ENVIRONMENTAL DURABILITY OF SILICON NITRIDE-BORON NITRIDE COMPOSITES

众所周知，Si3N4-BN复合材料具有广泛的应用前景，如汽车部件、耐火接管、管材、滑动浇口和水平连铸钢材的断环等。由于这些材料在化学工程等恶劣环境中容易受到腐蚀，因此研究这些材料的高温氧化和高温高压水腐蚀具有重要意义。本项目的目的是研究常压烧结和反应结合的Si3N4-BN复合材料在高温下在纯氧和纯水中的腐蚀行为。结合材料的微观结构，对实验结果进行了讨论。高温氧化试验是在900-1200ﾟC纯氧中进行的，水热腐蚀试验是在120-300ﾟC的高压灭菌器中进行的。结果表明：(1)高温氧化和水热腐蚀性能主要取决于BN含量；在常压烧结的Si3N4-BN复合材料中，SiO_2和Y_2O_3、Al_2O_3等添加剂形成的稳定的钝化膜抑制了进一步的氧化和腐蚀。(3)反应结合的Si3N4-BN复合材料的高温抗氧化性能和水热腐蚀性能优于常压烧结的Si3N4-BN复合材料。

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.