Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - WoS: 7Citation - Scopus: 7Mfm Imaging of Expanded Austenite Formed on 304 Ss and Cocrmo Alloys(Elsevier Ltd., 2014) Öztürk, Orhan; Fidan, Mehmet; Mändl, StephanNew data related to the magnetic nature of the expanded austenite layers on CoCrMo and austenitic stainless steel by nitrogen plasma immersion ion implantation (PIII) are presented. Implantations were performed in the temperature range between 300 and 550°C for a fixed processing time of 1h. Magnetic properties, nitrogen distribution, implanted layer phases, and surface topography were studied with a combination of experimental techniques involving magnetic force microscopy, SIMS, XRD, SEM and AFM. As a function of the processing temperature, phase evolution stage for both alloys follows the same trend: (1) initial stage of the expanded phase formation, γN; (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 CoCrMo and 304 SS 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.Article Citation - WoS: 2Citation - Scopus: 3Use of Combination of Accelerator-Based Ion-Beam Analysis Techniques To the Investigation of the Corrosion Behavior of Cocrmo Alloy(Elsevier Ltd., 2014) Noli, F.; Misaelides, P.; Lagoyannis, A.; Pichon, L.; Öztürk, OrhanNuclear Reaction Analysis - NRA in combination with d-RBS (Ed: 1.35 MeV) was applied in order to investigate the corrosion behavior of CoCrMo alloy. The corrosion resistance of the alloy was compared to that of modified CoCrMo samples by several techniques as plasma nitriding and oxidizing at moderate temperature (∼400 °C). Electrochemical techniques in simulated body fluid 0.9% NaCl (37 °C) were applied in order to accelerate the corrosion process. The nitrogen depth distribution before and after the corrosion was determined using the 14N(d,α)12C and the 14N(d,p)15N nuclear reactions whereas the oxygen by the 16O(d,p)17O. The surface morphology and microstructure was investigated using microscopy techniques. It was found that surface treatments produce thick nitrided layers (5-6 μm) consisting of a supersaturated nitrogen solution (nitrogen concentration is ∼30 at.%) in the matrix (expanded phase γN) and a thin oxygen solution (0.3 μm). The samples subjected to plasma nitridation and oxidation exhibited the lowest deterioration and better resistance to corrosion compared to the single nitrided or single oxidized and the untreated material. This could be attributed to the modified surface region with the high nitrogen content and the presence of oxygen.Article Citation - WoS: 11Citation - Scopus: 13Magnetic Layer Formation on Plasma Nitrided Cocrmo Alloy(Elsevier Ltd., 2011) Öztürk, Orhan; Okur, Salih; Pichon, L.; Liedke, M. O.; Riviere, Jean PaulIn this study structural and magnetic character of the expanded austenite phase (γN) layer formed on a medical grade CoCrMo alloy by a low-pressure Radio-Frequency plasma nitriding process was investigated. The formation of the expanded austenite phase is facilitated at a substrate temperature near 400°C for 1, 2, 4, 6 and 20h under a gas mixture of 60% N2-40% H2. The magnetic state of the γN layers was determined by a surface sensitive technique, magneto-optic Kerr effect (MOKE), and with a scanning probe microscope in magnetic force mode (MFM). Strong evidence for the ferromagnetic nature of the γN-(Co,Cr,Mo) phase is provided by the observation of stripe domain structures and the hysteresis loops. The ferromagnetic state for the γN phase observed here is mainly linked to large lattice expansions (~10%) due to high N contents (~30at.%). As an interstitial impurity, nitrogen dilates the host lattice i.e. the Co-Co (or Fe-Fe) distance is increased, which strongly influences the magnetic interactions. An analogy between the magnetic properties of the expanded phases, γN-(Fe,Cr,Ni) and γN-(Co,Cr,Mo), formed in austenitic stainless steel alloys and the CoCrMo alloy of this study is made, and it is suggested that the ferromagnetic states for the γN-(Co,Cr,Mo) and γN-(Fe,Cr,Ni) phases may be correlated with the volume dependence of the magnetic properties of fcc-Co/Co4N and fcc-Fe/Fe4N, respectively.Article Citation - WoS: 26Citation - Scopus: 24Cocrmo Alloy Treated by Floating Potential Plasma Assisted Nitriding and Plasma Based Ion Implantation: Influence of the Hydrogen Content and of the Ion Energy on the Nitrogen Incorporation(Elsevier Ltd., 2010) Pichon, L.; Okur, Salih; Öztürk, Orhan; Rivière, J. P.; Drouet, M.Nitriding was performed on a medical grade CoCrMo alloy at 400°C in N2 or N2-H2 atmosphere at a working pressure of 0.84Pa for 2h. Various surface treatment techniques were used to incorporate nitrogen into the CoCrMo alloy: without any plasma assistance, by floating potential radio-frequency plasma assisted nitriding (FPPAN), by plasma based ion implantation (PBII) with several high voltage accelerations (up to 20kV). Without plasma activation, no nitrogen is incorporated in the CoCrMo. On the contrary, all the plasma or PBII treated samples show the formation of a nitrogen-rich f.c.c. γN phase. The layer nitrided over few microns has a nitrogen composition ranging from 30at.% to about 20at.% near the nitrided layer-substrate interface, with an enhanced surface microhardness. Hydrogen is found to enhance the nitriding efficiency. Without hydrogen, a high voltage polarization provides a supplementary amount of implanted nitrogen available for further diffusion and the sputtering of the surface passive oxide. So, with limited high voltages, thicker layers with higher amounts of nitrogen can be achieved by PBII compared to plasma nitriding. However, with higher voltages, the sputtering becomes too important and the nitride layer is thinner. © 2010 Elsevier B.V.Article Citation - WoS: 31Citation - Scopus: 40Structural and Magnetic Characterization of Plasma Ion Nitrided Layer on 316l Stainless Steel Alloy(Elsevier Ltd., 2009) Öztürk, Orhan; Okur, Salih; Riviere, Jean PaulIn this study, an FeCrNi alloy (316L stainless steel disc) was nitrided in a low-pressure R.F. plasma at 430 °C for 72 min under a gas mixture of 60% N2-40% H2. Structural, compositional and magnetic properties of the plasma nitrided layer was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and magnetic force microscopy (MFM). The magnetic behaviour of the nitrided layer was also investigated with a vibrating sample magnetometer (VSM). Combined X-ray diffraction, cross-sectional SEM, AFM and MFM, as well as VSM analyses provide strong evidence for the formation of the γN phase, [γN-(Fe, Cr, Ni)], with mainly ferromagnetic characteristics. The uniform nature of the γN layer is clearly demonstrated by the XRD, cross-sectional SEM and AFM analyses. Based on the AFM and SEM data, the thickness of the γN layer is found to be ∼6 μm. According to the MFM and VSM analyses, ferromagnetism in the γN layer is revealed by the observation of stripe domain structures and the hysteresis loops. The cross-sectional MFM results demonstrate the ferromagnetic γN phase distributed across the plasma nitrided layer. The MFM images show variation in the size and form of the magnetic domains from one grain to another.Article Citation - WoS: 18Citation - Scopus: 22Microstructural and Mechanical Characterization of Nitrogen Ion Implanted Layer on 316l Stainless Steel(Elsevier Ltd., 2009) Öztürk, OrhanNitrogen ion implantation can be used to improve surface mechanical properties (hardness, wear, friction) of stainless steels by modifying the near-surface layers of these materials. In this study, a medical grade FeCrNi alloy (316L stainless steel plate) was implanted with 85 keV nitrogen ions to a high fluence of 1 × 1018N2+ / cm2 at a substrate temperature <200 °C in an industrial implantation facility. The N implanted layer microstructures, thicknesses and strengths were studied by a combination of X-ray diffraction (XRD), conversion electron Mössbauer spectroscopy (CEMS), atomic force microscopy (AFM) and nanohardness measurements. AFM was also used for the surface roughness analysis of the implanted as well as polished materials. The CEMS analysis indicate that the N implanted layer is ∼200 nm thick and is composed of ε-(Fe,Cr,Ni)2+xN-like nitride phase with mainly paramagnetic characteristics. The nanohardness measurements clearly indicate an enhanced hardness behaviour for the N implanted layer. It is found that the implanted layer hardness is increased by a factor of 1.5 in comparison to that of the substrate material. The increased hardness resulting from nitrogen implantation is attributed to the formation of ε nitride phase.Article Citation - WoS: 67Citation - Scopus: 80Metal Ion Release From Tin Coated Cocrmo Orthopedic Implant Material(Elsevier Ltd., 2006) Türkan, Uğur; Öztürk, Orhan; Eroğlu, Ahmet EminIn this study, a medical grade CoCrMo alloy was coated with TiN by means of physical vapor deposition (PVD) technique at 550 °C for 6 h. The TiN layer microstructure and thickness were studied by X-ray diffraction (XRD) and cross-sectional scanning electron microscopy (SEM). The adhesive strength of the TiN coatings on the CoCrMo substrate was studied by a commercially available Scanning Scratch Tester. Static immersion test was conducted to investigate the effectiveness of TiN coating in preventing the dissolution of metal ions into the simulated body fluid (SBF) from the substrate by atomic absorption spectrometry (AAS) and inductively coupled plasma optical emission spectrometry (ICP-OES). The XRD results showed that the PVD coated TiN films exhibited (111) preferred orientation, while the SEM analysis indicated quite uniform and highly dense TiN coated layer (about 3 μm thick) with a columnar growth mode reaching from substrate to coating surface. The scratch test results showed that the adhesive strength between the TiN film and the CoCrMo substrate was adequate. The AAS and ICP-OES results showed that the presence of the TiN coating prevented the release of cobalt and chromium metal ions from the substrate CoCrMo alloy whereas cobalt was preferentially dissolved from the as-polished material. Calcium phosphate precipitation was observed on the surface of the as-polished material, indicating a degree of bioactivity of the as-polished surface which is absent in the TiN coated substrate alloy.Article Citation - WoS: 82Citation - Scopus: 92Metal Ion Release From Nitrogen Ion Implanted Cocrmo Orthopedic Implant Material(Elsevier Ltd., 2006) Öztürk, Orhan; Türkan, Uğur; Eroğlu, Ahmet EminCoCrMo alloys are used as orthopedic implant materials because of their excellent mechanical and corrosion properties. However, when placed in vivo, these alloys release Co, Cr, Mo ions to host tissues, which may give rise to significant health concerns over time. Nitrogen ion implantation can be used to form protective layers on the surface of CoCrMo orthopedic alloys by modifying the near surface layers of these materials. In this study, medical grade CoCrMo alloy (IS0 5832-12) was ion implanted with 60 keV nitrogen ions to a high dose of 1.9 × 10 18 ions/cm 2 at substrate temperatures of 100, 200 and 400 °C. The N implanted layer microstructures, implanted layer phases, and thicknesses were studied by a combination of Bragg-Brentano (θ/2θ) and grazing incidence (Seeman-Bohlin) X-ray diffraction (XRD and GIXRD) and cross-sectional scanning electron microscopy (SEM). Atomic force microscopy (AFM) was used for roughness analysis of N implanted as well as as-polished surfaces. Static immersion tests were performed to investigate metal ion release into simulated body fluid (SBF) by electrothermal atomic absorption spectrometry (ETAAS) and inductively coupled plasma optical emission spectrometry (ICP-OES). XRD and SEM analyses indicated that the N implanted layers were ∼ 150-450 nm thick and composed of the (Co,Cr,Mo) 2+xN nitride phase and a high N concentration Co-based FCC phase, γ N depending on the substrate temperature. ETAAS analysis results showed that in vitro exposure of the N implanted surfaces resulted in higher levels of cobalt ion release into the simulated body fluid compared to the untreated, polished alloy. The higher Co release from the N implanted specimens is attributed to the nature of the implanted layer phases as well as to the rougher surfaces associated with the N implanted specimens compared to the relatively smooth surface of the untreated material. SEM analysis of N implanted and untreated specimens after immersion tests clearly indicated calcium phosphate formation on the as-polished CoCrMo alloy, indicating a degree of bioactivity of the untreated metal surface which is absent in the N implanted specimens.Article Citation - WoS: 16Citation - Scopus: 18Microstructural, Mechanical, and Corrosion Characterization of Nitrogen-Implanted Plastic Injection Mould Steel(Elsevier Ltd., 2005) Öztürk, Orhan; Onmuş, Ortaç; Willeamson, Don L.Nitrogen-ion implantation can be used to improve the wear and corrosion behaviour of moulds for plastic injection by modifying the near-surface layers of these materials. In this study, an FeCr ferritic stainless steel (X36CrMo17, similar to AISI-420F) was ion implanted with 85 keV nitrogen ions to low and high doses of 2×1017 and 1×1018 ions/cm2 at a substrate temperature <200 °C in an industrial implantation facility. The N-implanted layer microstructures, thicknesses, and hardnesses were studied by a combination of symmetric and grazing incidence X-ray diffraction (XRD and GIXRD), conversion electron Mössbauer spectroscopy (CEMS), cross-sectional scanning electron microscopy (SEM), and nanohardness measurements. The friction, wear, and corrosion behaviour were investigated by a pin-on-disc tribo tester and a salt spray corrosion analysis method. The XRD, CEMS, and SEM analyses indicate that the N-implanted layers are ∼0.05-0.08 μm thick and are composed of ε-(Fe,Cr,Mn)2+xN with paramagnetic and magnetic characteristics. The treated layer shows nearly two times better corrosion resistance than does the substrate. The wear and nanohardness measurements indicate that the wear behaviour and the N-implanted layer hardness are dose dependent and the latter is increased by more than a factor of 1.6 for the high-dose implanted specimen in comparison with that of the substrate material.Article Citation - WoS: 15Citation - Scopus: 15Microstructural, Mechanical, and Corrosion Characterization of Plasma-Nitrided Plastic Injection Mould Steel(Elsevier Ltd., 2005) Öztürk, Orhan; Onmuş, Ortaç; Willeamson, Don L.Plasma nitriding can be used to improve wear and corrosion behaviour of moulds for plastic injection by modifying the near-surface layers of these materials. In this study, a ferritic stainless steel (X36CrMo17) was plasma nitrided at 520-540 °C for 15-18 h under various gas mixtures of N2+H2 in an industrial nitriding facility. The nitrided layer microstructures, thicknesses, and strengths were studied by X-ray diffraction (XRD), 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. Combined Mössbauer, XRD, and SEM analyses demonstrate that (Fe,Cr,Mn)-nitrides, the ε- and γ′-nitrides, the Fe3C-like carbide, and CrN are distributed in the top nitrided layers of several micron thickness. The CEMS and CXMS analyses clearly show the nearly complete decomposition of the surface and deeper layers into phase separated mixtures of pure bcc-Fe, (Fe,Cr,Mn)-nitrides, and CrN. The nitriding conditions with the gas composition N2/H2=1 produces the thickest nitrided layer (∼135 μm) with enhanced corrosion protection. The nanohardness of the surface layers is found to be plateau-shaped and increased by about a factor of three in comparison to that of the substrate material.