Chemical vapor infiltration
Chemical vapour infiltration (CVI) is a ceramic engineering process whereby matrix material is infiltrated into fibrous preforms by the use of reactive gases at elevated temperature to form fiber-reinforced composites. The earliest use of CVI was the infiltration of fibrous alumina with chromium carbide. CVI can be applied to the production of carbon-carbon composites and ceramic matrix composites. A similar technique is chemical vapour deposition (CVD), the main difference being that the deposition process of CVD is on hot bulk surfaces, while the deposition process of CVI is on porous substrates.
During a chemical vapour infiltration process 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 process 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 is used in a conventional isothermal and isobaric CVI process.
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 process 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 process
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 process with forced flow process – 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 process (FCVI).
Types of ceramic matrix composites with process parameters
Table 1 : Examples of Different processes of CMCs.
|Fiber||Matrix||Common Precursor||Temperature(℃)||Pressure (kpa)||Process|
|Carbon||Carbon||Kerosene, Methane||Approximate 1000||1||Forced flow CVI|
|Carbon||Silicon Carbide||CH3SiCl3-H2||Approximate 1000||1||Forced flow CVI|
|Silicon Carbide||Silicon Carbide||CH3SiCl3-H2||900-1100||10-100||Isobaric – Forced flow CVI|
Some examples where CVI process is used in the manufacturing are:
Carbon / Carbon Composites (C/C) Based on previous study, a PAN-based[clarification needed] 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 was carried by hydrogen into the chamber. The preform is in SiC matrix for hours at 1000 ℃ under pressure.
Advantages of CVI process
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 less 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.
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.
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.
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