SiC/BN Composites by Spark Plasma Sintering (SPS) of Precursor-Derived SiBNC Powders

Preceramic polyborocarbosilazane was pyrolysed at 900 degrees C and 1400 degrees C, respectively, yielding amorphous SiBNC. The as-obtained powders were subsequently spark plasma sintered (SPS) at temperatures in a range between 1400 degrees C and 1800 degrees C for 10 minutes under uniaxial pressure of 50 MPa. The samples have been characterized by SEM, TEM, XRD, helium pycnometry and Vickers hardness measurements. Independent of the pyrolysis temperature of the starting materials, the most compact samples were obtained by SPS at 1800 degrees C. In SPS conditions the onset of crystallization and phase separation of the amorphous SiBNC ceramic is substantially reduced as compared to conventional heating. A composite microstructure is obtained, which consists of SiC crystallites with diameters smaller than 500 nm, embedded in a matrix of turbostratic BN. Hardness values increase to 3 GPa when the SPS temperature is raised. TGA in O-2 shows a high resistance against oxidation of the SPS compacts.