Preparation of polyorganoborosilazanes and conversion into ultra-high-temperature borosilicon carbonitrides

Polyorganoborosilazanes with a wide range of B/Si/C ratios were synthesized by co-ammonolysis of various organochlorosilanes and borontrichloride. By pyrolysis of SiMe2Cl2-based or SiHC2H5Cl2-based polyorganoborosilazane with limited B/Si/C ratios in an inert atmosphere, amorphous borosilicon carbonitrides stable at 1880 degrees C were prepared. The amorphous structure of borosilicon carbonitride was closely related to distributions of B-N bonds, substituent groups on Si atoms and contents of B and C atoms in the preceramic polymers. Three-dimensional BCN networks in the ceramics maintained amorphous structures up to higher temperatures. These prepared ceramics were not only thermally stable, but the oxidation resistance is also excellent at 1700 degrees C in dry air and at 1500 degrees C under reduced pressures.