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首页> 外文期刊>CERAMICS INTERNATIONAL >Microstructural evolution mechanism of porous reaction bonded silicon nitride ceramics heat-treated in two powder beds
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Microstructural evolution mechanism of porous reaction bonded silicon nitride ceramics heat-treated in two powder beds

机译：两种粉末床中多孔反应粘结氮化硅氮化硅陶瓷的微观结构演化机理

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摘要

Porous Si3N4 with various grain morphologies was prepared by direct nitriding of silicon powder and subjected to heat-treatment while embedded in a Si3N4 powder bed. The influence of MgO, added either to the starting silicon or to the powder bed, on the microstructural transformation and morphology of pores at temperatures between 1425 degrees C and 1700 degrees C are discussed. In the presence of MgO, alpha-Si3N4 grains with equiaxed morphology resulted in an interconnected microstructure with spherical pores. By contrast, in the absence of MgO, an alpha-whisker-dominant microstructure resulted in pores of various morphologies. During the post heat-treatment process, the alpha-whiskers gradually vanished, and grains recrystallized as very fine beta-Si3N4 rods. Consequently, pores became spherical, large and whisker-free. In agreement with the results of SEM-EDX along with XRD analysis, the observed morphology transition and full phase transformation occurred by vapor phase transport of MgO from the powder bed and a subsequent solution-precipitation mechanism. The presence of volatile MgO in the powder bed caused a substantial decrease in weight losses, while enhancing beta phase formation, grain coarsening and linear shrinkage. The development of coarse beta-rods from alpha-matte grains in a Si-Mg-O-N glassy phase was related to the presence of substantial liquid phase during the growth mechanism. Compared to the granular morphology, whiskers with high aspect ratio gave rise to a high sintering driving force and led to a maximum value of approximate to 2% linear shrinkage and porosity of approximate to 30 vol%. Consequently, this ceramic exhibited the highest compressive strength of approximate to 10 MPa.
机译：通过直接氮化硅粉末制备具有各种晶粒形态的多孔Si3N4，并在嵌入Si3N4粉末床中进行热处理。讨论了在起始硅或粉末床中添加MgO的影响，在1425℃和1700摄氏度之间的温度下的微观结构转化和孔的形态上。在MgO的存在下，具有等轴形态的α-Si3N4晶粒导致与球形孔的相互连接的微观结构。相比之下，在没有MgO的情况下，α-晶须显性微观结构导致各种形态的孔。在后热处理过程中，α-晶须逐渐消失，并且将晶粒重结晶为非常细的β-Si3N4棒。因此，毛孔变成球形，大而无须无类。同意SEM-EDX的结果以及XRD分析，观察到的形态转变和全相转变发生来自粉末床的MgO和随后的溶液沉淀机制。在粉末床中存在挥发性MgO引起重量损失的显着降低，同时增强了β相形成，晶粒粗化和线性收缩。在Si-Mg-O-N玻璃相中从α-哑光晶粒中的粗β-棒的发展与生长机制期间存在显着液相的存在。与粒状形态相比，具有高纵横比的晶须产生高烧结驱动力，并导致最大值近似为2％线性收缩和近似约30体积％的孔隙率。因此，该陶瓷表现出近似为10MPa的最高抗压强度。

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