Use of Gas Phase Impregnation and Densification

Chemical vapor deposition (CVD) involves heating a fiber preform in a gaseous environment to deposit the matrix, present in the gaseous phase, on to the fiber. The term chemical vapor infiltration (CVI) is used to describe CVD densification occurring within the fiber preform as distinct from a simple surface deposition technique. Kohno describes the carbon infiltration of carbon-carbon composites [65]. 

Although CH4 requires a temperature above 550°C to initiate carbon deposition, it is widely available and has excellent diffusion properties and is hence, widely used in the CVD process. Diffusion can be increased further by dilution with H2, He, N2 or Ar to increase the mean free path (the average distance a molecule moves between two successive collisions) of the CH4. 

Spear [66] outlined a basic model for the different stages that occur during CVD  

Forced flow of reactant gases into the reaction vessel 

Diffusion of reactants through laminar flow boundary layers around the substrate 

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