THE CVI-PROCESSING OF CERAMIC MATRIX COMPOSITES

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Abstract
Under specific conditions, ceramics can be deposited from gaseous precursors within porous substrates. This technique, referred to as chemical vapor infiltration (CVI) is particularly suited to the preparation of ceramic matrix composites (CMC). Pore filling by CVI results from two simultaneous phenomena : (i) a surface reaction and (ii) mass transfers of the reactants and products in the gas phase. In isothermal/isobaric CVI, mass transfers occur only by diffusion. As a result. ICVI has to be performed at low temperatures and under reduced pressures in order to lead to a deposit homogeneous in thickness along the pores. In forced-CVI, mass transfers are by forced convection due to a pressure gradient. Moreover, an inverse thermal gradient is applied resulting both in a much higher deposition rate. The feasibility of the CVI process is established for a number of ceramic matrices including carbon and Sic

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