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Öğe Fabrication of silicon nitride with MgO/MgF2 using pressureless sintering(Polish Ceramic Society, 2009) Çaliskan, F.; Tatli, Z.; Hampshire, S.; Sönmez, H.; Demir, V.Because of its superior properties, that is high hardness and strength, excellent creep, oxidation and corrosion resistance as well as low density, silicon nitride is a candidate material for high temperature applications and is frequently used as cutting tools, a typical application for hard materials. In this study, various silicon nitride compositions were prepared using oxide and fluoride additives (e.g. MgO, Y2O3, MgF2). After the green samples were obtained by manual pressing, the samples were cold isostatically pressed under 250 MPa. Cold pressed samples were pressureless sintered under nitrogen atmosphere in the range 1450-1700°C for 0.5-2.5 hour. Silicon nitride sintered with the additions of MgF2 and Al2O3 gave better density and pore free micrographs especially at lower temperatures compared with the samples containing MgO plus Al2O3. Densities of samples were measured by using Archimedes principle and analyses of resulting products were carried out using SEM and X-ray diffraction.Öğe Processing of vacuum heat-treated MgO-densified silicon nitride crucible for molten cast iron handling(Elsevier Sci Ltd, 2012) Demir, V.; Demir, A.Silicon nitride with 3% MgO powder mix is uniaxially and cold isostatically pressed to form a green Si3N4 crucible. Liquid. phase sintering was applied to the green Si3N4 crucible at 1600 degrees C for 30 min under the nitrogen atmosphere. Intergranular Mg-Si-O-N glass remained between the silicon nitride grains which reacted with the molten metal during melting. This grain boundary glass was removed by vacuum heat treatment at 1575 degrees C for 5 h. The vacuum heat treated crucible was used to melt cast iron to examine reactions between the molten metal and Si3N4 ceramic crucible. EDX spectra across the Si3N4-cast iron interface and XRD for silicon nitride sample after cast-iron melting side surface analysis were carried out. Optical microscopy and SEM image analysis were made to examine the interaction between Si3N4 crucible and cast iron melt. Surprisingly, no reaction was observed between the vacuum heat treated crucible and melted cast iron. (C) 2011 Elsevier Ltd. All rights reserved.Öğe Vacuum heat treatment of MgO-densified pressureless sintered silicon nitride ceramics(Trans Tech Publications Ltd, 2007) Demir, V.; Thompson, D. P.Silicon nitride samples were pressureless sintered with up to 5 w/o MgO to give densities in the range 98-99% of theoretical. After pressureless sintering, selected samples were placed in a vacuum heat treatment furnace surrounded by a carbon bed in a carbon crucible at a pressure of less than 4x10(-4) mbar, and vacuum heat treated at different temperatures and times to remove grain-boundary glass. The results showed that this was substantially achieved at 1575 degrees C for 3h and that increasing the time to 5 hours gave still further improvement. SEM images, EDX analysis and oxidation tests provided additional evidence for the removal of Mg from the samples.Öğe Vacuum heat treatment of MgO-densified pressureless sintered silicon nitride ceramics(Trans Tech Publications Ltd, 2007) Demir, V.; Thompson, D.P.Silicon nitride samples were pressureless sintered with up to 5 w/o MgO to give densities in the range 98-99% of theoretical. After pressureless sintering, selected samples were placed in a vacuum heat treatment furnace surrounded by a carbon bed in a carbon crucible at a pressure of less than 4×10 -4 mbar, and vacuum heat treated at different temperatures and times to remove grain-boundary glass. The results showed that this was substantially achieved at 1575°C for 3h and that increasing the time to 5 hours gave still further improvement. SEM images, EDX analysis and oxidation tests provided additional evidence for the removal of Mg from the samples.
