CVD process for Si-N-O fibre growth controlled by in-situ FTIR spectroscopic monitoring

High strength and high temperature composite materials such as CMCs represent a very promising family of future materials. A high temperature CVD-process has been developed to produce a new type of high-performance amorphous silicon-oxynitride (Si-N-O) fibres. The fibres were grown on a SiC substrate at 1450°C exposing a stoichiometric precursor powder mixture of SiO2 + SiC, doped with 10 wt% Ti powder to flowing NH3. To improve CVD process control an in-situ FTIR monitoring system is in development. For application of a FTIR based monitoring to the fibre growth process a specific optical adaptation has been designed onto the growth reactor. The optical set-up allows an almost simultaneous in-situ measurement of the transmission and emission of the hot gas atmosphere just above the precursor powder mixture. In addition to the decomposition of NH3, different reaction products have been identified, such as CO, HCN and CH4. Gaseous Si-O species could be detected which are responsible for the silicon transport in the gas phase from the solid SiO2 precursor powder to the fibre growth position. The assessment of the SiO bands has been supported by additional experiments which promote the formation of gaseous SiO.