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.

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.

Thermal barrier coatings (TBCs) thermally insulate the substrate from high temperature exposure. This work attempted to simulate real engine thermal cyclic conditions by designing a test method to evaluate the thermal cyclic fatigue (TCF) performance of different coatings applied inside exhaust manifold of a diesel engine. The coatings investigated in this work comprised of two plasmas-sprayed TBCs (conventional 8YSZ and nanostructured 8YSZ) and one bond coat (NiCoCrAlY). Additionally, these coatings were exposed to isothermal testing and their oxidation behavior was evaluated.   All the coatings along with only substrate were exposed to temperature around 525°C for 150 cycles in thermal cyclic testing carried out on Scania’s heavy-duty diesel engine. For isothermal testing, all coatings along with only substrate material were exposed to 650°C and 750°C for 168 hours respectively. Microstructural analysis by SEM/EDS was carried out to compare the microstructural evolution of the tested coatings with the as sprayed TBCs. In the case of thermal cyclic test, all coatings showed no failure and no TGO growth up to 150 cycles. In the EDS analysis for isothermally tested coatings, oxidation of the substrate at bond coat- substrate interface instead of TGO growth was observed. Bond coat showed lowest oxide layer thickness at 650°C and 750°C followed by conventional YSZ and then nanostructured YSZ. But, conventional YSZ showed microcracks in top coat near top coat- bond coat interface after isothermal testing. Thermal cyclic and isothermal exposure test results showed that bond coated substrate and nanostructured YSZ have the potential to be implemented inside the real manifold.

Electrophoretic deposition (EPD) has been used to produce the yttria-stabilized zirconia (YSZ) coatings on metal substrates. Sintering of YSZ with and without doping has been carried out at 1150 °C for 2hrs. The properties of these coatings have been examined in light of thermal barrier applications. For EPD, the green density increases with an initial increase in the HCl concentration and the EPD time. This suggests that particle packing was influenced by a time dependent re-arrangement, in addition to the initial suspension dispersion state. The green density peaks at a electrical conductivity of around 10×10-4 S/m achieved by an 0.5 mM HCl addition for the 20 g/l suspensions with the EPD time of around 8 ~10 minute. For sintered coatings, the HCl concentration had a marked effect on the neck size to grain size ratio of the 8 mol% yttria-stabilized zirconia (8YSZ) coatings. The presence of ZrCl4 and ZrOCl2, and a high concentration of oxygen vacancies at the grain boundaries are believed to promote neck growth in the early stage of sintering at 1150 °C. During sintering of 3 mol% and 8 mol% yttria-stabilized zirconia (3YSZ and 8YSZ) at 1150 ºC for 2hrs, the densification rate substantially increased with a small amount of Fe2O3 addition (0.5 mol%) to the 3YSZ/8YSZ deposits. A more pronounced graingrowth was present in the Fe2O3 doped 8YSZ deposits. The increased Zr4+ diffusion coefficient is mainly responsible to the rapid densification rate of the Fe2O3 doped 3YSZ/8YSZ deposits. A small grain growth observed in the Fe2O3 doped 3YSZ deposits is attributed to the Fe3+ segregation at grain boundary. A small amount of CeO2 doping was found to substantially inhibit the densification rate of the doped 3YSZ deposits with a minor grain growth. Fe2O3 doping reduced the thermal conductivities of 3YSZ/8YSZ. It is found that Rayleigh-type phonon scattering due to the mass difference alone is inadequate to explain the thermal conductivity of Fe2O3 doped YSZ systems. The lattice strain effects due to the ionic radius difference could more effectively reduce thermal conductivity of the Fe2O3-doped 3YSZ. A decrease in the growth rate of the TGO scale with the increasing Fe2O3 additions was observed for the oxidized FeCrAlY metal substrates with the Fe2O3-doped 3YSZ coating, which was found to be attributed to the early formation of the stable and dense α-Al2O3 phase due to the presence of Fe3+ ions.

Thesis (MTech (Mechanical Engineering))--Cape Peninsula University of Technology. / For the last few decades, methods to improve the engine efficiency and reduce the fuel consumption of jet engines have received increased attention. One of the solutions is to increase the operating temperature in order to increase the exhaust gas temperature, resulting in an increased engine power. However, this approach can be degrading for some engine parts such as turbine blades, which are required to operate in a very hostile environment (at ≈ 90% of their melting point temperature). Thus, an additional treatment must be carried out to protect these parts from corrosion, oxidation and erosion, as well as to maintain the substrate’s mechanical properties which can be modified by the high temperatures to which these parts are exposed. Coating, as the most known protection method, has been used for the last few decades to protect aircraft engine parts. According to Wolfe and Co-workers [1], 75% of all engine components are now coated. The most promising studies show that the thermal barrier coating (TBC) is the best adapted coating system for these high temperature applications. TBC is defined as a fine layer of material (generally ceramic or metallic material or both) directly deposited on the surface of the part In order to create a separation between the substrate and the environment to reduce the effect of the temperature aggression. However, the application of TBCs on surfaces of components presents a challenge in terms of the consistency of the thickness of the layer. This is due to the nature of the processes used to apply these coatings. It has been found that variations in the coating thickness can affect the thermodynamic performance of turbine blades as well as lead to premature damage due to higher thermal gradients in certain sections of the blade. Thus, it is necessary to optimise the thickness distribution of the coating.

Different methods to predict barrier failure in packaging board or paper after converting were investigated. The approach was to compare substrates before and after creasing/folding by applying different barrier tests and to propose a methodology to predict failure in the barrier layer.  Different coatings were used to develop and verify the methodology; a hemicellulose based dispersion barrier coating, a dispersion coated PVOH coating and an extrusion coated PE. Creasing was performed according to standard procedure using recommended creasing geometries. Folding of paper was performed by a gentle creasing with a board backing followed by folding the paper between two metal plates with a well defined distance. The first step in the evaluation was to visually inspect creased/folded substrates by light microscopy to search for coating failures in form of cracks. Both good and bad samples were then tested for grease resistance with a standard test, i.e. TAPPI 454. The TAPPI 454 test showed to be effective to expose barrier failure since oil would penetrate quite fast through the creasing line of cracked samples. Even some samples that appeared to have no cracks in the light microscope showed failure with the grease test. The results showed that only the PE coated samples could sustain a barrier after creasing and folding. This was probably due to a high ductility of the PE-coating combined with a high thickness. The water vapour transmission rate, WVTR, of the samples that passed the TAPPI 454 test was then measured on the samples that endured the grease resistance test. Since PE is a good water vapour barrier, WVTR-measurements were proper for detecting barrier defects. The VWTR of the creased/folded samples was slightly higher for the creased samples than the un-creased references despite the absence of cracks. This was probably due to that the barrier layer got thinner as a result of the strains applied on the coating during the creasing/folding operation.  A methodology to predict barrier failure in barrier coated packaging board and paper after creasing and folding was proposed. Well defined creasing and folding geometries were used in combination with screening for cracks in the barrier layer, first by visual inspection in light microscopy and then by a standard grease resistance test. The samples that passed then screening tests could then be analyzed using more exact but also more time consuming methods such as WVTR. / Olika metoder att förutspå skador i barriärskikt på kartong eller papper efter konvertering undersöktes. Tillvägagångssättet var att jämföra substrat före och efter bigning och vikning genom att tillämpa olika barriärtest och att föreslå en metod för att förutspå defekter i barriärlager. Olika barriärmaterial användes för att utveckla och bekräfta metoden: en hemicellulosa baserad dispersionsbestrykning, en dispersionsbestrykt PVOH barriär och en extruderad PE barriär. Bigandet utfördes enligt standard proceduren och rekomenderade biggeometrier användes. Vikningen av papret utfördes genom varsam bigning med kartong som stöd följt av vikning av pappret genom två metallplattor med ett bestämt avstånd. Utvärderingen började med visuell inspektion av bigade/vikta substrat i ljusmikroskop för att finna barriärdefekter i form av sprickor. Både bra och dåliga prover testades sedan för fettbeständighet med hjälp av ett standardtest, dvs TAPPI 454. TAPPI 454 testet visades sig att vara ett effektivt sätt att identifiera barriärdefekter på grund av att penetration av olja vid biglinjen skedde snabbt på de prov som uppvisade sprickor. Även några av de prov som ej uppvisade sprickor i ljusmikroskop klarade inte av fettbeständighetstest. Resultatet visade att det enda material som kunde bibehålla barriäregenskaper efter bigning och vikning var de PE belagda proven. Detta är antagligen tack vare PE-bestrykningens höga duktulitet och tjocklek. Vattenångspermeabiliteten, WVTR, uppmättes på de prov som uthärdade fettbeständighetstestet. Eftersom PE är en utmärkt vattenångbarriär, var WVTR-mätningar lämpliga för att upptäcka barriärfel. WVTR resultaten för de bigade/vikta proven visade ett något högre värde än de obigade referenserna även om de inte hade sprickor. Det något högre WVTR värdet beror antagligen på att barriärskiktet blev tunnare på grund av töjningen i barriärskiktet under big/vikningen. En metod för att förutspå skador i barriärbestrykt kartong och papper efter bigning och vikning föreslogs. Definierade big- och vikgeometrier användes i kombination av screening av sprickor i barriärskikten, först genom visuell inspektion i ljusmikroskop och sedan ett standarderiserat fettbeständighetstest. Proven som passerar screeningen kan sedan bli analyserade för mer exakta och tidskrävande metoder som WVTR.

State of the art thermal barrier coatings (TBCs) for gas turbine applications comprise (7 wt.%) yttria partially stabilized zirconia (7YSZ). 7YSZ offers a range of attractive functional properties – low thermal conductivity, high thermal expansion coefficient and high in-plane strain tolerance. However, as turbine entry temperatures are raised, the performance of 7YSZ coatings will be increasingly affected by sintering and environmental contamination, by calcia-magnesia-alumina-silica (CMAS) deposits. The effect of sintering-induced stiffening on the driving force for spallation of plasma-sprayed (PS) TBCs was investigated. Spallation lifetimes of TBC specimens sprayed onto alumina substrates were measured. A simple fracture mechanics approach was employed in order to deduce a value for the strain energy release rate. The critical strain energy release rate was found to be constant, and if this value had been known beforehand, then the rationale presented here could be used for prediction of coating lifetime. The effect of vermiculite (VM) and volcanic ash (VA) contamination on the sintering-induced spallation lifetime of PS TBCs was also investigated. The presence of both VM and VA was found to accelerate the rise in their Young’s modulus with sintering. Spallation results show that coating lifetime may be significantly reduced, even at relative low addition levels, due to the loss of strain tolerance caused by the penetration of glassy deposits. This result gives a clear insight into the role CMAS plays in destabilizing TBCs. Finally, the adhesion characteristics of ingested volcanic ash were studied using a small jet engine. The effects of engine speed and particle size were investigated. Deposition on turbine surfaces was assessed using a borescope. Deposition mainly occurred on the nozzle guide vane and blade platform. A numerical model was used to predict particle acceleration and heating in flight. It was observed that larger particles are more likely to adhere because they have greater inertia, and thus are more likely to impact surfaces. The temperature of the larger particles at the end of its flight was predicted to be below its softening point. However, since the component surface temperatures are expected to be hotter, adhesion of such particles is probable, by softening/melting straight after impact.

Les barrières thermiques (BT) élaborées par projection plasma sous air (APS) sont utilisées par l’industrie aéronautique pour protéger les pièces fixes des parties chaudes des turbines à gaz. Les BT consistent en un système bicouche composé d'une couche de liaison NiCrAlY de 200 µm d'épaisseur et d’un revêtement céramique de ZrO2-8%Y2O3 (YSZ) de 1 mm d'épaisseur, déposés sur le substrat métallique à protéger. Les principaux objectifs de cette thèse sont d’une part de comparer la tenue en oxydation cyclique de deux microstructures de barrières thermiques élaborées par APS et d’autre part de caractériser le comportement mécanique de celles-ci, afin de comprendre et de modéliser les mécanismes d’endommagement de ces dépôts afin d’en améliorer la conception. Tout d’abord, l’élaboration par projection plasma des différentes BT a été réalisée. Pour évaluer les performances de ces BT en termes de durée de vie et identifier les mécanismes d’endommagement, elles ont ensuite été testées en oxydation cyclique sous gradient, pour reproduire les conditions réelles d’utilisation en service. Ensuite une caractérisation exhaustive des propriétés physiques et mécaniques des différents matériaux a été menée. Ainsi, les - substrat seul, sous-couche NiCrAlY, les deux revêtements de YSZ à microstructures différents et les deux systèmes BT complets - ont été testés en flexion 3 points (F3P) et en flexion biaxiale Small Punch Test (SPT). A partir des propriétés obtenues et de ces résultats, des modélisations éléments finis ont été proposées : les modes d’endommagement observés pendant les essais de F3P et SPT ont été reproduits. La compréhension de des phénomènes d'endommagement et la prédiction de la durée de vie des BT sont des enjeux majeurs pour les motoristes qui souhaitent élaborer un modèle pertinent de durée de vie. / Thermal barriers coatings (TBC) developed by air plasma spraying (APS) are used by the aviation industry to protect the fixed parts of hot sections of gas turbines. The TBC system consists of a bilayer system composed of a 200 m thick NiCrAlY bondcoat and a 1 mm thick ZrO2-8% Y2O3 (YSZ) ceramic coating, deposited on the metal substrate to be protected. The main objectives of this PhD thesis are, on the one hand, to compare the cyclic oxidation behavior of two microstructures of TBC developed by APS and, on the other hand, to characterize the mechanical behavior of these TBC, in order to understand, model the damage mechanisms. and improve their design. Firstly, the plasma projection of the different TBCs was carried out. To evaluate their performance in terms of lifetime and identify the mechanisms of damage, they were then tested in cyclic oxidation with gradient to reproduce the actual conditions of use in service. Then, an exhaustive characterization of the physical and mechanical properties of the different materials was conducted. Thus, the - substrate alone, NiCrAlY sublayer, the two YSZ coatings with different microstructures, and the two complete TBC systems - were tested in 3-point bending (F3P) and biaxial flexion Small Punch Test (SPT). From the properties obtained and these results, finite element modelisations were proposed: the modes of damage observed during the F3P and SPT tests were reproduced. The understanding of damage phenomenon and the prediction of life of TBCs are major issues for engine makers who wish to develop a relevant model of service life.

Oxygen barrier coatings have the potential to greatly extend the lifetime of certain food products by incorporating them into existing food packaging. Present technologies face definite challenges of maintaining high performance, while attaining simple and inexpensive preparation methods. The oxygen barrier effect obtained with these coatings is also susceptible to a plasticization effect when exposed to high humidity, since water vapor molecules are readily soluble in typically hydrophilic resins. In this work, we demonstrate a 1 – 2 micron thick oxygen barrier coating, prepared on a 12 micron poly(ethylene terephthalate) substrate, that has oxygen transmission rates as low as 1.44 cc m-2 day-1 under standard conditions and can maintain similar oxygen barrier performance at high humidity. This degree of oxygen barrier meets the standard of 1 – 10 cc m-2 day-1 established for food packaging applications. The coating is prepared through use of sol-gel chemistry between poly(vinyl alcohol) and vinyltrimethoxsilane molecules, which form a strong network resin through hydrolysis and condensation reactions. The formulation of these oxygen barrier coatings allows for variability of solids percentage and viscosity without significant change in performance. The ability to scale up the preparation of these coated films was tested successfully on an industrial flexographic printing press.