therefore, as part of the present research work, a finite element method (FEM)-based study was carried out for ascertaining the dimensions of the specimens. The simulation studies predicted that failure of the specimens within the gage length can be ensured only when certain correlations between the dimensional parameters are satisfied. Further, the predictions from the simulation study were validated experimentally by carrying out actual testing of specimens of various dimensions. Details on the above mentioned aspects of specimen design are provided in Chapter 5. The PtAl coatings undergo brittle fracture at lower temperatures while ductile fracture occurs at higher temperatures. Further, the coatings exhibit a scatter in the yielding behavior at temperatures in the vicinity of BDTT. Therefore, the BDTT, determined as the temperature at which yielding is first observed in the stress-strain curves, may not be representative of the PtAl coatings. In Chapter 6, a method for the precise determination of BDTT of aluminide bond coats, based on the variation in the plastic strain to fracture with temperature, has been demonstrated. The BDTT determined by the above method correlated well with the variation in fracture surface features of the coating and was found representative of these coatings. In Chapter 7, the effect of temperature and strain rate on the tensile properties of a PtAl bond coat has been evaluated. The temperature and strain rate was varied between room temperature (RT)-1100°C and 10-5 s-1-10-1 s-1, respectively. The effect of strain rate on the BDTT of the PtAl bond coat has been examined. Further, the variation in fracture surface features and mechanism of fracture with temperature and strain rate are illustrated. The micro-mechanisms of deformation and fracture in the coating at different temperature regimes have also been discussed. The coating exhibited brittle-to-ductile transition with increase in temperature at all strain rates. The BDTT was strain rate sensitive and increased significantly at higher strain rates. Above BDTT, YS and UTS of the coating decreased and its ductility increased with increase in the test temperature at all strain rates. Brittle behavior occurring in the coating at temperatures below the BDTT has been attributed to the lack of operative slip systems in the B2-NiAl phase of the coating. The onset of ductility in the coating in the vicinity of BDTT has been ascribed to generation of additional slip systems caused by climb of dislocations onto high index planes. The coating exhibited two distinct mechanisms for plastic deformation as the temperature was increased from BDTT to 1100°C. For temperatures in the range BDTT to about 100°C above it, deformation was controlled by dislocations overcoming the Peierls-Nabarro barrier. Above this temperature range, non-conservative motion of jogs by jog dragging mechanism controlled the deformation. The transition temperature for change of deformation mechanism also increased with increase in strain rate. For all strain rates, fracture in the coating at test temperatures below the BDTT, occurred by initiation of cracks in the intermediate single phase B2-NiAl layer of the coating and subsequent inside-out propagation of the cracks across the coating thickness. Ductile fracture in the coating above the BDTT was associated with micro-void formation throughout the coating. The effect of Pt content on the tensile behavior of PtAl coating, evaluated at various temperatures ranging from room temperature (RT) to 1100°C and at a nominal strain rate of 10-3 s-1, is presented in Chapter 8. Irrespective of Pt content in the coating, the variation in tensile behavior of the coating with temperature remained similar. At temperatures below BDTT, the coatings exhibited linear stress-strain response (brittle behavior) while yielding (ductile behavior) was observed at temperatures above BDTT. At any given temperature, the elastic modulus decreased while the strength increased with increase in Pt content in the coating. On the other hand, the ductility of the coating remained unaffected with Pt content. The BDTT of the coating also increased with increase in Pt content in the coating. Addition of Pt did not affect the fracture mechanism in the coating. Fracture at temperatures below BDTT was caused by nucleation of cracks at the intermediate layer and their subsequent inside-out propagation. At high temperatures, fracture occurred in a ductile manner comprising void formation, void linkage and subsequent joining with cracks. The deformation sub-structure of the coating did not get affected with Pt incorporation. Short straight dislocations were observed at temperatures below BDTT, while, curved dislocations marked by jog formation were observed at temperatures above BDTT. The factors controlling fracture stress and strength in the PtAl coatings at various temperatures have also been assessed. The overall summary of the present research study and recommendations for future studies are presented in the last chapter, i.e. Chapter 9.