The behaviour of silica during the carbothermal reduction nitriding process at
temperatures between 1300-1500°C was studied by means of X-ray diffraction and scanning
electron microscope analysis. The experimental runs were allowed to proceed up to 1 h in presence
of nitrogen flow. The following mechanism of reduction nitriding of silica which was based on the
experimental observation was proposed. Initially the impurity of the starting material is reduced
before 1300°C. SiO2 was reduced into SiO gas phase by active carbon and it was vaporised out of
the mixture. The nucleation of α-Si3N4 was formed vapour-gas reaction took place and deposited on
the surface of the mixture as well as around the reaction crucible. In the third stage, α-Si3N4
transforms to one dimensional direction which was β-Si3N4 particle. This was followed by the
formation of SiC at temperature above 1450°C.
In the present work, laboratory experiments were carried out on the sintering of silicone nitride powder without any sintering additives. Silicone nitride powder was produced at own laboratory conditions by carbothermal reduction-nitriding of silica of which was presented elsewhere. It consists of mainly -Si3N4 (83%) and -Si3N4 (17%) phases with a small amount of minor constituents. Before compaction procedure, the powder was ball milled for 2 h and then 2 gr. of the sample was compacted uniaxially at a pressure of 25 MPa. The green compact was placed into furnace in a graphite crucible. The sintering process was carried out in a programmable muffle furnace at 1700 °C and held for 1 h. with a 20 °C/min. heating rate. The sintered compacts were analyzed using X-ray, SEM, SEM-EDS techniques. The results showed that the formation of liquid phase can accelerate the densification of the compact. The densification of the pure SIALON can be increased by adding of sintering aids that accelerates the formation of liquid phases in the sample. The densified sialons give better hardness and thermal properties of the sample. It was well demonstrated that for the densification of sialons large amount of liquid phase forming oxides are needed to add to initial composition. It is also suggested that there is linear relationships between densification and amount of liquid phase formed in the compact.
Although material science is advanced at present time, new technological developments require new generation and different materials that are commercially not available in global market. In order to make long distance space journeys possible in near future, innovation of multipurpose and reusable ceramic composite materials for at high temperature environments are needed. Ceramics that are useable at elevated temperatures as high as 2300˚C are called Ultra high temperature ceramics (UHTC). In this study, the processing of UHTCs based on ZrC/ZrB2/SiC/Al2O3 ceramic composite that has high melting point, chemical and thermal stability, at the laboratory conditions was investigated using zircon, boric acid, carbon and aluminium powders. Aluminothermic method was chosen, the starting mixtures and the obtained reaction products were analysed using SEM, XRD, XRF and TG/DTA analysis methods. The experimental results showed that, the obtained sample consists of ZrB2/ZrC as a matrix phases.
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