Microstructure and Properties of Nanosemicrystalline Si 3 N 4 Ceramics with Doped Sintering Additives: Part II. Phase Tr
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Microstructure and properties of nanosemicrystalline Si3 N4 ceramics with doped sintering additives: Part II. Phase transformation and microstructural control K. H. Ryu and J-M. Yang Department of Materials Science and Engineering, University of California – Los Angeles, Los Angeles, California 90095-1595 (Received 23 June 1997; accepted 29 January 1998)
The low temperature pressureless sintering of a nanosized Si3 N4 powder with doped sintering additives was investigated. The microstructural evolution during sintering at different temperatures was analyzed using x-ray diffraction and scanning electron microscopy. The effect of using nanosized Si3 N4 powder as a catalyst to accelerate the a ! b –Si3 N4 transformation of a commercial Si3 N4 powder with larger particle sizes was also investigated. Finally, two stage sintering was used to study the feasibility of controlling the microstructure and the mechanical properties of the nanosized silicon nitride.
I. INTRODUCTION
Silicon nitride ceramics are one of the most promising candidates for high temperature structural applications in corrosive environments. However, due to the strong covalent bonding between silicon and nitrogen atoms, the consolidation of silicon nitride ceramics is usually achieved through liquid phase sintering using extrinsic sintering additives such as metal oxide and rare earth metal oxide. Normally, liquid phase sintering through the dissolution-reprecipitation process gives rise to elongated b –Si3 N4 grains in the dense ceramics.1,2 The role of elongated b –Si3 N4 grains is to increase the fracture toughness by bridging or deflecting cracks. Previous studies3,4 indicated that the fracture toughness of silicon nitride was strongly dependent on the diameter of the elongated grains. Therefore, controlling the formation and growth of the elongated grains during the densification is critical to the mechanical properties of the Si3 N4 . Recently, ultrafine silicon nitride ceramics have been developed for a variety of applications due to the potential ability of reducing the defect size and density.5–10 The characteristics of the nanosized silicon nitride powders are strongly dependent on the fabrication routes such as the laser driven5,7 and the plasma assisted chemical reaction.6,9,10 Typically the nanosized powders are amorphous, spherical-shaped, with sizes of few tens of nanometers. However, the homogeneous mixing of extrinsic sintering aids with nanosized powders is difficult to achieve because the ultrafine powders have a tendency to agglomerate. The inhomogeneous mixing of sintering aids can cause localized liquid phase formation which leads to uneven shrinkage and distortion of the sintered body. To avoid inhomogeneous mixing of sinter2588
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J. Mater. Res., Vol. 13, No. 9, Sep 1998
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ing additives, several efforts have been pursued to modify the surface of the individual particles.11,12 Specifically, the surface coating with a thin layer of
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