PULSE CHEMICAL-VAPOR-DEPOSITION AND INFILTRATION OF PYROCARBON IN MODEL PORES WITH RECTANGULAR CROSS-SECTIONS .2. STUDY OF THE INFILTRATION

Model straight pores with rectangular cross-section (size ranging from 60 to 320 mum) have been infiltrated with pyrocarbon resulting from the cracking Of C3H8 or CH4 under pulse chemical vapour infiltration (P-CVI) conditions. Three main parameters control the quality of the pore infiltration: temperature and pressure, as previously known for regular CVI under isothermal/isobaric conditions (I-CVI) and, additionally, the residence time t(R), which appears to be the key parameter in P-CVI. There is a direct correlation between t(R), on the one hand, and both the PyC thickness gradient and anisotropy along the pores, on the other hand. The experimental results are explained on the basis of a qualitative model assuming two competing deposition mechanisms, depending on whether PyC is formed from small and H-rich molecules (akin to C3H8) (low t(R) values) or from large aromatic H-poor intermediates resulting from the maturation of the gas phase (high t(R) values). The use of CH4 (more stable thermally than C3H8) slows down the maturation process and favours in-depth infiltration. The best infiltrations, similar to and even better than those reported for I-CVI, are achieved under low t(R), T, P conditions but require a very large number of pulses. P-CVI is an efficient way to control the microstructure of the deposit.