Cvi

Multiple Cycles to Obtain Desired Density

Multiple Cycles to Obtain Desired Density

Final Oxidation Resistant Coatings

Machining to Open Surface

Final Oxidation Resistant Coatings

Machining to Open Surface

Fig. 10.14. Chemical Vapor Infiltration Fabrication Sequence

Initial Fiber Array Initial Fiber Coating Continued Fiber Coating

Fig. 10.15. Chemical Vapor Infiltration Growth21

Initial Fiber Array Initial Fiber Coating Continued Fiber Coating

Fig. 10.15. Chemical Vapor Infiltration Growth21

pumped into the chamber and flow around and diffuse into the preform. The gases decompose or react to deposit a solid onto and around the fibers. As the reaction progresses, the apparent diameter of the fibers increases and eventually fills the available porosity, as depicted in Fig. 10.15. Since this is essentially a deposition process, there is very little mechanical stress on the fibers. The primary processes involved are mass and heat transfer, with the objective to maximize the rate of matrix deposition while minimizing density gradients.20

One of the problems with CVI is that the reactions occur preferentially at, or near, the first surfaces contacted by the reactant gases, resulting in sealing-off the interior pores in the preform. To minimize this effect, it is necessary to run the process at low temperatures, reduced pressures, and to use dilute reactant concentrations, all of which translate into long processing times.

The most important uses of CVI are to produce carbon and silicon carbide matrix composites. A carbon matrix can be formed by the decomposition of methane (e.g., CH4+nitrogen+hydrogen) on a hot fibrous preform. For a silicon carbide matrix, methyltrichlorosilane (CH3SiCl3) is used, along with hydrogen at 1800° F, according to the following reaction:

To obtain the desired microstructure and delay the closing of porosity at the preform surface, it is important to control parameters such as the temperature, pressure, and flow rate of the gases, and the preform temperature. It should be noted that the microstructure produced by CVI is normally not as fine as that produced by hot pressing. In addition, carbon fiber and silicon carbide are incompatible from a coefficient of thermal expansion stand point, and the silicon carbide matrix will develop microcracks allowing oxygen penetration into the structure during service. Therefore, SiC fibers are often used with silicon carbide matrices.

Fig. 10.16. Chemical Vapor Infiltration Reactor

The equipment used for CVI includes gas handling and distribution equipment, a reactor or furnace to heat the substrates, a pressure control system, and scrubbers or traps to remove hazardous materials from the effluent gases. The details of a reactor are shown in Fig. 10.16, and a schematic of the equipment layout is given in Fig. 10.17. Reactants may be gases, liquids, or solids at room temperature. Direct evaporation of liquids and solids can be accomplished; however, liquids are usually swept through the reactor as vapors, by bubbling carrier gases such as hydrogen, argon, nitrogen, or helium through the reactant liquid. The reactors are normally heated by either resistance or induction. Graphite fixtures are used to support the preforms during the initial stages of infiltration. Vacuum pumps are used to control the process pressure.20

There are a number of variations to the CVI process, the most important being the isothermal, forced gradient and pulse flow processes. The objectives of the forced gradient and pulse flow processes are to utilize temperature (or temperature and pressure) gradients, or by pulsing the reactant gases, to reduce the long cycle times inherent in the isothermal process.

The isothermal process is by far the most common process and the only widely used commercially process. The gases flow outside the preform by convection and inside the preform by diffusion. To delay sealing-off the interior of the preform by surface crusting, deposition is performed at relatively low temperatures

Inert Gas for Pressure Control

<-Clean Water

Scrubber -►Waste Water

Vacuum Pump

Fig. 10.17. Chemical Vapor Infiltration Equipment and reduced pressures. The relatively low temperatures result in lower reaction rates, with the reduced pressures favoring diffusion. Since pore sealing on the surface occurs, sometimes called crusting or canning, it is necessary to periodically remove the part and machine the surface to allow further densification. An advantage of the isothermal process is that a furnace can be loaded with a number of parts with different configurations for processing at the same time.

In forced gradient CVI, a graphite tool that holds the preform is placed in contact with a water-cooled metallic gas distributor. The preform is heated on one side and the reactant gases are injected through the cooler side where almost no deposition occurs under pressure. The reactant gases pass unreacted through the preform because of the lower temperature. When the gases reach the hot zone, they decompose and deposit on the fibers to form the matrix. As the matrix material gets deposited in the hot portion of the preform, the preform density and thermal conductivity increases and the hot zone moves progressively from the hot side toward the cooler side. This process reduces the number of machining cycles required to obtain a dense part.

In the pressure pulse CVI process, the chamber is evacuated and reactant gases are then injected for a very short time followed by another evacuation. This cycle is repeated until the part is fully dense. The pulsing action speeds the deposition process by continually removing spent gases and supplying fresh reactant gases, but still does not eliminate all of the machining operations.

The CVI method offers several advantages. First, it is conducted at relatively low temperatures, so damage to the fibers is minimal. Since most interfacial coatings are applied using CVD, matrix infiltration can be conducted immediately after the interfacial coatings are applied. It can also be used to fabricate fairly large and complex near net shapes. The mechanical and thermal properties

Normal Operating Range 1550-2000°F 0.1-14 psi

CVI Reactor

Normal Operating Range 1550-2000°F 0.1-14 psi

CVI Reactor

<-Clean Water

Scrubber -►Waste Water

Fig. 10.17. Chemical Vapor Infiltration Equipment

Methyltrichlorosilane

Preform

Pressure Sensor

Methyltrichlorosilane

Preform

Pressure Sensor are good because high purity matrices with controlled microstructures can be obtained. In addition, a large number of ceramic matrices can be formed using CVI, a few of these are listed in Table 10.4. The major disadvantage of the CVI process is that it is not possible to obtain a fully dense part since the amount of residual porosity (Fig. 10.18) is around 10-15%, which adversely affects the

Table 10.4 CVI Reactions for Ceramic Matrices

Ceramic Matrix

Reactant Gases

Reactant Temperature (° F)

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