G. Celebi Efe

Investigation of Boronizing Kinetics of AISI 51100 Steel

In this study, some mechanical properties of borided AISI 51100 steel with high C concentration were investigated. Boronizing heat treatment was carried out in solid medium consisting of Ekabor-II at 850, 900, and 950xa0°C for 2, 4, 6, and 8xa0h. Morphology and mechanical properties of boride layer, and the effect of chemical composition on properties and kinetics of borides were investigated. The results of this study indicated that the morphology of the boride layer has a saw-tooth nature, and its hardness is over 1500 HV. Depending on process time and temperature, the depth of boride layer ranged from 30 to 106xa0μm. Optical and SEM studies and XRD analysis revealed that borides formed on the surface of steel substrates have dominantly single Fe2B boride phase in addition to small amount of Cr2B.

3D porous collagen/functionalized multiwalled carbon nanotube/chitosan/hydroxyapatite composite scaffolds for bone tissue engineering

In this study, we describe new collagen/functionalized multiwalled carbon nanotube/chitosan/hydroxyapatite (Col/f-MWCNT/CS/HA) composite scaffolds which were fabricated by freezing (-40u202f°C at 0.9u202f°C/min) and lyophilization (48u202fh, 0u202f°C and 200u202fmtorr). The compressive stresses (from 523 to 1112u202fkPa), swelling (from 513.9u202f±u202f27 to 481.05u202f±u202f25%), porosity (from 98u202f±u202f0.15 to 95.7u202f±u202f0.1%), contact angle (from 87.8 to 76.7°) properties examined before and after biomineralization for comparison 3D porous Col, CS, Col/f-MWCNT and Col/f-MWCNT/CS scaffolds. Biomineralization was performed by biomimetic method in concentrated SBF (10u202f×u202fSBF, at 37u202f°C and 6.5u202fpH). XRD, SEM, EDS, FTIR, TGA, Optical microscopy and BET results showed that compared to Col, CS and Col/f-MWCNT scaffolds, Col/f-MWCNT/CS scaffolds had higher in vitro bioactivity, large surface area (11.746u202fm2/g) and a good pore volume (0.026u202fcc/g), interconnected porous microstructure (with 20-350u202fμm pore size) and incorporates the advantageous properties of both Col, f-MWCNT, CS and HA. Finally, the methyl thiazolyl tetrazolium (MTT) assay was performed to evaluate scaffolds cytotoxicity which showed that Col/f-MWCNT/CS scaffolds have the best biocompatibility.

A comparison of pretreatments on hydroxyapatite formation on Ti by biomimetic method

In this study, hydroxyapatite coating on titanium material substrate was successfully performed by using biomimetic method. Titanium plates immersed in 1.5 SBF at pH 7.4 and 37xa0°C were analyzed at the end of the first, second, and fourth weeks. At the end of the first week, the immersion process was continued with the sample exposed to the optimum selected surface pretreatment. Three different treatments have been applied to determine the optimum surface treatment: each substrate immersed into NaOH, HCl, and NaOH + HCl solutions before heat treatment at 600xa0°C for 1xa0h and immersed in NaOH solution was selected the optimum surface treatment. The presence of biphasic hydroxyapatite (HA, Ca5(PO4)3(OH)) and tricalcium phosphate (TCP, Ca3(PO4)2) on Ti surfaces were confirmed by XRD. SEM studies showed that denser HA coating which have nano-sphere-like morphology formed on Ti pretreated with NaOH solution at the end of first week than other hydroxyapatite (HA)-coated Ti pretreated with NaCl and NaOH + NaCl solutions and coating thickness increased by increasing immersion time. The HA coating thickness of the samples immersed in optimum pretreatment solution was found as 178 and 340xa0μm for at the end of second and fourth weeks, respectively. The particle size analysis of the biphasic HA powders scraped from the coating layer on the substrate before and after sintering was carried out by Zetasizer and it showed that HA powders have 0.58xa0μm average particle size and their particle size distribution has less dimensional dispersion after sintering. Energy-dispersive X-ray spectroscopy (EDS) analysis revealed that the Ca/P ratio in HA powders was near to 1.5. Raman and Fourier transform-infrared spectroscopy (FTIR) results combined with the X-ray diffraction (XRD) indicated the presence of biphasic hydroxyapatite after biomimetic coating process and increment in crystallinity of the powders after sintering. It was found that HA nucleation on Ti pretreated with NaOH solution was higher than Ti pretreated with other solutions which is confirmed by electrochemical impedance spectroscopy (EIS).

Effect of Double Stage Nitriding on 34CrAlNi7-10 Nitriding Steel

In this study, effect of double stage gas nitriding of 34CrAlNi7-10 steel was analyzed. Nitriding processes were carried out in two stages. in the first stage, samples were nitridied at 500 ◦C for 10 h and then in second stage nitriding process was continued at 530 and 550 ◦C for 20 h. During both stages, the nitrogen activity on surface was controlled by the nitriding potential, which is a thermodynamical control parameter for controlled gas nitriding process. The nitriding potential was kept constant, at value of 10, during the first stage and was varied in the range of 3.2–0.2, for both nitriding temperatures, in the second stage. The presence of nitrides, formed on the surface of test materials was determined by XRD analysis technique. The morphology of nitrided layer was studied by optical microscopy. The microhardness of the surfaces of nitrided samples in HV1 was between 955 and 1029. The measurements have shown that the thickness of the compound layer (white layer) has varied between 6.85–23.90 μm and that the growth of the white layer and microhardness gradient were strongly affected by the nitriding potential and the temperature. Diffusion depths as Nht(HV) for Vickers Hardness were determined and compared.

Characterization of UHWPE-TiO2 Composites Produced by Gelation/Crystallization Method

Characterization of UHWPE-TiO2 Composites Produced by Gelation/Crystallization Method G. Celebi Efea,b,∗, C. Bindal and A.H. Ucisik Sakarya University, Engineering Faculty, Department of Metallurgy and Materials Engineering, Esentepe Campus, 54187 Sakarya, Turkey Sakarya University, Biomedical, Magnetic and Semi Conductive Materials Research Center (BIMAS-RC), Esentepe Campus, 54187 Sakarya, Turkey Atılım University, Board of Trustees, 06836 Ankara, Turkey