Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis The Effect of Metal Doping on Tio2 for Photocatalytic Applications(Izmir Institute of Technology, 2019) Alduran, Yeşim; Özyüzer, Lütfi; Öztürk, OrhanRecently, the photocatalysis method has been an active research area as a promising solution for environmental cleaning method, leading to self-cleaning and sterilization of solar cell surfaces to produce water dissociation reaction. Titanium dioxide (TiO2) is the most suitable semiconductor for photocatalytic applications due to its high oxidation potential and high efficiency when irradiated by ultraviolet light (UV). Undoped and Ruthenium (Ru+) doped TiO2 thin films were prepared using magnetron sputtering technique. All thin films were grown on SLG different ratios like 1 sec, 3 secs, 5 secs and 7 secs to set shutter position in magnetron sputter target. Transparent substrate SLG is coated with nearly 50 nm TiO2 thin films without compromising any optical properties. Samples were heat treated for two hours at 500°C to get the anatase phase crystal structure. The crystallization peaks of TiO2 are proved to get the anatase phase. Photocatalytic activity of TiO2 thin films are determined after 1, 3, 5 and 24 hours with organic pollution as a methylene blue dye degradation under UV light. The degradation of methylene blue was investigated kinetically and photocatalytic activity rate constants of the photocatalysts were calculated. All thin films could not reach super hydrophilicity state. Undoped TiO2 contact angle 47.309o and Ru doped TiO2 63.218o were evaluated. The photocatalytic degradation percentage of Methylene Blue was reached 87%, after 24 hours of UV irradiation, when using Ru-doped TiO2 thin film. Consequently, the anatase phase of Ru-doped TiO2 thin films are found best photocatalytic activity in self-cleaning performance.Master Thesis Structural and Nanohardness Behavior of Low Energy, High Flux Nitrogen Implanted Austenitic Stainless Steel(Izmir Institute of Technology, 2018) Dal, Refika; Öztürk, Orhan316 austenitic stainless steels (SSs) are one of the most commercial and technological alloys and extensively used in the field of defence, nuclear and biomedical applications due to its excellent corrosion resistance in abrasive and erosive environment. However, this type of steel is rather soft, and these results in poor durability, in particular when this material (316 SS) is in contact with other surfaces. In addition, 316 SS is nonmagnetic at room temperature. In order to make the surface of 316 SS harder, nitrogen ion beam implantation and wear resistant method is applied. Earlier studies of high dose nitrogen ion implantation into the surface of austenitic SSs around 400 °C substrate temperature showed that an expanded austenite phase (The Nitrogen phase in the FCC lattice of 316 SS) gives excellent wear resistance with high hardness value. In this study, type 316 stainless steel (SS) was implanted with low energy (700 eV), high flux (2.9 mA/cm2) nitrogen ions at 400 °C substrate temperature in order to harden its surface. Microhardness and nanohardness measurements were carried out on the nitrogen implanted surface and on the nitrogen implanted cross-section under the applied loads ranging from 6 mN to 30 mN. Both microhardness and nanohardness data suggest that the hardness of the N implanted 316 SS significantly increases compared to the hardness of the substrate material (by a factor of 3 to 4).The hardness increase is believed to be due to the high amount of nitrogen, the thick nitrogen implanted layer and macroscopic residual compressive stresses, the formation of which is verified by θ/2θ XRD scans as lattice expansions about 10 at. %. SIMS profiles suggest concentration-dependent diffusion behavior for the N implanted layers. Based on SIMS and SEM/EDX data, nitrogen implanted layers are 4-5 micron thick and constituting about 28 %.Master Thesis Magnetic Characterization of Expanded Austenite Phase Formed on Nitrogen Ion Implanted 316 Stainless Steel Alloy(Izmir Institute of Technology, 2015) Karataş, Özgün; Öztürk, Orhan; Selamet, YusufAustenitic stainless steels (SSs) are technologically important alloys and highly resistant to corrosion in a variety of environments. Nevertheless, these materials have a few drawbacks; they are rather soft materials and susceptible to wear. Correspondingly, an improvement of the surface properties is often desirable. Ion beam techniques are widely used to enhance surface properties of these alloys. Surface modification of austenitic SSs by nitrogen ion beams at moderate substrate temperatures near 400 ºC, leads to the formation of a high N content phase. This phase, known as an expanded austenite phase, γN, creates a hard and wear resistant layer on the stainless steel. Additional property of this phase is related to its magnetic structure due to the large amount of nitrogen insertion and corresponding lattice expansion. In the current study, new data corresponding to structural and magnetic nature of the expanded austenite layers on austenitic 316 SS by low-energy, high-flux nitrogen ion implantation are presented. Phase and compositional analyses, surface topography and magnetic features of the nitrogen ion implanted layers were studied by a combination of experimental techniques involving XRD, SEM, AFM, MFM, VSM and MOKE. Nitrogen implantations were performed for 30, 90 and 240 minutes of processing time, at a fixed temperature near 400 °C. Relatively low-energy (0.7 keV) and high-flux (2 mA/cm2) ion beam conditions were carried out during the implantation. Combination of the aforementioned techniques provides strong evidence for the formation of the γN phase with mainly ferromagnetic characteristics. MFM imaging reveals stripe-like domain structures of the nitrogen ion implanted layers. Both VSM and MOKE analyses display hysteresis loops of the layers. Ferromagnetism in the γN layers are manifested by MFM, M and MOKE analyses. Ferromagnetic structure is linked to large lattice expansions 0 due to high nitrogen contents at. . s an interstitial impurity, nitrogen dilates fcc lattice of 316 SS i.e. Fe-Fe distance is increased, which strongly influences the magnetic interactions.Master Thesis Microstructural and Mechanical Characterization of Nitrogen Ion Implanted and Plasma Ion Nitrided Plastic Injection Mould Steel(Izmir Institute of Technology, 2003) Onmuş, Ortaç; Öztürk, OrhanIn this study, high-chromium ferritic plastic injection mould steel (X36CrMo17, similar to AISI-420F) was subjected to plasma nitriding and nitrogen ion beam implantation under various conditions. The effectiveness of conventional plasma nitriding and nitrogen ion beam implantation conditions in improving the tribological properties and mechanical performance was investigated.The experimental results clearly show that plasma nitriding and nitrogen ion beam implantation lead to the development of the various near-surface microstructures and enhanced mechanical properties.Plasma nitriding was performed at a temperature range between 520-540 °C, with a bias voltage of 500 V for 15 to 18 hours under various gas mixtures of N2+H2 in an industrial nitriding facility (micro-pulsed DC). Nitrogen ion beam implantation was carried out with 2x1017 and 1x1018 ion doses with an 85 kV nitrogen ion energy at temperatures 200 a1C. Near-surface phases, compositions, plasma nitrided and nitrogen implanted layer thicknesses and the strength of these layers were studied by a combination of symmetric (0-2.) and grazing incidence x-ray diffraction (XRD and GIXRD), conversion electron and x-ray Mössbauer spectroscopies (CEMS and CXMS), cross-sectional scanning electron microscopy (SEM) and cross-sectional nanohardness measurements. The corrosion behaviour was investigated by a salt spray method and by observation of acid etching during SEM sample preparation. The tribological properties (friction and wear) were examined by a ball-on-disc tribometer.Combined Mössbauer, XRD, and SEM analyses clearly indicate that (Fe,Cr,Mn)-nitrides and CrN are distributed in the top nitrided layers of several micron thickness. The CEMS and CXMS analyses show the nearly complete decomposition of the near surface and deeper crystal structures into pure bcc (Fe), (Fe,Cr,Mn)-nitrides, consisting of Fe3N and Fe4N, and CrN. The XRD, CEMS and CXMS results also show an Fe3C-like carbide phase, (Fe,Cr,Mn)3C, in the nitrided layers, whose presence is attributed to C segregation to the near-surface region and to extra carbon being present in the nitriding system. The nitriding conditions with the gas composition N2/H2.1 produces the thickest nitrided layer (about 135 .m) with an excellent resistance to corrosion. The nanohardness measurements indicate that the cross sectional hardness-depth profiles under all nitriding conditions is found to be plateau-shaped, and the nitrided layer surface hardness values are found to be increased by a factor of about three in comparison to that of the substrate material. This study also showed that the hardening effect in plasma nitrided specimens was due to a fine and homogeneous chromium nitride precipitation. The wear analysis results showed that the nitrided layers have reduced friction coefficient values (20 to 25%) and possess excellent wear resistance in comparison to that of the bulk material.It was also found that nitrogen ion implantation into plastic injection mould steel produces N content rich layers (less than 0.1 .m thick) with enhanced wear and corrosion properties. The surface nanohardness measurements of the low and high dose implanted specimens indicate that the hardness values increase by factors of about 1.4 and 1.6, respectively, compared to the substrate material. The salt spray corrosion analysis (2% NaCl solution in distilled water) experiments showed enhanced corrosion behaviour for the N implanted layers, and that nitrogen ion implantation is a viable method for improving corrosion resistance of plastic injection mould steel.Combined Mössbauer and XRD data reveal that the N implanted layers consist of nitride phase, (Fe,Cr,Mn)2+xN, with both magnetic (x-1) and paramagnetic (x-0) characteristics. Based on the CEMS results, the N implanted layer thicknesses are found to be - 40 and 65 nm for the low dose (2x1017 ions/cm2) and high dose (1x1018 ions/cm2) dose N implanted specimens, respectively.Master Thesis Structural and Magnetic Characterization of Nitrogen Ion Implanted Stainless Steel and Cocrmo Alloys(Izmir Institute of Technology, 2014) Fidan, Mehmet; Öztürk, OrhanIon beam surface modification methods can be used to create hard and wear resistant surface layers with enhanced corrosion resistance on austenitic stainless steels (SS) and CoCr base alloys using nitrogen ions. This is mainly due to the formation of high N content phase, γN, at relatively low substrate temperatures from about 350 to 450 ºC. This surface layer is known as an expanded austenite layer. Different N contents and diffusion rates depending on grain orientations as well as anisotropic lattice expansion and high residual stresses are some peculiar properties associated with the formation of this phase. Another peculiar feature of the expanded austenite phase is related to its magnetic character. In this study, new data related to the magnetic nature of the expanded austenite layers on austenitic stainless steel (304 SS) and CoCrMo alloy by nitrogen plasma immersion ion implantation (PIII) are presented. Magnetic behaviour, nitrogen distribution, implanted layer phases, surface topography, and surface hardness were studied with a combination of experimental techniques involving magnetic force microscopy, SIMS, XRD, SEM, AFM and nanoindentation method. The experimental analyses indicate that the low temperature samples clearly show the formation of the expanded austenite phase, while the decomposition of this metastable phase into CrN precipitates occurs at higher temperatures. As a function of the processing temperature, phase evolution stage for both alloys follows the same trend: (1) initial stage of the expanded phase, γN, formation; (2) its full development, and (3) its decomposition into CrN precipitates and the Cr-depleted matrix, fcc γ-(Co,Mo) for CoCrMo and bcc α-(Fe,Ni) for 304 SS. MFM imaging reveals distinct, stripe-like ferromagnetic domains for the fully developed expanded austenite layers both on 304 SS and CoCrMo alloys. Weak domain structures are observed for the CoCrMo samples treated at low and high processing temperatures. The images also provide strong evidence for grain orientation dependence of magnetic properties. The ferromagnetic state for the γN phase observed here is mainly linked to large lattice expansions due to high N content.Master Thesis Structural Investigation and Wettability of Pvd Tin Caoted Cocrmo Orthopedic Alloy(01. Izmir Institute of Technology, 2007) Çağlar, Özlem; Öztürk, OrhanWettability, defined as the ability of any solid surface to be wetted when in contact with a liquid, is one of the most important properties of biomaterials since highly wettable surfaces are expected to disclose better adhesion of the cells.The wettability characteristics of a biomaterial.s surface can be improved by surface modification techniques.In this study, wettability characteristics of TiN coated CoCrMo orthopedic alloy materials were investigated. CoCrMo alloys were coated with TiN using physical vapor deposition (PVD) technique at 550 C for 6 h. The TiN coated layer microstructure,roughness, thickness, and composition were studied by X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscope (SEM). Wetting studies involved contact angle and surface tension measurements using distilled water and simulated body fluid (SBF) as test liquids. The contact angles of the coated and uncoated CoCrMo alloys were investigated by the sessile drop method and the surface tension of the test liquids was measured by the ring method. The XRD results show the PVD coated TiN films had (111) preferred orientation, while the SEM analysis indicated quite uniform TiN coated layers (about 3 m thick) with a columnar growth mode. The AFM results indicated much higher roughness values for the TiN coated surfaces compared to the uncoated ones. The experimental results show the contact angle values for the TiN coated samples is lower than the uncoated ones suggesting better wetting for the coated layer. Based on the contact angle and the surface tension values, the work of adhesion values were estimated and it was found that the TiN coated layers has better adhesion ability compared to the uncoated CoCrMo alloy. The lower contact angles for the TiN coated samples were attributed mainly to the rougher surfaces associated with the TiN coated surfaces in comparison with the relatively smooth surface of the uncoated samples. Also, based on the EDX analysis results, the surface oxygen content on the coated surface may be another factor contributing to the better wettability characteristics of the TiN coated CoCrMo alloy compared to the uncoated alloy.Master Thesis Biocompatibility and Microstructural Characterization of Pvd Coated and Nitrogen Implanted Co-Cr Alloy(Izmir Institute of Technology, 2004) Türkan, Uğur; Öztürk, OrhanIn this study, the effectiveness of nitrogen ion implanted and TiN coated layers on CoCrMo alloy (ISO 5832-12) in preventing metal ion release during in vitro exposure of these layers to simulated body fluid (SBF) was investigated. The experimental results clearly show higher levels of cobalt ion release from nitrogen implanted CoCrMo material surfaces into simulated body fluid as compared to the as-polished CoCrMo alloy. The results clearly indicate that nitrogen ion implantation used for modification of fcc CoCrMo alloy surfaces lead to the development of various near surface microstructures. Nitrogen ion implantation was carried out at 60 and 30 keV ion energies with the corresponding current densities of 0.1 and 0.2 mA/cm2, respectively, for the substrate temperatures of 100, 200 and 400 °C and implantation time of 30 minutes. Near surface crystal structures and phases, nitrogen ion implanted and TiN coated layer thicknesses were characterized by a combination of symmetric (.-2.) and grazing incidence x-ray diffraction (XRD and GIXRD) and cross-sectional scanning electron microscopy (SEM). Metal ion release into the simulated body fluid was analyzed by atomic absorption spectrometry (AAS) and inductively coupled plasma optical emission spectrometry (ICP-OES). The experimental XRD results clearly show the formation of a metastable, fcc, high-N phase (.N) in mainly fcc CoCrMo alloy for the ion beam conditions at 400 °C. The lower implantation temperatures, 100 and 200 °C, for both 60 and 30 keV ion energies, result in a nitride phase, (Co, Cr, Mo)2+xN. The cross sectional SEM results for the specimens implanted at the 60 and 30 keV ion energies at 400 °C reveal quite clearly the uniform nature of the .N layers. The .N layer thicknesses, based on the SEM data, were found to be 450 and 540 nm for the 60 and 30 keV implanted specimens, while the (Co, Cr, Mo)2+xN nitride layer has a thickness range from 150 to 250 nm for the 60 and 30 keV at 100 and 200 °C implantation conditions. The SEM results also indicate that the (Co, Cr, Mo)2+xN nitride and .N phase layers on the CoCrMo alloys have high etch resistance suggesting enhanced corrosion resistance for the N implanted specimens compared to the substrate material. The XRD and SEM results for the TiN coated (via PVD) specimens show that the fcc TiN coatings exhibit (111) preferred orientation and have a coating thickness of . 3 µm with a columnar type of growth mode. The experimental AAS results show that in vitro exposure of the N implanted layers result in higher levels of cobalt ion release into the SBF than the as-polished substrate CoCrMo alloy. This was attributed to the rougher surfaces of the N implanted specimens compared to that of the substrate material (i.e, rougher surface implying a larger area is available for metal ion release). It was also found that the specimens implanted at the lower substrate temperatures of 100 and 200 °C have lower levels of Co ion release compared to those specimens implanted at the substrate temperature of 400 °C. The limited dissolution of cobalt, in this case, was explained by the stronger bonds of metal-N in the nitride phase than those of .N phase. Furthermore, the AAS data indicate higher cobalt ion release rate for the N implanted specimens compared to the substrate alloy and suggest transport (diffusion) controlled dissolution reaction mechanism. The AAS results show no cobalt ions are released from the TiN coated specimens (i.e, the release levels were below the analytical detection limit of the AAS apparatus). This indicates that the TiN coated layer can be an effective barrier for reducing the metal ion release from the substrate alloy. The SEM/EDX study of the surface morphologies of the N implanted, TiN coated and as-polished CoCrMo alloy test specimens after the static immersion test clearly indicate calcium phosphate formation on the as-polished alloy, while there was almost no phosphate precipitates on the surface of N implanted and TiN coated specimens. While the experimental results show higher levels of cobalt ion release for the N implanted specimens compared to the substrate material, the overall release levels are found to be below toxic levels for the human body.Master Thesis Structural, Compositional and Mechanical Characterization of Plasma Nitrided Cocrmo Alloy(Izmir Institute of Technology, 2009) Okur, Serdal; Öztürk, OrhanPlasma nitriding techniques can be used to create wear and corrosion protective layers on the surface of CoCrMo alloys by modifying the near surface layers of these materials. In the present study, a medical grade CoCrMo alloy was nitrided in a low-pressure ( 60 mTorr) R. F. plasma at 400 Cfor 1, 2, 4, 6, and 20 hours under a gas mixture of 60% N2 . 40% H2. The structural as well as compositional characterization of the plasma nitrided layers were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and glow discharge optical emission spectroscopy (GDOES). The hardness and wear behaviour of the nitrided layers were performed by a microhardness tester and a pin-on-disk wear apparatus. The experimental analyses indicate that the expanded austenite phase, YN, with high N contents ( 30 at.%) is formed by the plasma nitriding process at 400 C. However, at longer nitriding times (6 and 20 h) there is decomposition into CrN in the YN matrix and a preferential (200) orientation of YN grains parallel to the surface develops. Based on the microscopy analyses of the electrochemically and Ar ion beam etched nitrided sample cross-sections and on the GDOES data, the YN layer thicknessesare found to be ranging from 2 to 10 microns. Based on the thickness data, an average N diffusion coefficient for the CoCrMo samples plasma nitrided at 400 C is estimated to be near 2x10-11 cm2/s. While significant improvements in hardness and wear volume reductions are observed for all the plasma nitrided alloys compared to the untreated alloy, the CoCrMo alloys with the YN structure only had the best combined wear-corrosion protection.