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Reverse-flow combustors have been used in heavy land-based gas turbines for many decades. A sheath is typically installed to provide cooling at an expense of large pressure losses, through small jet impingement cooling and strong forced convention channel flow. With the modern advancement in metallurgy and thermal-barrier coating technologies, it may become possible to remove this sheath to recover the pressure losses without melting the combustor chamber. However, without the sheath, the flow inside the dump diffuser may exert nonuniform cooling on the combustion chamber. Therefore, the objective of this project is to investigate the flow pattern, pressure drop, and heat transfer in the dump-diffuser reverse-flow combustor with and without sheath to determine if the sheath could be removed. The investigation was conducted through both experimental and computational simulation. The results show that 3.3% pressure losses could be recovered and the highest wall temperature will increase 18% without the sheath.

The thermal conductivity of various porous thermal barrier coating (TBC) systems used in elevated temperature for industrial applications has been evaluated using a proposed six-phase model. These porous TBC systems rely on microstructural properties and yield different types of porosity. These microstructural properties can influence the thermal conductivity of TBC systems. The purpose of this thesis is to assess the thermal conductivity of TBC systems based on microstructural attributes, particularly the effect of different types of porosity. Thus, the first component of this thesis investigates the microstructural characterization of various TBC systems using image analysis (IA) technique. In this technique, scanning electron microscopy (SEM) and light optical microscopy (LOM) micrographs were used to measure the porosity level of different TBC materials. The volumetric fraction of porosity along with orientation, shape, and morphology have a considerable impact on the total thermal conductivity of TBCs. The second component of this thesis evaluates the thermal conductivity of these porous TBC systems by taking into account the effect of the heat treatment process. The IA results reveal that as long as the porosity content increases, the thermal conductivity decreases for all of the TBC materials studied in this thesis. Further, while the content of microcracks and non-flat porosity play a crucial role in reducing the thermal conductivity of TBC materials, the other types of porosity (open randomly oriented, penny-shaped, and interlamellar) exert less impact on the thermal conductivity of TBCs. Comparing the results of the proposed six-phase model to experimental values and finite element analysis (FEA) values showed a relatively good agreement. The proposed six-phase model can predict the thermal conductivity of porous microstructure of TBC systems close to real measured values; therefore, the proposed six-phase model may be utilized to fabricate the porous microstructure of TBCs.

Thermal barrier coatings (TBCs) of ZrO2-7wt% Y2O3 were deposited by electron beam physical vapor deposition (EB-PVD) onto stationary flat plates and cylindrical surfaces in a multiple ingot coater. Crystallographic texture, microstructure, and deposition rate were investigated in this thesis. The crystallographic texture of EB-PVD TBCs deposited on stationary flat surfaces has been experimentally determined by comparing pole figure analysis data with actual column growth angle data. It was found that the TBC coating deposited directly above an ingot exhibits <220> single crystal type crystallographic texture. Coatings deposited between and off the centerline of the ingots the exhibited a <311>-type single crystal texture. For coatings deposited in the far corners of the coating chamber either a <111> fiber texture or a <311> single crystal type texture existed. The crystallographic texture of EB-PVD TBCs deposited on cylindrical surfaces was characterized using x-ray diffraction (XRD) at different angular positions on the cylinder substrate. XRD results revealed that crystallographic texture changes with angular position. Changes in crystallographic texture are attributed to the growth direction of the columns and substrate temperature. Growth direction is controlled by the direction of the incoming vapor flux (i.e. vapor incidence angle), in which competition occurs between crystallites growing at different rates. The fastest growing orientation takes over and dominates the texture. Substrate temperature variations throughout the coating chamber resulted in different growth rates and morphology. Morphology differences existed between cylindrical and flat plate surfaces. Flat cross sectional surfaces of the coatings exhibited a dense columnar structure in which the columns grew towards the closest vapor source. Surface features were found to be larger for coatings deposited directly above an ingot than coatings deposited away from the ingots. Morphological differences result from substrate temperature changes within the coating chamber, which influences growth kinetics of the coating. Cylindrical surfaces revealed a columnar structure in which columns grew towards the closest vapor. Porosity of the coating was found to increase when the angular position changed from the bottom of the cylinder. Change in angular position also caused the column diameter to decreases. Morphology changes are attributed to self-shadow effects caused by the surface curvature of the cylinder and vapor incidence angle changes. Overall, the microstructure and crystallographic texture of EB-PVD coatings was found to depend on the position in the coating chamber which was found to influence substrate temperature, growth directions, and shadowing effects. The coating thickness profiles for EB-PVD TBCs deposited on stationary cylinders have been experimentally measured and theoretically modeled using Knudsen's cosine law of emissions. A comparison of the experimental results with the model reveals that the model must to be modified to account for the sticking coefficient as well as a ricochet factor. These results are also discussed in terms of the effects of substrate temperature on the sticking coefficient, the ricochet factor, and coating density.

Residual stresses are generally considered to be the driving forces for the failure of APS TBCs. In this thesis, the residual stress distribution as a function of depth in APS TBC has been measured by synchrotron XRD and explained by image based modelling based on the microstructure detailed studied by SEM and CT. The residual stress/ strain distribution as a function of depth was measured by synchrotron XRD in transmission and reflection geometry. The residual stress/ strain values were analysed using full pattern Rietveld refinement, the sin square psi method and XRD2 method. For the reflection geometry, a new method was developed to deconvolute the residual stress value in each depth from the measured averaged values. Two types of residual stress/strain distribution were observed. The first kind of residual stress was found to be compressive and followed a non-linear trend, which increased from the surface to the interface, decreased slightly and increased again to the interface. This trend showed a jump feature near the interface. The second kind of residual stress distribution possessed two jump features: one near the interface similar to the first kind and another jump feature near the sample surface. The residual stress in both beta and gama phase in the bond coat were also investigated which showed a tensile stress state. The stress trend predicted by our analytical model followed a linear relationship. Comparing this with the first kind of residual stress distribution, two main differences were shown. Firstly the jump feature near the interface and secondly the much larger overall stress gradient. The 3D and 2D microstructure of the sample with the first kind of residual stress distribution was observed by X-ray CT and SEM. The effect of pores, inter-splat cracks and the rumpling interface on the residual stress distribution was investigated by image based modelling. It was proved that the pores and the inter-splat cracks had no large influence on the stress distribution and the jump feature near the interface was a result of the rumpling interface. The much larger stress gradient observed in the measured residual stress distribution was an indication of the stress relaxation in the coating which was proved by a specially designed mechanical test. To explain the jump feature near the sample surface in the second kind of stress distribution. 3D microstructures of the measured samples were observed using X-ray CT. The effect of vertical and the side cracks on the stress distribution were investigated by image based modelling. It was found that the vertical crack had no large influence on the residual stress distribution and the jump feature in the stress trend near the surface could be attributed to the side crack. The effect of other kinds of cracks that were not directly observed in our samples, such as middle or through side cracks, were also investigated. These results were used to develop a semi-destructive method to determine the existence and distribution of cracks in APS TBC.

The dissertation is split into two parts. The first part will be focused on changes in material properties found at the nanoscale, as miscibility and electronic structure can change significantly with size. The formation of classically-immiscible bimetallic nanoparticles (BNPs) becomes favorable at the nanoscale and novel catalytic properties can emerge from the bimetallic alloying. The formation of alloyed and non-alloyed BNPs is achieved through pulse laser ablation (PLA) and a significant increase in catalytic activity is observed for both. Recently discovered, the increased activity in the non-alloyed BNPs, deemed multicomponent photocatalysis, is examined and the proposed mechanism discussed. The second part of the talk will focus on thermal barrier coatings (TBCs), which are advanced, multi-layered coatings used to protect materials in high temperature environments. MCrAlY (M=Ni, Co) bond coats deposited via atmospheric plasma spray (APS) are intrinsically rough and initially the roughness provides a high surface area platform for the mechanical interlocking of the yttria stabilized zirconia (YSZ) top coat, which provides the bulk of the thermal insulation. After high temperature exposure, a protective oxide scale forms at the top coat/bond coat interface however the convex asperities of the bond coat can grow non-α-Al2O3 type oxides that can be detrimental for coating lifetime. A surface modification technique that removes the asperities while leaving intact the concavities is used to examine the role that roughness distribution has on 1100°C APS coating lifetime. Lastly, recent work validating a modelling strategy for evaluating 900°C TBC lifetimes, which can typically surpass 25 kh, is presented. Differences in coating-substrate interdiffusion behavior over 5-20 kh of 900°C exposure are discussed and reproduced with Thermo- Calc/DICTRA for three superalloys (1483, 247, X4) deposited with high velocity oxy fuel (HVOF) NiCoCrAlY coatings.

The aim of this thesis is to investigate the effects of additives, reactive elements and impurities, on the lifetime of thermal barrier coatings. The thesis consists of a number of studies on interface adhesion, impurity diffusion, grain boundary sliding and cleavage processes and their impact on the mechanical behaviour of grain boundaries. The effects of additives and impurity on interface adhesion were elaborated by using total energy calculations, electron localization and density of states, and by looking into the atomic separations. The results of these calculations allow the assessment of atomic level contributions to changes in the adhesive trend. Formation of new bonds across the interface is determined to improve the adhesion in reactive element(RE)-doped structures. Breaking of the cross interface bonds and sulfur(S)-oxygen(O) repulsion is found responsible for the decreased adhesion after S segregation. Interstitial and vacancy mediated S diffusion and the effects of Hf and Pt on the diffusion rate of S in bulk NiAl are studied. Hf is shown to reduce the diffusion rate, and the preferred diffusion mechanism of S and the influence of Pt are revealed to be temperature dependent. Finally, the effects of reactive elements on alumina grain boundary strength are studied. Reactive elements are shown to improve both the sliding and cleavage resistance, and the analysis of atomic separations suggest an increased ductility after the addition of quadrivalent Hf and Zr to the alumina grain boundaries.

Yttria Partially Stabilized Zirconia (Y-PSZ) plasma-sprayed coatings are widely used in turbine engines as thermal barrier coatings. However, in diesel engines Y-PSZ TBCs have not met with wide success. To reach the desirable temperature of 850-900˚C in the combustion chamber from the current temperature of 400-600˚C, a coating with a thickness of approximately 1mm is required. This introduces different considerations than in the case of turbine blade coatings, which are on the order of 100µm thick. Of the many factors affecting the durability and failure mechanism of TBCs, in service and residual stresses play an especially important role as the thickness of the coating increases. For decreasing the residual stress in the system, a multi-layer coating is helpful. The design of a multilayer coating employing relatively low cost materials with complementary thermal properties is described. Numerical models were used to describe the residual stress after deposition and under operating conditions for a multilayer coating that exhibited the desired temperature gradient. Results showed that the multilayer coating had a lower maximum stress under service conditions than a conventional Y-PSZ coating. Model validation with experiments showed a good match between the two.

The effects on heat transfer of piston crown surface finish and use of a metal based thermal barrier coating (TBC) on the piston crown were studied in an SI engine. Measured engine parameters such as power, fuel consumption, emissions and cylinder pressure were used to identify the effects of the coating and its surface finish. Two piston coatings were tested: a baseline copper coating and a metal TBC. Reducing surface roughness of both coatings increased in-cylinder temperature and pressure as a result of reduced heat transfer through the piston crown. These increases resulted in small improvements in both power and fuel consumption, while also having measurable effect on emissions. Oxides of nitrogen emissions were increased while total hydrocarbon emissions were decreased. Improvements attributed to the TBC were found to be small, but statistically significant. At an equivalent surface finish, the TBC performed better than the baseline copper finish.

The effects on heat transfer of piston crown surface finish and use of a metal based thermal barrier coating (TBC) on the piston crown were studied in an SI engine. Measured engine parameters such as power, fuel consumption, emissions and cylinder pressure were used to identify the effects of the coating and its surface finish. Two piston coatings were tested: a baseline copper coating and a metal TBC. Reducing surface roughness of both coatings increased in-cylinder temperature and pressure as a result of reduced heat transfer through the piston crown. These increases resulted in small improvements in both power and fuel consumption, while also having measurable effect on emissions. Oxides of nitrogen emissions were increased while total hydrocarbon emissions were decreased. Improvements attributed to the TBC were found to be small, but statistically significant. At an equivalent surface finish, the TBC performed better than the baseline copper finish.