Chemistry and kinetics of chemical vapor infiltration of pyrocarbon -: VI.: Mechanical and structural properties of infiltrated carbon fiber felt

The mechanical and structural properties of carbon fiber felts infiltrated with pyrolytic carbon at a temperature of 1100 degrees C and pressures of 20 and 30 kPa using methane and a methane/hydrogen-mixture were investigated. Flexural strength, modulus, strain-to-failure, pore size distribution, crystallite parameters, and microstructural properties, using scanning electron microscopy and polarized-light microscopy, have been determined and analyzed, respectively. These properties were correlated with deposition chemistry and kinetics. The individual fibers are coated with a smooth laminar carbon layer showing low optical reflectivity, followed by another smooth laminar carbon layer showing medium reflectivity, and a rough laminar carbon sheath showing high reflectivity. These changes of carbon structure with progressive infiltration are discussed on the basis of a parameter [A(S)/V-R] describing the ratio of internal surface area A(S) and pore volume V-R of the felt. This parameter continuously increases with progressive infiltration whereby the deposition chemistry is changed from polynuclear aromatic to mononuclear aromatic and finally to light, linear hydrocarbons as the major carbon forming species. An autocatalytic mechanism is proposed to explain sharp transitions between the various carbon structures. (C) 1999 Elsevier Science Ltd. All rights reserved.