Synthesis and characterization of Si/Si₂N₂O/Si ₃N₄ composites from solid-gas precursor system via CVD

Si/Si₂N₂O/Si₃N₄ composites were prepared via chemical vapor deposition (CVD) using a solid-gas precursor system. In this method the Si-F gas species generated from the thermal decomposition of a solid precursor react with a nitrogen precursor gas to produce in situ silicon nitride and oxynitride. Cylindrical preforms (3 cm in diameter x 1.25 cm long) with 50 % porosity - prepared by the uniaxial compaction of Si powders with average particle size of 12.4 μm - were infiltrated in a multiple step mode with high purity nitrogen (HPN) and the Si-F gas species in a chemical vapor deposition (CVD) reactor. The specimens were heated at a rate of 15°C/min up to a processing temperature of 1300°C, and maintained isothermally for 70 minutes; then they were cooled down to room temperature. Results from the characterization by XRD and SEM show the deposition of Si₂N₂O and Si₃N₄ on the Si particles, occupying the interstices of the porous preforms. Si ₃N₄ is typically deposited with a sponge-like structure and compact deposits, while Si₂N₂O is formed with a pin-like morphology and as whiskers. Depending on the number of infiltration steps, Si/Si₂N₂O/Si₃N₄ composites with tailorablc porosity can be prepared.