Plasma electrolytic oxidation, or PEO, is a surface modification process for the production of ceramic oxide coatings upon substrates of metals such as aluminium, magnesium and titanium. Two methodologies for the quantitative study of electrical breakdown (discharge) events observed during plasma electrolytic oxidation processes were developed and are described in this work. One method presented involves direct measurement of electrical breakdowns during production of an oxide coating within an industrial scale PEO processing arrangement. The second methodology involves the generation and measurement of electrical breakdown events through coatings pre-deposited using full scale PEO processing equipment. The power supply used in the second technique is generally of much lower power output than the system used to initially generate the sample coatings. The application of these techniques was demonstrated with regard to PEO coating generation on aluminium substrates. Measurements of the probability distributions of discharge event characteristics are presented for the discharge initiation voltage; discharge peak current; event total duration; peak instantaneous power; charge transferred by the event and the energy dissipated by the discharge. Discharge events are shown to increase in scale with the voltage applied during the breakdown, and correlations between discharge characteristics such as peak discharge current and event duration are also detailed. Evidence was obtained which indicated a probabilistic dependence of the voltage required to initiate discharge events. Through the scaling behaviour observed for the discharge events, correspondence between the two measurement techniques is demonstrated. The complementary nature of the datasets obtainable from different techniques for measurement of PEO discharge event electrical characteristics is discussed with regards to the effects of interactions between concurrently active discharge events during large scale PEO processing.

Nanoindentation has emerged as a leading technique for the investigation of mechanical properties on small volumes of material. Extensive progress has been made in the last 20 years in refining the nstrumentation of nanoindentation systems and in analysis of the resulting data. Recent development has enabled investigation of materials under several dynamic conditions. The palladium-hydrogen system has a large miscibility gap, where the palladium lattice rapidly expands to form a hydrogen-rich β phase upon hydrogenation. Nanoindentation was used to investigate the mechanical effects of these transformations on foils of palladium. Study of palladium foils, which had been cycled through hydrogenation and dehydrogenation, allowed the extent of the transformed region to be determined. Unstable palladium foils, which had been hydrogenated and were subject to dynamic hydrogen loss, displayed significant hardening in the regions which were not expected to have transformed. The reason for this remains unclear. Impact indentation, where the indenter encounters the sample at relatively high speeds, can be used to probe the strain rate dependence of materials. By combining impact indentation and elevated temperature indentation, the strain rate dependence of the superelasticity of nickel-titanium was probed over a range of temperatures. Similar trends in elastic energy ratios with temperature were observed with the largest elastic proportions occurring at the Austenite finish transformation temperature. Multiple impact and scratch indentation are two modes of indentation which are thought to approximate erosive and abrasive wear mechanisms, respectively. These were utilised to investigate the wear resistance of several novel coatings formed by plasma electrolytic oxidation (PEO) of Ti-6Al4-V. Multiple impact indentation results appear to subjectively rank the erosive wear performance of both ductile and brittle materials. Comparison of normalised performance of coating systems on aluminium in abrasive wear to scratch hardness showed similar degrees of resistance.

This work presents information obtained from high-speed video and electrical monitoring of electrical breakdown (discharge) events during plasma electrolytic oxidation (PEO) of aluminium alloy substrates. Discharges were found to occur in extended sequences termed "cascades" at particular locations. This was a feature common to all the substrates and processing frequencies investigated. As the coating thickness increases, the characteristics remained broadly similar, although discharges become more energetic and longer-lived. Short PEO treatments were applied to existing PEO coatings in order to investigate the microstructural effects of discharge cascades. It was found that cascades persist at particular locations due to the residual deep pore channel left by previous discharges in the cascade. Observations were made of the way the coating was restructured around a cascade location. Samples were illuminated with very high intensity flashes during PEO processing, revealing that relatively large (1 mm diameter) bubbles form where a discharge emerges from the surface of a coating. Analysis of the overall energy consumption, as well as the energetic processes occurring within an individual discharge, indicate that the bubble growth occurs due to rapid volatilisation of water originating from the electrolyte. It is postulated that the growth of this bubble causes the electrical resistance to rise and is responsible for the termination of the discharge current. Investigations of high frequency (2,500 Hz) processing lead to the discovery of discharges occurring during the cathodic half-cycle, after a certain coating thickness had been achieved. Cathodic discharges were more energetic than anodic discharges, and created large craters in the coatings. Gas evolution was found to exceed the electrochemical Faraday yield, and was similar at low and high frequency initially. Once cathodic discharges began, the gas evolution rate increased and the coating mass gain levelled off.

Made available in DSpace on 2014-06-11T19:23:28Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-03-28Bitstream added on 2014-06-13T20:30:13Z : No. of bitstreams: 1 antonio_ca_me_bauru.pdf: 1720399 bytes, checksum: 396bef6efc3f71eb7d1f9f3cad7c2279 (MD5) / Apesar da excelente relação resitência/peso das ligas de alumínio, a aplicação tecnológica destas ligas é limitada pela baixa resistência ao desgaste. Neste trabalho, amostras de uma liga de alumínio (AA 5052) foram tratadas pelo processo de oxidação por plasma eletrolítico, com tempo de exposição variando de 150 a 900 s. A composição e a estrutura química dos revestimentos assim produzidos foram analisadas por espectroscopia de absorção no infravermelho. Um método baseado na medida de correntes parasitas e a perfilometria foram usados, respectivamente, na determinação da espessura e da rugosidade das camadas depositadas. O revestimento formado porssui espessura de até 9,2um. Análises da morfologia dos revestimentos foram feitas com microscopia eletrônica de varredura enquanto a resistência a desgastte das superfícies foi avaliada com um sistema pino-sobre-disco. Os resultados revelaram a deposição de um revestimento cerâmico, que conferiu expressivo aumento à resistência a desgaste da liga, o qual mostrou que as amostras tratadas suportaram uma carga aplicada de 13,44 vezes em comparação com amostras sem tratamento / Despiste the excellent strengh/weight ratio, technological applications of aluminum aloys are limited by their low wear resistance. In this work, samples of AA 5052 aluminum alloy have been modified by plasma electrolytic oxidation, with exposure time ranging from 150 s to 900 soconds. Compositional characterization has been performed by fourier transform infrared spectroscopy. Eddy current and profilometry have been used, respectively, to evaluate thickness and roughness of the deposited layers. The coating formed has a thickness of up to 9,2 micrometers. Morphological investigations have been performed with scanning electron microscopy while wear resitance has been assessed using a pin-on-disk devide. The results have revealed the deposition of ceramic layers with significant enhancement of wear resistance, which showed that the treated samples resistance, which showed that the treated samples resist an applied load 13.44 times more compared with untreated samples

Submitted by ROSANA FERNANDES ANTONIO null (rosanafernandes@terra.com.br) on 2016-05-31T17:28:39Z No. of bitstreams: 1 TESE-revisão final completa.pdf: 3847881 bytes, checksum: e10b8224324efd0b693acb74474081b5 (MD5) / Approved for entry into archive by Ana Paula Grisoto (grisotoana@reitoria.unesp.br) on 2016-06-01T13:12:10Z (GMT) No. of bitstreams: 1 antonio_rf_dr_bauru.pdf: 3847881 bytes, checksum: e10b8224324efd0b693acb74474081b5 (MD5) / Made available in DSpace on 2016-06-01T13:12:10Z (GMT). No. of bitstreams: 1 antonio_rf_dr_bauru.pdf: 3847881 bytes, checksum: e10b8224324efd0b693acb74474081b5 (MD5) Previous issue date: 2016-04-28 / O tântalo vem se destacando entre os biomateriais metálicos devido a propriedades como baixa reatividade química, ótima duc¬tibilidade, elevada resistência à corrosão e ótima bioatividade. Neste trabalho foi utilizada a técnica de oxidação eletrolítica assistida por plasma (PEO- do ingês, Plasma Electrolytic Oxidation) em substrato de tântalo, utilizando-se como eletrólito uma solução de acetato de cálcio e glicerofosfato dissódio, com a finalidade de se crescer um revestimento de hidroxiapatita sobre a superfície metálica. No processo PEO empregou-se pulsos de tensão contínua com frequência de 100 Hz e foram avaliados os efeitos da variação da diferença de potencial aplicada entre os eletrodos, que foi variada de 350 a 500 V, e do tempo de tratamento, que variou de 60 a 600 s. Para análises da composição química e estrutural dos revestimentos foram utilizadas espectroscopias de reflexão/absorção no infravermelho (IRRAS) e de energia dispersiva de raios X (EDS) e difração de raios X (XRD), enquanto que a morfologia da superfície foi avaliada por microscopia eletrônica de varredura (MEV). Empregando-se a técnica de perfilometria determinou-se a rugosidade das superfícies tratadas. A bioatividade dos revestimentos foi avaliada a partir de ensaios de adesão e crescimento de células osteoblásticas. Os melhores resultados foram obtidos com amostras tratadas em 500V com um tempo mínimo de 300 s. Nestas condições foi possível a formação de um revestimento de hidroxiapatita cristalina, confirmada por XRD. Os espectros obtidos com IRRAS também confirmaram a presença de espécies químicas responsáveis pela formação de hidroxiapatita como fosfatos (PO43-), hidroxilas (OH-) e carbonatos (CO3-2). As amostras tratadas apresentaram bioatividade e viabilidade celular maiores que as observadas com o tântalo como recebido, devido às estruturas cristalinas de hidroxiapatita juntamente com a elevada rugosidade superficial obtida com o tratamento. A energia térmica gerada nas amostras durante o tratamento foi um fator determinante para se explicar a formação da hidroxiapatita. / Tantalum is becoming increasingly important amongst metallic biomaterials due to its special properties such as low reactivity, excellent ductility, high resistance to corrosion as well as high bioactivities. In this work Plasma Electrolytic Oxidation technique PEO, was employed on Tantalum substrate using a calcium acetate and glycerophosphate sodium with the goal of generating a coating of hydroxyapatite on the metallic surface. In this PEO process, DC pulses of 100 Hz were used. The differential of potential between the electrodes in the range of 350 V to 500 V were applied that lasted from 60s to 600s and their *effects were evaluated. For chemical composition and structural analysis of coating, Infrared Reflection Absorption Spectroscopy (IRRAS) were employed together with energy dispersive X-ray spectroscopy (EDS) and diffraction of X-ray (XRD). The morphology of the surface was evaluated with Electronic Scanning Microscopy. With profilometry techniques the roughness of the treated surface was determined. The bioactivity of the coating was assessed by testing the adhesion and development of osteoblastic cells. The best results were obtained with samples undergoing a tension of 500 V lasting for a minimum of 300 s. Under these conditions it was possible to form hydroxyapatite crystal that was confirmed by DRX. The spectra obtained with IRRAS also confirmed the presence of chemical species responsible for the formation of hydroxyapatite such as phosphate groups (PO43-), hydroxyls (OH-) and carbonates (CO32-). The treated samples show greater cellular bioactivity and viability than those observed in raw tantalum due to the crystal structure of hydroxyapatite together with high surface roughness obtained as a result of the treatment. The thermal energy generated in samples during the treatment was a determining factor for explaining the formation of hydroxyapatite.

Orientador: Nilson Cristino da Cruz / Resumo: O alumínio é um dos materiais mais utilizados em vários setores da indústria. Devido ao baixo custo, se comparado ao cobre, e considerando sua abundância de oferta, o interesse nas propriedades térmicas do alumínio tem aumentado. Neste contexto, a proposta desta pesquisa é avaliar as propriedades térmicas de amostras de alumínio tratadas por oxidação eletrolítica plasmática (PEO), em eletrólitos de silicato de sódio, utilizando espectroscopia fotoacústica. A rugosidade, morfologia e a composição química dos revestimentos foram analisados, respectivamente, por perfilometria, Microscopia Eletrônica de Varredura (MEV) e Espectroscopia de Energia Dispersiva (EDS). A porosidade da camada superficial do revestimento foi estimada pelo software de processamento de imagem digital, utilizando imagens da análise de MEV, enquanto difração de Raios X (DRX) foi utilizada para determinar a estrutura cristalina das superfícies tratadas. Os resultados das análises demonstraram que os revestimentos, predominantemente constituídos por γ-Al2O3, são rugosos, com superfícies porosas e apresentam boa resistência mecânica. A espectroscopia de absorção de UV-VIS demonstrou que a amostra tratada absorveu em média 18% mais radiação na região infravermelha quando comparada com amostras sem tratamento. Em consequência, como revelado pela espectroscopia fotoacústica a difusividade térmica das amostras tratadas é pelo menos 30% maior se comparada ao alumínio não tratado e 700% maior do que a alumina conven... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Aluminum is one of the most widely used materials in several areas of the industry. Owing to lower cost, if compared to copper, and considering its abundance, the interest on the thermal properties of aluminum has increased. In this context, the proposal of this research is to evaluate the thermal properties of aluminum samples treated by Plasma electrolytic oxidation (PEO), in sodium silicate electrolytes, using photoacoustic spectroscopy. Roughness, morphology and chemical composition of the coatings have been analyzed, respectively, by profilometry, scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS). The porosity of the surface layer was estimated by digital image processing using SEM micrographs while X-ray diffraction (XRD) was used for determining the crystalline structure of the treated surfaces. The results of the analyses have shown that the coatings, predominantly constituted by γ-Al2O3, are rough, with porous surfaces and present good mechanical resistance. UV-Vis absorption spectroscopy has shown that the treated sample absorbed on average 18% more radiation in the infrared region than the pristine samples. In consequence, as revealed by photoacoustic spectroscopy the thermal diffusivity of the treated samples is at least 30% larger if compared to untreated aluminum and 700% larger than that of conventional Alumina / Mestre

L’oxydation par plasma électrolytique (ou oxydation micro-arc) est un procédé de traitement des alliages légers (Al, Mg, V, Ti, etc.) apte à pallier les limites de l’anodisation, en particulier au regard des contraintes environnementales. Bien que connu depuis de nombreuses années, les mécanismes sous-jacents à ce procédé assisté par des micro-décharges restent peu ou mal compris. L’objectif de ce travail est de cerner les mécanismes de formation et de développement des micro-décharges et d’associer leurs caractéristiques aux propriétés des couches d’oxyde élaborées sur l’alliage d’aluminium Al2214.La démarche adoptée consiste à associer étroitement l'étude des micro-décharges, la caractérisation des couches élaborées, et les mécanismes de claquage de la couche d'oxyde en cours de croissance. A l’aide de moyens originaux de vidéo rapide (> 125 000 images/s) et d'ombroscopie, la dépendance de l’évolution des micro-décharges aux paramètres macroscopiques du procédé a clairement été établie. L’importance de la présence et de la position de contre-électrodes a été mise en évidence et étudiée. Il est également montré que le choix judicieux de la fréquence et de la densité de courant anodique améliore la qualité des couches obtenues. Une fréquence de l’ordre du kHz semble la mieux appropriée.Enfin, à partir de mesures synchrones, un retard à l’apparition des micro-décharges par rapport au front montant des impulsions de courant a été mis en exergue. Très sensible aux paramètres du procédé, ce retard est probablement lié aux mécanismes de claquage de la couche d'oxyde isolante. Des scénarios concernant ces mécanismes ont ainsi été proposés. / Plasma electrolytic oxidation is a surface treatment process applied to light weight alloys (Al, Mg, V, Ti, etc.) which may advantageously replace conventional anodizing, especially regarding environmental issues. Though this process has been known for many years, the underlying mechanisms that govern this micro-discharge assisted process remain poorly understood. This work aims at better identifying the breakdown and development mechanisms of the micro-discharges and at correlating the micro-discharge characteristics to the properties of the layers grown onto Al2214 aluminium alloy samples. The approach consists in coupling the study of the micro-discharges, the characterization of the grown layers and the breakdown mechanisms. By means of high rate video recording (> 125 000 frames/s) and shadowgraph techniques, the dependence of the evolution of the micro-discharges with the macroscopic process parameters has been clearly established. The important role of counter-electrodes and their respective position with respect to the sample have been identified and studied. It is also shown that the suitable choice of current frequency and anodic current density may greatly improve the quality of the resulting oxide layers. Current frequency in the kHz range seems most appropriate to grow thick and defect-free homogeneous layers.Finally, from synchronous measurements, it has been pointed out a delay in the onset of micro-discharges with respect to the rising edge of the current pulses. Besides this delay is strongly sensitive to the process parameters, it is probably related to the breakdown mechanisms of the insulating layer. Scenarios for these mechanisms have been proposed.

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

In the present investigation, surface coatings employing laser surface alloying (LSA) and plasma electrolytic oxidation (PEO) processes have been prepared on Mg alloys. The coatings have been investigated for corrosion and wear behaviour. Two important Mg alloys based on Mg–Al system were selected namely, MRI 230D and AM50 as substrates. LSA coatings have been prepared employing Al and Al2O3 as precursors using different laser scan speeds. PEO coatings were prepared in standard silicate and phosphate based electrolytes employing unipolar, pulsed DC. Hybrid coatings using a combination of the two processes were also produced and investigated for corrosion and wear behaviour. Hybrid coatings of LSA followed by PEO (LSA+PEO) were investigated for effectiveness of sealing the cracks in the LSA coatings by subsequent PEO process and consequent improvement in the corrosion resistance. Hybrid coatings of PEO followed by LSA (PEO+LSA) were prepared with an objective of sealing the pores in the PEO coating LSA treatment. In an attempt to produce more compact PEO coatings, electrolyte containing montmorillonite clay additives was employed for the PEO process of AM50 Mg alloy. The coatings were produced employing different current densities and the effect of current density on the microstructure and corrosion behaviour of coating was investigated. Electrochemical corrosion tests of uncoated and coated alloys were carried out in 3.5 wt.% (0.6M)NaCl, neutral pH, solution with an exposed area of 0.5 cm2 for a time duration of 18.5 h. For the PEO coatings with clay additives, corrosion tests were conducted additionally in 0.5 wt.% (0.08 M) NaCl, neutral pH, solution for a time duration of 226.1 h. Wear behaviour of LSA coatings was analyzed by employing a pin on disc tribo–tester conforming to ASTM G–99 standard at ambient conditions with ground EN32 steel disc of hardness Rc 58 as the counterface. Tests were conducted under dry sliding conditions for a sliding distance of 1.0 km at a sliding velocity of 0.837 m/s employing normal loads of 10, 20, 30 and 40 N. Friction and wear behavior of PEO and PEO+LSA coatings were analyzed at ambient conditions by employing a ball−on−flat linearly oscillating tribometer conforming to ASTM G–133 standard. AISI 52100 steel ball of diameter 6 mm was employed as the friction partner. Wear tests were conducted under dry sliding conditions for a total sliding distance of 100 m at normal loads of 2 N and 5 N with oscillating amplitude of 10 mm and mean sliding speed of 5 mm/s. LSA coatings could not improve the corrosion resistance of MRI 230D Mg alloy. This was attributed to the presence of cracks in the LSA coating, which resulted in the accelerated galvanic corrosion of the substrate. LSA coatings improved the wear resistance at all loads. The improved wear resistance was attributed to β (Mg17Al12) phase and Al2O3 particles in the coating which increased the hardness of the LSA layer. No trend in corrosion and wear resistance with laser scan speed was observed for LSA coatings. PEO coatings improved the corrosion resistance of the MRI 230D Mg alloy significantly. The improved corrosion resistance was attributed to the enhanced barrier protection provided by dense barrier layer formed at the substrate/coating interface and to the insoluble phase constituents in the coatings. PEO coating was effective in improving the wear resistance at low loads/contact pressures. At higher loads, the coating underwent micro–fracture as a result of the porosity in the coatings. Hybrid coatings of LSA followed by PEO (LSA+PEO) in silicate based electrolyte improved the corrosion resistance of LSA coatings. However, the corrosion resistance was not improved to the extent of PEO coatings on as–cast alloy as a result of cracks in the primary coatings, which were not fully sealed by the plasma conversion products. No trend in corrosion resistance with laser scan speed was observed for LSA+PEOcoatings. In hybrid coatings of PEO followed by LSA (PEO+LSA), primary PEO coating was completely melted and mixed with applied precursor to form a single composite LSA layer. The corrosion resistance of the hybrid coatings was observed to be lower than that of the as–cast alloy. The presence of solidification cracks reduced the barrier properties and resulted in the accelerated galvanic corrosion of the substrate similar to LSA coatings. Hybrid (PEO+LSA) coatings exhibited improved wear resistance as compared to as–cast alloy at lower loads as a result of increase in the hardness due to β (Mg17Al12) phase and oxide/ceramic particles in the hybrid layer. At higher loads, hybrid coatings exhibited higher wear rate as compared to as–cast alloy and PEO coatings. This was attributed to three–body abrasive wear as a result of dislodged hard oxide/ceramic particles in the wear tracks. No trend in corrosion and wear resistance with laser scan speed was observed for PEO+LSA coatings. PEO coatings on AM50 Mg alloy by employing clay additives in the electrolyte resulted in the reactive uptake of clay particles producing a predominantly amorphous coating at low current density. Clay additives were effective in improving the compactness of the coating at lower current density. At higher current densities, the porosity of the coatings increased. The clay particles got re–constituted producing increasing amount of crystalline phases with increase in current density. Long term impedance measurements showed that clay addition as well as increased current density employed for the PEO process was not effective in improving the corrosion resistance of the coatings. At low current density, even though the coating with clay additives was more compact, it was deficient in MgO and consisted predominantly of an amorphous phase, which underwent fast dissolution in electrolyte thereby resulting in an early loss of barrier properties. At higher current densities, even though the coatings consisted of increased amount of MgO and crystalline phases, which resist dissolution in the electrolyte, the increased porosity and defective barrier layer resulted in easy permeation of the electrolyte into the substrate/coating interface, which resulted in much earlier loss of barrier properties and inferior corrosion resistance.

This doctoral thesis deals with the determination of the influence of plasma electrolytic oxidation (PEO) on fatigue behaviour of extruded AZ61 magnesium alloy in air and in the 3.5% NaCl solution. The coatings were formed in the silicate-phosphate electrolyte under pulsed current conditions at a frequency of 50 Hz. The influence of current density on coating formation was examined under current densities of 70, 130 and 200 mA cm-2 for different durations up to a maximum of 1800 s. 8 g dm-3 of KF were added to the electrolyte to study the influence of fluoride ions in plasma electrolytic oxidation. It is shown that fluoride ions inhibit localised oxidation in the initial stage of the process, associated with the secondary particles based on Al–Mn. The presence of fluoride also modified the sparking characteristics, decreased the rate of coating growth and changed the morphologies of the coatings. The influence of fluoride on the coating hardness, and the corrosion resistance of the alloy during exposure to salt spray, was negligible. Based on previous optimised PEO conditions, coatings formed under a current density of 130 mA cm-2 for 300 s in the electrolyte containing KF were chosen for fatigue testing. The high-cycle fatigue tests were carried out on cylindrical samples under a force controlled sinusoidal tension-tension cycle with asymmetry parameter R=0. The experimental data were fitted with Kohout & Věchet function. The fatigue limit of uncoated alloy in air for 107 cycles was determined at 145.4 MPa and the combination of PEO coating with chloride ions caused a reduction of ~55 %. Attention was paid to the fatigue crack initiation in different conditions of cyclic loading. The fracture surfaces underwent detailed fractography analysis including secondary crack observation on the gauge length. The contribution of Al–Mn particles were confirmed on the uncoated alloy in air and the presence of chloride ions were observed as another influential contributor to local corrosion attack. The cyclic loading caused spalling of the outer layer, and the multiple initiation was observed on PEO coated alloy, caused by cracks in the coating and stress transferring to the alloy.