In simple terms, chemical vapour infiltration (CVI) involves a solid material being deposited from gaseous reactants via thermal decomposition on to a heated substrate, in this case the fibrous preform. The preform is placed in a high temperature furnace and gaseous reagents such as methane are introduced. Thermal degradation of the gas occurs at approximately 1000°C on the heated fibre surfaces forming a pyrolytic carbon deposit. This continues until sufficient material has been deposited to form a matrix that binds the fibres.
Advantages include relatively low processing temperatures and potential for near-net-shaping. Difficulties include a tendency to deposit preferentially near the component surface, leading to pore blockage or crusting, so inhibiting further densification. It can also lead to prolonged processing times because intermittent machining operations are needed to remove the crusting which adds to the long time needed for product manufacture.
Various techniques have been devised to speed up the production process, including plasma enhancement, directing the gas through the component, and thermal gradient techniques (where the sample is heated from underneath, whilst facing a cold wall, hence setting up a localised thermal gradient).