LEE- Malzeme Bilimi ve Mühendisliği Lisansüstü Programı
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Çıkarma tarihi ile LEE- Malzeme Bilimi ve Mühendisliği Lisansüstü Programı'a göz atma
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ÖgeSynthesis and characterization of various tungsten carbide powders from tungsten hexachloride powders via mechanochemical reaction and autoclave/pressure vessel methods( 2020) Özkan Aytekin, Nihan ; Öveçoğlu, Mustafa Lütfi ; 632586 ; Malzeme Bilimi ve MühendisliğiTransition metal carbides have been studied for approximately last half century due to their extraordinary characteristics that make them convenient to be utilized for industrial applications such as tool and structural materials under special conditions. Tungsten carbide has a special location amongst transition metal carbides because of its superior properties such as high hardness, high density, high melting temperature, high fracture toughness, good electrical and thermal conductivity and high elastic modulus etc. These attractive properties make it preferred to be used for tips for cutting and drilling tools, WC-Co hard metals, wear-resistant surfaces for machines, scratch-resistant jewelry materials, erosion-resistant coatings for aerospace elements, extrusion and pressing molds, wear resistant parts in wire drawing, platinum-like catalytic materials for polymer electrode membrane fuel cells and thin film diffusion barriers in microelectronics. In W-C binary system, two phases such as W2C and WC exist. These two phases possess various polymorphic modifications that are stable in different temperature and composition ranges. Many production techniques have been used in the sythesis of tungsten carbides. The production of WC was firstly achieved in a conventional way by high temperature. A solid state direct reaction occured between W and C powders at a temperature range of 1200°C-2000°C under controlled atmosphere to synthesize WC. Also there are different high temperature methods such as carbothermal reduction, chemical vapour condensation, self propagating high-temperature synthesis, molten salt synthesis, calcining and thermal processing at high temperatures. In addition to these techniques there are relatively low temperature methods such as low temperature autloclave processing, combined process of calcination, nitridization and and carburization at low temperatures, electrochemical sytnhesis from halide-oxide melts under pressure, mechanically alloying and mechanochemical synthesis. Especially mechanically alloying and mechanochemical synthesis are advantegous methods due to having simple and low-cost equipment and performing reactions at room temperatures in relatively short reaction times. The main aim of this dissertation is to produce WC powders with an effective technique that overcomes the limitations of traditional production methods and provides advantages of time, energy saving, simplicity and low equipment cost. In order to achieve this aim, elemental W and C powders were first milled in a spex at different durations by mechanical alloying method to obtain tungsten carbide powders. The aim of the first stage is to obtain the tungsten carbide powders with the starting powders traditionally used, and to compare the results with the results of the study in which tungsten carbide was synthesized with the alternative starting powders. In the second stage, mechanical chemical reaction was used to synthesize high purity and sub-micron sized WC powders using WCl6 (as W source), Na2CO3 (as C source) and Mg (as a reducing agent) powders. In a very short time the chemical reaction started and the WC phase occurred. In a very short time chemical reaction started to form and WC phase occured. Amounts of C source (Na2CO3) and reducing agent (Mg) were varied to optimize raw materials reacted and to obtain high-purity WC powders. After purification of the obtained powders, nano-sized tungsten carbide powders, which are the only WC phase, were achieved in high purity without any intermediate phase and impurity. The other purpose of this dissertation is to use an alternative production technique in order to produce WC powders, and compare the products, especially those produced by mechanical alloying / mechanochemical synthesis and autoclave methods. Therefore in the third stage low temperature autoclave processing was employed for the fabrication of WC powders from the same raw materials. The autloclave processing was used with mechanical activation to reduce the reaction temperature by activating the reactive particles and to homogenize the distribution of the microstructure. In this method powder blends were placed in a hydrothermal reactor and the hydrotermal reactor was heated in a furnace. This autoclave processing was implemented by varying the temperature and the duration of synthesis, the type of excess carbon source, the amounts of carbon sources and reducing agent. High purity and nano-sized tungsten carbides were obtained by optimizing the production conditions. Furthermore, autoclave synthesis is accepted as an eco-friendly method due to carrying out in a closed / isolated system and saving energy. The final powders of two different methods were characterized and compared each other using X-ray diffractometer (XRD), particle size analyzer (PSA), pycnometer, stereo microscope (SM), scanning electron microscope/energy dsipersive spectometer (SEM/EDS), transmission electron microscope (TEM) and differential scanning calorimetry/thermogravimetric analysis(DSC/TGA). WC powders were produced from WCl6-Na2CO3-Mg powder blends via mechanochemical synhtesis for the fist time.in the literature. In this way the results of mechanochemical snthesis from new raw materials for WC powder production contibuted the literature. Also, this thesis provides a comparison of mechanochemical method which is a simple, energy saving and room temperature method and autoclave method which is a more traditional method for producing tungsten carbide.
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ÖgeDoğal taşlara yüzey koruyucu olarak sol-jel yöntemiyle nano katkılı kaplama geliştirilmesi( 2020) Tekin Eroğul, Gözde ; Yücel, Onuralp ; 643023 ; Malzeme Bilimi ve Mühendisliği Ana Bilim DalıDoğal taşlar, tuz kristalizasyonu, atmosferik etkileşim, taş gözeneklerinde suyun donması, taşın sürekli ıslanıp kuruması, rüzgâra bağlı etkiler, taş üzerinde mikroorganizmaların büyümesi, rüzgâra bağlı aşınma ve insan etkisi gibi faktörler yüzünden zaman içinde bozulmaya uğrarlar. Su doğal taşların bozulmasına yol açan en önemli sebeplerden biridir; çünkü su, tuzların taş içerisinde kristalleşmesine yol açarak taşların pul pul dökülmesine neden olur, atmosferik kirliliklerle reaksiyona girip asidik bileşikler oluşturarak taşların yüzeyini bozar, soğuk iklimlerde donarak taşların çatlamasına sebebiyet verir ve mikro organizmaların taş üzerinde büyümesine neden olur. Bu sebepten doğal taşların suyun aşındırıcı etkisine karşı korunması gerekmektedir. Doğal taşlara koruyucu kaplamalar uygulamak taşlara su girişini engellemenin en etkin yöntemlerinden biridir. Bunun için doğal taşlara koruyucu kaplama olarak hidrofobik ve süperhidrofobik kaplamaların her ikisi de uygulanabilmektedir. Hidrofobik kaplamalarda, temas açısı 90°'den büyük olduğundan damlalar küreler şeklinde yüzeyde kalır ve hidrofilik kaplamalardaki gibi emilmezler. Süper hidrofobik kaplamalarda ise temas açısı 140°nin üzerinde olup damlalar hava paketlerinin üzerinde kalırlar. Bu tip kaplamalar doğal taşlara su girişini önleyerek taşlarda suya bağlı bozunmaların olmasını engellerler. Süperhidrofobik ve hidrofobik kaplama üretmenin en basit ve düşük maliyetli yöntemlerinden biri sol-jel yöntemidir. Sol-jel yöntemi, moleküler başlangıç madde-lerinin hidroliz ve yoğunlaşmasına dayanan bir düşük sıcaklık prosesidir. Bu proses ile inorganik, inorganik-organik hibrit kaplamalar, yüksek saflıkta tozlar, fiberler, aerojeller, seramik ve camlar gibi çeşitli malzemeler üretilebilmektedir. Sol-jel yöntemiyle sentezlenen inorganik-organik hibrit kaplamalar doğal taşların yüzeylerinin korunmasında sıklıkla kullanılmaktadır. Burada, inorganik bileşen kaplamaya kimyasal direnç ve ısıl kararlılık sağlarken polimerik organik bileşen de kaplamaya hidrofobiklik sağlamaktadır. Nano tozlar da kaplama yüzeylerinde yarattıkları mikro-nano pürüzlülük nedeniyle kaplamaların hidrofobikliğini arttırlar. Bu tez çalışmasında, mermer yüzeyler için nano silika ve nano alümina katkılı inorganik-organik hibrit kaplamalar geliştirilmiştir. Organik bileşen olarak polidimetilsiloksan (PDMS), inorganik bileşen olarak da tetrametoksisilan (TMOS) kullanılmıştır. İlk yapılan çalışmalarda, PDMS oranı %10'da sabit tutularak nano silika katkı miktarları değiştirilmiş ve farklı nano silika miktarının temas açısına etkisi incelenmiştir. İkinci setteki çalışmalarda, yine PDMS oranı %10'da sabit tutulmuş ancak bu sefer nano silika tozu yerine nano alümina tozu kullanılarak farklı nano alümina miktarlarının ve farklı toz kullanımının temas açısına etkisi incelenmiştir. Üçüncü setteki çalışmalarda ise, nano silika oranı %1'de sabit tutularak farklı PDMS oranının temas açısına etkisi incelenmiştir. Daha sonra geliştirilen kaplamalar nem direnci ve UV yaşlandırma testlerine tabi tutularak optimum koşulları sağlayan kaplamalar belirlenmiştir. Yapılan deneyler sonucunda, optimum temas açısı değeri nem direnci testi öncesinde %1 nano silika %10 PDMS katkılı kaplama ile 145° olarak ölçülmüşken, nem testi sonrasında ise %3 nano silika %10 PDMS katkılı kaplamalı numunelerde 140° olarak ölçülmüştür. Nano silika katkılı formülasyonların çoğu test sonrasında da hidrofobik özelliklerini korurken nano alümina katkılı formülasyonlar nem direnci testi sonrasında bu özelliklerini kaybetmiştir. UV yaşlandırma testi öncesinde ve sonrasında numunelerin hiçbirinde gözle görülebilecek renk değişimi olmadığı gözlenmiştir.
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ÖgeYüksek mukavemetli düşük alaşımlı petrol boru çeliklerinin üretimi ve sıcak haddeleme proses optimizasyonu( 2020) Tütük, Ramazan ; Kayalı, Eyüb Sabri ; 629124 ; Malzeme Bilimi ve MühendisliğiGünümüzün hızla sanayileşen dünyasında artan enerji taleplerine bağlı olarak yeni enerji kaynaklarının bulunması kadar mevcut kaynakların ekonomik bir şekilde taşınması da çok önemli bir konu haline gelmiştir. Petrol ve doğalgaz gibi akışkan enerji kaynaklarının boru hatları yardımıyla yüksek miktarlarda, bir bölgeden diğer bir bölgeye, hatta bir kıtadan diğer bir kıtaya iletilmesi, deniz ve karayolu ile yapılacak taşımaya göre çok daha ekonomik ve hızlı bir şekilde yapılabilmektedir. Artan debiler yüksek basınçlarla birlikte daha kalın ve daha yüksek mukavemetteki çeliklere olan talebi artırmıştır. Kurulan boru hatlarında kullanılan düşük alaşımlı, yüksek mukavemetli, sorunsuz ve hızlı kaynaklanabilirlik özelliklerine sahip çeliklere olan bu talep, demir-çelik sektörüne de ürün geliştirme konusunda bir itici güç oluşturmuştur. Yüksek mukavemet ve tokluk gereksinimlerinin düşük maliyetlerle karşılanmasına yönelik olarak geliştirilen yüksek mukavemetli düşük alaşımlı (HSLA-High Strength Low Alloy) çelikler, sıcaklık ve deformasyon kontrolü yöntemi ile termomekanik olarak haddelenmekte ve bu sayede üründeki tokluk ve kaynaklanabilirlik özellikleri bozulmadan düşük alaşımlamalar ile yüksek mukavemet ve tokluk değerlerine erişilebilmektedir. Termomekanik haddeleme, çeliklerde ostenit fazının yeniden kristalleşme sıcaklığının üzerinde veya kısmen üzerinde yapılan kaba haddeleme (rough rolling) ve bu fazın yeniden kristalleşme sıcaklığının altında veya kısmen altında yapılan nihai haddeleme (finish rolling) olmak üzere iki ezme kademesinden ve duşlu masalarda belirli bir sıcaklığa soğutulmasından oluşmaktadır. Yeniden kristalleşme sıcaklığının altında yapılan ezme işlemi ile deforme edilmiş ostenit taneleri elde edilmekte ve ostenit taneleri içerisindeki dislokasyon yoğunlukları artırılmakta, bu sayede, sonrasında oluşacak ferrit tanelerine çok sayıda çekirdeklenme bölgesi yaratılmaktadır. Ferrit tanelerinin çok sayıda çekirdeklenmesi, büyüme sırasında birbirlerinin tane sınırlarına çok daha çabuk ulaşmalarını ve büyümelerinin durmasını sağlamakta ve böylece nihai üründe homojen ince taneli bir yapı elde edilebilmektedir. HSLA çeliklerinde mukavemet artışında katı çözelti sertleşmesi, dislokasyon yoğunluk artışı, faz dönüşümleri, çökelti sertleşmesi ve tane küçültme mekanizmaları etkin rol oynamaktadır. Katı çözelti sertleşmesi manganez ve molibden gibi alaşım elementlerinden, faz dönüşümü ve dislokasyon yoğunluğu kaynaklı artıştan, düşük dönüşüm sıcaklıklarına sahip iğnesel ferrit ve beynit fazlarından; çökelti sertleşmesi alaşımda yer alan niyobyum, vanadyum ve titanyum gibi elementlerin oluşturdukları karbür, nitrür ve karbonitrür fazlarından; tane küçültme ise termomekanik haddeleme ile elde edilen çok sayıda tane sınırından kaynaklanmaktadır. Niyobyum elementi sadece çökelti sertleştirmesi ile değil düşük sıcaklıklarda yapılan yüksek ezme miktarları ile nihai küçük tane yapısına da katkıda bulunmaktadır. Titanyum elementi ise, oluşturduğu TiN fazı ile sıcak haddeleme öncesinde yapılacak tavlama işleminde ostenit tanelerinin irileşmesini engellemekte (tavlama sıcaklıklarında çözeltiye geçmeyerek) ve haddelemeye başlangıç anındaki tane boyutunu düşürerek yine son üründeki küçük taneli yapıya katkıda bulunmaktadır. Petrol ve doğalgaz boru hatlarında kullanılan mikroalaşımlı çeliklerin mikroalaşımlandırma ve termomekanik haddeleme parametrelerinin incelenmesi amacı ile yapılan bu çalışmada, iki farklı haddeleme teknolojisi kullanılarak (tersinir ve tandem sıcak haddeleme) yüksek mukavemetli düşük alaşımlı API X70M PSL2 kalite, Cr, Mo, Nb, V ve Ti ile mikroalaşımlandırılmış 2 mikroalaşımlı çelikde, ostenit fazının yeniden kristalleşme sıcaklığının altında, farklı ezme miktarlarında termomekanik haddeleme denemeleri yapılmış, bu denemelerin, üretilen çeliğin özellikleri (akma-çekme-uzama, darbe direnci, sünek-gevrek geçiş sıcaklığı, Ağırlık Düşürmeli Yırtma Testi- DWTT-, katlama, sertlik, mikroyapı, kırılma karakteristiği) üzerine etkileri incelenmiştir. Tersinir sıcak haddeleme metodu ile yapılan deneme üretimi öncesinde Gleeble proses simülasyon cihazı ile düzlem deformasyon testleri gerçekleştirilmiş, deneysel yöntemle ostenitin yeniden kristalleşmediği sıcaklık (Tnr) 960 °C olarak tespit edilmiştir. Endüstride Tnr sıcaklığının tespiti için yaygın olarak kullanılan Boratto, Fletcher ve Bai tarafından geliştirilen ampirik formüller ile söz konusu sıcaklık değeri sırasıyla 1272 °C, 1079°C ve 1066°C olarak tespit edilmiştir. Deneysel yöntemle elde edilen bu sıcaklık değeri kullanılarak yapılan tersinir sıcak haddeleme deneme üretiminde Tnr sıcaklığı altında tersinir hadde hattının kabiliyetlerine bağlı olarak %42'lik bir ezme oranı uygulanabilmiştir. Deneme sonrasında yapılan testlerde akma, çekme, uzama ve darbe değerlerinin standart gereklilikleri karşıladığı ancak 0°C'de yapılan ağırlık düşürmeli yırtma testlerinde %10'luk sünek kırılma alanı ile minimum değer olan %85 sünek kırılma alanı beklentisinin çok altında kalındığı tespit edilmiştir. Görece düşük tokluk ve çok düşük sünek yırtılma değerlerinin elde edilmesinde yüksek alaşım içeriğinin (yüksek C, Nb ve Mo değerleri) yanısıra tersinir şerit hadde teknolojisi gereği haddeleme sırasında iş parçasının hadde ayaklarının iki tarafında yer alan tambur fırınlara sarılarak ısıtılmasının, termomekanik haddeleme sırasında hedeflenen yüksek deformasyon hızlarına ulaşılmasını engellediği, ısıtma sırasında ostenit tanelerinin toparlanmasına neden olduğu ve bu nedenle nihai üründe hedeflenen küçük taneli yapıya ulaşılamadığı tespit edilmiştir. Tandem hadde denemelerinde tersinir hadde denemesine kıyasla daha yalın bir kimyasal analiz (daha düşük C, Nb ve Mo değerleri) kullanılarak Tnr altında farklı ezme oranları (%60,7-72,8) ile üretimler yapılmış, bu değişkenlerin üretilen çeliğin özellikleri (akma-çekme-uzama, darbe direnci, sünek-gevrek geçiş sıcaklığı, Ağırlık Düşürmeli Yırtılma Testi-DWTT, katlama, sertlik, mikroyapı, kırılma karakteristiği) üzerine etkileri incelenmiştir. Tandem hadde denemeleri sırasında Tnr sıcaklığının tespiti için yaygın olarak kullanılan Boratto ampirik formülünden yararlanılmış, yapılan hesaplamada söz konusu sıcaklık değeri 1134°C olarak tespit edilmiştir. Fletcher ve Bai tarafından geliştirilen diğer ampirik formüllerden ise Tnr sıcaklığı sırasıyla 1057°C ve 1020°C olarak bulunmuştur. Yapılan çalışmalar neticesinde yeniden kristalleşme sıcaklığı altında artan ezme miktarları ile nihai üründeki tane boyutunun azaldığı, mukavemet ve tokluk değerlerinin arttığı tespit edilmiştir. Aynı zamanda ürünlerin sünek-gevrek geçiş sıcaklığı değerlerinin de bu sayede çok düşük sıcaklıklara (-80 °C'nin altına) indiği tespit edilmiştir. Tnr altında en yüksek ezmenin verildiği deneme üretimi sonrası yapılan tüm testlerin standart gerekliliklerini karşıladığı tespit edilmiştir.
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ÖgeNovel approaches for protection of light metals under various wear conditions via micro arc oxidation process(Lisansüstü Eğitim Enstitüsü, 2021) Muhaffel, Faiz ; Çimenoğlu, Hüseyin ; 675450 ; Malzeme Bilimi ve MühendisliğiLight metals such as magnesium, aluminium and titanium have recently developed great attention for automotive, aerospace, transportation and many other industries. Their low densities make them more favourable than heavier steel and cast iron alloys. In particular, they offer weight reduction and cost-efficient performance and therefore, they bring a subsequent revolutionary change in the design and manufacturing of metallic components. Despite their beneficial specific strength, light metals mostly exhibit low hardness and insufficient wear resistance. Although they pose a native oxide protective layer on their surface after exposure to air, this layer only provides minimal protection to wear- and corrosion-related damages. Thus, surface engineering technologies become necessary to satisfy the specific properties and long service life of fabricated engineering components. Compared to other surface modification processes, micro arc oxidation (MAO) is distinguished by providing a thick, hard and well-adhered oxide coating on the magnesium, aluminium and titanium alloys without jeopardizing the mechanical properties of the substrate because of negligible heat input. Moreover, the MAO technique has advantages like processing materials with complex geometries, no need for extensive preparations of the surface of the substrate and applying in environmentally friendly electrolytes. In the scope of these drawbacks of light metallic alloys, wear performance was aimed to be enhanced against various service conditions such as at high loads, in a corrosive environment, at high temperatures, etc. In the first chapter of the thesis, a study was initiated to extend the usage of magnesium alloys, especially in corrosion and wear-related engineering applications, by coating their surfaces via MAO. Considering the individual influence of phosphate- and aluminate-based electrolytes on the corrosion and wear performances of the synthesized MAO coatings, AZ91 magnesium alloy has been subjected to MAO in aluminate-based reference electrolyte with and without additions of Na3PO4 at concentrations of 5 and 10 g/l. Unlike dry sliding conditions, MAO coatings synthesized in the aluminate-based reference electrolyte did not provide reasonable protection of AZ91 magnesium alloy against wear in corrosive media (0.9 wt% NaCl solution). This study revealed that the addition of 5 g/l Na3PO4 into this reference electrolyte was sufficient for enhanced resistance of MAO coating against chemical and mechanical degradations (i.e. corrosion-wear) without altering its features in terms of surface roughness and thickness. In the second chapter of the thesis, a study has been initiated to increase the success of MAO coatings fabricated on aluminium alloys against degradation under sliding contact conditions at high temperatures. For this purpose, the 7075 Al alloy has been micro-arc oxidised in an aluminate-based electrolyte with or without the adding monoclinic zirconia (ZrO2) particles. Microstructural analyses revealed that the coating synthesised in a ZrO2-added electrolyte consisted of a ZrO2 particles participated alumina (Al2O3) based outer layer and a monolithic Al2O3-based inner layer, which exhibited similar features with that of the synthesised in the ZrO2-free aluminate-based electrolyte. Moreover, the coating fabricated in the ZrO2-added electrolyte exhibited enhanced wear resistance during the dry sliding wear tests conducted at room temperature and had higher durability during the tests done at 300 ℃. Since the examined coatings were worn by the fatigue wear mechanism, their durability during high temperature wear tests was analysed using the conventional stress-based fatigue approach. From the derived equations, the maximum contact pressures at which coatings can endure 106 contact cycles at 300 ℃ were estimated as 851 and 331 MPa for the coatings fabricated in the ZrO2-added and ZrO2-free aluminate-based electrolytes, respectively. In the third chapter of the thesis, the influence of Al2O3 and ZrO2 incorporation on the structural properties and wear resistance of titania (TiO2) based MAO coatings fabricated on Ti6Al4V alloy was studied. For this purpose, MAO was employed in a silicate-based electrolyte with and without additions of Al2O3 and ZrO2 particles. The structural properties were determined via X-ray diffraction (XRD) and X-ray photoelectron (XPS) spectroscopy analysis and an energy dispersive spectrometer (EDS) equipped scanning electron microscope (SEM). Furthermore, thermochemical simulations were made by using FactSage 7.3. Mechanical properties of the MAO coatings were determined by hardness measurements and dry sliding reciprocating wear tests. Structural examinations revealed that the MAO coatings fabricated in Al2O3 and ZrO2 added electrolytes comprised of these oxides and their complex forms (Al2TiO5 and ZrTiO4, respectively) along with TiO2 and amorphous silica (SiO2). Although incorporations of Al2O3 and ZrO2 did not remarkably improve the hardness of the MAO coatings, the highest wear resistance was obtained from the one formed in the ZrO2 added electrolyte. On the other hand, the MAO coating fabricated in the Al2O3 added electrolyte exhibited lower wear resistance than that fabricated in the particle-free silicate-based electrolyte. In the fourth chapter of the thesis, the structural features, biocompatibility, and mechanical performance of a TiO2 layer with incorporated ZrO2 formed by micro arc oxidation on commercially pure titanium have been examined. In comparison to the ZrO2-free TiO2 layer, the ZrO2-incorporated oxide layer was dense and contained ZrTiO4 as a new oxide as well as ZrO2 particles. Associated changes in the microstructure enhanced the mechanical durability of the TiO2 layer. Owing to the incorporation of identical biocompatible compounds and almost similar surface roughness, no remarkable difference in bioactivities of the ZrO2-free and ZrO2-incorporated oxide layers was detected after simulated body fluid tests. In the fifth chapter of the thesis, a work was initiated to examine the role of counterface materials on the tribological behaviour of Mg alloys subjected to the MAO process and focus on reducing the sliding coefficient of friction (COF) values of MAO treated surfaces. Surfaces of AZ31 grade Mg alloy were subjected to an MAO treatment in a sodium metasilicate and potassium hydroxide containing electrolyte. Unlubricated sliding wear tests were conducted using counterfaces made of nitride-based (N-based), TiN, TiCN, CrN, and hydrogenated diamond-like carbon (HDLC) coated as well as uncoated SAE 52100 grade bearing steel balls. MgO based coatings formed on the surfaces of samples during the MAO process increased the wear resistance of AZ31, regardless of the type of counterface material used. Sliding the MAO treated surfaces against counterfaces made of uncoated steel and N-based coatings yielded high COF values of 0.6–0.8 that exceeded those of the uncoated AZ31 against the same counterfaces. During dry sliding of the H-DLC counterface on the MgO coating, smooth and stable friction curves with a low steady-state COF value of 0.13 were recorded. Therefore, a significant drawback of MAO treatment that gives rise to surfaces with high COF could be addressed by running them against H-DLC coated counterfaces, a method that could be applied to the development of lightweight tribological components that are both sliding wear-resistant and have low COF. In the sixth chapter of the thesis, the wear and corrosion performances of AZ91D magnesium alloy have been examined after MgO- and Al2O3-based coatings fabricated. While the MAO process was applied to generate a magnesia layer, cold spraying (CS) and MAO processes were combined to obtain a novel alumina layer. CS was conducted to cover the substrate by depositing an aluminium layer (monolithic or composite). Afterwards, application of the MAO process produced an alumina layer on the deposited aluminium layer (monolithic or composite), forming a multilayered coating on the examined magnesium alloy. The experiments revealed that the alumina layer formed on the alumina reinforced aluminium matrix composite layer ensured superior protection for AZ91D alloy against mechanical and chemical degradations compared to the magnesia layer. In brief, within the scope of the thesis, it has been shown that MAO coatings can be modified especially for wear-related applications to work efficiently at high temperatures, at high contact pressures, in corrosive environments and even in environments with biological fluids. Generally, the primary method in improving the properties of MAO coatings is to generate a composite oxide coating that forms on the base metal substrate by various ions or particles as the additives introduced into the electrolyte. Our studies have shown that when the MAO process is combined with another surface modification process, it can produce only oxides with better properties without forming its oxides of the base metal, which exhibits low hardness and wear resistance. With the combination of CS and MAO methods, magnesium alloys widely used today have gained a much higher wear resistance than the magnesium alloys coated by the MAO process. These coating methods, which are applied at low temperatures, also do not alter the microstructure and mechanical properties of the magnesium alloys having a low melting temperature. However, the general feature of MAO coatings is their high COF values. Therefore, within the scope of the thesis, it has been revealed that it is beneficial to modify contact surface to be used in applications where a low friction coefficient is required, and thus, H-DLC coating was applied on the steel balls, which were the counterfaces of MAO coating. In future works, it is planned to reduce the COF of MAO coatings to improve the wear properties of MAO coatings by adding lubricant additives into the electrolyte.
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ÖgeInvestigation of the growth kinetics and morphology transitions during porous anodization of titanium in ethylene glycol based electrolytes(Lisansüstü Eğitim Enstitüsü, 2021) Seçkin, Eren ; Ürgen, Mustafa Kamil ; 725516 ; Malzeme Bilimi ve MühendisliğiTitanium dioxide (TiO2, titania) is a transition metal oxide that has been widely used as a pigment since the early 1900s. The discovery of the photocatalytic properties of the material has made it highly attractive for photovoltaics, photocatalysis, and sensor applications. Besides, titania-based materials have been increasingly employed in the biomaterials industry due to their chemical stability and bio-compatibility. Recently, the production and use of TiO2 nanomaterials has become popular. The high surface area and possible quantum effects of titania nanostructures have provided significant advantages over micro-size use. TiO2 nanotubes have attracted great interest among various titania nanostructures, and intensive research has been carried out on their production and use. Although the anodic formation of non-porous barrier oxide films on titanium has been known for decades, the discovery of self-organized nanoporous titania production by anodization in fluoride-containing electrolytes opened a new pathway; and numerous studies have been conducted on the subject. Preliminary studies on the nanoporous anodization of titanium concentrated on understanding the anodization reactions and controlling the process. Subsequent studies mainly focused on the fabrication of nanotube structures and their use in various applications. Recently, sophisticated studies such as advanced nanotube geometries and doped nanotubes have been prominent. However, the literature shows that the reproducible fabrication of titania nanotubes of desired length and morphology, in a well-ordered structure with a clean surface, is still a challenge. Although many different morphologies have been obtained during anodization, the influence of this morphological variance on nanotube growth has not been defined with a consistent model. One of the main reasons for this deficiency is the insufficient understanding of the anodization process, which reduces effective control over the process. The necessity of a clear understanding of the formation, growth kinetics, and morphological transformations of the nanotubes constituted the main reason for starting this thesis study. In the first part of experimental studies, investigation and optimization of the anodization parameters (surface condition, stirring, aging of the electrolyte, voltage, temperature, and time) were conducted by utilizing an ethylene glycol-based anodization electrolyte containing 0.6 wt % NH4F, 1 vol % H2O. The two-step anodization procedure was optimized to improve the pre-anodization surface. The high surface roughness of the untreated Ti foil could be reduced significantly by the optimized first anodization procedure with an anodization at 50 V and 30 °C for 180 min and subsequent removal of the anodized layer. In the anodization studies carried out under different stirring rates (0 to 1000 rpm), it was observed that the stirring rate has a strong effect on the morphology. The barrier oxide film on the sample surface disintegrated faster as the stirring rate during anodization increased. It was attributed to the increase in the chemical dissolution rate due to increased agitation. In addition, it was revealed that the increase in stirring rate facilitates effective temperature control. Experimental studies on the aging and reuse of the anodization electrolyte have shown that current density behavior and morphology can vary significantly depending on the condition of the electrolyte. As the fluoride is consumed by forming water-soluble [TiF6]2- species and be solvated during anodization, the amount of free fluorine in the electrolyte gradually decreases. It was experimentally verified that the current density values shift down, and the chemical dissolving power of the electrolyte decreases in anodization conducted after extended usage or short but repetitive usage of the anodization electrolyte. However, it has also been shown that when the electrolyte is kept unused overnight after anodization, almost equivalent current density values and morphological results can be obtained as the anodization performed the previous day. We estimate that when the electrolyte is kept unused for a convenient duration after anodization, [TiF6]2- species decompose into TiF3, releasing three fluorides (for each molecule); thus, the amount of free fluoride in the electrolyte increases again. In studies examining the effects of anodization voltage and temperature on growth and morphology, it was verified that an increase in either parameter results in an increase in current density and hence an increase in growth rate. In addition, the increase in either parameter affects the morphology by accelerating the chemical dissolution. It was demonstrated that although the voltage has a slight effect on the dissolving power of the electrolyte, the temperature has a strong effect and the chemical dissolution in the electrolyte increases strongly with increasing temperature. A series of experiments were conducted to observe the effects of the anodization duration on the surface morphology. Three morphologies were obtained in the anodization experiments conducted at 50 V and 30 °C for different durations (5 min to 40 min). For 5 min and 10 min anodized samples, barrier oxide morphology, for 20 min anodized sample open-top nanotube morphology, and 40 min anodized sample nanograss morphologywere obtained. It was deduced that the surface morphology during anodization progresses in order of barrier layer, open nanotubes, and nanograss structure, showing that these morphologies are successive processes. Anodization experiments conducted at 50 V and 25 °C for different durations confirmed that the surface morphology changes in the suggested order. It was revealed that there is a correlation between the temperature and the dissolution time of the barrier oxide, which formed the basis of the proposed kinetic model. Additionally, ultrasonication and stripping with scotch tape methods were studied for clearing the sample surface from barrier layer residues and so-formed nanograss. In the second part of experimental studies, the formation and growth kinetics of the titania nanotubes were investigated by utilizing an ethylene glycol-based anodization electrolyte containing 0.3 wt % NH4F, 1 vol % H2O. A model, combining growth with morphology, has been proposed by utilizing the findings obtained in the first part. According to this model, growth is divided into three stages (I. Growth under initial barrier layer, II. Nanopore to nanotube transition, III. Nanograss formation). Stage 1 is defined as the period which includes initial barrier oxide formation, pore initiation, and nanotube growth under gradually dissolving initial barrier oxide. Formation studies conducted at 50 V and 25 °C for different durations (5 sec to 30 sec) demonstrated that ordered nanotubular formation under initial barrier oxide occurs in the early periods of the anodization. We determined that the field-assisted growth of the nanotubes occurs at a constant rate (µm C-1 cm2) at the bottom, under the gradually dissolving and still protective initial barrier oxide layer. Titanium samples were anodized at 25 °C for different durations (20 min and 80 min) remaining in Stage 1 to determine field-assisted growth rate. In both anodized samples average growth rate of ~0.7 µm C-1 cm2 was obtained. It was experimentally observed that the total dissolution time of the initial barrier oxide layer during anodization performed at 25 °C was within the range of 100 – 150 min. We have defined Stage 2 as the short period after the initial barrier oxide layer is wholly dissolved in which open tube top ordered nanotubes are seen, and nanopore/nanotube transition occurs. As Stage 2 is the sudden transition phase between Stage 1 and Stage 3, it was ignored in the kinetic calculations. Stage 3 is defined as the period for the chemical dissolution of the tube tops and the initiation and progress of nanograss formation. In this part, the nanotube film's growth and the film's shortening are examined together. Field-assisted growth continues at the bottom with the same efficiency and rate (per coulomb) (µm C-1 cm2) as it is in Stage 1. However, the simultaneous shortening of the tubes due to chemical dissolution occurs at the top at a constant rate (per time) (nm min-1). A kinetic model has been proposed to determine the persistence time of this top barrier layer. According to model, the chemical shortening rate of the nanotubes due to chemical dissolution is determined experimentally by using two different anodization durations remaining in Stage 3, which allows us to calculate the barrier oxide dissolution time (BDT). Barrier oxide dissolution times for 25 °C and 5 °C was calculated as 130.1 min and 414.1 min, respectively, and verified experimentally by anodization conducted for the corresponding BDT values. The dissolution of the top barrier layer is a chemical process, and its activation energy can be calculated using the experimentally determined parameters that allows us to determine the temperature dependence of BDT. By utilizing 25 °C and 5 °C anodization data and taking BDT-1 as the rate constant, the activation energy equivalent to 9.53 kcal mol-1 was obtained for the process. Additional experiments conducted at 35 °C, 15 °C, and -5 °C by using the theoretically calculated BDT values confirmed the model's validity. These calculations and experiments verified that temperature dependence of the barrier oxide dissolution time (BDT) shows Arrhenius-like behavior. Besides controlling the final morphology, it is shown that the plot of nanotube thickness that corresponds to BDT at different temperatures indicates a linear relation; and open-top nanotubes ranging from 6.8 µm to 10.4 µm can be obtained by tuning anodization temperature and duration according to this model. To summarize, this model gives the opportunity to tailor titania nanotube thickness (within a specific range) and desired nanotube morphology (barrier top, open tube top, or nanograss) by tuning anodization temperature and duration according to the proposed model.