Chemical vapor infiltration (CVI) is a fiber-reinforced composite manufacturing process in which the matrix is introduced into the fibrous preform through the use of high-temperature reactive gases.
Chemical vapor infiltration (CVI) is a fiber-reinforced composite manufacturing process in which the matrix is introduced into the fibrous preform through the use of high-temperature reactive gases. The CVI process is fairly simple – a fibrous preform is supported on a porous metallic plate and a carrier gas (mixed with matrix material) is then passed through at a high temperature. However, after leaving the reactor, an effluent treatment plant is used to treat the gases and unreacted matrix material. That is the simplest form of CVI, known as the ‘hot wall’ technique and used with isothermal and isobaric CVI processes. However, this process can typically take a very long time because the deposition rate is very slow. For that reason, new processes are being developed to shorten the time required. For example, thermal gradient CVI uses a forced flow process (FCVI) to speed up the rate of deposition. The final material achieved is less porous and more uniformly dense because the gases and matrix material are forced to flow under higher pressure with a large temperature gradient from 1050 °C at the water-cooled zone to 1200 °C at the furnace zone.
How is Chemical Vapor Infiltration Used?
Chemical vapor infiltration manufacturing processes can be used for the production of many different composite types. For example, carbon/carbon (C/C) composites can be formed through FCVI with kerosene as the precursor matrix material. Alternatively, silicon carbide/silicon carbide (SiC/SiC) composites can be formed in two steps. First, methane gas is introduced to a preform of SiC fibers to form the interlayer between the fiber and matrix. Then, methyltrichlorosilane is carried by hydrogen into the chamber to form the silicon carbide matrix.
Advantages of Chemical Vapor Infiltration
There are many advantages of chemical vapor infiltration composite manufacturing processes. For example, the residual stresses in the material are much lower because the infiltration temperature required is lower than other manufacturing methods. Larger, more complex shapes can be produced with little damage to the fibers and geometry of the preform due to the low infiltration temperatures and pressures required. Additionally, there is a large degree of flexibility when choosing fibers and matrices and very pure matrices can be produced if the purity of the gases are well-controlled. However, there are a few downsides to this method of composites manufacturing. The composite materials produced have fairly high porosities and the process itself is slow with high capital investment and operating costs.
In the future, perhaps more methods of chemical vapor infiltration composites manufacturing will be developed to help mitigate these disadvantages and further the advantages of the process. Once the process becomes more economical it is probably that it will become a much more popular technique for the manufacturing of composites – particularly large and complex composite parts.