Two new mechanisms of droplet splashing and breakup during impact have been identified and analyzed. One is the internal rupture of spreading droplet film through formation of holes, and the other is the splashing of droplet due to its freezing during spreading. The mechanism of film rupture was investigated by two different methods. In the first method, circular water films were produced by directing a 1 mm diameter water jet onto a flat, horizontal plate for 10 ms. In the second method, films were produced by making 0.6 mm water droplets impact a solid surface mounted on the rim of a rotating flywheel. Substrate wettability was varied over a wide range, including superhydrophobic. In both cases, the tendency to film rupture first increased and then decreased with contact angle. A thermodynamic stability analysis predicted this behavior by showing that films would be stable at very small or very large contact angle, but unstable in between. Film rupture was also found to be promoted by increasing surface roughness or decreasing film thickness. To study the effect of solidification, the impact of molten tin droplets (0.6 mm diameter) on solid surfaces was observed for a range of impact velocities (10 to 30 m/s), substrate temperatures (25 to 200°C) and substrate materials (stainless steel, aluminum and glass) using the rotating flywheel apparatus. Droplets splashed extensively on a cold surface but on a hot surface there was no splashing. Splashing could be completely suppressed by either increasing the substrate temperature or reducing its thermal diffusivity. An analytical model was developed to predict this splashing behavior. The above two theories of freezing-induced splashing and film rupture were combined to predict the morphology of splats typically observed in a thermal spray process. A dimensionless solidification parameter, which takes into account factors such as the droplet diameter and velocity, substrate temperature, splat and substrate thermophysical properties, and thermal contact resistance between the two, was developed. Predictions from the model were compared with a wide range of experimental data and found to agree well.

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.

An approach to arsenic and selenium removal from fly ash is studied. This research includes a comparison of the leaching ability of ammonium oxalate, ammonium citrate, ammonium nitrate and EDTA to extract arsenic and selenium; use of common agricultural waste as a source of oxalate anion to remove arsenic and selenium from fly ash and estimation of additional calcium effects on arsenic and selenium leaching behaviors. This research shows that extraction strength order is EDTA > ammonium oxalate > ammonium citrate > ammonium nitrate > water, achieving arsenic extraction efficiencies of 94.18%, 84.17%, 4.50%, 2.89% and 0.18%, respectively; achieving selenium extraction efficiencies of 96.14%, 96.26%, 84.34%, 26.60% and 0.71%,respectively, in single-stage extraction. Tall fescue is applied as a source of natural oxalate resource and is able to remove over 70% of arsenic and selenium from fly ash. Additional calcium is found to make 82.20% of total arsenic in free oxalate leachate drop to 1.65% of total arsenic in free oxalate and free calcium leachate. All samples were analyzed using HG-AFS. Hopefully, this research will be helpful when a large scale, cheap and sustainable fly ash clean-up approach is needed for power plants prior to landfilling. Also, calcium effects will enable arsenic and selenium to move to the solid phase and could possibly solve the problem of toxic wastewater generated from the clean-up process. The enriched toxic solid waste could be used for pesticide applications.

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.

No description available.

El presente trabajo tiene como objetivo desarrollar en solución acuosa y a tem-peratura ambiente, rutas de síntesis química coloidal de nanopartículas de plata y nano-partículas compuestas estables. Se obtienen nanopartículas de plata reproducibles, con un control morfológico de tamaño y forma durante el proceso de síntesis. Llevamos a cabo el estudio de las propiedades ópticas (espectros de absorción de las resonancias de plasmones superficiales (SPR)) que caracterizan a una determinada forma y tamaño. El análisis incluye estructuras nanométricas de plata de diferentes tamaños, en ambientes diversos y formas diferentes, como esferas, prolates, y prismas de diferente sección transversal, etc Se ha demostrado que la síntesis química produce coloides de nanopartículas de plata esféricas y anisotrópicas estables. La morfología y estabilidad de las nanopartícu-las coloidales son estudiadas mediante técnicas de espectroscopia y microscopía elec-trónica. El rol y concentración necesaria de cada uno de los reactivos para producir co-loides estables mediante síntesis química son determinadas. Se ha demostrado que, con-trariamente a las opiniones actualmente expresadas en la literatura, es posible controlar el tamaño de las nanopartículas de plata y obtener coloides de nanopartículas de plata esféricas y anisotrópicas estables por largo tiempo, utilizando una ruta de síntesis quí-mica sencilla y una baja concentración de reactivos estabilizadores (PVP). Recubrimientos de nanopartículas esféricas de plata estabilizadas con polivinilpirroli-dona (PVP) sobre substratos de vidrio óptico son preparados mediante el proceso de spin-coating y un posterior tratamiento térmico. Diferentes morfologías tipo core-shell de Ag@SiO2 son preparados mediante un método químico simple y rápido, sin necesidad de adicionar reactivos de acoplamiento o modificadores superficiales de la sílice. Proponemos mecanismos de reacción para la preparación de diferentes nano-estructuras tipo core-shell de plata-sílice. Las nanopartí-culas compuestas de sílice-plata muestran unas propiedades de absorción de resonancia plasmónica muy evidentes. El trabajo de éste capítulo ha sido realizado en colaboración con Juan C. Flores, quien desarrolló la ruta de síntesis como parte de sus estudios de doctorado. Por último, una modificación del método sol-gel es empleada para la prepara-ción de nanopartículas de TiO2, y partículas compuestas de Ag@TiO2, SiO2@TiO2-Ag y SiO2@Ag@TiO2. Diferentes morfologías tipo core-shell son preparadas mediante un método químico simple y rápido sobre un substrato óxido, sin necesidad de adicionar agentes de acoplamiento o modificaciones superficiales. Las evidentes propiedades de absorción plasmónica de las nanopartículas de plata mostradas por las partículas com-puestas han demostrado la presencia de plata metálica sobre la titania, sin la posterior oxidación de la capa de plata por el contacto directo con la titania (TiO2). Esta evidencia es confirmada por la técnica de microscopía electrónica de alta resolución. Las propie-dades de absorción plasmónica de las partículas compuestas hacen a estos materiales muy prometedores para aplicaciones foto-catalíticas. / The present work aims to develop chemical synthesis routes of stable colloidal silver nanoparticles and composites nanoparticles in aqueous solution at room tempera-ture. We obtain reproducible morphological control of silver nanoparticles size and shape during synthesis solely by solution chemistry and carry out the study of the opti-cal properties (surface plasmon resonances (RPS) absorption spectra) which character-ize a specific shape and size. The analysis includes silver nanosized bodies of different size, in diverse environments and of various shapes, as spheres, prolates, and prisms of different transversal section, etc. Synthetic wet chemistry routes yielding stable colloids of spherical and aniso-tropic silver nanoparticles are demonstrated, and the morphology and stability of the colloidal nanoparticles studied extensively through spectroscopy and electron micros-copy techniques. The role of each reagent and the concentrations required to obtain sta-ble colloid via these wet chemical routes is determined. It was shown that, contrary to commonly expressed opinions in the literature, it is possible to control the particle size of silver nanoparticles and obtain long-term sable colloids of both spherical and aniso-tropic silver nanoparticles using simple chemical routes and low concentration of stabi-lizing agent (PVP). Films of polyvinylpyrrolidone (PVP) stabilized spherical silver nanoparticles are also prepared, by using spin coating on standard optical glass plates and subsequent thermal processing. Different core-shell type morphologies of Ag@SiO2 are also produced using a simple and rapid chemical method, without using added coupling agents or surface modifications of silica. We propose reaction mechanisms for the formation of the dif-ferent silica-silver core-shell nanostructures. The silica-silver composite nanoparticle display clear plasmonic resonance absorption properties. This chapter work has been done in collaboration with PhD student Juan C. Flores who developed the synthesis route as part of his doctoral studies. Finally, a sol-gel chemistry approach was used to fabricate nanoparticles in the systems TiO2, Ag@TiO2, Ag@TiO2-SiO2 and TiO2@Ag@SiO2. Different core-shell morphologies are produced using a simple and rapid chemical method. without using added coupling agents or surface modifications of the oxide substrate. Clear silver na-noparticle plasmonic absorption properties shown by the composite nanoparticles demonstrate the formation of metallic Ag, without the oxidation of Ag nanoshell in di-rect contact with TiO2, evidence confirmed also by high resolution electron microscopy. The plasmonic absorption properties of the composites nanoparticles make them a promising material for photocatalytic applications.

<p>This work considers the transport and attachment of nanoscale particles to surfaces and the associated phenomena that dictate particle-surface interactions. A consideration of the deposition of nano-scale particles on surfaces is a natural outgrowth of more than a century of research in the area of colloid science, and has taken on new pertinence in the context of understanding the fate and transport of engineered nanoparticles in aqueous environments. More specifically, the goal of this work is to better understand the effects of particle size, surface polymer coatings, and aggregation state on the kinetics of nanoparticle deposition. Theoretical tools such as those developed by Derjaguin-Landau-Verwey-Overbeek (DLVO) and Flory-Krigbaum , as well as the soft particle theory and surface element integration scaling methods are employed to address certain problems that were not considered with the existing theoretical frameworks for the conventional colloidal problems. Consequences of theoretical predictions are evaluated experimentally using column experiments or the quartz crystal microbalance techniques to monitor deposition kinetics. One of the key findings of this work is the observation that polymer coatings may stabilize nanoparticles against deposition or increase deposition, depending on whether the polymer coatings exist on both of the interacting surfaces and the interaction between the polymer and the collector surface. Both steric and bridging mechanisms are possible depending on whether contact between the polymer and collector surface can result in successful attachment. In addition, limitations in the use of conventional, equilibrium-based DLVO theory to describe the deposition of nano-scale particles at very low ionic strength are also identified and discussed. Moreover, it is demonstrated that the interaction between the aggregated nano-scale particles and environmental surfaces is controlled by the characteristic size of the primary particles rather than that of the aggregates. Thus despite an increase in hydrodynamic diameter, aggregation is predicted to reduce deposition only from the hydrodynamic aspects, but not from the colloidal interaction aspect. The affinity between aggregated nanoparticles and a surface may be increased at the initial stage of deposition while being unaffected by aggregation state during later stages of deposition. The results of this study lead to better understandings, at least on a qualitative level, of the factors that controlling the kinetics of deposition and, in a broader sense, the fate and transport of nanoscale particles in the aqueous environment.</p> / Dissertation

The use of laser welding in the automotive industry in the past few decades has facilitated joining of hot-dip galvanized (HDG) steel sheets at high production rates and low cost. The recent development of tailor welded blanks (TWB) using laser welding allowed combinations of sheet grades and thicknesses to “tailor” the vehicle part for optimized design, structural integrity and crash performance but more importantly, reductions in weight. Welded blanks are further subjected to stamping or stretch forming prior to final assembly. Unfortunately, both welding and stretch forming cause the galvanized coating to deteriorate, and thereby, undermine the long term corrosion protection. Despite existing publications on zinc coated steel and advances in processing techniques, there is a lack of understanding on the influence of laser welding and stretch forming on the corrosion performance of HDG steel. Hence, the purpose of this study was to determine how welding speed and biaxial strain affect interstitial-free (IF) and high strength low allow (HSLA) steel coupons when they are subjected to continuous immersion and accelerated corrosion tests. The corrosion rates of the coupons were evaluated using electrochemical techniques and gravimetry. Changes in the galvanized coating were characterized using scanning electron metallography. It was observed that, the original zinc layer transformed into the delta and gamma Fe-Zn intermetallic phases locally in the heat affected zone (HAZ) after laser welding. The resulting microstructure was similar to that of a commercially galvannealed coating and exhibited superior corrosion resistance than that of pure zinc. Linear polarization resistance (LPR) measurements revealed that the zinc coating was able to protect a chemically exposed region of steel in 0.1 M NaCl solution. While the Nd:YAG laser welded coupons with narrow HAZs performed equally well as the non-welded ones, diode laser welded coupons, with a wide locally annealed coating in the HAZ, exhibited a decrease in the peak corrosion rate of zinc. Moreover, minimal amounts of rust were observed on the surface of the HAZ after testing. With biaxial strain, welded and deformed coupons generally demonstrated higher peak corrosion rates than that of undeformed welded ones. When subjected to cyclic corrosion testing according to SAE J2334, rust formed in the exposed region after one 24 hour test cycle due to wet-dry conditions. However, zinc corrosion products on the surface provided substantial corrosion resistance to the remaining zinc coating and to the steel substrate. Gravimetric measurements of welded coupons showed a linear increase in weight gain with increased exposed widths of the steel after 30 cycles but biaxial strain further increased the weight gain on deformed coupons. After 60 cycles, the trend became exponential for both welded and deformed coupons. There was a negligible difference between the corrosion performance of IF and HSLA steel. Using X-Ray diffraction and Raman spectroscopy, species of both iron and zinc corrosion products were identified. Without the application of paint coatings, zinc oxide (ZnO), zinc hydroxy chloride (ZnCl2[Zn(OH)2]4), and hydrozincite ([ZnCO3]2[Zn(OH)2]3) were responsible for passivating the surface and reducing the overall corrosion rate of the galvanized coating.

This thesis work was performed at AB Sandvik Coromant and aimed to enhance the knowledge about the relationships between the performance of TiN and TiAlN-coated cutting tools in metal turning and their mechanical and chemical properties. Measurements of coating material properties and turning wear tests in annealed tool steel Sverker 21, stainless steel 316L, grey cast iron V314 and nodular cast iron SS0727 were performed. The cutting temperatures were estimated from FEM-simulations. To find the dominant wear mechanism and identify the properties that are most important for the resistance against that particular wear, a correlation analysis was performed together with a wear study using LOM, SEM and EDS. The results show that relations between cutting performance and mechanical properties and/or composition of the coatings can be established. The FEM-simulations suggested that the peak tool temperature was highest, ~750°C, for turning in 316L and lowest for turning in Sverker 21, ~300°C. Turning in cast iron resulted in temperatures around 500-550°C. A mechanism for the growth of the crater on inserts tested in stainless steel 316L is proposed. Wear due to thermo-mechanical load and adhesion are believed to be the dominating wear mechanisms. The performance of the tool showed a high correlation to the composition of the coatings, with a decreased tool life for higher Al-contents. The reason for this might lie in an increased brittleness of these coatings, accelerating formation of lateral cracks above the crater. Calculated ratios of bulk and shear modulus suggests an increased brittleness for higher Al-contents. A higher tendency to stick to the work piece material might also contribute to a decrease in tool life. An Increased Al-content could also drive the formation of c-AlN to h-AlN, causing even higher wear rates. The coatings with higher substrate bias showed an enhanced performance, even though the crack pattern was worsened for these variants. The reason for the enhanced performance seen for these variants might instead originate in an enhanced adhesion to the substrate. In the flank wear resistance test in Sverker 21 the Al-content proved to be important, with an improved performance for higher Al-contents. In contrast to the test in 316L, a change in bias or hardness had no effect on the performance in this test. Scratch patterns on the flank supports that an abrasive wear mechanism is present, but no correlation between hardness and tool life could be obtained. Either some other material property than hardness is of importance for the abrasive resistance in this test, or another wear mechanism, occurring simultaneously with abrasion, is the wear rate deciding. The second part of this thesis work was to evaluate the ability of a quantum mechanical computational method, density functional theory, to predict material properties. The method predicts the lattice parameters and bulk moduli in excellent agreement with experimental values. The method also well predicts other elastic properties, with results consistent with reference values. There seems to be a constant shift of about 50-100 GPa between the calculated elastic modulus and the experimentally measured values, probably originating in contributions from grain boundaries, texture, stresses and defects present in the real coatings, and possibly also in errors in the experimental method due to an influence from the substrate. The calculated hardness values did not follow the trend of an increased hardness for TiAlN compared to TiN, which is seen in experiments.

The use of laser welding in the automotive industry in the past few decades has facilitated joining of hot-dip galvanized (HDG) steel sheets at high production rates and low cost. The recent development of tailor welded blanks (TWB) using laser welding allowed combinations of sheet grades and thicknesses to “tailor” the vehicle part for optimized design, structural integrity and crash performance but more importantly, reductions in weight. Welded blanks are further subjected to stamping or stretch forming prior to final assembly. Unfortunately, both welding and stretch forming cause the galvanized coating to deteriorate, and thereby, undermine the long term corrosion protection. Despite existing publications on zinc coated steel and advances in processing techniques, there is a lack of understanding on the influence of laser welding and stretch forming on the corrosion performance of HDG steel. Hence, the purpose of this study was to determine how welding speed and biaxial strain affect interstitial-free (IF) and high strength low allow (HSLA) steel coupons when they are subjected to continuous immersion and accelerated corrosion tests. The corrosion rates of the coupons were evaluated using electrochemical techniques and gravimetry. Changes in the galvanized coating were characterized using scanning electron metallography. It was observed that, the original zinc layer transformed into the delta and gamma Fe-Zn intermetallic phases locally in the heat affected zone (HAZ) after laser welding. The resulting microstructure was similar to that of a commercially galvannealed coating and exhibited superior corrosion resistance than that of pure zinc. Linear polarization resistance (LPR) measurements revealed that the zinc coating was able to protect a chemically exposed region of steel in 0.1 M NaCl solution. While the Nd:YAG laser welded coupons with narrow HAZs performed equally well as the non-welded ones, diode laser welded coupons, with a wide locally annealed coating in the HAZ, exhibited a decrease in the peak corrosion rate of zinc. Moreover, minimal amounts of rust were observed on the surface of the HAZ after testing. With biaxial strain, welded and deformed coupons generally demonstrated higher peak corrosion rates than that of undeformed welded ones. When subjected to cyclic corrosion testing according to SAE J2334, rust formed in the exposed region after one 24 hour test cycle due to wet-dry conditions. However, zinc corrosion products on the surface provided substantial corrosion resistance to the remaining zinc coating and to the steel substrate. Gravimetric measurements of welded coupons showed a linear increase in weight gain with increased exposed widths of the steel after 30 cycles but biaxial strain further increased the weight gain on deformed coupons. After 60 cycles, the trend became exponential for both welded and deformed coupons. There was a negligible difference between the corrosion performance of IF and HSLA steel. Using X-Ray diffraction and Raman spectroscopy, species of both iron and zinc corrosion products were identified. Without the application of paint coatings, zinc oxide (ZnO), zinc hydroxy chloride (ZnCl2[Zn(OH)2]4), and hydrozincite ([ZnCO3]2[Zn(OH)2]3) were responsible for passivating the surface and reducing the overall corrosion rate of the galvanized coating.