Papers by Author: Yücel Onüralp

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: In this study, SiC films were deposited by reactive DC magnetron sputtering of high purity (99.999%) Si target in Ar/CH4 gas mixture. Three types of substrates, AISI M2 grade high speed steel, glass, and Si (100) were used in each deposition. Films were grown with different compositions at 50 oC and 250 oC by varying (0–50 %) CH4/Ar processing gas ratios. Microstructural properties of SiC films were characterized by cross-sectional FE–SEM (Field–Emission Scanning Electron Microscope) observations. XRD (X–Ray Diffractometer) results indicated that films were amorphous. Friction coefficients as low as 0.1 were obtained from SiC coatings against Al2O3 balls, according to the tribological tests. Optical investigations showed that the transparency and opacity of SiC films could easily be tailored by modifying Si and C concentrations in the coatings.

Abstract: SiC films were deposited by reactive DC magnetron sputtering of high purity (99.999%) Si target. 3 types of substrates, AISI M2 grade high speed steel, glass and Si (100) wafer were used in each deposition. The effect of different CH4 flow rates on the microstructural properties and surface morphologies were characterized by cross-sectional FE-SEM (Field-Emission Scanning Electron Microscope) observations. SIMS (Secondary Ion Mass Spectrometer) depth profile analysis showed that the elemental film composition was constant over the whole film depth. XRD (X-Ray Diffraction) results indicated that films were amorphous. Nanomechanical properties of SiC films were also investigated.

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: In this study, the self-propagating high-temperature synthesis (SHS) and following acid leaching techniques were carried out to produce zirconium diboride (ZrB2) powder. In the SHS experiments, technical grade ZrO2 powder, and different amounts of B2O3 and Mg powders were used. The SHS products were obtained in the form of black, spongy solid. In the leaching step, the effect of different acid concentrations on selective leaching was investigated by using optimum SHS product to eliminate the impurities such as MgO, Mg3B2O6 and Mg2B2O5. The products obtained were characterized by using XRD, ICP and SEM techniques.

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.