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Chemical vapour infiltration (CVI) is a
ceramic engineering Ceramic engineering is the science and technology of creating objects from inorganic, non-metallic materials. This is done either by the action of heat, or at lower temperatures using precipitation reactions from high-purity chemical solutions ...
process whereby matrix material is infiltrated into fibrous preforms by the use of reactive gases at elevated temperature to form
fiber-reinforced composite __NOTOC__ A fiber-reinforced composite (FRC) is a composite building material that consists of three components:Serope Kalpakjian, Steven R Schmid. "Manufacturing Engineering and Technology". International edition. 4th Ed. Prentice Hall, Inc. 2001 ...
s. The earliest use of CVI was the infiltration of fibrous alumina with
chromium carbide Chromium(II) carbide is a ceramic compound that exists in several chemical compositions: Cr3C2, Cr7C3, and Cr23C6. At standard conditions it exists as a gray solid. It is extremely hard and corrosion resistant. It is also a refractory compound, ...
. CVI can be applied to the production of carbon-carbon composites and ceramic-matrix composites. A similar technique is
chemical vapour deposition Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (substra ...
(CVD), the main difference being that the deposition of CVD is on hot bulk surfaces, while CVI deposition is on porous substrates.


Process

During chemical vapour infiltration, the fibrous preform is supported on a porous metallic plate through which a mixture of carrier gas along with matrix material is passed at an elevated temperature. The preforms can be made using yarns or woven fabrics or they can be filament-wound or braided three-dimensional shapes. The infiltration takes place in a reactor which is connected to an effluent-treatment plant where the gases and residual matrix material are chemically treated.
Induction heating Induction heating is the process of heating electrically conductive materials, namely metals or semi-conductors, by electromagnetic induction, through heat transfer passing through an induction coil that creates an electromagnetic field within th ...
is used in a conventional
isothermal In thermodynamics, an isothermal process is a type of thermodynamic process in which the temperature ''T'' of a system remains constant: Δ''T'' = 0. This typically occurs when a system is in contact with an outside thermal reservoir, an ...
and
isobaric Isobar may refer to: * Isobar (meteorology), a line connecting points of equal atmospheric pressure reduced to sea level on the maps. * Isobaric process In thermodynamics, an isobaric process is a type of thermodynamic process in which the pr ...
CVI. A typical demonstration of the process is shown in Figure 1. Here, the gases and matrix material enter the reactor from the feed system at the bottom of the reactor. The fibrous preform undergoes a chemical reaction at high temperature with the matrix material and thus the latter infiltrates in the fiber or preform crevices. The CVI growth mechanism is shown in Figure 2. Here, as the reaction between fibre surface and the matrix material takes place, a coating of matrix is formed on the fibre surface while the fibre diameter decreases. The unreacted reactants along with gases exit the reactor via outlet system and are transferred to an effluent treatment plant.


Modified CVI

The ‘hot wall’ technique – isothermal and isobaric CVI, is still widely used. However, the processing time is typically very long and the deposition rate is slow, so new routes have been invented to develop more rapid infiltration techniques: Thermal-gradient CVI with forced flow – In this process, a forced flow of gases and matrix material is used to achieve less porous and more uniformly dense material. Here, the gaseous mixture along with the matrix material is passed at a pressurised flow through the preform or fibrous material. This process is carried out at a temperature gradient from 1050 °C at water cooled zone to 1200 °C at furnace zone is achieved. The Figure 3 shows the diagrammatic representation of a typical Forced-flow CVI (FCVI).


Types of ceramic matrix composites with process parameters

Table 1 : Examples of Different processes of CMCs.


Examples

Some examples where CVI process is used in the manufacturing are: Carbon / Carbon Composites (C/C) Based on previous study, a PAN-based carbon felt is chosen as preform, while kerosene is chosen as a precursor. The infiltration of matrix in the preform is performed at 1050 ℃ for several hours at atmospheric pressure by the FCVI. The inner of the upper surface of preform temperature should be kept at 1050 ℃, middle at 1080 ℃ and the outer at 1020 ℃. Nitrogen gas flows through the reactor for safety. Silicon Carbide / Silicon Carbide (SiC/SiC) Matrix:CH3SiCl3 (g) SiC(s)+ 3 HCl(g) Interphase: CH4(g) C(s)+ 2H2(g) The SiC fibers serve as a preform which is heated up to about 1000 ℃ in vacuum and then CH4 gas is introduced into the preform as the interlayer between fiber and matrix. This process lasts for 70 minutes under pressure. Next, the
methyltrichlorosilane Methyltrichlorosilane, also known as trichloromethylsilane, is a monomer and organosilicon compound with the formula CH3SiCl3. It is a colorless liquid with a sharp odor similar to that of hydrochloric acid. As methyltrichlorosilane is a reacti ...
was carried by hydrogen into the chamber. The preform is in SiC matrix for hours at 1000 ℃ under pressure.


Advantages of CVI

Residual stresses are lower due to lower infiltration temperature. Large complex shapes can be produced. The composite prepared by this method have enhanced mechanical properties, corrosion resistance and thermal-shock resistance. Various matrices and fibre combination can be used to produce different composite properties. (SiC, C, Si3N4, BN, B4C, ZrC, etc.). There is very little damage to fibres and to the geometry of the preform due to low infiltration temperature and pressures. This process gives considerable flexibility in selecting fibers and matrices. Very pure and uniform matrix can be obtained by carefully controlling the purity of gases.


Disadvantages

The residual porosity is about 10 to 15% which is high; the production rate is low; the capital investment, production and processing costs are high.


Applications

CVI is used to build a variety of high-performance components: * Heat-shield systems for space vehicles. * High-temperature systems like combustion chambers, turbine blades, stator vanes, and disc brakes which experience extreme thermal shock. * In the case of burners, high-temperature valves and gas ducts, oxides of CMCs are used. Components of slide bearings for providing corrosion resistance and wear resistance.


References

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External links


Center for Composite Materials

World Academy of Ceramics
Ceramic materials Chemical processes Industrial processes Plastics industry