ENVIRONMENTAL DURABILITY OF SILICON NITRIDE-BORON NITRIDE COMPOSITES

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It is well known that Si_3N_4-BN composite materials are attracting considerable interest for a range of diverse applications such as components for automobiles to refractory nozzles, tubes, sliding gates, and break rings for the horizontal continuous casting of steel. It is important to investigate the high-temperature oxidation and the high-temperature high-pressure water corrosion of these composites because these materials are subject to corrosive attack in severe environments such as chemical engineering. The purpose of this project is to investigate the corrosion behavior of pressureless-sintered and reaction-bonded Si_3N_4-BN composite in pure oxygen and pure water at elevated temperatures. The results are discussed with respect to the microstructure of these materials. The high-temperature oxidation tests were conducted in pure oxygen at 900-1200ﾟC.The hydrothermal corrosion tests were conducted in an autoclave at 120-300ﾟC.The following results were obtained.(1) The high-temperature oxidation resistance and the hydrothermal corrosion resistance were mainly dependent on BN content, and the material with higher amount of BN had more poor oxidation and corrosion resistance.(2) The depression of further oxidation and corrosion due to the stable passive film which were made from silica and additives such as Y_2O_3 and Al_2O_3 was recognized in pressureless-sintered Si_3N_4-BN composite.(3) The high-temperature oxidation resistance and the hydrothermal corrosion resistance for reaction-bonded Si_3N_4-BN composite were superior to those for pressureless-sintered Si_3N_4-BN composite.