Papers by Author: Filiz Çinar Şahin

Abstract: In this paper, the correlation between structural and mechanical properties of nanostructured austenitic oxide dispersion strengthened (ODS) steel prepared by powder metallurgy were studied. High efficient milling process was applied to achive an optimal dispersion of ceramic oxide particles in the steel matrix. The austenitic powder with or without Y₂O₃ addition was milled by two different milling process (wet and dry). Spark plasma sintering as a fast and effective compaction method was used as sintering equipment. The morphology and structural properties of ODS were studied by X-ray diffraction and scanning electron microscopy. Mechanical properties, such as microhardness, bending strength and modulus of nanostructured ODS were also determined.

Abstract: This study was carried out to produce and characterize B4C - TiB2 ceramics by spark plasma sintering. Initial B4C and TiB2 powders were mixed in ethanol solution with YSZ balls for 1 hour. Sintering was performed with 5, 10, 15 and 20 volume % TiB2 containing mixtures respectively in disc-shaped graphite dies. The applied pressure was 40 MPa at 1760 °C for 5 minute sintering duration. The improvement of low sinterability and low strength of B4C was investigated as well as strength of flexibility, hardness and fracture toughness. The obtained products were characterized by X-ray diffraction and SEM imaging. The hardness values were measured under 1000 g load and the density values were measured with Archimedes' principle. The 3-point bending tests and modulus of elasticity measurements were also conducted.

Abstract: The aim of this study was to produce dense and mechanically strong boron carbide ceramics with the help of different oxide additives. Physical properties of two different grades of pure boron carbide powders were analysed and sintered by Spark Plasma Sintering method. Starting powders were prepared by ball milling with the addition of 5 wt. % Y2O3, Al2O3, SiO2 and Y2O3 + Al2O3. In the sintering step, powder mixtures were sintered by SPS method in round-shaped graphite dies under 50 MPa for 5 minutes in the range of 1700-1800oC. In the characterization step, sintered sample compositions and microstructures were characterized by XRD and SEM analysis respectively. The hardness values were measured under 1000 g load and the density values were measured with Archimedes' principle. The fracture toughness analysis were also carried out.

Abstract: Nanocrystalline alumina (Al2O3) powders were sintered by Spark Plasma Sintering (SPS) method in a vacuum atmosphere to obtain highly dense and fine grained final ceramic products. In the first section of experiments, 0.4 % wt MgO doped and 0.4 wt % Y2O3 doped Al2O3 were sintered at high temperatures and under high pressure with a SPS system. Later sintering procedures were carried out with codoping Y2O3 with the cathodic ratio of 0.4 wt % in order to investigate dopant effects on spark plasma sintered alumina. The microstructures of all samples were observed using scanning electron microscope and the properties such as density, hardness and fracture toughness were examined.

Abstract: Si3N4 based composites containing different amounts of SiC and SiAlON were produced by reactive spark plasma sintering of Si3N4, SiO2 and C black. Y2O3 and AlN were added as sintering additives. The SPS process was carried out at sintering temperature of 1650 0C for 5 min with uniaxial pressure of 40 MPa in a nitrogen atmosphere.The mechanic properties and morphology of Si3N4 ceramic composite were determined. Microstructures of the sintered samples were observed by SEM images and phase compositions were analysed by XRD.

Abstract: In the present study, a method based on sulfuric acid dehydration of sugar was developed to synthesis a precursor material, which can yield B4C/ TiB2 composites at much lower temperatures compared to traditional carbothermal methods. The precursor material for pure B4C and B4C / TiB2 composites were heat treated at 1650oC under Ar and Ar+H2 atmosphere. Then the samples were characterized by X-ray diffraction (XRD) and crystallized B4C and B4C / TiB2 composites can be obtained at 1650oC

Abstract: In the present study, a method based on sulfuric acid dehydration of sugar was developed to synthesize a precursor material, which can yield B4C/SiC composites at much lower temperatures compared to traditional carbothermal methods. The precursor material for pure B4C was heat treated at the temperatures between 400 and 1600oC under inert atmosphere. The precursor material for B4C /SiC composites was heat treated only at 1600oC under an inert atmosphere. Then the samples were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The results showed that B-C bonds formed as low as 400oC. On the other hand, crystallized B4C and B4C / SiC composites can be obtained at the heat treatment temperatures between 1400 and 1600oC.

Abstract: Titanium, zirconium and magnesium alloys are considered to be biocompatible, and can be used as implants such as hip ball and sockets and to make medical equipments. Biomaterials with hybrid structures in some applications utilizing the beneficial properties of different metals together are considered potential implant materials. Therefore, in this study, experimental trials were attempted to bond pure magnesium, AM60 (6 wt% Al-0.27 wt% Mn), and AZ31 (3 wt% Al-1 wt% Zn) alloys to pure zirconium and Ti6Al4V (6 wt% Al-4 wt% V) alloy to experimentally evaluate the forming bimetallic structures by diffusion bonding technique by vacuum hot pressing. SEM analysis showed the presence of a significant diffusion zone and a presence of diffusion bonding in some metallic couples. It may be suggested that novel hybrid implant materials, composed of diffusion couples of magnesium, zirconium and titanium alloys may emerge in the future.

Abstract: Silicon nitride based composites have been fabricated by carbon addition. Carbon black nanograins and graphite micrograins were used as second phase additions. Alumina and yttria were used as sintering aids. Mixing of powders was performed in a ball mill and for comparison in a high efficient attritor mill. For sintering the hot pressing technique has been applied. Experiments at 2 MPa uniaxial pressure have been performed. The Si/N mass fraction after sintering were determined by prompt gamma activation analysis (PGAA). The amount of carbon black and graphite introduced in the silicon nitride matrix increased the porosity and decreased the hardness and bending strength of composites. Lower modulus, and lower strength was obtained for composites with carbon black addition in comparison to graphite added samples. The microstructure of composites consisted mainly of alpha and beta silicon nitride. The formation of silicon carbide was observed only at 10 wt% carbon black addition.