Zafer Tatli

Carbothermal reduction and nitridation of quartz mineral for the production of alpha silicon nitride powders

In this study, α-Si3N4 powder was produced by carbothermal reduction and nitridation (CRN) of quartz from Can-Canakkale. Carbon with a specific surface area of 110 m2g−1 and quartz powders were mixed then the powder mix was placed in an alumina tube furnace and reacted in between 1300-1500°C for 4 hours under nitrogen flow. The quartz powder was carbothermally reduced and nitrided to form silicon nitride powders. XRD results showed that the reaction product was mainly α-Si3N4 and contained some β-Si3N4 and residual quartz. In order to reduce amount of unreacted quartz, the raw materials mixture was grinded either with carbon black or with no carbon. After CRN reactions of separate grinded quartz powders with carbon, residual quartz was disappeared, reaction temperature was decreased and α-Si3N4 rate was increased. Hence, a better mixing of carbon and fine silica enhanced the α phase formation. SEM images and XRD pattern showed that sub micron particles (0.6–0.87m), high α-phase content Si3N4 powders can be produced at 1450°C for 4 h in flowing nitrogen gas during the CRN process.

ROBOTİK MIG KAYNAK YÖNTEMİ İLE BİRLEŞTİRİLEN 5754 ALÜMİNYUM ALAŞIMININ MEKANİK VE MİKROYAPI ÖZELLİKLERİNE KAYNAK HIZININ ETKİSİ

MIG method has become the most preferred and the most wanted method in the welding of all the nonferro metals and alloys due to its convenience. In the welding operations with the usage of robotic MIG method that there are no defects which arise from welding operator has been the factor that increased the availability and reliability of this method. In a study done by starting from this point, AA 5754 aluminium sheets has been welded with robotic MIG method at different welding speed and their microstructure and mechanical properties has been analyzed. In this study optimum welding parameters and sufficient mechanical properties have been reached. Also it was shown that the changed welding speed affected the mechanical properties. These ideal parameters are believed to be important in their usage in industry

Gözenekli Biyoaktif Hidroksiapatit Seramiklerinin Üretimi

The main subject of this study was to produce a net shaped micro porous hydroxyapatite ceramics. For this aim, initially fabrication of hydroxyapatite powder was achieved by a thermal extraction method which is inexpensive processing route. Phase determination of the yield of the process was carried out by X-ray diffraction method. Replica technique was used to produce a reticulated structure from hydroxyapatite powder. The foams were produced with various rates of additive as binder and additive-free compositions. After the HAp daubed on surfaces of substrate polymer was fired and sintered in the process at > 1100°C. The obtained ceramic foam was characterized using optical microscopy. The preform exhibited different pore sized structures depending binder rate and sintering temperature. As a result, hydroxyapatite preforms was produced with high porosity and their properties were improved by adding the binder which supports biocompability of HAp.

α-Si3N4-β SiAlON Seramiklerinin Kesici Takım Potansiyelinin İncelenmesi

One of the ceramic products is ceramic cutting edges and these are used as cutting tools for fabrication of metal products (ferrous and non ferrous metals) at very high speeds. These cutting tools can mainly be classified silicon nitride, alumina, mixed-ceramics, titanium carbonitride, cubic boron nitride and hard ceramic particle reinforced metals. Ceramic cutting tools offer a high productivity because of their superior hardness, fracture toughness and resistance of elevated temperature which allows higher cutting speeds. In addition to these, resistance to chemical attacks is important in selection of cutting tool materials. In this research, high dense Si3N4 ceramic parts were fabricated, and cutting performance of the produced α/β Si3N4 ceramics were investigated. The microstructure analyses were carried out by SEM and optical microscopy.

Fabrication of Mixed-Oxide Sequential Deposited Silicon Nitride Powders

Silicon nitride can easily be hot-pressed with small additions of metal oxide to give a product with high strength and zero porosity but the disadvantage of hot-pressing is that it is an expensive process limited to the formation of fairly simple shapes and small batches. The present work has therefore focused on preparing MgO-Al2O3 densified samples of silicon nitride by the much cheaper and more versatile technique of pressureless sintering. Coating studies using mixed solutions of Mgand Al-alkoxides showed a tendency for mutual polymerisation, rather than codepositing homogeneously on the silicon nitride powder. Improved coating was achieved by sequential deposition, and this also afforded better control of the total thickness of the deposited layer. A particular advantage at the coated powders was that they could be sintered to >99% density at temperature as low as 1450°C. It is concluded that the powder coating technique is an excellent method of homogeneously incorporating minor amounts of sintering additive into a powder. Introduction Silicon nitride based ceramics are widely used in high temperature applications due to their excellent mechanical, physical, and chemical properties [1]. Because of the covalent bonding character, the poor sinterability of Si3N4 ceramics is generally encountered during fabrication. However, the application of this materials is still limited, mainly because of its low reliability and high machining costs [2]. The densification of Si3N4 can be by using oxide additives, such as yttria, alumina, and magnesia, which allow sintering to proceed via a transient liquid phase [3-4]. Silicon nitride ceramics are conventionally prepared by mechanical mixing of silicon nitride powder and sintering additive [5]. However, this processing does not guarantee a homogeneous distribution of the additives. An alternative method involves the coating of the silicon nitride particles with precursors of the sintering additives [6-7]. Further more conventional method is not well suited for the doping of Si3N4 with small amounts of sintering additive. Particle coating techniques are receiving increasing attention as they offer several advantages. They provide a convenient means of incorporating sintering aids and dopants more uniformly than can be obtained by conventional powder mixing [8-9]. Usually, Multiphase oxide mixtures of alumina, magnesia, calcia, yttria or rare earth oxides are used as sintering aids [10] In this work mixtures of MgO+Al2O3 were used. These were deposited sequentially on the surface of silicon nitride powders, with the aim of simulating grain-boundary eutectic liquid compositions, which would achieve lower-temperature densification.