Dissertations / Theses on the topic 'Aluminum oxide coating'

L’objectif de cette thèse consiste en une étude fondamentale de différentes approches pour la modification de surfaces d’alliages d’aluminium. Elle s’inscrit dans le cadre du projet FLYCOAT, subventionné par la région Wallonne. Ce dernier avait pour objectif le développement d’alternatives au couplage classique d’un procédé d’anodisation utilisant des bains de Cr (VI) aux résines époxy pour la protection des alliages d’aluminium contre la corrosion. Dans un premier temps, la synthèse par plasma atmosphérique dans un réacteur de type décharge à barrière diélectrique (DBD) de films riches en groupements carboxyliques à partir de 8 précurseurs organiques est étudiée. Une attention particulière est portée à la compréhension fondamentale des mécanismes de polymérisation de ces précurseurs. L’influence significative de minimes variations de la structure chimique du précurseur est étudiée. Concrètement, nous démontrons l’impact de la présence et de la position de doubles liaisons ou encore le ratio C/O dans le monomère injecté sur le mécanisme de synthèse des couches déposées. Pour ce faire, une méthodologie combinant des analyses de la phase plasma et des films déposés est proposée. Les propriétés électriques de la DBD d’argon sont évaluées par oscilloscope avant et durant l’injection des différents précurseurs. La quantité d’énergie transférée de la décharge vers le précurseur est évaluée par spectroscopie d’émission optique et corrélée à sa structure. Une fragmentation réduite est mise en évidence par spectrométrie de masse pour les monomères contenant une double liaison. Ces analyses de la phase plasma sont alors corrélées avec les propriétés physiques et chimiques des films synthétisés. Les compositions chimiques de surface et de la matrice des couches minces sont étudiées par spectroscopie à photoélectrons X (XPS) et infrarouge. Le rôle essentiel de la présence et de la position de la double liaison dans la molécule injectée est démontré. Les vitesses de dépôt et la rugosité des films déposés par plasma atmosphérique avec l’injection des 8 précurseurs sont évaluées par profilométrie à stylet. Dans la seconde partie, le couplage de deux méthodes de plasma atmosphérique est proposé pour la synthèse de couches d’alumine aux propriétés adaptables. Le premier traitement consiste en un nombre varié de passages d’une torche plasma opérant dans un régime d’arc. L’effet du nombre de passages sur les propriétés physiques et chimiques du substrat est étudié par XPS, angle de contact, microscopie électronique à balayage et mesures de diffraction à rayons X. Une corrélation est suggérée entre le nombre de passages de la torche et les propriétés électrochimiques du substrat. L’influence de ce premier traitement sur les propriétés de la couche d’oxyde d’aluminium synthétisée par oxydation par plasma électrolytique est mise en évidence. Dans un troisième temps, le plasma pouvant être considéré comme un réservoir d’énergie, une étude de faisabilité est réalisée afin d’évaluer sa potentielle utilisation pour la réticulation d’une résine de type benzoxazine. L’efficacité du traitement par DBD atmosphérique d’argon ou hélium est comparée et discutée.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished

Aluminum oxide (A1203, or alumina) is a conventional ceramic known for applications such as wear resistant coatings, thermal liners, heaters, crucibles, dielectric systems, etc. However applications of A1203 are limited owing to its inherent brittleness. Due to its excellent mechanical properties and bending strength, carbon nanotubes (CNT) is an ideal reinforcement for A1203 matrix to improve its fracture toughness. The role of CNT dispersion in the fracture toughening of the plasma sprayed A1203-CNT nanocomposite coating is discussed in the current work. Pretreatment of powder feedstock is required for dispersing CNTs in the matrix. Four coatings namely spray dried A1203 (A-SD), A1203 blended with 4wt.% CNT (A4C-B), composite spray dried A1203-4wt.% CNT (A4C-SD) and composite spray dried A1203-8wt.% CNT (A8CSD), are synthesized by plasma spraying. Owing to extreme temperatures and velocities involved in the plasma spraying of ceramics, retention of CNTs in the resulting coatings necessitates optimizing plasma processing parameters using an inflight particle diagnostic sensor. A bimodal microstructure was obtained in the matrix that consists of fully melted and resolidified structure and solid state sintered structure. CNTs are retained both in the fully melted region and solid-state sintered regions of processed coatings. Fracture toughness of A-SD, A4C-B, A4C-SD and A8C-SD coatings was 3.22, 3.86, 4.60 and 5.04 MPa m1/2 respectively. This affirms the improvement of fracture toughness from 20 % (in A4C-B coating) to 43% (in A4C-SD coating) when compared to the A-SD coating because of the CNT dispersion. Fracture toughness improvement from 43 % (in A4C-SD) to 57% (in A8C-SD) coating is evinced because of the CNT content. Reinforcement by CNTs is described by its bridging, anchoring, hook formation, impact alignment, fusion with splat, and mesh formation. The A1203/CNT interface is critical in assisting the stress transfer and utilizing excellent mechanical properties of CNTs. Mathematical and computational modeling using ab-initio principle is applied to understand the wetting behavior at the A1203/CNTinterface. Contrasting storage modulus was obtained by nanoindentation (~ 210, 250, 250-350 and 325-420 GPa in A-SD, A4C-B, A4C-SD, and A8C-SD coatings respectively) depicting the toughening associated with CNT content and dispersion.

Selbstreinigende, ultrahydrophobe Oberflächen lassen sich in der Technik vielfältig einsetzen. Das ultrahydrophobe Verhalten beruht einerseits auf einer Rauigkeit im μm-Bereich und andererseits auf der chemischen Zusammensetzung der Oberfläche. Durch den gegebenen Oberflächenaufbau sind derartige Materialien jedoch empfindlich gegen Verschleiß. In diesem Beitrag wird ein Schichtverbund bestehend aus Aluminiumoxid und zwei polymeren Komponenten vorgestellt. Die Aluminiumoxidschicht wird auf dem Wege der anodischen Oxidation erzeugt. Dieses seit langem bekannte Verfahren ermöglicht nicht nur die Oxidation der Aluminiumoberfläche, sondern gestattet es, auch, definierte Oberflächenprofile einzustellen. Durch den gezielten Einbau des hochmolekularen Polymers Chitosan in die mikroprofilierte Aluminiumoxidschicht wurde eine mechanische Stabilisierung der Schicht im Sinne eines anorganisch-organischen Composites erreicht. Außerdem dienten die Amino-Seitengruppen des Chitosans als reaktives Interface für die notwendige chemische Hydrophobierung und als Reaktionszentrum für Vernetzungen, wodurch eine weitere mechanische Stabilisierung bewirkt wird. Der Schichtaufbau hat wesentlichen

Selbstreinigende, ultrahydrophobe Oberflächen lassen sich in der Technik vielfältig einsetzen. Das ultrahydrophobe Verhalten beruht einerseits auf einer Rauigkeit im μm-Bereich und andererseits auf der chemischen Zusammensetzung der Oberfläche. Durch den gegebenen Oberflächenaufbau sind derartige Materialien jedoch empfindlich gegen Verschleiß. In diesem Beitrag wird ein Schichtverbund bestehend aus Aluminiumoxid und zwei polymeren Komponenten vorgestellt. Die Aluminiumoxidschicht wird auf dem Wege der anodischen Oxidation erzeugt. Dieses seit langem bekannte Verfahren ermöglicht nicht nur die Oxidation der Aluminiumoberfläche, sondern gestattet es, auch, definierte Oberflächenprofile einzustellen. Durch den gezielten Einbau des hochmolekularen Polymers Chitosan in die mikroprofilierte Aluminiumoxidschicht wurde eine mechanische Stabilisierung der Schicht im Sinne eines anorganisch-organischen Composites erreicht. Außerdem dienten die Amino-Seitengruppen des Chitosans als reaktives Interface für die notwendige chemische Hydrophobierung und als Reaktionszentrum für Vernetzungen, wodurch eine weitere mechanische Stabilisierung bewirkt wird. Der Schichtaufbau hat wesentlichen

The development of nanostructured coatings which exhibit enhanced mechanical properties is currently of interest due to the importance of high performance coatings in a large range of applications. Single layer coatings have predominantly been used for these demanding applications, however the promising mechanical properties observed in multilayer coatings has shifted the focus of current research. In particular, there has been reports of the use of alternating materials with opposing mechanical properties, as seen in the abalone shell, which have exhibited hardness and toughness values significantly greater than either of their constituent materials. The main objective of this thesis was to fabricate Al/W nanostructured multilayers and determine if they exhibit enhanced mechanical properties. The Al/W nanostructured multilayers were fabricated using two different deposition techniques: pulsed magnetron sputtering and cathodic arc deposition. These two techniques differ in the energy of the depositing species and this results in significant differences in film properties. The indentation hardness of the coatings was measured using a Hysitron Nanoindenter. The relationship between the mechanical properties and microstructure was obtained using a range of characterisation techniques. Auger electron spectroscopy (AES), energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) were used to determine the chemical composition and stoichiometry, while cross-sectional transmission electron microscopy (XTEM) and energy filtered transmission electron microscopy (EFTEM) were used to explore the microstructure. The findings of this thesis showed very different results for the two deposition techniques. Although sputtering successfully produced well defined multilayers, no evidence of enhanced hardness was found for periods between 5 and 200 nm. On the other hand, arc deposited samples with intended periods between 1 and 200 nm showed a hardness enhancement above that of pure W, however the samples of highest hardness did not contain Al layers for much of their thickness. Arc deposited samples with the finest nominal periods (1 and 2 nm) contained W-Al intermetallics and were soft. The hardening mechanism was not attributed to a multilayer structure, rather to the introduction of defects in the W layers which acted as pinning sites for dislocations. A modified Hall-Petch equation for hardness enhancement fitted the data for W films prepared by pulsed cathodic arc in which the grain diameter was replaced by the nominal multilayer period. The difficulty producing Al layers on W surfaces in the cathodic arc was overcom e by changing the film growth mechanism by introducing Ar or O2 at the W/Al interface. In the latter case, Al2O3/W multilayers were formed but again showed no hardness enhancements. Complete microanalysis and characterisation of the multilayer structures is vital in determining the mechanisms which govern the hardness enhancements. The evidence in this thesis suggests that the defect density, and not the presence of interfaces are responsible for the hardness enhancement effect.

The main goal of this dissertation was to show that the principle of atomic layer deposition (ALD) can be applied to “endless” fibers. A reactor of atomic layer deposition has been designed, especially for coating depositions onto meter long bundles of fibers. Aluminum oxide (alumina), titanium oxide (titania), double layers of alumina and titania, as well as aluminium phosphate have been deposited onto bundles of carbon fibers using the home-built reactor. Scanning electron microscopic (SEM) and transmission electron microscopic (TEM) images indicate that the coatings were uniform and conformal onto fiber surface. There was a good adhesion of the coatings to the fibers. Alumina has been deposited using two separate aluminum sources (aluminum trichloride and trimethylaluminum), and water as a source of oxygen. In case of alumina deposition using aluminum trichloride and water, initial deposition temperature was 500 °C. In these conditions, a part of the fiber bundle has been damaged. Thus, the deposition temperature was decreased to 300 °C and the fibers were unaffected. In addition, during this process hydrochloric acid is formed as a byproduct which is a corrosive substance and affects the reactor and there was a chloride impurity in the coatings. Thus, aluminum trichloride precursor was replaced by trimethylalumium. Alumina deposition onto carbon fibers using trimethylaluminum and water was carried out at a temperature of 77 °C. SEM images revealed that the fibers were unaffected and the coatings were uniform and conformal. Oxidation resistance of the carbon fibers was improved slightly after alumina deposition. Oxidation onset temperature of the uncoated fibers was about 630 °C. The resistance was linearly increased with the coating thickness (up to 660 °C) and getting saturated over a thickness of 120 nm. Titania coatings have been deposited using titanium tetrachloride and water. The physical appearances of the titania coatings were similar to the alumina coatings. The oxidation onset temperature of the titania coated carbon fibers was similar to the uncoated fibers but the rate of oxidation was decreased than the uncoated fibers. Two double layer coatings were deposited, alumina followed by titania (alumina/titania), and titania followed by alumina (titania/alumina). If the fibers were coated with the double layer of alumina/titania, they had almost same oxidation onset as alumina coated fibers but the rate of oxidation was decreased significantly compared to alumina coated fibers. This feature is independent of the thickness of the titania layers, at least in the regime investigated (50 nm alumina followed by 13 nm and 40 nm titania). On the other hand, the oxidation onset temperature of fibers coated with titania/alumina (20 nm titania /30 nm alumina) was approximately 750 °C. The fibers were burned completely when temperature was further increased to 900 °C and held another 60 minutes at 900 °C. This is significantly better than any other coating used in this dissertation. ALD of titania and alumina in principle was known beforehand, this dissertation here applies this knowledge for the first time to endless fibers. Furthermore, this dissertation shows for the first time that one can deposit aluminum phosphate via ALD (planar surface as well as fibers). Aluminum phosphate might be special interest in the fiber coating because it is a rather soft material and thus might be used to obtain a weak coupling between fiber and matrix in composites. Aluminum phosphate was deposited using trimethylaluminum and triethylphosphate as precursors. Energy dispersive X-ray spectroscopy and solid state nuclear magnetic resonance spectra confirmed that the coating comprises aluminum phosphate (orthophosphate as well as other stoichiometries). Scanning electron microscopic images revealed that coatings are uniform and conformal. In cases of alumina and titania, it was observed that the coatings were delaminated from the ends of cut fibers and thus formed of clear steps. On the other hand, for aluminum phosphate coating it was observed that the border between coating and underlying fiber often being smeared out and thus formed an irregular line. It seems in case aluminum phosphate cohesion is weaker than adhesion, thus it might be act a weak interface between fiber and matrix. Alumina, titania, and double layer microtubes have been obtained after selective removal of the underlying carbon fibers. The carbon fibers were selectively removed via thermal oxidation in air at temperatures exceeding 550 °C. SEM and TEM images indicate that the inner side of the tube wall has the same morphology like the fibers. In addition, it was observed that the individual microtubes were separated from their neighbors and they had almost uniform wall thicknesses. The longest tubes had a length of 30 cm
Das Hauptziel dieser Dissertation bestand darin nachzuweisen, dass die Atomlagenabscheidung (engl. atomic layer deposition (ALD)) auf „endlose“ Fasern angewendet werden kann. Es wurde ein Reaktor zur Atomlagenabscheidung gestaltet, der speziell für die Beschichtung meterlanger Faserbündel geeignet ist. Aluminiumoxid, Titanoxid, Doppelschichten aus Aluminiumoxid und Titanoxid sowie Aluminiumphosphat wurden mit Hilfe des selbstgebauten Reaktors auf Kohlefaserbündel abgeschieden. Rasterelektronenmikroskopische (REM) und transmissionselektronenmikroskopische (TEM) Aufnahmen zeigten, dass die Beschichtung auf den Fasern einheitlich und oberflächentreu war. Des Weiteren wurde eine gute Adhäsion zwischen Beschichtung und Fasern beobachtet. Das Prinzip der Beschichtung mit Titanoxid und Aluminiumoxid mit Hilfe der ALD war bereits vorher bekannt und im Rahmen dieser Dissertation jedoch erstmals auf "endlose" Fasern angewendet. Des Weiteren wird in dieser Dissertation erstmals gezeigt, dass es möglich ist, Aluminiumphosphat mittels ALD abzuscheiden (sowohl auf planaren Oberflächen als auch auf Fasern). Aluminiumphosphat könnte von besonderem Interesse in der Faserbeschichtung sein, da es ein relativ weiches Material ist und könnte daher als eine Art „schwacher“ Verbindung zwischen Faser und Matrix in Kompositen dienen. Die Oxidationsbeständigkeit von beschichten Kohlefasern wurde im Vergleich zu unbeschichteten Fasern bis zu einem gewissen Grad erhöht. Monoschichten von Aluminiumoxid und Titanoxid waren dafür wenig effektiv. Aluminiumphosphatbeschichtete Fasern waren deutlich besser geeignet als die beiden anderen. Eine Doppelschicht aus Titanoxid gefolgt von Aluminiumoxid verbesserte die Oxidationsbeständigkeit nochmals deutlich gegenüber allen anderen Beschichtungen, die in dieser Dissertation verwendet wurden. Mikroröhren aus Aluminiumoxid, Titanoxid und Doppelschichten wurden durch die selektive Entfernung der zugrunde liegenden Kohlefasern erhalten. Einzelne Mikroröhren waren von benachbarten Röhren getrennt und sie weisen eine nahezu einheitliche Wanddicke auf

The unprecedented ability of plasmonic metal nano-structures to concentrate light into deep-subwavelength volumes has propelled their use in a vast array of nanophotonics technologies and research endeavors. They are used in sensing, super-resolution imaging, SPP lithography, SPP assisted absorption, SPP-based antennas, light manipulation, etc. To take full advantage of the attractive capabilities of CMOS compatible low-cost plasmonic structures based on Al and Cu, nanolaminate coatings are investigated to improve their long-term stability in corrosive physiological environments. The structures are fabricated using phase-shifting PDMS masks, e-beam deposition, RIE, Atomic Layer Deposition and Rapid Thermal Annealing. An alternate approach using Nanosphere Lithography (NSL) was also investigated. Films were examined using ellipsometry, atomic force microscopy and transmission measurements. Accelerated in-situ tests of Hafnium Oxide/Aluminum Oxide nanolaminate shells in a mildly pH environment with temperatures akin to physiological environments emulated using PBS show greatly enhanced endurance, with stable structures that last for more than one year.
Master of Science

Ultra-thin films (UTFs) are important in many applications, seen in the semiconductor industry, in chromatography, in sensing, in microfluidics, in aerospace, and in robotics. They also protect materials from corrosion, change surface energies, limit water intrusion into materials, allow material self-cleaning and self-healing, provide scratch resistance, and impart other specific chemical properties. In many cases, UTFs drastically alter surface properties and therefore their applications. It is imperative that proper and consistent characterization be performed on coatings to confirm and understand their desired properties. In Chapter two, Al oxidation under MgF2 protective layers is studied using real time X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE). These tools allowed me to monitor Al oxidation for both short (hours) and long (months) periods of time. XPS revealed the chemical changes that took place in these materials as a function of time, and these changes were verified with SE. These studies help increase an understanding of aluminum changes under MgF2 protective layers. The third chapter demonstrates ab initio calculations guided X-ray photoelectron spectroscopy (XPS) analysis of surfaces functionalized with fluorinated silanes. This study addresses deficiencies in the literature where CF2:CF3 ratios from experimental XPS data do not match theoretical CF2:CF3 ratios. In a systematic approach, I developed semi-empirical models directed both by ab initio calculations and adjustable, empirical parameters. These models were effective in describing the raw data and exceeded fitting methods used in literature. In Chapter four, SiO2 UTFs with variable thicknesses deposited on Eagle XG® glass substrates are characterized. Challenges associated with this work consisted of similar optical functions of the film and substrate as well as backside reflections from the substrate. These obstacles were met using a multi-sample analysis (MSA), a variable angle spectroscopic ellipsometric approach, and mechanical abrasion/roughening of the substrate backside. With these approaches, I developed a model that precisely fit the data collected from all the samples and gave the correct optical function of the material along with thickness values for each film. Surface characterization represents a commitment of resources. It takes time to make measurements, and it takes time to analyze and understand the results. As presented in this work, I increase understanding of ultra-thin films at interfaces using both a multi-tool approach as well as using multiple analytical methods on data collected from each tool.

Plasma sprayed aluminum oxide ceramic coating is widely used due to its outstanding wear, corrosion, and thermal shock resistance. But porosity is the integral feature in the plasma sprayed coating which exponentially degrades its properties. In this study, process maps were developed to obtain Al2O3-CNT composite coatings with the highest density (i.e. lowest porosity) and improved mechanical and wear properties. Process map is defined as a set of relationships that correlates large number of plasma processing parameters to the coating properties. Carbon nanotubes (CNTs) were added as reinforcement to Al2O3 coating to improve the fracture toughness and wear resistance. Two novel powder processing approaches viz spray drying and chemical vapor growth were adopted to disperse CNTs in Al2O3 powder. The degree of CNT dispersion via chemical vapor deposition (CVD) was superior to spray drying but CVD could not synthesize powder in large amount. Hence optimization of plasma processing parameters and process map development was limited to spray dried Al2O3 powder containing 0, 4 and 8 wt. % CNTs. An empirical model using Pareto diagram was developed to link plasma processing parameters with the porosity of coating. Splat morphology as a function of plasma processing parameter was also studied to understand its effect on mechanical properties. Addition of a mere 1.5 wt. % CNTs via CVD technique showed ~27% and ~24% increase in the elastic modulus and fracture toughness respectively. Improved toughness was attributed to combined effect of lower porosity and uniform dispersion of CNTs which promoted the toughening by CNT bridging, crack deflection and strong CNT/Al2O3 interface. Al2O3-8 wt. % CNT coating synthesized using spray dried powder showed 73% improvement in the fracture toughness when porosity reduced from 4.7% to 3.0%. Wear resistance of all coatings at room and elevated temperatures (573 K, 873 K) showed improvement with CNT addition and decreased porosity. Such behavior was due to improved mechanical properties, protective film formation due to tribochemical reaction, and CNT bridging between the splats. Finally, process maps correlating porosity content, CNT content, mechanical properties, and wear properties were developed.

The lap shear strength of chromic acid anodized, primed, Ti-6Al-4V alloy bonded with a high performance FM-5 polyimide adhesive has been investigated as a function of thermal treatment for selected times at various temperatures in air. The research findings indicate that the lap shear strength decreases with the increase in duration of the thermal treatment at constant temperature and with the increase in temperature at constant time. The bond fails increasingly in the oxide coating with increasing treatment temperature and time of treatment. Surface analysis results for debonded specimens suggest that the process leading to failure is the formation of fluorine-containing materials within the oxide, which weakens the adherend-adhesive bond. The formation of the fluorine components is facilitated by treatment at elevated temperatures. This study suggests that the presence of fluoride ions in the anodic oxide coating, prior to bonding, is detrimental to the bond strength of adhesively bonded Ti-alloy when exposed to high temperatures. The wedge test configuration was used to investigate the influence of temperature on the bond durability of adhesively bonded chromic acid anodized Ti-6Al-4V alloy in air. Based on the average crack length vs. exposure time data, the bond durability varied in the order -25°C > 24°C > 177°C. In each case, the bonded joint failed cohesively within the adhesive, irrespective of the temperature of exposure. XPS analysis and scanning electron photomicrographs of failure surfaces revealed that the failure occurred at the scrim cloth/adhesive interface. The influence of thermal treatment history on the bond durability of adhesively bonded chromic acid anodized Ti-6Al-4V alloy immersed in boiling water was also investigated. The average crack length vs. immersion time indicated no significant differences for specimens that were thermally treated and then bonded compared to the non-thermally treated specimens. In addition, the failure mode was cohesive within the adhesive for specimens prepared using various thermal treatment conditions. The crack growths for samples treated for 0.5 hour and 1.0 hour and for non-thermally treated specimens for any given exposure time were equivalent. In addition, cohesive failure (failure within adhesive) was observed for each specimen under each treatment condition. The specimens that were bonded and then thermally treated for 3 hours, failed in the oxide coating immediately upon insertion of the wedge. Surface analysis results for debonded specimens suggest that the process leading to failure is the formation of fluorine-containing materials within the oxide. The measured average activation energy for the formation of aluminum fluoride species is 149 kJ/mol. The high activation energy suggests that the rate of aluminum fluoride formation is substantial only at high temperatures. In summary, the presence of fluorides in the anodic oxide coatings prior to bonding is detrimental to the overall strength and durability of adhesively bonded chromic acid anodized Ti-6Al-4V joints which have been exposed to high temperatures (350°C-399°C).
Ph. D.

Materiais cerâmicos, como a alumina e a zircônia têm sido explorados ao longo dos anos na aplicação como biomateriais. Sua natureza bioinerte, tem estimulado o desenvolvimento de novas alternativas, como tratamentos químicos para tornar o desempenho biológico destas cerâmicas mais adequadas. O recobrimento biomimético de cerâmicas bioinertes a partir de soluções com concentrações iguais às do plasma sanguíneo humano, SBF, pemiite o crescimento de uma camada bioativa na superfície dos implantes. A bioatividade do material está relacionada com a formação de uma camada constituída por cristais de hidroxiapatita carbonatada de baixa cristalinidade, similar à apatita biológica. A biocompatibilidade associada às propriedades estruturais da alumina e da zircônia tem estimulado o uso clínico destes materiais, inclusive em regiões de maiores solicitações mecânicas. Neste trabalho foram produzidas amostras de alumina, zircônia dopada com ítria (3% mol) e compósitos de alumina e zircônia dopada com ítria (3% mol) preparadas pelo método de co-precipitação, calcinadas, sinterizadas, tratadas quimicamente com soluções de ácido fosfórico e hidróxido de sódio e imersas em 1,0M e 1,5M SBF. Os pós calcinados foram analisados por difração de raios x (DRX), microscopia eletrônica de varredura (MEV), análise de área de superfície específica (BET) e análise de distribuição granulométrica (CILAS). Os resuhados por DRX indicam que as amostras possuem baixa cristalinidade. Observou-se que por BET as amostras apresentam alta área de superfície específica. Os resultados de CILAS e MEV mostraram que os pós encontram-se na forma de aglomerados. As amostras sinterizadas, foram analisadas por difração de raios x (DRX), microscopia eletrônica de varredura (MEV) e fluorescência de raios x (FRX), apresentando 96% da densidade teórica e as fases analisadas pelo método de Rietveld foram identificadas e quantificadas como: cúbica, tetragonal e monoclínica da zircônia, além da fase alfa da alumina. O tratamento químico com ácido fosfórico não apresentou uma tendência de maior formação de apatita em relação às amostras não tratada quimicamente. O tratamento com hidróxido de sódio provocou transformação acentuada da fase cúbica para tetragonal e da fase tetragonal para a fase monoclínica da zircônia. Os ensaios biomiméticos proporcionaram a formação de apatita sobre as superfícies das amostras, identificadas por DRX, e sua espessura foi medida por FRX.
Ceramic materials, as akimina and zirconia have been explored along the years as biomateriais application. The bioinert nature has been stimulating the development of new alternatives, as chemical treatments to improve the biological application of these ceramics. The biomimetic process of bioinerts ceramics for coating apatite is based on soaking the implant in a simulated body fluid, SBF, with ion concentrations nearly equal to those of human blood plasma. The bioactivity of the material is related with the formation of a layer constituted of hidroxiapatite low crystalline, similar to the biological apatite. The biocompatibility associated to the structural properties of the alumina and zirconia has been stimulating the clinical use of these materials, mainly in areas of larger mechanical requests, places not recommended for bioactive hidroxiapatite, for instance. In this work samples of alumina, zirconia doped with yttria (3% mol) and composites of alumina and zirconia doped with yttria (3% mol) were prepared by co-precipitation method, calcinated, sintered, chemically treated with solutions of acid phosphoric and sodium hydroxide and them immersed in l.OM and 1.5M SBF. The calcinated powders were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), gas adsorption (BET) and laser diffraction. The XRD resuhs indicate that the samples are low crystalline. It was observed for BET that the samples present high specific surface area. The resuhs of laser diffraction and SEM showed that the powders are agglomerates. The sintered samples were analyzed by XRD, SEM and X-ray fluorescence (XRF). The phases quantified by Rietveld method were: cubic, tetragonal and monoclinic of the zirconia, besides the phase alpha of the alumina. The chemical treatment with phosphoric acid didn\'t present a tendency of larger apatite formation in relation to the samples no chemically treated. The treatment with sodium hydroxide provoked accentuated transformation of the cubic phase for tetragonal phase and of tetragonal phase for the monoclinic phase of the zirconia. The immersion of samples in SBF provided apatite coating formation on the surfaces of the samples, identified by XRD, and the coating thiclmess was measured by XRF.

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Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

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Dissertação (Mestrado)
IPEN/D
Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

Doutoramento em Ciência e Engenharia de Materiais
A síntese eletroquímica de filmes finos de óxido usando a técnica de oxidação de alta tensão e a investigação da estrutura, propriedades físicas e químicas dos filmes obtidos são os principais objetivos desta tese. A anodização de metais sob a ação de vários kilovolts produz filmes com espessura pequena (até 180 nm) e com propriedades diferentes dos filmes formados usando técnicas eletroquímicas convencionais. As camadas de óxido depositadas desta forma conferem, frequentemente, melhores propriedades de proteção, semicondutoras e fotoeletroquímicas. No âmbito deste trabalho filmes finos sobre titânio e alumínio foram preparados em diferentes eletrólitos, incluindo soluções de ácidos e sais, bem como em água desionizada e peróxido de hidrogénio. Mostra-se que os filmes preparados por oxidação com descarga pulsada de alta tensão têm estrutura superficial e propriedades elétricas mais uniformes em comparação com os obtidos por anodização convencional. Outro objetivo do trabalho é a dopagem dos filmes anódicos com diferentes dopantes, por incorporação de espécies do eletrólito durante a formação do filme. Os filmes preparados por oxidação de descarga pulsada de alta tensão no titânio mostram uma melhor resposta de fotocorrente a comprimentos de onda pequenos e uma concentração mais baixa de dadores ionizados, relativamente aos filmes obtidos por anodização convencional. Os filmes preparados por descarga no alumínio e titânio são formados por uma camada compacta. Estudos sobre o processo de descarga revelaram que o principal fator que influencia a cinética de crescimento do filme de óxido é a concentração de defeitos pontuais, que por sua vez é determinada pela composição do eletrólito. Também se mostrou que as técnicas usando alta tensão permitem preparar filmes anódicos não só em soluções convencionais, mas igualmente em outros meios, tal como água desionizada, água destilada e peroxido de hidrogénio, onde a anodização por métodos convencionais (potenciostático ou galvanostático) é impossível. Além disso é revelado que a técnica da descarga pulsada de alta tensão é um método eficiente para encapsulação de nanocilindros de metal, preliminarmente depositados em nanoporos de titânia e alumina alinhados verticalmente.
Electrochemical synthesis of thin oxide films by using the high-voltage oxidation technique and investigation of structure, physical and chemical properties of the obtained films are the main objectives of this thesis. Anodisation of metals under action of several kilovolts allow to produce films with rather low thickness (up to 180 nm) and with properties different from the films created by using conventional electrochemical approaches. The oxide layers deposited in this way often confer advanced protective, semiconductor and photoelectrochemical properties. In the frame of this thesis thin films on titanium and aluminium were prepared in several electrolytes, including solutions of acids and salts as well as in deionised water and hydrogen peroxide. It is shown that the films prepared by powerful pulsed discharge oxidation are characterized by more uniform surface structure and electrical properties in comparison to those obtained by conventional anodization. Another aim of the work is doping of the anodic films with different dopants by incorporation of species from the electrolyte during the film formation. Films prepared by powerful pulsed discharge oxidation technique on titanium demonstrate a significantly improved photocurrent response at short wavelengths and an essentially lower concentration of ionized donors as compared with the films obtained by conventional anodization. The discharge-prepared films on both aluminium and titanium are composed by one compact layer. Studies of the discharge processes revealed that the main factor influencing the kinetics of the oxide film growth is the concentration of point defects which, in turn, is determined by the composition of electrolyte. Also, it was shown that the high voltage techniques allow to prepare anodic films not only in conventional solutions, but also in other media such as deionised water, distilled water and hydrogen peroxide, where anodisation by conventional (potentiostatic or galvanostatic) methods is impossible. Furthermore, the powerful pulsed discharge technique is shown as efficient method for encapsulation of metal nanorods preliminarily deposited into the vertically aligned titania and alumina nanopores.

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CAPES
O petróleo, por sua relevância na economia global, demanda um volume crescente de pesquisas no setor e com isso a necessidade do uso de materiais resistentes à ambientes agressivos como é o caso do petróleo cru, gerando procedimentos eficazes e seguros que minimizem impactos ao meio ambiente. Revestimento termicamente aspergido tem sido intensamente utilizado na indústria de petróleo, aeroespacial, elétrica entre outros. Os revestimentos podem ser feitos de metal, cerâmica, vidros e a maioria dos plásticos, e o uso de um compósito adequado pode melhorar a resistência à corrosão em altas temperaturas. As cerâmicas apresentam alto ponto de fusão e são bons isolantes térmicos, porém apresentam baixa tenacidade e fragilidade. Estudos mostram que o uso de óxidos como ZrO2, TiO2 como reforços e de aditivos como óxidos de elementos de terras raras Y2O3, La2O3 podem melhorar a tenacidade da cerâmica à base de alumina. A primeira etapa desta pesquisa teve como objetivo a produção de compósitos cerâmicos à base de alumina, reforçados com 5%, 10%, 15%, 20% e 30% em peso de zircônia estabilizada com óxidos de terras raras La2O3 e Y2O3 variando entre 1 e 7%. A metodologia experimental utilizada consistiu da preparação do compósito cerâmico através da moagem em moinho de bolas, confecção de pastilhas por prensagem uniaxial em prensa hidráulica, e sinterização das amostras que foi realizada nas temperaturas de 13000C, 1350 0C e 14000C por 48h, com posteriores análises de absorção, dureza vickers, microscopia ótica, DRX e MEV. De posse dos resultados, numa segunda etapa a pesquisa foi direcionada à aplicação do compósito cerâmico Al2O3 – ZrO2 reforçado com óxido de ítrio em revestimento de chapas metálicas usadas na fabricação de tanques de armazenamento e transporte de petróleo depositado pelo processo de aspersão térmica a plasma – Atmospheric Plasma Spray, com e sem Bond coat (método de aspersão HVOF – High Velocity Oxi-Fuel), seguidos de ensaios para caracterização e pré-qualificação dos revestimentos como ensaios de adesão, riscamento e MEV. De um modo geral, os valores obtidos nos ensaios de adesão e as imagens de MEV indicam que o compósito estudado é adequado para revestimentos tipo barreira térmica para aplicação em ambientes corrosivos ao petróleo cru.
Oil, for its relevance in the global economy requires a growing body of research in the industry as well as need for the use of materials resistant to harsh environments such as crude oil, generating effective and safe procedures that minimize impacts on the environment . Thermally sprayed coating has been intensively used in the oil industry, aerospace, electrical and more. The coatings may be made of metal, ceramic, glass and most plastics, and the use of an appropriate composite can improve the corrosion resistance at high temperatures. Ceramics have a high melting point and are good insulators, but have low toughness and fragility. Studies show that the use of oxides such as ZrO2, TiO2 as fillers and additives such as oxides of rare earth elements Y2O3, La2O3 can improve the toughness of alumina based ceramic. The first step of this research was aimed at the production of ceramic composites based on alumina reinforced with 5%, 10%, 15%, 20% and 30% zirconia weight stabilized with rare earth La2O3 and Y2O3 oxides ranging from 1 and 7%. The used experimental methodology consisted of the preparation of ceramic composite by grinding in ball mill, preparation of pellets by uniaxial pressing in a hydraulic press, and sintering of the samples was performed at temperatures of 13000C, 13500C and 14000C for 48 hours with subsequent analysis absorption, Vickers hardness, optical microscopy, XRD and SEM. With the results in a second step the research was directed to the application of the ceramic composite Al2O3 - ZrO2 reinforced with yttrium oxide coating of sheet metal used in the manufacture of storage tanks and transport of oil deposited by thermal spray process the plasma - Atmospheric Plasma Spray, with and without bond coat (HVOF spraying method - High Velocity Oxy-Fuel), followed by assays for characterization and classification of pre-testing coatings such as adhesion, scratching and SEM. In general, the values obtained in the adhesion tests and SEM images indicate that the composite studied is suitable for thermal barrier-type coatings in corrosive environments for application to the crude oil.

The master thesis focuses on the preparation of nickel coating on ceramic (Al2O3) substrate. The deposition of nickel was carried out by the electroless plating method in bath at various kinetic conditions. An impact of varied size, shape and roughness on the quality of the coated surface was investigated. The main goal was to find optimized conditions of the plating process of the thin metal coatings. Prepared nickel coatings were analysed by SEM, EDX analysis, mechanical profilometry and the plating bath was analysed by UV-VIS spectrophotometry. The continuous nickel coatings were achieved by modifying the deposition process, and the kinetic mechanism of experimental conditions was described.

Combustion chemical vapor deposition (combustion CVD) is a thin film deposition process that uses a flame created by the ignition of an aerosol containing precursors dissolved in a flammable solvent. Combustion CVD is a relatively new technique for creating thin film oxide coatings. Combustion CVD has been successfully used to deposit high quality thin oxide films for potential applications such as thermal barrier coatings, dielectric thin films, composite interlayer coatings, etc. The present work involved developing the optimum parameters for deposition of thin films of yttria-stabilized zirconia (YSZ), alumina (Al₂O₃), and YSZ-alumina composites followed by a determination of the mechanical properties of the films (measured using nanoindentation) as a function of composition. The optimized parameters for deposition of YSZ, alumina, and YSZ-alumina composites onto single crystal a-plane alumina involved using an organic liquid as the flammable solvent and Y 2-ethylhexanoate, Zr 2-ethylhexanoate and Al acetylacetonate as the metal precursors at a 0.002 M concentration delivered at 4 ml/min at flame temperatures of 155 ℃ and substrate temperatures of 105 ℃. The resulting films were grown with deposition rates of ~ 1.5 μm/hr. Measurement of the mechanical properties (hardness, elastic modulus and fracture toughness) of the films was performed using a mechanical properties microprobe called the Nanoindenter®. In order to obtain valid results from nanoindentation, the combustion CVD films were optimized for minimum surface roughness and grown to a thickness of approximately 0.8 μm. With the penetration depth of the indenter at approximately 150 nm, the 800 nm thickness of the film made influences of the substrate on the measurements negligible. The hardnesses and elastic moduli of the YSZ-alumina films did not vary with the composition of the film. The fracture toughness, however, did show a dependence on the composition. It was found that second phase particles of alumina grown into a YSZ matrix increased the fracture toughness of the films (on average, 1.76 MPa• m⁰.⁵ for 100% YSZ to 2.49 MPa• m⁰.⁵ for 70 mol% YSZ/30 mol% alumina). Similarly, second phase particles of YSZ grown into an alumina matrix also increased the fracture toughness (on average, 2.20 MPa• m⁰.⁵ for 100% alumina to 2.45 MPa• m⁰.⁵ for 37.2 mol% YSZ/62.8 mol% alumina). Modeling of the fracture toughness of the YSZ-alumina films was successfully achieved by using the following toughening mechanisms: crack deflection from the second phase particles, grain bridging around the particles, and residual stress from the CTE mismatch between the film and the substrate and between the second phase particles and the matrix of the film.

Durch elektrochemische Impedanzspektroskopie während der anodischen Oxidation von Aluminium werden in der vorliegenden Arbeit die elektrochemischen Vorgänge während der Oxidbildung quantitativ und zeitabhängig untersucht. Es wird ein Modell vorgeschlagen und diskutiert, welches das Impedanzverhalten während der anodischen Oxidation in Schwefel-, Oxal- und Phosphorsäure über einen großen Bereich von Konzentrationen und Stromdichten abbilden kann. Aus den gewonnenen Ergebnissen werden die kapazitive Wirkung der Sperrschicht am Porengrund, der Eintritt von Ladungsträgern in die Sperrschicht, der Ionentransport durch die Sperrschicht sowie die Oxidbildungsreaktion selbst als wesentlich für das Impedanzverhalten identifiziert. Die ermittelten Zusammenhänge und Konstanten können als Grundlage für Modellvorstellungen dienen, welche das Verhalten elektrischer Prozessgrößen und die Ausbildung der charakteristischen Oxidstruktur bei der anodischen Oxidation von Aluminium verknüpfen
In the present work, the electrochemical subprocesses of the oxide formation on aluminium by anodic oxidation are investigated using electrochemical impedance spectroscopy. The time dependence of the impedance behaviour and the quantitative relations between the process parameters and the impedance behaviour are considered. A model for the representation of the electrochemical behaviour during the anodic oxidation in sulphuric, oxalic and phosphoric acid is proposed and discussed for a wide range of anion concentrations and current densities. On the basis of the obtained results, the capacitive effect of the barrier layer, the charge transfer resistance of the barrier layer, the ion transport within the barrier layer and the oxide formation are identified as the dominating effects for the impedance behaviour. The established relations can serve as a basis for models, which interrelate both the electrochemical behaviour and the geometrical formation of the characteristic pore structure

Bien que les composites carbones représentent une avancée technologique notoire dans le domaine des matériaux thermostructuraux, leur utilisation reste limitée par une forte réactivité en atmosphère oxygénée. L'objectif de cette étude était donc de revêtir la surface de tels composites d'une couche de céramique capable de s'opposer à la diffusion des composes oxygénés vers les sites actifs. La méthode choisie consiste en un dépôt, sur les pièces à revêtir, d'un réactif en suspension ou en milieu sol-gel, suivi d'une étape de céramisation. De part leurs propriétés physico-chimiques, les ß'-SiAlON conviennent à la réalisation d'une telle barrière. Classiquement ils sont obtenus par frittage de poudres, à haute température, procédé inadapté à l'obtention de films. Il est donc apparu nécessaire, dans un premier temps, de mettre au point une nouvelle méthode permettant la synthèse de ces oxynitrures a basse température. Elle consiste en une réduction nitrurante de composes aluminosilicates par un flux gazeux mixte d'azote et d'hydrogène. L'élaboration des ß'-SiAlON sur des surfaces dépourvues de carbone libre (sic. Alumine) apparaît impossible. Le couplage de l'analyse thermogravimétrique avec la spectrométrie de masse démontre, dans le cas de creusets en graphite, la formation de méthane dans la phase gazeuse. Lorsque ce compose est associe à l'hydrogène et à l'azote, il est possible d'obtenir du ß'-SiAlON en l'absence de carbone libre. Des dépôts de kaolin obtenus par sédimentation lente des particules mises en suspension ont été transformes en films denses de ß'-SiAlON à la surface de composites

Le procédé d’oxydation par plasma électrolytique (PEO) un procédé électrochimique de traitement de surface permettant d’élaborer des couches d’oxydes protectrices à la surface d’alliages métalliques légers (Al, Ti, Mg). Contrairement aux procédés d’anodisation, le procédé PEO utilise une densité de courant et une tension élevées qui donnent lieu au développement de micro-décharges (MDs) sur la surface traitée. L’objectif de cette étude était d’étudier les caractéristiques de ces MDs sous différentes conditions opératoires et de les corréler avec la microstructure des couches d’oxyde élaborées, pour in fine mieux comprendre les mécanismes de croissance. Tout d’abord, des traitements PEO séquencés ont été menés en changeant certains paramètres électriques en cours de traitement. Les résultats ont révélé un comportement particulier des MDs qui dépend non seulement des conditions électriques appliquées mais aussi de la morphologie de la couche en croissance. Les résultats ont également mis en évidence une transition anticipée vers le régime bénéfique de décharges « soft » contribuant ainsi à une amélioration significative des microstructures de la couche d’oxyde et de la consommation énergétique du procédé PEO. Des mesures optiques résolues en temps ont permis d’établir une corrélation entre le développement des MDs et la dynamique des bulles de gaz à l’interface oxyde / électrolyte. Particulièrement, les résultats ont prouvé l’existence de MDs internes à la couche lors du régime de croissance « soft ». De plus, une caractérisation multi-échelle de la structure dite en « pancake », structure typique du régime «soft », a révélé la formation d’une structure lamellaire nanocomposite. Elle consiste en une alternance de lamelles d’alumine pure avec des lamelles de mullite 1:1 métastable. Enfin, deux études prospectives ont été conduites autour du procédé PEO. La faisabilité de réaliser des traitements duplex combinant les procédés cold spray et PEO a été démontrée ainsi que la possibilité de produire des (nano-) particules d’oxyde métallique via le procédé PEO
Plasma electrolytic oxidation (PEO) is an electrochemical surface processing technique that allows the growth of protective oxide films on lightweight metals (Al, Ti, Mg). Contrary to conventional anodising, PEO operates at high current density and voltage which results in the ignition of micro-discharges (MDs) over the processed surface The aim of this work was to investigate the characteristics of the MDs under different processing parameters and to correlate these characteristics with the microstructure of the produced oxide layers in order to better understand the oxide growth mechanisms. Firstly, PEO sequenced treatments were conducted by changing the electrical parameters in the course of a treatment. Results revealed a particular behaviour of the MDs which depends not only on the applied electrical parameters but also on the morphology of the growing layer. Results also evidenced an earlier transition to the beneficial “soft” sparking regime, contributing to a significant improvement of the microstructure of the oxide layer as well as process energy consumption. Time-resolved optical characterizations of the PEO process pointed out a correlation between ignition of MDs and the dynamic of the surrounding gas bubbles at the oxide / electrolyte interface. Particularly, results proved the existence of inner MDs during the “soft” sparking regime. Secondly, a multi-scale characterization of the typical “pancake” structure formed during the transition to the “soft” regime revealed the formation of a lamellar nanocomposite structure consisting of periodical alternations of alumina and metastable 1:1 mullite lamellae. Finally, two new opportunities for the PEO process were explored. The feasibility of duplex treatment involving cold-spray and PEO technologies was demonstrated and the possibility to produce metallic oxide (nano-) particles was proposed

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Revestimentos funcionais compósitos são um atrativo tecnológico crescente, pois possibilitam a combinação de materiais metálicos, poliméricos ou cerâmicos, resultando em propriedades superiores as dos materiais individuais, sendo por este motivo, largamente aplicados na engenharia de materiais. Na presente dissertação, foram produzidos revestimentos compósitos por eletrodeposição através da codeposição de uma matriz metálica de cobre e de partículas de óxidos de alumínio incorporadas (g - Al2O3 ou AlO(OH)), sobre substratos de aço carbono, a partir de diferentes banhos eletrolíticos. Três etapas foram efetuadas, na primeira realizou-se o estudo da influência do modo de agitação e da presença ou não de ligantes (citrato de sódio 1,00 mol/L) nos teores de cobre e alumina nos revestimentos produzidos. Em seguida foi avaliada a ação de complexantes (citrato de sódio 1,00 mol/L e pirofosfato de potássio 0,90 mol/L) usando polarização potenciodinâmica e voltametria cíclica, em conjunto com microbalança eletroquímica de cristal de quartzo (EQCM) e a posterior produção de revestimentos compósitos a partir de banhos contendo CuSO4 0,02 mol/L + pirofosfato de potássio 0,90 mol/L + 20 g/L de alumina, variando a densidade de corrente aplicada (I), a velocidade de agitação do eletrodo rotatório (A) e o do tempo de agitação prévia (t). Por fim, na terceira etapa, fez-se a substituição de alumina por Boehmita e a produção dos revestimentos a partir de banhos contendo CuSO4 0,02 mol/L + pirofosfato de potássio 0,90 mol/L + 20 g/L de Boehmita, empregando um planejamento composto central, em que os parâmetros citados também foram variados. Os resultados mostraram que a presença de um ligante e a agitação prévia e continuada do eletrólito durante o experimento foram fundamentais para a produção dos revestimentos compósitos. Ensaios de EQCM mostraram que o citrato se adsorveu na superfície do eletrodo de ouro, diferentemente do pirofosfato. Os teores de Boehmita e cobre nos revestimentos produzidos, assim como a morfologia, resistência de polarização e densidade de corrente de corrosão dos revestimentos foram influenciados pelos parâmetros avaliados.
Functional coatings composites are an increasing technologic attractive, as they allow the combination of metallic, polymeric or ceramic materials resulting in properties superior than those of the individual materials. They are, therefore, widely used in engineering materials. In this dissertation, composite coatings were produced by electrodeposition through co-deposition of a copper metal matrix and particulate aluminum oxides embedded (g - Al2O3 or AlO (OH)) on carbon steel substrates from different electrolytic baths. Three steps were taken: first the influence of the stirring mode and the presence or absence of ligands (sodium citrate 1.00 mol/L) in the amount of copper and alumina in the produced coatings were studied. After it, was evaluated the action of complexing agents (Sodium citrate 1.00 mol/L and potassium pyrophosphate 0.90 mol/L) using potentiodynamic polarization and cyclic voltammetry together with electrochemical microbalance quartz crystal (EQCM), and further producing composite coatings from baths containing CuSO4 0.02 mol/L potassium pyrophosphate + 0.90 mol/L + 20 g/L of alumina, varying the applied current density (I), the stirring speed of the rotating electrode (A) and the stirring time prior to electrodeposition (t). Finally in the last step was performed the substitution of alumina by bohemite and production of coatings from baths containing CuSO4 0.02 mol/L potassium pyrophosphate + 0.90 mol/L + 20 g/L bohemite, employing a central composite design, in which these parameters were also varied. The results showed that the presence of a ligand and the electrolyte stirring prior and during the experiment were critical to the production of composite coatings. EQCM tests showed that the citrate is adsorbed on the surface of the gold electrode, unlike pyrophosphate. The results showed that the contents of boehmite and copper in the coatings produced, as well as the morphology, polarization resistance and corrosion current density of the coatings were influenced by the evaluated parameters

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Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

L’Oxydation par Plasma Electrolytique (PEO) est un procédé de traitement de surface des alliages métalliques légers (Al, Mg, Ti) qui permet de faire croître des couches protectrices d’oxydes épaisses et dures sur ces matériaux. Pour dépasser les limites de l’anodisation, le procédé PEO repose sur la génération de micro-Décharges anodiques résultant du claquage de la couche diélectrique dans un électrolytique aqueux sous l’effet d’une densité de courant ou d’une différence de potentiel élevées (typ. 20 A/dm2 ; 700 V). Les objectifs de ce travail sont d’une part d’étudier les caractéristiques des micro-Décharges (composition chimique, densité et température électronique) et leur comportement macroscopique (conditions d’amorçage, densité surfacique, taille, durée de vie), et d’autre part de corréler ces études aux mécanismes de croissance des couches d’oxydes dans différentes conditions électriques (forme du courant bipolaire pulsé) et de composition d’électrolytes alcalins. Ces études couplées ont permis notamment de mettre en évidence que le passage en régime d’autorégulation (précédemment identifié) s’accompagne de la croissance d’une couche spongieuse, vraisemblablement amorphe, autour et dans les fissures de structures composées d’alumine cristallisée et résultant des claquages diélectriques. De même, la caractérisation de couches traitées PVD + PEO a conduit à améliorer la compréhension de certains mécanismes de claquage intervenant dans le procédé PEO, et en particulier les processus à l’interface couche d’oxydes - substrat. Enfin, une étude spécifique des micro-Décharges cathodiques (inhabituellement observées en PEO) a conduit à proposer des mécanismes de claquage de la couche diélectrique durant cette demi-Période cathodique du courant. Il a en outre été montré que, bien que l’alternance négative du courant soit nécessaire pour améliorer la croissance des couches d’oxydes, les micro-Décharges cathodiques ont un effet néfaste sur celle-Ci. Il est ainsi nécessaire de contrôler la forme d’onde du courant appliqué afin d’éviter la génération de telles décharges
Plasma Electrolytic Oxidation (PEO) is a surface treatment of light metallic alloys (e.g Al, Mg, Ti) that makes possible to grow thick and hard oxide protective coating on those materials. To overcome the limitations of anodizing the PEO process takes benefit of anodic micro-Discharges resulting from the dielectric breakdown in an aqueous electrolyte under a high applied current density or voltage (typ. 20 A/dm2; 700 V). Therefore this work aims first at studying both the macroscopic parameters (breakdown conditions, surface density, lifetime, size) of such micro-Discharges and their behavior, and second to correlate these studies to the growth mechanisms of the oxide coatings within various electrical (applied current waveform) conditions and alkaline electrolyte composition. These coupled studies allowed us to evidence that the transition from arc regime to soft regime (previously determined) corresponds to the growth of a loose spongy silicon-Rich phase which is likely amorphous, inside and around cracks of the pancake structures issued from the dielectric breakdown and composed of crystalline alumina. Meanwhile, analyses of combined PVD + PEO coatings lead us to improve our understanding of some breakdown mechanisms occurring during the PEO process, with a particular attention to the phenomena at the coating-Substrate interface. Finally, a particular study of cathodic micro-Discharges (unusually observed in PEO) allowed us to propose breakdown mechanisms of the dielectric layer during that negative half-Period of the current. Besides it has been shown that those cathodic micro-Discharges are detrimental to the layer growth though the cathodic half-Period of the current is mandatory to improve the coating growth. It is therefore necessary to manage the current waveform to avoid creating such detrimental discharges

Determination, par une analyse statistique deduite de la methode de weibull, de la charge critique moyenne comme parametre fiable de l'adhesion entre un depot et son substrat. Etude, dans le cas du couple cu/al etr de depots de sio::(2), de l'influence de la preparation de la surface du substrat sur le parametre d'adhesion

Thesis (M.Sc.(Engineering)--University of the Witwatersrand, Faculty of Engineering and the Built Environment, School of Chemical and Metallurgical Engineering, 2012,.
The objective of this research was to improve alumina (Al2O3) mechanical and electrical properties by reinforcement using multi-walled carbon nanotubes (MWCNTs) and titanium carbide (TiC). The objective of the study was achieved with interesting and challenging difficulties along the way. The MWCNTs were initially coated with boron nitride (hBN) in order to improve the Alumina-CNTs interface which was previously discovered to be weak and also to protect them from reacting with Al2O3 during sintering. The coating of CNTs with hBN was done using nitridation method. This method was unsuccessful since it was not possible to coat each CNT individually. Dispersing hBN coated CNTs proved to be impossible without pealing the off the hBN coating. The “flaking off “of the hBN coating from the CNTs revealed that the CNT-hBN interface was weak; therefore uncoated CNTs were used for this study. The starting powders (Al2O3, TiC and CNTs) were individually dispersed before they were mixed together. TiC and Al2O3 were dispersed using an ultrasonic probe which was done successfully. The CNTs were dispersed by an ultrasonic probe and then attritor milled with the use of polyvinylpyrolidone (PVP) as a dispersant. The dispersed Al2O3 and TiC (30 wt%) powders were mixed in a planetary ball mill. The composite powder was sieved and sintered using SPS with temperature and pressure programmed to be 1700˚C, 35MPa respectively. In making the Al2O3+CNT composite powder, the already dispersed Al2O3 and CNTs (1 wt%) were mixed in a planetary ball mill, after sieving the powder it was sintered using SPS at 1600˚C, 35MPa (programmed conditions). Lastly in making the Al2O3+CNT+TiC composite, the already dispersed TiC, CNTs and Al2O3 were all mixed in a planetary ball mill, after sieving it was sintered using SPS at 1650˚C, 35MPa (programmed conditions). For comparison of properties, dispersed monolithic Al2O3 was also sintered using SPS at 1600˚C, 35 MPa. The density results showed that the monolithic Al2O3 was 99.8% dense, , Al2O3+CNTs was 99.4%, Al2O3+TiC+CNTs was 99.2% and Al2O3+TiC sample was 99.0%. The mechanical properties of the samples were measured using the indentation method. The hardness and fracture toughness of the samples were; Al2O3= 3.3MPa√m (17 GPa), Al2O3+CNTs = 4.2MPa√m (18 GPa), Al2O3+TiC = 4.8 MPa√m (23 GPa) and Al2O3+TiC+CNT= 5.0 MPa√m (23 GPa). The electrical properties showed that incorporating CNTs and TiC into Al2O3 improved Al2O3 electrical conductivity. The measured electrical conductivity of the ceramic samples were; Al2O3 iii ≈ 0 Sm-1, Al2O3+CNTs= 30 S.m-1, Al2O3 +TiC + CNTs = 6855 S.m-1 and Al2O3+TiC = 9664 S.m-1. The CNTs improved Al2O3 mechanical properties slightly inhibiting grain growth by pinning the grain boundary movement and also by crack bridging. The Al2O3 electrical conductivity was increased by the CNTs network that was located along the alumina grain boundaries. The TiC improved Al2O3 mechanical properties slightly inhibiting grain growth and through crack deflection mechanism. The addition of TiC into Al2O3 increased the electrical conductivity by serving as a conducting continuous secondary phase. The results show that the CNT-hBN interface is weak. The addition of CNTs and TiC into monolithic Al2O3 slightly improved its mechanical and electrical properties but it density was slightly compromised. CNTs and TiC slightly improved monolithic alumina hardness by in inhibiting Al2O3 grain growth and the fracture toughness through crack deflection and crack bridging mechanisms. The CNTs network located at the Al2O3 grain boundaries not only aided in improving Al2O3 hardness but also served as transport medium for electrons hence increasing the Al2O3 electrical conductivity. Addition of TiC into Al2O3 increased its electrical conductivity by conducting electrons from one TiC grain to the adjacent grain. The large increase in electrical conductivity upon addition of TiC is due to the presence of a continuous TiC phase within Al203.

The main goal of this dissertation was to show that the principle of atomic layer deposition (ALD) can be applied to “endless” fibers. A reactor of atomic layer deposition has been designed, especially for coating depositions onto meter long bundles of fibers. Aluminum oxide (alumina), titanium oxide (titania), double layers of alumina and titania, as well as aluminium phosphate have been deposited onto bundles of carbon fibers using the home-built reactor. Scanning electron microscopic (SEM) and transmission electron microscopic (TEM) images indicate that the coatings were uniform and conformal onto fiber surface. There was a good adhesion of the coatings to the fibers. Alumina has been deposited using two separate aluminum sources (aluminum trichloride and trimethylaluminum), and water as a source of oxygen. In case of alumina deposition using aluminum trichloride and water, initial deposition temperature was 500 °C. In these conditions, a part of the fiber bundle has been damaged. Thus, the deposition temperature was decreased to 300 °C and the fibers were unaffected. In addition, during this process hydrochloric acid is formed as a byproduct which is a corrosive substance and affects the reactor and there was a chloride impurity in the coatings. Thus, aluminum trichloride precursor was replaced by trimethylalumium. Alumina deposition onto carbon fibers using trimethylaluminum and water was carried out at a temperature of 77 °C. SEM images revealed that the fibers were unaffected and the coatings were uniform and conformal. Oxidation resistance of the carbon fibers was improved slightly after alumina deposition. Oxidation onset temperature of the uncoated fibers was about 630 °C. The resistance was linearly increased with the coating thickness (up to 660 °C) and getting saturated over a thickness of 120 nm. Titania coatings have been deposited using titanium tetrachloride and water. The physical appearances of the titania coatings were similar to the alumina coatings. The oxidation onset temperature of the titania coated carbon fibers was similar to the uncoated fibers but the rate of oxidation was decreased than the uncoated fibers. Two double layer coatings were deposited, alumina followed by titania (alumina/titania), and titania followed by alumina (titania/alumina). If the fibers were coated with the double layer of alumina/titania, they had almost same oxidation onset as alumina coated fibers but the rate of oxidation was decreased significantly compared to alumina coated fibers. This feature is independent of the thickness of the titania layers, at least in the regime investigated (50 nm alumina followed by 13 nm and 40 nm titania). On the other hand, the oxidation onset temperature of fibers coated with titania/alumina (20 nm titania /30 nm alumina) was approximately 750 °C. The fibers were burned completely when temperature was further increased to 900 °C and held another 60 minutes at 900 °C. This is significantly better than any other coating used in this dissertation. ALD of titania and alumina in principle was known beforehand, this dissertation here applies this knowledge for the first time to endless fibers. Furthermore, this dissertation shows for the first time that one can deposit aluminum phosphate via ALD (planar surface as well as fibers). Aluminum phosphate might be special interest in the fiber coating because it is a rather soft material and thus might be used to obtain a weak coupling between fiber and matrix in composites. Aluminum phosphate was deposited using trimethylaluminum and triethylphosphate as precursors. Energy dispersive X-ray spectroscopy and solid state nuclear magnetic resonance spectra confirmed that the coating comprises aluminum phosphate (orthophosphate as well as other stoichiometries). Scanning electron microscopic images revealed that coatings are uniform and conformal. In cases of alumina and titania, it was observed that the coatings were delaminated from the ends of cut fibers and thus formed of clear steps. On the other hand, for aluminum phosphate coating it was observed that the border between coating and underlying fiber often being smeared out and thus formed an irregular line. It seems in case aluminum phosphate cohesion is weaker than adhesion, thus it might be act a weak interface between fiber and matrix. Alumina, titania, and double layer microtubes have been obtained after selective removal of the underlying carbon fibers. The carbon fibers were selectively removed via thermal oxidation in air at temperatures exceeding 550 °C. SEM and TEM images indicate that the inner side of the tube wall has the same morphology like the fibers. In addition, it was observed that the individual microtubes were separated from their neighbors and they had almost uniform wall thicknesses. The longest tubes had a length of 30 cm.:Bibliographische Beschreibung und Referat 2 Abstract 4 List of abbreviations 10 1. General introduction and outline of this dissertation 12 1.1 References 20 2. Atomic layer deposition: Process and reactor 25 2.1 Introduction 25 2.2 Principle of atomic layer deposition 26 2.3 Materials and methods 29 2.3.1 Precursors 29 2.3.2 Precursors transportation 31 2.3.3 Carrier and purge gas 32 2.3.4 ALD reactors 32 2.4 Flow-Type ALD reactor for fiber coating 33 2.5 Conclusion 35 2.6 References 35 3. Single layer oxide coatings 38 3.1 State of the art 38 3.2 Alumina coating using non-flammable precursors 39 3.2.1 Introduction 39 3.2.Result and discussion 39 3.3 Alumina coating using organometallic precursor 46 3.2.1 Introduction 46 3.2.2 Results and discussion 46 3.4 Titania coating using titanium tetrachloride and water 59 3.4.1 Introduction 59 3.4.2 Results and discussion 59 3.5 Experimental Part 67 3.5.1 General experiments 67 3.5.2 Alumina coating using aluminum chloride and water 69 3.5.3 Alumina coating using trimethylalumium and water 69 3.5.4 Titania coating 72 3.6 Conclusions 72 3.7 References 74 4. Coating thickness and morphology 78 4.1 Introduction 78 4.2 Results and discussion 80 4.2.1 Purge time 15 s 81 4.2.2 Purge time 30 s 85 4.2.3 Purge time 45 s to 100 s 85 4.3 Experimental part 88 4.4 Conclusions 89 4.5 References 89 5. Alumina and titania double layer coatings 91 5.1 Introduction 91 5.2 Results and discussion 92 5.3 Experimental part 102 5.4 Conclusions 103 5.5 References 103 6. Atomic layer deposition of aluminum phosphate 105 6.1 Introduction 105 6.2 Results and discussion 106 6.3 Experimental part 113 6.4 Conclusions 114 6.5 References 115 7. Alumina microtubes 117 7.1 Introduction 117 7.2 Results and discussion 118 7.2.1 Fibers before coating deposition 118 7.2.2 Coatings on the carbon fibers 118 7.2.3 Microtubes 121 7.3 Experimental part 127 7.4 Conclusions 128 7.5 References 128 8. Conclusions 131 Acknowledgements 136 Curriculum Vitae 138 Selbständigkeitserklärung 142
Das Hauptziel dieser Dissertation bestand darin nachzuweisen, dass die Atomlagenabscheidung (engl. atomic layer deposition (ALD)) auf „endlose“ Fasern angewendet werden kann. Es wurde ein Reaktor zur Atomlagenabscheidung gestaltet, der speziell für die Beschichtung meterlanger Faserbündel geeignet ist. Aluminiumoxid, Titanoxid, Doppelschichten aus Aluminiumoxid und Titanoxid sowie Aluminiumphosphat wurden mit Hilfe des selbstgebauten Reaktors auf Kohlefaserbündel abgeschieden. Rasterelektronenmikroskopische (REM) und transmissionselektronenmikroskopische (TEM) Aufnahmen zeigten, dass die Beschichtung auf den Fasern einheitlich und oberflächentreu war. Des Weiteren wurde eine gute Adhäsion zwischen Beschichtung und Fasern beobachtet. Das Prinzip der Beschichtung mit Titanoxid und Aluminiumoxid mit Hilfe der ALD war bereits vorher bekannt und im Rahmen dieser Dissertation jedoch erstmals auf "endlose" Fasern angewendet. Des Weiteren wird in dieser Dissertation erstmals gezeigt, dass es möglich ist, Aluminiumphosphat mittels ALD abzuscheiden (sowohl auf planaren Oberflächen als auch auf Fasern). Aluminiumphosphat könnte von besonderem Interesse in der Faserbeschichtung sein, da es ein relativ weiches Material ist und könnte daher als eine Art „schwacher“ Verbindung zwischen Faser und Matrix in Kompositen dienen. Die Oxidationsbeständigkeit von beschichten Kohlefasern wurde im Vergleich zu unbeschichteten Fasern bis zu einem gewissen Grad erhöht. Monoschichten von Aluminiumoxid und Titanoxid waren dafür wenig effektiv. Aluminiumphosphatbeschichtete Fasern waren deutlich besser geeignet als die beiden anderen. Eine Doppelschicht aus Titanoxid gefolgt von Aluminiumoxid verbesserte die Oxidationsbeständigkeit nochmals deutlich gegenüber allen anderen Beschichtungen, die in dieser Dissertation verwendet wurden. Mikroröhren aus Aluminiumoxid, Titanoxid und Doppelschichten wurden durch die selektive Entfernung der zugrunde liegenden Kohlefasern erhalten. Einzelne Mikroröhren waren von benachbarten Röhren getrennt und sie weisen eine nahezu einheitliche Wanddicke auf.:Bibliographische Beschreibung und Referat 2 Abstract 4 List of abbreviations 10 1. General introduction and outline of this dissertation 12 1.1 References 20 2. Atomic layer deposition: Process and reactor 25 2.1 Introduction 25 2.2 Principle of atomic layer deposition 26 2.3 Materials and methods 29 2.3.1 Precursors 29 2.3.2 Precursors transportation 31 2.3.3 Carrier and purge gas 32 2.3.4 ALD reactors 32 2.4 Flow-Type ALD reactor for fiber coating 33 2.5 Conclusion 35 2.6 References 35 3. Single layer oxide coatings 38 3.1 State of the art 38 3.2 Alumina coating using non-flammable precursors 39 3.2.1 Introduction 39 3.2.Result and discussion 39 3.3 Alumina coating using organometallic precursor 46 3.2.1 Introduction 46 3.2.2 Results and discussion 46 3.4 Titania coating using titanium tetrachloride and water 59 3.4.1 Introduction 59 3.4.2 Results and discussion 59 3.5 Experimental Part 67 3.5.1 General experiments 67 3.5.2 Alumina coating using aluminum chloride and water 69 3.5.3 Alumina coating using trimethylalumium and water 69 3.5.4 Titania coating 72 3.6 Conclusions 72 3.7 References 74 4. Coating thickness and morphology 78 4.1 Introduction 78 4.2 Results and discussion 80 4.2.1 Purge time 15 s 81 4.2.2 Purge time 30 s 85 4.2.3 Purge time 45 s to 100 s 85 4.3 Experimental part 88 4.4 Conclusions 89 4.5 References 89 5. Alumina and titania double layer coatings 91 5.1 Introduction 91 5.2 Results and discussion 92 5.3 Experimental part 102 5.4 Conclusions 103 5.5 References 103 6. Atomic layer deposition of aluminum phosphate 105 6.1 Introduction 105 6.2 Results and discussion 106 6.3 Experimental part 113 6.4 Conclusions 114 6.5 References 115 7. Alumina microtubes 117 7.1 Introduction 117 7.2 Results and discussion 118 7.2.1 Fibers before coating deposition 118 7.2.2 Coatings on the carbon fibers 118 7.2.3 Microtubes 121 7.3 Experimental part 127 7.4 Conclusions 128 7.5 References 128 8. Conclusions 131 Acknowledgements 136 Curriculum Vitae 138 Selbständigkeitserklärung 142

Indiana University-Purdue University Indianapolis (IUPUI)
Repair is an alternative treatment option in many cases to replacement of resin matrix composite restoration. However, aged resin matrix composites have a limited number of carbon-carbon double bonds to adhere to a new layer of rein. Therefore, surface treatments of the aged resin matrix composite surface prior to repairing could improve the repair bond strength. The objectives of this study were to: 1) To evaluate various surface treatments on shear bond strength of repair between aged and new microhybrid resin matrix composite, and 2) To assess the influence of applying a silane coupling agent after surface treatments. Eighty disk-shaped resin matrix composite specimens were fabricated and thermocycled 5000 times prior to surface treatment. Specimens were randomly assigned to one of the three surface treatments (n = 20): 1) Airborne abrasion with 50 μm aluminum oxide, 2) Tribochemical silica coating (CoJet), or 3) Er,Cr:YSGG laser and control group (n = 20). Specimens were cleaned with 35-percent phosphoric acid, rinsed, and dried. Each group was assigned into two subgroups (n =10): a) no silanization, and b) with silanization. Adhesive agent was applied and new resin matrix composite was bonded to each conditioned surface. Bond strength was evaluated by shear test. Data were analyzed with a two-way ANOVA model. The interaction between conditioning and silanization was significant(p = 0.0163), indicating that comparisons of silanization must be evaluated for each conditioning method, and that comparisons of conditioning methods must be evaluated separately with and without silanization. Airborne particle abrasion showed significantly higher repair bond strength than Er,Cr:YSGG laser without silanization (p < 0.0001) and with silanization(p = 0.0002), and higher repair bond strength than the control without silanization (p < 0.00001) and with silanization (p < 0.00001). Airborne particle abrasion did not have significantly different in repair bond strength than Tribosilica coating without silanization (p = 0.70) or with silanization (p = 0.33). Tribosilica coating had significantly higher repair bond strength than Er,CR:YSGG laser without silanization (p < 0.0001) and with silanization (p < 0.0001), and significantly higher repair bond strength than control without silanization (p < 0.0001), but not with silanization (p =0.16). Er,CR:YSGG laser and control did not have significantly different repair bond strength without silanization (p = 1.00) or with silanization (p = 0.11). There was no effect of silanization on repair bond strength overall (p = 0.34) for any of the surface conditioning methods (p = 0.76 for airborne particle abrasion; p = 0.39 for tribosilica coating; p = 1.00 for Er,Cr:YSGG laser, or p = 0.39 for control). Airborne particle abrasion with 50-μm aluminum oxide particle and tribochemical silica coating followed by the application of bonding agent provided the highest shear bond strength values, suggesting that they might be adequate methods to improve the quality of the repairs of resin-matrix composites.

Microtrusses are hybrid materials composed of a three-dimensional array of struts capable of efficiently transmitting an externally applied load. The strut connectivity of microtrusses enables them to behave in a stretch-dominated fashion, allowing higher specific strength and stiffness values to be reached than conventional metal foams. While much attention has been given to the optimization of microtruss architectures, little attention has been given to the strengthening mechanisms inside the materials that make up this architecture. This thesis examines strengthening mechanisms in aluminum alloy and copper alloy microtruss systems with and without a reinforcing structural coating. C11000 microtrusses were stretch-bend fabricated for the first time; varying internal truss angles were selected in order to study the accumulating effects of plastic deformation and it was found that the mechanical performance was significantly enhanced in the presence of work hardening with the peak strength increasing by a factor of three. The C11000 microtrusses could also be significantly reinforced with sleeves of electrodeposited nanocrystalline Ni-53wt%Fe. It was found that the strength increase from work hardening and electrodeposition were additive over the range of structures considered. The AA2024 system allowed the contribution of work hardening, precipitation hardening, and hard anodizing to be considered as interacting strengthening mechanisms. Because of the lower formability of AA2024 compared to C11000, several different perforation geometries in the starting sheet were considered in order to more effectively distribute the plastic strain during stretch-bend fabrication. A T8 condition was selected over a T6 condition because it was shown that the plastic deformation induced during the final step was sufficient to enhance precipitation kinetics allowing higher strengths to be reached, while at the same time eliminating one annealing treatment. When hard anodizing treatments were conducted on O-temper and T8 temper AA2024 truss cores, the strength increase was different for different architectures, but was nearly the same for the two parent material tempers. Finally, the question of how much microtruss strengthening can be obtained for a given amount of parent metal strengthening was addressed by examining the interaction of material and geometric parameters in a model system.

Durch elektrochemische Impedanzspektroskopie während der anodischen Oxidation von Aluminium werden in der vorliegenden Arbeit die elektrochemischen Vorgänge während der Oxidbildung quantitativ und zeitabhängig untersucht. Es wird ein Modell vorgeschlagen und diskutiert, welches das Impedanzverhalten während der anodischen Oxidation in Schwefel-, Oxal- und Phosphorsäure über einen großen Bereich von Konzentrationen und Stromdichten abbilden kann. Aus den gewonnenen Ergebnissen werden die kapazitive Wirkung der Sperrschicht am Porengrund, der Eintritt von Ladungsträgern in die Sperrschicht, der Ionentransport durch die Sperrschicht sowie die Oxidbildungsreaktion selbst als wesentlich für das Impedanzverhalten identifiziert. Die ermittelten Zusammenhänge und Konstanten können als Grundlage für Modellvorstellungen dienen, welche das Verhalten elektrischer Prozessgrößen und die Ausbildung der charakteristischen Oxidstruktur bei der anodischen Oxidation von Aluminium verknüpfen.
In the present work, the electrochemical subprocesses of the oxide formation on aluminium by anodic oxidation are investigated using electrochemical impedance spectroscopy. The time dependence of the impedance behaviour and the quantitative relations between the process parameters and the impedance behaviour are considered. A model for the representation of the electrochemical behaviour during the anodic oxidation in sulphuric, oxalic and phosphoric acid is proposed and discussed for a wide range of anion concentrations and current densities. On the basis of the obtained results, the capacitive effect of the barrier layer, the charge transfer resistance of the barrier layer, the ion transport within the barrier layer and the oxide formation are identified as the dominating effects for the impedance behaviour. The established relations can serve as a basis for models, which interrelate both the electrochemical behaviour and the geometrical formation of the characteristic pore structure.