The thermal decomposition of hydrocarbons has been investigated under particular conditions in a low-pressure chemical vapour deposition reactor in order to select suitable carbon sources for the preparation of carbon-rich or graduated Si , C, --x layers. Growth of pyrolitic carbon thin films (pyro-C) starts at ca. 1050 K and only above 1273 K using C,H,Pr' and CH, , respectively. The microstructure of the pyro-C layers is more dependent on the deposition temperature than on the nature of hydrocarbons. Their co-pyrolysis with SiEt, used as Sic precursor has been achieved in the temperature range 1050-1250 K. As expected, the film composition does not change significantly at 1173 K using CH, as an additional C source. By contrast, the C content of the films deposited by co-pyrolysis of SiEt4 and C6H5Pri increases continuously from 0.48 to 1 by increasing the mole fraction ratio x(C6H5Pri)/[x(C,H,Pri) +x(SiEt,)] from 0 to 1. Multilayers and compositional gradient layers can be prepared by discrete or continuous changes of the gasphase composition, respectively. These films were successfully used as interphase in ceramic-ceramic composite materials to weaken the fibre/matrix bond and to improve their ductility.
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