Tesi sul tema "Electrolytic plasma oxidation"

Plasma electrolytic oxidation is used to prepare corrosion- and wear-resistant coatings on light metals. The extensive literature reports on coatings formed under a wide range of different electrical regimes and in diverse electrolyte compositions. However, little work is available that investigates systematically PEO of titanium under a range of electrical variables in a particular electrolyte. In the present work, coatings are formed in a silicate electrolyte under a range of current densities, duty cycles and rates of positive to negative current density. The coatings were found to contain anatase, rutile and amorphous silicate-rich material, with comparatively minor influences of the PEO parameters. Further, coatings were limited in thickness to 40 μm due to a decrease in voltage and intensity of sparks at longer treatment times. The coatings were relatively soft with poor wear- and corrosion- resistances, and a high coefficient of friction although the last could be reduced by incorporation of PTFE particles into the coating. The study also investigates coatings formed in aluminate-phosphate based electrolytes, which generated wear-resistant and corrosion-resistant coatings of increased hardness. A focus was on the use of high-resolution electron microscopy, which has not been reported previously, to determine the details of the coating composition and structure. The findings revealed the distribution of coating species, showing an aluminum-rich outer layer and a titanium-rich inner layer, with phosphorus enriched in a band near the base of the coating. However, the coatings also revealed highly localized variations in composition within their noanocrystalline structures, due to the melting and rapid solidification of the coating material. The study also examined the role of electrolyte purity on the formation and properties of the coatings, which has not been examined elsewhere. Importantly chlorine species from the lower purity electrolyte were shown to enrich near the substrate, resulting in a cracked interfacial layer and reduced adhesion of the coating. Such observations may account for reports of poor coating adhesion in the literature. Further, a reduced purity of the electrolyte results in an erratic voltage response, due to cycles of mechanical breakdown and healing of the coating, with high levels of chloride resulting in a highly porous coating. The distributions of phosphorus and chlorine species within the coatings suggest that these species migrate inwards, with chlorine species migrating faster than phosphorus species.

This thesis summarizes the work carried out during the three-year Ph.D in Industrial Engineering and involve the study and characterization of coatings obtained on light alloys with the technique known as Plasma Electrolytic Oxidation (PEO). PEO process is, from the practice point of view, similar to the traditional anodic oxidation process as it's based on the electrochemical growth of a protective oxide layer on a metal surface. Compared with the traditional anodizing, PEO process works at higher currents and higher voltages, thus modifying the characteristics of the obtained layer. In recent years the importance of PEO process is increasing both in the research and in the industrial world. In fact the potentiality of the coatings obtained with this type of process are higher than those of the coatings obtained with the traditional techniques of chemical conversion or anodizing. However, the relatively high cost and some problems related to the process (in particular the need of a post treatment to ensure galvanic corrosion) have now slowed to the widespread use on an industrial scale. So the scientific research on one hand is looking for new solutions to further improve the properties of the coatings, in order to justify the higher costs, on the other is trying to modify the existing process to reduce the above-mentioned costs. The obtained results explained in this thesis have allowed an expansion in the knowledge regarding the PEO coatings and in particular to move towards greater industrial development of the technique. In fact new process parameters that permit to reduce the total time for the obtainment of good PEO coatings maintaining good corrosion resistance were found, especially working with higher current densities if compared with the ones reported in literature. Moreover the addiction of molybdenum and lanthanum salts as additives in the electrolyte used in the PEO process, has permitted to improve the performances of the coating in terms of corrosion resistance. The addiction of graphite nanoparticles and silver particles has permitted to obtain respectively coatings with improved corrosion and wear resistance and coatings with an intrinsic antimicrobial effect. PEO process was also successfully applied on steels.
Questo lavoro di tesi riassume il lavoro svolto durante i tre anni di dottorato in Ingegneria Industriale e riguarda lo studio e la caratterizzazione di rivestimenti ottenuti mediante la tecnica denominata Plasma Electrolytic Oxidation (PEO) su leghe leggere. Il processo PEO è, dal punto di vista operativo, molto simile ai tradizionali processi di ossidazione anodica in quanto si basa sulla crescita per via elettrochimica di uno strato di ossido protettivo sulla superficie del metallo. Rispetto al tradizionale processo di anodizzazione il processo PEO lavora però a correnti e voltaggi più elevati, modificando così le caratteristiche dello strato ottenuto. Il processo PEO sta assumendo negli ultimi anni sempre maggiore rilevanza sia nell'ambito della ricerca che in quello industriale. Le potenzialità, infatti, dei rivestimenti ottenuti con questo tipo di processo sono molto più elevate rispetto a quelle dei rivestimenti ottenibili con le tradizionali tecniche di conversione chimica o di anodizzazione. Tuttavia il costo abbastanza elevato ed alcune problematiche relative al processo ne hanno per ora frenato la diffusione su larga scala a livello industriale. Dal punto di vista della ricerca scientifica quindi, da un lato si stanno cercando nuove soluzioni che consentano di migliorare ulteriormente le proprietà dei rivestimenti, in modo da giustificare i costi più elevati, dall'altro si stanno cercando delle variazioni al processo che consentano di ridurre i costi sopracitati. I risultati ottenuti durante il dottorato di ricerca e descritti in questo lavoro di tesi hanno permesso di ampliare le conoscenze inerenti i rivestimenti PEO e in particolare di procedere verso un maggiore sviluppo industriale della tecnica. Infatti è stata sviluppata una nuova sequenza di parametri di processo, basata sul lavorare ad elevate densità di corrente, che permette di ottenere rivestimenti di ottima qualità con tempi inferiori rispetto a ciò che viene attualmente realizzato. Inoltre l'aggiunta di sali di molibdeno e lantanio, come additivi dell'elettrolita usato nel processo PEO, ha permesso di incrementare notevolmente la resistenza a corrosione dei rivestimenti in modo tale da consentire la realizzazione di componenti a più alto valore aggiunto. L'aggiunta di nanoparticelle di grafite ha permesso di ottenere rivestimenti con buona resistenza a corrosione e ad usura. L'inserimento di altre tipologie di additivi (particelle d'argento) ha poi permesso di conferire proprietà battericide al rivestimento. Infine la tecnica PEO è stata anche con successo applicata agli acciai basso legati aprendo un importante filone di sviluppo a livello tecnologico.

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.

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.

Principles of plasma electrolytic oxidation of the AL25 aluminum alloy in diphosphate alkali solutions containing cobalt(2+) cations are discussed. It has been established that a variation in the concentration of the electrolyte components provides the formation of mixed-oxide coatings consisting of the basic matrix materials and the cobalt oxides of different content. An increase in the cobalt oxide content in the coating is achieved by the variation in electrolysis current density as well as the treatment time due to both the electrochemical and thermo-chemical reactions at substrate surface and in spark region. Current density intervals that provide micro-globular surface formation and uniform cobalt distribution in the coating are determined. The composition and morphology of the surface causes high catalytic properties of synthesized materials, which confirmed the results of testing in model reaction CO and benzene oxidation as well as fuel combustion for various modes of engine operation.

The high susceptibility to corrosion limits the broad application of magnesium alloys, and therefore, the corrosion protection of magnesium is of major concern in practical conditions. A great effort has been made in the last few decades to solve this problem. Various types of surface coatings have been developed to provide corrosion protection for magnesium alloys, among which plasma electrolytic oxidation (PEO) is one of the most promising techniques. The PEO treatment can produce a hard ceramic-like oxide coating on magnesium and its alloys, leading to significantly enhanced wear and corrosion resistance. However, the intrinsic porous morphology of the PEO coatings still limits their effect of corrosion protection. The objective of the present work is to overcome this microstructural drawback, and further improve the corrosion protection ability of PEO coatings on magnesium and its alloys. Different approaches have been adopted to reduce the degradation rate of PEO coatings, including optimisation of the PEO treatment itself, sequential processing combining PEO coating with various post-treatments and formation of smart self-healing PEO coatings inspired by biological systems. The PEO process was investigated by analysing the current/voltage transients, and the PEO coatings were systematically characterised by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). Fourier transform infrared spectroscopy (FTIR) was also used to study the chemical composition of the plasma enhanced chemical vapour deposition (PECVD) coatings on the PEO coated magnesium alloy. The corrosion resistance of the PEO coatings in 3.5 wt.% NaCl solution was investigated by the electrochemical methods, including open circuit potential (OCP) monitoring, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation scans (PDP). In addition, the mechanical behaviour of PEO coatings was also examined by scratch testing. It was found that both voltage and frequency have significant effect on the properties of PEO coatings, and a more compact coating was produced by pulsed bipolar voltage mode. The PECVD post-treatment was proven to be an effective method of improving the corrosion protection ability if appropriate precursors were used, as this method could cause both positive and negative effects. Coating degradation could also occur during immersion post-treatments, although the corrosion resistance of the PEO coating was also improved by the Ce deposition and benzotriazole (BTA) adsorption. In this case, a better corrosion protection was achieved by combining the PEO coating with Ce-based immersion post-treatment, as the insoluble Ce-containing compounds provided both sealing effect and the inhibition of cathodic reaction. Finally, the self-healing PEO coating incorporated with inhibitor loaded nanocontainers was developed and shown a good potential for providing a long-term corrosion protection for magnesium alloys, even though the corrosion resistance was not significantly increased compared with conventional PEO coating. However, none of the above approaches was perfect, indicating that there is still plenty of work to be done in the future.

Plasma electrolytic oxidation (PEO) is a relatively novel surface modification technique that provides excellent wear- and corrosion-resistant coatings on light-weight metals, in particular on aluminium. Formation of PEO coatings involves complex processes and mechanisms that are difficult to study. In this work, a new method of in-situ impedance spectroscopy is used to provide new insights into fundamental characteristics of PEO processes and coating formation mechanisms as well as to develop new means of process control. This method is based on application of a variable frequency voltage perturbation signal to obtain impedance characteristics of the electrolyser during the PEO processing. The applied voltage signal and the current response are collected and post-processed to verify the system linearity, refine phase, reduce noise and spline the impedance spectra. The obtained spectra are then fitted with appropriate equivalent circuits to reveal mechanisms underlying the PEO process. Physical meaning of various circuit components is verified using specially designed experiments in which certain system characteristics are set up in such way to obtain controllable processing conditions, such as electric field distribution in the electrolyser, electrolyte resistance or coating thickness. The circuit analysis reveals specific RC and RL loops that can be related to individual processes associated with interfacial charge transfer and transport phenomena. Characteristic time constants corresponding to these processes are evaluated and their evolution with PEO treatment time is considered. Correlations of the process kinetics with phenomena observed during the PEO treatment of Al and coating growth characteristics are discussed. Several experiments involving complimentary methods and devices are also designed and carried out to assist the main method in investigation of the PEO processes. COMSOL Multiphisics software package is used for modelling the distribution of electric field and electrolyte resistance in the electrolyser; ex-situ EIS analysis- for impedance spectra comparison with ISIS results; fractal analysis- for studying the effects of coating morphology on impedance spectra; specialized imaging multi-channel (SIM) framing camera- for real-time observation of discharge events; and FFT analysis of high-resolution current signals- for studying the information on individual discharge events. Based on these studies, the characteristics of PEO process are discussed from different aspects and these better understanding is eventually achieved.

Permanently implanted biomaterials may cause problems to the host body associated with long term chronic inflammation which would eventually require revision surgery. The development of biodegradable materials which can be absorbed, consumed and excreted by the patient is therefore of interest. Magnesium alloys have for a long time been considered as potential biomaterials for load-bearing applications due to their excellent biological properties including superior biochemical and biomechanical compatibility compared to other alternatives such as biodegradable polymers and bioceramics. However, the application of magnesium material in the biological area is still limited due to its intrinsically poor corrosion performance in the biological environments. Therefore, various methods have been explored to control the degradation rate of magnesium in biological fluid, of which plasma electrolytic oxidation (PEO) is the most promising method. PEO is a plasma-assisted anodising process that can convert the surface of magnesium into a ceramic layer, thus preventing the corrosive medium contacting the substrate; therefore, the degradation rate can be reduced. Furthermore, highly biocompatible coatings can be produced when appropriate electrolytes are used in the PEO process. Motivated by the beneficial properties of magnesium and corrosion protection provided by the PEO technique, considerable efforts have been devoted towards the development of magnesium implants based on PEO protection. Nevertheless, the corrosion rate of magnesium has not been reduced to an acceptable level and a universal PEO process appropriate for magnesium has not yet been established. In the present study, PEO processes on commercially pure (cp) magnesium and the resulting coating characteristics have been systematically studied. Through this progressive study, a biologically friendly electrolyte containing Ca and P compounds have been developed. An appropriate current regime for this electrolyte has also been studied. Finally, a hydroxyapatite layer, intended to enhance the sample bioactivity, was deposited on the PEO coated cp magnesium. The PEO process was studied according to key electrical characteristics including voltage transient, and voltage/current waveforms. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) were employed to study the surface and cross-sectional morphology, elemental composition, phase composition of the coatings. Residual stress induced by the PEO process is also studied using XRD method. The corrosion properties of the coated samples in simulated body fluid (SBF) were studied using electrochemical methods including open circuit potential (OCP) monitoring, electrochemical impedance spectroscopy (EIS) measurement, and potentiodynamic polarisation scans. The mechanical properties, including static tensile properties and cyclic fatigue performance of the coated samples were also studied to verify the applicability of magnesium in biological areas from the mechanical point of view. The results indicated that the combination of a pulsed unipolar (PUP) current regime of 3000 Hz and an electrolyte composed of 12 g/l Na3PO4•12H2O and 2g/l Ca(OH)2 provides the best process stability and success of Ca and P incorporation. Moreover, the corrosion resistance of cp magnesium in the SBF could be improved by more than 10 times. Nevertheless, such protection is very limited as the coating was degraded rapidly in the simulated body fluid, which is due to the chemical instability of MgO at the pH of SBF. Tensile and cyclic fatigue tests demonstrated that the PEO coated cp magnesium possesses sufficient mechanical properties for general load-bearing biomedical applications even though the fatigue strength is significantly deteriorated by the surface modification. Further work required to achieve better control over the biodegradation process of Mg implants can be outlined as follows: (i) robustness of the developed PEO process should be explored on other corrosion resistant magnesium alloys containing biologically friendly elements (like Ca, Zn, Mn); (ii) addition of F-, SiO32- in the electrolyte to facilitate the formation of stable compounds besides MgO in the PEO coating, thus reducing the degradation rate of magnesium based implants.

Aluminium and its alloys are broadly used in a wide variety of applications. Aluminium's main advantages include: lightness, high specific strength, high thermal and electrical conductivities, good formability, excellent machinability, diversity of aluminium alloys, extensive range of forms and options (e.g. rolling, extrusions, stampings, forgings and castings), and suitability for a diverse range of joining techniques, surface treatment and recyclability. A number of surface treatment technologies are available. They produce thicker oxide coating layers that can be used to combat corrosion and wear of aluminium alloys under aggressive environments, such as petroleum extraction environments. Coating processes to surface modify e.g. with alumina include Plasma Electrolytic Oxidation (PEO), Plasma Spray Ceramic (PSC) and Hard Anodizing (HA). In this thesis, aqueous slurry erosive wear characteristics of oxide coatings produced using the aforesaid three processes have been compared and benchmarked against the uncoated aluminium substrate. This research investigates the extent of erosion resistance, in particular, the effect of sand concentration and temperature variations to the aqueous slurry impingement against material properties such as adhesion, ductility and roughness. Also, a series of electrochemistry tests has been conducted. Coatings properties have been characterised using various tools including optical microscopy, scanning electron microscopy (SEM), x-ray diffraction (XRD), surface profilometry and hardness indentations. The results indicate that PEO coating can offer the most effective barrier to the aluminium substrate as it has the lowest weight loss under erosion tests and the highest enhancement factor under electrochemistry tests compared to the other materials. Therefore, PEO surface treatment should be considered as one of generation materials and innovative problem-solving technologies to be introduced in oil and gas industry.

Plasma electrolytic oxidation (PEO) attracts increasing interest in biomedical applications as it can form porous TiO2 coatings incorporated with Ca and P on Ti alloy substrates. However, the formation of PEO coatings containing crystalline apatite phases using a single stage PEO process is difficult. The aim of the present study was two-fold. Firstly, to expand the cyclic voltammetry (CV) approach into high anodic potentials to achieve a better understanding of the electrochemical behaviour of Ti alloy electrodes under the conditions of PEO. Secondly, to develop a range of novel electrolytic plasma processes to produce biocompatible coatings for Ti implants. Initially, the CV method was employed to identify characteristic potential ranges corresponding to differential behaviour of Ti alloy electrodes in the sodium orthophosphate electrolyte containing additives of Ca salts of various carboxylic acids. The two Ca salts have then been identified to be suitable for PEO electrolytes and during this progressive study a method to produce uniformly porous CaP containing PEO coatings using a single stage PEO treatment has been developed. Finally, this mode has been adopted for PEO treatments of Ti in electrolytes containing particulate additives of hydroxyapatite (HA). Coatings produced were examined using scanning electron microscopy (SEM), electron dispersive spectroscopy, X-ray diffraction analysis and glow discharge optical emission spectroscopy. Minkowski functionals were used to analyse the morphological features in the surface-plane SEM images, which allowed subjective interpretation of the coating morphology to be eliminated. The proliferation and viability of human osteosarcoma (MG-63s) and mouse osteoblast (MC3T3-E1) cells cultured on selected PEO coatings were studied and showed these characteristics to be comparable to those of the cells cultured on tissue culture plastic control. This study has substantially expanded the understanding of electrochemical processes that occur during the PEO treatment of Ti and broadened the range of electrolytes for production of apatite containing PEO coatings for orthopaedic and orthodontic implant applications. These results provide a benchmark against which further work investigating different aspects of PEO processing of Ti and other materials could be undertaken.

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.

Lo scopo della tesi è stato quello di sviluppare mono- e multi-strati PEO (Plasma Electrolytic Oxidation) su lega di titanio Ti6Al4V, in sostituzione della lega CrCoMo nelle protesi d'anca e di ginocchio. Ad oggi le leghe a base di cobalto rappresentano il prodotto di riferimento nell’industria biomedicale, poiché garantiscono contemporaneamente elevate prestazioni meccaniche, grazie all’elevato modulo di Young, proprietà tribologiche di resistenza ad usura, resistenza alla corrosione e biocompatibilità. Recentemente però a tali leghe sono stati associati fenomeni di allergia da parte di un numero crescente di pazienti, a causa del rilascio ionico degli elementi di lega che la compongono, con conseguenti infiammazioni ai tessuti circostanti e successivo intervento di sostituzione della protesi. Il titanio e le sue leghe, date le ottime caratteristiche meccaniche, ma soprattutto le ottime capacità di resistenza alla corrosione e superiore biocompatibilità sono ideali per la produzione di protesi articolari. Purtroppo però il titanio presenta una scarsa resistenza ad usura, e ciò lo rende attualmente inutilizzabile per la realizzazione di protesi articolari. Il lavoro di tesi si è quindi focalizzato sulla produzione di mono- e multi-strati PEO, su substrato di Ti6Al4V, ottimizzando i parametri di processo dell’ossidazione anodica al plasma, per conferire elevate proprietà di resistenza all’usura, senza compromettere la resistenza alla corrosione. Gli strati PEO sono poi stati caratterizzati attraverso analisi microstrutturali e tribologiche. L’attività di sviluppo e realizzazione dei rivestimenti è stata svolta presso l'azienda "NanoSurfaces Industries" di Cadriano (BO), mentre la caratterizzazione microstrutturale e le prove tribologiche in configurazione block-on-ring (strisciamento non lubrificato) e la successiva caratterizzazione post-prova, si è svolta presso il Dipartimento di Ingegneria Industriale (DIN) dell’Università di Bologna (gruppo Metallurgia).

Orientador: Nilson Cristino da Cruz
Banca: Francisco Trivinho Strixino
Banca: José Roberto Ribeiro Bortoleto
O Programa de Pós Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi
Resumo: Neste trabalho, a oxidação eletrolítica assistida por plasma (PEO) foi utilizada para produzir um revestimento cerâmico na superfície de substratos da liga (AA 5052) de alumínio. As propriedades elétricas deste revestimento foram analisadas através da espectroscopia de impedância elétrica (EIE). As espessuras dos revestimentos foram determinadas pelo método de correntes parasitas e microscopia eletrônica de varredura (MEV). O MEV foi empregado também na avaliação da topografia dos revestimentos. Para a determinação da estrutura e composição químicas foram empregadas as espectroscopias de absorção no infravermelho (IRS) e de energia dispersiva de raios x (EDS). As estruturas cristalinas foram determinadas pela técnica da difração de raios x. Os resultados revelaram que as superfícies foram recobertas por um revestimento complexo, contendo principalmente alumínio, oxigênio e silício, que aumento a resistividade elétrica 10-10 vezes em comparação com o alumínio como recebido
Abstract: In this work, plasma electrolytic oxidation (PEO) has been used to produce ceramic coating on AA 5052 aluminum alloy substrates. The electrical properties of coatings were analyzed using electrical impedance spectroscopy (EIS). The thickness of the coatings was determined by the eddy current method and scanning electron microscopy (SEM). SEM was also used to evaluate the topography of the coatings. Infrared absorption (IRS) and energy dispersive x-ray (EDS) spectroscopy have been employed to determine structural and the chemical composition and the crystalline structures were determined by X-ray diffraction. The results have revealed that the surfaces were covered by complex coatings, composed by aluminum, oxygen and silicon, which increased the electrical resistivity by 1010 times in comparison with the as-recebived aluminum
Mestre

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)
Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Neste trabalho, a oxidação eletrolítica assistida por plasma (PEO) foi utilizada para produzir um revestimento cerâmico na superfície de substratos da liga (AA 5052) de alumínio. As propriedades elétricas deste revestimento foram analisadas através da espectroscopia de impedância elétrica (EIE). As espessuras dos revestimentos foram determinadas pelo método de correntes parasitas e microscopia eletrônica de varredura (MEV). O MEV foi empregado também na avaliação da topografia dos revestimentos. Para a determinação da estrutura e composição químicas foram empregadas as espectroscopias de absorção no infravermelho (IRS) e de energia dispersiva de raios x (EDS). As estruturas cristalinas foram determinadas pela técnica da difração de raios x. Os resultados revelaram que as superfícies foram recobertas por um revestimento complexo, contendo principalmente alumínio, oxigênio e silício, que aumento a resistividade elétrica 10-10 vezes em comparação com o alumínio como recebido
In this work, plasma electrolytic oxidation (PEO) has been used to produce ceramic coating on AA 5052 aluminum alloy substrates. The electrical properties of coatings were analyzed using electrical impedance spectroscopy (EIS). The thickness of the coatings was determined by the eddy current method and scanning electron microscopy (SEM). SEM was also used to evaluate the topography of the coatings. Infrared absorption (IRS) and energy dispersive x-ray (EDS) spectroscopy have been employed to determine structural and the chemical composition and the crystalline structures were determined by X-ray diffraction. The results have revealed that the surfaces were covered by complex coatings, composed by aluminum, oxygen and silicon, which increased the electrical resistivity by 1010 times in comparison with the as-recebived aluminum

This work presents theoretical and experimental findings pertaining to the possible replacement of conventional, automotive fuels by solid, light alkaline-metal borohydrides (EM+,B3+/H-). On a complete oxidation basis, and under certain arrangements, the latter fuels have volumetric energy capacities that exceed the most recognized, automotive constraints; furthermore, the solid metal oxide products (EM+,B3+/02-) have the potential to be regenerated off-board to their respective hydrides via electrolytic reverse complete oxidation processes. Electrolytic reverse complete oxidation processes are conceptualized as water electrolysis and electrolytic reverse combustion or electrolytic reverse hydrolysis unit operations in series. To more clearly express these mostly electrochemical fuel-cycles, a thermodynamic reaction model was contrived. To investigate the electrolytic reverse hydrolysis hypothesis, attempts were made to prepare metal-supported, electrolytic reverse hydrolysis anode compartments. The glycine nitrate process facilitated the synthesis of nickel iron oxide (anode) and select doped ceria fluorite and double-doped LaGaO3 perovskite (solid oxygen anion electrolytes) powders. Hydridic electrolyte compositions belonging to the Na2BH5-Na413205 quasi-binary system were synthesized from NaH, NaBH4 and NaB02. Analyses for the materials' compatibilities and solubilities studies (823 + 10 °K, 1.00 f 0.01 MPa) included induction coupled plasma, inert x-ray diffraction, and scanning electron microscopy. NaH reduces magnetite to austenite; hence, the most promising solid oxygen anion electrolyte, Lao7Sro3GauFeo3Mgo 103.6 (LSGFM), cannot be in direct contact with these hydridic electrolytes. The other oxides of Lao8Sro2Ga08Mg0203.6 are significantly soluble in these melts, but either a quenching or an electrochemical technique will be required to more accurately assess their values. For the preparation of small, metal-supported, electrolytic reverse hydrolysis anode compartments, suspension plasma spraying was used for the depositions of the anode and solid oxygen anion electrolyte layers. Energy dispersive x-ray spectroscopy, scanning electron microscopy, and x-ray diffraction were used for the analyses of the resultant coatings' characteristics. In consequence of LSGFM's remarkable specific conductivity at — 673 °K, a solid oxygen anion electrolyte-supported, electrolytic reverse combustion or electrolytic reverse hydrolysis anode compartment design may be considered, but this will require the addition of a protective, solid oxygen anion electrolyte layer. The thermodynamic analyses has identified scandia stabilized zirconia as the most auspicious solid oxygen anion electrolyte; hence, understanding the nature of the anhydrous Sc)+,Zr4*,B3+,Na+,H+/H",02" system at 723 ± 50 °K and < 1.0 MPa is paramount to further efforts regarding electrolytic reverse hydrolysis or electrolytic reverse combustion proof-of-concept studies. [symboles non conformes]

Metal foams have attracted wide range of interest from researchers and industries because of their unique combinations of properties. Of particular interest, open cell metallic foams have good weight-specific mechanical properties, and improvements could make these materials highly desirable for lightweight structural and energy absorption applications. These properties could potentially be increased for open cell foams by treatments affecting their large surface areas. The effect could be very significant, especially when the dominant deformation mode is bending of the foam struts, as the coating will be located away from the neutral bending axis of these struts maximizing its effect. This has been previously found after the application of surface treatments, such as electroplating. The technique of Plasma Electrolytic Oxidation (PEO) is another process that shows an even greater effect on foam specific mechanical properties. In this work, Plasma Electrolytic Oxidation (PEO) coating treatment is applied to open celled aluminium foams with different structures, aiming to improve the mechanical and weight-specific properties of the hybrid material. Open cell aluminium foams of different types, both investment cast (Duocel foam) and replicated (produced in the laboratory) have been produced and PEO coated using a range of different processing parameters. Two pore sizes of Duocel aluminium foam (measured as 2.2 mm and 2.5 mm average pore diameter) with porosity of 90–91%, and a single pore size (1.6 mm diameter) of the pure aluminium replicated foam with porosity around 60–64% have been examined. The PEO treatment of foams was carried out in the pulsed bipolar current mode, with a range of processing times (20, 40, 60 and 80 minutes), pulse frequencies (50 to 6250 Hz) and duty cycles (different ON/OFF waveform ratios). These processing parameters were explored in the present work in four different stages of investigation, as will be explained in detail later. The mechanical properties (yield stress, specific strength, Young’s modulus and energy absorption) of the coated foams produced are assessed experimentally, both in tension and compression, and simple models developed to describe the elastic behaviour, based on either the Gibson-Ashby model of foams as a regular cellular array, or the Markaki-Clyne model of randomly intersecting fibres are used to make predictions to compare to these results. Complimentary characterisation was carried out using SEM, EDX, XRD and nanoindentation techniques to understand the nature of PEO coatings on foams (including coating thickness, growth rate, mechanical properties, porosity, elemental and phase compositions), and the effect this has on mechanical properties. Thereby, the process can be optimised to improve the mechanical performance of the foams. It was demonstrated that PEO coatings can be successfully applied to open cell foams (of low and high level of porosity) and the coating penetrates completely into the structure up to several millimetres depth, with thickness diminishing with depth. The presence of this coating is of benefit for uniaxial mechanical properties as well as specific foam properties. PEO pulse frequency influences coating thickness, porosity and the measured mechanical properties. The major effect on coating hardness and elastic modulus as well as on the strength and stiffness of the coated foams is associated with the volume fraction of porosity within the coating. The effect of using different duty cycles (associated with the ON and OFF times in each cycle in the current pulse frequency used) results in different coating morphology, thickness, distribution and deposition rate. Very fast coating growth rate has been shown to be not always beneficial, whereas low coating growth rate may be useful for the formation of good quality coatings (containing fewer microcracks and possibly lower intrinsic stresses), with potential for a very even distribution into the foam internal structure. An assessment based on strength increase (∆σ) and density increase (∆ρ) of the coated foams shows that the benefits of the application of PEO coatings to metal foams are greater than those shown in other metal foams coated by different techniques. The primary reason for this is that the oxide ceramic coatings formed on foams have low density, excellent mechanical properties and good adhesion to the substrate. These properties have been improved for foams by the PEO optimization process carried out in the present work.

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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.

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

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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

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

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A fotocatálise heterogênea utilizando óxido de titânio tem se mostrado promissora na degradação de poluentes orgânicos. Neste contexto, a oxidação eletrolítica assistida por plasma (do inglês, Plasma electrolytic oxidation – PEO) é um método bastante compatível por possibilitar a formação de superfícies fotocatalíticas com porosidade e com composição química controladas. Neste trabalho, a oxidação por PEO foi utilizada na geração de superfícies porosas em um substrato de titânio, usando uma solução eletrolítica contendo 5 ou 10 g/L de oxalato amoniacal de nióbio, com a finalidade de incorporar nióbio às superfícies de óxido de titânio, visando de aumentar sua atividade fotocatalítica sob luz visível. No processo de PEO foram utilizadas as tensões de 500 e 600 V, com frequência fixada em 60 Hz e tempo de tratamento variando de 60 a 600 s. As morfologias, topografias superficiais e estruturas cristalinas das amostras foram analisadas por microscopia eletrônica de varredura (MEV), microscopia de força atômica (AFM) e difração de raios X (DRX), respectivamente. Energia dispersiva de raios X (EDS) foram empregadas para determinação de estrutura cristalina e composição química, respectivamente. O gap óptico das amostras foi determinado a partir de espectros de reflexão na região do ultravioleta visível (UV-Vis). Rugosidade foi determinada por perfilometria e um goniômetro automatizado foi usado para determinação do ângulo de contato e energia de superfície. A atividade fotocatalítica foi avaliada a partir da degradação de azul de metileno. A amostra que apresentou a melhor atividade fotocatalítica foi a produzida usando 500 V, 600 s e 10 g/L, a qual resultou na degradação de 70% de azul de metileno após 120 min sob irradiação com uma lâmpada de luz ultravioleta.
Heterogeneous photocatalysis using titanium oxide has shown to be promising in degradation of organic pollutants, reducing their toxicity and allowing them to be degraded by the environment. In this context, Plasma Electrolytic Oxidation (PEO) is a very interesting method because it allows the formation of photocatalytic surfaces with controlled porosity and chemical composition. In this work, PEO oxidation was used for the generation of porous surfaces of titanium substrates, using an electrolytic solution containing 5 or 10 g / L of niobium ammonium oxalate, with the purpose of incorporating niobium to titanium oxides, aiming the enhance of its photocatalytic activity under visible light. In the PEO process, voltages of 500 and 600 V were used, with a frequency set at 60 Hz and treatment time ranging from 60 to 600 s. The samples surface morphologies; topography and crystallographic structure were analyzed by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD), respectively. X-ray dispersive energy spectroscopy (EDS) have been employed for crystallographic structure and chemical composition evaluations, respectively. The optical gap of the samples was determined from reflection spectra in the visible ultraviolet region (UV-Vis). Roughness was determined by profilometry and an automated goniometer was used for contact angle and surface energy measurements. The photocatalytic activity was evaluated from the degradation of methylene blue. The sample with best photocatalytic activity was produced using 500 V, 600 s and 10 g / L, which resulted in the degradation of 70% methylene blue after 120 min under irradiation with ultraviolet light.

Les traitements de surface sont souvent nécessaires pour améliorer les performances des matériaux métalliques et élargir les spectres de leurs applications. L'alliage de titane TA6V (ou grade 5) est ainsi utilisé dans de nombreux domaines (du biomédical à l'aéronautique) car il est léger et possède de bonnes propriétés en anticorrosion et de résistance à la température. Cependant, son comportement tribologique est en fait médiocre et doit donc être amélioré à l'aide d'un revêtement. L'objectif de ces travaux a été d'élaborer par Oxydation Micro-Arcs (OMA), des revêtements sur substrat TA6V, afin d'en améliorer les propriétés tribologiques. L'OMA est un procédé d'oxydation électrochimique innovant et récent, pour lequel restent à clarifier les mécanismes de croissance des revêtements et l'influence exacte de certains paramètres opératoires, en particulier la composition de l'électrolyte ou le signal électrique imposé. L'étude systématique de différents électrolytes a tout d'abord permis d'élaborer des revêtements adhérents, avec des épaisseurs comprises entre 5 et 60 µm. Ces revêtements résultent de la conversion électrochimique du substrat et de l'incorporation d'espèces depuis l'électrolyte. Ils sont composés d'une phase amorphe, dont la proportion dépend directement de la quantité de silicates dans le bain, et de phases cristallisées formées suite à l'élévation importante de la température surfacique lors du traitement. La compréhension des corrélations entre électrolyte et revêtement a notamment permis de limiter la formation de la phase amorphe molle et de favoriser celle de structures cristallines dures, menant au final à un électrolyte optimisé. L'étude des paramètres électriques, tels que la fréquence ou le temps de traitement, a mis en évidence par ailleurs leur influence importante sur la composition et la morphologie des revêtements. La modification du rapport cyclique a ainsi permis de modifier la composition chimique du revêtement, en favorisant la formation d'alumine cristallisée au sein de celui-ci. Enfin, ces revêtements ont été caractérisés du point de vue mécanique. La présence de phases cristallisées dans le revêtement a permis d'en augmenter la dureté, en comparaison du substrat de TA6V. Toutefois, à cause de la rugosité élevée des revêtements élaborés par OMA, il est apparu indispensable d'effectuer un post-traitement de polissage mécanique pour réduire le coefficient de frottement et le volume de matière usée. Au final, la compréhension des corrélations entre paramètres opératoires et propriétés du revêtement a permis d'élaborer avec succès un revêtement aux propriétés tribologiques très prometteuses, c.-à-d. un coefficient de frottement inférieur à 0,3 et un volume usé inférieur à 0,01 mm3 après 100 000 cycles, démontrant une amélioration significative des propriétés mécaniques surfaciques du substrat TA6V
Surface treatments are often needed for metallic materials in order to improve their performances and broaden their scope of applications. TA6V (or grade 5) titanium alloy is used in many fields (going from biomedical to aeronautical parts) because it is light and possesses good anticorrosion and thermal properties. Nevertheless its tribological behavior needs substantial improvements that a coating may provide. The aim of this work was to prepare, using Plasma Electrolytic Oxidation (PEO), coatings on TA6V in order to improve its tribological properties. PEO is a recent and innovative electrochemical oxidation process for which growth mechanisms and accurate influence of operating parameters such as electrolyte composition or applied electrical signal, still need clarification. Systematic study of several electrolytes led to the preparation of adherent coatings with thicknesses between 5 and 60 µm. These coatings result from both electrochemical conversion of the substrate and incorporation of compounds from the electrolyte. They are composed of an amorphous phase, its proportion depending directly on the silicates quantity in the bath, and crystalline phases formed after the important rise of surface temperature during treatment. The understanding of correlations between electrolyte and coatings have limited the formation of the soft amorphous phase and favored hard crystalline structures, leading to an optimized electrolyte. The study of electrical parameters, such as frequency or treatment time, highlighted their strong influence on the coatings composition and morphology. The duty cycle influenced the chemical composition of the coatings, promoting the formation of crystalline alumina. Finally coatings prepared with PEO were mechanically tested. The presence of crystalline phases allowed the increase of the coatings Vickers hardness compared to the bare TA6V. Nevertheless, due to the PEO coatings roughness, a step of mechanical polishing post-treatment appeared necessary in order to reduce the friction coefficient and wear loss. Finally, the understanding of correlations between process parameters and coatings properties, has successfully led to the preparation of a coating with promising tribological properties, namely a friction coefficient below 0.3 and a wear loss inferior to 0.01 mm3 after 100,000 cycles, demonstrating a significant improvement in surface mechanical properties of the TA6V substrate

Das Ziel dieser Dissertation besteht darin, einen Beitrag zur technologischen, ökonomischen und ökologischen Weiterentwicklung der anodischen Verfahren zur Oberflächenkeramisierung von Aluminium zu leisten. Die Arbeit ist in zwei thematische Schwerpunkte untergliedert. Im ersten Teil wird für die Hartanodisation eine hinsichtlich eines geringeren Energieeinsatzes optimierte Elektrolytzusammensetzung identifiziert und mit einem optimierten galvanostatischen Pulsmuster simultan appliziert. Im Ergebnis kann die Gesamtleistungsaufnahme um ca. 6 % reduziert werden, ohne die mechanischen Eigenschaften der Oxidschichten zu mindern. Im zweiten Schwerpunkt werden das Lichtbogen- und das Flammspritzen mit der plasmaelektrolytischen anodischen Oxidation kombiniert, um verschleißbeständige Aluminiumoxidschichten auf Stahl-, Titan- und Magnesiumsubstraten zu applizieren. Neben einer umfangreichen Mikrostrukturanalyse (REM, EDX, XRD, EBSD) werden die mechanischen Eigenschaften der Schichten untersucht und mit atmosphärisch plasmagespritzten Al2O3-Schichten verglichen. Insbesondere Oxidschichten auf lichtbogengespritztem AlCu4Mg1 zeigen dabei eine hohe Härte sowie eine sehr gute Verschleißbeständigkeit
The aim of the present work is to contribute to the technological, economic and ecological improvement of the anodic processes for the surface ceramization of aluminum. The work is subdivided into two thematic priorities. In the first part, for the hard anodizing process an optimized electrolyte composition for a lower energy input is identified and applied simultaneously with an optimized galvanostatic pulse regime. As a result, the total power consumption can be reduced by approximately 6% without reducing the mechanical properties of the oxide coatings. In the second focus, arc and flame spraying are combined with plasma electrolytic anodic oxidation to apply wear resistant aluminum oxide coatings on steel, titanium and magnesium substrates. In addition to a comprehensive microstructural analysis (SEM, EDX, XRD, EBSD), the mechanical properties of the layers are investigated and compared with atmospheric plasma sprayed Al2O3 coatings. In particular, oxide layers formed on arc sprayed AlCu4Mg1 coatings show a high hardness as well as very good wear resistance

In questo lavoro è stato valutato il comportamento tribologico di leghe prodotte mediante i più diffusi processi di Powder Bed Fusion: SLM (Selective Laser Melting) ed EBM (Electron Beam Melting). Nella prima parte è stata studiata l’influenza di processi di anodizzazione innovativi come PEO (Plasma Electrolytic Oxidation) ed ECO (ElectroChemical Oxidation) sulla lega di alluminio A357 prodotta mediante SLM con due diverse combinazioni di parametri di processo volte ad ottimizzare rispettivamente, la microstruttura (M) e la produttività (P). Nella seconda parte è stato studiato il comportamento tribologico della lega Ti6Al4V prodotta per EBM, in relazione al processo produttivo, alla finitura superficiale e all’orientazione derivante dal processo EBM. Lo studio tribologico è stato effettuato con prove di strisciamento non lubrificato (geometria pattino-su-cilindro contro acciaio EN 100Cr6), con carichi applicati nell’intervallo 20-60 N, nel caso della lega A357 SLM anodizzata, e di 10 N per la lega Ti6Al4V EBM, in entrambi i casi con velocità di strisciamento di 0,3 m/s. Nel primo caso i risultati ottenuti hanno dimostrato come entrambi i processi di anodizzazione abbiano migliorato la resistenza ad usura della lega SLM A357 as-built rispetto al substrato non rivestito. Il rivestimento ECO ha dato luogo ad una maggiore resistenza ad usura rispetto al PEO, grazie alle migliori proprietà di coesione ed adesione. Nel secondo caso i risultati ottenuti hanno evidenziato come il processo EBM sulla lega di Ti6Al4V abbia prodotto resistenza ad usura maggiori rispetto a quella convenzionale, grazie alla maggiore finezza microstrutturale. Nei casi studiati, un aumento di rugosità superficiale ha condotto sempre ad un incremento del tasso d’usura. Al contrario, la sezione di crescita, con orientazione dei grani perpendicolare alla direzione di strisciamento, ha portato ad un’usura minore, rispetto alla sezione a grani equiassici perpendicolare alla direzione di crescita.

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

Плазмово-електролітичне оксидування (ПЕО) є ефективним способом підвищення функціональних властивостей робочих поверхонь деталей, що працюють в умовах значних теплових та механічних навантажень, зокрема із силумінів, які широко використовуються в автомобільній промисловості та двигунобудуванні. За результатами проведених досліджень встановлено, що із пірофосфатних електролітів на основі сульфату кобальту в ПЕО-режимі формуються рівномірні оксидні композиційні покриви характерного фіолетового кольору внаслідок включення до складу поверхневих шарів нестехіометричних оксидів кобальту.

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.

Le procédé le plus courant pour améliorer les propriétés de surface de l'aluminium est le procédé d'anodisation en bain acide, permettant la formation d'une couche protectrice d'oxyde d'aluminium en surface. Cela confère une meilleure résistance à l'usure et à la corrosion grâce à la grande dureté et la stabilité chimique de l'alumine. En revanche, l'alumine est un excellent isolant électrique, augmentant considérablement la résistance de contact des pièces anodisées. Traditionnellement, un traitement électrolytique à base de nickel, cadmium et chrome est appliqué aux pièces nécessitant une bonne conductivité en surface. Cependant, l'électrolyte contient des métaux lourds et notamment du chrome hexavalent, substance cancérigène, mutagène et reprotoxique que l'agence européenne de la chimie compte interdire au sein de l'Union Européenne. Par conséquent, des traitements alternatifs sont recherchés, conduisant à l'oxydation par plasma électrolytique. Ce procédé de conversion électrochimique diffère de l'anodisation en bain acide par l'utilisation de forts courant/tensions et l'emploi d'électrolytes basiques faiblement concentrés. La couche d'oxyde résultante est poreuse, permettant l'incorporation de particules solides. Ces dernières sont dispersées dans l'électrolyte et progressivement incorporées dans la couche d'oxyde pendant sa croissance. Il est alors concevable d'incorporer des particules conductrices dans l'objectif de créer des chemins de percolation à travers la couche, formant une couche composite oxyde d'aluminium-particules qui protège l'aluminium sous-jacent tout en maintenant une faible résistance électrique. A cet effet, les nanotubes de carbone ont été choisis pour leur excellente conductivité électrique et leur géométrie favorable à la percolation. L'étude vise à incorporer des nanotubes de carbones dans la couche d'oxyde générée pendant le traitement par oxydation plasma électrolytique de l'aluminium en vue de produire des couches d'oxydes conductrices. Les investigations expérimentales établissent les connaissances fondamentales sur les mécanismes d'incorporation, l'impact des paramètres électriques de traitement, l'effet des nanotubes de carbone sur le procédé et sur les propriétés des couches, notamment le comportement électrique. Les résultats révèlent que les nanotubes de carbone accélèrent la croissance de la couche et augmentent la porosité de celle-ci. A forte concentration, les couches formées deviennent excessivement poreuses avec des défauts (fissures, délamination) qui fragilisent l'intégrité de la couche d'oxyde. Les nanotubes de carbone dans l'électrolyte et dans l'oxyde influencent considérablement le procédé. La transition vers le régime de micro-décharges « soft » intervient plus tôt avec des concentrations en nanotubes plus élevées. Des concentrations excessives de nanotubes de carbone inhibent le procédé, empêchant la formation de la couche d'oxyde. Les traitements en régime « d'arc » favorisent l'incorporation des nanotubes de carbone dans l'oxyde devant les traitements en régime « soft ». Bien que l'incorporation des nanotubes de carbone améliore significativement de la conductivité électrique des couches d'oxyde, le seuil de percolation n'est pas encore atteint, et les couches demeurent isolantes pour l'instant. Cependant, les résultats obtenus sont très prometteurs, encourageant le financement de recherches supplémentaires pour optimiser la conduction électrique des couches ainsi formées, en s'appuyant sur les découvertes ici rapportées
The most common method to enhance the surface properties of aluminum is acid-based anodization, forming a protective layer of aluminum oxide on the metal surfaces. This imparts improved wear and corrosion resistances due to alumina's high hardness and chemical stability. However, aluminum oxide is a strong electrical insulator, substantially increasing contact resistance in anodized components. Traditional electrolytic surface treatments involving nickel, cadmium, and chromium maintain electrical conductivity but involve heavy metal-containing electrolytes, including carcinogenic hexavalent chromium, a substance facing European Union import restrictions. Consequently, alternative treatments are sought, leading to electrolytic plasma oxidation. This electrochemical conversion process differs from acid anodization, using higher current/voltage and dilute basic electrolytes. The resulting oxide layer is porous, enabling the incorporation of solid particles. These particles are dispersed in the electrolyte and gradually incorporated within the growing oxide layer. By adding conductive particles it is conceivable to create percolation paths, forming a composite aluminum oxide-particle layer that protects the underlying aluminum while maintaining low electrical contact resistance.Carbon nanotubes were chosen for their excellent electrical conductivity and high form factor, enabling percolation at low volume concentration. The study aims at incorporating carbon nanotubes into the oxide layer generated during aluminum plasma electrolytic oxidation to produce conductive oxide layers. Experimental investigations establish fundamental insights into incorporation mechanisms, impact of electrical parameters, the influence of carbon nanotubes on the process, and coating properties, especially electrical behavior. Results reveal that carbon nanotubes accelerate layer growth and increase oxide coating porosity. High concentrations yield excessively porous layers with defects (cracks, delamination), compromising layer integrity. Carbon nanotubes in both the electrolyte and the growing oxide substantially affect the process. Transition to "soft" micro-discharge regime shifts earlier with higher nanotube concentrations under suitable electrical conditions. Excessive nanotube concentrations inhibit the process, preventing oxide layer formation. "Arc" regime treatments favor nanotube incorporation in the oxide compared to "soft" regime treatments. Although carbon nanotube incorporation significantly enhances oxide layer electrical conductivity, the percolation threshold isn't reached, and layers remain insulating for now. Despite this, the results are highly promising, prompting further research to optimize electrical conductivity in these composite coatings, building upon the findings reported here

Дисертація на здобуття наукового ступеня доктора технічних наук за спеціальністю 05.17.03 – технічна електрохімія (161 – хімічні технології та інженерія). ‒ Національний технічний університет "Харківський політехнічний інститут", Харків, 2020. Об’єкт дослідження ‒ електрохімічні та хімічні процеси на міжфазовій межі та в оксидному шарі при формуванні гетерооксидних покриттів на сплавах алюмінію і титану. Предмет дослідження – механізм процесу поверхневої обробки сплавів алюмінію та титану у лужних розчинах електролітів, технологічні параметри плазмо-електролітного оксидування, склад, структура та функціональні властивості гетерооксидних покриттів. Дисертацію присвячено розробці наукових засад технології плазмо-електролітного формування гетерооксидних покриттів заданого складу і функціональних властивостей на сплавах алюмінію (титану) для екотехнологій. Висунуто та експериментально доведено гіпотези щодо гомогенізації поверхні багатокомпонентних сплавів алюмінію (титану) та формування заданого рельєфу оксидної матриці плазмо-електролітним оксидуванням у лужних розчинах дифосфатів та формування міцноадгезованих гетерооксидних покриттів із широким спектром функціональних властивостей на сплавах алюмінію (титану), що реалізацується в одному технологічному процесі плазмо-електролітним оксидуванням у лужних розчинах дифосфатів за присутності сполук металів-допантів. За результатами комплексного дослідження плазмо-електролітного оксидування багатокомпонентних сплавів запропоновано нову парадигму інженерії поверхні, за якою в одному технологічному процесі проводять гомогенізацію поверхні оброблюваних матеріалів із мінімізацією вмісту їх легувальних елементів, утворення наперед заданої топографії монооксидної матриці Al₂O₃ (TiO₂) та одночасною інкорпорацією цільових допувальних компонентів. Запропоновано використання комплексних електролітів на основі дифосфатів лужних металів для прискорення електрохімічного розчинення, зв’язування та видалення легувальних елементів із поверхневих шарів багатокомпонентних сплавів алюмінію (титану), встановлено шляхи керування гомогенізацією поверхні та доведено, що ПЕО в розчині 0,5–1,0 моль/дм³ K₄P₂O₇ за густини струму 5–7 А/дм² дозволяє зменшити вміст легувальних елементів у поверхневих шарах в 4–5 разів та сформувати розвинену оксидну матрицю металу-носія, що склало підґрунтя для розробки узагальненої технологічної схеми процесу. Запропоновано стратегію синтезу гетерооксидних покриттів плазмо-електролітним оксидуванням легованих сплавів алюмінію (титану) з формуванням в одному процесі оксидної матриці металу-носія та інкорпорації оксидів металів-допантів; доведено, що співвідношення компонентів електроліту впливає на вміст допанта, морфологію та топографію поверхні гетерооксидного покриття. З використанням диференціальних залежностей dU/dt–U для опису кінетичних закономірностей та встановлення стадійності процесу плазмо-електролітного оксидування сплавів різного хімічного складу доведено, що відмінність кута нахилу таких залежностей на початкових ділянках ПЕО зумовлена формуванням оксидів різної природи, а домінанта реакцій розчинення компонентів сплаву над реакціями формування оксидів з високим питомим опором обумовлює появу плато на залежності dU/dt–U, протяжність якого відбиває формування гетерооксидного шару. Обґрунтовано концепцію інкорпорації оксидів Mn та Co до складу покриттів і доведено, що в лужних електролітах на основі дифосфатів при додаванні солей металів-допантів в режимі "спадаючої потужності" з варіюванням густини струму формуються гетерооксидні покриття Al₂O₃·MnOₓ із вмістом мангану до 36,0 ат.% та Al₂O₃·CoOᵧ із вмістом кобальту до 24,0 ат.%, що дозволило визначити оптимальні умови синтезу. Підтверджено утворення в запропонованих режимах матриці металу-носія із фазовою структурою корунду, в яку інкорпоровані оксиди металів-допантів змінної валентності. Встановлено, що значне зростання мікротвердості для системи Al | Al₂O₃ CoOᵧ зумовлено не тільки утворенням α-Al₂O₃ в каналах пробою, а і формуванням структури сапфіру CoAl₂O₄ за рахунок хімічного заміщення і доведено, що термообробка гетерооксидних покриттів при температурах 300–500°С зумовлює зміну співвідношення оксидних форм допувальних компонентів при збереженні високих показників мікротвердості. Встановлено, що одностадійна плазмо-електролітна обробка поршня двигуна КамАЗ-740 у розчинах дифосфату з додаванням манганатів (VII) та солей кобальту (ІІ) дозволяє сформувати рівномірні міцноадгезовані каталітичні і теплозахисні гетерооксидні покриття оксидами мангану та кобальту, високу активність яких доведено в робочому процесі каталітичного горіння палива. Знайшли подальший розвиток уявлення про систему чинників впливу на склад, морфологію, топографію та структуру гетерооксидних покриттів на легованих сплавах алюмінію (титану) і залежність функціональних властивостей оксидних шарів від режиму формування та складу поверхні. Практичне значення одержаних результатів полягає в розробці варіативних технологічних схем плазмо-електролітної обробки багатокомпонентних сплавів алюмінію (титану) у розчинах дифосфатів із мінімізацією вмісту легувальних елементів у поверхневих шарах та формуванням гетерооксидних покриттів з підвищеним вмістом активних компонентів й заданими функціональними властивостями. Тестуванням розроблених покриттів на випробувальних стендах кафедри двигунів внутрішнього згоряння НТУ "ХПІ" встановлено зменшення викидів оксидів азоту й вуглецю та підвищення паливної економічності двигунів за рахунок внутрішньоциліндрового каталізу. Результатами випробувань гетерооксидних покриттів у Харківському науково-дослідному експертно-криміналістичному центрі МВС України встановлено їх підвищену корозійну стійкість та механічну міцність, що дозволило рекомендувати одержані матеріали для захисту від корозійного руйнування та підвищення механічної міцності капсюлей-детонаторів, які використовуються для проведення вибухових робіт. Підвищені механічні властивості та висока адгезійна міцність оксидних покриттів до основного металу підтверджено випробуваннями на АТ "УКРНДІХІММАШ". Теоретичні матеріали та практичні результати дослідження використано в освітньому процесі Національного аерокосмічного університету ім. М.Є. Жуковського "Харківський авіаційний інститут" при підготовці фахівців за спеціальністю "Теплоенергетика" та Військового інституту танкових військ НТУ «ХПІ» при підготовці курсантів за спеціальностями "Забезпечення військ (сил)" та "Озброєння та військова техніка". Науково-технічна новизна розробок підтверджується 7-ма патентами України та патентом Респубілки Казакстан, частина з яких відзначена дипломами Всеармійського конкурсу "Кращий винахід року", а саме: патент України № 116176 "Спосіб зниження токсичності газових викидів двигунів внутрішнього згоряння" (диплом I ступеня у номінації "Автомобільна техніка", 2017 рік); патент України № 117765 "Спосіб обробки поршнів двигунів внутрішнього згоряння" (диплом II ступеня у номінації "Автомобільна техніка", 2018 рік); патент України № 135696 "Поршень двигуна внутрішнього згоряння з каталітичним термостійким покриттям" (диплом "За оригінальність технічного рішення", 2019 рік).
Dissertation for the Degree of the Doctor of Engineering Sciences in the Specialty of 05.17.03 – technical Electrochemistry (161 – Chemical Technology and Engineering). – National Technical University "Kharkіv Polytechnic Institute", Kharkіv, 2020. The object of research is chemical and electrochemical processes in the volume of electrolyte, oxide coating and interface in the formation of heteroxide coatings on aluminum and titanium alloys. The subject of research is the mechanism of the surface treatment of aluminum and titanium alloys in alkaline solutions of electrolytes, technological parameters of plasma-electrolyte oxidation, composition, structure and functional properties of heteroxide coatings. The thesis is devoted to the development of scientific bases of plasma-electrolytic formation of heterooxide coatings of a given composition and functional properties on aluminum (titanium) alloys for ecotechnologies. Hypotheses were generated and experimentally proved concerning the homogenization of the surface of aluminum (titanium) multicomponent alloys and the formation of a given relief of the oxide matrix by plasma-electrolyte oxidation in alkaline solutions of diphosphates and the formation of strongly adhesed heteroxide coatings with a wide range of functional properties on aluminium (titanium) alloys by executing plasma-electrolytic oxidation in alkaline solutions of diphosphates with the presence of dopant metal compounds in one technological process. As a result of a comprehensive study of plasma-electrolytic oxidation of multicomponent alloys, a new paradigm of surface engineering is proposed, according to which in one technological process the surface of processed materials is homogenized with minimization of their alloying components, formation of predefined topography of Al₂O₃ (TiO₂) monoxide matrix and simultaneous incorporation of target alloying components. The use of complex electrolytes based on alkali metal diphosphates for acceleration of electrochemical dissolution, binding and removal of alloying components from the surface layers of multicomponent aluminum (titanium) alloys is proposed, ways to control surface homogenization are established and it is proved that PEO 1.0 in 0.5 mol/L K₄P₂O₇ solution at a current density of 5–7 A/dm² allows to reduce the content of alloying components in the surface layers by 4–5 times and to form developed oxide matrix of the metal-carrier, which became the basis for the development of a generalized flow chart. It is proposed to use a strategy for the synthesis of heteroxide coatings by plasma-electrolyte oxidation of alloyed aluminum (titanium) alloys with the formation of the oxide matrix of the metal-carrier and the incorporation of oxides of metal-dopants in one process; it is proved that the ratio of electrolyte components affects the content of dopant, morphology and topography of the heteroxide coating surface. With the use of differential dependences dU/dt–U in order to describe the kinetic laws and establish the stages of the process of plasma-electrolytic oxidation of alloys of different chemical composition, it is proved that the difference in the slope of such dependences at the initial sites of PEO is due to the formation of oxides of different nature, and the dominant of dissolution reactions of alloys components over the reaction of oxide formation with high resistivity cause the appearance of a plateau on the dU / dt – U dependence, the length of which reflects the formation of a heteroxide layer. The conception of incorporation of Mn and Co oxides into the coatings was substantiated and it is proved that in alkaline electrolytes, which are based on diphosphates, with the addition of metal-dopant salts in the mode of "decreasing power" with variation of current density heteroxide oxide coatings Al₂O₃·MnOₓ with manganese content up to 36 % and Al₂O₃·CoOᵧ with cobalt content up to 24.0 %, are formed that allowed to determine the optimal synthesis conditions. The formation of matrix of metal-carrier in proposed modes with a phase structure of corundum, in which oxides of dopant metals of variable valence are incorporated, is confirmed. It is established that a significant increase in microhardness for the system Al | Al₂O₃·CoOᵧ is caused not only by the formation of α-Al₂O₃ in breakdown paths, but also by the formation of the structure of CoAl₂O₄ sapphire due to chemical substitution and it is proved that heat treatment of heteroxide coatings at temperatures of 300–500 °C causes a change in the ratio of oxide forms of alloying components while maintaining high microhardness values. It is established that one-stage plasma-electrolyte treatment of the KamAZ-740 engine piston in diphosphate solutions with the addition of manganates (VII) and cobalt (II) salts allows to form uniform strongly adhered catalytic and heat-protective heteroxide coatings by oxides of manganese and cobalt, high activity of which was proved in the process of catalytic fuel combustion. The idea of the system of factors influencing the composition, morphology, topography and structure of heteroxide coatings on alloyed aluminum (titanium) alloys and the dependence of the functional properties of oxide layers on the mode of formation and surface composition was further developed. The practical significance of the obtained results lies in the development of variable technological schemes of plasma-electrolyte treatment of multicomponent aluminum (titanium) alloys in diphosphate solutions with minimization of alloying components in surface layers and formation of heteroxide coatings with high content of active components and given functional properties. Testing of the developed coatings on the test benches of the Department of Internal Combustion Engines of NTU "KhPI" revealed a reduction in emissions of nitrogen and carbon oxides and increase in fuel efficiency of engines due to internal cylinder catalysis. The results of tests of heteroxide coatings in the Kharkiv Scientific Research Forensic Center of the Ministry of Internal Affairs of Ukraine established their increased corrosion resistance and mechanical strength, which allowed to recommend the obtained materials to protect against corrosion damage and increase the mechanical strength of detonator caps used for blasting. Increased mechanical properties and high adhesive strength of oxide coatings to the base metal were confirmed by tests at JSC "UKRNDIHIMMASH". Theoretical materials and practical results of the research were used in the educational process of the National Aerospace University named after M.E. Zhukovsky "Kharkiv Aviation Institute" in the training of specialists in the specialty "Thermal power" and the Military Institute of Armored Forces of NTU "KhPI" in the training of cadets in the specialties "Provision of troops (forces)" and "Armament and military equipment". The scientific and technical novelty of the developments is confirmed by 7 patents of Ukraine, some of which were awarded diplomas of the All-Army competition "Best Invention of the Year", namely: patent of Ukraine # 116176 "Method of reducing toxicity of gaseous emissions from internal combustion engines" (first-degree diploma certificate in nomination "Automotive Equipment", 2017); patent of Ukraine # 117765 "Method of processing pistons of internal combustion engines" (second-degree diploma certificate in nomination "Automotive Equipment", 2018); patent of Ukraine # 135696 "Piston of an internal combustion engine with a catalytic heat-resistant coating" (diploma certificate "For the originality of the technical solution", 2019).

Дисертація на здобуття наукового ступеня доктора технічних наук за спеціальністю 05.17.03 ‒ технічна електрохімія (161 – хімічні технології та інженерія). ‒ Національний технічний університет "Харківський політехнічний інститут", Харків, 2020. Дисертацію присвячено розробці наукових засад технології плазмо-електролітного формування гетерооксидних покриттів заданого складу і функціональних властивостей на сплавах Al (Ti) для екотехнологій. Висунуто та експериментально доведено гіпотези щодо гомогенізації поверхні багатокомпонентних сплавів та формування заданого рельєфу оксидної матриці плазмо-електролітним оксидуванням в лужних розчинах дифосфатів та формування міцноадгезованих гетерооксидних покриттів із широким спектром функціональних властивостей за присутності сполук металів-допантів. З використанням диференціальних залежностей dU/dt–U описано кінетичні закономірності та встановлено стадійність процесу ПЕО сплавів різного хімічного складу. Встановлено, що використання розчинів дифосфатів дозволяє зменшити вміст легувальних елементів у поверхневих шарах в 4–5 разів та сформувати розвинену оксидну матрицю металу-носія. Доведено, що в лужних електролітах при додаванні солей металів-допантів в режимі "спадаючої потужності" з варіюванням густини струму формуються гетерооксидні покриття із Ѡ(Mn) до 36,0 ат. % та з Ѡ(Co) до 24,0 ат. %, що дозволило визначити оптимальні умови синтезу. Підтверджено утворення в запропонованих режимах матриці металу-носія, в яку інкорпоровані оксиди металів-допантів змінної валентності. Доведено, що термообробка гетерооксидних покриттів при температурах до 600°С зумовлює зміну співвідношення оксидних форм допувальних компонентів при збереженні високих показників мікротвердості. Встановлено, що одностадійна плазмо-електролітна обробка поршня двигуна КамАЗ-740 у розроблених електролітах та режимах дозволяє сформувати рівномірні міцноадгезовані каталітичні і теплозахисні гетерооксидні покриття оксидами мангану та кобальту, активність яких доведено в робочому процесі каталітичного горіння палива. Розроблено варіативні схеми плазмо-електролітної обробки багатокомпонентних сплавів Al (Ti) з підвищеним вмістом активних компонентів й заданими функціональними властивостями. За результатами комплексу експериментальних досліджень та тестувань властивостей покриттів в модельних середовищах й технологічних умовах визначено перспективні області застосування одержаних матеріалів.
Thesis for scientific degree of Doctor of Technical Sciences in the Specialty of 05.17.03 – Technical Electrochemistry (161 – Chemical Technology and Engineering). – National Technical University "Kharkіv Polytechnic Institute", Kharkіv, 2020. The thesis is devoted to the development of scientific bases of plasma-electrolytic formation of heterooxide coatings of a given composition and functional properties on Al (Ti) alloys for ecotechnologies. Hypotheses were generated and experimentally proved concerning the homogenization of the surface of aluminum (titanium) multicomponent alloys and the formation of a given relief of the oxide matrix by plasma-electrolyte oxidation in alkaline solutions of diphosphates and the formation of strongly adhesed heteroxide coatings with a wide range of functional properties on aluminium (titanium) alloys by executing plasma-electrolytic oxidation in alkaline solutions of diphosphates with the presence of dopant metal compounds in one technological process. With the use of differential dependences dU/dt–U in order to describe the kinetic laws and establish the stages of the process of plasma-electrolytic oxidation of alloys of different chemical composition. It found that the use of complex electrolytes based on alkali metal allows to reduce the content of alloying components in the surface layers by 4–5 times and to form developed oxide matrix of the metal-carrier. It proved that in the mode of "decreasing power" with variation of current density heteroxide oxide coatings with Ѡ(Mn) up 36.0 аt. % and with Ѡ(Co) up 24.0 аt. %, that allowed to determine the optimal synthesis conditions. The formation of matrix of metal-carrier in proposed modes, in which oxides of dopant metals of variable valence are incorporated, is confirmed. It is proved that heat treatment of heteroxide coatings at temperatures up 600°C causes a change in the ratio of oxide forms of alloying components while maintaining high microhardness values. It is established that one-stage plasma-electrolyte treatment of the KamAZ-740 engine piston in in developed electrolytes and modes allows to form uniform strongly adhered catalytic and heat-protective heteroxide coatings by oxides of manganese and cobalt, high activity of which was proved in the process of catalytic fuel combustion. The variation schemes of of plasma-electrolytic treatment of multicomponent Al (Ti) alloys with the increased content of active components and the set functional properties were suggested. The perspective areas of the application of the obtained materials according to the results of experimental researches and tests of properties in model environments and technological conditions are determined.

碩士
明志科技大學
材料工程系碩士班
106
Recently, plasma electrolytic oxidation (PEO) process has been widely studied and applied in both of academic research and industries due to its great ability to fabricate specific functional oxide layers on Al (aluminum), Mg (magnesium), Zr (zirconium) and Ti (titanium) alloys. In this study, we successfully prepared the porous PEO coating on titanium via plasma electrolytic oxidation (PEO) in aluminate-phosphate electrolyte under unipolar mode with 50% duty cycle, 1000Hz and 4A. The microstructural, mechanical and corrosion behaviors of PEO coating were investigated by X-ray diffractometer (XRD), scanning electron microscope (SEM), (scanning) transmission electron microscope (STEM), energy-dispersive X-ray spectroscopy (EDS), α-step profilometer, scratch adhesion test and potentiodynamic polarization measurement. The good adhesion of PEO coating on titanium is achieved by using the electrolyte with 3g/L sodium aluminate and 10g/L sodium phosphate. According to XRD and cross-sectional SEM-EDS, the PEO coatings on titanium are mainly composed of Al2TiO5 and rutile-TiO2. We also find that the corrosion resistance of PEO coatings in Hank’s solution is independent of the ratio of aluminate and phosphate electrolytes. Furthermore, the growth mechanism of the PEO coating was systematically studied on the samples after PEO process in the electrolyte with 3g/L sodium aluminate and 10g/L sodium phosphate after processing durations of 20s, 40s, 90s, 300s and 600s, respectively. It can be found that anatase-TiO2 formed on pure Ti at the early stage (<20s) and the structure of PEO coating was changed from "anatase-TiO2 single phase" to "anatase and rutile-TiO2 mix-phases" at the sparking stage (~40s). While the PEO voltage reached above 450 volts (>90s), the aluminum titanate (Al2TiO5) was formed on the top layer of PEO coating. According to STEM-EDS, the formation of Al2TiO5 was due to the PEO solution with the addition of sodium aluminate. However, the crystallinity of Al2TiO5 was transformed from amorphous to crystalline while PEO voltage reached at microarc stage.

碩士
國立臺灣科技大學
機械工程系
106
In this experiment, the plasma electrolytic oxidation experiment was carried out on the AZ91 magnesium alloy using an aluminate-based electrolyte in a constant current DC bipolar pulse mode.In this experiment, various electrical parameters (including current density J (mA/dm2) [(43.48/-50.72) ~ (54.35/-61.59)], positive and negative charge ratio (Q+/Q-, Charge Ratio, CR) [ 0.89~2.22; 0.35~0.86] and positive and negative current ratios (I+/I-, Current Ratio, IR) [0.89; 1.13] correlate with discharge behavior and accompanying film properties during PEO and establish a soft regime during PEO The relationship between the (sr) phenomenon and the nature of the oxide. Adjust current density under fixed CR (~0.87) [(43.48/-50.72) ~ (54.35/-61.59) (mA/dm2)]; Fixed IR = 0.89 by adjusting anode working time [TON+ = 200~500 (μs)] and fix cathode working time [TON-= 200 (μs)] to change CR [0.89 to 2.22]; fixed IR=1.13 by adjusting anode working time [TON+= 200~500 (μs)] and fix Cathode working time [TON-= 650 (μs)] to change the film properties accompanying the discharge behavior during CR [0.35~0.86] will be discussed in this paper.The correlation between discharge intensity and CR has been established in this paper. Increasing CR (positive and negative charge ratio = degree of polarization) can inhibit the excessive discharge phenomenon of the formed film layer during the PEO process and eliminate the discharge channel in the film layer. The s.r. phenomenon in the PEO process has a significant correlation with the electrical properties of the oxides. The s.r. phenomenon is caused by the defect in the film layer that changes the oxide insulation properties, and the hydrogen in the PEO process may be the key. When the s.r. phenomenon occurs, it mainly depends on the oxygen diffusion mechanism to form a film and grow. At a fixed CR, the current density will affect the output of the material during discharge, so the greater the current density, the smaller the difference in thickness between the center and edge layers. The IR (positive and negative current ratio = oxidation rate) will affect the rate of oxidation after the material is ejected during the discharge process. Increasing the oxidation rate allows the material to rapidly form oxides after eruption, filling the pores after discharge, so IR>1 The film layer obtained in the case has a thicker outer oxide film than IR<1. Keywords: Plasma electrolytic oxidation(PEO) Plasma Aluminate AZ91

For many decades, the implantation of Ti based biomedical implants has been extensively utilized to improve and restore the patients’ quality of life. However, despite advances in technology, failures of the implant do occur. In many cases, the failed Ti implants require immediate removal or correction through surgical operation, which, apart from the substantial economic impact on patients and governments, would cause prolonged suffering for patients. Moreover, the rapid increase in the population’s life expectancy necessitates the fabrication and engineering of more reliable orthopaedic implants based on Ti and its allosys for the aged population to address the current issues associated with implant failure. Several factors are involved in bone implant failures, such as bacterial infection and inflammation and poor integration of the bone with the implant surface. Among them, the infection problem requires special attention, as almost two-thirds of the infected implants are not treatable and eventually fail. The emergence of multi-drug resistant (MDR) bacteria is another horrifying issue, which poses a real threat to humankind and is responsible for high mortality rates among vulnerable patients. Fortunately, the emerging advances in nanotechnology and surface engineering can be a promising solution to this crisis. The ideal bone implant should possess antibacterial properties as well as the high bioactivity required for osseointegration improvement. In this regard, novel surface modification treatments such as the plasma electrolytic oxidation technique (PEO) has proven to be effective in the bioactivity improvement of titanium-based implants. However, the development of the PEO treated surface with antibacterial properties is still challenging. This study aims to implement innovative approaches to address the critical factors associated with titanium implant failure by applying post-PEO treatments such as hydrothermal process and functionalizing the surface with novel 2D materials. This thesis is presented in eight chapters, including a comprehensive literature review and several published, under review or confidential unpublished papers. In brief, the significant contributions of this work fall into four categories as follows: •Engineering of nanostructured titania surfaces with tunable and mixed topography (paper 1). • Optimal fabrication of antibacterial titania nanostructures, a combined approach of plasma electrolytic oxidation and hydrothermal treatment (paper 2). • Antibacterial development of titania surface with the application of graphene oxide and PEO-EPD technique (paper 3). • Comparative antibacterial activity of 2D materials coated on the porous-titania against gram-positive (S. aureus) and gram-negative (E.coli) bacteria (paper 4).
Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering and Advanced Materials, 2021

碩士
國立聯合大學
機械工程學系碩士班
105
Barium titanate (BaTiO3) thin film is a commonly used electronic materials, but also the development of the earliest perovskite-type ferroelectric materials, in physics, chemistry, materials and other disciplines have an important position, because of its high The electrical constant is therefore the focus of the field of dielectric research. Plasma electrolytic oxidation is an emerging film preparation technology, with simple equipment, easy operation, process control and other advantages, and thus by the scientists at home and abroad a wide range of interest, but on the barium titanate film plasma electrolysis oxidation research is not enough. In this study, barium titanate thin films were prepared by plasma electrolysis and oxidation, and the effects of electrolyte concentration, composition and electrical parameters on film formation and morphology were investigated. A barium titanate thin film was prepared by using a mixed solution of 0.3 M barium acetate and 2 M sodium hydroxide as an electrolytic solution. And optimize the process parameters of plasma electrolysis oxidation. The influence of different parameters on the morphology and properties of the films was studied by means of characterization. The results showed that the higher the temperature of the electrolyte was, the surface roughness of the films increased, and the low temperature electrolytes were easy to be formed. In addition, the concentration of the electrolyte increased Will also cause the surface of barium titanate film roughened, and will increase the thickness of the film. The effect of the increase in current density is similar to that of the increase in electrolyte concentration. The difference is that as the microarc oxidation frequency increases, the film surface becomes more flat and the film becomes thinner. The longer the oxidation time, the more rough the surface of the film will become, the thickness will increase. The dielectric constant of the barium titanate thin film showed that the dielectric constant of the film at room temperature and the dielectric constant of barium titanate decreased with the increase of frequency. In this paper, the growth kinetics of barium titanate thin films was also studied. By summarizing the empirical formula of film growth and comparing the experimental data, the empirical formula was further modified.

碩士
國立聯合大學
機械工程學系碩士班
102
Hydroxyapatite (HAp) is a bioactive ceramic with chemical composition close to the composition of the human skeleton. Many research attempt to deposit HAp on surface of titanium metals to replace artificial dental implants and artificial joints in single process. this paper, barium titanate was synthesized by Plasma Electrolytic Oxidation (PEO) method . The experimental results shows that electrolyte composition, pH, and ultrasonic vibration have profound influence in HAp synthesis. The electrolytes are the mixture of calcium acetate (Ca (CH3COO) 2.H2O), sodium dihydrogen phosphate (NaH2PO4) and sodium hydroxide (NaOH). This study result showed that the concentration ratio of calcium/sodium dihydrogen phosphate affect . The larger of the ratio, the large of the C/P ratio. The optimal process parameters of electrolyte concentration are to use 0.2 M calcium acetate and 0.1 M sodium dihydrogen phosphate, applying voltage 430 V under 10 minutes. pH also influenced the synthesis.in acid environment.The formed phase is CaTiO3 and TiO2, while in alkaline solution it formed single phase HAp. Ultrasonic vibration also make significantly contributions on increasing the film thickness, surface roughness and micro hardness. The corrosion potential Ecorr and Icorr demonstrate the improved corrosion rate by deposition of HAp.

碩士
國立中興大學
材料科學與工程學系所
105
In this research, titanium dioxide was produced by plasma electrolyte oxidation on Ti foil substrate with the electrolytic solution of sodium hexametaphosphate (NaPO3)6 and sodium hydroxide (NaOH) to control the formation of titania in the pure anatase phase and to evaluate the application of dye-sensitized solar cells. The formed titania was explored in the crystalline phase, surface microstructure, hydrophilicity and other factors. Using the electrolyte of 0.04 M sodium hexametaphosphate and 0.125 M sodium hydroxide, with Ti bulk and Ti foil as reaction substrates, fixed the current frequency, duty cycle, reaction time and temperature of electrolyte was 980 Hz, 10%, 10 minutes and 2℃, respectively. By controlling the reaction voltage in plasma electrolyte oxidation method, the pure anatase titanium dioxide can be prepared. Furthermore, 0.02 M sodium hexametaphosphate with 0.125 M sodium hydroxide and 0 M sodium hexametaphosphate with 0.125 M sodium hydroxide were modified to carry out the same previous process on Ti foil reaction substrate. From the results of modification, we can know that changing the substrate, composition of electrolyte and power parameter can control the phenomenon of spark discharge to affect the growth oxide layer and control the existence of the pure anatase titania. Compare the difference between Ti foil and Ti bulk, it can be seen that the oxide layer of the crystalline phase, the thickness and roughness were similar. The most obvious difference is when the breakdown voltage of Ti bulk is 250 V while the breakdown voltage of Ti foil is 200 V. The time of reaction current on Ti foil and flow through the specimen longer than Ti bulk, thus the oxide film has a higher surface energy to obtain the bigger surface pore size and excellent hydrophilicity. In the application evaluation, the photoelectric conversion efficiency of dye-sensitized solar cells is related to titanium dioxide photoelectrode, dye molecules and redox electolytes. When the sodium hexametaphosphate and sodium hydroxide are used as electrolytes, the pure anatase titanium dioxide is produced. When the sodium hydroxide was used as electrolyte, the crystalline phase of titanium dioxide layer were anatase and rutile. Due to the difference of the surface microstructure, the amount of the dye adsorption was affected. Therefore, the photoelectric conversion efficiency is more excellent under the sodium hydroxide electrolyte. If the microstructure of pure anatase titanium dioxide can be improved to form micro-nanopores and to increase the adsorption specific surface area of the dye in the future, the photoelectric conversion efficiency will be enhanced which can increase the potential of dye-sensitized solar cells.

碩士
國立中興大學
材料工程學系所
98
In contrast to traditional anodic oxidation, plasma electrolytic oxidation (PEO) can produce better crystalline ceramic coatings with quick deposition rates. The objective of this study is to prepare alumina coatings in an electrolyte with different additive concentrations by PEO on aluminum alloys. It was conducted in 0.1 M NaAlO2 electrolytes at current densities of 1- 20 A/dm2 with reaction time for 10- 120 minutes. Observe the formation of α-Al2O3 phase formed on the influence of differences；and then at fixed the current density of 10 A/dm2 by adding Al(NO3)3 concentration from 1.0 mM-5.3 mM with reaction time for 10- 120 mins, then to compare their results with without adding the difference. As for voltage characteristics, we found the spark discharge was delayed obviously while adding the concentration of Al(NO3)3 in 1.6 mM- 2.1 mM range. The decreasing of electrolyte conductivity could be observed. When the Al(NO3)3 concentration was above 5.3 mM, the discharge voltage varied dramatically but no spark discharge could be found. Obtained alumina coatings were mainly α-Al2O3 and γ-Al2O3 phases by X-ray diffraction. The relative amount of α-Al2O3 increased with the aluminum nitrate concentration while decreased by adding 1.0 mM~ 2.1 mM aluminum nitrates. Moreover, the highest relative amount of α-Al2O3 was obtained by adding 1.6mM Al(NO3)3. The relative amount of α-Al2O3 gradually increased as the current density and the reaction time increased in 0.1 M NaAlO2. The morphology of PEO coatings revealed hemispherical pits on the surface in 2.1 mM Al(NO3)3 by field emission scanning electron microscopy. In contrast to non-add Al(NO3)3 additives, the thickness and surface roughness of coatings were all increased by adding Al(NO3)3 additives. Once the Al(NO3)3 concentration is above 1.6 mM, the coating surface roughness become slowly increasing, but thickness gradually decreasing. AS for cooling applications for aluminum substrate treated by PEO, the thermal resistance was about 7.0±1.2 ℃/W. It was close to the bulk alumina (5.0±1.6 ℃/W) of the thermal resistance although higher than the aluminum substrate (3.4±0.0 ℃/W). Thus, it can be used as insulation materials and heat sink. The hemispherical pits on the surface with less dense film might cause electrolyte penetrating into PEO coatings easily. Therefore, it is difficult to increase the interface impedance between the anode and the electrolyte due to the reaction of electrolytes with Al directly, which makes the delayed discharge voltage. Adding aluminum nitrate caused the α-Al2O3 of coatings increasing with the spark discharge process. It is because there was sufficient energy to make other phase transforme to stable α-Al2O3 phase, which was 5 times higher than that without the additive (15 A/dm2-60 minutes). Besides, α-Al2O3 and γ-Al2O3 in the XRD relative intensity of the signal front was 1.1 times than the literature.