Dissertations / Theses on the topic 'Microarc oxidation'

Дисертація на здобуття наукового ступеня кандидата технічних наук за спеціальністю 05.17.03 – технічна електрохімія. – Національний технічний університет "Харківський політехнічний інститут", Харків, 2016 р. Дисертація присвячена удосконаленню технології оксидування сплаву титану ОТ4–0 для створення матеріалів з протикорозійними та каталітичними властивостями. Обґрунтовано склад електролітів і експериментально визначено вплив режимів оксидування на склад, морфологію та властивості оксидних покриттів. Розроблено технологію мікродугового оксидування сплаву титану ОТ4–0 для одержання оксидних композицій Ti/TiOx∙CeOy, Ti/TiOx∙CeOy∙ZrOz, Ti/TiOx∙CeOy∙ZrOz∙CuOn. Експериментально встановлено, що церійвмісні оксидні шари виявляють високу каталітичну активність у реакціях окиснення бензолу та конверсії СО, а введення іонів міді та цирконію у церійвмісні МДО- покриття знижує температуру конверсії СО. Запропоновано склади електролітів для формування цирконій- та молібденвмісних оксидних покриттів із підвищеною термостійкістю. Визначено корозійну стійкість синтезованих матеріалів та їх каталітичну активність в модельній реакції конверсії монооксиду карбону та бензолу.
Thesis for granting the Degree of Candidate of Technical sciences in speciality 05.17.03 – Technical Electrochemistry. – National Technical University "Kharkiv Politechnical Institute", 2016. The thesis is dedicated to improvement of titanium alloy ОТ4–0 oxidizing technology for the creation of materials with anticorrosive and catalytic properties. The electrolytes composition and oxidation mode influence on the oxide coating composition, morphology and properties was established. The technology of titanium alloy ОТ4–0 microarc oxidation, the oxide composition Ti/TiOx∙CeOy, Ti/TiOx∙CeOy∙ZrOz, Ti/TiOx∙CeOy∙ZrOz∙CuOn synthesis was developed. The cerium-containing oxide layers show high catalytic activity in oxidation benzene and monoxide carbon; insertion cuprum and zirconium ions in cerium-containing oxide layers decrease monoxide carbon conversion temperature experimentally established. The electrolytes composition for zirconium- and molybdenum-containing thermostable oxide coatings synthesis is proposed. The synthesized materials corrosion resistance and catalytic activity in the model reaction of carbon monoxide conversion were determined.

Дисертація на здобуття наукового ступеня кандидата технічних наук за спеціальністю 05.17.03 – технічна електрохімія. – Національний технічний університет "Харківський політехнічний інститут", Харків, 2016 р. Дисертація присвячена удосконаленню технології оксидування сплаву титану ОТ4–0 для створення матеріалів з протикорозійними та каталітичними властивостями. Обґрунтовано склад електролітів і експериментально визначено вплив режимів оксидування на склад, морфологію та властивості оксидних покриттів. Розроблено технологію мікродугового оксидування сплаву титану ОТ4–0 для одержання оксидних композицій Ti/TiOx∙CeOy, Ti/TiOx∙CeOy∙ZrOz, Ti/TiOx∙CeOy∙ZrOz∙CuOn. Експериментально встановлено, що церійвмісні оксидні шари виявляють високу каталітичну активність у реакціях окиснення бензолу та конверсії СО, а введення іонів міді та цирконію у церійвмісні МДО- покриття знижує температуру конверсії СО. Запропоновано склади електролітів для формування цирконій- та молібденвмісних оксидних покриттів із підвищеною термостійкістю. Визначено корозійну стійкість синтезованих матеріалів та їх каталітичну активність в модельній реакції конверсії монооксиду карбону та бензолу.
Thesis for granting the Degree of Candidate of Technical sciences in speciality 05.17.03 – Technical Electrochemistry. – National Technical University "Kharkiv Politechnical Institute", 2016. The thesis is dedicated to improvement of titanium alloy ОТ4–0 oxidizing technology for the creation of materials with anticorrosive and catalytic properties. The electrolytes composition and oxidation mode influence on the oxide coating composition, morphology and properties was established. The technology of titanium alloy ОТ4–0 microarc oxidation, the oxide composition Ti/TiOx∙CeOy, Ti/TiOx∙CeOy∙ZrOz, Ti/TiOx∙CeOy∙ZrOz∙CuOn synthesis was developed. The cerium-containing oxide layers show high catalytic activity in oxidation benzene and monoxide carbon; insertion cuprum and zirconium ions in cerium-containing oxide layers decrease monoxide carbon conversion temperature experimentally established. The electrolytes composition for zirconium- and molybdenum-containing thermostable oxide coatings synthesis is proposed. The synthesized materials corrosion resistance and catalytic activity in the model reaction of carbon monoxide conversion were determined.

Les microARNs (miRNAs) sont des régulateurs majeurs de l'expression génique et ont été impliqués dans un grand nombre de processus biologiques et pathologies, dont le cancer. Le stress oxydant est une accumulation des espèces réactives de l'oxygène et génère une réponse cellulaire dans laquelle le miRNAs peuvent jouer un rôle important. Dans cette étude nous nous intéressons au rôle et à la régulation des miRNAs de la famille miR-200 (miR-141, 200a, 200b, 200c and 429) qui accumulent en réponse au stress oxydant. Dans la première partie, nous invetiguons la fonction de miR-141 et 200a dans la tumorigénèse et la réponse au stress oxydant. Nous montrons que ces miRNAs ciblent une protéine clé de cette réponse, la p38alpha. Ils sont pro-tumorigènes mais leur surexpression augmentent la sensibilité à la chimiothrapie. Dans les tumeurs ovariennes humaines, l'expression du miR-200a est associée à une signature de Stress oxydant, ce qui corrèle avec une meilleure survie des patientes. La deuxième partie est dédiée à l'étude de deux miRNAs issus d'un même locus génomique, miR-200c et 141. Nous démontrons que leur transcription est associée à celle du gène localisé en amont, PTPN6. Nous proposons deux mécanismes originaux de co-régulation de ces gènes. Le premier est que la transcription de PTPN6 ne s'arrête pas à son site de polyadénylation, mais continue jusqu'aux miRNAs. Le deuxième mécanisme est l'association physique entre les deux promoteurs à travers une boucle de l'ADN, mis en évidence par des expériences de 3C (Chromosome Conformation Capture)
MicroRNAs (miRNAs) are key regulators of gene expression and habe been implicated in a vast number of biological processes and pathologies, including cancer. Oxidative stress is generated by an accumulation of reactive oxygen species and elicits a cellular response in which miRNAs can play an important role. By investigating miRNAs affected by oxidative stress, we identified the miR-200 family of miRNAs (miR-141, 200a, 200b, 200c and 429) and studied their function and regulation. In the first part, we investigated the miR-141 and 200a function in oxidative stress response and their impact on tumorigenesis. We showed that they directly target a key stress sensor, the p38alpha. These miRNAs are pro-tumorigenic but their overexpression in xenografted tumors lead to an increased sensitivity to paclitaxel chemotherapy. In an human ovarian cancers, miR-200a expression is associated with an Oxidative Stress signature, which correlates with a better clinical outcome. These findings have potential future clinical applications in prediction and treatment of ovarian cancer. The second part is focused on two genetically clustered miRNAs, miR-200c and 141, which are co-upregulated upon oxidative stress. We demonstrated that their transcription is intimately associated to this one of the upstream gene, PTPN6. We propose two original mechanisms for the tight co-regulation of these genes. The first is a transcriptional read-through of the PTPN6 polyadenylation site leading to the transcription of the downstream miRNAs. The second mechanism, supported by our 3C (Chromosome Conformation Capture) experiments, is a physical interaction between the two promoters by a DNA-loop

La pollution atmosphérique est un problème de santé publique. En 2013, le Centre International de Recherche sur le Cancer a classé la pollution de l'air, ainsi que les particules fines (PM₂.₅), de taille inférieure à 2,5 µm, comme cancérogène de groupe I pour l'homme. Les PM₂.₅ ont la capacité de pénétrer en profondeur dans l'appareil respiratoire. En se déposant au niveau des alvéoles pulmonaires, elles entraînent une réponse inflammatoire importante notamment via la libération de médiateurs de l'inflammation et des vésicules extracellulaires (EV) par les cellules immunitaires infiltrantes ou résidentes. Dans ce contexte, ce projet de thèse comportait deux objectifs majeurs : i) déterminer les caractéristiques chimiques des PM₂.₅ collectées en milieu industrialo-urbain, en identifier les origines et la variabilité saisonnière de leur composition chimique ; ii) étudier la réponse et les EV produites par les macrophages suite à l'exposition aux PM₂.₅ et observer les effets des EV sur les cellules pulmonaires.Pour cela, des macrophages primaires ont été exposés aux PM₂.₅ prélevées à Dunkerque et leur réponse cellulaire (stress oxydant, inflammation, polarisation, miARN) a été mesurée. Les EV produites en réponse à cette exposition ont été isolées et caractérisées. Enfin, des cellules épithéliales pulmonaires, les BEAS-2B, ont été exposées aux EV libérées en réponse à l'exposition aux PM₂.₅ et les effets de cette exposition (inflammatoire, stress oxydant, miARN) ont été mesurés. Nous avons mis en évidence une inflammation et une réponse anti-oxydante dans les macrophages, ainsi qu'une modification de leur polarisation. Les PM₂.₅ entrainent une libération plus importante d'EV
Air pollution is a major public health problem. In 2013, The International Agency for Research on Cancer classified air pollution and fine particle (PM₂.₅), with size lower than 2.5 µm, as carcinogenic to humans (group I). PM₂.₅ are able to penetrate deeply in lungs. When PM₂.₅ settle in pulmonary alveolar, they lead to strong inflammatory response, with inflammatory mediators ans extracellular vesicles release by infiltrating or resident immune cells. In this context, this thesis included two major aims : i) evaluate the physico-chemical characteristics of PM₂.₅ sampled in an industrial-urban site, identify their origin and study the seasonal variability of their composition ; ii) investigate the cellular response and EV produced by macrophages in response to PM₂.₅ and study EV's effects on epithelial cells. To achieve this, macrophages are exposed to PM₂.₅ collected in Dunkerque, and cellular response (oxidative stress, inflammation, polarization, miRNA) was quantified. EV released in response to PM₂.₅ exposition was isolated and characterized. Finally, epithelial cells, BEAS-2B, are exposed to EV released by exposed and non exposed macrophages to evaluate effects from this exposure (inflammation, oxidative stress, miRNA). We observed inflammation and anti-oxidant response in macrophages after PM₂.₅ exposure, as well as polarization modification. PM₂.₅ lead to increased number of EV by macrophages

碩士
中華大學
機械工程學系碩士班
100
Microarc oxidation (MAO) is an electrochemical surface treatment process for generating oxide coatings on metals. It involves the modification of a conventional anodically grown oxide film by the application of an electric field greater than the dielectric breakdown field for the oxide. This process has been widely used to grow thick oxide layer on aluminum which can offer protection against wear and corrosion. This study investigates the effects of adding graphite and nano-diamond powders into the electrolyte solution on structure and thermal properties of grown aluminum oxides by MAO process. The 6061 aluminum alloy was used as the substrate material and pulsed DC power supply was utilized to grow oxide layer. The results show that Al4C3, Al4O4C and Al2O3 are the main phases in grown layer when graphite particles are added into the electrolyte solution. The addition of graphite particles into the electrolyte solution significantly increases the thermal emissivity and hence decreases the heat spreading resistance of the grown layers. In the case of using nano-diamond particles as the additives, only diamond, Al2OC and Al2O3 phases are observed in the grown layer. The measured heat spreading resistance decreased obviously, which can be attributed to the high thermal conductivity of diamond particles trapped inside the coatings.

碩士
龍華科技大學
工程技術研究所
97
Aiuminum alloy has been widely used in the industry because the characteristic of light mass and easy manufacturing for complicated components. Byuse of micro-arc oxidation (MAO) method, producing an oxidized ceramics coating on the surface of aluminum alloy substrate can enhance the hardness and corrosion resistance of the surface. This study aims to improve the substrate’s properties further by adding insoluble particle in the electrolyte separately. The results of hardness and corrosion resistance of the ceramic coating both show that the addition of AlO3 particles is superior to that of TiO2,or SiC and the hardnwss of the ceramic coating may reach to 1248 Hv. Add SiC particles has the best of the corrosion resistance. The particles can be added to decrease the roughness of ceramic coatings.

碩士
龍華科技大學
工程技術研究所
99
With different methods, this study aims to fabricate hydroxyapatite on the titanium and its alloy surface by use of microarc oxidation treatment. The paper is composed of two parts. The first part is that RF magnetron sputtering technology and self-made targets were used to produce hydroxylapatite covering the surface of titanium and its alloy. The second part is that a coating containing rich calcium and phosphorus composition was formed on the surface of titanium and its alloy by means of Ca2+, PO43-ions added in the electrolyte; and then, the surface was treated with a hydrothermal method in order to obtain rich content of hydroxylapatite. The study also aims to prepare the titanium and its alloy with better corrosion resistance, hydrophilic property and biologically active structure. The corrosion resistance of oxide coating was analyzed by Electrochemical Impedance Spectroscopy (EIS). Hydrophilic property was investigated by water contact angle. Furthermore, the morphology, microstructure, chemical composition and phase structures of coating were studied by FSEM-EDS and XRD. Surface roughness and thickness were examined by micro-shape analyzer (α-step) analysis. In addition, the MTT assay was used to explore the cell proliferation rate. The first part of the study shows that the surface made from the RF magnetron sputtering treatment has amorphous hydroxyapatite structure. Moreover, its cell proliferation is worse than that of the secondary part; however, it is still superior to the surface of substrate. More importantly, the surface obtained from the second part appears better hydrophilic property, richer calcium and phosphorus content, favored surface roughness and higher cell proliferation.

碩士
龍華科技大學
工程技術研究所
100
This study proposes the micro-arc oxidation and electroless copper plating two methods, the formation of a new process - micro-arc oxidation of direct copper plating process (Micro-the arc Oxidation Direct Plate Copper MAODPC) to improve the thermal resistance of the LED radiating ceramic substrate. The results of this technique can be applied to the ceramic printed circuit boards, multi-chip packaging,microelectronics and precision machinery manufacturing industry, and even anti-electromagnetic interference (EMI), so the implementation of this plan can expand the application fields of micro-arc oxidation film.technology to upgrade traditional industries, increase competitiveness and foreign. This study is divided into two parts: the first part of the current density 2、4、6、8、10 A/dm2 micro-arc oxidation, and then use the best conditions for the micro-arc oxidation ceramic layers of Electroless Copper Plating. The second part to replace commonly used in the past industrial formaldehyde as a reducing agent, the Taguchi method (hereinafter referred to as the Taguchi method) to change the operating conditions of temperature, time, copper sulfate concentration and pH of the electroless copper plating using sodium hypophosphite. The first part of the study results show that the know: According to the micro-arc oxidation film thickness, roughness and micro-arc oxidation surface electroless copper plating thickness to select current density 4A/dm2, electroless copper plating was better. The second part of the study results show that: at 85°C, CuSO4‧5H2O 6g / L, 60min, pH5 - conditions, the availability of thick copper film 22μm, and the measured resistance value of 5.8 × 10-4Ω‧cm. The results of the study to overcome the micro-arc ceramic layer resistive and ceramic layer and the copper bonding problems, the successful preparation of the three-layer structure of aluminum alloy - ceramic - metal copper.

碩士
大葉大學
醫療器材設計與材料碩士學位學程
107
Magnesium is one of the essential elements in the human body. Magnesium alloy has good biocompatibility and degradability, and has the potential to become a temporary orthopedic implant material, providing temporary mechanical support for wound or diseased tissue healing. However, magnesium alloy implants have the problem of excessive corrosion in the human body. Therefore, in order to reduce the corrosion rate of magnesium alloy in the human body and explore the factors and biocompatibility of magnesium alloy degradation, and avoid complete degradation before the repair of the implant site. Surface treatment technology is considered to be the main improvement direction of magnesium alloys. 　　In order to improve the degradation rate of AZ31B magnesium alloy in human plasma, this study used Micro-Arc Oxidation (MAO) to improve corrosion resistance. The electrolyte was prepared by using citrate, sodium hydroxide and sodium citrate. The micro-arc oxidation treatment is performed by adjusting the time by a direct current pulse method. The experimental results show that AZ31B magnesium alloy is treated by micro-arc oxidation (MAO-AZ31B), analyzed by corrosion polarization curve of Simulated Body Fluid (SBF), and AZ31B micro-arc oxidation for 30 min has the best corrosion resistance. MAO-AZ31B was immersed in SBF for 7 days, from Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Cross-alignment shows that SBF will degrade from the surface pores and cracks into the micro-arc oxidation layer, so that the pores of the porous layer are filled with Apatite, and a layer of Apatite deposit covers the MAO-AZ31B, according to In the depth analysis of atomic concentration, there was no significant difference in Ca2p and P2p between MAO-AZ31B and SBF solution for 1 day and 7 days. It was considered that there was a certain thickness of Apatite deposit for 1 day. Survival of Neuro2a mouse neuroblastoma cells with different numbers of AZ31B test strips for 48 hours showed significant inhibition of cell growth and death.

碩士
龍華科技大學
工程技術研究所
95
Aluminum alloy has been widely used in the industry because the characteristic of light mass and easy manufacturing for complicated components. By use of microarc oxidation (MAO) method, this study produces an oxidized ceramics coating on the surface of aluminum alloy substrate. The objective aims to enhance the hardness and corrosion resistance of the aluminum alloy substrate. This study divides into theoretical and applicable parts: the theoretical results indicate that the α-Al2O3 content and hardness of the ceramic coating are increase with the current density. The results of AC impedance method and polarization curve both show that the corrosion resistance of the ceramic coating is superior to the substrate at different current density and the corrosion rate may be retarded to three orders or more. The applied results indicate the optimal operational condition by use of Taguchi experimental plan: sodium tungstate concentration 8g/L, TiO2 0.5g/L, current density 7A/dm2 and reaction time 60min. Furthermore, the optimal hardness of the ceramic coating may reach to 1380 Hv.

碩士
國立臺灣科技大學
機械工程系
102
Zirconia composite coating was prepared on 6061 aluminum alloy by microarc discharge oxidation. Tthe microstructure, hardness, XRD and toughness have been investigated in this study. First, the the experiments were carried out in alkaline electrolyte containing K2ZrF6 at different negative voltage. The results indicated that it is easier to form alumina than Zirconia for MDO coating on aluminum alloy, because Zr(OH)4 partilce were formed in alkaline easily. Compare with formation of alumina MDO coating, Zr(OH)4 partilce made a lot of pores during coating and which not uniform coating. K2ZrF6 dissolves in acidity electrolyte to prevent formation of Zr(OH)4 partilce. But the acidity electrolyte will corroded the cathode lead zirconia composite coating not to be compact. The problem of Zr(OH)4 partilce precipitation was solve by adding dispersant (NaPO3)6. There is a good experimental result that was carried out at negative voltage 100 V what XRD result indicated t-ZrO2 mainly. In order to get more compact coating, the sodium aluminate (NaAlO2) 2 g/L was added into the K2ZrF6 electrolyte. There is also a good experimental result was carried out at negative voltage 100 V and the coating was more compact than before and XRD result indicated t-ZrO2 mainly. When the contain of aluminum sodium aluminate in the K2ZrF6 electrolyte is greater than 4 g / L, the thickness and density were decreasing . Because sodium aluminate is weak alkaline, it caused the problem of Zr(OH)4 partilce precipitation when sodium aluminate is too much. XRD result indicated ??Al2O3 mainly. Above MDO result were carried by voltage control. When the coating thickness is increasing ,the current is decreasing , and than there is no energy to let coating to grow and to be more compact . In order to get more compact and more thicker coating, experiment process was change from voltage control to current control. The result show that the thickness were increase without sodium aluminate, but there were still some big pores in the coating. Better result were carried out with sodium aluminate by 0.3~0.5A in K2ZrF6 electrolyte.. And then the density were more compact by increasing contain of K2ZrF6. The voltage was limited at prosess of later stage to avoid damage from larger size of arc. XRD result indicated t-ZrO2 mainly, and the more m-ZrO2 were forming, when the contain of K2ZrF6 increasing in electrolyte. Zirconia MDO coating have higher toughness than alumina MDO coating, but it cause toughness decrease when K2ZrF6 contain too much in electrolyte . Corrosion resistance test results show that when the contain of K2ZrF6 increasing in electrolyte, the thickness and density of the coating increase , and the corrosion resistance increased. The corrosion resistance decreases due to the large pores between the surface layer and the substrate ,and the cracks were throughout the coating, when K2ZrF6 reached 22g / L.

博士
逢甲大學
材料科學所
94
The generation and collapse of bubbles at the surface of a component caused by repeated sudden pressure changes of fluid, can lead to cavitation erosion. The collapsing bubbles result in the emission of shock waves and micro-jets which leads to plastic deformation, fatigue, fracture or material loss. It is hard to observe the damage of cavitation erosion until substantial material removal for a long service time. There are many surface modification technologies to improve cavitaiton erosion for structural materials today, such as thermal spray, ion nitriding and laser surfacing. Most of them are high cost and difficult to scale up. Wet process maybe an alternative to the above mentioned techniques in cavitaiton erosion resistant surface modification. In this study, Ni-P-SiC composite coating and microarc oxidation (MAO) are applied on AISI 1045 carbon steel and AA 7075 aluminum alloy, respectively. The Ni-P-SiC composite coating can provide high surface hardness and the Ni-P matrix presents high corrosion resistance to be expected for synergistically improving cavitation erosion resistance. Ni-P, Ni-P-(micro)SiC and Ni-P-(nano)SiC with and without post heat treatment were evaluated. Microarc oxidized surface layer with extreme high hardness may also be expected to the improvement cavitation erosion resistance for metal aluminum. However the high surface roughness of microarc oxidized layer was suspicious to be negative to cavitation erosion resistance, an elastomer infiltration is proposed to the cavitation resistance improvement by absorbing the shock waves and micro-jets energy. The cavitation erosion test was carried out in distilled water and 3.5 wt.% NaCl solution. The results show that electroless nickel plating without post heat treatment show poor cavitation erosion resistance due to the electroless nickel film peeled off easily during cavitation erosion and it can be improved by post heat treatment. The failure of the heat treated specimen is due to crack extend along the surface defect and fatigue. The specimens of Ni-P-(micro)SiC composite coating can prevent from the crack extend but high surface make high mass loss. The specimens of Ni-P-(nano)SiC composite coating with post heat treatment has best cavitation erosion resistance due to the (nano) SiC can prevent from the crack extend in the matrix and also have smooth surface. After microare oxidation treatment, the surface hardness of AA 7075 aluminum alloy can reach 1000 HV in both silicate and phosphate electrolytes. The oxide layer thickness of the silicate electrolyte is about 100 mm and phosphate electrolyte is about 20 mm. However, the high surface roughness is caused by discharge during microarc process and show poor cavitation erosion resistance. The great hardness difference between the oxide layer and Al substrate may be the reason for poor cavitation erosion resistance. Elastomer infiltration give much improvement of the cavitaiton erosion resistance of micoarc oxidized treatment specimen with bubbles presented at the interface of elastomer and oxide layer. The number of bubbles was affected by the surface roughness In this study, Ni-P-(nano)SiC composite coating was found to effectively protect steel substrate from cavitation erosion due to synergistically strengthening of nano-SiC and high hardness matrix that prevent crack propagation. The high surface roughness of microarc oxidized, although high hardness gives rise to poor cavitation erosion resistance due to easily peel off of the oxidized layer. After elastomer infiltration on the microarc oxidized surface, the elastomer absorbs micro-jets energy to improve the cavitation erosion resistance of the microarc oxidized as well as blank specimens.

碩士
龍華科技大學
工程技術研究所
101
This study attempted to utilize microarc oxidation to prepare titanium oxide layer containing silver. The present paper was divided into two parts: the first part was to prepare titanium oxide layer via microarc oxidation. The second part tried to use various approaches for adhering silver to the surface of titanium oxide layer. Field Emission Scanning Electron Microscopy (SEM) was used to observe the surface morphology produced by different approaches. Energy Dispersive Spectrometer (EDS) was employed to observe surface composition, whereas Micro-Shape Analyzer (α-step) for analyzing surface roughness and thickness and X-ray generator (XRD) for observing structure and phase composition of microarc oxidation layer. Finally, the MTT assay was conducted to compare the cell activity and antibacterial effects of Titanium Oxide layer containing silver. Research findings of the first part showed that: the experiment successful prepared the crystalline phase of rutile and anatase with pores in the titanium oxide layer, in which the thickness, size of surface pore, roughness and hydrophilic of titanium oxide will be elevated as the reaction voltage increased. Results of the second part of the study indicated that: the silver reduction treatment method on titanium oxide layer prepared by microarc oxidation could adhere silver particles to the surface of the titanium oxide layer. After bioassay, it was found that the biological activity of titanium oxide layers containing silver particles was lower than that of titanium oxide layer without containing silver, but both biological activities hadbetter biocompatibility. Throughout bacterial culture, the quantity of bacteria around titanium oxide layer with the addition of silver began to reduce, and the higher the silver content resulted in the less the quantity of bacteria around.

This thesis research investigates the surface properties and performances of gold nanoparticles, microarc oxidation coating, and epitaxial nano-twinned copper film. The research aims to understand the critical behavior of material surfaces in order to facilitate design and development of new materials for tribological applications. The research will focus on improving of the gun barrel performances. Experimental approaches will be used for combining analysis with basic thermal energy transfer principles. Results obtained here will be used for developing new materials to be used in facilitating gun barrels. Experimental approach includes scanning calorimetry-thremogravimetric analysis, tribological testing, and potentiodynamic polarization. The fundamental understanding obtained here will be beneficial for the gun barrel design, manufacturing, and military technologies followed by the results of experiments with different three types of materials. The results of this research showed that the coatings with microarc oxidation and nano-twinned structure improved wear resistance from the tribological examinations and size of AuNPs affected their thermal behaviors measured by differential scanning calorimetry and thermogravimetric analysis method.

碩士
大同大學
材料工程學系(所)
97
Aluminum alloys exhibit low density, high specific strength, good wear resistance, anti-corrosion and great workability, it can be applied in wide application on commodity sector, automobile, aerospace and national defense industry. Due to the concepts of environmental consciousness and energy saving, valve metal and its alloys has increasingly used in automobile and bicycle recently. Meanwhile, aluminum is an active element easily combined with oxygen, leading to a formation of 5 nanometer inert thin film. However, the thin and uneven naturally-formed thin film exhibits poor corrosion and wear resistance. The corrosion resistance can be improved by suitable electrochemical reaction processes through reducing the corrosion current density and raising the reductive electrochemical potential of the oxidation layer. The microarc oxidation (MAO) technique is a promising method for the valve metal in terms of the improvement of corrosion resistance and wear resistance. The microarc discharge formed on some specific area at high temperature will lead to the oxidation and sintering reaction on the surface of anode workpiece. The insulating MAO thin film can provide a barrier for the passage of current, which will be beneficial to prevent the substrate from corrosion. The various high voltages and pulsed bipolar waveforms of the MAO experiment are employed on the surface of 7075-T6 Al alloy in this work. The aqueous electrolytes used in MAO process include NaAlO2 and KOH. In the same electrolytic solution, Ton(+) is the most influential parameter on film growth rate under the same pulsed frequency. The deposition rate increased while Ton(+) is increasing. But it will spall off the MAO film when raising the pulsed frequency. It seems that the spallation phenomenon is related to the over-shooting part of the pulsed wave on the anodic workpiece. When both the frequency and duty ratio is low, the more compact layer is formed. The compact layer will lower the corrosion current density. However, the corrosion resistance of samples with compact layer can also be increased even if the duty ratio is high. The improvement can be attributed to the superior coverage at high duty ratio. Moreover, the outer porous layer was composed of γ-Al2O3, and the inner compact layer was composed of α-Al2O3. To increase the cathodic pulse ratio will get higher growth rate. Furthermore, the film thickness is nearly 60μm and the average growth rate approaches 3 μm/min while the electric parameter is 2000Hz, Ton(–) 16% and working time 20 minute. In brief, Ton(-) has positive effect on promoting the growth rate. The lower ratio of Ton(-) produced much more porous layer in contrast to thick MAO films prepared at the high ratio of Ton(-). The experiment shows: the surface of substrate will form the alumina phase by MAO technique. The MAO coatings can decrease the corrosion current density by 4 order (10-10A/cm2) and raise the corrosion potential by 0.5 volt. The growth rate of MAO film is nearly to 3μm/min and micro-hardness of 1800Hv.

Magnesium and its alloys are extensively used for various industries such as aerospace, automobile and electronics due to their excellent properties such as low density, high strength and stiffness and electromagnetic shielding. However, the wide spread applications of these alloys are limited due to the undesirable properties such as poor corrosion, wear and creep resistance and high chemical reactivity. These alloys are highly susceptible to galvanic corrosion in sea water environment due to their high negative potential (-2.37 V vs SHE). The effective way of preventing corrosion is through the formation of a protective coating, which acts as a barrier between the corrosive medium and the substrate. Many surface modification methods such as electro/ electroless plating, conversion coating, physical and chemical vapour depositions, thermal spray coating etc., are available currently to improve the corrosion resistance of Mg alloys. Of these methods, the electroless nickel plating has gained considerable importance because of its excellent properties such as high hardness, good wear and corrosion resistance. The properties of binary electroless nickel coating have been further improved by the addition of a third element such as cobalt, tungsten, tin and copper etc. It has been reported that the addition of tungsten as the third element in the Ni-P improves the properties such as hardness, wear and corrosion resistance, thermal stability and electrical resistance. Magnesium alloys are categorized as a “difficult to plate metal”, because of their high reactivity in the aqueous solution. They react vigorously with atmospheric oxygen and water, resulting in the formation of the porous oxide/ hydroxide film which does not provide any protection in the corrosive environment. Further, the presence of this oxide film prevents the formation of a good adhesive bond between the coating and the substrate. The surface treatment process for removal of the oxide layer is very much essential before plating the Mg alloy. Currently two processes such as zinc immersion and direct electroless nickel plating are adopted to plate Mg alloys. Etching in a solution of chromate and nitric acid followed by immersion in HF solution to form a conversion film is necessary for direct electroless nickel (EN) plating of Mg alloy. However, strict environmental regulations restrict their usage because of hazardous nature. Expensive palladous activation treatment is a well-known process as a replacement for chromate-HF pretreatments for Mg alloys. It has been reported that EN plating has been carried out over Mg alloys by using conversion coating followed by HF treatment. Formation of an intermediate oxide layer by electrolytic methods is also one of the ways these toxic pretreatments can be avoided. Microarc oxidation (MAO) is an environment friendly surface treatment technique which provides high hardness, better corrosion and wear resistance properties for the Mg alloys. EN coating has been prepared on MAO layer for improving the corrosion resistance. These MAO/EN composite coatings have been prepared using chromic acid and HF pretreatment process. As the replacement for the chromate-HF pretreatment, SnCl2 and PdCl2 sensitization and activation procedures respectively were adopted over MAO layer for the deposition of Ni-P coating. From the above reported literature, it can be inferred that for the activation of inert MAO layer to deposit electroless nickel coating, the hazardous chromate/HF and highly expensive PdCl2 activation processes were followed. Therefore, there is a need for identifying an alternative simple and cost effective pretreatment process for the deposition of electroless nickel. It is well known that borohydride is a strong reducing agent that has been used for the deposition of Ni-B coatings. In the present study, an attempt has been made to utilize borohydride in the pretreatment process for the reduction of Ni2+ ions over the MAO interlayer, which provides the nucleation sites for the deposition of Ni-P coating. Ni-P and Ni-P/Ni-W-P duplex coatings were deposited from stabilizer free carbonate bath on AZ31B Mg alloy to improve the corrosion resistance of the base substrate. The conventional chromate and HF pretreatment processes were followed for the deposition of electroless nickel coating. In order to improve the corrosion resistance of the duplex coating, post treatments such as heat treatment (4 h at 150°C) and chromate passivation were adopted. EDX analysis of AZ31B Mg alloy showed the presence of 2.8 wt.% of Al and 1.2 wt. % Zn with the balance of Mg for AZ31B Mg alloy. After the chromic acid and HF treatment, the magnesium content was reduced from 90.0 wt % to 54.9 wt%, which could be due to the incorporation of chromium on the surface layer. The surface showed about 17.8 wt. % of F. The alloy exhibited the roughness of about 0.29± 0.01µm after mechanical polishing. The roughness value was significantly changed after the chromic acid treatment processes. The maximum roughness of about 1.28±0.06 µm was obtained after the HF activation. XPS analysis confirmed the existence of chromium in +3 oxidation state after the chromic acid treatment. The Ni-P coating thickness of about 25 microns was obtained in 1 h and 15 min. In the case of duplex coatings, Ni-P plating was done for 45 min. to obtain approx. 17 microns thickness and Ni-W-P plating was done for 1.15 h to obtain a thickness of approx. 10 microns, resulting in a total thickness of 25 ± 5 microns. Ni–P coating exhibited nodular morphology with porosity. The size of these cluster nodules were of about 10 µm in diameter. On the other hand, the duplex coating exhibited a less nodular, dense and smooth appearance. From the compositional analysis it was found that Ni–P coating contained about 6 wt. % P. In the case of duplex coating, the P content was reduced to 3 wt % due to the incorporation of about 2 wt% of tungsten. In corrosion studies, the potentiodynamic polarization data obtained for bare Ni-P coating in 0.15 M NaCl solution exhibited a higher current of about 218 μA/cm2 as compared to the substrate due to the porosity of the coating. However, the Ni-P/Ni-W-P duplex showed 55 times improvement in corrosion resistance, vis-a-vis Ni-P due to the dense nature of the coating. The corrosion resistance of the coatings increased in the following order: Ni-P < bare alloy < duplex < duplex-passivated < duplex-heat treated passivated. In EIS study, the Nyquist plot obtained for the bare substrate and Ni–P coating showed the presence of inductance behavior at the lower frequency region due to the adsorption of electroactive species over the substrate through the porous oxide layer. However, the passivated and duplex passivated coatings exhibited only capacitive behavior due to their compact nature. From the above, it can be concluded that, direct deposition of Ni-P coating over the chosen Mg alloy using chromic acid and HF pretreatment process resulted in porous morphology, which affected the corrosion resistance of the coating. As an alternative strategy, the microarc oxidation conversion coating was developed on Mg alloy and characterized. The MAO coating was developed using silicate electrolyte at three different current densities (0.026, 0.046 and 0.067 A/cm2) for about 15 min. With respect to the MAO coating, an increase in the current density increased the pore diameter and decreased the pore density. The surface of the coating became coarser and rough. The cross-sectional morphology of the coating showed two district layers namely the dense and thin inner layer and a porous thick outer layer. The thickness of the coating increased with increase in current density. MAO coating prepared at an intermediate current density of 0.046 A/cm2 exhibited a higher thickness of about 12 µm and a further increase in current density showed a decrease in thickness, due to the greater rate of dissolution of Mg, relative to the rate of deposition. The surface roughness of the MAO coatings also increased with increase in current density. The Ra value increased from 1.39±0.06 to 3.52±0.17 µm with increase in current density. XRD peaks obtained for the Mg substrates corresponded predominately to magnesium. However, the coated specimens showed the presence of peaks corresponding to Mg2SiO4 along with Mg and MgO. The corrosion measurements for the bare substrate and MAO coatings were carried out in 3.5% NaCl medium (0.6 M). Based on potentiodynamic polarization studies, the MAO coating prepared at 0.046 A/cm2 exhibited a lower corrosion current density with a higher Rp value, which was about five orders of magnitude higher than the bare substrate, due to the dense nature of the coating. In EIS study, MAO coatings were fitted with the two time constants equivalent circuit containing outer porous layer and inner barrier layer. The barrier layer resistance values were higher than that of porous layer resistance, which indicated that the resistance offered by barrier layer was higher than the porous layer. The total resistance value obtained for the coating prepared at 0.046 A/cm2 were higher compared to the other coatings, which attested to its better corrosion resistance. The electrochemical noise measurement was carried out for longer immersion durations upto 336 h in 3.5% NaCl solution. The noise resistance value obtained for the base Mg alloy was about 100 Ω at 1h immersion, whereas for the MAO coating prepared at 0.04 A/cm2 a maximum value of about 34.8 MΩ was achieved and it was retained even after 96 h of immersion. Mott–Schottky analysis showed that the oxide layer on magnesium substrate acted as a n-type semiconductor, whereas the MAO coatings exhibited p-type semiconductor behavior. The MAO coating obtained at an intermediate current density showed a higher acceptor density and the flat band potential, which resulted in the better performance of the coating in corrosive environment. In another set of investigations, the Ni-P and Ni-P/Ni-W-P coatings were deposited on AZ31B Mg alloy with MAO coating as an interlayer. The MAO layer was activated by a simple borohydride pretreatment process. During the pretreatment process, the MAO coating was subjected to mild alkali treatment, immersion in the Ni-P plating solution and finally immersion in borohydride solution. During each pretreatment step, the sample was characterized for their surface morphology and composition. The surface morphology showed the distribution of spherical particles over the surface of MAO coating after immersion in the Ni-P plating solution. EDX analysis showed the presence of 2.4 wt. % of Ni, which confirmed that Ni ions were adsorbed over the surface of the MAO coating during the pretreatment process. XPS analysis carried out after immersion in the Ni-P plating solution indicated that Ni existed in +2 oxidation state. The surface became smooth and uniform with flake- like morphology after the borohydride treatment, which indicated that the surface was etched by the borohydride solution. EDX analysis showed the presence of 1.8 wt.% of Ni after borohydride reduction. XPS analysis confirmed the reduction of nickel to the zero oxidation state. Additionally, MAO/Ni-P and MAO/Ni-P/Ni-W-P duplex coatings were developed on MAO coating after a simple borohydride pretreatment. Ni-P and duplex coatings showed uniform and dense nodular morphology without any defects, which clearly indicated that the borohydride treatment provided a uniform and homogeneous active surface for the deposition of electroless nickel based coatings. Borohydride pretreatment process resulted in excellent bonding between MAO/Ni-P layers in the cross section. Based on potentiodynamic polarization studies, the corrosion current values obtained for MAO/ Ni-P and MAO/Ni-P/Ni-W-P duplex coatings were about 1.44 and 1.42 µA/cm2, respectively. The coating showed about 97 times improvement in corrosion resistance compared to the bare substrate, attesting to the dense nature of the coating. In EIS study, the single time constant equivalent circuit was used for fitting the spectra, which pertained to the coating /electrolyte interface. The single time constant could be attributed to the pore-free dense, uniform coatings developed over the MAO interlayer. For the MAO/Ni-P and MAO/Ni-P-Ni-W-P duplex coatings, the charge transfer resistance of about 15 and 11 kΩcm2 were obtained for duplex and Ni-P coatings, which reinforce the better corrosion protective ability of the coating. The above investigation confirms that MAO coatings have good corrosion resistance in the aggressive chloride medium. Consequently, they can serve as an ideal interlayer for the deposition of the electroless nickel coating. Even if the electroless nickel coating is found to fail in harsh environments, the MAO interlayer can protect the base substrate due to its higher corrosion resistance. It is also noteworthy that the borohydride treatment provides better adhesion between the MAO/Ni-P interlayer.