Bond coats used in thermal barrier coatings (TBCs) for gas-turbine engine applications are studied in this thesis, with a focus on oxidation behaviour, surface rumpling and stress evolution. Bond coats made of γ/γ’ Ni-Al-Pt alloys have been widely used in TBCs and it has been found that addition of platinum greatly improves the oxidation resistance of the coatings. The mechanisms behind this benefit, however, are not well understood. For this reason, the oxidation behaviour of four γ/γ’ Ni-20Al-xPt (x= 0, 5, 10 and 15 at. %) alloys at 1150 °C is studied and compared in terms of oxide spallation, oxide microstructure and growth, residual stress in the oxide scale and oxide/alloy interface morphology. The progressive increase of platinum addition into the alloys results in (1) greater resistance to oxide spallation, (2) reduction in oxidation of nickel, (3) lower stresses in the α-Al2O3 scale and (4) more planar oxide/alloy interfaces. It is found that the selective oxidation of aluminium promoted by platinum plays a central role in the evolution of the oxidation behaviour of the alloys. Surface rumpling of a NiCoCrAlY bond coat deposited on a Ni-base superalloy during cyclic oxidation at 1150 °C is studied. The extent of rumpling is found to depend on thermal history, coating thickness and exposure atmosphere. While the coating surface progressively roughens with cyclic oxidation, bulk NiCoCrAlY alloys with the same nominal composition show a much less tendency to rumple under the same thermal cycling condition. The coatings, especially the thin ones, experience substantial degradation (e.g. β to γ phase transformation and exhaustion of yttrium) induced by oxidation and coating/substrate interdiffusion during thermal exposure. The observations together suggest that rumpling is driven by the lateral growth of the thermally grown oxide (TGO) and the coating deforms in compliance with the TGO. While the dependence of rumpling development on experimental conditions is generally in agreement with the prediction of the existing model, it is suggested that the degradation of the NiCoCrAlY coating and its dependence on coating thickness need to be taken into consideration when predicting the rumpling development of NiCoCrAlY coatings. The residual stresses in a NiCoCrAlY bond coat deposited on a Ni-base superalloy are studied by X-ray diffraction using the sin2Ψ technique. The stresses at room temperature are found to be tensile; they first increase and then decrease with oxidation time. The stress develops and builds up upon cooling, predominantly within the temperature range from 1150 °C to 600 °C. Due to the limited penetration depth into the bond coat, the X-ray only probes the stress in a thin surface layer consisting of a single γ-phase formed through aluminium depletion during oxidation. Above 600 °C, the volume fraction of the β-phase in the bond coat increases with decreasing temperature. The mechanisms of stress generation in the coating are examined and discussed based on experiments designed to isolate the contribution of possible stress generation factors. It is found that the measured bond coat stresses are mainly induced by the volume change of the bond coat associated with the precipitation of the β-phase upon cooling.

MCrAlY coatings are commonly applied as the bond coat in TBCs used in modern gas turbines. Cold spray (or CS), characterized by low process temperature and high particle impact velocity, has been demonstrated as a promising alternative to thermal spray processes, such as air plasma spray (APS) and high velocity oxygen fuel (HVOF), for manufacturing MCrAlY coatings. The general objective of the thesis research is to characterize CS deposition on a single-crystal nickel-base superalloy and to develop low-cost/high-performance NiCoCrAlTaY coatings using the CS technique. Several individual studies were carried out with each having a specific focus towards achieving the general research objective. CS deposition of NiCoCrAlTaY coatings using nitrogen was first examined to verify the feasibility of replacing the expensive helium gas typically used as the CS process gas. Several materials were used as the substrates, and the effects of substrate materials and surface preparation on coating microstructure and properties were investigated. Recycling of non-deposited powder particles was then explored to reduce the costs associated with the feedstock powder. A cost model that includes the economics of powder recycling was developed for the CS process, showing that the use of nitrogen and powder recycling could potentially be cost-effective for CS deposition of MCrAlY coatings. A CS process that can produce in-situ nanocrystalline NiCoCrAlTaY coatings was proposed to develop coatings with enhanced oxidation performance. This CS approach utilizes conventional commercial powders instead of pre-milled nanocrystalline powders. Detailed characterization using the scanning electron microscope (SEM), scanning transmission electron microscope (STEM), and X-ray diffraction (XRD) was carried out to investigate the microstructure of the resulting CS NiCoCrAlTaY coatings, single-crystal substrate, and their interface. Isothermal oxidation performance of the CS NiCoCrAlTaY coatings was evaluated at 1100°C for 1h to 500h. Results revealed that the nanostructure promoted the α-Al2O3 scale formation and sustained α-Al2O3 scale growth, suggesting good isothermal oxidation performance. Finally, the effects of different processing sequences on CS NiCoCrAlTaY coating characteristics and short-term isothermal oxidation performance were investigated. Specifically, CS deposition of NiCoCrAlTaY coatings was carried out on single-crystal superalloy substrates that underwent various degrees of full heat treatments prior to being coated. The remaining superalloy heat treatments required were then performed on the coated samples after the CS deposition. The microstructures of the superalloy substrates and CS NiCoCrAlTaY coatings were characterized after each heat treatment. Isothermal oxidation performance of the coated samples following different sequences was evaluated at 1100°C for 2 hours. The results suggested a promising processing sequence that could potentially further improve the oxidation performance of CS NiCoCrAlTaY coatings.

No description available.

Superalloys used in aeroengines are designed to offer superior strength at increasingly higher operating temperatures. In order to optimise the working efficiency and provide additional protection to the components such as turbine blades; a thermal barrier coating (TBC) system is applied. The TBC is a multilayer system consisting of mainly two layers i.e. bond coat (BC) and topcoat (TC). In addition, a third layer grows between the TC and BC during oxidation known as a reaction layer or thermally grown oxide (TGO). The function of the TC (usually, yttria stabilised zirconia (YSZ)) is to provide thermal insulation to aeroengine parts or reduce their surface temperatures; whereas, the BC provides binding between the TC and the substrate, and oxidation resistance to the underlying alloy by forming an adherent and continuous oxide i.e. α-Al2O3. During service, in the absence of mechanical damage to the TBC, most failures are attributed to the BC performance. The most frequently adopted BCs are; β-(Pt, Ni)Al, Pt-γ-Ni/γ’-Ni3Al and MCrAlY. In addition, reactive elements (REs) are incorporated in the BCs due to their ability to enhance oxidation resistance significantly. In the present study βNiAl based coatings/BCs and alloys with and without REs (Zr and Hf) and Pt were prepared. For the coatings CMSX-4 single crystal superalloy was used as a substrate material and pack aluminising/cementation or in-situ chemical vapour deposition (CVD) as a coating process. The isothermal oxidation testing was carried out at 1150oC for 50 and 100 hours in air. The preparation and oxidation performance of a δNi2Al3 coating was carried out, as, this is a starting material for βNiAl matrix based coatings/or BCs. The oxidation of δNi2Al3 coating showed large volumetric changes (thickness variations), multiphase TGO, TGO/coating interface melting and spallation during oxidation. In contrast, the ‘simple βNiAl’ coating (or βNiAl matrix) was found to exhibit comparably enhanced thermal stability than that of the δNi2Al3 coating. Moreover, a detailed study of the simple βNiAl coating was also carried out in order to understand the oxidation performance. The coating before oxidation in the as-deposited condition was found to contain residual compressive stresses of 140 – 200 MPa. In contrast, after oxidation analysis exhibited substantial interdiffusion between the coating and the substrate resulting in a large reduction of the Al content and influx of substrate elements into the coating. This in turn caused coating transformation from βNiAl to the γ’-Ni3Al phase and formation of a multiphase TGO (TiO2, NiAl2O4, and ϴ-Al2O3 intrusion in α-Al2O3). Moreover, the degree of the TGO spallation and residual stresses increased with the oxidation time. In order to enhance the oxidation performance of the βNiAl coatings, the substrate pre-treatment was carried out i.e. CMSX-4 superalloy was electrolytically etched to remove the γ-Ni phase and fabricate βNiAl coatings on the remaining γ’-Ni3Al. This coating is termed as E-βNiAl. In comparison to simple βNiAl, the E-βNiAl coating showed improved spallation resistance. However, E-βNiAl revealed increased surface area due to etching of the substrate and triggered fast TGO growth rates when tested in an un-polished condition. Furthermore, simple βNiAl coatings were doped with Zr and Hf separately using a two-step aluminising method. The appropriate addition of either Zr or Hf was found to reduce the substrate elements (W, Ta, Cr and Ti etc.) in the coating before and after oxidation. After oxidation, examination of the presence of Zr or Hf in the coating was found to confirm the commonly reported beneficial effects. The TGOs grown on these coatings were almost pure α-Al2O3 which subsequently reduced growth and stresses. In addition to Zr/& Hf doped coatings, a study on Hf and Zr doped βNiAl bulk alloys was also carried out in order to understand the dopant effects on the oxidation resistance of βNiAl alloys in the absence of interdiffusion (as in case of coatings). In general, the commonly reported oxidation benefits were confirmed by the addition of these elements such as reduced TGO growth, oxide pegging, a columnar morphology of the TGO and segregation of REs at alumina grain boundaries etc. In addition, two more beneficial effects are suggested to be the ‘TGO crack filling up (or crack-healing)’ and formation of the ‘dense-TGO’. Within this study, the investigation of commercially available Pt-βNiAl BC was also carried out in air and vacuum atmospheres. The results demonstrated that the initial chemistry and elemental distribution (particularly Al/& Pt) was found to affect the TGO growth and phases significantly. In addition to its well established beneficial effects, the main effect of a Pt addition is suggested to be the stabilisation of the βNiAl structure even at a lower Al content.

Thermal barrier coatings (TBCs) have been widely used in gas turbine engines to protect the underlying metal from high operating temperature so as to improve the durability of the components and enhance the engine efficiency. However, since the TBCs always operate in a demanding high-temperature environment of aircraft and industrial gas-turbine engines, a better understanding of this complex system is required to improve the durability and reliability. The objective of this study is to investigate the effects of surface modification for the NiCoCrAlY bond coats on the thermal cycling lifetime and failure characteristics of TBCs. Parameters of modification for the bond coats included as-sprayed, barrel-finished, hand-polished and pre-oxidation heat treatment at 1100[degrees]C in P=10O2-8 atm up to 4 hours, carried out prior to the electron beam physical vapor deposition (EB-PVD) of ZrO2-7wt% Y2O3 (7YSZ) ceramic topcoat. The resulting characteristics of the bond coat and the thermally grown oxide (TGO) scale were initially documented by surface roughness, phase constituents of the TGO scale, and residual stress of the TGO scale. The thermal cycling test consisted of 10-minute heat-up to 1121°C, 40-minute hold at 1121°C, and 10-minute forced air-quench. As-coated and thermally-cycled TBCs were characterized by optical profilometry (OPM), photo-stimulated luminescence spectroscopy (PSLS), optical microscopy, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), and scanning/transmission electron microscopy (TEM/STEM) equipped with high angle annular dark field (HAADF) and X-ray energy dispersive spectroscopy (XEDS). TBC specimens for TEM/STEM analysis were prepared by focused ion beam (FIB) in-situ lift-out (INLO) technique. Superior thermal cycling lifetime was observed for TBCs with as-sprayed bond coats regardless of pre-oxidation heat treatment, and TBCs with hand-polished bond coats only after pre-oxidation heat treatment. With pre-oxidation heat treatment, relative photostimulated luminescence intensity of the equilibrium α-Al2O3 increased. Thus, the improvement in TBC lifetime can be correlated with an increase in the amount of α-Al2O3 in the TGO scale, given a specific surface modification/roughness. The lifetime improvement due to pre-oxidation was particularly significant to TBCs with smooth hand-polished bond coats and negligible for TBCs with rough as-sprayed bond coats. Spallation-fracture paths depended on the lifetime of TBCs. Premature spallation of TBCs occurred at the interface between the YSZ and TGO. Longer durability can be achieved by restricting the fracture paths to the TGO/bond coat interface. Small particulate phase observed through the TGO scale was identified as Y2O3 (cubic) by diffraction analysis on TEM. While small addition of Y in the NiCoCrAlY bond coat helps the adhesion of the TGO scale, excessive alloying can lead to deleterious effects.

The continued focus to include high hydrogen fuels such as Syngas in aircraft operation to reduce emissions and increase engine efficiency has led to an ongoing investigation into bond coat materials capable of withstanding unfavourable oxidation in high temperature humid environments. The increased presence of water in the engine exhaust leads to increased oxygen activity in the hot section of the engine. In this work, four commercially available MCrAlY bond coat materials were oxidized in high temperature environments with various humidities to understand the behaviours of different reactive element inclusions in resisting high temperature oxidation. Oxidation tests were done at 0%, 18%, and 33% water by volume at 1100C in a 1atm environment to simulate conditions expected in engines using high hydrogen fuels. Oxidation was done for 2h and 20h to observe transient oxide formation behaviour. The surfaces and cross sections of the specimens were examined using SEM and EDS analysis, along with XRD analysis. The progression of surface oxides, TGO thickness, and element depletion zones were observed. Two opposing mechanisms are observed: the upward diffusion of metal cations to the free surface and the inward diffusion of oxygen to the alloy. The presence of water is shown to increase internal oxidation of the bond coat alloy and delay the formation of a protective alumina TGO. Tantalum inclusion in the alloy composition is shown to produce the most stable alumina TGO with the least internal oxidation after 20h exposure in 33% H2O (%vol); the most hostile oxidation environment tested.

The continuing quest for higher performance levels of modern gas turbine engines has been accompanied by the demand for higher engine operating temperatures. The use of Thermal Barrier Coatings (TBCs) enabled gas turbines to operate at higher temperatures by protecting the blade material (nickel superalloy) while operating in extreme environments. The TBC system typically consists of a bond coat for protection of the nickel–based superalloy against oxidation followed by a top coat consisting of a thermally insulating zirconia-yttria. In addition to the complex gradation in phases, the coatings are subjected to continuous oxidation with service exposure, mechanical loading on rotating parts, fatigue, thermal mis-match and temperature gradients. Hence, the study of failure mechanisms of TBCs become important in deciding operational reliability and service life of the coating. As there are many systems in which the operating temperatures are not high enough to warrant the use of the top coat (ceramic layer), the study of failure mechanisms in superalloys coated with only the bond coat continue to be of great interest. The present work concentrates on the fracture behavior of NiAl bond coats on nickel superalloy and seeks to evaluate the fracture toughness of the coating through the use of micro-machined samples. A review of the relevant literature indicated that while a considerable body of work exists on bulk tests of failure (spalling, splitting, etc.), not much has been reported in the open literature on the evaluation of basic quantities such as the toughness of the coating itself. The present thesis seeks to establish a protocol for the evaluation of toughness and crack propagation mechanisms in coatings through a combination of micro-sample testing that allows fracture to be correlated with location in the film and the use of an analytical model to quantitatively evaluate stress intensity factors in a bi-material system. A system of NiAl coating produced by pack aluminizing is studied for the fracture properties of the coating. Specimen geometries are optimized to enable micro-cracks to be machined and propagated in a low load testing system, such as a depth sensing indenter, so that crack lengths (and position relative to the interface) can be correlated with load. To enable linear elastic theory to be used, dimensions are determined that allow fracture before general yielding. A three point bending test using miniaturized micro-beam specimens of ~ 4 X 0.3 X 0.3 mm is found to be suitable for the above purpose. The technique is a challenging one that requires focused ion beam machining (FIB) along with careful handling and alignment of small samples. The coatings are characterized for their microstructure by electron microscopy to identify compositional variation across the thickness and to determine the thickness of the coating and inter diffusion zone (IDZ). The crack advancement is monitored with increments of loading and the stress intensity factor is evaluated using a program written in “MAPLE” for an edge crack subjected to bending in a bilayered material. Surprisingly, fracture in this system is found to be stable owing to a gradual increase in toughness from the coating surface to the interface. Such an increase from less than 2 to more than 9 MPa m0.5 may be due to the increasing Ni/Al ratio across the thickness of the bond coat. Crack branching is observed as the crack approaches the IDZ and the reasons for such behaviour are not fully understood. This work establishes the viability of this technique to determine fracture properties in highly graded coated systems and may be readily extended to more complex coating architectures and other forms of loading such as cyclic, mixed mode, etc.

Carbon fibre reinforced composites are known for their high specific strength-to-weight ratio and are of great interest to the aerospace industry. Incorporating these materials into the fuselage, like in Boeing's 787 "Dreamliner", offers considerable weight reduction which increases flying efficiency, and reduces the cost of flying. In flight, aircraft are often subject to lightning strikes which, in the case of composites, can result in localized melting given the high resistive nature of the material. Aerospace carbon fibre composites often incorporate a metallic mesh or foil within the composite layers to dissipate the electrical charge through the large aircraft. The damage to the aircraft is minimized but not always eliminated. This research aims to elaborate a practical technique to deposit thin layers of conductive material on the surface of aerospace grade composites. Using Cold Gas Dynamic Spray (CGDS), such coatings could be used to repair damaged components. An experimental research approach was used to develop metallic coated composites. Using the CGDS equipment of Centerline (SST-P), specific parameters (such as gas temperature and stagnation pressure) were determined for each type of metallic coating (tin-based & copper-based). The use of bond coats was explored in order to attain the desired coatings. Once optimized, these coatings were evaluated with respect to their corrosive, adhesive, and electrical properties following industry standards.

The purpose of this research work was to develop a nozzle capable of depositing dense CoNiCrAlY coatings via cold gas dynamic spray (CGDS) as well as compare the oxidation performance of bond coats manufactured by CGDS, high-velocity oxy-fuel (HVOF) and air plasma spray (APS) at temperatures of 1000°C and 1100°C. The work was divided in two sections, the design and manufacturing of a CGDS nozzle with an optimal profile for the deposition of CoNiCrAlY powders and the comparison of the oxidation performance of CoNiCrAlY bond coats. Throughout this work, it was shown that the quality of coatings deposited via CGDS can be increased by the use of a nozzle of optimal profile and that early formation of protective α-Al2O3 due to an oxidation temperature of 1100°C as opposed to 1000°C is beneficial to the overall oxidation performance of CoNiCrAlY coatings.

The purpose of this research work was to develop a nozzle capable of depositing dense CoNiCrAlY coatings via cold gas dynamic spray (CGDS) as well as compare the oxidation performance of bond coats manufactured by CGDS, high-velocity oxy-fuel (HVOF) and air plasma spray (APS) at temperatures of 1000°C and 1100°C. The work was divided in two sections, the design and manufacturing of a CGDS nozzle with an optimal profile for the deposition of CoNiCrAlY powders and the comparison of the oxidation performance of CoNiCrAlY bond coats. Throughout this work, it was shown that the quality of coatings deposited via CGDS can be increased by the use of a nozzle of optimal profile and that early formation of protective α-Al2O3 due to an oxidation temperature of 1100°C as opposed to 1000°C is beneficial to the overall oxidation performance of CoNiCrAlY coatings.