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

The synthesis and processing of dense silicon-based bulk ceramic mater ials derived from the thermal decomposition of preceramic organosilico n polymers such as polysilazanes and polysilanes was achieved by three different routes A, B, and C. Additive-free silicon carbonitride bodi es of the composition Si1.7C1.0N1.6 were produced according to route A , with up to 94% relative density by the pressureless pyrolysis of com pacted infusible polysilazane powders at low processing temperatures ( 1000 degrees C). The resulting silicon carbonitride was single-phase a nd amorphous according to X-ray- and TEM-investigations and exhibited a low solid-phase density of 2.33 g/cm(3). The maximum room temperatur e fracture strength of additive-free silicon carbonitride was 370 MPa. Crystallization of the as-synthesized silicon carbonitride bulk sampl es occur red at temperatures exceeding 1400 degrees C. In a second pro cess, route B, densification of polysilazane-derived amorphous silicon carbonitride powder was achieved by liquid phase sintering with alumi na and yttria as sintering additives in nitrogen atmospheres and at te mperatures up to 1900 degrees C. This process resulted in the formatio n 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 obtain ed in a third process, route C, by the pyrolysis,and subsequent sinter ing of alpha-Si3N4-powder/polysilane blends. The Si3N4-powder serves a s an inert filler reducing the volume shrinkage associated with the po lymer-to-ceramic transformation. Dense Si3N4 and Si2N2O bulk ceramics were formed according to route B by the liquid phase sintering of amor phous silicon nitride powder synthesized by the polysilazane pyrolysis under ammonia.