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.