Metalurji ve Malzeme Mühendisliği Lisansüstü Programı - Yüksek Lisans

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  • Öge
    Characteristics of cold sprayed titanium based coatings
    (Institute of Science And Technology, 2017-08) Paksoy, Ahmet Hilmi ; Baydoğan, Murat ; Atar, Erdem ; 506141430 ; Metallurgical and Materials Engineering ; Metalurji ve Malzeme Mühendisliği
    Metallic biomaterials are the most common biomaterial group among synthetic biomedical applications. This material group differs from the other biomaterials owing to high dynamic and static strength, enough resistance to corrosion, wear, and fatigue. Moreover, cobalt-chromium-based alloys are one of the most favorable metallic biomaterials with titanium alloys and stainless steels due to well combination of high strength, corrosion resistance, and formability. Nevertheless, localized corrosion, ion releasing from the worn surfaces to the human body, and other undesirable situations may lead to early failure and limit their service life to 10-12 years. Abrasion in the human body is induced ion releasing and when the amount of the released ions reach a specific level, harmful effects can be generated. Another disadvantage of these material groups is poor osteointegration properties and low biocompatibility. Biocompatibility is the crucial biologic property for biomaterials because it combines almost all the biologic properties such as bioactivity and biodegradability. Higher service life with sustainable superior properties enables the comfort patient and does not necessitate painful reimplantation operations. In order to overcome the disadvantages of cobalt-chromium alloys without sacrificing their superior properties, surface modification techniques are attractive research of a topic in recent years. In this master thesis study, I preferred to modify the surface of cobalt-chromium alloy with the deposition of titanium-based feedstock by the cold gas dynamic spray process. Additionally, an improvement on bioactive property aimed at the formation of titanium oxide layer on the outermost surface via the application of thermal oxidation process. Titanium was chosen for the based material because of its high biocompatibility. Although natural oxidation of titanium provides good bioactivity and higher corrosion resistance, this oxide layer has low mechanical properties and cannot preserve stability against any wear condition. Hence, in the literature, lots of studies focused on the development of more stable oxide layers on the surface of titanium and titanium alloys. Both depositional and diffusional surface modification techniques such as micro-arc oxidation and thermal oxidation etc. are used for developing titanium oxide on the surfaces. It is known from the binary phase diagram of titanium and oxygen, oxidation reaction starts at temperatures above 300oC. Specific to thermal oxidation, increasing process temperature enables the formation of the more stable oxide layer and a thicker oxygen diffusion zone beneath it. Additionally, the hardness of the oxygen diffusion zone gradually decreases with respect to the depth and this situation allows a smooth transition from the main material to the brittle oxide layer and increases the wear resistance. All these advantages highlight thermal oxidation compared to the other oxidation techniques. The cold gas dynamic spray (GCDS) process was preferred because of its simplicity and high efficiency. The possibility to form coating without any chemical reaction and/or melting, renders GCDS one of the most sufficient techniques among the other thermal spray techniques, especially for surface modification of heat-sensitive metals with high affinity to oxygen such as titanium. Moreover, extreme kinetic energies of particles xxii generate high impact and plastic deformation during the process. Hence, it is possible to produce coating harder than substrate material. As a part of a research project (TUBITAK Project No: 214M246), in this master thesis study effect of additives on the structural feature, mechanical and biological properties of titanium-based coatings were investigated with the aim of improving surface properties of the cobalt-chromium alloy. In that respect, feedstock containing titanium, aluminum, copper, zinc, and iron oxide (in magnetite form) were prepared at different ratios and deposited onto ASTM F75 biomedical grade cobalt-chromium alloy by the cold gas dynamic spray process. Then, cold sprayed substrates were thermally oxidized at 600oC for 60 hours in order to obtain a stable bioactive titanium-based oxide layer on the outermost surface with different biological behaviors with respect to compositions. Characterization of the coating was executed under the title of three different groups; structural characterization, mechanical properties, and biological tests. Structural surveys made by X-Ray diffraction analysis, optical and scanning electron microscope observations including area and line scan energy dispersive X-Ray spectroscopy. Additionally, roughening effect of thermal oxidation was investigated. In order to obtain mechanical properties; hardness measurements from cross-section and surface, tribological tests against alumina balls in dry, serum, and SBF (at 36.5 oC) testing environment were performed. In the last part of the study, biological tests were done to determine the effect of additives on bioactivity. In the early stage of the study, ASTM F75 grade cobalt-chromium alloy was deposited by six different feedstock compositions. This feedstock consists of 92 wt.% Ti – 5 wt.% Cu – 3wt.% Fe3O4, 94 wt.% Ti – 3 wt.% Cu – 3wt.% Fe3O4, 95.5 wt.% Ti – 1.5 wt.% Cu – 3wt.% Fe3O4, 87 wt.% Ti- 8 wt.% Al – 5 wt. % Zn, 89 wt. %Ti – 8 wt.%Al – 3 wt.% Fe3O4, 84 wt. %Ti – 8 wt.%Al – 3 wt.% Fe3O4 - 5 wt.% Zn. Aluminum was used to deposit titanium successfully which has low deformation capability, without sacrificing bioactive property. While copper and zinc were added to feedstock as an antibacterial agent, iron oxide (in magnetite form) was added to improve bioactivity and enlarge the application area of a coating such as magnetic resonance imaging contrast agent, targeted drug delivery due to magnetic behavior. After deposition of feedstock, coatings thermally oxidized at 600 oC for 60 hours in normal atmospheric conditions. Characterization of the coatings started with cross-sectional optical microscope investigations of Cu-containing coatings. The copper content in the feedstock was decreased to avoid from possible formation of toxic copper oxide phases during thermal oxidation. Initial obtained results showed that when the copper content decrease, coatings show high porosities and discontinuities on the interface of coating and substrate. Hence, 5 wt.% Cu containing feedstock was chosen with the composition 92 wt.% Ti – 5 wt.% Cu – 3wt.% Fe3O4 for further structural investigations, determination of mechanical properties, and biological tests. Scanning electron microscope examinations showed that all the coatings before thermal oxidation exhibit almost no porosity. On the other hand, porosity level increased to rational values after thermal oxidation while around 2 μm thick oxide layer formed on the outermost surface. Moreover, area and line scan energy dispersive X-Ray spectroscopies showed a good distribution of additives in the coating and oxide layer. Once more for all coating, determination of oxygen-containing phases even in close regions to substrate shows well diffusion of oxygen during thermal oxidation. More specifically, in the observation of Cu-containing coating, titanium-oxygen solid xxiii solution regions were detected around all the copper particles and these phases probably inhibit the oxidation of copper which is the favorable result for the aim of the study. For the Al-containing coatings, possible titanium aluminum intermetallic compounds were detected around aluminum particles. Moreover, titanium-zinc intermetallic compounds and preservation of metallic zinc were observed. Lastly, particle boundaries were established as settlement places for iron oxide particles. It is understood from the result of X-ray diffraction analyses; before thermal oxidation, only peaks of feedstock elements were determined which proves no chemical reaction during deposition (except iron oxide it may be because of presence less than detachable amount 3 wt. %. ) took place. On the other hand, after thermal oxidation, all of the coatings commonly have titanium dioxide (TiO2) in rutile form and alfa titanium, which was detected probably under the oxide layer by the penetration of x-rays during the test. Furthermore, roughness measurements are done before and after thermal oxidation and 5-6 times increment was observed. This increment in roughness provides the bio-activity of the coating via increasing the interaction of the surface and the surrounding tissues. Cross-sectional and surface hardness, measurements were executed with Vickers hardness and depth-sensing micro-nano hardness tester with the Vickers indenter under the load of 25 g and 100 mN respectively. All the coatings showed similar hardness values; before thermal oxidation around 80 HV0.025, after thermal oxidation from the crossection around 450 HV0.025 from the surface around 900 HV. These results showed that different kinds of additives did not directly affect the hardness of coatings and/or oxide layer. Tribological performance of coatings and substrate were performed under 1N load for 25m sliding distance in dry, serum, and simulated body fluid (SBF) sliding conditions. While for the substrate, significant wear track was obtained, for the coatings wear tracks could not be detected by a profilometer. Only trimming of the rough surface was observed and the depth of tracks was not possible to measure on a profilometer. These results also were supported by optical micrographs of worn surfaces of coatings and alumina balls. The surface of counter bodies is generally clean with the existence of some wear scratches and these observations may be thought just simple sliding occurred between the coating and alumina balls for all the cases. Comparative steady-state friction coefficients for coatings and substrate were obtained after wear test and results are showing in dry sliding conditions coatings present lower value than substrate in contrary to serum and SBF environment. This behavior may be explained by chemical reactions on the surface of coating in liquid media and extreme contact pressures (around 840 MPa). Lastly, when the coatings are individually compared with each other's, Zn-containing coatings exhibit the lowest steady-state friction coefficient due to the possible lubricant effect of metallic zinc or/and zinc-titanium intermetallic compounds. When the in vitro bioactivity tests were of concern, all the coatings showed high bioactivity after 4 weeks of immersion in SBF. SEM micrographs of the surfaces demonstrate the formation of structures in the same morphology as hydroxyapatite. Among all four different compositions, copper-containing coating showed earlier interaction with SBF, as a result, hydroxyapatite-like structures formed even after 1 week of immersion. In general, mechanical properties and bioactivity of ASTM F 75 grade cobalt-chromium alloys were improved by the formation of titanium-based multilayered coating via sequential application of cold gas dynamic spray and thermal oxidation. These observations show a promising future for the possible biomedical application of cobalt-chromium alloys without sacrificing superior properties.
  • Öge
    Gümüş oksit-çinko pil üretimi
    (Fen Bilimleri Enstitüsü, 2011-08-04) Varol, Utku Can ; Timur, Servet İ. ; 506081216 ; Metalurji ve Malzeme Mühendisliği ; Metallurgical and Materials Engineering
    Gümüş oksit-çinko pilleri çok yüksek enerji yoğunlukları ve kararlı deşarj eğrileri sayesinde diğer pillerden ayrılmaktadır. Bu özellikleri 20. yüzyılın ortalarından itibaren araştırmacıların dikkatini çekmiştir. Geliştirilen piller yüksek maliyetleri nedeniyle ticari uygulamalarda kullanılamasalar da özellikle askeri amaçlarla ve uzay çalışmalarındaki kullanımları oldukça yaygındır.İlk üretimlerinden bu yana yapısal olarak birçok değişim geçiren gümüş oksit-çinko pilleri dünya çapında belirli şirketler tarafından üretilir ve üretilen bu piller genellikle askeri açıdan güçlü olan ABD, Almanya, Fransa gibi ülkelerin ürettiği torpido ve füzelerde enerji sağlayıcı birimler olarak kullanılır.Ticari olarak kol saatleri, işitme cihazları, hesap makinaları vb. uygulamalarda kısıtlı kullanımları bulunmasına karşın son yıllarda dizüstü bilgisayarlarda kullanılan pillerde gümüş oksit-çinko pillerin kullanılmasının daha avantajlı olacağı fark edilmiş ve bu alanda çalışmalar yapılmaya başlanmıştır.Bu çalışmada, gümüş oksit-çinko pillerin gümüş oksit pozitif elektrotunun üretimi, gümüş oksit toz presleme yöntemiyle gerçekleştirilen geleneksel yöntemden farklı bir şekilde yapılmıştır. Gümüş oksit elektrotlar karbon elyaf mat üzerine gümüş elektrolizi ve anodik oksidasyon işlemleri gerçekleştirilerek üretilmiş, daha sonra üretilen bu elektrotların elektrokimyasal özellikleri kurulan pil hücrelerinde test edilmiştir. Bu çalışmada ülkemizde ilk kez gümüş oksit-çinko pillerinin, geleneksel üretim yöntemlerinden farklı bir yöntemle üretilen kompozit elektrotlar ile laboratuvar ortamında oluşturulması amaçlanmıştır.
  • Öge
    Yoğrulabilir (6063) alaşımın yenilikçi kısmi - katı yüksek basınçlı döküm teknolojisi ile üretimi
    (Fen Bilimleri Enstitüsü, 2019-06-13) Süslü, Yekta Berk ; Keleş, Özgül ; 506161425 ; Metalurji ve Malzeme Mühendisliği ; Metallurgical and Materials Engineering
    Alüminyum alaşımları; düşük yoğunlukları, yüksek özgül mukavemetleri, iyi korozyon dirençleri ve mükemmel şekilalabilirlikleri sayesinde mühendislikte yaygın olarak kullanılmaktadır. Gelişen teknoloji ile birlikte otomotiv, havacılık, savunma ve elektrik-elektronik gibi endüstriyel alanlarda kullanımı artmaktadır. Yenilenemez enerji kaynaklarının gittikçe azalması ve temininin zaman geçtikçe zorlaşması, enerji sarfiyatlarında yapılacak gelişmeleri tetikleyen başlıca unsurlardan olmuştur. Bununla birlikte birçok endüstride ürün hafifletme, yapılan çalışmaların ana konusu haline gelmiştir. Alüminyum alaşımları; içerdikleri silisyum oranına bağlı olarak döküm alaşımları ve yoğrulabilir (dövme) alaşımları olarak sınıflandırılabilmektedir. Döküm alaşımları; ergiyik halde yüksek akıcılığa sahiptir ve kum kalıba döküm, kokil kalıba döküm ve basınçlı döküm gibi teknolojiler ile şekillendirilir. Yoğrulabilir alaşımlar ise çok daha az silisyum içeren (% ağ 0-4), düşük akıcılık özelliklerinden ötürü döküm ile şekillendirilebilmesi güç alaşımlardır. Çoğunlukla ekstrüzyon ve dövme gibi plastik şekil verme teknikleri ile şekillendirilirler. Yoğrulabilir alaşımların döküm ile şekillendirilebilmesi, yarı-katı şekillendirme yöntemleri ile mümkün hale gelmektedir. Yarı-katı şekillendirmenin akıcılığı arttırmasının yanı sıra, dökümde görülmesi muhtemel porozite ve çekinti hatalarının da önlenmesinde büyük katkısı bulunmaktadır. Mekanik ya da elektromanyetik bir kuvvet ile karıştırılan ve soğutulan ergiyik alaşımlar, küresel birincil fazlar oluşturur. Küresel fazlar, enjeksiyon kuvveti ile birbirleri üzerinden daha kolay kayabildiğinden, ergiyik akıcılığı artar. Tiksotropi ile açıklanan planar/laminar akış, ergiyiğin türbülans olmadan akmasını ve kalıbı daha sakin doldurmasını sağlamaktadır; planar/laminar akış sayesinde hazne ve kalıp içerisinde bulunan hava, türbülanslı akışın aksine ergiyik içinde hapsolmaz ve bu da yüksek basınçlı dökümde yaygın bir şekilde görülen porozite hatasınının önlenmesini sağlar. Kalıp ile ergiyik arasındaki sıcaklık farkı da azaldığından, hem çekinti sorunu önlenir hem de kalıbın maruz kalacağı ısıl deformasyon şiddeti azaltılır, bu da kalıp ömrünü olumlu şekilde etkiler. Katılaşma sonrası küreselleşmiş mikroyapı, mekanik dayanım değerlerinin ve tokluğun artmasını sağlar. Bu çalışmada, geleneksel döküm yöntemleri ile şekillendirilmesi güç olan ısıl sertleştirilebilir EN AW 6063 (AlMg0,7Si) alaşımı, 1,5 mm kesit kalınlığına sahip parça olarak yenilikçi bir yarı-katı şekillendirme yöntemi olan GISS (Gas Induced Semi-Solid) kullanılarak yüksek basınçlı döküm tekniği ile üretilmiştir. GISS yöntemi, ergiyik halde akıcılığı düşük 6063 alaşımını yarı-katı çamur formuna getirmek ve yüksek basınçlı dökümde uygulanan enjeksiyon kuvveti ile akıcılığını arttırarak alaşımın döküme uygun duruma getirilmesi için kullanılmıştır. Çıktı olarak, geleneksel sıvı faz döküm ile yarı-katı dökümün parça kalitesi üzerindeki etkileri incelenmiştir. Porozite, çekinti, akıcılığa bağlı dolum ve parçanın kalıptan sağlam çıkma yetisi, geleneksel yöntem ile yarı-katı döküm yöntemi ile üretilen parçalarda dökülebilirlik anlamında sonuç değişken olarak irdelenmiştir. Sertleştirme için 220°C sıcaklıkta 150 dakika yapay yaşlandırma ısıl işlemi gerçekleştirilmiştir. Geleneksel döküm ile yarı-katı dökümün mekanik özellikler üzerindeki etkilerinin görülebilmesi için ısıl işlem öncesi ve sonrası parçalara sertlik testi uygulanmıştır. Sonuçlar incelendiğinde, yarı-katı döküm yöntemi ile üretilmiş parçaların, geleneksel yöntem ile üretilmiş parçalara göre çok daha az döküm hatası içerdiği saptanmıştır. Dökülebilirliğin, yarı-katı şekillendirme ile geliştirilebildiği gözlemlenmiştir. Bununla birlikte, iki farklı yöntem ile üretilmiş parçaların sertlikleri arasında belirgin bir fark gözlenmeyip ortalama 45 HB olarak ölçülmüştür. Uygulanan yaşlandırma ısıl işlemi ile sertlik değerleri 55 HB civarına yükseltilebilmiştir. Çalışma kapsamında, geleneksel döküm yöntemleri ile şekillendirilmesi dökülebilirlik anlamında güç yoğrulabilir EN AW 6063 alaşımı, yenilikçi GISS yarı-katı döküm yöntemi kullanlılarak başarılı bir şekilde üretilmiştir. Yaşlandırma ısıl işlemi ile sertlik artıtılmıştır.
  • Öge
    Contribution of mxene coatings to the performance of nickel based electroactive materials
    (Institute of Science And Technology, 2020-06-15) Karaman, Berke ; Ürgen, Mustafa ; 506181408 ; Metallurgical and Materials Engineering ; Metalurji ve Malzeme Mühendisliği
    Energy storage devices are gaining more and more attention due to the increasing need for more and faster energy storage. Energy storage devices can work with two different kinds of storage mechanisms in general which are electrical double layer and faradaic reactions. Electrical double layer mechanism works with electrostatic interactions within the oppositely charged ions while faradaic reactions are based on charge transfer between the particles. When compared to other energy storage devices like batteries and fuel cells, supercapacitors forefront with their high power density and high cycling stability however, their energy density is much lower than other energy storage devices. Therefore, new studies aim to increase energy density while keeping high power density and high cycling stability. Three traditional material types are used as supercapacitor electrodes which are carbon based materials, metal oxides and polymers. Carbon based materials' charge storge mechanism is solely based on double layer mechanism which provides high power density but very low energy density. Polymers provide high energy density however, their shrinkage and swelling during charge storage degrades their performance. Meanwhile, metal oxides energy storage mechanism is based on faradaic reactions which provide high energy density but their cycling stability is relatively low due to possible phase transformations. Nickel hydroxides are one of these materials that are used in supercapacitor electrodes. Their ability to possess high specific capacitance, ease of production and environmentally friendly nature brings attention. Nickel hydroxides have three different phases which are alpha beta and gamma. Alpha phase provides better electrochemical performance while beta phase has lower capacitances and gamma phase is considered the intermediate phase in cycling. However, alpha phase when cycled in KOH electrolyte, transforms into beta phase due to aging which results in poor cycling stability. In addition to three traditional electrode materials types, a new type of electrode materials emerges as 2 dimensional materials including graphene, phosphorene and mxenes. Mxenes are forefront with their high capacitance that could already reach to the highest -3 capacitance obtained by metal oxides which is 1500 Fcm . However, none of these materials are enough the met required needs therefore combinations of these materials are used in order to combine the best aspects of these materials and close the deficts of each other. In this thesis, Ti3C2Tx mxene is produced by LiF+HCl method and coated with electrophoretic deposition on the NiOOH that is produced with anodic oxidation. Production methods are specially selected as binder-free methods in order to eliminate using binders that would reduce the performance of the electrode. Parameters of production of mxenes are optimized as 12M LiF+9M HCl at 45oC for 24 hours while NiOOH are produced in KOH at 200oC for 30 minutes with anodic oxidation method. Electrophoretic deposition parameters set at 50 V for 5 minutes in 50 ml Acetone with the addition of 0.003g I2. In each production step, binder-free production methods are preferred in order to enhance the electron transfer between electroactive materials and current collector. In order to see the direct effect of the combination of NiOOH and Ti3C2Tx, all NiOOH, Ti3C2Tx and composite NiOOH/Ti3C2Tx are produced as separate electrodes and characterized before and after 1000 cycling to observe morphological, structural and electrochemcial changes as well as the performance of these electrodes. As a result, capacitance of the composite electrode is almost twice (1.91 F/cm2) of the NiOOH electrode (1.15 F/cm2) and four times of the mxene electrode (0.41 F/cm2). However, the addition of mxene coating on the NiOOH was not impactful in the capacity retention aspect as both NiOOH and composite electrode lost around 60% of its capacitance after 1000 cycles. After structural and morphological characterizations, it is observed that mxene coating could not prevent the phase transformation of the NiOOH from gamma to beta and also, mxenes are oxidized after 1000 cycles. Even though flowerlike morphology of the NiOOH is preserved, the flaky structure of mxenes diminishes after 1000 cycles in both composite electrode and bare mxene electrode which can be a result of the oxidation. Further, performance boost of the composite electrode is inspected with electrochemical techniques. Electrochemical impedance spectroscopy has been used in order to further evaulation of electrochemical properties. Solution resistance of composite electrode (0.329 Ω) found to be lower than both mxene (0.474 Ω) and NiOOH electrode (0.642 Ω) . Meanwhile, Nyquist plots clearly show that diffusion properties of the composite electrode is enhanced as slope of the vertical line on the low frequency region is the highest. Rate capabilities which is greatly affected by the diffusion capabilities, shows that while NiOOH electrode can keep 12% of its capacitance when scan rate is increased from 1mv/s to 100 mv/s, mxene electrode can keep 24% of its capacitance which is reflected as 16% in the composite electrode. Meanwhile, electrodes are characterized by the scan rate and peak current relations and observed that all three electrodes reaction mechanisms are heavily dependent on the diffusion properties. It is concluded that surface area increase and the structure of the mxenes significantly improve the electrode's diffusion properties and result in high capacitance and high rate capability. As a result, it is observed that mxene deposition on NiOOH flowerlike structures dramatically increases the surface area and enhances diffusion properties, which reflects to the performance of the electrode.
  • Öge
    Şekilli ince filmlerin optik ve metamalzeme davranışının incelenmesi
    (Fen Bilimleri Enstitüsü, 2012-01-26) Ongun, Erhan ; Kazmanlı, Kürşat ; 506081425 ; Metalurji ve Malzeme Mühendisliği ; Metallurgical and Materials Engineering
    Metalik nano-yapılı ince-film malzemelerde, nano-yapı morfolojisinin mühendislik tasarımı ve üretimi nanoteknolojinin önde gelen araştırma konularındandır. Çünkü, morfoloji (boyut ve şekil), uygulamaya özel olarak üretilecek olan malzemenin optik, elektrik, mekanik, kimyasal ya da manyetik özelliklerini belirleyen önemli bir tasarım parametresidir.Bu çalışmada, modern yüzey modifikasyon tekniklerinden Eğik-Açılı Fiziksel Buhar Biriktirme tekniği kullanılarak eğik-kolonsal morfolojiye sahip metalik (Ag, Cu, Ti) nano-yapılı ince-film malzemeler üretilmiştir. Nano-porozite ve morfolojik özelliklerin optik cevap üzerindeki etkileri incelenmiştir.Normal ve eğik-açılı kaplama durumlarında ve farklı kaplama kalınlıklarında üretilen numunelerin yüzey ve kesit görüntüleri FEG SEM yöntemi ile alınarak metalik nano-yapıların morfolojisi ve film-porozitesi incelenmiştir. Kristal yapı analizleri XRD yöntemi ile yapılmıştır. Optik özellikleri incelemek için [280,?,1000] nm dalga boyu aralığında iki lineer-polarizasyon durumunda R (yansıtma) ve T (geçirme) ölçümleri NKD-7000 spektrofotometre kullanılarak yapılmıştır.Farklı kaplama şartlarında üretilen nano-poroziteli ve eğik-kolonsal morfolojiye sahip metalik nano-yapılı ince-filmlerin anizotropik optik özelliklere sahip olduğu ve morfolojik özelliklerinin optik cevap üzerinde etkili olduğu görülmüştür.