POLYMER-DERIVED SI-BASED BULK CERAMICS .1. PREPARATION, PROCESSING AND PROPERTIES

The synthesis and processing of dense silicon-based bulk ceramic materials derived from the thermal decomposition of preceramic organosilicon polymers such as polysilazanes and polysilanes was achieved by three different routes A, B, and C. Additive-free silicon carbonitride bodies of the composition Si1.7C1.0N1.6 were produced according to route A, with up to 94% relative density by the pressureless pyrolysis of compacted infusible polysilazane powders at low processing temperatures (1000 degrees C). The resulting silicon carbonitride was single-phase and amorphous according to X-ray- and TEM-investigations and exhibited a low solid-phase density of 2.33 g/cm(3). The maximum room temperature fracture strength of additive-free silicon carbonitride was 370 MPa. Crystallization of the as-synthesized silicon carbonitride bulk samples occur red at temperatures exceeding 1400 degrees C. In a second process, route B, densification of polysilazane-derived amorphous silicon carbonitride powder was achieved by liquid phase sintering with alumina and yttria as sintering additives in nitrogen atmospheres and at temperatures up to 1900 degrees C. This process resulted in the formation of dense polycrystalline beta-Si3N4/beta-SiC-composites. The average room temperature fracture strength and fracture toughness of the gas pressure sintered composite were in the range of 650 MPa and 10.2 MPa root m, respectively. Polycrystalline Si3N4/SiC-composites were obtained in a third process, route C, by the pyrolysis,and subsequent sintering of alpha-Si3N4-powder/polysilane blends. The Si3N4-powder serves as an inert filler reducing the volume shrinkage associated with the polymer-to-ceramic transformation. Dense Si3N4 and Si2N2O bulk ceramics were formed according to route B by the liquid phase sintering of amorphous silicon nitride powder synthesized by the polysilazane pyrolysis under ammonia.